Auxiliary Boiler Survey

Auxiliary Boiler Survey

Boiler survey through here! MTPNO 867 Machinery SiO Høvik, 2006.08.26

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Table of Content Preface

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Introduction

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Chapter 1: Boilers, Understanding the Basics • • • • • • •

Introduction Steam Fundamentals Heat Transfer Circulation Feed and Boiler water Treatment Material applications in marine boilers Conclusion Page 49

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012 012 016 020 024 035

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056 056 058 061 062 064 066 067 068 070 071 072

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074 074 076 077 078 082 088 096 103 135 144

Chapter 2: Guide to Boiler Failure Modes • • • • • • • • • • • •

Introduction Deposit or Scale Formations, Water Side Long Term Overheating Short Term Overheating Caustic Corrosion, Water Side Low pH Corrosion, during service, Water Side Low pH Corrosion, during Acid Cleaning, Water Side Oxygen Corrosion, Water Side Oil Ash Corrosion, Fire Side Cold End Corrosion, Fire Side Corrosion Fatigue Cracking Stress Corrosion Cracking

Chapter 3: Auxiliary Boiler Survey • • • • • • • • • • •

Introduction Survey Preparation Survey Safety Measures Shell Type Boilers Horizontal Shell Type Boilers Vertical Shell Type Boilers Water Tube Boilers Types of Horizontal Shell Boilers Types of Vertical Shell Boilers Types of Vertical Composite Boilers Types of Two Drum Water Tube Boilers

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Chapter 4: Combustion and Atomizers • • • • • • • • • • •

Introduction Combustion Atomizers Spill Type Pressure Jets Atomizers Spinning Cub Atomizers Steam assisted Pressure Jets Atomizers Ignition Burner Burner Safety Systems Oil Fired Combustion System Survey Visual Survey Function Test

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146 146 148 148 150 151 153 154 155 155 157

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159 159 159 159 160

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161 161 172 174 176 177 178

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182 182 189 190 192

Chapter 5: Refractories and insulation • • • • •

Introduction Refractories Survey of Refractory Insulation Survey of Insulation

Chapter 6: Boiler Mountings and Fittings • • • • • • •

Introduction Safety Valves Boiler Valves Water Level Gauges Pressure Gauges Boiler Plate Soot Blowers

Chapter 7: Boiler Control and Monitoring • • • • •

Introduction Automated Feed Water Regulation Automated Combustion Control Monitoring of Auxiliary Boilers Testing of Control and Monitoring System

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Preface Det Norske Veritas has experienced a number of incidents where different types of steam boilers have caused fatal accidents and material damages, especially involving oil-fired auxiliary boilers older than ten years. Considering the severe consequences that equipment failure may have on crew’s safety and ship operations, steam boilers may represent a potentially high risk factor if improperly maintained. These poorly maintained boilers can result in furnace explosions, as well as rupturing of the pressurized parts. We have noted that some of our Surveyors need to increase their competence related to performance of boiler surveys. Based on this and the impression that competence related to boiler operation and maintenance is decreasing now days among seafarers, it was decided to develop this course. We hope the content will be of interest to you as a surveyor. As a reminder of the possible catastrophic consequences of boiler failure, please see examples below.

Result of an exhaust gas boiler failure, capacity 1.5 Ton/Hr, steam pressure 5 bars.

Result of an exploded smoke tube boiler of 14 years old, the remains of the boiler are indicated by the red circle.

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As you see the consequences are huge. That's why it is so important that you as a DNV surveyor know what you are dealing with, and are able to take the correct decisions based on your observation.

Good luck with the course! Kim Rolfsen Head of Section Machinery Ships in Operation Maritime Technology and Production Centre

On behalf MTPNO 867 Machinery SiO I like to convey our gratitude to all the Surveyors at Høvik, and at the stations who supported us in realizing the course by reviewing the content, supplying pictures, and giving valuable comments. Frans Paardekooper Project Manager, Auxiliary Boiler Survey Course MTPNO 867 Machinery SiO Høvik, 2006.08.26

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Introduction Scope of Course The purpose of this course is to equip the Surveyor with the basic, minimal amount of knowledge necessary in order to competently survey a marine boiler. The content of this course is focussed on oil fired auxiliary boilers for marine use, since this is the type we most frequently encounter for survey. Nevertheless the material presented is also applicable for other types of marine boilers, such as main and exhaust gas boilers. Boiler History In 200 B.C. a Greek named Hero designed a simple machine that used steam as a power source, named aelopile meaning rotary steam engine. It took many centuries before this invention was put into practical use.

Hero´s aelopile.

Steam generation as an industry began in the 17th century and the development was sparked off by the rising demand for ore, minerals, and coal. In order to satisfy this demand mines became deeper and as a result were often flooded with ground water. The first commercially successful steam engine, including boiler was patented by Thomas Savery in 1698, and its purpose was to pump water from the mines. These early boilers were made of copper and riveted construction, they delivered steam just above atmospheric pressure.

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Savery´s engine, 1700.

Developments in material, production, engine technology, and the ever increasing demand for higher power output and efficiency led to boiler designs with higher steam production and pressures. Turning our attention now to marine engineering, it took until 1803 before the first steam propulsion plant was installed on the paddle wheeled vessel Charlotte Dundas. This was quickly superseded by the passenger vessel Chermont in 1807, and in 1811 by the famous Comet.

The paddle wheel vessel Charlotte Dundas.

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As a result of many boiler explosions with fatal consequences the first safety regulations were issued in 1817. Boilers had to be made of wrought iron or copper (no cast iron), and were subjected to pressure testing and inspections. Successful introduction of the screw propeller in 1837 gave a great impetus to steam propulsion. The vertical compound engine appeared in 1854, and this engine required higher steam pressures. Improved boiler designs permitted working pressures of 1.7 bars, and with the introduction of the triple expansion engine in 1871 this was raised to 4 bars.

The Scotch boiler (Tank type boiler). By the end of the nineteenth century (1880) it was realised that a new type of boiler had to be used due to: 1. Introduction of the steam turbine which required higher steam production and pressures 2. The maximum working pressure for a tank (Scotch) type boiler was at that time considered to be 11 bar, this in view of plate thickness and associated weight. 3. To be able to rise steam pressure more quickly, important for warships. 4. Limit the consequences of pressure part rupture. In the year 1880 it was reported that there were 170 boiler explosions in the US, with a loss of 259 lives, and 555 people injured.

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Sectional header main boiler (water tube boiler)

The way forward was considered to be the water tube boiler design. The earliest patent is from William Blakey in 1766, but the first successfully used types are from James Rumsey in 1788. The first designs suffered from circulation deficiencies, inadequate water treatment, and poor tube arrangement. It took until 1889 before the water tube boiler was first tried on the yacht Reverie, and its success caused a rapid development of this concept for naval and merchant vessels. The drum type water tube boiler came into practical being in the 1890`s, this was made possible by the availability of rolled steel plates making economical drum production feasible.

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Two drum, D Type main boiler.

Further developments have lead to the two and three drum water tube boilers so that ever rising steam temperature could be controlled in an economical way. Modern use of Steam The ability of mankind to generate steam in a safe and dependable manner is one of those few technologies that initiated a series of events. It started the industrial revolution in the late 17th century and is still shaping today’s world. Most of the electricity we consume today is produced by steam, it’s also used in numerous production processes. At present we operate land based water tube boilers for electricity production of 1300 MW, with a steam production of 1232 kg/s, 265 bars, and 543˚C.

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A steam boiler of a modern power plant, make VGB Power Tech.

Focusing on the maritime industry we can divide the boilers as follows: •

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Main boilers, supplying steam for the propulsion and auxiliary turbines. These are two drum water tube boilers, an example is the B&W Radiation boiler type MRR, steam production 24.5 kg/s, 104 bar, and 513˚C. Today we find steam propulsion mostly on LNG tankers and some older VLCC’s. Auxiliary boilers supplies steam for heating of fuel and cargo. On oil tankers the generated steam is used to drive cargo pump turbines.

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Exhaust gas boilers or economisers are installed on almost all vessels with oil fired auxiliary boilers and increase the plants overall efficiency by utilising the waste heat in the main engine exhaust. The produced steam is generally used for heating purposes.

We can also divide the boilers according to their construction, shell, horizontal, vertical type, and water tube type, this will be addressed later in the course.

B&W Radiation main boiler type MRR, steam production 24.5 kg/s, 104 bars, and 513˚C.

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Chapter 1: Boilers, Understanding the Basics Introduction Many people unfamiliar with boilers have the impression that they are basically just large water kettles, however a boiler is complex and there are many comprehensive books written on the subject. The scope of this chapter is to summarize much of that vast amount of information about boilers into the important, basic facts relevant to Surveyors.

Steam Fundamentals Steam maintained its dominant position as a working fluid in thermodynamic cycles because of its unparalleled combination of high thermal capacity, high critical temperature, wide availability, and nontoxic nature. Key properties of a working fluid are: • Pressure and temperature. • Enthalpy, which can be described as the internal stored energy per unit of mass. • Entropy, which can be explained as a measure of the thermodynamic potential of a system in units of energy, per units of mass. • Specific volume. In a steam process or cycle we may find steam in the following conditions: • Saturated steam or sometimes called dry steam. In this condition there is a unique relationship between pressure and temperature as tabulated in the steam tables. When one property (temp. or Pressure) is known one can find the corresponding enthalpy, temp. / pressure in the steam tables. Most auxiliary boilers generate saturated steam which is utilised for heating of cargo, fuel, accommodation, and other utilities. Chosen steam pressures are usually between 6 to 18 bars, this represents the most optimal combination of the steam thermal capacity (enthalpy) and necessary material thickness of boiler and system. • Superheated steam (sometimes named live steam) is created by heating saturated steam of a given pressure, above the saturation temperature. Superheated steam is found on vessels equipped with steam turbines. The advantage of this is an increased thermal efficiency of the installation, higher thermal capacity of the steam, and steam expansion can be continued longer in the turbine. This due to the delayed formation of water droplets in the steam, which starts at the saturation temperature. • Wet or finished steam is a mixture of steam and water. It is found in the last stages of the turbine before the condenser. Taking a closer look at figure 1, temperature / enthalpy diagram in which the steam generation process is set out. The following areas are distinguished. • Line A to B: Water is in the liquid phase, left of the line is the liquid region. • Line B to C: Liquid and vapour phases coexist (wet steam), water is evaporating at constant temperature. In point C we have 100% vapour, saturated steam. • Line C to D: Steam is superheated in superheat region. • Line A, B, C, and D is a line of constant pressure.

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•

•

The total enthalpy can be split in a liquid enthalpy (A-B), and enthalpy for evaporation (A-C). In the lower pressure regions of the diagram the evaporation enthalpy is a bigger part of the total enthalpy as in the higher pressure regions. This evaporation energy is released during condensation of the steam in heating coils, this is why high pressure steam (40-100 bars) is not used for heating purposes. The saturated liquid and vapour line meet at the indicated critical point, 221 bars and 374 ˚C. At this point water no longer exhibits boiling behaviour, it changes instantly to steam. The difference in density between liquid and vapour phase is zero, one cubic meter of water weighs the same as one cubic meter of steam. Therefore natural circulation is not possible, this is expanded upon in chapter on circulation. An important point to remember is, the closer the boiler is operated near the critical point the more problematic it becomes to achieve a good natural circulation.

Fig. 1 Temperature / Enthalpy diagram of steam generation process.

Although the process of boiling water is a familiar phenomenon, in general terms it may be described as a heat transfer process where heat addition to a liquid no longer raises its temperature, but heat is absorbed as the liquid becomes a gas. If the boiling process in a simple water cooker is examined, the following stages can be differentiated (see Fig. 2). 1. Incipient boiling: The temperature of the water adjacent to the heated surface slightly exceeds the local saturation temperature of the water while the bulk of the water remains sub cooled. Very small bubbles are formed adjacent to the heated surface, which periodically collapse as they come in to contact with the cooler water. 2. Nucleate boiling: As head transfer rate increases the temperature of the water reaches saturation temperature and the bubbles are no longer confined to the heated surface, they move into the fluid. Steam generation has started.

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3. Film boiling: Further increase of the heat flux causes larger surface evaporation rates which eventually restrict the liquid flow to the surface. It will cover the heated surface with an insulating layer of steam and the ability of the surface to transfer heat drops.

Fig. 2 Transition from heating to boiling (ebullition) as wall temperature increases.

In designing boilers care must be exercised to control film boiling. In high heat input locations such as furnaces it is important to maintain nucleate boiling in order to adequately cool the surface and prevent material failure. Film boiling may occur in existing boilers caused by a disturbance of the circulation, resulting in insufficient cooling. Or an increase of the head input due to flame impingement. Figure No. 3 below illustrates a boiling curve of a heated wire in a pool, although the characteristics are similar for most situations. The heat transfer rate per unit area (heat flux) is plotted versus the temperature difference between metal surface and bulk fluid. Incipient boiling is called subcooled nucleate boiling in this illustration, the following points are noted. • At point C film boiling has commenced. This transition is referred to as the “critical heat flux” CHF, “departure from nucleate boiling” DNB, burn out, dry out, or boiling crisis. • Line C-D: This is the onset of film boiling, more of the heated surface is blanketed with steam.

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•

•

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Line D-D´: In fossil fuel boiler furnaces the head input is effectively independent of the surface temperature. Therefore a reduction in the heat transfer rate results in a corresponding increase in surface temperature from point D to D`. In some cases the elevated metal temperature is so high that the metal surface may melt. Line D-E: If the heat transfer rate is dependent upon the surface temperature, typically for a nuclear steam generator, the average local temperature of the surface increases as the local heat transfer rate declines. This region (D-E) is referred to as unstable film boiling or transition boiling. Because a large surface temperature increase does not occur, the main consequences are a decline in heat transfer performance. Line E-D´-F: The surface is effectively blanketed by an insulating layer of steam or vapour. The energy is transferred from the solid surface through this layer by radiation, conduction, and micro convection to the vapour interface. From this interface, evaporation occurs and bubbles depart. This heat transfer region is called stable film boiling.

Fig. 3 Boiling curve-heat flux versus applied temperature difference.

From the above it is clear that an oil fired boiler must not be operated beyond point C, once point D is reached there may be the possibility to jump to D´ while the head flux remains unchanged. This will result in a substantial rise of the surface temperature which will most likely lead to damages.

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Heat Transfer Introduction Heat transfer deals with the transmission of thermal energy and plays a central role in boilers. The following are the three basic modes of heat transfer. 1. Conduction: Transfer of thermal energy due to a temperature difference between adjacent molecules in a solid, for example a steel plate or tube. 2. Convection: Transfer of thermal energy within a liquid or gas by a combination of molecular conduction and macroscopic fluid motion. In boilers it occurs adjacent to heated surfaces as a result of fluid motion (water or gas) passing that surface. 3. Radiation: Transfer of thermal energy between bodies by electromagnetic waves. This transfer requires no intervening medium as with conduction and convection. One or more of these modes may takes place simultaneously in a boiler, at one location, and controls the amount of heat transferred. Conduction The laws of physics concerning heat flow by conduction are. • • • •

Heat flows in the direction of decreasing temperature. The flow of heat per unit of time is proportional to change of temperature in the direction of the heat flow, and the dimensions of exposed area. The heat transferred per unit of time is inversely proportional to the wall, material thickness. Heat flow also depends on a material property named thermal conductivity which differs for various materials.

Convection In boilers convection occurs during heat transfer between flue gas/tube on the gas side, and tube/feed water on the steam side. Convection has two forms. 1. Natural convection: Fluid motion is due to local density differences alone, heated lighter fluid rises and is replaced by cooler fluid. 2. Forced convection: A mechanical force from a fan or circulation pump gives motion to the fluid. Natural convection takes place on the steam / water side and forced convection on the gas side in marine auxiliary boilers. As with conduction the heat flows in the direction of descending temperature, and is proportional to the temperature change and area exposed. Also here the heat flux depends on a fluid property called “convection heat transfer coefficient”, which is a function of the thermal and hydrodynamic properties (pressure, temp., flow) of the liquid or gas and surface geometry.

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Radiation The amount of radiant energy emitted by a body is determined by its temperature and the nature of the surface. In order for a body to absorb radiant energy its absolute temperature needs to be lower than that of the emitting body. Two forms of radiation are encountered in a boiler. 1. Flame radiation is found in the furnace, this is mainly caused by glowing carbon particles which are created during the combustion. 2. Flue gas radiation is encountered outside the furnace in the convection part of the boiler, and is contributed by the large presence of CO and HO in the flue gases. Contamination by soot of the heated surface will result in a lesser amount of radiant energy being absorbed, leading to higher flue gas temperatures in other parts of the boiler and the funnel. Contamination of heated surface In boilers we experience fouling of the heated surface by soot on the flue gas side, and by scale and deposits on the steam / water side. Occasionally we are confronted with steam / water side contamination by oil as a consequence of a leaking fuel tank heating coil. Contamination of flue gas side Fouling by soot of the heated surface will reduce the heat transfer rate, and thereby less steam will be generated. It will however not lead to higher tube wall temperatures since an insulating soot layer is formed. In practice it will normally cause an increase of wall temperature since a certain amount of steam is necessary for the vessel’s operation, therefore this steam reduction will be compensated by burning more fuel. Contamination of steam / water side Fouling of the steam / water side will also decrease the transferred heat flux and result in less steam production. But more importantly it will lead to substantial higher tube wall temperatures. Contamination by oil is especially dangerous, since oil isolates 20 times better than a layer of scale of the same thickness. This leads to overheating and reduced material strength. An oil deposit of only 0.5 mm is calculated to lead to a 1/3 strength reduction of its original design value. Also here one should keep in mind that reduced steam generation is commonly compensated by combusting more fuel, adding an additional temperature increase.

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Fig. 1 Heat transfer in a furnace wall by conduction.

Table 1 Calculated surface temperatures (T1, T2, T0) for clean and contaminated tubes.

The above is illustrated in figure No. 1, in conclusion the furnace wall temperature is increased from 256˚C to 633˚C with an oil film of just 0.5 mm. Furthermore the transferred heat is reduced from 137 kW/m to just 82.3 kW/m, a reduction of 40%. Consequently the thermal efficiency of a fouled boiler will be reduced, fuel consumption is multiplied, and the risk for damages significantly increased.

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Aalborg AQ 3 boiler, damaged area in pictures below is indicated by red circle.

Furnace tope plate deformed and weld to flue gas uptake pipe fractured, due to contamination of heated surface.

Result of oil contamination, deformed furnace plate viewed from in side the furnace.

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Circulation Introduction For a system to generate steam continuously and keep material temperatures within design limits, water must circulated through the tubes. Two different approaches are commonly used. 1. Natural or thermal circulation, encountered in marine boilers. 2. Forced or pumped circulation, utilised in land based power plant and exhaust gas boilers. Natural Circulation In an unheated downcomer no steam is present (Fig. 1). Heat addition generates a steam water mixture in the riser. Because the steam water mixture in the riser is less dense than the water in the downcomer, gravity will cause the water to flow downward in the downcomer and will cause the steam water mixture to move upwards into the steam drum.

Fig. 1 Natural circulation loop.

The rate of circulation depends upon the difference in average density between the unheated water and the steam water mixture. The total circulation rate potentially depends primarily upon four factors.

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1. Height of the boiler: Taller boilers result in a larger total pressure difference between the heated and unheated legs and therefore produce larger total flow rates 2. Operating pressure: Higher operating pressures provide higher density steam and steam water mixtures. Thus reducing the total weight difference between the two and reducing the flow rate. 3. Heat input rate: Higher heat input typically increases the amount of steam in the heated riser and reduces the average density of the water steam mixture, increasing total flow rate. 4. Free flow area of the components: An increase in cross sectional (free flow) areas for the water and water steam mixture may increase the circulation rate. Forced Circulation As illustrated below (see Fig. 2), a mechanical pump is added to the flow loop and the pressure difference created by the pump controls the water flow rate.

Fig. 2 Simple forced circulation loop.

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Unlike natural circulation, forced circulation does not enjoy an inherent flow compensation effect when heat input changes. Flow rate does not increase significantly with increasing heat flux. Two different systems are distinguished. 1. Re-circulating system: The circulation pump suction is supplied by gravity from the drum and forces water through the heated riser, a water steam mixture is generated and discharged in to the steam drum. Steam is separated from the mixture and the water re-circulates. 2. Once through system: This system provides continuous evaporation of slightly sub cooled water to 100% steam, without steam water separation, a steam drum is not required. Forced circulation is mainly used where boilers are designed to operate near or above the critical pressure of 221.3 bars. The forced re-circulation system is also utilised in exhaust gas boilers. Circulation in Marine Boilers Natural circulation is predominately found in today’s marine boilers. The figure (Fig. 3) below illustrates a circulation loop for a two drum boiler. Blue, yellow, and red coloured downcomers (arrows) supply the water drum and headers with relative cool feed water (sub saturation temperature), via the generating and furnace tubes a steam water mixture is returned to the steam drum.

Fig. 3 Circulation loop, two drum boiler.

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Because of the high constant heat flux with these boilers an uninterrupted cooling especially of the furnace tubes is essential. Inadequate cooling can result in rapid overheating, cycling thermal stress failures or material failures from different tube expansions. Circulation can be locally disturbed by tube blockage due to deposits or flow interruption. Also in tank or shell type boilers a natural circulation is generated within the water content of the boiler. The circulation in these boilers is less critical on account of a lower constant heat flux and operating pressure.

Also in vertical shell type boilers natural circulation takes place.

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Feed and Boiler Water Treatment Introduction In boilers, water is converted into steam, which leaves the boiler in a relatively pure state. Impurities (other than gases) which enter with the feed water are retained and concentrated in the boiler water. If left unattended this may result in the following: •

Formation of hard scales, these are formed by certain constituents in zones of high heat input leading to a retardation of heat flow, and raising the metal temperature above normal operating temperatures. This can cause overheating and ultimately failure of pressure parts.

Two examples of poor boiler water treatment, hard scale and sludge deposits.

• •

Sludge, or solid particles normally carried in suspension, may settle locally and restrict the flow of cooling water, or in some cases, may deposit in the form of insulating layers with an effect similar to that of hard scale. Oil and grease prevent adequate wetting of the internal surface, and in areas of high heat input causes overheating. Or the oil / grease may carbonize and form a tightly adherent insulating coating.

Two examples of oil contaminated boilers, both need chemical cleaning.

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• • •

Dissolved gases and acidic conditions result in corrosion which can weaken the boiler by removal of metal. This usually occurs in localized areas in the form of cavities and pits. Certain chemicals if present in specific concentrations may produce intergranular attack on the metal, leading to embrittlement and failure. High concentrations of foam producing solids in the boiler water results in water carry over and contaminate the steam.

From the above it is evident that the ultimate purpose of feed and boiler water treatment is to keep the internal surfaces free of scale or sludge, and prevent the corrosion of these surfaces, thereby maintaining the integrity and performance of the boiler.

Magnetite layer on internal surface of a steam drum (left) and split water tube (right), sign of good water treatment.

The permissible amount of contaminants and treatment chemicals entering the boiler decreases with rising boiler pressures and heat transfer rate, therefore the required boiler water quality level increases with higher steam pressures. Boiler water quality has a significant influence on deposition, of which the insulating effects become less tolerable as pressures rises, because overheating is more likely. Feed water The total feed water flow to a boiler normally comprises of a small quantity of make up water, to replace water lost from the system due to leakage or by blow down, together with the condensate recovered from the system. Make up water Virtually all ocean going vessels use make up water evaporated from seawater, thus contaminants / feed water treatment is minimized. Some contaminants

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may be encountered in the distillate due to carry over of water particles with the vapor, and re-absorption of non-condensable gases. Additional solids (scale formers) removal is not required, however dissolved gases (oxygen, CO2) must be removed to prevent corrosion. Condensate In a well maintained feed system the amount of make up water required will be minimal and the bulk of the feed water will be returned condensate. The main problem arising with the use of condensate is the possible pick up of copper from copper alloys used for condenser tubes. Corrosion of aluminium-brass or cupronickel may take place, with the result that copper corrosion products will be returned to concentrate in the boiler. This problem is aggravated by any ingress of sea water to the system. The copper oxides deposit on the heated surfaces and cause galvanic corrosion. Scale Scale formation in boilers leads to lower efficiency because of a reduction of heat transfer rate. Overheating and tube failure may result, and often high cost of chemical cleaning may be entailed.

Accumulation of scale deposits will reduce heat transfer and boiler efficiency.

The salts of calcium and magnesium are the main source of scale problems. It is possible to eliminate these contaminants from the make up water before entry into the system, but for most marine boilers the alternative is to use chemicals to modify the scale formers so that they are precipitated as a relative non adherent sludge, which can be blown out of the boiler before any scale is formed. The common chemicals used to prevent formation of scale are: • Sodium phosphate. This is used to precipitate the calcium (lime) salts from the solution as calcium phosphate sludge. • Sodium Hydroxide. This is also known as caustic soda and it precipitates magnesium salts from the solution as magnesium hydroxide sludge. These chemicals are normally added as a dilute solution, fed to the boiler either by means of a proportioning pump, or by injection from a pressure pot direct into the boiler.

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Accumulation of sludge has a comparable effect as scale build up, reduced heat transfer and overheating.

Corrosion The presence of dissolved gases such as oxygen and carbon dioxide in feed and boiler water will cause corrosion. However, it does not always occur in the form of general wastage, but often as localized deep pitting which can readily lead to tube failure. Oxygen One of the most common reasons for boiler corrosion remains the action of dissolved oxygen in make up and feed water. Generally, oxygen pitting will occur near or above the waterline in the steam drum of an operated boiler, or very close to the feed water entry point.

An oxygen corrosion pitting damaged a Sunrod pin tube.

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The oxygen content in feed and boiler water can be reduced by the following means. • Thermal deaeration: The solubility of gases such as oxygen and carbon dioxide in water reduces with increasing water temperatures. Oxygen is removed in a vented deaerating heater where steam and condensate are mixed, or by heating the cascade /hotwell to approximately 90 ˚C. Thermal deaeration will remove up to 75% of the unwanted oxygen, the remaining oxygen needs to be absorbed chemically. • Chemical deaeration (scavenging): The following chemicals are added to the boiler water to remove the remaining oxygen. 1. Sodium Sulphite: This will combine with oxygen to form sodium sulphate, which results in the formation of additional dissolved salt. 2. Hydrazine: This will react chemically with oxygen to form nitrogen and water but will not form dissolved solids.

The oxygen content in water decreases with rising temperature, keep cascade or hotwell tank at a minimum of 85 °C.

Both chemicals are toxic, and hydrazine is considered to be carcinogenic to humans. Carbon dioxide As a result of the chemical reaction between sodium hydroxide (caustic soda) and magnesium (scale former) carbon dioxide is formed, this will combine with water to form carbonic acid. This acid can dissolve ferrous metals both in the boiler and the condensate system. The most common method used to eliminate carbon dioxide is by adding chemicals to the feed water such as hydrazine, and volatile amine.

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Metal passivation The water/steam side metal surface of a boiler is passivated when the right conditions are created (pH 9.5 to 11), this will inhibit further corrosion. Metal passivation is the process by which a base metal surface forms a protective oxide film. For boilers this means that the loose non protective film of hematite (Fe2O3) readily formed when an excess of oxygen is present, is reduced to magnetite (Fe3O4) or black iron oxide. This is a dense, tight protective oxide film which inhibits corrosion because it is a less reactive iron oxide. Over time, this thin mono-molecular film formed by passivators becomes self-repairing and its growth is self-limited because corrosion products necessary for the process are unavailable, as corrosion is inhibited.

Graph is showing the attack of steel at 310 °C by water of varying degrees of acidity and alkalinity.

Carry Over and Priming The term “Carry Over” is the phenomenon of water droplets being carried over with the steam into the steam system. Priming relates to contamination of the steam by injection of gross quantities of water. The effects of the above can be serious, in that water droplets containing suspended and dissolved solids can evaporate later in the steam system, and deposit their entrained solids in the superheater section, or perhaps eventually on the turbine blades.

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This phenomenon is caused by high concentration of impurities in the boiler water which causes foaming to occur. Correct boiler water treatment, and regular blowdown to reduce boiler water impurities will prevent this from happening. Water Treatment Sometimes auxiliary boilers are regarded as “kettles”, and no corrective water treatment for scale prevention or blowdown for sludge ejection is considered necessary. This of course is a fallacy, and adoption of a feed / boiler water treatment procedure will pay dividends in the long run. Treatment for low pressure boilers Suitable feed and boiler water treatment for small low pressure boilers (6-30 bars) can be provided by so called combined (multi) chemical treatment products. This entails one product being added to the boiler water which, precipitates hardness, providing the water with the necessary alkalinity, and scavenges dissolved oxygen. In order to maintain feed and boiler water within the desired quality levels the following tests may be carried out daily. • • • •

Phenolphthalein (P) alkalinity test (100-300 ppm CaCo3): The dosage level of combined treatment product is based on the P alkalinity value. Chloride value test (200 ppm max): This is a reference point for controlling the rate of blowdown, and an indication of seawater contamination. Boiler water pH test: Recommended limits are 9.5 to 11 in order to prevent corrosion attack. Condensate pH test: Recommended limits are 8.3 to 9.0 to control corrosion after the boiler.

Depending on the water analysis results a certain quantity of treatment product is supplied to the boiler via a potfeeder, proportioning pump, or directly in to the hotwell. Chloride values will determine the rate and amount of blowdown necessary to bring the boiler water within recommended levels. Treatment for Medium & High pressure Boilers The use of combined chemical treatment products for these boilers is not adequate, since higher pressures and temperatures increase the tendency for scale and corrosion, making it necessary to have the possibility of changing the chemical conditions and test parameters individually. A coordinated treatment program including single function chemical dosage and monitoring is essential. Feed and boiler water testing are carried out more frequently with these boilers. Normally, this is done twice to four times a day in order to maintain the required water quality level. Also the extent of testing has increased as can be seen in below example. • • • •

Phenolphthalein (P) alkalinity test (100-130 ppm CaCo3): Alkalinity control. Total (M) alkalinity test (below 2 x P alkalinity): Alkalinity control. Phosphates test (20–40 ppm PO4): precipitates hardness. Hydrazine test (0.03-0.15 ppm N2H4): Oxygen control.

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• • •

Chloride test (<30 ppm): Controlling rate of blowdown. Boiler water pH test (9.5-11): Corrosion control. Condensate pH test (8.3-9): Corrosion control.

Depending on the test results, different chemicals are added to the feed and boiler water. Solid concentrations in the boiler water may also be determined by a conductivity meter, it displays a visual readout of the ability of the water to conduct an electrical current. The greater the solid concentration the higher the reading will be. Once a threshold value is reached or exceeded (<2000 microΏ, 300μS/cm) the boiler has to be blown down to reduce solid content to within acceptable limits. Test results, administered chemical dosages, make up water quantity, and blow down rate are normally recorded in a feed / boiler water treatment log. Most major suppliers (Nalco, Drew Asland, Unitor) of treatment chemicals provide the additional service of reviewing these treatment logs and revert back with comments and advice. Boiler Cleaning New Boilers, Initial Water Side Cleaning The initial cleaning of new boilers for service, or that of older equipment after major alterations or repairs entails the boiling out of the unit with a caustic solution, to remove grease and other deposits, which may be present in the steam generating part. During boil out the unit is fired at a low rate to maintain 50 % of the normal operating pressure. The boiling out period is usually from 12 to 36 hours, during which the boiler is blown down periodically through all blow down connections. If necessary, the boil out may be supplemented by an inhibited acid cleaning to remove mill scale. Following the boil out it is general practice to drain the boiler, and as soon a possible start flushing the boiler with hot fresh water. After this operation the boiler is cooled down and thoroughly inspected. If the results are satisfactory the boiler is fired up, if not the cleaning process is repeated. Boiler in Operation For satisfactory and efficient operation a boiler must be kept clean on both the waterside and fireside. With adequate attention to the pre boiler feed system and by maintaining the boiler water chemistry within prescribed limits, there should be little need to clean the waterside. The fireside, on the other hand requires daily attention if the steam temperature and boiler efficiency are to be maintained at their optimum values. Water Side Cleaning If conditions are allowed to deteriorate to the point that scale or baked on sludge are found during waterside inspections, chemical analysis of the deposits will indicate the cleaning method best suitable for their removal. Tubes may be cleaned by passing air turbine driven brushes and scale cutters through each tube, and flushing with high pressure water hose. Scale may be removed from internal surfaces by pneumatic chipping hammer, mechanical wire brush, and washing down with high pressure water hose.

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It may be necessary to de-scale the boiler by acid cleaning if access for mechanical cleaning is limited, or the unit is heavily contaminated. A specialized firm should be consulted to accomplish this process, which entails the use of acid and neutralizing rinsing agents. The acid strength, neutralizer, and the temperature at which they are used are of vital importance if the cleaning process is to be kept within safe limits. Excessive acid strength or un-neutralized acid remaining after cleaning will attack the metal, possibly to the point that parts need to be renewed. The acid cleaning operation normally takes 8 to 36 hours, and is concluded with a thoroughly inspection of the waterside. Fire Side Cleaning Soot blowers are used to clean the boiler fire side, and air or steam is used as the blowing medium. Depending on the boiler type and fuel burned the boiler is at least soot blown every 24 hours, but usually every 4 or 12 hours. Most oil fired auxiliary boilers are not equipped with soot blowers, these boilers need to be periodically washed down with high pressure water hose. Degreasing In the event the waterside is found contaminated with oil, it needs be removed with the help of degreasing chemicals. First the boiler is completely drained and as much oil mopped out as possible, subsequently it is filled with fresh water and degreasing chemicals added in the prescribed quantities. The degreasing period is usually 12 hours after which the boiler is drained and flushed with fresh water. After satisfactory inspection the boiler is fired up and a small dosage of degreasing chemicals is added to the boiler water in order to remove the last traces of oil during operation.

Appendix Reference is made to the below appendixes regarding: • •

Boil out procedure from Asland Boiler Acid Cleaning procedure from Asland

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Drew Marine Boiling Out Newly Constructed and Retubed Marine Boilers LAC™ liquid alkaline cleaner is a combination of fast-acting detergents, wetting agents and alkaline cleaners blended in a water-based carrier. LAC liquid alkaline cleaner can be used in boiling out new or retubed boilers as well as in cleaning contaminated boilers and associated systems. Application Any specific recommendations and procedures given by the boiler manufacturer should be followed regarding initial boil out of new boilers. The procedure is usually necessary to remove initial oil and grease from tube storage and expansion if applicable, plus mill scale formed during construction. Dosage Dosage is normally 2-6% of LAC liquid alkaline cleaner by volume. Procedure NOTE: READ MATERIAL SAFETY DATA SHEET BEFORE HANDLING CHEMICALS AND MAKE SURE THAT GOOD VENTILATION IS PROVIDED. 1. Drain and flush out any loose material with a high pressure water hose. Remove as much oily matter as possible from the boiler by wiping with clean rags. 2. Replace any gauge glasses having mica backing with another type for the boil out or valve off the water column gauge cocks so that no cleaning solution enters the gauge. Important: Make sure that the cocks are tagged “closed” and that they are returned to the normal “open” position after cleaning and flushing cycles. 3. Start filling the boiler with a 2-6% solution by volume of LAC liquid alkaline cleaner and fresh water at 60-70°C. 4. Close manhole openings using plain material gaskets. Fill the boiler to the normal steaming level. 5. Open drum vents and drains on superheater outlets if superheaters are fitted. 6. Start firing the boiler. No steam is to be generated. When steam comes out of vents, indicating a definite pressure, close the vents. Close the superheater inlet drain. Leave outlet drain or outlet vent slightly open. Take care that this highly alkaline solution does not flood over into superheaters if fitted as this can cause high alkaline concentrations on service firing and could be detrimental to superheater material. 7. Raise boiler pressure at a rate not greater than 7 kg/cm2 (100 psig) per hour to half of normal operating pressure. 8. Maintain the above condition for 24 hours. During the 24-hour period, make short blowdowns from drums and headers. Add water as needed to maintain the necessary level.

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MAKE SURE THAT ALL OPERATING PERSONNEL UNDERSTANDS AND FOLLOWS SAFETY PROCEDURES CAREFULLY MENTIONED IN THE MATERIAL SAFETY DATA SHEETS.

MODULE 2.1.4

REMOVAL OF SCALE AND CORROSION PRODUCTS FROM OIL FIRED BOILERS

CHEMICALS:

DESCALE-IT (DESCALANT) GC (NEUTRALIZATION) AMERZINE (PASSIVATION)

EQUIPMENT:

1. 2. 3.

SOLUTION STRENGTH: SOLUTION STRENGTH: SOLUTION STRENGTH:

20 % 1% 0.1%

CHEMICAL CIRCULATION TANK CHEMICAL CIRCULATION PUMP HEATING EQUIPMENT

CLEANING PROCEDURE: PLEASE NOTE: DEPOSITS SUCH AS OIL OR ORGANIC MATERIALS SHOULD BE REMOVED PRIOR TO ACID CLEANING. REFER TO MODULE 2.1.1 OR 2.1.2! • SECURE EQUIPMENT TO BE CLEANED FROM SERVICE, SEGREGATE OR BLANK OFF FROM SYSTEM AS A WHOLE AND COOL BEFORE DRAINING. • OPEN ALL ACCESS PORTS, MANHOLE COVERS AND AS MANY HANDHOLE CAPS AS PRACTICAL. • REMOVE AS MUCH DEBRIS AND DEPOSIT ACCUMULATION AS POSSIBLE BY FLUSHING WITH HIGH VELOCITY WATER FLOW OR MANUALLY. • MAKE NECESSARY CONNECTIONS FOR FILLING AND ATMOSPHERIC VENT LINES. • FILL EQUIPMENT WITH DISTILLATE UNTIL THE TOP ROW IS COVERED WHILE ADDING DESCALE-IT TO ESTABLISH RECOMMENDED STRENGTH. SECURE REMAINING OPENING IN EQUIPMENT MAKING CERTAIN THAT VENT IS FULLY OPEN (HYDROGEN GAS DEVELOPS). • APPLY HEAT FROM AN EXTERNAL SOURCE. TEMPERATURE RANGE TO BE KEPT 50 - 70 °C. DO NOT EXCEED ACID SOLUTIONS OVER 70 °C. • OVERALL CLEANING TIME WILL BE IN THE ORDER OF 4 - 12 HOURS, BUT DO NOT EXCEED 24 HOURS CONTACT TIME WITH ACID SOLUTIONS. • WHEN THE CLEANING IS COMPLETE, COOL AND DRAIN THE EQUIPMENT. THOROUGHLY FLUSH WITH DISTILLATE. • DRAIN THE ACID WASTE SOLUTION TO A TANK OR BILGE WHERE NEUTRALIZATION CAN BE ACCOMPLISHED BEFORE DISCHARGE. • REMOVE ALL LOOSENED DEPOSITS BY FLUSHING OR MANUAL CLEANING. • REFILL THE UNIT WITH DISTILLATE TO THE TOP ROW. • FOR NEUTRALIZATION ADD THE PRECALCULATED AMOUNT OF GC TO THE EQUIPMENT. HEAT TO 50 - 70 °C AND CIRCULATE AS MENTIONED ABOVE FOR 1 - 2 HOURS OR UNTIL THE PH OF THE SOLUTION IS NEUTRAL OR HIGHER. • COOL DOWN AND DRAIN THE SOLUTION TO THE SAME TANK OR BILGE HOLDING THE WASTE ACID FROM THE CLEANING AND FLUSH THE UNIT WITH DISTILLATE. • SECURE OPENING, REFILL SYSTEM WITH WATER, AND CIRCULATE FOR 30 MINUTES TO 1 HOUR. DRAIN AND FLUSH SYSTEM AGAIN. • FOR PASSIVATION REFILL THE BOILER WITH DISTILLATE AND ADD THE PRECALCULATED AMOUNT OF AMERZINE TO THE BOILER. FIRE THE BOILER IN IDLE CONDITION FOR 12 HOURS. THE AMERZINE SOLUTION CAN REMAIN IN THE BOILER. • INITIAL DOSE BOILER WATER TREATMENT CHEMICALS. • EQUIPMENT IS NOW READY FOR RETURN IN SERVICE.

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Material applications in marine boilers Introduction The vast majority of materials used in the construction of boilers are carbon steels. Carbon steels may be defined by the amount of carbon retained in the steel or by the steelmaking practice. These steels are commonly divided into four classes by carbon content. 1. 2. 3. 4.

Low carbon steels: Medium – low carbon steels: Medium – high carbon steels: High carbon steels:

0.15 % carbon maximum between 0.15 and 0.23 % C. between 0.23 and 0.44 % C. more than 0.44 % C

However, from a design viewpoint, high carbon steels are those over 0.35% because these can not be used as welded pressure parts. Medium to Low carbon steels see extensive use as pressure parts particularly in the low pressure application where strength is not a significant design issue. Carbon steels are also referred to as killed, semi killed, rimmed, and capped depending on how the carbon – oxygen reaction of the steel refining process was stopped. During the steel making process, oxygen, introduced to refine the steel, combines with carbon to form a gas. If the oxygen introduced is not removed or combined prior to or during casting by the addition of Si, Al, or some other deoxidizing agent, the gaseous products continue to evolve during solidification of the metal in the mold. The amount of gas evolved during solidification determines the type of steel and the amount of carbon left in the steel. If no gas is evolved and the liquid lies quietly in the mold, it is known as killed steel. With increasing degrees of gas evolution, the products are known as semi killed and rimmed steels. Virtually all steels used in boilers today are fully killed because these steels have the lowest number of volumetric defects, giving a high quality steel. Steel can be altered by modifying its microstructure through heat treatment. Various heat treatments may be used to meet hardness or ductility requirements, improve machinability, refine grain structure, remove internal stresses, or obtain high strength levels or impact properties. The most common heat treatments used during steel fabrication are the following. •

•

Annealing, or also called full annealing is done by heating ferritic steel above the upper critical transformation temperature (A3, 870 - 700 °C), holding it there long enough to fully transform the steel to austenite and then cool it at a controlled rate in the furnace below 316 °C. A full anneal refines the grain structure and provides a relatively soft, ductile material that is free of internal stresses. Process annealing, better known as stress relieving and some times called sub-critical annealing is performed at a temperature just below the lower critical temperature (A1), usually between 510 – 704 °C. Stress relieving neither refines grains nor re-dissolves cementite, but it improves the ductility and decreases residual stresses in work hardened or welded steel.

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Equilibrium diagram showing phase solubility limits, carbon in iron.

•

•

Normalizing is a variation of full annealing. Once it has been heated above the upper critical temperature, normalized steel is cooled in air rather than in a controlled furnace. Normalizing is sometimes used as a homogenization process; it assures that any prior fabrication or heat treatment history of the material is eliminated. Normalizing relieves the internal stresses caused by previous hot or cold working and, while it produces sufficient softness and ductility for many purposes, it leaves the steel harder and with a higher tensile strength than full annealing. To remove cooling stresses, normalizing is often followed by tempering. Tempering is applied after normalizing or quenching of steels. These preliminary heat treatments impart a degree of hardness to the steel but also make it brittle. The object of tempering, a secondary treatment, is to remove some of that brittleness by allowing certain transformations to proceed in the hardened steel. It involves heating to a predetermined point below the lower critical temperature (A1) and is followed by any desired cooling rate. Some hardness is lost by tempering, but toughness

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•

is increased, and stresses induced by quenching are reduced or eliminated. Higher tempering temperatures promote softer and tougher steels. Hardening or quenching occurs when steels of higher carbon grades are heated to produce austenite and then cooled rapidly (quenched) in a liquid such as water or oil. Upon hardening, the austenite transforms into martensite. Martensite is formed at temperatures below about 204 °C, depending on the carbon content and the type and amount of alloying elements in the steel.

Normalizing and tempering are frequently used in the production of boiler materials. Stress relieving (process annealing) is carried out after cold forming, rolling and welding of boiler parts. Pure iron lacks the required mechanical properties to be usable in modern boilers, therefore alloying elements are deliberately added in a controlled quantity to modify the material properties to match a particular specification. The most important elements used in boiler materials with their specific effects are mentioned below. •

Carbon is the most important alloying element in steel. In general an increase in carbon content produces higher ultimate strength and hardness but lowers the ductility and toughness of steel alloys. In low alloy steels for high temperature application the carbon content is usually restricted to a maximum of about 0.15% to assure optimum ductility for welding, expanding and bending operations. But the carbon content should not be lower than 0.07% for optimum creep strength.

General effect of carbon on the mechanical properties of hot rolled carbon steel.

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•

•

•

Molybdenum, when added to steel increases its strength, elastic limit, resistance to wear, impact qualities and hardenability. Mo contributes to high temperature strength and is the most effective single additive to increase creep strength. Chromium raises the yield and ultimate strength, hardness, and toughness of steel at room temperature. It is virtually irreplaceable in resisting oxidation at elevated temperatures and also increases high temperature strength. The optimum chromium content for creep strength in annealed alloy steels is about 2.25%. Nickel increases toughness when added to steel and improved resistance to corrosion. The most important use of nickel as an alloying element in steel is its combination with chromium. The various combinations of chromium and nickel in iron produce alloy properties that can not be obtained with equivalent amounts of a single element. These steels are resistant to corrosion and oxidation at high temperature. In addition they offer greatly enhanced creep strength.

Steels of different properties are used in boilers, each selected for one or more specific purposes. Each steel must have properties for both manufacturing and satisfactory service life. When evaluating the material properties of steel grades intended for the construction of boilers, it is not only the ambient temperature properties such as, tensile, yield strength, hardness, and toughness that needs to be reviewed. But also the material properties at high temperatures (operation temp.) need to be considered. Tensile and yield strength data determined at ambient temperature, can not be used as a guide to mechanical properties of metals at higher temperatures. Even though such tests are made at the higher temperature, the data is inadequate for designing equipment for long term services at these temperatures. This is true because, at elevated temperatures, continued application of load produces a very slow continuous deformation, which can be significant and measurable over a period of time and may eventually lead to fracture, this phenomenon is called creep. The maximum allowable working stresses for these ferrous materials, to be used for high temperature application are based partially on long term creep rupture tests. Elevated temperature material properties At elevated temperatures, the service life of a metal component subjected to either vibratory or non vibratory loading, is predictably limited. In contrast, at lower temperatures and in the absence of a corrosive environment, the life of a component in non vibratory service is unlimited, providing the operational loads do not exceed the yield strength of the material. Elevated temperature behaviour begins, approximately at the following temperatures for the below listed alloys: • • • • •

Aluminium alloys Titanium alloys Low carbon steels Austenitic iron based high temperature alloys Nickel based high temperature alloys

205 315 370 540 650

°C °C °C °C °C

Tensile and Yield Strength

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Higher temperatures have a significant effect on the mechanical properties of metals. Ferrous materials lose much of there tensile / yield strength at these elevated temperatures. At a temperature of 430 °C plain carbon steel sees a rapid decline of its yield strength. As an example, at 300 °C the yield strength of plain carbon steel is approximately 450 MPa, this has decreased to approximately 100 MPa at 600 °C. This temperature increase results in a yield strength reduction of 77.8 %, it will be obvious that the material will yield, deform, and rupture if the operational stresses remain constant. The above described occurs in boilers that have experienced sudden increase in operating temperature, for instance by starvation of feed water causing deformation of the overheated parts. Creep Stress imposed at elevated temperatures, produces a continuous strain in the component and results in a phenomenon called creep. Creep, by definition, is a time dependent strain, occurring under stress. After a period of time, creep will terminate in fracture by stress rupture, also called creep rupture. The conditions of temperature, stress, and time under which creep and stress rupture failures occur, depend on the metal or alloy, its microstructure, and on the service environment. At lower temperatures, a steel grade with very small grains (fine grain size) may be stronger than the same steel grade with fewer large grains (coarse grain size) because the grain boundaries act as barriers to slip. At elevated temperatures, where thermally activated deformation can occur, a fine grain structure material may be weaker, because the irregular structure at the grain boundary promotes local creep. This allows grains to rotate by grain boundary sliding. Creep deformation and creep strength are grain size sensitive properties. Thus a larger grain size improves creep strength. Most creep curves consist of three distinctive stages, see figure below.

Schematic creep curve showing the three stages of creep.

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1. Primary creep, also known as transient creep, represents an initial elastic strain resulting from the immediate effect of the applied load. This is a region of increasing plastic strain at a decreasing strain rate. 2. Secondary Creep, also known as steady state creep is usually characterized by extremely small variations in rate of deformations. This period is essentially one of constant creep rate and represents the predictable service life of a component, therefore being of particular interest to designers. 3. Tertiary creep, refers to the region of increasing rate of extension, that is followed by fracture. In service tertiary creep may be accelerated by a reduction of cross section, resulting from cracking or localized necking. Environmental effects such as oxidation, that reduces cross section, may also initiate tertiary creep or increase the tertiary creep rate. Under certain conditions some metals may not exhibit all three stages of plastic extension. For example, at high stresses or temperatures, the absence of primary creep is not uncommon. At the other extreme, notably in cast alloys, no tertiary creep can be observed, and fracture may occur with only minimum extension.

Creep curves showing no primary creep and no tertiary creep.

A component under creep loading will eventually fracture, if the strain occurring under creep does not relief the stress. Depending on the alloy, the appearance of the stress rupture fracture may be microscopically brittle or ductile. A brittle fracture occurs with little or no elongation or necking, and a ductile fracture is typically accompanied by discernible elongation and necking. Stress rupture ductility is an important factor in boiler material selection. As shown by the schematic creep curves in below figure, a higher rupture ductility for the same load and temperature conditions means a higher safety margin. Creep damage occurs at the grain boundaries, by the formation of internal micro fracture and voids. The process of creep damage is different form fatigue

41

damage, in that the former starts inside the material, whereas fatigue damage occurs due to the formation and propagation of a surface micro crack.

Schematic creep curves for alloys having low and high stress rupture ductility.

The first micro structural evidence of creep damage will be noted somewhere along the linear portion of the secondary creep period. The microstructure of a metal (grain boundaries) including evidence of creep damage can be made visible by etching a smooth polished surface and examining the specimen under high magnification in a microscope. Alternatively one can take a replica of the etched surface and view this under the microscope.

Voids that form on the grain boundaries at the early stages of creep with little deformation visible.

When considering suitability of a metal alloy for use in steam boiler construction, creep data of the applicable alloy needs to be evaluated to inshore satisfactory service life. To simplify the practical application of creep data it is customary to establish two values of

42

stress (for a material at a temperature) that will produce two corresponding rates of creep (elongation): 10% per 10000 h and 100000 h, respectively.

Creep rate curves for 2-1/4Cr-1 Mo steel.

For any specified temperature, several creep rupture tests must be run under different loads. The creep rate during the period of secondary creep is determined from these curves and is plotted against the stress. When this data is plotted on logarithmic scale, the points of each specimen often lie on a line with a slight curvature.

Typical creep rupture curves for 2-1/4Cr-1 Mo steel.

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The minimum creep rate for any stress level can be obtained from this graph, and the curve can also be extrapolated to obtain creep rates for stresses beyond those for which data is obtained. Because tests are not normally conducted for more than 10000 h, the values for rupture times longer than this are determined by extrapolation. Elevated Temperature Fatigue In service, the steady loads, or strains, to which components are subjected, are often accompanied by mechanically induced cyclic loads that are responsible for failure by fatigue. The effect of temperature on fatigue strength is marked: fatigue strength decreases with increasing temperature. However the precise relationship between temperature and fatigue strength varies widely, depending on the alloy and the temperature to which it is subjected. At high temperatures, the fatigue strength often depends on the total time the stress is applied rather than solely on the number of cycles. This behaviour occurs because of continuous deformation under load at high temperatures. Under fluctuating stress, the cyclic frequency affects both the fatigue life and the amount of creep. This is shown in the figure below, at room temperature the curves converge at the tensile strength plotted along the mean stress axis. At high temperature, the curves terminate at the stress rupture strength along the mean stress axis.

Effect of temperature on the fatigue life of S-816 alloy tested under a fluctuating axial load at a frequency of 216000 cycles per hour.

Combined creep and fatigue loads result in substantially decreased life at elevated temperatures as compared with that of anticipated simple creep loading.

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Thermal Fatigue Mechanical vibration is not the only source of cyclic loads. Transient thermal gradients within a component can induce plastic strain, if these gradients are repeatedly applied, the resulting cyclic strain can induce component failure. This process is known as thermal fatigue. The two conditions necessary for thermal fatigue are: 1. Some form of mechanical constraint. 2. A change in temperature.

Thermal fatigue crack in weld of tube plate and boiler shell.

Thermal expansion or contraction caused by a temperature change, acting against a constraint, causes thermal stress. A constraint can be imposed by for example a rigid pipe mounting or tube plate. In thick sections, temperature gradients are likely to occur both along and through the material causing highly triaxial stresses and reducing material ductility, even though the uniaxial ductility often increases with increasing temperature. In the event a component is exposed to creep and thermal fatigue at the same time, than creep strains are superimposed on thermal strains and thus account for a further reduction in life expectancy. General Oxidation Oxidation (also called metal burning) as well as all other reactions of metals with gaseous environments, has long been recognized as a severe limitation to the use of metals at high temperature. The temperature (scaling temperature) at which carbon steel appreciably oxidizes is approximately 550 °C. It can be

45

recognized as a thick, brittle, dark oxide layer which often contains longitudinal fissures and cracks. In other areas, patches of oxide may have exfoliated. Cracks and exfoliated patches result from component expansion and contraction.

Thermally deteriorated metal on a failed wall tube. Note the spalled and cracked oxide resembling tree bark caused by expansion of the tube during bulging and thermally induced stresses.

Metallurgical Instabilities Stress, time, temperature, and environment may change the metallurgical structure during service. They may thus contribute to failure by reducing strength, although some changes may enhance strength. These structural changes are also referred to as metallurgical instabilities. The elevated temperature at which creep occurs also leads to micro structural changes, and metallurgical instabilities in the metal. Creep damage and micro structural degradation of the metal occur simultaneously, resulting in a further reduction of the service life.

a) Section through the failure lip showing a complete spheroidization of the carbide phase in ferrite.

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(b) Section in the same plane as the failure, but 180° around the circumference of the tube structure is nearly normal pearlite and ferrite. (c) Section taken 205 mm (8 in.) away from the failure is also normal ferrite and pearlite. These three structures indicate the elevated temperature is confined to a small area of the failure. All etched with nital. 500 X. Material is exposed to a temperature of 650 °C.

Material Hardness Hardness may be defined as resistance to indentation under static or dynamic loads, and also as the resistance to scratching, abrasion, cutting or drilling. To the metallurgist, hardness is important as an indicator of the effect of heat treatment, fabrication process, or service exposure. Hardness values are also roughly indicative of the ultimate tensile strength of steels. Hardness testing is the simplest of the mechanical tests, can easily be preformed on location, and is often the most versatile tool available in the field. Among its many applications hardness testing can be used to assist in evaluating the combined effects of creep and micro structural degradation, in order to determine whether the material is fit for further service. Comparing the measured hardness values of the failed / damaged component with that prescribed by the material specification provides an approximation of the tensile strength changes, and also indicates extent of softening or hardening caused by overheating. Material application Det Norske Veritas, along with other Classification Societies and National Boiler Authorities have the responsibility for identifying and approving material specifications, for those metals deemed suitable for boiler construction and for development of the allowable design values for these metals as function of their service temperature. For Det Norske Veritas these steel grades are mentioned in Part 2, Chapter 2, Section 2 “Rolled Steel Plates”, and Section 4 “Tubes” (January 2005 publication) of the DNV Rules. Material supplied for construction or repair of boilers needs to be delivered with the following documentation: • Plates with a NV Material Certificates issued by DNV. • Tubes / pipes and flanges with a Material Works Certificate, which is issued by the manufacturer. In both cases the material manufacturer of the plate and tubes needs to be DNV Approved Manufacturer for the applicable products. The DNV designation of boiler steel grades is not widely used, it is more common to be confronted with a steel grade according to an industry standard, as for example DIN, ISO; BS; ASTM, or JIS. Reference is made to enclosed table with comparable steel grades. In general the following materials are used in auxiliary boilers encountered on board merchant vessels. • Furnaces are usually made of carbon steel suitable for high temperature with carbon content of 0.15 to 0.20%. Frequently used steel grades for these parts are NV 410, H ll (DIN), P 265GH, or (ASTM) A 516GR60. In regions of high heat input and more fluctuating temperatures such as flue gas pipes, burner mouth, and furnace crown, low alloyed steels (19Mn6) are sometimes chosen.

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•

•

•

The boiler shell and tube plates are also usually made of carbon steel of medium carbon content. In the 80`s some manufacturers (Sunrod) used high strength steel grades for the boiler shell but, this is not practiced at present (Several boiler codes restrict the us of high tensile steels). Flame and water tubes for auxiliary boilers are usually made of carbon steel, for example grades St. 37.8I or St. 35.8I. Most employed tubes are still seamless tubes, although today’s electric resistance welded tubes are of equal quality. The highest metal temperatures occur in the superheater and reheater. Consequently, materials need to have superior high temperature properties and resistance to oxidation. Mild steel is considered permissible for superheaters up to steam temperatures of 399 °C, above this temperature alloy steel grades are used. The alloying elements are usually molybdenum and chromium.

When for some reason the original material can not be obtained during a boiler repair, and one is confronted with the situation of choosing a comparable steel grade. Please exercise the necessary prudence as will be evident by the following example: Part of a boiler shell needs to be cropped and renewed. The original boiler shell is made of steel grade NV 1Cr/2Mo, and since this material could not be supplied the decision has been made to use steel grade NV D36 for the boilers shell repair. Material grade

NV 1Cr/2Mo Original boiler shell material

NV D36 Proposed steel grade for repair

Tensile strength

470 / 620 MPa

490 / 630 MPa

Yield strength

305 MPa

355 MPa

Elongation

20%

21%

Charpy value

20 J at 20 °C

34 J at -20 °C

The novice may conclude from the above data that steel grade NV D36 is even superior to the original steel grade and perfectly suitable for the intended shell repair. As mentioned earlier mechanical properties determined at ambient temperature can not be used as a guide to mechanical properties at higher temperatures. Also NV D36 is only suitable for structural application and not for boiler / pressure vessel application. We may conclude NV D36 is not a comparable steel grade to NV 1Cr/2Mo and should not be used, its use will lead to early unexpected failure of the shell plate by creep. Comparable steel grades to NV 1Cr/2Mo are for example 14CrMo45, or 13CrMo44. In general one may state that apart from the obvious differences in chemical composition between structural steel and boiler steel grades, boiler steels have been tested with regard to their elevated temperature behaviour, creep rupture

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tests. Comparable steel grades should therefore be found in the appropriate sections of steel grades for boiler pressure vessel application of the different industrial standards. Principle stresses in boilers Stress is defined as the internal force between two adjacent elements of a body, divided by the area over which it is applied. The main significance of a stress is its magnitude, however, the nature of the applied load and the resulting stress distribution are also important. The designer must consider whether the loading is mechanical or thermal, whether it is steady state or transient, and whether the stress pattern is uniform. The principle stresses during boiler operation may be divided as follows. • Pressure stresses (steady state load) which are classified as primary membrane stresses since they remain as long as the pressure is applied. • Thermal stresses result, from restricting a member that is attempting to expand or contract, due to a temperature change. • Alternating stresses (transient load) resulting from cyclic pressure vessel operation, this may lead to fatigue cracks at high stress concentration. When the wall thickness is small compared to other dimensions, the stress acting over the thickness of the vessels wall and tangential to its surface, can be represented by mathematical formulas for a common shell form. The basic equations for the longitudinal stress σ1 and hoop or circumferential stress σ2 in a cylindrical vessel with a wall thickness tw, diameter D, and subject to a pressure p are:

σ1 = p.D / 4.tw

σ2 = p.D / 2.tw

From the above formulas, it is evident that the hoop stress has twice the magnitude of the longitudinal stress. This translates in to the fact that longitudinal welds are twice as highly loaded as circumferential welds. Therefore they should receive extra attention during inspections, and be a main candidate for any NDT examination after welding repairs.

Longitudinal weld seam in a steam drum.

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Conclusion In concluding this chapter on “Boilers, Understanding the Basics” please find below some considerations, which may be worth keeping in mind during boiler surveys. •

Most auxiliary boilers we inspect produce saturated steam, mainly used for heating of fuel oil and cargo. The steam pressures involved are relatively low (6 to 13 bars) and therefore some people underestimate the energy contained in a boiler, and eventual consequences incase of sudden release of the water steam mixture. The frequently heard fraise “it is only 6 bars steam pressure” should not be taken seriously. Bear in mind that even if a ruptured boiler shell does not result in an explosion, sudden release of the water content will flood the engine room space with steam. This will among other consequences (steam temperature, visibility loss), lead to oxygen depletion of the engine room space resulting in a life threatening situation for the engine room crew.

Dramatic failure of an exhaust gas boiler shell

•

•

Contamination of the heat transfer surface by only a thin layer (0.5 mm) of oil raises the furnace wall temperature to dangerously high values (633 °C). Operating the boiler in this condition for any prolonged time will lead to overheating, material degradation, and failure of these pressure parts. When discovered during inspections imminent remedial actions are called for, to prevent future disasters. Theoretically the effect of water side contamination will be a raise in furnace wall temperature, and reduction of heat transfer rate which translates in less steam being produced. The steam consumption system still requires the original steam output and therefore the loss of steam

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•

•

•

•

• • • • •

•

production is compensated by burning more fuel. Practically therefore, contaminated heat transfer surfaces will lead to an additional wall temperature raise and increased fuel consumption. Feed and boiler water treatment is necessary for every type of boiler. It will keep the water / steam surfaces free from scale, sludge, and prevents corrosion. If water treatment is carried out properly, the waterside of a 20 year old boiler looks just as clean as that of a 5 year old boiler. High pressure boilers are less forgiving with regard to negligent water treatment, than low pressure boilers. Also, the boiler water quality requirements become more stringent, with raising pressures and heat transfer rates. A general rule as to what extent of scale contamination a boiler can safely operate with can not be given, due to the complexity of the subject. First of all, the weight and thickness of the scale layer alone does not always accurately indicate the tendency to overheating. Scale composition and morphology also influence heat transfer. Basically, a boiler should be without scale in order to operate at peak performance. The negative effects of scale also largely depend on its location, high heat transfer surfaces such as furnace crown are very susceptible to scale formations. Scale also prevents a proper visual inspection, since it hides defects, such as corrosion pits and cracks. Marine auxiliary boilers are generally made of carbon steel grades. These materials start to be affected by elevated temperature behavior at 370 °C. Affects such as creep and metallurgical instabilities deteriorate the micro structure of the material, resulting in reduced strength. The material will have a predictable time to failure depending on the stress, time, and temperature of exposure. The areas around the furnace in marine auxiliary boilers are generally affected by elevated temperature behaviour, due to scale or oil contamination. General oxidation of carbon steels occurs when they are exposed to temperatures of 550 °C or above. It can be recognised by longitudinal fissured scale on the exposed surface or completely cracked surface, so called crazy cracking. Plain carbon steel sees a rapid decline of its yield strength at a temperature above 430 °C. Look for thermal fatigue cracks at any point of restraint such as, stay connections, internal supports, and tube plate connection. The stress in longitudinal welds is twice as high as that in circumferential welds of a cylindrical object. Therefore longitudinal welds of the boiler shell deserve extra attention during inspections. Very small deformations in boiler plating may be the result of stress redistribution by plastic deformation, setting of the material. Larger deformations in boiler plating are always the result of overheating. Most probably this will be long term overheating in which case the heated surface will be contaminated by scale, mud, soot, or you find evidence of flame impingement or missing brickwork. If the heated surfaces are completely clean and burner is operating properly then it is short term overheating, frequently caused by feed water starvation. Boiler parts exposed to long term overheating are affected by creep and possibly other elevated temperature phenomena. Deformation of the exposed parts clearly indicates that creep is in it’s 3rd stage (tertiary

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•

•

•

creep) and failure is imminent. It is not known when failure will occur (a week, a year), or how much the material strength has decreased. As a general rule deformed boiler parts should be renewed as soon as possible. A deformed furnace crown or cylindrical plate is not as strong as when it had its original shape. Stresses in the material at the deformation have increased, especially at the edges of the deformation. When dealing with deformed boiler parts, the following questions needs to be answered in the order listed below. 1. Is the boiler structurally still sound? If deformations are too extensive, sharp corners or large areas, it needs to be repaired before boiler operations are resumed. 2. Is the deformed material fit for further use? This question has to be answered if one considers the boiler structurally fit for further operation, until repairs are carried out. The deformed material can be evaluated with help of the following techniques. 1. Measuring the material hardness, this can be done by a portable hardness measurer. Take several hardness readings of the deformed area and also of not affected material. Compare the values with the material specification. Too low readings indicate loss of material strength, and too high readings imply brittleness of the material. Please keep in mind that measurements should be taken of the base material. Sometimes overheated material has a hard oxide layer, or the surface has hardened, this hard layer has to be removed by grinding, before accurate hardness readings can be obtained.

Portable hardness measurement set from Equotip.

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2. Make a replica of the materials microstructure and examine this under the microscope. An area of the deformed plate needs to be polished and etched to produce a replica, this is not always easy in a boiler. Creep starts from within the material, if creep damage is seen on the surface, the material has to be renewed immediately.

A replica of the surface (polished and etched) is made by applying a softened plastic foil to the surface. This foil moulds itself to the metal surface when pressed. After its removal from the metal,

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the plastic replica provides an exact copy of the etched surface microstructure, which can then be examined under our laboratory’s high-quality and very high-resolution microscopes.

•

•

•

•

•

In case the boiler is considered fit for a short time of operation until repairs can be implemented, it is important to clean the boiler. Removal of scale, mud and soot will drastically decrease the material operation temperature of the damaged areas. One should also accurately record the dimensions of the deformed area, length, breath, and depth. It will be possible then to monitor the damage, progressing creep will be indicated by increased dimensions of the deformed area. Welded connections in way of deformations should be carefully inspected during damage surveys. Often these weld seams are overstressed by the deformation and develop cracks. Cleaning of the area and examination by Magnetic Particle Inspection (MPI) is a wise course of action. Hydraulic pressure test of damaged boiler only guarantees us that it can withstand the test load at room temperature. The effect of long term operation at service temperatures is not taken into account, therefore if the damage is too extensive a successful hydraulic pressure test is not a guarantee for safe operation. When performing a hydraulic pressure test, the test pressure should remain on the boiler for approximately 60 minutes before final inspection. It was customary in the past to heat the test water to approximately 80 °C in order to prevent leaking of the expanded tube connections. At present with all welded boilers this is not applicable any more, water at room temperature is in order. Caution should be exercised when accepting pressure down grading of damaged boilers. Reducing the working pressure with 1 or 2 bars will reduce the material stresses due to pressure, but the thermal and alternated stresses are not considered. Also the strength reduction of the overheated material due to creep and other elevated temperature phenomena is not known. In general pressure down grading looks good on paper, but it will not give the desired operational safety.

Appendix Reference is made to below tables of comparable boiler steel grades.

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Chapter 2: Guide to Boiler Failure Modes Introduction This chapter is a field guide to the most commonly encountered boiler failures. It will help the novice and the more experienced observer to identify the failure and pinpoint its likely locations. The failures discussed are found in boilers of virtually all pressures and construction.

Deposit or Scale Formations, Water side Locations Basically deposits can occur at any location in the boiler where water or steam is present. Scale formation tends to concentrate in the hottest steam generation regions, high heat flux areas. Therefore combustion chamber plates, wall and screen tubes are usually more heavily fouled. Also deposition often occurs immediately down stream from circumferential-weld backing rings, which disturbs the flow and are favored sites for steam blanketing.

Tube sections virtually plugged with deposits. The tube on the right is from a low pressure boiler and is fouled with almost pure calcium carbonate. The centre tube contains silicates, phosphates, and other components. And the left tube section is rendered almost 20 % copper.

Superheater deposits are caused by carry over of boiler water. Scale formation will usually be concentrated near superheater inlets or in nearby pendant U – bends.

Severely corroded blades from a steam turbine.

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General Description Boiler deposits originate from four sources: water borne minerals, treatment chemicals, corrosion products, and contaminants. Deposits from these sources may interact to increase deposition rate, which produces a more tenacious layer, and serves as nucleation site for new deposit formation. One deposition process involves the concentration of soluble and insoluble substances in a thin film bordering the metal surface during steam-bubble formation.

Five instants in the life of a steam bubble.

Material segregates at the steam / water interface, moves along the interface, and is deposited at the bubble base as the bubble grows. Other deposit mechanisms involve precipitation from solution and settling of large particulate matter. Inverse-temperature solubility leads to deposition where heat transfer is great. The tendency to form deposits is related to localized heat input, water turbulence, and water composition at or near the tube wall. When the steam bubble becomes dislodged from a tube wall, the deposits are washed with water (re-dissolved). The rate at which the deposit builds depends on the rate of bubble formation and the effective solubility of the deposit. In case of high heat input a stable steam blanket (film boiling) can be formed and cause concentration of water soluble material.

Cross section of bulged tubes cause by overheating due to heavy deposits.

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Steam-blanket deposits do not re-dissolve, because the surface cannot be washed while blanketed with steam. Prolonged operation above the maximum deposit loadings may produce serious corrosion, overheating failures, and increase fuel consumption. However, the weight and thickness of the deposit layer alone does not always accurately indicate the tendency to overheat. Deposit composition and morphology also influence heat transfer. Elimination All deposits are undesirable, and ultimately result from water-chemistry properties and boiler operation. Proper water treatment can reduce depositions. The most important boiler operating characteristic influencing deposition is firing practice. Also elimination of hot spots, correct monitoring of water levels, proper burner position, and appropriate blowdown practices contribute to reduced deposition.

Long Term Overheating Locations Failures resulting from long term overheating occur in combustion chamber plates, wall, screen, and superheater tubes. Tubes and plates especially subject to overheating often contain significant deposits, have reduced coolant flow, experience excessive fire side heat input, or are near or opposite burners. Tubes adjacent to restricted or channeled furnace gases suffer from long term overheating. Other tubes and areas subjected to overheating include sections in which refractory has been spalled. Slanted tubes, such as nose arches, are particularly susceptible to long term overheating due to steam channeling.

Tube failure in a nose arch in the furnace (left) and a massive thick walled tube fracture caused by creep (right).

Fire tubes are rarely affected. Failures usually occur in relatively broad areas and involve many tubes that are either ruptured or bulged.

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General Description Long-term overheating is a condition in which metal temperatures exceed design limits for days, weeks, months, or longer. This type of overheating is the cause of more boiler failures than any other mechanism. Because steel loses much strength at elevated temperatures, rupture caused by normal internal pressure becomes more likely as temperatures rises.

Yield stress of plain carbon steel as function of temperature. Please note the rapid strength reduction above 430 °C.

The maximum allowable design temperature is primarily a function of tube / plate metallurgy. As the amount of alloying element, particularly chromium and molybdenum, is increased, higher temperatures can be tolerated. Long term overheating depends on temperature, length of time at temperature, and tube / plate metallurgy. A mild steel tube subjected to temperatures above 454˚C for more than a few days may experience long-term overheating. If temperatures remain elevated for a prolonged period, overheating will certainly occur. Furnace and gas temperatures often exceed 1093 ˚C, if the heat transfer rate on the water side is markedly influenced by deposits, film boiling, and coolant flow restriction this will result in increased metal temperatures above design condition. Thermal oxidation (metal burning) One sign of long term overheating can be a thick, brittle, dark oxide layer on both internal and external surfaces. If the metal temperature exceeds a certain

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value for each alloy, thermal oxidation will become excessive. Often the thermally formed oxide layer contains fissures and cracks.

Thermally deteriorated metal on a failed wall tube, note the spalled and cracked oxide resembling tree bark caused by expansion of the tube during bulging.

In other areas, patches of oxide may have exfoliated. Cracks and exfoliated patches result from tube expansion and contraction caused by deformation during overheating and / or thermal stressing. Tube wall and plate thinning can result from cyclic thermal oxidation and spalling. Creep rupture (stress rupture) Stress rupture usually produces a thick-lipped rupture at the apex of the bulge and a deformation. Creep produces slow plastic deformation and eventual coalescence of micro voids in metal during overheating. Often a small longitudinal fissure will be present at the apex of a heavily oxidized bulge.

Small, ragged creeps rupture at the apex of a bulge, note thick rupture edges.

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Elimination Long term overheating is a chronic, rather than a transient problem. Therefore it requires removal of a chronic system defect. Excess deposits should be removed by chemical or mechanical cleaning and recurring prevented by proper water treatment. Improper boiler operation, and excessive heat input should be avoided.

Short Term Overheating Locations Failures caused by short term overheating are confined to steam/water cooled tubes (wall, screen, roof tubes) and combustion chamber plates. When low water level is the cause, failure will occur near the top of water tube wall headers, near steam drums. Also roof tube walls and top of furnace plates are usually found deformed.

Longitudinal tube rupture (left), large fish mouth rupture of rifled nose arch tube (right). Note the absence of any deposits.

General Description Short term overheating occurs when material temperature rises above design limits for a brief period. In all instances, metal temperatures are at least 454 ˚C and often exceed 730 ˚C. Failure is usually caused by a boiler operation upset. Conditions leading to short term overheating are partial or total tube pluggage, and insufficient coolant flow due to upset conditions, or excessive fire side heat flux. Several factors which often present the failures caused by short term overheating are uniform tube expansion, absence of significant internal deposits, absence of large amounts of thermal formed magnetite, and violent rupture.

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Elimination The solution of short term overheating, which is often caused by a brief upset condition, is to eliminate the upset. If restricted coolant flow due to tube pluggage is suspected, drums, headers, and other areas should be inspected and cleaned.

Caustic Corrosion, Water side Locations Generally, caustic corrosion is confined to; 1. Water cooled tubes in regions of high heat flux. 2. Slanted and horizontal tubes. 3. Locations beneath heavy deposits. 4. Heat transfer regions at or adjacent either to backing rings at welds or to other devices that disrupt flow. General Description The term caustic and ductile gouging refers to the corrosive interaction of sufficiently concentrated sodium hydroxide with a metal to produce distinct hemispherical or elliptical depressions. At times a crust of hard deposits and corrosion products will surround and / or overlie the attacked region.

Patch of hard iron oxides on internal surface (left) and cratered region beneath (right).

The susceptibility of steel to be attacked by sodium hydroxide is based on the amphoteric nature of iron oxide; that is, oxides are corroded by both low pH and high pH environments. High pH substances, such as sodium hydroxide will

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dissolve magnetite. When this happens sodium hydroxide may react directly with the iron. Two critical factors contribute to caustic corrosion. 1. The availability of sodium hydroxide or of alkaline producing salts. 2. A mechanism of concentration. Sodium hydroxide is often intentionally added to the boiler water at non corrosive levels. Alkaline producing salts may contaminate the condensate by in-leakage through the condenser. Poorly controlled water treatment may also cause excessive alkalinity. Because sodium hydroxide and alkaline producing salts are rarely present at corrosive levels in the bulk environment there are basically three concentration mechanisms. 1. Departure from nucleate boiling (DNB): During nucleate boiling a minute concentration of boiler water solids will form at the metal surface. As the steam bubble separates from the metal surface the water will re-dissolve soluble solids as sodium hydroxide. At the onset of film boiling, the rate of bubble formation exceeds the rinsing rate. Under these conditions, sodium hydroxide, as well as other dissolved solids or suspended solids will begin to concentrate. 2. Deposition: A similar situation occurs when deposits shield the metal from the bulk water. Steam that forms under these thermally insulating deposits escapes and leaves behind a corrosive residue that can deeply gouge the metal surface. 3. Evaporation at a waterline: Where a waterline exists, corrosives may concentrate by evaporation, resulting in gouging along the waterline. Elimination When the availability of sodium hydroxide or alkaline producing salts and the mechanism of concentration exist simultaneously, they govern susceptibility to caustic corrosion. Caustic corrosion is counteracted by reducing or eliminating the availability of sodium hydroxide and alkaline producing salts.

Caustic gouging beneath deposits.

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Low pH Corrosion, during service, Water side Locations Generally, in-service acid corrosion is confined to water cooled tubes in regions of high heat flux; slanted or horizontal tubes, location beneath heavy deposits; and heat transfer regions adjacent to backing rings at welds, or other devices that disrupted flow.

Appearance of deposits covering groove (left), and contour of groove after deposit removal (right).

General Description Although relatively rare, a general depression of bulk water pH may occur if certain contaminants gain access to the boiler. Boilers using water of low buffering capacity can realize a bulk pH drop to less than 5 if contaminated with sea water, hydrochloric acid, or sulfuric acid. The concern of this chapter, however, is with the more common creation of localized pH conditions. Two circumstances must exist simultaneously to produce this condition. 1. The boiler must be operated outside of normal, recommended water chemistry parameters. This may happen in case of a leaking condenser, in-leakage of sea water takes place. 2. A mechanism for concentrating acid producing salts. This condition exists where boiling occurs and adequate mixing is hindered by the presence of porous deposits or crevices. Where deposits and crevices are present, a

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concentration of acid-producing salts may induce hydrolysis to produce localized low pH conditions, while the bulk water remains alkaline. There are basically three concentration mechanisms, and they are similar to high pH corrosion (caustic corrosion). 1. Departure from nucleate boiling, film boiling. 2. Deposition. 3. Evaporation at the waterline. Where low pH conditions exist, the thin film of iron oxide is dissolved and the metal is attacked. It is very difficult to distinguish localized attack by low pH substances, from those by high pH substances simply by visual examination. Distinguishing between the two may require a metallographic examination.

Low pH gouging

Elimination When the availability of free acids or acid producing salts and the mechanism of concentration exist simultaneously, they govern susceptibility to localized low pH corrosion. Low pH corrosion is counteracted by reducing or eliminating the availability of free acids or acid producing salts.

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Low pH Corrosion, during Acid Cleaning, Water side Locations In general any surface exposed to acid is susceptible. One of the first areas to be effected is the tube ends inside mud and steam drums. Hand-hole covers, drum manholes, and shell welds may also be affected. Heat-transfer surfaces and weldments may experience vigorous attack. Shielded regions within crevices, behind backing rings, and under remaining deposits may prevent proper neutralization of the cleaning acid. This result in vigorous localized attacks of the metal once the boiler is returned to service.

Acid corrosion on the internal tube surface (left) and jaggedness associated with severe acid corrosion (right).

General Description Attack of a metal surface by strong acid is generally unmistakable. The surface usually has a rough or jagged appearance, depending on the severity of the attack. Corrosion of steel by acids is a natural consequence of steel’s thermodynamic instability in these environments. Steel will corrode spontaneously in most acids. During the corrosion reaction, iron displaces hydrogen from the solution. That is, iron is oxidized and iron ions go into the solution. Hydrogen ions are reduced and form hydrogen bubbles at the metal surface. To shift this corrosion process, inhibitors are added to acid-cleaning solutions used in boilers. Uncontrolled acid corrosion of the boiler during cleaning generally results from an unanticipated deviation from the standard conditions or practices. Many

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deviations are possible and may include events such as thermally induced brackdown of the inhibitor, inappropriate selection of cleaning agent or cleaning strength, excessive exposure times and temperatures, and failure to neutralize completely. Elimination Mitigation of low pH corrosion of boiler equipment during acid cleaning requires close monitoring of the entire cleaning procedure. The following are a few examples of parameters to be monitored and evaluated during the procedure. 1. Deposit weight determination; Deposit weight measurements at a number of locations will aid in determining the proper acid strength, exposure time, and total quantity required to adequately clean the boiler. 2. Deposit analyses; This will help in determining the appropriate cleaning agent and the sequence in which the agent should be used. 3. Temperature of cleaning; Both the solution and metal temperature should be safely below the thermal breakdown point of the inhibitor. 4. Monitoring; Chemistry of neutralizer should be monitored following the boilers exposure to the acid. 5. Visual inspection; Tubes, mud drums, steam drum should be inspected after cleaning.

Oxygen Corrosion, Water side Locations Although relatively uncommon in an operating boiler, oxygen attack is a problem frequently found in idle boilers. In an operating boiler the first areas to be affected are the economizer and feed water headers. In cases of severe oxygen contamination other areas of the boiler may be affected, such as the surface along the waterline in the steam drum, and the steam separation equipment. In all cases, considerable damage can occur even if in a short period of oxygen contamination.

Oxygen pits inside a tube section (left and right), external surface pits on fire tube (middle).

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General Description Since the oxides of iron are iron’s natural, stable state, steels will spontaneously revert to this form if conditions are thermodynamically favorable. Generally, conditions are favorable if steel, which is not covered by a protective form of iron oxide, is exposed to water containing oxygen. The corrosiveness of water increase as temperature and dissolved solids increases, and pH decreases. Aggressiveness generally increases with an increase in oxygen. Fractures in the protective magnetite are caused by thermal or mechanical stresses during operation, there fractures furnish anodic regions where oxygen containing water can react with the bare, unprotected metal. Oxygen corrosion often occurs as pitting which is covered by non protecting iron oxides. In addition to wall perforation, oxygen pits can act as stress concentration sites, thereby fostering the development of corrosion fatigue cracks, caustic cracks, and other stress related failures. Elimination Since water is always present in an operating boiler, and the protective magnetite coating exists in a state of continuous breakdown and repair, mitigation of oxygen corrosion is achieved by sufficient diminishing dissolved oxygen. This is achieved by proper operating dearators, and admission of adequate quantities of oxygen scavenging chemicals to the feed water.

Oil Ash Corrosion, Fire side Locations Oil ash corrosion is a high temperature, liquid phase corrosion phenomenon generally occurring where metal temperatures are in the range of 593 to 816 ˚C. It may affect superheater tubes, water cooled tubes, and support / attached equipment which operate at a higher surface temperature than the tubes.

Wall thinning as a result of oil ash corrosion.

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General Description Oil ash corrosion occurs when molten slag containing vanadium compounds forms on the tube wall according to the following sequence: 1. Vanadium and sodium compounds present in the fuel are oxidized in the flame to V2O5 and Na2O. 2. Ash particles stick to the metal surface, with Na2O acting as a binding agent. 3. V2O5 plus Na2O react to the metal and form a liquid (eutectic). 4. The liquid formed dissolves the magnetite, exposing the underlying metal to rapid oxidation. It is believed that corrosion occurs by catalytic oxidation of the metal by vanadium pentoxide (V2O5) or complex vanadates. This corrosive slag may develop when fuels containing high levels of vanadium, sodium, sulfur, or a combination of these elements are used; when excessive amounts of excess air is available for the formation of V2O5; where metal temperatures exceeding 593 ˚C are achieved. As the metal temperature increases, the range of compositions of Na2O and V2O5 that form the liquids expands considerably. Hence, in units with relatively thick layers of internal scale, the metal temperature will increase and may exceed temperatures at which sodium–vanadium complexes form liquids. If this occurs, sudden, unexpected problems with oil ash corrosion may appear, even though operating parameters and fuel chemistry remain unchanged.

High temperature corrosion at the base of an attachment.

Elimination Oil ash corrosion is eliminated by controlling the critical factors that govern it. First, if fuel containing very low quantities of vanadium, sodium, and sulfur cannot be specified one has the second option of firing the boilers with a low excess of air to retard V2O5 formation. Third possibility is to prevent metal temperatures from exceeding 593 ˚C.

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Cold End Corrosion, Fire side Locations Cold end corrosion occurs wherever the metal temperature drops below the sulfuric acid dew point of the flue gas. It occurs in relatively low temperature sections of the boiler such as economizers and air heaters.

Corrosion and perforations of a finned economizer tube resulting from exposure to sulfuric acid.

General Description In general the problem is associated with the combustion of fuels containing sulfur or sulfur compounds. Sulfur in the fuel is oxidized to sulfur trioxide which as the flue gas cools reacts with water vapor to form vapor phase sulfuric acid. If the sulfuric acid vapor contacts a relatively cool metal surface, it may condensate as liquid sulfuric acid. The temperature at which sulfuric acid condenses varies from 116 to 166 ˚C, or higher depending on sulfur trioxide and water vapor concentrations in the flue gas. The critical factors governing cold end corrosion include the presence of corrosive quantities of sulfur trioxide, the presence of moisture in the flue gases, and the presence of metals whose surface temperature is below the sulfuric acid dew point. Elimination Cold end corrosion is eliminated by gaining control of the critical factors governing it. The oxidation of sulfur trioxide is prevented by specifying low sulfur fuel and operating the boiler with a low air excess. Also, by raising the metal temperature above sulfur acid dew point will prevent occurrence of cold end corrosion.

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Corrosion Fatigue Cracking Locations Corrosion fatigue occurs in any location where cyclic stresses of sufficient magnitude are operative. Rapid boiler start up and shutdown can greatly increase the susceptibility to corrosion fatigue. Common locations are at points of attachment or rigid constraint, such as connections to inlet or outlet headers, tie bars, and buckstays. General Description The term refers to cracks propagating though a metal as a result of cyclic tensile stresses operating in an environment that is corrosive to the metal. The term and definition above are somewhat misleading in the case of boilers, since normal oxidation of metal to magnetite is sufficient to induce corrosion fatigue in the presence of sufficient cyclic tensile stresses. Cracks develop according to the following sequence; 1. During the first phase of cyclic stress, the tube wall undergoes expansion. Since the oxide layer is brittle relative to the tube wall, the oxide layer may fracture, opening microscopic cracks through the oxide to the metal surface. 2. The exposed metal surface at the root of the crack oxidizes, forming a microscopic notch in the metal surface. 3. During the next expansion cycle, the oxide will tend to fracture along this notch, causing it to deepen. 4. As this cyclic progress continues, a wedge shaped crack propagates though the wall, until rupture occurs or the tube wall is penetrated.

Developing corrosion fatigue crack formed at base of cracked layer of iron oxide (magnification 400X) (left), Mature corrosion fatigue crack (mag.200), (right).

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The crack always propagates in a direction perpendicular to the direction of the principal stress. Hence, if the principal cyclic stress is produced by fluctuations in internal pressures, longitudinal cracks are produced. If the principal stress is a bending stress produced by thermal expansion and contraction of the tube, cracks will be transverse. Corrosion fatigue cracks commonly occur adjacent to physical restrains, and are often associated with pits which serve as stress concentrating notchs.

Longitudinal cracks resulting from internal pressure (left) and transverse cracks due to bending stresses (right).

Elimination Corrosion fatigue cracking is eliminated by controlling cyclic stresses, and environmental factors. Reducing or limiting cyclic stresses by extended start up and shutdown times. Controlling pH and excessive levels of dissolved oxygen can be useful in eliminating pitting corrosion, which will eliminate a common point of initiation for corrosion fatigue cracking.

Stress Corrosion Cracking Locations In principle, stress corrosion cracking could occur wherever a specific corrodent and sufficient tensile stress coexist. Because of improved water treatment programs and boiler designs the occurrence of caustic stress corrosion cracking (caustic embrittlement) is much less frequent than in the past. General Description The term stress corrosion cracking refers to a metal failure resulting from a synergistic interaction of tensile stress and a specific corrodent to which the metal is sensitive. The tensile stresses may be either applied, such as those caused by internal pressures, or residual, such as those introduced by welding. In boilers, carbon steel is specifically sensitive to concentrations of sodium hydroxide, while stainless steel is specifically sensitive to both sodium hydroxide

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and to chlorides. The combination of concentrated sodium hydroxide, some soluble silica, and tensile stresses will cause continuous intergranular cracks to form in carbon steel. As the cracks progress the strength of the remaining intact metal is exceeded, and a brittle, thick walled fracture will occur. Instances of caustic stress corrosion cracking in boiler metal operating below 149 ˚C are rare.

Crack on the internal surface, note the proximity to the weld.

Elimination To eliminate problems with stress corrosion cracking it is necessary to gain control of either tensile stresses or concentrations of corrodents. 1. Tensile stress can be either applied or residual. Applied stresses are service generated stresses and can only be partly influenced by proper operation of the boiler. Residual stresses are the result of manufacturing and construction processes such as welding or tube bending. These stresses can be relieved by proper annealing techniques. 2. Avoiding concentrated corrodents is generally the most successful means of reducing or eliminating stress corrosion cracking. This is accomplished by avoiding departure from nucleate boiling and keeping internal surfaces free from deposits, in other words proper water treatment.

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Chapter 3: Auxiliary Boiler Survey Introduction Det Norske Veritas in common with all other Classification Societies require all boilers, whether main or auxiliary boilers to be surveyed periodically. The scope of this inspection is to confirm that the boiler can be safely operated for the duration of the upcoming period, and identify the need for immediate repairs or maintenance. At these surveys, the boiler, superheater, economizer, and air heater are to be examined internally and externally, and principal boiler mountings are to be opened up and inspected. The survey is finalized with an examination of the fuel oil burning system under operation, and testing of the safety functions.

Survey Preparation For the quality of the survey it is of paramount importance that the boiler is prepared properly. In preparing a boiler for survey there are certain fundamentals which should be observed, the most important ones are listed below. • •

The boiler is to be taken off-line, completely drained, and cooled down. All mountings should be isolated safely from any live feed or steam ranges so that these can be opened up for inspection and overhaul. Blanks should, if necessary, be fitted to secure safe isolation. The ships side blow down valve should be shut.

Deposits on the steam / water side need to be removed in order to carry out a proper survey.

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•

•

In multi-boiler installations extra vigilance should be observed in safely isolating the off line boiler, from the online boiler. Common pipe connections such as blowdown lines and drain pipes from safety valves chests must be taken into consideration. All water /steam side manhole covers must be opened in order to facilitate internal survey. Also handhole covers of headers are to be removed for inspection.

The boiler water side is not properly cleaned, deficiencies may be overlooked when inspecting a boiler in this condition.

• • •

Access to the fire side / furnace should be provided by removing the burner and opening doors to smoke boxes. The boiler should not be entered until it is sufficiently ventilated, and cooled down. Before a boiler can be surveyed it should be thoroughly cleaned. Failure to do this may result in serious defects being overlooked. In well maintained boilers some wire brushing and a good hose-down of the water steam side is sufficient. If hard scale has been allowed to form mechanical cleaning by chipping hammer, or chemical cleaning may be necessary. Removal of loose soot and deposits on fire side tube walls and furnace floor is normally adequate. If the fire side is very dirty then mechanical cleaning and water washing may be needed.

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Smoke tubes are blocked by soot, this has to be cleaned before commencing the survey.

Survey Safety Measures For personal safety reasons, the following must be considered before entering a boiler for internal inspection. • It is advisable to start with the external boiler survey, during which it can be confirmed that the boiler is safely isolated. • A boiler is a confined space and therefore applicable safety instructions and policies need to be adhered to. Make certain the boiler space is well ventilated and the atmosphere is safe. No person should enter the boiler without authority. Some responsible person should always be standing by the manhole door when another person is in the boiler. • Before entering a boiler empty all coverall pockets, sometimes it can be extremely difficult to recover lost property in a boiler. • Make sure internal boiler parts are cooled down sufficiently for entry. Apart from the discomfort while doing the survey, boiler exit may be more difficult or impossible due to body expansion and sweating. • Since auxiliary boiler spaces are normally very small, we recommend that only one person enters the boiler. • If feeling claustrophobic before entry, we strongly advice not to enter the boiler. Before entry, give the necessary consideration on how to exit the boiler. • Once in a boiler and experiencing difficulties to come out, it is essential to stay calm and not to panic. Take a small break and try again, remember when you managed to enter, it is possible to get out.

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•

Goggles and filtration masks may be worn during fire side inspection. This in order to protect one self against soot inhalation and eye irritation by soot.

When the boiler is of a type that can not be satisfactory internally examined due to inaccessibility, a hydrostatic pressure test may be called for. Please, take account of the fact that a liquid pressurizing medium is far less dangerous than a pneumatic pressurizing medium. Therefore, pressure testing with steam or compressed air should not be attempted.

Shell Type Boilers These boilers are of moderate steam capacity and have been evolved to work with feed water of medium quality. They are suitable for relatively simple steam installations. Tank type boilers have relatively large water content in relation to its volume, heated surface, and steam production. Normally the heated surface consists of a cylindrical furnace or fire box located in the lower part of the shell. Shell or tank type boilers can be classified by their construction in two groups. 1. Horizontal shell type boilers. 2. Vertical shell type boilers.

Horizontal shell type boiler, Steambloc packaged boiler.

Today we mostly find vertical shell type boilers installed onboard, which are utilized as auxiliary boilers.

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Vertical tank type boiler, Mission OS

Horizontal Shell Type Boilers The famous Scotch boiler has by now been replaced by modern designs such as the Steambloc package boilers, and Mission 3 phase boilers. However, the basic design of these boilers is still the same as the Scotch boiler. Figure 1 represents a horizontal shell type boiler and indicates the various points were defects may be commonly found. These defects will now be discussed.

Fig. 1, position of defects.

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1. Distortion of combustion chamber tube plate (Fig. 1, b): This defect, if slight, can be detected by placing a straight edge across the tube ends and sighting the face of the tube plate. The pushing of the tube plate inwards is caused by overheating of the tubes, usually attributable to excessive scale or oil deposits. In extreme cases, signs of leakage of the welded tube ends may be evident and when this is found, cracks in welded tube connections may have occurred.

Distortion of tube plate.

2. Wastage of combustion chamber or furnace stays (Fig. 1, d): This common defect, often referred to as “necking”, is accelerated by the straining action imposed by continual expansion and contraction of the combustion chamber, or furnace, by temperature fluctuations. When dealing with wasted stays it should be remembered that their strength varies with the square of their diameter.

Wastage and fracture of stays.

3. Overheated chamber top (Fig. 1 e): The first part to suffer from water shortage is the combustion chamber crown.

Overheated top plate.

An accumulation of mud, scale, or other insulating material such as oil can also be the cause of overheating. When a distortion of a combustion

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chamber top occurs, special attention should be given to the welded connections (possible cracks). 4. Wastage of door flanges and landings (Fig. 1, l): This type of defect remains all too common. Manhole and handhole door spigot clearance should receive special attention at each boiler survey. Leakage from man / hand hole doors can cause serious shell wastage. Particular attention should always be given to the fit of the doors and to obtaining a good joint. A careful check should be made for strained door studs, slack fitting or stripped nuts, and distorted manhole door dogs.

Picture of leaking hand hole on the left and wastage of door flange on the right.

5. Grooving of end plates (Fig. 1, n): Grooving has been found in the boiler shell plating adjacent to the welded connection of the end plate to the shell.

Grooving in way of weld of boiler shell plate.

6. Cracking of furnace at welded connection to end plate (Fig. 1, s): Serious furnace failures have occurred in the past when circumferential corrosion fatigue cracking resulted in the rupture and collapse of the furnace. In view of the serious consequences of such a defect particular attention should be given to this weld connection during boiler surveys.

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Cracks in furnace at connection to end plate.

7. Uniform furnace distortion (Fig. 1, u): Overheating and subsequent deformation is always caused by the presence of some insulating medium, scale, mud, or oil contamination. This effect may be further increased by faulty combustion. The usual way of getting some idea as to whether a furnace is possibly round, is to sight along the corrugations with a torch from inside the combustion chamber.

Checking furnace for distortion drawing on the left, and a collapsed furnace on the right.

8. Unidirectional thermal cracking (Fig. 1, v): Thermal cracking sometime occurs in furnaces of package boilers. This can be attributed to flame impingement from a faulty burner, which was allowed to operate in this condition for some time.

Thermal cracks on peaks of corrugations right and local bulge in furnace left.

9. Local bulge in furnace (Fig. 1, w): Local bulges in furnaces are caused by overheating and are remedied by cutting out the bulged piece and welding in a new piece. On no account should repair by fitting doubler plates be contemplated.

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10.General internal wastage of shell bottom (Fig. 1, y): In neglected boilers corrosive deposits may have accumulated at the bottom for a long period. This may result in pitting and wasted areas. In general the only satisfactory method of repair consists of fitting a new boiler shell insert. The building up of wasted shell areas by welding should not be permitted. 11.Overheated combustion chamber back plate (Fig. 1, z): It is quite common, especially in boilers which are not kept clean, to find the back plate bulged between the stays. A bulged plate accumulates scale and mud, and promotes further overheating and extension of the bulge. Provided the bulging of the plate between the stays is not very extensive, and has not stretched the material in way of the stay holes to cause leakage. The obvious remedy is to keep the waterside as clean as possible, in order to prevent further overheating.

Overheated combustion chamber back plate.

Vertical shell type boiler AQ 3.

Vertical Shell Type Boilers The original vertical boilers were riveted and were either of cross tube design with central uptake, or of the horizontal smoke tube design. The present day all welded vertical boilers with fireboxes and smoke tubes are basically developments of the earlier designs. A type different from the foregoing is the one fitted with water tubes, which has gained in popularity. The modern vertical shell type boilers nowadays incorporate membrane tube walls (mono walls) around the furnace. The following contains some common defects found in vertical shell tube boilers. 1. In general vertical boilers are big enough for internal access, although the lower parts around the fire box (red circle at bottom, see sketch below) are often very restricted and resort has to be made to the best possible examination through small hand holes. 2. In case of poor water treatment, pitting in firebox and to a lesser extent the shell can be expected. In case of smoke tubes boilers, the tubes, shell, and furnace crown may be affected. It is also important to ascertain in such a case that the stay tubes which tie the flat tube plates together are still in a good condition.

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Pitting on top of the firebox (left), area affected by pitting and deformation as mentioned below are marked by a red ellipse in the sketch (right).

3. While concentrating on the internal examination it is always advised to make a special point of looking for distortions of the heated surfaces. Especially, the flatter parts where mud or scale can accumulate leading to overheating. Overheating through water shortage usually results in serious deformations of the furnace or firebox.

Deformed furnace plate seen from the water side (left) and the furnace side (right).

4. The firebox or furnace of vertical boilers is usually connected to the bottom of the cylindrical shell by what is known as an “ogee” ring. In service this ring is located in a zone of little if any circulation and it forms a receptacle for sludge and corrosive sediments. As it is subject to considerable straining due to pressure and temperature fluctuations, it is prone to “grooving” which can be of a serious nature. The presence of this groove can, it will be appreciated, only be found by careful examination through bottom shell hand hole doors.

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Grooving of shell and firebox plate in way of ogee and foundation ring.

5. As mentioned previously in this chapter, one type of vertical boiler is utilizing water tubes in lieu of smoke tubes. These boilers consist of an upper and lower cylindrical section joined to another by straight water tubes, enclosed in the lower section of the firebox, the gases from which pass through the water tubes on route to the uptake. Several fatal accidents have occurred involving the detachment of the top half shell of such boilers. While making internal examination of this type, special attention should be paid to the circumferential welded seam between upper tube plate and shell.

Crack in circumferential weld between upper and lower section, sketch on the right is indicating the position.

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Fatigue cracking of boiler shell and tube plate connection may result in top half being detached from the boiler as illustrated above.

6. The flue gas pipe is an area where heat concentrations are likely to occur, which may result in thermal cracking. Therefore, this area should have special attention during internal survey.

Cracks found in the lower and upper weld of the flue gas seen from the steam / water side. The position of the cracks is indicated in the sketch on the right.

7. Boilers having hemispherical furnace crowns incorporate sometimes bar stays between the crown and the flat lower tube plate. It will be readily seen that any distortion of the furnace crown may result in overstressing the welded connections of the stays. Ultimately, this may lead to failure of the weld. 8. With regard to the external examination, it is sometimes found that wastage exists beneath damp lagging around leaking boiler mountings. Therefore any signs of boiler mountings leakage should be further investigated if the boiler shell is affected by corrosion. Also soot stains on the lagging, or flaked off paint is an indication of flue gas leakage and must be investigated.

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Crack in weld of stay between furnace crown plate and lower tube plate, position is indicated in the sketch on the right.

Deformation of the crown plate overstresses the stay weld which results in cracks, this is also applicable for the welds of the flue gas pipe.

9. Foundation and boiler supports should be examined at every survey. Sometimes these are exposed to overheating caused by defective brickwork. Or when it is situated below the engine room platform, it may be subjected to the corrosion effects of occasional bilge water and a general damp atmosphere. It should be remembered that the combined weight of a boiler and its content, which may be as much as 30 tons, is supported by this structure. 10.Some vertical boilers are designed with a toroidal header which is situated at the bottom of the combustion chamber. This header forms a ring and supports the weight of the boiler. Normally, it is welded to a T or L section support ring and further secured by a number of triangular shaped brackets. Cracks have been found at the toes of these brackets.

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Leaking valves as shown above may lead to serious damage and dangerous situations as illustrated in the left picture where the extension tube and boiler shell are affected by corrosion.

Fig. 1 Tube plug to be used on fire tubes and as a general rule not more than 10 % of tubes should be plugged due to unfavorable head distribution.

11.When isolated tube failures occur in service it is the practice to fit stoppers. Such stoppers should be removed and the defective tube renewed as soon as possible. Figure 1 shows a stopper with bar which is used for plain smoke tubes, some manufactures supply these stoppers

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with a chain for curved or bent tubes. Smoke tube stoppers should always be removed during boiler surveys, whether or not the defective tubes are to be renewed. This in order to examine that the tread and the rod are in a good condition. Failure in service of a smoke tube stopper could result in serious injuries to the personnel. On no account should welded blanks or, worse still, driven in tapered plugs be fitted in plain smoke tubes. 12.It sometimes happens that thermal cracks develop in the ends of plain smoke or stay tubes, at their combustion chamber or firebox ends. This may be accelerated by the use of over-long tubes, resulting in protruding (un-cooled) ends. When this defect is recurring, even after the protruding ends have been cut off, it has sometimes been effective to fit protective heat resistant ferrules after the tubes have been renewed.

Heat resisting ferrules or simply cutting off the uncooled portion is protecting the fire tube ends.

Water Tube Boilers Water tube boilers came into extensive use in the merchant navy as main boilers during and immediately following the 1914-1918 war. Today we only find them utilized as auxiliary boilers on vessels in need of large steam quantities of a certain pressure and temperature, normally not produced by shell type boilers. The generated superheated steam is used to drive cargo pump turbines and turbo generators. The main reasons for adapting water tube boilers instead of shell type boilers are: • Saving weight: The relative weight of a Scotch boiler to a water tube boiler installation for equivalent heating surface area is approximately 3:1. • The possibility of using higher temperatures and pressures: The limit of working pressures for scotch boilers, for practical reasons, such as shell thickness and lack of flexibility was 21 bars. Water tube boilers did not have this constraint, and therefore due to higher pressures and temperatures machinery size and weight for a given output was reduced and thermal efficiency increased. • Greater mechanical flexibility: The water tube boiler is not so sensitive to fluctuating pressures. • Higher steam outputs: The good circulation and the ability to withstand higher pressures have enabled higher steam outputs compared to a scotch boiler.

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•

Wider safety margin in event of explosions: The possibility of a serious explosion is considered to be far more remote with a water tube boiler than with a shell type boiler. In the former, tube diameters are wisely limited and drums are protected from direct radiation and flame impingement. If a tube fails, the content of the boiler escapes at a rate determined by the tube bore. Whereas in the latter, serious rupture of an overheated furnace can almost instantaneously release the content into the engine room.

A thorough conscientious examiner in any walk of life knows the values of working to a definite routine and, in case of a boiler survey, where it is of the utmost importance that nothing is missed, this is essential. The total heating surface of each individual boiler embodies generating, superheating, feed and / or air heating surfaces. The boiler design varies from one installation to the next. Initially, therefore, it is practically essential that the Surveyor makes a brief scrutiny of the boiler arrangement plan, noting in particular the super heater design and the method of steam temperature control. The layout of the boiler unit having been ascertained, a convenient survey route should be planed as suggested in figure 2.

Fig. 2 Suggested survey route for a D type water tube boiler.

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Steam drum Access to the steam drum is often rendered extremely difficult due to the presence of internals such as cyclone separators, feed troughs, perforated plates, etc., and the extent of removal of these can be the cause of controversy at survey times.

Steam drum interior of a Kawasaki main boiler, equipment installed depends on steam production and size of steam drum.

The primary purpose of the drum internals (separators, scrubbers, baffle plates) is to permit separation of the saturated steam from the water-steam mixture leaving the boiling heat transfer surface. The steam free water is then recirculated with the feed water to the heat absorbing surface for further steam generation. Other internals are feed pipes to mix the feed water with the saturated water, and blow down pipes to remove solids from the water surface. Below some common defects found in steam drums will be discussed. 1. Internal pitting of the steam drum surface, if not deemed serious on account of the drum thickness, should not be ignored, but taken as an indication of what may exist in the tube bores, where it could well be serious. When pitting is present in the short length of the bore normally visible in a bent boiler tube, the first consideration should be, “is it active or not?”. If the pitting is of shallow depth and inactive (no corrosion products) no further action is needed. On the other hand, visible active pitting of substantial depth (approximately 40 % tube thickness) definitely requires investigation further down the tubes. Generally, the most seriously affected tubes are those in close proximity to the furnace.

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Above corrosion in a water drum (right) may indicate severe corrosion in tubes, The left picture of a steam drum shows the longitudinal weld which should receive particular attention due to higher stress levels then the circumferential welds.

2. While examining the internal steam drum attention should be paid to the condition and fastening of any fittings. Cracks have been experienced in the welds to the drum of such internal equipment supports. 3. All drum openings to mountings should be sighted; some times tools are forgotten and left in passages leading to gauge glasses or other mountings. 4. Drum welds, in particular the longitudinal welds should be carefully examined, since corrosion / cracks have been found in way of the welds heat affected zone. The tangential stresses in a boiler drum are twice the magnitude of the axial stresses. Therefore the longitudinal welds are more critical than the circumferential welds. Furnace After entering the furnace, it is prudent to pause in the middle of the floor and get an overall impression of the general condition. Look at the screen, water wall, roof tubes, and the refractory. Bear in mind that a shortage of cooling as a rule results in a general distortion of the furnace tubes. The roof tubes are the once first affected by this in case of water shortage. Membrane or mono water walls are nowadays almost without exception used in all modern water tube boilers. They have resulted in great savings in refectory, and provided a gastight furnace wall.

Different methods of welding a membrane or mono tube wall.

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However, there are also disadvantages, repair of a failed tube is more difficult. If not repaired immediately the un-cooled tube will burn and gases can escape into the engine room. Secondly, in the event of a furnace explosion the damage is likely to be more serious since the pressure built is much greater before release. In the following some points for consideration will be discussed. 1. Screen tubes as their name implies screens the superheater from the radiant heat from the furnace. Overheating of these tubes is usually shown by distortion or occasionally by swelling. By shining with a torch sideways across the face of the bank it is easy to see which screen tubes are distorted. The tube bank should also be inspected for cleanliness, gasses should have free passage through the bank.

Gas passage is blocked by soot at the bottom of screen tubes, position is indicated in the sketch on the left.

Checking for distorted screen and wall tubes with the help of a flash light.

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2. Water leakages in the furnace are usually shown by white stains down the outside of the pipe.

White stains are indicating a leaking fin tube bent of a Sunrod CHS boiler, item 2 applies for all boilers.

3. The tubes in the proximity of the burner flame envelope should be examined for flame impingement. Which can lead to distortion, tube bulging, and thermal fatigue cracks. 4. The same observations made regarding screen tubes apply equally to water wall tubes (side, roof, and floor tubes). 5. Openings in the tube wall for soot bowers, flame peep holes, need to be given special attention since they find themselves more exposed and fail more frequently.

Failed tube in way of a so called nose arch.

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6. Refractory is part of the boiler heat retaining envelope, and protects the underneath steel parts against overheating. Probably the most important refractory material in the furnace is that installed to protect parts of the steam and water drum against direct heat exposure from the furnace. Failure of this refractory has on occasions resulted in circumferential thermal fatigue cracking of the drums, which necessitated renewal. 7. Heaters must be surveyed through handholes for corrosion, pitting and cleanliness. Sometimes deposits accumulate in the middle of bottom headers which disturbed the flow through the tubes.

Deposits have accumulated at mid length of water wall header.

8. When outside the boiler a quick glance should be given to the flatness of the boiler casing, any distortion may be caused by a minor furnace explosion. Soot spots or peeled off paint may indicate leakages which are in need of further investigation. Water drum Normally, the internal examination of water drums produces little to note in way of defects. If however pitting, corrosion or deposits were observed in the tubes ends in the steam drum or headers then the lower parts of the tube bores need special attention from inside the water drum. Superheaters Due to the fact that the superheater and its supports are exposed to high steam and flue gas temperatures, it is most likely to find defects in this part of the boiler. Mild steel is used for superheater tubes with steam temperatures up to 399 ˚C, above this temperature alloy steels are used. The alloying elements used being molybdenum and chromium in various proportions. The following contains some common defects found in superheaters. 1. Distortion of superheater banks as a result of overheating. This can be caused by reduced flow during start up or shut down, or the built up of deposits on the internal tube surface. 2. Cracked and burned supports, these parts are not cooled and therefore subjected to high temperatures.

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3. The safe working of the superheater is to some extent allied to its external cleanliness and this must be considered during survey. In case of operating the superheater in dirty condition, special attention should be paid to those parts through which there is still a gas path, as these have probably been operated at excessive metal temperatures.

Newly installed superheater on the right and on the left a picture of the supports.

4. Failure of severely overheated superheat tubes has in several cases lead to iron burns which completely destroyed the boiler. Steam drums, water drums, and headers are usually stress relieved upon completion of the welding work. Several incidents have occurred where attachments have been welded to the drums after stress relief, and eventually cracks have developed in way of these welds of such a serious nature that renewal of the drum was necessary. Therefore the above should be kept in mind when welding work on drums or headers is considered.

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Types of Horizontal Shell Boilers Different configurations Horizontal shell boilers have several different combinations of tube layout. This involves the number of passes the hot flue gasses make through the boiler before they are discharged. Also they are distinguished in dry and wet back configurations.

Dry back configuration.

Above is shown a dry back boiler where the hot gasses are reversed by a refractory lined chamber on the outer plating of the boiler.

Wet back configuration.

A more efficient method of reversing the hot gases is obtained in the wet back configuration. The reversal chamber is contained entirely within the boiler. Modern packaged boilers operate therefore according to the wet back principle.

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Early horizontal shell type boilers had compared to today’s standard, a very poor efficiency. The flue gases were moving from the furnace more or less straight in to the chimney. This lead to the development of two passes dry back boilers which had a considerably improved thermal efficiency.

Two pass, dry back boiler.

A further development of the two pass boiler was the three pass boiler, with a wet back. This improved the thermal efficiency again, and therefore is the standard configuration in use today.

Three pass, wet back boiler.

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Cochran Chieftain Packaged Boiler Particulars (Various steam production and pressure size ranges are offered within a design.) Make: Cochran Type: Chieftain Steam capacity range: 2000 - 15000 kg/h Steam pressure: 17 bar maximum Description This is a three pass semi wet back design of a package boiler which has a large heating surface in relationship to its volume. As indicated above it is made in a wide range of sizes varying in output and working pressure.

Cut-away view of a Cochran Chieftain packaged boiler.

The first pass consists of a single plain furnace which is wasted at about two thirds of its length. This in order to accelerate the furnace gases before they enter the combustion chamber, and also provides structural flexibility. The furnace is self supporting due to the thick (21 mm) furnace plate used. There are two passes of the smoke tubes. The first pass consists entirely of plain tubes which are curved at one end so they may be received radially into the holes in the hemispherical combustion chamber tube plate. These tubes are lightly expanded and then seal welded at their combustion chamber ends, whilst at the front tube plate they are attached by expanding only.

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Section through the pressure shell of a Chieftain boiler, 17.2 bars, 3068 kg/h

The second pass of tubes is entirely convectional and contains a proportion of stay tubes which are supporting the front and rear tube plate. Plain tubes are expanded into the tube plate whilst stay tubes are expanded and afterwards welded.

Cochran Wee Chieftain Packaged Boiler Particulars (Various steam production and pressure size ranges are offered within a design.) Make: Cochran Type: Wee Chieftain Steam capacity range: 1000 - 5000 kg/h Steam pressure: 10.34 bar (available up to 28 bars) Description The range of smaller boilers based on the Chieftain boilers is generally referred to as Wee Chieftain boilers. Its three pass wetback design is renowned for reliability, durability and its robust design. The construction of the pressure shell is very similar to that of the Chieftain although scantlings will generally be found to be lighter. Various designs of furnace and combustion chamber are adopted depending on the rating and size of the particular boiler. Nowadays these boilers are delivered as a total package, incorporating combustion equipment, feed water pump and controls, control panel, all necessary valves and fittings. Packaged boilers are available for immediate use once they are fastened down to the ships structure and connected to the various systems.

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Cochran Wee Chieftain boiler.

Cochran Borderer Package Boiler Particulars (Various steam production and pressure size ranges are offered within a design.) Make: Cochran Type: Borderer Steam capacity range: 5000 - 6800 kg/h Steam pressure range: 6.9 – 10.34 bar Description The Cochran Borderer features a three pass, full wet back design for maximum efficiency. Key elements are that the boiler is compact, and its robust design.

Cochran Borderer Package Boiler.

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Steambloc Package Boiler Particulars (Various steam production and pressure size ranges are offered within a design.) Make: Babcock & Wilcox Ltd Type: Steambloc Steam capacity range: 590 - 10000 kg/h Steam pressure: 17 bar Description Designed as a dry back, single furnace, return tube, horizontal boiler with a high efficiency rating, it is available in a variety of evaporation capacities and pressures. In its most simple form, it may be encountered as a two pass unit having a plain furnace. It should be noted that the pressure shell plating thickness is considerably less than that of the furnace, while the tube plates are the heaviest plates. Larger units of the design incorporate a part corrugated furnace and are manufactured with three passes of gases.

Steambloc 3 pass Packaged boiler, skid mounted.

In the above depictured boiler the rear smoke box is constructed with a division wall or baffle, to separate the second and third passes of the smoke tubes. This baffle forms an inner smoke box, and is fitted with a separate door to completely

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seal the flue gases, which have traversed the first pass from the cooler exhausting from the third pass.

Rear smokebox of a Steambloc 3 pass boiler.

Parat Halvorsen B 5, Smoke Tube Boiler Particulars (Various steam production and pressure size ranges are offered within a design.) Make: Parat Halvorsen AS Type: B 5 Smoke Tube Steam capacity range: 1000 - 15000 kg/h Steam pressure: 12 bar

Parat B5 smoke tube boiler.

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Types of Vertical Shell Boilers Clayton Steam Generator Particulars (Various steam production and pressure size ranges are offered within a design.) Make: Clayton Industries Number of sizes: 15 different sizes Power output range: 100 – 6800 kW Steam capacity range: 144 - 10944 kg/h Steam pressure range: 1.08 – 204.84 bar Description All Clayton boilers employ the following principles: 1. Controlled or forced circulation. 2. Counter flow heat transfer. A pump continuously supplies feed water to a helical coil heat exchanger, which transfers its heat to the water. The flow of feed water is counter to the flow of combustion gases, an engineering principle that contributes to high fuel-to-steam efficiency. Water leaving the heat exchanger passes through a mechanical separator where the liquid and vapour are separated. Steam exits the separator to the steam header.

Flow diagram through a Clayton steam generator.

The employed principles of forced circulation and counter flow and the resultant low water content result in many advantages. Quick start and response capabilities result primarily due to the low water content, forced circulation, and helical coil design. Another benefit of this design is reduced blowdown. The amount of water removed from the system to maintain an acceptable level of total dissolved solids (TDS) is greatly reduced compared to conventional boilers. Further, a Clayton steam generator provides an automatic indication of scale build-up: indicated by an increase in feed pump pressure. The Clayton design is inherently safe with no possibility of a hazardous steam explosion. The heated pressure part steam generator is a spiral spring coil.

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Clayton steam generator, burner is in the bottom. Right schematic drawing of the steam separator.

The feed water flow is provided by a heavy duty positive displacement diaphragm pump. A single high-grade carbon steel, continuous coil heat exchanger employs a staggered configuration and spacing of coil sections, to help ensure turbulent and high velocity gas flows that facilitate high rates of heat transfer.

Staggering and spacing of the coil sections, on the right Clayton circulation pump.

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Sunrod CPDB Boiler Particulars Make: Type: Steam capacity: Steam pressure:

Sunrod CPDB 1500 kg/h 12 bar

Description This type of boiler is constructed with a complete water cooled furnace, no refractory liner whatsoever being required. The patent pin tube elements are laced in a large diameter uptake tubes. These elements claim to increase the efficiency of the boiler and at the same time enable a compact design to be achieved. It should be noted that these types of boilers have relatively thin shell plating in comparison with the furnace and crown plates. Such boiler shells are sometimes referred to as pressure envelopes, the total weight of the full boiler is taken by the furnace structure rather than the shell.

Upper right, new pin tube elements in factory. See arrow for corresponding area in below drawing, of furnace support brackets (yellow), Pin tubes waterside (red), pin tube from furnace (blue).

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Sunrod CPDB boiler.

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Sunrod CPH Boilers Particulars (Various steam production and pressure size ranges are offered within a design.) Make: Sunrod Type: CPH Steam capacity range: 700 - 35000 kg/h Steam pressure: up to 18 bar Description The Sunrod CPDB type was further developed into the CPH type, one of the incentives was to cope with the increasing demand for boilers with larger steam capacity.

Sunrod CPH boiler

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This boiler incorporates a water cooled furnace formed by a fusion welded tube panel, membrane or mono tube walls. The tubes are connected at their lower ends to a toroidal heater, and their upper ends are attached to the steam chamber. A number of large downcomers ensures good circulation. The fire tube (burner tube) is water cooled by a separate heater. Each of the uptake tubes contains a pin tube element. As an example the CPH 140 type has 39 uptake tubes and can produce 30000 kg steam per hour. Survey points and defects The following depicts typical defects and areas for attention of the Sunrod CPH and CPDB type boilers. The locations of these defects are indicated by corresponding coloured circles in the above drawing of the CPH boiler. Picture of defect

Description Pin tube element The lower connection between the pin tube element and uptake tube is found leaking. Indicated on the picture by white debris. Sometimes this weld connection is found corroded. Close up inspection of these areas is advised.

Cracks at down comers Cracks have been experienced in way of the connection between the down comers and the steam chamber. Also cracks have been found in way of the weld between the U section and shell plating.

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Deformed uptake tube An overheated and deformed uptake tube which needs to be renewed. As a temporarily repair the uptake tube can be closed by inserting a cylindrical plate at the top and bottom (right side in picture). If possible the pin tube element should be removed, and holes cut in the uptake tube for cooling, otherwise the fitted closing plates will overheat.

New uptake tube.

Grooving weld uptake tube Corrosion and grooving is found at the weld connection between the uptake tube and the furnace crown plate.

Deformation of furnace crown Deformation is found in way of outer ring of uptake tubes. In severe cases cracks between uptake tubes are found.

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Aalborg AQ 3 Boiler Particulars (Various steam production and pressure size ranges are offered within a design.) Make: Aalborg Type: AQ 3 Heated surface range: 15 – 175 m Steam capacity range: 800 – 8750 kg/h Steam pressure range: 7 - 12 bar Boiler weight range: 2900 – 180000 kg Description The principle heated surface of the AQ 3 boiler is a cylindrical furnace, or firebox, enclosed within the water space in the lower part of the boiler shell. Basically, these boilers consist of a lower, or water chamber and an upper, or steam/ water chamber. These two chambers are connected by a large number of vertical water tubes and two large down comers. These down comers are essential to ensure a high rate of circulation when maximum steam production is required.

Aalborg AQ 3 boiler

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About one third of the tubes are stay tubes, most of these are situated in a ring as near as possible to the periphery of the tube plates because any outward deflections of these plates, when under pressure, results in stress concentrations in this area. The flue gases ascend through the elliptical flue pipe into the smoke box.

Aalborg AQ 3 boiler

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Where they are by means of baffle plates which are attached to the first row of water tubes, evenly dispersed throughout the smoke box before escaping to the chimney. The boiler is fitted with a large stay bar between the upper tube plate and the shell crown plate. A further stay bar is fitted between the lower tube plate and the furnace crown plate. Survey points and defects The following depicts typical defects and areas for attention of the Aalborg AQ 3 boilers. The locations of these defects are indicated by corresponding coloured circles or arrows in the above drawing of the boiler. Picture of defect

Description Deformed furnace crown plate Furnace crown plate is the first area affected by overheating. Indents accumulate mud and sludge which result in further overheating.

Deformation seen from furnace

Cracking of welds Deformation of the furnace crown plate may cause cracking of stay and flue pipe welds. Check these welds with MPI if indents are found.

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Crack flue pipe welds Since this is an area of high heat input cracks are often found in the weld connections of the elliptical flue gas pipe.

Crack in weld upper tube plate Cracks have been found in the weld between the shell plate and the upper tube plate. If allowed to progress they may lead to very dangerous situations.

Previous repair

Crack

Cracks in burner tube When the brickwork, quarl around the burner is missing or in poor condition the fire tube gets overheated and cracks.

A number of manufacturers produce boilers of similar design to the Aalborg AQ 3 type. One of these types of boilers is the Osaka OVE. Also the Hitachi Zosen HV, and the Helsingoskibs vertical boiler are very similar to the AQ 3 boiler from Aalborg.

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Osaka boiler, type OVE.

Aalborg AQ 9 Broiler Particulars (Various steam production and pressure size ranges are offered within a design.) Make: Aalborg Type: AQ 9 Thermal output range: 5.7 – 32.3 MW Steam capacity range: 8000 – 45000 kg/h Standard steam pressure range: 9 - 12 - 18 bar Boiler weight range: 24500 – 90500 kg Description The AQ 9 is widely used in diesel powered ships in conjunction with an exhaust gas boiler installation.

Flow diagram of an oil fired AQ 9 boiler working in conjunction with an AV 6N exhaust gas boiler.

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The oil fired boiler automatically supplements the steam supply, when the demand exceeds the production of the exhaust gases. Furthermore the AQ 9 may be used as steam space for the exhaust gas boiler.

Aalborg AQ 9 boiler

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The boiler consists of a cylindrical furnace, a convection section and a water and steam space. In this boiler the furnace is surrounded by a closely placed row of membrane wall tubes. The furnace tubes are welded at the bottom into a ring (toroidal) heater. At the top the furnace tubes are welded into the lower tube plate of the intermediate water chamber surrounding the cylindrical uptake through which the hot flue gases pass to the convection chamber above. The flue gases are drawn through the central uptake and, by means of baffle plates attached to the vertical water tubes in the smoke box, are caused to flow spirally across the tubes to the chimney.

Baffle plates

Flue gas flow in the convection tube bundle, flow is directed by baffle plates, black dots are stay tubes.

The top of the intermediate water chamber acts as the lower tube plate for the vertical water tubes forming the convection heating surface. The upper tube plate, together with the cylindrical shell and flat top, form the steam / water space of the boiler. A gastight membrane wall encloses the convection heating surface and this wall is fitted with a sufficient number of cleaning doors and a flanged exhaust gas outlet.

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Effective natural circulation is secured by means of eight external unheated down comers, connecting the steam / water chamber and the bottom ring heater.

Aalborg AQ 9 boiler

Survey points and defects The following depicts typical defects and areas for attention of the Aalborg AQ 9 boilers.

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The locations of these defects are indicated by corresponding coloured circles or arrows in the above drawing of the boiler. Picture of defect

Description Cracks underside uptake tube The under side of the uptake tube is exposed to hot flue gases, cracks have been found at this location. It is very difficult to see a possible crack from the furnace floor, close up inspection of the uptake tube is absolutely necessary.

From the bottom all is in order

Deformation of tube bends These tube bend should be inspected for bulging / deformation, since they are exposed to the hot flue gases.

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Picture of defect

Description Baffle plates in convection space In time these baffle plates are found damaged and sometimes burned away since their cooling is very limited. Missing or damaged baffle plates, causes the flue gases to go directly into the exhaust channel.

Baffle plate

Baffle plates Toriodal heathers Ring heaters to be inspected via hand holes. The pictures are taken during an inspection with an endoscope.

Aalborg AQ 12 Boiler Particulars (Various steam production and pressure size ranges are offered within a design.) Make: Aalborg Type: AQ 12 Thermal output range: 0.89 – 4.50 MW Steam capacity range: 1250 – 6300 kg/h Standard steam pressure range: 9 - 12 - 16 bar Boiler weight range: 6800 – 24400 kg

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Aalborg AQ 12 boiler

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Description The AQ 12 has a cylindrical furnace which is completely water cooled, except for the furnace bottom which is ready cast with refractory. Short stays are fitted between the shell plate and furnace side wall for support on the larger designs. Also a number of long stays are fitted between the furnace crown plate, and the shell crown plate. From the furnace the hot flue gases enter the convection section, which consists of a rectangular bank of water tubes between the furnace and shell crown plates. This convection bank is fitted inclined, in order to secure strong water circulation.

Illustration shows a sectional view of an Aalborg AQ 12 boiler.

Survey points and defects The following depicts typical defects and areas for attention of the Aalborg AQ 12 boilers. The locations of these defects are indicated by corresponding coloured circles or arrows in the above drawing of the boiler.

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Picture of defect

Description Cracked, leaking stays Due to the inaccessibility it is not possible to inspect these stays during inspection of the water space. In order to detect a cracked stay, a tell tale hole is drilled on the ends of the stay. Tell tale hole

Furthermore these tell tale holes are normally covered with insulation material. The first sign of a cracked stay is water leakage through the insulation material.

New stay

Corrosion furnace crown plate Mud and sludge accumulates on the flat furnace crown plate, when left unattended this may cause corrosion pitting. Especially in way of the welds, the heat affected zone.

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Picture of defect

Description Cracks in convection section Cracks have been found at the entry to the rectangular convection section. Especially at the four corners of the box. This part of the convection section is exposed to the hot flue gases and cooling is limited.

New convection section

Blocked gas passage The flue gas passage through the convection section is blocked with soot.

Aalborg AQ 18 Boiler Particulars (Various steam production and pressure size ranges are offered within a design.) Make: Aalborg Type: AQ 18 Thermal output range: 4200 – 3210 kW Steam capacity range: 6300 – 45000 kg/h Standard steam pressure range: 9 - 18 bar

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Boiler weight range: Fuel consumption range: Thermal efficiency: Flue gas flow: Flue gas outlet temp (max/min load)

14200 – 63800 kg 450 – 3210 kg/h 84 % at 100% MCR 7200 – 52000 kg/h 370/220 – 390/250 °C

Description The AQ 18 boiler comprises many of the design details of the well known AQ 9 boiler. The water drum and the steam drum are cylindrical vessels with flat top and bottom plates. The new features of the AQ 18 are that it has two drums, the burner is top mounted and the furnace is located in the centre of the generating tube bank. The furnace consists of membrane wall panels.

Flow diagram for a steam atomizing burner plant.

The boiler is designed for steam atomizing oil burners, and as a standard the AQ 18 is supplied with a KBSA burner from Vesta AS. The KBSA utilizes the Y jet atomizing principle in the burner nozzle. The layout of the top mounted burner combined with the furnace which fits the flame makes it possible to obtain a complete combustion of even the lowest fuel grades. The advantages of the design with cylindrical vessels that have flat plates of equal thickness are that stress concentrations in corners are minimized. As opposed to designs with different plate geometry and thickness, cracks are avoided in connections after many starts and stops of the burner during the boilers lifetime.

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Aalborg AQ 18 boiler

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The furnace and outer casing walls are made of polygonal membrane wall panels. This gives a very sturdy construction which is gastight.

Sketch of the AQ 18 boiler

The generating bank is located between the furnace wall and the casing wall. The generating bank consists of vertical tubes arranged in a staggered configuration. The large size water drum supports the boiler. The advantage of the design with a water drum is that headers are avoided, and therefore there is a safe circulation to all tubes with no risk of blocking and subsequent burning of the furnace tubes.

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The drum gives optimal space for the heating coil, this ensures a quick start up and eliminates the risk of corrosion on the water and fire side. The upper drum is furnished with the necessary internal fittings to ensure even distribution of feed water and a sufficient dryness of the steam. The upper drum carries all essential boiler mountings. Survey points and defects The following depicts typical defects and areas for attention of the Aalborg AQ 18 boilers. The locations of these defects are indicated by corresponding coloured circles or arrows in the above drawing of the boiler. Picture of defect

Description Deformed lower steam drum Plate The bottom plate of the steam drum is exposed to the hot flue gasses and deformations are found in between the stay tubes. The deformations also cause overstressing of the stay tube welds and the burner tube welds.

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Deformed leaking tubes Bulged and leaking tubes are sometimes found in the membrane wall.

Leaking tube

Check tube walls with torch.

Mission OS Boiler Particulars (Various steam production and pressure size ranges are offered within a design.) Make: Aalborg Type: Mission OC Steam capacity range: 1600 – 6500 kg/h Steam pressure: 10 bar Boiler weight range: 7000– 17200 kg Description Mission OS is a high performance oil fired auxiliary boiler in the low capacity range up to 6.5 t/h. It is designed as a vertical cylindrical boiler. The boiler shell surrounds the cylindrical furnace and the convective section consisting of pintube elements. The pin tube elements are an integrated part of the boiler design. The pin tube elements support the furnace and boiler top plate. The design has been significantly optimised to achieve lower weight and improved strength. The result is a longer lifetime of the pin tube by a factor of 8. The boiler pressure part is made of well proven mild steel with elevated temperature properties. The burner housing is mounted on the boiler front, angled 15 degrees downward against the furnace bottom. This design allows for a long flame and gives better

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utilisation of the furnace and the flow of the combustion particles becomes optimal. The result is high performance combustion even with the lowest grades of fuel.

Mission OS Boiler

Mission OM Boiler Particulars 11 bar version Make: Type: Steam capacity range: Thermal output range: Steam pressure: Boiler weight range:

(Various steam production size ranges are offered within a design.)

Particulars 18 bar version Make: Type: Steam capacity range: Thermal output range: Steam pressure: Boiler weight range:

(Various steam production size ranges are offered within a design.)

Aalborg Mission OC 8000 – 20000 kg/h 5600 – 14000 kW 11 bar 19000– 35000 kg Aalborg Mission OC 14000 – 45000 kg/h 9800 – 31500 kW 18 bar 27300– 62700 kg

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Sketch of the Mission OM boiler, 11 bar version.

Mission OM is a vertical cylindrical medium size steam boiler in the 8 - 45 t/h capacity range. The boiler is constructed from standard modules and comes preassembled in order to minimise installation time.

Sketch of the Mission OM boiler, 18 bar version.

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The furnace features gas tight membrane walls. A sufficient number of down comers ensure safe natural circulation. The boiler is equipped with integrated pin-tube elements in the convection section. Furthermore, the pin tube elements support both the furnace and the boiler top plate.

Above improved pin tube design used in the Mission OS and OM, right burner opening in furnace wall.

The standard boiler is delivered with an Aalborg Industries rotary cup or steam atomizing burner and a control system, ensuring fully automatic operation and control. The burner operates on diesel oil, heavy fuel oil or gas and is designed for a high turn down ratio with complete combustion at low oxygen levels.

Mission OL Boiler Particulars (Various steam production and pressure size ranges are offered within a design.) Make: Aalborg Type: Mission OL Steam capacity range: 12500 – 55000 kg/h Thermal output range: 8800 – 38800 kW Steam pressure: 18 bar Boiler weight range: 25400– 78000 kg Description The slim top-fired design makes the boiler perfect for use in e.g. tankers.

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The Mission OL is a lightweight boiler, requiring a minimum of deck space.

Aalborg Mission OL boiler

The two drum cylindrical boiler design, features straight tubes directly connecting the water and steam drums. This ensures safe circulation with no risk of overheating and subsequent burnout of the tubes. Water circulation is further enhanced by external non heated down comers.

On the left tube with extended pins used in the Mission OC, and right a Mission OC boiler under construction.

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Thanks to gastight, polygonal membrane wall panels the sturdily constructed furnace is resistant to gas pulsations. The integrated burner design reduces refractory to a protective layer at the bottom of the furnace and around the access doors. The convective section consists of straight tubes. These tubes have been extended by pins bent to create a unique flue gas flow, enhancing the heat transfer and lowering the pressure loss across the convective section. The result is a compact and efficient heat transfer surface. The Mission OC is designed with a high flue gas velocity in the convection tube bank, which causes a self cleaning effect on the heat transfer surface. Soot blowing equipment is supplied with the boiler in a standard delivery. Water washing of the heating surface can easily be carried out from the top of the tube bank. The advanced KBSD steam atomising burner is used. It has been specially designed to fit the Mission OL boiler, and providing excellent burner performance. Operation and control of the Mission OL boiler plant are facilitated by a reliable and user friendly microprocessor based control system. A PC, conveniently placed in the control room, with a graphical user interface enables remote control and monitoring of the boiler and burner plant. From the system all kinds of diagnostics and statistical data may be retrieved, which is an invaluable help in fault tracing situations.

Parat Halvorsen MW Boiler Particulars (Various steam production and pressure size ranges are offered within a design.) Make: Parat Halvorsen AS Type: MW Water Tube Boiler Steam capacity range: 8000 – 25000 kg/h Steam pressure: 7 bar Boiler weight range: 21000– 45000 kg Description The Parat MW is designed for chemical tankers, small to medium size crude oil tankers, FPSO`s, and cruise liners. It is a fully automated steam boiler, with a well dimensioned steam drum which makes the boiler suitable for evaporating steam from other exhaust gas boilers. The boiler is supplied with a circular furnace, has a convection section and is designed for forced draught. It operates with natural circulation, and a sufficient number of down comers are installed ensure proper circulation. As insulation material, rock wool is used jacketed with galvanized sheet metal plates.

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Parat MW water tube boiler

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Types of Vertical Composite Boilers A composite boiler is able to be operated on either diesel engine exhaust gases or oil, or both, when necessary. They normally consist of an exhaust gas fired and oil fired section, built within one boiler shell.

Aalborg AQ 5 Boiler

Aalborg AQ 5 Composite boiler.

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Particulars (Various steam production and pressure size ranges are offered within a design.) Make: Aalborg Type: AQ 5 Steam pressure range: 7 – 18 bar Heated surface oil fired section: 75 m² Heated surface of exhaust gas section: 200 m² Steam production oil fired section: up to 4000 kg/h Steam production exhaust section: up to 2500 kg/h The steam production of exhaust gas section depends on the amount and temperature of the exhaust gases. Description This boiler is a composite version of the AQ 3 type, in which the pressure shell is in the form of three cylindrical drums. The lower drum contains the oil fired furnace, the centre drum, which is made with two tube plates, is essentially a water space, whilst the upper drum is a steam and water space.

Sectional view of an AQ 5 boiler

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The tube plate of the lower drum and the lower tube plate of the centre drum form the boundaries of the smoke box and oil fired section, whilst the upper tube plate of the centre drum and the tube plate of the upper drum form the boundaries of the exhaust gas section. The three sections are connected by straight plain and stay tubes, and two large down comers which connect the upper and lower drums to promote efficient circulation. Survey points and defects The following depict typical defects and areas for attention of the Aalborg AQ boilers. The location of these defects is indicated by corresponding coloured circles in the above boiler drawing. Picture of Defect

Description Cracks in down comers Differential expansion between the stay plate, generating tubes, and the down comers, result in cracks in the down comers. These cracks are frequently found in way of the welds.

Crack in down comer

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Stay arrangement The stay arrangement between the shell sections and tube plates requires careful attention.

A number of manufacturers produce boilers of similar design as the Aalborg AQ 5. One of these types of boilers is the Osaka Composite boiler type OEVC.

Osaka OEVC Boiler

Aalborg AQ 16 Boiler Particulars (Various steam production and pressure size ranges are offered within a design.) Make: Aalborg Type: AQ 16 Steam production oil fired section: up to 6300 kg/h Description The AQ 16 is a combination of two well known boiler types out of the QA series. In principle the AQ 16 is a combination of the AQ 12 and AQ 7 exhaust gas

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boiler. The oil fired part is of the AQ 12 design, and the exhaust part of the AQ 7 design which is installed adjacent to this. As an option the boiler can be combined with a compact silences. Since the exhaust part is equipped with smoke tubes no circulation pumps are necessary.

Schematic drawing of a AQ 16 boiler with silencer

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Aalborg Mission OC Boiler Particulars (Various steam production and pressure size ranges are offered within a design.) Make: Aalborg Type: Mission OC Steam pressure: 9 bar Steam production oil fired section: 750 – 5000 kg/h Steam production exhaust section: up to 3000 kg/h Description MISSION™ OC is a vertical boiler with an exhaust gas section consisting of smoke tube. The cylindrical shell surrounds the smoke tubes, the furnace, the steam space and the convective section consisting of pin-tube elements. These pin tube elements support both the furnace and the boiler top plate. The boiler pressure part is made of well-proven mild steel with elevated temperature properties. Stress concentrations in corners are minimised by the simple design of cylindrical shells with flat plates of equal thickness. It operates with a mono bloc type pressure-jet burner. The burner housing is mounted on the boiler front, angled 15 degrees downwards against the furnace bottom. This allows for a long flame and offers better utilisation of the furnace. The flow of the combusting particles becomes optimal, and this results in a high performance combustion, even with the lowest grades of fuel. The MISSION™ OC is also available with a rotary-cup burner. The composite boiler can be supplied with a compact silencer to suit any type of diesel engine.

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Schematic drawing of a Mission OC boiler

Mission OC boiler at different stages during building.

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Survey points and defects The following depicts typical defects and areas for attention of the Mission OC boilers. The location of these defects is indicated by corresponding coloured arrow in the above drawing. Picture of Defect

Description Blocked smoke tubes Smoke tubes blocked by soot needs to be cleaned.

Parat MC Boiler Particulars (Various steam production and pressure size ranges are offered within a design.) Make: Parat Halvorsen AS Type: MC Combined Oil / Exhaust fires Steam production oil fired section: 5000 kg/h Steam production exhaust section: 3500 kg/h Description The principle of the boiler is based on a common water and steam space and separate sections for the oil fired and exhaust gas. If several engines are utilized, the boiler can be delivered with several exhaust gas sections. The boiler comprises of smoke tubes for both the oil fired and exhaust gas sections.

Parat MC Boiler

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Schematic view of a Parat MC boiler

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Types of Two Drum Water Tube Boilers Mission D Type Boiler Particulars (Various steam production and pressure size ranges are offered within a design.) Make: Aalborg Type: Mission D Type Steam capacity range: 25000 – 120000 kg/h Thermal output range: 17600 – 84700 kW Steam pressure: 18 bar Thermal efficiency: 84% at 100% MCR Boiler weight range: 39600 – 120900 kg Description The D-type boiler is designed especially for tanker operation, it produces steam for: • Cargo pump turbines • Heating • Tank cleaning, as well as the production of inert gas.

Schematic drawing of a Mission D Type boiler.

The boiler consists of a steam and water drum connected by a generating tube bank. The furnace is made of membrane walls forming a fully water cooled gastight furnace. Refractory is limited to a protective layer on the deflected tubes around the inspection and access doors, and around the burner opening. Large unheated external down comers, welded to the drums, secure natural water circulation with a large circulation ratio.

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The convective section is arranged between the drums. The heating surface in the centre of the convective section has been extended with pins, enhancing heat transfer and creating a unique lengthwise flue gas flow. The result: a compact, highly effective heat transfer surface. High flue gas velocity in the convection tube bank causes a self cleaning effect on the heat transfer surface. Soot blowing equipment is supplied with the boiler in a standard delivery. The Mission D-type boiler comes with the advanced KBSD steam atomising burner. This burner has been designed to fit large capacity Mission boilers, including the Mission D-type boiler. Thorough analysis with the CFD (Computational Fluid Dynamics) program, a unique flame structure and geometrically optimum wind box and register was designed. This ensures even air distribution, minimum pressure loss and a large turn down ratio.

Right, Used steam generating tube with pins, Left a CFD analysis of the burner.

Mission D Type boiler ready for shipping

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Chapter 4: Combustion and Atomizers Introduction A boiler requires a source of heat at a sufficient temperature to produce steam. Fossil fuels, heavy fuel oil or diesel oil are burned directly in the furnace to provide this heat, although waste energy from another source (e.g. exhaust gases) may also be used.

Combustion Combustion is defined as the rapid chemical combination of oxygen with the combustible elements of a fuel. There are just three combustible elements of significance in fossil fuels used onboard ships: carbon, hydrogen and sulfur. Sulfur is of minor significance as a heat source, but a major contributor to corrosion and pollution problems. The objective of good combustion is to release all of the energy in the fuel while minimizing losses from combustion imperfections and excess air. The combination of the combustible fuel elements and compounds in the fuel with all the oxygen requires temperatures high enough to ignite the constituents, mixing or turbulence to provide intimate oxygen / fuel contact, and sufficient time to complete the process. This is sometimes referred to as the three Ts (temperature, turbulence and time) of combustion. In contrast to diesel engines, boilers operate with a continuous combustion process. Consequently, ignition performance of the fuel is irrelevant, and due to the non cyclic performance there is no need to sweep away the products of combustion before more fuel can be introduced, as the whole process is one of continuous supply of air and fuel. Combustion can therefore take place with much lower air to fuel ratios than with diesel engines. Although only a limited level of fuel oil preheating (when operating on heavy fuel oil) is necessary with boilers plants compared to diesel engines, it is of equal importance to satisfactory combustion performance. With boiler plants the intended level of atomization is relatively coarse with a droplet size of around 50 to 100 μm. This slightly increases the allowable viscosity, usually in the range of 15 to 30 cSt depending on the burner design, with a corresponding reduction in pre-heat temperature. Irrespective of the burner type, the overall intention is that the finely divided and swirling fuel oil spray produced by the atomizer, will be thoroughly mixed with the turbulent primary air (motion imparted by the air register swirl or diffuser plate) to give a short wide flame, enveloped initially by a equally turbulent secondary air supply. When the fuel is sprayed into the combustion chamber, its temperature rises as it approaches the previously introduced fuel oil, which is now burning. The outer shell of an individual fuel oil droplet evaporates to form a surrounding cloud of vapour. When this vapour reaches its auto ignition temperature, combustion commences at those points where the ratio is stoichiometric, surrounding the droplets with a zone of combustion. Further evaporation of the droplet (now accelerated by the heat from the combustion zone) will supply combustible material to one side of this zone, while the inward diffusion of air supplies the necessary oxygen to the other. The supply of this air

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changes from primary to secondary air as the kinetic energy imparted on injection propels the droplets through the furnace.

Schematic diagram of an atomizer with air register showing air flow, primary and secondary flame.

If the viscosity of the oil leaving the burner unit is too high, there will be an increase in droplet size, leading to unstable combustion control, poor combustion, and smoke and deposit formation. Too low viscosity results in poor distribution (penetration) of the flame within the furnace. Flame blow off and flashback restrict the maximum and minimum fuel oil flow rates through any given burner. In case of flame blow off, the air / fuel mixture velocity exceeds the flame speed, while in flashback the converse applies. With boilers, other than some small package boilers the combustion air is normally pre-heated, this can be up to 130˚C. Pre-heating the air raises the boilers overall thermal efficiency, gives a higher specific steam generation capacity, enhanced load control and improves combustion performance by ensuring a more complete utilization of the fuel oil and decreases fouling. Increased pre-heating also acts to decrease the tendency of SO2 to further oxidize to SO3 in the exhaust gas stream, reducing the cold corrosion potential. Boiler tube fouling, whether a result of incorrect pre-heating or the use of fuel oils with a high carbon residue value, not only inhibits tube surface heat transfer, making regular soot blowing necessary, but can also accumulate on the air swirl plates and atomizer leading to poor air / fuel mixture and increasing the probability of yet further fouling. In the most extreme case of poor combustion or failure to ensure regular and thorough soot blowing, the accumulation of carbonaceous deposits particularly in way of the cooled sections of the uptake can result in soot and hydrogen fires.

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Conventional multi boiler fuel oil system from Saacke.

Atomizers The following atomizers are commonly employed on auxiliary boilers. • • •

Spill type pressure jets. Spinning cub atomizer. Steam assisted pressure jets.

Spill type pressure jets atomizer The pressure head of the fuel oil is converted into a velocity head as it passes through the small tangential holes at the atomizer tip. In addition, the holes impair a swirling motion to the oil, the discharge from the nozzle being broken up into a fine mist by centrifugal force. The burner is provided with a leak off from the swirl chamber. By increasing the amount of leak off, the amount of fuel oil that is delivered to the furnace is reduced without seriously impairing the atomization.

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Aalborg KBPJ spill type pressure jet burner, capacity range 0.35 to 5.25 MW.

Spill type burner, the oil enters centrally and spill oil leaves through annular passage in burner body.

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Spinning cub atomizer The working principle of rotary cub burners is based on atomising by centrifugal force. The atomising cub is driven at high speed via a heavy-duty belt drive. The oil is gently positioned at low pressure into the spinning cub where gradually, and forced by the centrifugal action of the cub, it moves forward until it is thrown off the cub rim as a very fine, uniform film. The high velocity primary air discharged around the cub strikes the oil film, breaks it up and converts it into a mist of fine particles which are introduced into the combustion zone, and burner. The secondary air necessary for complete combustion is supplied by a forced-draught fan through the wind box and burner air register.

Aalborg KB rotary cub burner, capacity range 0.55 to 38 MW.

Aalborg Rotary cub burner.

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Rotary cup burner from Saacke used for fuel oil.

Front and back of Aalborg KB Rotary cub burner.

Steam assisted pressure jets Atomizers Low pressure steam is used in this type of atomiser to increase the effectiveness of fuel oil pressure as a means of obtaining atomization. These atomisers have several advantages, it is claimed that their use results in a cleaner boiler and they require lower fuel pressures. But their main disadvantage is the absorption of up to 1% of the steam output and this of course is loss of fresh water.

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Aalborg KBSA Steam atomised burner, capacity range 1.7 to 46.9 MW.

Burner tip of a skew jet atomizer.

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The KBSD steam atomising burner is use on top-fired boilers, capacity range 1.7 to 46.6 MW.

Ignition burner Starting initial combustion or lighting the main burner requires an independent source of ignition. As a minimum the ignition burner assembly consist of a fuel nozzle, spark ignition source and an energy source to produce the spark. The ignition burner is operated on gas oil, and the arrangement can be stationary or equipped with a retracting mechanism for protection from furnace radiation. Programmable ignition controls are installed that automatically sequence all functions including on / off of the fuel, atomizing and purge medium, lighting and spark probe insertion and retraction. A manual ignition control is normally fitted for emergencies.

Ignition burner seen from the furnace (right) and from the burner front (left).

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Burner Safety Systems Automated sequence of operation, interlocking and alarm systems are of prime importance in safe and reliable operation of the oil fired combustion system. In observance of the DNV rules and recommended practice of operation, an automated control system should include the following. • Purge interlock requiring a specified minimum air flow for a specific time period to purge the furnace before the fuel trip valve is opened. • A spark producing device must be in operation before introducing any fuel to the furnace (fuel trip valve is opened). The ignition source must continuously provide a flame or spark of adequate size until combustion of the main burner is self sustaining or a specified time period has elapsed, resulting in ignition failure alarm and fuel trip valve closure. • Flame detector connected to an alarm (flame failure) and interlocked to shut off the burner fuel valve upon loss of flame. • After flame failure or unsuccessful ignition automatic restart is not to take place until local reset on the control panel has taken place. • The burner door or front is equipped with a limit switch which shuts off the fuel supply and gives an alarm when the burner is swinged out or retracted off its position.

Right, limit switch of burner door. Left, flame detector.

• • •

A positive air flow through the burner into the furnace and up the stack must be maintained at all times. Therefore failure of the forced draft fan will cause an alarm and shutoff of fuel supply. Adequate fuel pressure and temperature for proper atomization must be maintained at all times. Shutoff of fuel supply and applicable alarm must be raised in case of low fuel pressure and temperature. Apart from the above items which are directly connected to the oil firing combustion system, the following conditions will also result in closure of the fuel trip valve and sounding of the applicable alarm ¾ Water level low. ¾ Steam pressure high.

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•

¾ Forced circulation failure. Automatic start and stop of the oil fired combustion system controlled by a set upper and lower value of the steam pressure.

Oil Fired Combustion System Survey The complete inspection can be divided into the following two separate parts. 1. Visual survey of the combustion system. 2. Function test of the burner unit and its safety functions. Visual Survey The following contains some common defects and points to be taken in to consideration while inspecting oil fired combustion systems. •

Inspect the burner front for signs of fuel oil leakages at flanges and filters, fuel oil soaked insulation material should be renewed. Also drip pans underneath burner, fuel filters and fuel pumps should be clean and free of fuel oil.

Check burner front, fuel pipes, and drip tray for fuel leakages.

•

Fire detection and extinguishing equipment in way of burner front to be inspected. Above the burner unit there is normally a fire detector and a fixed fire extinguishing system (CO2 or foam nozzle) installed. Also in the vicinity of the burner there should be a red sand box, check the content because sometimes this is used to store other items or the sand is used and contaminated in which case it has to be renewed.

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•

Swing out or retract the burner unit and inspect the atomiser tip, it must be free from carbon residues which will impair fuel atomization leading to poor combustion. Also the air register swirl vans and tip plates (swirler) are prone to carbon residue clogging, sometimes swirl vanes and tip plates are found partly or completely burned away. Needless to say that this will result in a deficient air / fuel mixture causing further fouling of the burner unit and heated surfaces. And incase the flame is deflected there is the risk of flame impingement, overheating of furnace walls.

Air register with swirl vans and tip plates, inspect for clogging, bent and burned vanes.

•

Survey the refractory quarl around the burner from inside the furnace, sometimes it is found cracked or big pieces of refractory material are missing. This refractory quarl serves two purposes, firstly to protect the steel underneath the refractory against radiation heat and secondly to improve combustion by deflecting the heat back into the combustion zone.

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Refractory quarl around the burner which is due for renewal.

Function Test During function test of the oil fired combustion system the following to be observed. •

•

Start the burner unit and check that the automated start up sequence is followed, purging the furnace and ignition of main burner, in case of unsuccessful ignition an alarm should be raised and fuel trip valve closed. Most furnace explosions are caused by improper purging of the furnace before ignition, therefore this is an important safety function and it should not be by-passed as sometimes seen. Once the burner is in operation watch the flame through the peep hole opposite the burner. The flame should burn stable without flickering, or producing black smoke. It should have a bright yellow colour, a dark yellow flame indicates too high CO emission levels meaning reduced combustion efficiency. Flame blow off or blow back are caused respectively by too high or too low combustion air flow.

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Peep hole to observe the flame.

•

The following safety function can be tested as described below. ¾ Flame failure alarm and fuel valve trip by removing the flame detector from its housing.

Ignition burner with flame detector.

¾ Emergency stop burner by operating the stop button on the local control panel or near the burner unit. ¾ Burner door open alarm by opening the door, while this alarm is activated it should not be possible to start the burner. ¾ Low combustion air flow alarm by closing off (if valve is fitted) the air to the pressostat and drain it, fuel trip valve should close. ¾ High / low fuel pressure and temperature by closing the valves to the pressostat and thermostat and drain them, alarm should be activated. Set points of the pressostat and thermostat can be checked with calibration equipment.

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Chapter 5: Refractories and insulation Introduction There is perhaps the tendency when surveying auxiliary boilers to think too much of potential dangers through deterioration of pressure parts and not pay enough attention to the heat retaining envelope which consists of refractory in the furnace and an insulation system on the outside.

Refractories Refractory materials are used for their insulating and erosion resisting properties. It is a material that will retain its solid state even at very high temperatures, which in boiler furnaces can be as high as 1300 °C. Although in recent years the amount of refractory in boilers has been drastically reduced by the introduction of membrane tube walls, heat resistant steel grades and better designs. Still in every boiler one finds a small amount of refractory in the furnace which fulfils one or all of the following purposes. • •

Protect the underneath boiler material from overheating and distortion. Refractory is found in way of headers, boiler drums, floors and usually around the burner. From baffles to direct the gas flow, this application is mainly used in case of water tube boilers.

Refractory materials are made of naturally occurring clay composed of alumina, silica and quartz. The material properties vary considerably and are largely dependent on the proportion of alumina present. In present day auxiliary boilers we find firebricks only on the furnace floor. Mouldable or plastic refractory is nowadays widely used, it must be pounded into place during installation and is found in way of headers, floors, burners etc. During start up care must be taken with the fire rate to allow the newly installed mouldable refractory to properly dry and cure. Survey of refractory When visually inspecting refractory inside the furnace, look for large cracks and missing pieces, small hairline cracks are to be expected. For safe and reliable boiler operation the refractory should be in a good condition, this in order to prevent overheating and deformations which may result in costly repairs.

Insulation Heat loss from a boiler is reduced by fitting insulation material on the outside, it is an economical balance between the value of heat loss and cost of insulation work. Insulation systems are designed to provide both safety for the personnel and minimum heat loss. A much used material is mineral wool which is comprised of molten slag, glass or rock blown into fibers or spun. It is supplied in the form of bocks or blankets and when installed covered with a sheet metal lagging for protection.

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Survey of Insulation Upon inspecting the boiler insulation take note of the following. • Damaged or missing insulation or sheet metal lagging should be repaired or placed back. This is particularly important where exposed temperatures are over 220 °C and there is a risk of fuel impingement of the insulation material. • Soot spots on the sheet metal lagging indicate a flue gas leakage and should be investigated. • Corrosion of the sheet metal lagging indicate a water leakage underneath the insulation, this should be further investigated. Also pay particular attention too leaking valve glands, if this is allowed to continue for some time it may lead to sever corrosion of the boiler shell in way of the penetrations. In the past this has lead to very dangerous situations since this corrosion was not noticed due to the fact it is covered by insulation material.

Leaking valves or manholes as illustrated above can cause serious corrosion of the boiler shell.

•

Burned and flaked off paint from the sheet metal lagging may indicate a hot spot caused by steam leakage which must be investigated.

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Chapter 6: Boiler Mountings and Fittings Boiler Introduction All marine boilers are required to be fitted with certain essential mountings with the objective to improve boiler, operation, efficiency, and safety. In this chapter these required mountings, their inspection and testing will be discussed. Det Norske Veritas, other Classification Societies, and National Boiler Authorities have rules and standards for these mountings. These rules and Standards govern the type, number, construction, and certification of the required mountings. Nowadays these governing rules are, to a large extent, harmonized between the different Authorities, especially with regard to type and number of required mountings. For Det Norske Veritas these requirements can be found in Part 4, Chapter 7, Section 6 of the DNV Rules (January 2003 publication), and are the basis for this chapter.

Safety Valves The most critical valve on a boiler is undoubtably the safety valve. Its primary function is to protect life and property by limiting the internal boiler pressure to a point below its safe operating level. Proper maintenance, adjustment, and testing of these valves is essential to guarantee their correct operation when needed. It is literally the last safety before a boiler explosion. Popping of the safety valves means that all other safety systems such as burner control, burner shut down, high steam pressure alarm, have failed to stop the steam pressure from rising above normal operation levels. Basic operation of a safety valve The basic spring loaded safety valve also called standard or conventional type is a simple and reliable self acting overpressure protecting device.

Typical safety valve designs for steam application according to DIN and ASME Standards.

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When the steam pressure rises above the set pressure of the safety valve, the disc begins to lift from its seat. This however causes a contraction of the spring and results in an increase of spring force, meaning that the pressure has to continue to rise before any further lift is possible. The additional pressure rise required before the safety valve will discharge at its rated capacity is called overpressure. All spring loaded safety valves make use of a shroud (lip) on the periphery of the disk (valve lid) for the purpose of giving the disk additional uplift once raised from their seat by the steam.

Typical disk and shroud arrangement used on rapid opening safety valves.

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The volume contained within this shroud is known as the control or huddling chamber. As lift begins the steam enters this chamber and a larger area of the shroud is exposed to the steam pressure. This incremental increase in opening force overcompensates for the increase in spring force, causing rapid opening. Simultaneously, the shroud reverses the flow direction of the stream which provides a reaction force, which further enhances the lift.

Operation of a conventional safety valve.

Once normal steam pressure is restored the valve is required to close again, but due to the increased lift this will not happen until the pressure has dropped under the original set pressure. The difference between the set pressure and the reseating pressure is known as blowdown.

Relationship between pressure and lift for a safety valve.

The blowdown rings found on most ASME type safety valves are used to make fine adjustments to the overpressure and blowdown values of the safety valve. The lower blowdown ring is a common feature on many high capacity direct loaded spring valves and its adjustment governs the blowdown value, thereby limiting usable steam losses. The upper ring is usually factory set and essentially negates manufactures tolerances which affect the geometry of the huddling chamber.

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The blowdown rings on a ASME type safety valve.

Industry standards and rules that govern the design of safety valves, generally only define the following three dimensions that relate to the discharge capacity of the safety valve. 1. Flow area: The minimum cross sectional area between the inlet and the seat, at its narrowest point (d in below figure). 2. Curtain area: The area of the cylindrical or conical discharge opening between the seating surfaces created by the lift of the disk above the seat (d1 in below figure). 3. Discharge area: This is the lesser of the curtain and flow area, which determines the flow through the valve.

Illustration of the standard defined areas.

Types of safety valves The valve inlet design can be either a full nozzle or a semi nozzle type.

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A full nozzle valve (a) and a semi nozzle valve (b).

Full nozzles are usually incorporated in safety valve designs for high pressures. The approach channel and disk are the only part of the valve exposed to high steam pressures and temperatures when in closed position. This makes it possible to use less expensive materials for the body. The advantage of semi nozzle design is that the seat can be easily replaced, without renewing the whole inlet. The terms full lift, high lift and low lift valves refer to the amount of travel the disc undergoes as it moves from its closed position to the position required to produce the full discharge capacity, and how this affects the discharge capacity of the valve. 1. Full lift: The disk lifts sufficiently so that the curtain area no longer influences the discharge area. Therefore the discharge capacity is subsequently determined by the bore diameter. This occurs when the disks lifts a distance of at least a quarter of the bore diameter. 2. High lift: The disk lifts a distance of at least /12th of the bore diameter. This means that the curtain area, and ultimately the position of the disk, determines the discharge area. 3. Low lift: The disk lifts only a distance of /24th of the bore diameter. The discharge area is determined entirely by the disk position. The discharge capacity of high lift valves tends to be significantly lower than those of full lift valves. For a given capacity therefore one can usually select a full lift valve that has a nominal size several times smaller than the corresponding high lift valves. The size of the waste steam pipe and that of the apertures in the boiler shell are also correspondingly reduced. This ability of the valve to lift fully is achieved by allowing the steam from initial lift to impinge on additional lifting surfaces, either in the form of a lip, shroud, or pistons in guided cylinder. For auxiliary boilers of moderate pressures the conventional full lift safety valve is today commonly encountered in service.

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Installation As per rule requirement each boiler has to be equipped with two safety valves. Sometimes these are fitted in one chest which is directly connected to the boiler shell. When the heated boiler surface is less than 10 m², only one safety valve is required, this is not often encountered in the field. If the boiler is equipped with a superheater then at least one safety valve must be fitted on the outlet side. The later also applies for an exhaust gas boiler (economizer), at least one safety valve is necessary with the discharge capacity equal to the maximum steam production. Easing gears need to be fitted on all safety valves which can be operated from the boiler control position. The operating handles are normally found in way of the burner, or steam pressure gauge.

Examples of safety valve installation.

Each safety valve has its own waste steam pipe discharging to the atmosphere, it has to be suitably supported and where necessary fitted with expansion joints to prevent undue loading of the valve chest. In the past accidents have happened as a result of safety valves connected to a common waste steam pipe. The blowdown of one safety valve will cause a back pressure in the common waste steam pipe, and this will influence the functioning of the other connected safety valves, resulting in delayed popping (or not opening at all) of the valve. The rules permit the use of a common waste steam pipe but have stipulated the installation of so called balanced safety valves. Balanced safety valves are those that incorporate means of eliminating the effect of backpressure. On board ships we do not see many balanced safety valves since it is common practice to install individual waste seam pipes.

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A drain pipe is fitted on the lowest part of each valve chest on the discharge side, which is usually led by a continuous fall to the bilge or below the floor plate. This prevents accumulation of condensate in the valve chest causing corrosion, and in large amounts may influence opening of the valve. It is obvious that these drain lines should be kept free of blockage and no valves or chocks must be fitted. Certification and standards Most countries have their independent authorized approval bodies who examine the design and performance of safety valves to confirm conformity with the relevant standard or code. For steam boiler application there are very specific requirements for safety valve performance in the different standards and codes. Examples of standards related to safety valves are, BS 6759, JIS B 8210, ASME I, DIN 3320, ISO 4126. Safety valve standards are normally very specific about the information that must be carried on the valve. Marking is mandatory on both the shell, usually cast or stamped, and the name plate which must be securely attached to the valve. A general summary of the information required is listed below. On the shell: • Size, pressure designation • Material designation • Direction of flow by arrow On identification plate: • Manufacturer, model type. • Date of manufacturer and serial number. • Certified discharge capacity. • Number of relevant standard.

On the left marking on the valve body, and on the right an example of an identification plate.

In the event a safety valve has to be exchanged with one of another brand or type, is of utmost importance to confirm that the new valve has the same or greater discharge capacity as the previous one. Also the inlet and outlet connections should be checked that they by no means restrict the steam flow.

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According to the DNV Rules (July 2004), the safety valves should be provided with the following documentation. • All valves with Dn> 100 mm and p>16 bar need to be supplied with a DNV Product Certificate. • The manufacturer has to provide a certificate stating the rated valve capacity at the approved boiler pressure and temperature. • All valves need to be hydrostatically pressure tested at 1.5 times the nominal pressure. If not witnessed by a Surveyor this is confirmed by a Test Report from the maker. • Material Certificates are required to be provided as follows. 1. Steel valves Dn > 100 mm, Td > 400 °C – NV Material Certificate. 2. Steel valves Dn<= 100 mm, Td>400 °C – Material Test report. 3. Steel or nodular cast iron valves Dn > 100 mm, Td <= 400 °C – Material Work Certificate. 4. Steel or nodular cast iron valves Dn <= 100 mm, Td <= 400 °C – Material Test report. 5. .Copper alloy valves Dn > 50 mm, – Material Test report. 6. Copper alloy valves Dn <= 50 mm, – Material Work Certificate. The pressure and temperature ratings of the safety valve should be in accordance with a recognized national standard, as for instance DIN 2401 or ANSI 16.5-1958, API STD 600-1957. Reference is made to the enclosed pressure / temperature ratings according to mentioned standards at end of this chapter. Survey of safety valves Part of the boiler survey is opening up and inspection of the individual parts of the safety valves. Occasionally it happens that people feel it’s not necessary to disassemble the spring house assembly. A reason given for this is to save the original setting, so adjustment of the valve can be omitted. From the above it is clear that inspection of these critical parts (spring, spindle) is absolutely necessary and therefore the complete valve is to be disassembled. When overhauling safety valves it is good engineering practice not to mix parts of different valves. All parts need to be cleaned and inspected and as applicable measured to manufactures instructions. The following general guide lines may be of use during these inspections. • Spindles to be checked for straightness, erosion or corrosion, and damages. Bent valve spindles are frequently the cause of sluggish operation. • Springs to be checked for any permanent set or corrosion. Ideally the length of the spring should be compared with the length of a spare spring. Alternatively with the spring length of other safety valves, or sometimes acceptance criteria is mentioned in the instruction manual. • The valve or disk to be checked for damages to the shroud / lip and sealing surface. If valves are of the winged type, and with the narrow seating recommended by the maker for steam tightness, very little wear is permissible on the valve wings if effective sealing is to be maintained. The makers recommendations should be followed, if these specified dimensions are not maintained it may lead to feathering of the waste

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• •

steam pipe considerably below the blow off pressure. Feathering safety valves result in damaged valve seats. Seat to be checked for damage. Normally the valve and seat are ground, and when properly done, a light gray continuous line on the circumference of the seat and disk is displayed. Valve body to be checked for corrosion and erosion. If after visual inspection reduction of the body thickness is suspected one may determine the remaining thickness with a pair of outside calipers.

Adjustment and testing of safety valves Before attempting to adjust the safety valves it is essential to verify the accuracy of the boiler pressure gauge. According to the DNV Rules and other standards the safety valves must be set at a pressure not exceeding 3% above the approved design pressure of the boiler. Normally the approved design pressure of piping and equipment in the steam system is equal to that of the boiler, if this is not the case than the lowest design pressure has to be set. The approved design pressure can be found on the boiler plate which is permanently attached in a prominent place on the boiler, which is generally around the burner. Normally the approved design pressure is taken approximately 1 bar higher than the maximum allowable working pressure, rule wise the design pressure is not to be less than the maximum working pressure. Sometimes we see the boiler is operated below the approved design pressure, and then one may adjust the safety valves at any value above the operating pressure as long as it does not exceed 3% of the approved design pressure. Example: Approved design pressure, Maximum Setting of safety valves, Operating pressure,

13 bars 13.39 bars 9 bars

Safety valves can be set at any pressure between 9 and 13.39 bars. The hydraulic test pressure is 19.5 bars so the fear of some Chief Engineers to bring the pressure up to 13.39 bars is unfounded. The principal that the safety valves are the last safety against an over pressure failure of the boiler, should be kept in mind when adjusting the overpressure safety system. The correct sequence of alarms and shut down is as follows: 1. Burner stops at its normal set pressure for start and stop of burner. 2. High steam pressure alarm. 3. High high steam pressure alarm with simultaneous burner shut down. 4. Lifting of safety valves. For boilers equipped with a super heater, it is a recognized practice to set the safety valves on the superheater at a stipulated value below the set pressure of the safety valves on the steam drum. This is done to ensure that the superheater is circulated with `cooling steam´ at all times. Adjustment in this manner results in the superheater safety valve first lifting, and thus preventing overheating of the superheater. The DNV Rules and other standards prescribe that the discharge capacity of the safety valves has to be such that the pressure does not rise more than 10 %

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above the design pressure with closed boiler stop valve and maximum firing condition. This may be confirmed at new building by an accumulation test, with a duration of 15 minutes for smoke tube boilers, and 7 minutes for water tube boilers. With regard to the above example this means that the pressure should not rise above 14.3 bars. Normally the only time it is necessary to adjust a safety valve is immediately after a boiler survey. The initial setting may be performed by water or air on a test bench, but the final adjustment and function test has to be done on the boiler with steam. Since the safety valve is such a critical safety component it is essential function tests are performed under realistic operating conditions. Occasionally great reluctance is encountered in performing safety valve function test on the boiler. The arguments given for this reluctance are completely unjustified, and when the following is observed it should not create a dangerous situation. • Before any adjustment / test, accuracy of the pressure gauge to be confirmed. • Internal inspection of water / steam and flue gas side, confirmed structural soundness of the boiler. • Steam pressure gauge to be constantly observed during the test. Remember that the hydraulic test pressure is far above set pressure of the safety valves and allowable accumulation test pressure (10 % pd). • The test should be properly prepared, adequately manned, and conducted in a controlled and calm manner. Below some general guide lines are listed when conducting safety valve testing. • Set points of burner stop and shut down to be raised above safety valve pressure. Alternatively one can close the valve to the relevant pressure switches. • During steam pressure rise closely watch the pressure gauge, popping of the valve is clearly indicated by slight decrease in pressure followed by an increase indicating full lift, completed by blowdown at a pressure below the set pressure. Full lift of the valves is indicated by the spindle raise for some valve designs, or the free play of the easing gear. Raising boiler pressure is best done by closing boiler stop valve and manually controlling the burner while monitoring the steam pressure. • During the test the safety valve must lift smartly and fully at its adjusted pressure, and after it has relieved excess pressure, shuts with equal smartness. Sluggish or feathering valves indicate malfunction of the moving parts, also feathering valves result in seat damage. • Only popping of the safety valves is not advisable since this may result in dirt from the boiler being trapped in-between the sealing surfaces, leading to leaking valves. To avoid this it is better to allow full lift and blowdown of the valve so that any possible dirt is blown out. Claims that safety valves are of a design that can not be tested (or only a number of times) due to leaking afterwards, should not be accepted as an excuse to omit testing under steam. If subject valves are of such poor quality they must be renewed since they are not fit for normal service. This means after a pressure release in service one has to shut down and depressurize the boiler to exchange the safety valve, because the original valve no longer closes properly.

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While the safety valves are blowing off excessive pressure, inspect the waste steam pipes and drain pipes leading to the bilge. The drain pipes are sometimes found blocked or damaged. After reseating of the safety valves, test the easing gear from the burner position. It is not uncommon to find them wrongly assembled or seized. It is good engineering practice to measure the distance between spring compression nut and upper face of spindle and record this value in the engine log book for later reference.

Boiler valves The main valves fitted on a boiler as per DNV Rules and relevant codes are listed below. • Steam stop valve: Each boiler is to be fitted with a stop valve as close to the shell as possible. On multi boiler installation connected to common steam header an additional stop valve needs to fitted in series with the first one. The additional valve is generally a globe valve of the screw down, non return type which prevents one boiler pressurizing another. • Feed water check valve: This may be a globe valve of the screw down, non return type or a separate check and stop valve placed as near to each other as possible. The check valve prevents the boiler content blowing into the engine room, in the event of a feed line failure.

Location of the feed water check valve and on the right a boiler check valve.

•

•

Blow-down valves: Restrictions are in place for the use of cocks with tapered plugs. They are to be bolted type with separate packing gland which may only be used up to 13 bars. As with steam stop valves on multi boiler plants, an additional valve needs to be fitted in series with the first one. Test valve: Every boiler has at least one test valve, sometimes combined with a sampling cooling tank. This arrangement makes it possible to obtain boiler water test samples in a safe manner.

Steam stop valves fitted to ordinary shell type boilers are normally right angle globe valves made of nodular or spheroidal graphite cast iron or cast steel. All other valves are generally conventional globe valves.

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Certification The certification requirements for valves according to the DNV Rules (July 2004) are similar to those of safety valves, they should be provided with the following documentation. • All valves with Dn> 100 mm and p>16 bar need to be supplied with a DNV Product Certificate. • All valves need to be hydrostatically pressure tested at 1.5 times the nominal pressure. If not witnessed by a Surveyor this is confirmed by a Test Report from the maker. • Material Certificates are required to be provided as follows. 1. Steel valves Dn > 100 mm, Td > 400 °C – NV Material Certificate. 2. Steel valves Dn<=l to 100 mm, Td>400 °C – Material Test report. 3. Steel or nodular cast iron valves Dn > 100 mm, Td <= to 400 °C – Material Work Certificate. 4. Steel or nodular cast iron valves Dn <= to 100 mm, Td < equal to 400 °C – Material Test report. 5. .Copper alloy valves Dn > 50 mm, – Material Test report. 6. Copper alloy valves Dn <= to 50 mm, – Material Work Certificate. Also valves should be used in accordance with an internationally recognized standard of pressure and temperature rating, for instance DIN 2401 or ANSI 16.5-1958, API STD 600-1957. Reference is made to the enclosed pressure / temperature rating tables at end of this chapter. The material grade and pressure rating is generally cast in the valve body and bonnet.

DN 15, PN 40 angel steam stop valve and spring loaded feed water stop valve.

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General use globe valve design which is typically suitable for any on / off, and throttling services.

Survey of valves All valves adjacent to the boiler shell need to be opened up in connection with the survey. It is common practice to leave the valve body in place and remove the bonnet which is completely disassembled in the workshop. After all the parts are cleaned, inspected, and overhauled they are presented to the attending Surveyor. Commonly the bonnets are presented assembled after new stuffing box packing rings are fitted and the disk sealing surfaces are ground. Some general inspection hints can be found below. • Stem, spindle to be checked for straightness and erosion wear in way of the stuffing box packing. • Valve body to be checked for corrosion and erosion. • Disk and seats sealing surface must be in good condition to obtain a tight valve.

Water level gauges The DNV Rules prescribe that every boiler needs to have two independent means of indicating the water level. In practice this is usually two water gauge glasses, but one could be replaced by an approved equivalent device. This may be an approved electronic level gauge indicator. The water gauge glasses must be so positioned on the boiler that the lowest visible water level corresponds with the lowest, safe working water level. Also the location of combustion chamber top or furnace crown of tank type boilers

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needs to be marked adjacent to the water gauge glass. Cocks or valves need to be fitted on each end of the gauge glass which can be closed from a safe position in the event a glass brakes.

Water gauge glass with round glass for low pressures.

For boiler pressures up to about 20 bars it is normal practice to use round glass tubes. Above 20 bars the glass tube is replaced by what is in effect a built up rectangular section box with a thick glass plate on the front and back. Since gauge glasses are prone to impact damage a protector is often fitted around them. Cock handles should always be fitted in such a manner that they are pointing vertically downwards in normal working, open position. With the cock handles disposed in this way, it can be seen in a glance that all are correct, and there is no danger of vibration causing one to shut. Survey of gauge glasses The following general points may be considered during survey. • •

The water level should be easy to read in both glasses. It is not uncommon that the glasses become cloudy or discoloured with age and in such case renewal is required. The water level in both gauge glasses should be the same, if not this indicates a blockage. Testing of the gauge glasses to be performed by blowing them through with water and steam, by opening and closing the valves on each end.

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Reflective water level gauge, DN 26, PN 25, the body is made of carbon steel.

•

The double plate glass type of gauge is normally illuminated from the rear by an ordinary lamp. This arrangement must be in good working condition otherwise water level readings are not possible.

Pressure gauge Per rule requirement every boiler must be equipped with a pressure gauge at an easy readable position. The highest permissible working pressure is to be marked on the gauge in red. A pressure gauge of the Bourdon tube type is usually fitted. Sometimes it’s connected to the steam space by a ring type siphon tube, which fills with condensate and protects the dial mechanism from high temperatures.

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Pressure gauge mounted on a ring siphon and right two coiled Bourdon tubes, C shaped (a) and coiled (b).

Survey of pressure gauge The survey consists of confirming the gauge accuracy by sending it ashore for calibration, or using the pressure calibration equipment on board.

Boiler plate Each boiler is required to have a boiler plate permanently attached to the boiler in a prominent location, usually around the burner. This plate displays the following information which is hard stamped on the plate.

Example of a boiler name plate.

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• • • • • • •

Name and domicile of the manufacturer Manufactures type destination and serial number Year of manufacture Design pressure Design temperatures Hydraulic test pressure DNV identifying mark

All the major standards and codes require fitting of a boiler plate, some standards require the maximum working pressure and output to be displayed.

Soot blowers Soot blowers are mechanical devices used for periodical on line cleaning of the boiler flue gas side to remove ash and slag deposits. They direct a cleaning medium through nozzles against the soot or ash accumulations on the heated surface. Furthermore they prevent plugging of the gas passages. Mostly compressed air is used as a cleaning medium on auxiliary boilers, but also steam can be used. There are many different types of soot blowers in use, for example, manual fixed position blowers, fully automated blowers and retractable blowers. On most auxiliary boilers they are in general not installed, but we usually find them fitted on exhaust gas boilers. In general it is the manual fixed position type, which uses compressed air as a cleaning medium. Survey of soot blowers Survey of soot blowers entails verifying that they are in workable condition. Check that nozzles are not blocked, blow pipe properly supported, and operating mechanism not seized. Operate the blower rotation hand wheel without air, it should move easily.

On the right a picture of the blower rotation wheel and left the nozzle tube with support.

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Appendix Inspection Documents The surveyor will meet different kinds of inspection documents specified by international and national standards. In order to be able to compare these with the certificate types defined by DNV, a comparison of the international standards for inspection documents is given. The documents are listed in increasing order, i.e. from 1 where only a confirmation from the manufacturer is necessary to 7 where the highest level of documentation is required. Note that whereas ISO 10474 still include 3.1B and 3.1C type inspection documents, the latest edition of EN 10204 does not. ISO 10474 is seldom (if ever) used by purchasers. Consequently, DNV shall not issue 3.1C inspection certificate unless ISO 10474 is explicitly referenced by the purchaser.

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Pressure and temperature ratings for valves. Pressure / Temperature-Rating Druck und Temperaturabstufung nach DIN 2401 / ANSI B 16.5 Description GS-CK 14 V TT-St 35 V GS-26 Cr Mo 4 26 Cr Mo 4 GS-10 Ni 14 10 Ni 14

GG-20 GG-25 R-St 37-2

group TT-Stahl

3,5 Ni

GG

C-Stahl

GS-C 25 N C 22.3 C 22.8 C-Stahl, warmfest

GS-22 Mo 4 15 Mo 3 Mo 0,5

GS-17 Cr Mo 55 13 Cr Mo 44 Cr Mo 1,0 0,5

GS-12 Cr Mo 9.10 10 Cr Mo 9.10 1.4408

Cr Mo 2,25 1,0

Cr Ni Mo

bar / - -60 -50 °C 196 110 10 10 10 10 16 16 16 16 25 25 25 25 40 40 40 40 63 63 63 63 100 100 100 100 160 160 160 160 250 250 250 250 10 16 10 16 25 40 63 100 160 250 320 25 40 63 100 160 250 320 400 25

-10 +20 10 16 25 40 63 100 160 250 10 16 10 16 25 40 63 100 160 250 320 25 40 63 100 160 250 320 400 25

10 16 25 40 63 100 160 250 10 10 8 7 6 16 16 13 11 10 10 10 8 7 6 4 2 16 16 14 13 11 8 6 25 25 22 20 17 13 8 40 40 35 32 28 21 12 63 63 50 45 40 32 19 100 100 80 70 60 50 30 160 160 130 112 96 80 49 250 250 200 175 150 125 89 320 320 250 225 192 160 25 25 25 25 22 19 17 40 40 40 40 35 30 28 63 63 63 63 56 47 45 100 100 100 100 87 74 70 160 160 160 160 139 118 112 250 250 250 250 217 185 174 320 320 320 320 278 236 222 400 400 400 400 348 296 278 25 25 25 25 25 23 21 18

40 63 100 160 250 320 400 160

40 63 100 160 250 320 400 160

40 40 40 40 40 36 34 29 15 63 63 63 63 63 58 56 47 25 100 100 100 100 100 91 87 74 38 160 160 160 160 160 146 139 118 62 250 250 250 250 250 227 227 184 97 320 320 320 320 320 292 279 237 124 400 400 400 400 400 364 348 295 155 160 160 160 160 160 160 130 90 70 52

250 320 400 16

250 250 250 250 250 250 250 200 150 108 81 320 320 320 320 320 320 320 230 180 139 104 400 400 400 400 400 400 400 300 215 174 130 16 16 14 13 11 9 7 5 4 3

16 16

50 120 200 250 300 400 450 500 530 550 600

9

181

18 10 2 1.4401 1.4571

1.4308 1.4301 1.4541

Cr Ni 18 8

25 40 63 100 160 250 320 16 25 40 63 100 160 250 320

PN nach DIN 2401 Äquivalent PN ~API 600

16 25 40 63 100 160 250 400

16 150 10,5 bar 260 0C 15,0 bar 120 0C

16 25 40 63 100 160 250 400

25 25 40 40 63 63 100 100 160 160 250 250 400 400 16 16 25 25 40 40 63 63 100 100 160 160 250 250 400 400

25/40 300 21,1 bar 450 0C 48,5 bar 120 0C

25 40 63 100 160 250 400 16 25 40 63 100 160 250 400

63 400 28,1 bar 450 0C 64,7 bar 120 0C

25 24 20 18 16 12 10 8 6 40 39 38 36 34 30 28 26 24 21 20 55 53 50 48 45 39 37 36 33 29 25 82 79 76 73 69 59 56 54 50 43 40 124 119 115 109 96 88 85 80 74 65 60 206 199 191 182 170 151 140 136 124 108 100 320 315 310 300 280 249 235 228 208 181 170 16 15 12 11 9 7 5 4 3 25 22 18 15 14 10 8 7 5 40 36 30 30 26 23 22 21 20 16 14 54 48 44 40 35 31 29 28 26 22 20 81 72 66 60 53 48 44 40 38 34 32 122 110 99 90 80 71 67 64 60 53 50 204 182 165 150 133 119 111 107 100 88 80 320 303 275 250 222 199 186 179 175 150 140 100 160 250 320/400 600/800 900 1500 2500 43,4 bar 63,3 bar 105,5 bar 175,8 bar 450 0C 450 0C 450 0C 4500C 97,0 bar 145,5 bar 242,6 bar 404,3 bar 120 0C 120 0C 120 0C 120 0C

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Chapter 7: Boiler Control and Monitoring There are three major reasons to install control and monitoring equipment on an auxiliary boiler. 1. Safety: Automation of combustion and feed water control, and the implementation of automated safety protocols have greatly contributed to increase overall operational safety of steam plants. 2. Stability: The boiler operates more steadily, and predictable, without fluctuations and shutdowns when control systems are in place. 3. Accuracy: Automated processes are more accurate than those that are manual controlled. The process can be optimized with increased economic efficiency. Boiler controls in general consist of three variably interconnected systems, combustion, feed water, and superheated steam temperature control. From a historic perspective the feed water regulation was the first one to be automated, quickly followed by combustion controls. Superheated steam controls are rarely fitted on superheated steam producing auxiliary boilers, but they are essential for propulsion boilers. Nowadays all auxiliary boilers are equipped with a form of feed water and combustion control and a monitoring system.

Automated feed water regulation The boiler maker has designed the boiler to safely operate within an upper and lower normal water level. Sometimes these normal water levels (NWL) are marked in the vicinity of the gauge glass. A feed water control system therefore has the flowing tasks. 1. Monitor and control the water level by supplying more or less feed water to the boiler. 2. Detect the water level, and when an unsafe level is reached take appropriate action. Depending on the detected level this action may be sounding an alarm (high, low level), or shutting down the feed water supply or burner. In order to be able to monitor and control the boiler water level, the level must first be accurately detected. The following principal types of level detection devices are appropriate to steam boilers. Conductivity probes When considering a tank filled with water in which a probe is immersed. This probe is connected by electric cable via a voltage source and ampere meter to the tank. As long as the probe is immersed in water there will be current flow through the circuit. When the water level is lowered and the probe tip is out of the water the current flow will stop. This point measurement (on / off of current) when the probe tip touches water can be used to trigger an action through an associated controller, for example start / stop pump or open / close valve. A single probe can only provide a single action, therefore four or more probes are built into a common housing, which are cut to their appropriate lengths on installation.

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Operating principle of a conductivity probes, single tip.

On the left a conductivity probe with controller, on the right a probe arrangement to switch a feed pump on and off.

Capacitance probes The capacitance level probe consists of a conducting, cylindrical probe which acts as the first capacitor plate and is covered with a dielectric material. The second capacitor plate is formed by the boiler shell together with the water content. Therefore, by changing the water level, the area of the second capacitor plate is changed, which changes the overall capacitance of the system. The change in capacitance is however small, so the probe is used in conjunction with a amplifier, which feeds the amplified signal to a suitable controller.

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On the left capacitance in water, and on the right a typical level control.

Float control This is one of the simplest forms of level measurement, but still widely used on auxiliary boilers.

Float control directly in the boiler and with external chamber.

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On the left typical mounting arrangements and left different types of Mobrey level controls.

The float moves up and down as the boiler water level changes. At the end of the float rod is a magnet which operates the magnetic switches. A more sophisticated type uses a coil wrapped around a yoke inside the cap and can be used for modulating control systems. The float control can be mounted in an external chamber or directly within the boiler shell.

Sketch of a level control system using a differential pressure cell.

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Differential pressure cells The differential pressure cell is installed with a constant head of water on one side, and the other side is so arranged that the water head varies with the boiler water level. Variable capacitance, strain gauge, or inductive techniques are used to measure the deflection of the diaphragm which is converted and fed to a controller. Automatic on / off level control systems All the methods of level detection described above can be used for on / off level control. The signal produced is used to start the feed pump at a predetermined low boiler water level and allow it to run until the set high level is reached. This type of control system is only found on small boilers with a steam generation rate of below 5000 kg/h. The disadvantages of this system are frequent start and stops of feed pump, temperature cycling of the boiler due to relatively high flow rate of “cold” feed water.

Sketch of an on / off level control system.

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Automatic modulated level control system With this type of control the feed pump runs continuously, and an automatic valve controls the feed water flow rate to match the steam demand.

Sketch of a modulated level control system.

When operating correctly, modulating control can dramatically smooth the steam flow and ensures greater water level stability. To protect the feed pump from overheating when pumping against a closed modulation valve, a recirculation or spill line is provided to ensure a minimum flow rate through the pump. For modulated level control only floats with a continuous signal output, capacitance probes, and differential pressure cells can be used as water level sensor. Most auxiliary boilers on board have a form of modulated feed water control system installed. Single element water level control The standard single boiler level control system, with proportional control, gives excellent feed water regulation for the majority of marine auxiliary boiler plants. However with single element proportional control, the water level must first fall before the feed water control valve opens. This means that the water level must be higher at low steaming rates, and lower at high steaming rates. This translates in a falling level control characteristic.

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The limitation of single element level controls is its inability to react quickly onto sudden large load changes. In water tube boilers this may lead to unstable steam pressures and water levels. Two element water level control Two element control systems reverse the falling water level control characteristic, and ensure the water level is made to rise during high steaming rates.

Level control characteristics.

This strives to ensure that the quantity of water in the boiler stays constant during all loads, and during periods of increased, sudden steam demand, the feed water control valve opens. The system works by using the signal from a steam flow meter installed in the steam line to increase the level controller set point during high steam loads. The two elements of the signal are: 1. First element: Boiler water level signal from level detection device. 2. Second element: Steam flow signal from flow meter in discharge line. Any boiler installation which experiences frequent, sudden load changes may operate better with a two element feed water control system.

On / off control mode

Proportional plus integral mode

Proportional control mode

Proportional plus derivative control

Typical system responses for different control modes. The above example is for a temperature control, but same applies for level control system.

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Sketch of a two element boiler water level control system.

Automated combustion control The basic requirement for a combustion control system is to provide efficient combustion during all power levels. This is achieved by supplying the correct relationship of air and fuel to the furnace at the requested steam generation rate. In addition to the chapter “Combustion and Atomizers” we will broadly outline the basic burner control systems in use on marine auxiliary boilers. On / off burner control This is the simplest combustion control system, and it means that either the burner is firing at full rate, or it is shut off. The major disadvantage of this control method is that the boiler is subjected to large and frequent thermal shocks, every time the boiler fires. It is also not able to cope with large and sudden load increases and therefore this control system is limited to small boilers up to a steam production of 500 kg/h. High / low / off burner control This is a slightly more complex system where the burner has two firing rates. The burner first operated at a lower firing rate and than switches to full firing rate on increased steam demand, thereby overcoming the worst thermal shock. Also it switches from full, to intermediate firing rate, and burner stop upon reducing loads. This type of burner control is usually fitted on boilers with a steam production of up to 5000 kg/h.

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Modulating combustion control Modulated burner control will alter the firing rate to mach the boiler load over the whole steam production range. Every time the burner shuts down and re-starts, the furnace must be purged, this wastes energy and reduces efficiency. Full modulation, however, means that the boiler keeps firing over the whole steam generation range to maximise thermal efficiency and minimize thermal stresses. This type of combustion control is fitted on most auxiliary boilers with a medium to large steam production.

A mechanical and electronic modulated combustion control units for Saacke burners, a steam pressure signal moves the servo motors.

Monitoring of auxiliary boilers Different rules and standards vary in their monitoring requirements, but the minimum extent of monitoring according to the DNV Rules (January 2003 publication) is found below. •

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Water level low alarm: This signal has to come from an independent level detection device. Therefore boilers must have two sets of level detection devices, one for controlling the water level and shutting down the burner (Level low low alarm), and the other set for low and high level alarms. Water level lower (level low low): This alarm is raised after water level low alarm and also shuts down the burner automatically. After normal water level (NWL) is restored one has to manually reset this alarm in order to start the burner. Water level high alarm: Although not required but installed on most boilers this alarm stops the feed water pump, or closes the feed water control valve. Circulation failure alarm: This alarm is found when the boiler has forced circulation. The signal usually comes from a pressure sensor, if the circulation pressure reaches the lower set point the alarm is raised and the burner shut down. Also here manual reset is required before boiler start up is possible.

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Combustion fan failure alarm: The signal comes from a pressure sensor, if the combustion air pressure reaches its lower set point the alarm is raised and the burner tripped. Manual reset is required before boiler start up is possible. Heavy fuel oil temperature high alarm: A temperature sensor in the fuel supply line is supplies the signal for this alarm. Heavy fuel oil temperature low alarm: It is very uncommon to find a viscosity meter in boiler fuel oil systems. Mostly the fuel oil viscosity is controlled by setting a certain temperature on the controller of the heater. Steam pressure high alarm: A pressure sensor fitted on the boiler raises this alarm. Steam pressure higher (pressure high high) alarm: Usually a second pressure sensor raises this alarm and trips the boiler after steam pressure high alarm has sounded. Also here manual reset is required before start up of the plant is possible. According to the DNV Rules this alarm is only necessary “When the automatic control system does not cover the entire load range from zero load”, practically all boilers are fitted nowadays with a burner trip. Superheated steam temperature high alarm: This is only applicable when superheated steam above 350 °C is produced. Therefore this alarm is very seldom encountered on auxiliary boilers. Flame failure alarm: One some installations this alarm is split into an ignition failure alarm, only active during start up, and a flame failure alarm active during operation. Both alarms will trip the burner and manual reset of the alarm is required before restart of the plant.

Existing installations still extensively use thermostats and pressostats to control and trigger the various alarms and safety functions. Modern installations tend to favor central electronic cabinets which control and monitor the complete boiler operation.

Central control cabinet on the right and alarm / control panel on the left from Saacke.

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Testing of control and monitoring system The last part of a boiler survey is generally testing of the control and monitoring system. Also these tests should be properly prepared, adequately manned, and conducted in a controlled and calm manner. Below you find some general guidelines listed, which might be helpful during execution of these tests. •

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By simply observing water level gauge glass, steam pressure, and burner operation, the proper functioning of control systems can be confirmed. Load changes can be simulated by closing, throttling, and opening of the steam stop valve. After an initial load change the boiler parameters should return to a stabile condition. Feed water control valves or burner controls should not hunt. Steam pressure high, steam pressure high high, and burner shut down can best be tested during steam pressure rising for safety valves testing (popping). Water level low, level low low alarm, and boiler trip can be tested in the following ways. 1. If the boiler is fitted with an external chamber in which the level detection sensors are fitted, it is a simple matter of draining this chamber after isolating valves are closed. It is a prudent cause of action to open this chamber during the course of the survey. Inspection of the float and cleaning of the chamber is then made possible.

Low water level test on a boiler with an external chamber.

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Internally mounted level sensor.

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2. If the level detection sensors are fitted within the boiler shell, then the only proper way of testing is to lower the water level. This may best be done by, closing the steam stop valve, manually control the burner and set it at minimal firing rate, close the feed water control valve or stop the feed pump, and while closely observing the level gauge glass, slowly open the blow down valve until low level alarms sound and the boiler trips. Remember that level alarms and boiler trip should activate, with water in the lower visible regions of the level gauge glass. Combustion fan failure can be easily tested by stopping the fan, and incase of a pressostat by draining the line to it. Incase the boiler has forced circulation this alarm can be tested by tripping the circulation pumps. Pressure and temperature alarms may also be tested by use of calibration equipment found on board vessel with unmanned engine rooms.

Please be aware that wherever DNV Rules are mentioned in this course hand out we have not quoted the exact complete rule text, and have used the publication at the time of writing this course. Therefore we refer to the DNV Rules for complete and exact text and possible updates.

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