Water Circulation System 1

Water Circulation System 26 May 2012

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Contents of Presentation • • • • • •

Theory of circulation Types of circulation Boiling fundamentals Economizer Water walls Drum and its internals

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Circulation in Boiler The steam generator has to produce steam at highest purity, and at high pressure and temperature required for the turbine. Water must flow through the heat absorption surface of the boiler in order that it be evaporated into steam Natural circulation is the ability of water to circulate continuously, with gravity and changes in temperature being the only driving force known as "thermal head“.

The ratio of the weight of water to the weight of steam in the mixture leaving the heat absorption surfaces is called Circulation Ratio.

Cold feedwater is introduced into the steam drum where, because the density of the cold water is greater, it descends in the 'downcomer' towards the lower bottom ring header, displacing the warmer water up into the front tubes. Continued heating creates steam bubbles in the front tubes, which are naturally separated from the hot water in the steam drum, and are taken off. 26 May 2012

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Criticality of Drum Level control • Boiler drum level control is critical for both plant protection and equipment safety and applies equally to high and low levels of water within the boiler drum.The purpose of the drum level controller is to bring the drum up to level at boiler start-up and maintain the level at constant steam load. A dramatic decrease in this level at constant steam load. A dramatic decrease in this level may uncover boiler tubes, allowing them to become overheated and damaged. An increase in this level may interfere with the process of separating moisture from steam within the drum,

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Types of Drum level control • The Three main options available for drum level control are Single element drum level control • Two element drum level control • Three-element drum level control

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Forced Circulation However, when the pressure in the water-tube boiler is increased, the difference between the densities of the water and saturated steam falls, consequently less circulation occurs. To keep the same level of steam output at higher design pressures, the distance between the Bottom ring header and the steam drum must be increased, or some means of forced circulation must be introduced. Therefore natural circulation is limited to boiler with drum operating pressure around 175 Kg/cm2. 26 May 2012

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Natural Circulation & Forced Circulation Natural Circulation  The downcomer contain relatively cold water, whereas the riser tube contain steam water mixture ,whose density is comparitively less .this density difference is the driving force ,for the mixture. (thermo-siphon principle)  Circulation takes place at such a high rate that the driving force and frictional resisitance in water wall are balanced.

Natural Circulation

Forced Circulation

Forced Circulation  Beyond 180 Kg/cm2 of pressure, circulation is to be assisted with mechanical pumps, to overcome frictional losses. To regulate the flow through various tubes, orifice plates are used. This system is applicable in the high sub-critical regions (say 200 Kg/cm2). 26 May 2012

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Why controlled circulation is required? • AS THE PRESSURE INCREASES,THE DIFFERENCE IN DENSITY BETWEEN WATER AND STEAM REDUCES . • THUS THE HYDROSTATIC HEAD AVAILABLE WILL NOT BE ABLE TO OVERCOME THE FRICTIONAL RESISITANCE FOR A FLOW CORRESPONDING TO THE MINIMUM REQUIREMENT OF COOLING OF WATER WALL TUBES. • NATURAL CIRCULATION 26 May PMI Revision 00 IS2012 LIMITED TO 175KSC

Diff in Density 8

2.Controlled circulation system: • use of controlled circulation pump • used for pressure up to 194kg/cm2 (sub critical pr.) • circulation ratio=6-9

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3.Combined circulation system: Circulation ratio=2 No use of drum Use of controlled circulation pump Use for pressure above 200kg/cm2.

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Difference in the Circulation System

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Nucleate boiling, DNB and Film Boiling • Boiling is the formation of vapor bubbles at the heating surface. These bubbles form at nucleation sites whose number and location depend upon the surface roughness or cavities, fluid properties, and operating conditions. The boiling heat transfer coefficient is very sensitive to the temperature difference.PMI Revision 00 26 May 2012

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Economiser Boiler Economiser are feed-water heaters in which the heat from waste gases is recovered to raise the temperature of feed-water supplied to the boiler.

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Economiser • The economizer preheats the feed water by utilizing the residual heat of the flue gas. • It reduces the exhaust gas temperature and saves the fuel. • Modern power plants use steel-tube-type economizers. • Design Configuration: divided into several sections : 0.6 – 0.8 m gap

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Tube Bank Arrangement

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Finned Economizers

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Thermal Parameters of Economizer • • • • • •

Out side diameter : 25 – 38 mm. Tube thinckness: 3 – 5 mm Transverse spacing : 2.5 – 3.0 Longitudinal spacing : 1.5 – 2.0 The water flow velocity : 600 – 800 kg/m2 s The waterside resistance should not exceed 5 – 8 %. Of drum pressure. • Flue gas velocity : 7 – 13 m/s.

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Advantages of Economiser 6oC raise in feed water temperature, by economizers corresponds to a 1% saving in fuel consumption

220 C reduction in flue gas temperature increases boiler efficiency by 1%

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6oC raise in feed water temperature, by economizers corresponds to a 1% saving in fuel consumption

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Location and Arrangement • • • • •

Ahead of air-heaters Following the primary super-heater or re-heater Counter-flow arrangement Horizontal placement (facilitate draining) Supported to prevent sagging, undue deflection and expansion . • Stop valve and non-return valve incorporated to ensure recirculation in case of no feed-flow • Ash hopper below as flue gas takes a turn

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Type of Construction • Plain Tube : Several banks of tubes with either-in-line or staggered type formation. Staggered arrangement induces more turbulence than the in-line arrangement. This gives a higher rate of heat transfer and requires less surface but at the expense of higher draught loss. • Welded Fin-tube : Fin welded design is used for improving the heat transfer.

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Boiler Drum • It is an enclosed Pressure Vessel • Heat generated by Combustion of Fuel is transferred to water to become steam • Process: Evaporation Steam volume increases to 1,600 times from water and produces tremendous force

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DRUM Lifting in progress

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DRUM SERVES TWO MAIN FUNCTIONS  SEPERATING STEAM FROM THE MIXTURE OF WATER AND STEAM  2. IT HOUSES ALL EQUIPMENTS USED FOR PURIFICATION OF THE STEAM AFTER BEING SEPERATED FROM THE WATER 26 May 2012

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SPECIFICATION500MW MATERIAL - CARBON STEEL LENGTH - 22070 MM OD - 2130 MM ID - 1778 MM DESIGN.PR - 204 KSC WEIGHT(INT) -246 TONNES WITHOUT(INT) –224 TONNES

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Drum Internals The steam drum contains steam separating equipment and internal piping for distribution of chemicals to the water, for distribution of feedwater and for blowdown of the water to reduce solids concentration.

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DRUM INTERNALS 1.

PRIMARY SEPERATORS

CONSISTS OF BAFFLE ARRANGEMENT DEVICES WHICH CHANGE THE DIRECTION OF FLOW OF STEAM AND WATER MIXTURE 2. SECONDARY SEPERATORS SEPERATORS EMPLOYING SPINNING ACTION

3. SCREENING DRYERS

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Drum Internals

Steam Separator

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Downcomers • There are six down comers in (500 MW) which carry water from boiler drum to the ring header. • They are installed from outside the furnace to keep density difference for natural circulation of water & steam.

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WATER WALLS • HEATING AND EVAPORATING THE FEED WATER SUPPLIED TO THE BOILER FROM THE ECONOMISERS. • THESE ARE VERTICAL TUBES CONNECTED AT THE TOP AND BOTTOM TO THE HEADERS. • THESE TUBES RECEIVE WATER FROM THE BOILER DRUM BY MEANS OF DOWNCOMERS CONNECTED BETWEEN DRUM AND WATER WALLS LOWER HEADER. • APPROXIMATELY 50% OF THE HEAT RELEASED BY THE COMBUSTION OF THE FUEL IN THE FURNACE IS ABSORBED BY THE WATER WALLS. 26 May 2012

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Construction of Water Walls • Tangent tube The construction consists of water wall placed side by side nearly touching each other. An envelope of thin sheet of steel called "SKIN CASING" is placed in contact with the tubes, which provides a seal against furnace leakage.

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Construction of Water Walls Membrance Wall A number of tubes are joined by a process of fusion welding or by means of steel strips called 'fins pressurised furnace is possible with the related Advantages •Increase in efficiency •Better load response simpler combustion control. •Quicker starting and stopping •Increased availability of boiler. •Heat transfer is better •Weight is saved in refractory and structure •Erection is made easy and quick 26 May 2012

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WATER WALL SPECIFICATION NO

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OD(MM)

FRONTWALL

283

51

SIDEWALLS

444

51

REAR WALLS

283

51

ROOF

142

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Hydro Testing • Need: • Method: – Ensure pressure instrumentation is ready. – Water Filling(10ppm Ammonia, 200 ppm Hydrazine, pH 10) – Drainable parts (economizer, water wall and drum) – Non Drainable portions filling – First pressurization through boiler Fill pump – Later pressurization through special purpose pumps. – Test pressure and rate of pressure rise and rate of pressure drop as per IBR. – After completion system drained and kept under wet preservation. 26 May 2012

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IBR Requirement for Hydraulic test • Every boiler shall be hydraulically tested after erection at site in presence of the Inspector to 1¼ times the maximum working pressure as certified by the Inspecting Authority, to be stamped on the boiler, as free from any indication of weakness or defects. • The test pressure shall be raised gradually under proper control at all times so that it never exceeds by more than 6% of the required pressure and maintained for 30 minutes whereupon the pressure shall be reduced to maximum allowable working pressure and maintained for sufficient time to permit close visual inspection for leakage of pressure parts. • The boiler shall satisfactorily withstand such pressure without appreciable leakage or undue deflection or distortion of its parts for at least ten consecutive minutes.

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IBR Requirement for Hydraulic test • At the first hydraulic test of a boiler prior to the issue of an original certificate deflection measurements shall be made before, during and after test of each furnace length, fire-box and flat end or other plates. • After the application of the hydraulic test the Inspector shall carefully examine the boiler inside and outside and satisfy himself that it has satisfactorily withstood the test.

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THANK YOU

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