Boiler Tube

ABSTRACT   The power generation plays a vital role for any country development. One of such power generation unit is thermal power plant which is the main source for largescale production of electrical energy. The cycle used is “MODIFEID RANKINE CYCLE” which includes super heated steam, regenerating feed water and reheated steam. Raw materials used in thermal power plant are coal, water, oil and air.   The main objective behind my project is to study about the thermal plant, how power is generated, what are its sources, working of boilers and boiler tube failures.   This work concentrates much about boiler tubes, different type of tube failures and main cause for failure of tubes and preventive measures in boiler tubes.

CONTENTS 

Principle of power generation

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Boiler

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Boiler accessories

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Boiler tube failures

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Case study of boiler tube failures

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Conclusion

PRINCIPLE OF POWER GENERATION 

It is known fact that when coal is burnt releases heat energy. The same phenomenon when chemically represented.

C + O2 = CO2 + Heat Energy (395 KJ/Mole) Energy Transfer:  In the boiler chemical energy of fuel is converted into thermal energy by heating water and converting it into steam. The steam produced in the boiler is expanded in the turbine where thermal energy is converted to kinetic energy. 

As the steam expands it rotates the turbine this motion of the turbine is transmitted to generator in which mechanical energy is converted into electrical energy, which is transmitted to various load centers through transmission lines .

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The basic theoretical working cycle of a steam power plant on which it works is “ RANKINE CYCLE” the modern steam power plant uses modified Rankin cycle which includes reheating, super heating and regenerative feed water heating.

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RANKINE CYCLE:-

 ENERGY CONVERSON DIAGRAM:  

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Chemical Energy  BOILER Thermal Energy  TURBINE Kinetic Energy  GENERATOR Electrical Energy

BOILER  Boiler

is the main part in the power generation.

 Boiler

is a sealed vessel in which water is converted into steam.

 Boiler

acts as a medium in which water is converted into steam by using the heat released in the process of combustion of coal in the presence of oxygen.

 Before

combustion process the coal is collected from bunkers into millers.

 This

coal is converted as powder (pulverized) in the millers

 During

this process if their is any wet coal ,it is converted into dry coal.

 And

finally It sent to the boiler by using the air.

BOILER ACCESSORIES

BOILER DRUM 

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It is the main part of the boiler, in which steam and water separates. Inside the boiler drum, saturated steam is separated from water and then directed in to super heating tubes for further temperature increase. Water separated from steam will combine with the incoming boiler feed water and then re-enter the down comers to repeat the cycle. Thus the boiler drum acts as a steam separator.

DOWNCOMERS:

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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.

DRUM INTERNALS:

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PRIMARY SEPERATORS – Consists of baffle arrangement devices which change the direction of flow of steam and water mixture  SECONDARY SEPERATORS- Separators employing spinning action

WATER WALLS 

Almost all modern power boilers are equipped with water walls .

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In large boilers water walls completely cover the interior surfaces of the furnace.

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These tubes receive water from the boiler drum by means of down comers connected between drum and water walls lower headers.

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In a boiler the water walls absorbed approximate it 50%of heat released by the combustion of the fuel in the furnace.

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Heat so absorbed by the water walls used in the evaporation of water supplied to boiler.

ECONOMIZER

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The purpose of economizer is to preheat the boiler feed water before it is introduced into the steam drum by recovering heat from the flue gases leaving the boiler. Feed water after heated goes in to the boiler drum. The economizer is used for improving the efficiency of the boiler and it is the one of the important for the accessories for the boiler.

ADVANTAGES OF ECONOMISER: 

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220 C reduction in flue gas temperature increases boiler efficiency by 1%. 6oC raise in feed water temperature, by economizers corresponds to 1% saving in fuel consumption .

SUPER HEATER 

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A super heater is a device which heats steam to high temperatures. A super heater consists of group of tubes made of special alloys such as chromium –molybdenum. The wet steam from the boiler drum enters in to the super heater and gets heated up to a temperature of 535 deg cen and pressure of 170kg/sq cm by gaining heat from the flue gases leaving the furnace . The steam entering the super heater and then the secondary super heater to the tertiary super heater.

RE-HEATER:  The function of re-heater is to increase the

temperature of the steam leaving the HPT again to 540 deg C and the pressure up to the 42kg/cm2.

BURNERS: 

500 MW boiler is a tangential corner fired boiler having 32 burner assemblies i.e. 8 burner assemblies at 4 different elevations.

M.S

C.R.H

H. R. H

FROM F.R.S BOTTOM RNG HDR

1

ST

PASS W.W

1ST PASS W.W O/L HDRS

ROOF I/L HEADER

2ND PASS UPPER C-HDR

2nd PASS LOWER C-HDRS

LTSH I/L HEADER

LTSH O/L HEADER

D.P.I/L HEADER

D.P.O/L HEADER

S.H. HEADER

R.H.HEADER

2ND PASS ROOF O/L HDR(REAR

ECONOMISER

BOILER TUBE FAILURES  Overview:  

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Boiler tube failures are the leading cause of forced outages of fossil-fired power plants. Boiler tubes are subjected to various failure mechanisms driven by temperature, stress and environmental hazard, depending on their designs and functions. Super heater tubes, re heater tubes & furnace water wall tubes always experienced high temperature ductile failures such as stress-rupture failure, initiated by creep process or sudden overheating due to steam starvation or flue gas disruption. Suspended tubes with a lack of support or exposed to vibration / cyclic loading would bear a risk of mechanical fatigue. 

BOILER TUBE FAILURES Major reasons contributing for boiler tube failures are :  Short Term Overheating  Long term overheating  Dissimilar Metal Welds  Hydrogen Damage  Pitting  Falling Slag Erosion  Soot Blower  Material Defects  Welding Defects

SHORT TERM OVER HEATING

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Short term overheating in water cooled tubes occurs because of abnormal coolant flow are excessive combustion gas temperature. As a result, the tubes are subjected to excessively high temperature often hundreds of degrees which results in rapid failure. A considerable increase (> 5%) in the inside or outside diameter of affected tubes. Once flow conditions are interrupted as described above the normal cooling effects of the water no longer occur and tube metal temperature rises rapidly.

MAIN CAUSES: 

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Partial blockage caused by maintenance activities. Poor control of drum level.

CORRECTIVE ACTIONS:  

The deposits can be blown off which is very time consuming, or The unit can be chemically cleaned.

LONG TERM OVER HEATING Long term overheating can caused by deposits attached on inner surface of tubes.

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Long term overheating can occurs in all of pressure part in steam boiler such as water wall tubes, header, super heater, re heater, economizer, steam drum, water drum Almost 90% failure can occurs in super heater Definition of long term overheating is a condition in which pressure parts have metal temperature higher than temperature design in long term

CORRECTIVE ACTIONS: 

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Temporary pad welds should not be used because of the uncertainty associated with base metal condition. Tube shielding.

DISSIMILAR WELD JOINTS 

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As the elevation of the boiler is very much higher we have to join the different tubes to that much of higher elevation according to the temperatures inside the boiler. Quality control needs to be applied to assure sound weld. Results from application of high temperature and stress beyond design value

CORRECTIVE ACTIONS:    Replacement  Frequent inspection.

HYDROGEN DAMAGE 

Hydrogen damage in boiler tubes is caused by a corrosive reaction between steam and steel Shown below: Fe +H2o =Fe304 + H2

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The hydrogen that is released reacts with carbides to decarburize the steel and forms methane gas at the boundaries. Results in wall loss due to corrosion and decreases the strength in material attacked by hydrogen. The damage usually occurs in areas of high heat flux and flow disturbances includes tubes opposite to burners and tube bends

Corrective Action:  Control Boiler Water Chemistry   

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Check corrosion product ingress Chemical cleaning Replace affected tubes

 

PITTING

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The formation of small pits in a surface as a consequence of corrosion. Pitting corrosion may takes place in the environment of stagnant, oxygenated water formed during shutdown. Most familiar form of oxygen attack in boiler and condensate system. Dissolved oxygen is essential to corrosion of copper alloy tubes. At higher temperatures oxygen weakens steel.

CORRECTIVE ACTIONS: 

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If D.O in feed water is >10ppb,its source needs to be identified and arrested may be mechanically or chemically. Proper operation of deareator,L.P.Feed water Heater.

FALLING SLAG ERROSION

Typical Locations:    Lower furnace sloping wall near bottom opening

Probable Root Cause Slagging of coal and massive clinker fall Coal properties and boiler design 

Corrective Action  Change in fuel

SOOT BLOWER 

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A soot blower is a system for removing the soot that is deposited on the furnace tubes of a blower during combustion. Soot deposited on the heating surfaces of a boiler acts as a heat insulator. The result is that less heat is transferred to the water to raise steam and more heat is wasted up the chimney. This leads to higher fuel consumption and/or poor steaming.

Various types of soot blowers such as wall blowers, long retractable blowers and air heater blowers are used for cleaning. Steam is normally used as a medium for blowing away the soot.

PROBLEMS CAUSED BY SOOT 

Reduce efficiency

CASE STUDY FLY ASH EROSSION:    

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UNIT NO: 7 Location: Z- Panel Corner 1 Tube No. 1 Failure Date: 30.01.2012 Reason of failure: Falling slag erosion. Action: All the 7 tubes replaced. Radiography taken found OK. Action proposed: Plastic refractory will be provided in this location during next overhaul.

Long term over heating:      

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UNIT NO: 5 Location: Final Re heater Coil No. 17 Tube No. 19 .Failure Date: 31.01.2012 Reason of failure: Long term overheating. Action: All tubes replaced. Total no. of joints 14 radiography done and cleared. Vacuum pulling done and found OK. Action proposed: Oxide scale will be measured in this area in next overhaul. Other action plan: will be decided based on R&D report.

MECHANICAL RUBBING: 

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     

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UNIT NO: 5 Location: Spacer Tube of Final Super heater near Coil No. 21 Reason of failure: Erosion due to mechanical rubbing of Super heater cleat with spacer tube. Failure Date: 13.12.2011 Action taken: All the three tubes replaced. Total number of joints 6. All joints radiography done and cleared. Action proposed: Spacer tube locking and alignment to be ensured in all overhauls.

 

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The boiler efficiency for stage(3) in NTPC Ramagundam is 87.145 % with out any “FAILURES” in boiler tubes. If there was any failures in boiler tube then its efficiency will be lower than required one. Daily NTPC Ramagundam supplies 62.4MU If Stage(3) overhaul for few hours. i.e24hrs then power loss will be 12MU In order to overcome the efficiency and power generation. Boiler tubes will be in good condition . To check over them vacuum pulling test and radiography test will be done