describes about boiler tube failuresFull description
Important information about boilers and pressure vessel for mechanical engineersFull description
Descripción completa
Descripción: Important information about boilers and pressure vessel for mechanical engineers
describes about boiler tube failuresDescrição completa
describes about boiler tube failuresDescription complète
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Deskripsi lengkap
Experiment No: - 01 Aim:-Study of High Pressure Boilers Date:-
Aim: - Study of high pressure boiler. Objective:-
The term term boil boiler er or stea steam m gene genera rato torr is appl applie ies s to devi device ce for for Introduction: - The generating steam which may be used for power generation as in case of steam engines or turbines, for textile industries, for heating as in case of heating installation for building. Mainly there are two types of boiler:1. Low Low pres pressu sure re boi boile ler r 2. High High press pressur ure e boil boiler er
Why
high pressure boilers?
The demand for the high power output from the boiler and associated plants has increased in the last ten years. It is a common practice to use high pressure and temperature steam in power plants to increase the efficiency of the plant and to reduce the cost of electricity production. The high pressure boiler which is also known as modern boilers used for power generation generation are for steam capacities 30 to 650 tons/hr and above with a pressure up to 160 bar and maximum steam temperature of about 540°C.
Salient 1.
features of high pressure boilers:-
Methods of circulation of water and steam.
2. Arrangem Arrangement ent of of tubing tubing and and boiler boiler drum. drum. 3. Methods Methods of super superheat heating ing the steam. steam. 2
4. Wate Waterr soft softeni ening ng plant. plant.
Types of high pressure boiler:1. La Mont Mont boil boiler er 2. Bens Benson on boil boiler er 3. Loef Loeffl fler er boil boiler er 4. Schm Schmidt idt-Ha -Hartm rtman ann n boiler boiler 5. Velo Velox x boil boiler er 6.
Supercharged boiler
1.
La Mont boiler : -
Construction: - La Mont boiler is vertical boiler. It has two types of evaporator namely convective evaporator and radiant evaporator. Radiant evaporator are mounted as near as possible to the combustion chamber. The economizer is also used in La Mont boiler to heat the feed water from flue gases. This heated water passed to evaporating drum. At the top of the boiler air preheater is installed to heat the air which is required for combustion in combustion chamber.
Working: - The feed water from hot well is supplied to a storage and separating drum through the economizer. The most of the sensible heat is supplied to the feed water passing through the economizer. A centrifugal pump circulates the water water equa equall to 8 to 10 ties ties the weight weight of steam steam evapor evaporate ated. d. This This wate waterr is circulat circulated ed through through the evaporator evaporator tubes and part part of the water evapora evaporated ted is separated in the separator drum. The large quantity of water circulated prevents the tubes from being overheated. The centrifugal pump delivers the feed water to the headers at pressure of 2.5 bars above the drum pressure. The distribution header distributes 3
the water through the nozzle into the evaporator. The steam separated in the boiler is further passed through the superheater and then supplied to the turbine. The air drawn by the blower is preheated in the air preheater by the flue gases before these are discharged through the chimney. The heated air from air prehe preheate aterr is suppli supplied ed to the the comb combus ustio tion n chamb chamber er and and it impro improve ves s the combustion efficiency.
Advantages:1.
The boiler can generate steam up to a pressure of 150 bars and the generation rate of steam ranges from 30000 to 45000 kg per hour.
2. Startin Starting g of Lamon Lamontt boiler boiler is quick quick..
Disadvantages:1. Bubbles are formed formed at the inner face face of heating heating tube. Because of which it reduces the heat transfer rate.
2.
:Benson boiler :Construction: - Bens Benson on boil boiler er is also also vert vertic ical al boil boiler er.. It has has two two type types s of evaporators viz. Convective evaporator and radiant evaporator. Radiant evaporator are mounted as near as possible to the combustion chamber. The economizer is also used in Benson boiler to heat the feed water from flue gases. This heated water passed to evaporating drum. At the top of the boiler air preheater is installed to heat the air which is required for combustion in combustion chamber.
Working: - The main difficulty of Lamont boiler is the formation of bubbles at the inner side of heating tubes which reduces the heat flow and steam generation as 4
it offers high thermal resistance than water film. Benson argued in 1922 that if the boiler pressure was raised to critical pressure (225 bars) the steam and water have the sane density and therefore the danger of bubble formation can be easily eliminated. The arrangement of boiler is as shown below. The water as passed through the economizer into the radiant evaporator where most of the water is converted into the steam. The remaining water is evaporated in the convective evaporator absorbing the heat from hot gases by convection. The saturated high pressure steam (at 225 bar) is further passed through the super heater as shown.
Advantages:1. As the generatio generation n of steam is carried out in the the evaporating evaporating tubes tubes at pressure pressure higher than critical pressure it doesn’t require any evaporating drum. 2. The boiler boiler can be started started in short time time in 10 to 15 minutes minutes only. only. 3. Benson boiler is is lighter lighter in weight weight with with high generation generation rate of steam. steam. 4. Due Due to absenc absence e of the evapora evaporatin ting g drum drum the total weight weight is 20% less than other boilers. 5. The superhe superheater ater of the Benson Benson boiler boiler is the integra integrall part of forced forced circulati circulation on system therefore no special starting arrangement for superheater is required. 6. The cost cost of the the boiler boiler is reduces reduces as as there there is no evaporating evaporating drum. drum. 7. Bubble Bubble formatio formation n is eliminat eliminated ed in Benson boiler boiler which which is critical critical problem problem in Lamont boiler.
Disadvantages:1. The evaporation evaporation process will leave leave small deposits during conversion conversion of water water into steam due to which it requires frequent cleaning. To obviate this problem, the water softening plant is require. 2. Tubes are likely likely to be be overheated overheated in case of water flow flow is insufficient. insufficient.
3. Loeffler boiler ::-
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Construction: - Components in Loeffler boiler are air pre heater, evaporating drum radiant and convective evaporator feed pump etc. in Loeffler boiler H.P. and L.P. boilers are added with steam circulating pump. The economizer is used to heat the incomin incoming g water water from feed pump. pump. Steam Steam circulat circulating ing pump draws draws superheated steam from evaporating drum.
Working: - The major difficulty in Lamont boiler is the deposition of salt and sediment on the inner surface of the water tubes. This difficulty was solved in Loeffler boiler by preventing the flow of water into the boiler tubes. Most of the steam is generated outside from the feed water using part of the superheated steam coming out from the boiler. The arrangement is shown in the following diagram. The pressure feed pump draws the water through the economizer and delivers it into the evaporator drum as shown. About 35% f the steam coming out from the superheater is supplied to the H.P. steam turbine. The steam coming out from H.P. turbine is passed through reheater before supplying to L.P. turbine. The amount of heat generated in the evaporator drum is equal to the steam tapped (65%) from the superheater. The nozzles which distribute the superheated superheated steam throughout the water into the evaporator drum are of special desi design gn and and avoi avoid d prim primin ing g and and nois noise. e. This This boil boiler er can can carr carry y high higher er salt salts s concentration than any other type.
Advantages:1.
Loeffler boiler can carry higher salt concentration than any other type of boiler.
2.
Since evaporating tubes of Loeffler boiler carries only superheated steam there is no salt deposition so it is suitable for marine applications.
3. Boiler Boiler is is comp compac actt in desig design. n.
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4. Schmidth-Hartmann :Schmidth-Hartmann boiler :Construction: - Schmidth-Hartm Schmidth-Hartmann ann boiler consists of air preheater to heat the surrounding air which is required for combustion chamber. It also consist steam drum, superheater, and feed pump. Feed water preheater, primary separator is also used in this boiler. In Schmidth-Hartmann boiler there are two circuits one uses distilled water and other uses impure water.
Working: - The Schmidth-Hartmann boiler is different from other boiler because in this boiler distilled water is used for generation of the high pressure steam which is recalculated without any wastage in the circuit. This high pressure steam is utilized for generation of low pressure steam from impure water. Distilled water from water drum enters into the primary evaporating tubes by natural circulation. Steam at 95-100 bar pressure is generated in the evaporating tubes with the help of hot flue gases circulated over the tubes from combustion chamber. This steam enters via primary separator into the tubes submerged in impure water of the evaporator or steam drum. The exchange of heat between high pressure steam and impure water allows the water to be converted into steam at a pressure of 55-60 bar which is further passed into superheater for superheating purposes. The superheated stem is finally supplied to the steam turbine. The condensate of high pressure steam collected from the drum passes through the feed water preheater where it heat the low pressure feed water up to its saturation temperature. The low pressure (L.P.) feed water is pumped into preheater with the help of feed pump.
Advantages:-
1. It can use use impure impure water water for gener generatio ation n of steam steam.. 2. As it can use impure impure water water there there is no requireme requirement nt of water water softening softening plant plant ultimately reduces cost. 3. Any deposit deposits s in the evaporator evaporator drum drum due to impure water water can easily easily brushed brushed off by removing the submerged tube from the drum or by blowing off the water. 7
4. Wide fluctuations fluctuations of load are easily easily taken by this boiler boiler without without undue undue priming priming or abnormal increase in the primary pressure due to high thermal and water capacity of the boiler.
Disadvantages:1. Due to deposits deposits in in the evaporator evaporator drum because because of impure water plant plant has to stop frequently for cleaning. 2. Evaporating Evaporating drum is used which which increase increase the bulkiness bulkiness of of plant with with cost.
5. Velox boiler ::Construction: Construction: - In Velo Velox x boil boiler er pres pressu suri rize zed d comb combus usti tion on is used used so axia axiall flow flow comp compre ress ssor or is used used whic which h is ran ran by gas gas turb turbin ine. e. The The comb combus usti tion on cham chambe berr and and evaporating tubes are arranged in such a way that the hot products from combustion chamber chamber are directly directly passed passed to annulus annulus of evaporat evaporating ing tubes. tubes. Steam Steam separat separator or is arranged tangential to evaporating tubes. Hot flue gases are used for running gas turbine, for economizer, and for superheater.
Working: - As we know heat transfer rate is much higher in sonic velocity than that of the subsonic velocity. Same concept is utilized in the design of Velox boiler in order to reduce the surface area hence the size of evaporating tubes. The salient feature of the Velox boiler is pressurized combustion. Air drawn from surroundings by axial flow compressor is compressed up to 2.5 to 3 bar pressure. Compressed air at above sonic velocity and the fuel are injected into the vertical vertical combustion combustion chamber. chamber. The hot products products of combust combustion ion at high high velocity velocity are passed into the annulus of the concentric evaporating tubes. These hot gases transfer heat to feed water flowing inside the concentric evaporative tubes. Hot gases circulated about 10 to 20 times that of feed water circulation. The wet steam generated in the evaporative tubes is passed to a steam separator in tangential direction. It forms the vortex vortex in the separator separator and due to centrifu centrifugal gal action moisture moisture content content of steam steam is separated. This separated water is pumped back into feed water line. The dry steam separated is passed through the superheater tubes where it is superheated with the help of hot flue gases collected from evaporating tubes. The hot gases discharged from the superheater are expanded in gas turbine. The power developed by the turbine is used to drive the compressor. The exhaust of the gas turbine is used to heat the feed water in the economizer before these are discharged to the surroundings through the chimney.
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Advantages:1.
Velox boiler is compact.
2. Very high high combust combustion ion rates rates are possibl possible e as 35 to 45 million kJ per per cu. m. of combustion chamber volume. 3. Low excess excess air air is required required as the pressu pressurize rized d air is used and the the problem problem of draught is simplified.
Disadvantages:1. It can only only opera operate te on liqui liquid d or gaseou gaseous s fuels. fuels.
6. Supercharged boiler ::Construction: - In supercharged boiler there are two feed pumps one for feed water and one for boiler drum. Gas turbine supplied the used flue gases to economizer to heat the the feed feed wate water. r. This This gas gas turb turbin ine e also also run run comp compre ress ssor or.. Comp Compre ress ssor or supp suppli lies es compressed air to combustion chamber.
Working: - Combust Combustion ion in superch supercharg arged ed boiler boiler carried carried by pressur pressurized ized combusti combustion on process at 5-6 bars. The heated feed water from economizer economizer and also from boiler drum are mixed and supplied to the evaporating tubes where it is converted into wet steam and further heating is carried out in evaporating tube and the steam is discharged in the boiler drum. Almost dry steam from the drum is superheated in the superheater before it is supplied to the steam turbine. The high pressure exhaust gases from the boiler are used to run a gas turbine. The power developed by gas turbine is used turbine is used to run the compressor and other auxiliaries. Turbine exhaust gases are used to heat the feed water in the economizer and finally these are exhausted to surrounding through the chimney.
Advantages:1. The The heat heat transfe transferr surfac surface e requ require ired d is hardly hardly 30 to 25% of the heat transfe transfer r surfa surface ce of a conve conventi ntiona onall boile boilerr due due to very very high high over overall all heat heat transf transfer er coefficient.
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2.
A supercharged boiler of 150 tons/hr. generating capacity can be brought to full steam pressure within 25 minutes.
3.
These boilers are constructed to generate steam up to 2 lakh kg/hr. up to a pressure of 180 bars.
Disadvantages:1. The only disadvantage disadvantage of this this boiler is tightness of high pressure pressure gas gas passage passage is essential.
