Basics of Thermal Power Plant

7/30/2019 Basics of Thermal Power Plant

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Page 1 Thermal Training Institute

STEAM GENERATOR

OBJECTIVES

To acquaint the participants with the safe and efficient

operation of Boiler and its Auxiliaries and also Preventive & Breakdown

maintenance procedures.

PROGRAMME PROFILE

Working principle, function and classification of Boilers.

Description of Boiler components.

Function and working principle of Boiler auxiliaries-Mills andFeeders, Fans, Air Preheaters, Soot blowers etc.,

Boiler Mountings; safety valves, drains and vents.

Operation of Boiler Auxiliaries.

Alkali boil out and Acid cleaning.

Hydraulic Test.

Boiler startup and shut down.

Interlocks and Protections including F.S.S.S.

Efficiency and Performance monitoring of Boiler and its auxiliaries.

Important do's and Don'ts under emergency condition


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Page 2 Thermal Training Institute

CONTENTS

CHAPTER DESCRIPTION PAGE

01 STEAM GENERATOR3

02AIR PREHEATING SYSTEM 20

03 DRAUGHT SYSTEM

26

04 FUEL FIRING SYSTEM41

05 ELECTROSTATIC PRECIPITATOR76

06 SOOT BLOWER SYSTEM

90


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Page 3Thermal Training Institute STEAM GENERATOR

STEAM GENERATOR

INTRODUCTION

The term steam generator has come into existence to replace the term boiler

as it includes the furnace, boiler drum, water walls, super heaters,

reheaters, economizer etc. The boiler in our Power Station are all conventional

single drum natural circulation, radiant, dry bottom, balanced draft, reheat out

door type. The scarcity of fuel, steep increase in fuel costs and worsening of

quality of fuel lead to pay more attention for the increase in efficiency of the

steam generating system. The heat released in the furnace is received by the

water / steam / air to the maximum extent to have boiler efficiency of the order of

86%.

DRUM & DRUM INTERNALS:

The boiler drum serves the purpose of storing water for conversion to steam

through the water wall connections. The water and steam mixture entering the

drum through the up risers may contain one part of steam with about 5 to 8

parts of water by weight. Hence natural circulation in the boiler water walls is based

on thermo siphon principles. The circulation number, which is the ratio of quantity of

water & steam mixture flowing through the circuit to the quantity of the steam

produced, is designed to be around 8 for natural circulation.

The water from boiler drum takes a path through 6 nos down corners to

the bottom ring header and rises through the water wall tubes, absorbing heat

in the furnace. The steam and water mixture are collected at the upper

header and through the up rises enter the drum again.

CHAPTER - I


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Inside the drum, turbo separators with screen driers are placed for

separating saturated steam from water. In the axial flow turbo separator rotary

motion is accomplished by the configuration of the vanes. The spinner

blades impart centrifugal motion in the flowing mixture and the plate type

corrugated sheet placed at the outlet of the turbo separator provides torturous

path for the separation of the steam. The final separation of the dry

saturated steam is achieved in the screen drier, having wire mesh boxes, with

more wetting surfaces. For proper operation of drum internals, controlling the

normal water level is utmost important, because once the turbo separators are

immersed in water proper separation of steam will not be possible.

The drum is provided with spring-loaded safety valves to relieve the

pressure in case of abnormal increase in pressure above a present value. Drum

level transmitters are provided for remote indication in UCB, in addition to 2 Nos.

Inside the drum, turbo separators with screen driers are placed for

separating saturated steam from water. In the axial flow turbo separator rotary

motion is accomplished by the configuration of the vanes. The spinner

blades impart centrifugal motion in the flowing mixture and the plate type

corrugated sheet placed at the outlet of the turbo separator provides torturous

path for the separation of the steam. The final separation of the dry

saturated steam is achieved in the screen drier, having wire mesh boxes, with

more wetting surfaces. For proper operation of drum internals, controlling the

normal water level is utmost important, because once the turbo separators are

immersed in water proper separation of steam will not be possible.

The drum is provided with spring-loaded safety valves to relieve the

pressure in case of abnormal increase in pressure above a present value. Drum level

transmitters are provided for remote indication in UCB, in addition to 2 Nos.

hydrasteps (L) & (R). Local gauge glasses on both sides of the drum have also

been provided. To maintain the required phosphate value and PH in the

boiler water, the phosphate dozing line is connected to the drum. Both these

lines are extended almost throughout the length of the drum with perforatedholes, but in opposite directions, so that the phosphate will have much time to react.

Drum vents are provided on both (L) (R) sides, for controlling drum level,


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during swelling, the emergency blow down is connected to the drum and is

protruding inside the drum up to the normal working level, which is 250mm

below the drum geometric center line. The drum is inclined down by 2 deg on the

right side.

FURNACE:

The furnace of the boiler is formed by water walls for combustion of fuel

and the opening at the bottom for ash removal. The ent ire latent heat

of vaporization of water is added here. The space between the tubes is fusion

welded to form a complete gas tight water wall, thus eliminating there factory

work for insulation of the furnace. Sufficient height between the top row of fuel

nozzle and the furnace exit is provided to obtain proper furnace retention time

for the fuel to have complete combustion. The boiler is a tangentially fired boiler

having the fuel burners in all the four corners. There are four elevations of oil

burners for firing heavy oil with the bottom most elevation has the option of firing

light oil also. Six sets of coal burners are arranged corresponding to each mill,

with provision to admit combustion air around the burners.

The bottom ash hopper is open hopper type where in heavier products

of combustion (ash and slag) are collected. The front and rear walls slope

towards the center of the furnace to form the inclined sides of the hopper. The

bottom ash is continuously removed by two sets of scrapper feeders and

disposed by clinker grinders. Fouling of the water wall by gradual building up

of slag is cleared by wall blowers using superheated steam, to avoid

increase in furnace exit gas temperature. The water wall assembly is free

to expand downwards, as the furnace temperature increases. But to avoid

air ingress into the furnace, which is maintained at a negative pressure, suitable

water seal assembly by providing a 'seal through is arranged.

ECONOMIZER:

The economizers were initially introduced to recover the heat available

in flue gas that leaves the boiler, there by increasing the efficiency of the

steam generator. This resulted in saving in fuel consumption and hence

the name `Economizer'. The heat from flue gas is absorbed to raise the sensible

heat to feed water before the same enters the evaporative circuit 'of the


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boiler.The economizer is located just above air preheaters. The feed water flow

is in counter flow direction with respect to flue gas, giving higher heat transfer rate.

Feed water is supplied to the inlet header of the economizer after the feed

isolation valve and NRV. From the outlet header, it is fed to the d rum through

economizer outlet links. Drain and vents are also provided for the

economizer. Economizer recirculation line taken from the down comer to eco

inlet is provided to prevent steaming in economizer coil for which they are

not designed. Through the recirculation line flow is created during start up,

whenever there may not be any feed water flow through the eco, f rom BFP. Ash

hoppers below the economizer are provided as the flue gas duct takes from

vertical to collect and dispose off the ash.

SUPERHEATERS:

Super heaters are meant to raise the steam temperature above the saturation

temperature by absorbing heat from the furnace and flue gas to increase the

cycle efficiency. The maximum temperature to which the steam can be heated

is limited by the metallurgy and economy in init ial cost. Present t rend is to

limit the temperature at 540. A super heater which can 'see' the furnace

flame and absorbs heat y radiation is radiant and low temperature super

heaters receive the heat by convection, and they are placed either horizontally

or allowed to be hung vertically. The vertical arrangement is simpler in

supporting and allowing for expansion and this type is called pendant SH. The

superheaters are also arranged as ceiling for the furnace (ceiling SH) and as

wall of the second pass (steam cooled wall SH). The horizontal or low

temperature SH (LTSH) is located in the rear vertical gas path above the

economizer. The pendant SH is the final assembly of superheaters from which the

main steam lines (L) & (R) to turbine are taken.

REHEATERS:

Reheater is used to raise the temperature of the comparatively cold

steam, which is exhausted from HP turbine after doing work (Cold Reheat

steam). This is another method of raising the cycle efficiency. Because of

higher initial costs involved, reheater system will be used as the unit sizes

increase beyond 100MW. The reheater is composed of two stages viz. The


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front pendant and rear pendant sections. The f ront pendant reheater is located

above the furnace, between the rear water wall hanger tubes and the platen SH

section. The rear pendant section is located between the water wall screen

tubes and before the final SH section. In other words the reheater coils are placed

in between the platen and final SH.

STEAM TEMPERATURE CONTROL:

The metal temperature at all section in the super heater and reheater also in

the turbine should be maintained within permissible values. Attemperation

by feed water injection is used to limit the temperatures of steam at boiler outlet.

The design margin in temperature control may get deviated and may result in

higher outlet temperatures due to slag formation in water wall, high excess air,

low feed water temperature, low load operations, and top tier mills in service.

The cause has to be identified and action taken to keep the SH and RH

temperature at 540C at boiler out let always. Wall blower operation and burner

tilt adjustment are in addition to adjusting total air flow, coal firing etc.

Temperature control by feed water injection has to be resorted to as a final

measure. For SH steam temperature control, the required quantity of feed

water is injected at the SH connecting links between LTSH and platen SH and

platen to final SH into the path of the steam, through a nozzle at the entry

end of the desuperheater. Normally spray type desuperheaters are provided

with renewable liners to protect the main super heater shelled with renewable

liners to protect the main super heater shell from erosion of spray water and

thermal shock. Since they are placed before platen super heater, the water

particles entry to turbine is remote.

For reheater temperature control, burner tilting is carried out.

Reheater spray injection is carried out as a last resort at the cold reheat pipe

before entering the reheater coils.

A control valve linked to an automatic control drive unit regulates the flow of

spray water quantity supplied to the desuperheaters for main steam and RH

steam temperature control. A 100% stand by spray control valve is provided for

RH & SH temperature control. Both the main and stand by spray control valve are

provided with electrical isolation valve at the upstream side and hand valve at

the down stream end.


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SAFETY VALVES:

The pressure in the drum, super heater or reheater may suddenly rise

to abnormal limits, due to operational emergencies or due to load throw

of f conditions. To safeguard the pressure parts, safety valve are provided. The

boiler drum is provided with 3 Nos. spring loaded safety valves, to relieve

approximately 547T/hr.

Similarly the super heaters are safeguard by providing one no. spring

loaded and one no. electromatic safety valves, at the super heater outlet pipes. For

reheating system there are totally 4Nos safety valves, one no in cold reheat

line and 3 Nos. in hot reheat line at the boiler end.

VENTS AND DRAINS

These are important in the sense that proper operation of vents and drains

is essential during initial operating condition and during shut down. The steam

line drains should be kept open before charging, in order to avoid water

hammering and uniform warming up. The drum, the economizer, super

heaters and reheaters are provided with vents at suitable locations. They are

grouped and the valves are provided at drum level for ease of operation.

Similarly the super heater drains are provided at 16m level and the drains are

connected to the IBD expander.

For controlling drum level during start up, the emergency drum drain

valves and down comer drain are provided. These valves can be operated

from UCB. The main steam line is provided with a drain before the boiler stop

valve, so that the line can be warmed up to the stop valves during start up.

The continuous blow down from boiler drum is provided to remove the

sludge's and impurities continuously. The silica in steam should be kept below

0.02 ppm and in the boiler water, by suitable blow down it shall be maintained

below 0.25 ppm for a drum pressure of 165 kg/sq.cm. The normal drain

through CBD will be around 1% of rated steam flow. In order to use the steam

from the CBD quantity, a CBD expander is provided, while the condensate

is wasted through IBD tank, by means of a level controller.


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MAIN PARAMETERS

BOILER PROTECTIONS:

A boiler trip command stops all fuel inputs by tripping all the pulveriser, all

the feeders and all the oil guns. Closing the HO trip valve, IO trip valves and

LP trip valve and tripping of PA fans also will accompany boiler trip out, boiler trip

is initiated by any one of the following conditions.

a. Loss of DC power (2 sec)

b. Less than fire ball and loss of AC at any elevation in service

c. Drum level very low for more than 5 sec

d. Drum level very high for more than 10 sec

Fuel: CoalUnit 100%

MCR

100%

TMCR

60%

TMCR

HPH OUT

Generator MW 210 126 210

Flow Primary SH OIL T/hr 693.6 693.6 384.5 589.3

Flow RH outlet T/hr 618.6 572 349.1 582.2

Total heat to SH/RH M Kcal/hr 420.6 420.6 263.8 439.6

Pr. at SH OIL Kg/sq.cm 155 154.1 150.8 153.2

Temp at SH O/L deg.C 540 540 540 540

Pr. At RH I/L Kg/sq.cm 41.3 38.1 23 39.4

Temp a RH I / L deg.C 350 347 334 357

Pr. At RH OIL deg.C 39.7 36.6 22 37.8

Temp at RH OIL deg.C 540 540 540 540

Feed water temp deg.C 250 248 222 167

Ambient air temp deg.C 30 30 30 30

Combustion air temp

(sec)deg.C 317 312 281 268

Fuel quantity T/hr 135.2 125.8 78.9 130.8

Air quantity T/hr 837 779 550 806

FG temp at boiler deg.0 133 132 119 112

Efficiency based on % 86.71 86.74 86.72 87.66


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e. Both FD fans off

f. Both ID fans off

g. Unit air flow < 30%

h. Furnace pressure very low

i. Furnace pressure very high

j. Loss of all fuel trip

k. Flame failure trip

l. Emergen cy trip

m. Loss of reheat protection trip

BOILER FILLING UP

The boiler may be required to be filled up for conducting hydraulic test

or for lighting up or for wet preservation. For hydraulic test and preservation,

the ent ire system has t o f i l led be up v iz. Economizer, water wal l , drum

an d superheaters, upto boiler stop valves. But for the lighting up of the boiler,

it is sufficient that the economizer, water walls and drum upto normal level are

filled up. It is always preferable to fill up the boiler, with maintaining some

pressure in deaerator by pegging steam for deaeration. However,

sometime, if it is not possible to have deaerated water for filling up, water

which is chemically treated for removal of dissolved oxygen will be used. In

this case, hydrazine is used for this purpose.

For fil l ing up the system with boiler feed pump, deaerator shall be

pressurized with steam, obtained from other unit, and make up for feed tank

carried out through condensate pump. Boiler fill pump cannot be used in this

case for making up deaerator level. Since it is admitted into feed tank only and

not to the deaerator. But while conducting, hydraulic test, the hot water can not

be used for filling up, because the boiler tube leak can not be checked up by

entering into the boiler. In this case, it is preferable to use chemically treated

cold water. After the hydraulic test is declared OK, the entire water shall be

drained and then fil led up with deaerated water.

Filling the system for wet preservation can be carried out either with boiler fill

pump or with boiler feed pump. The required chemical (200 ppm hydrazine

and required ammonia for raising PH) shall be fed in CST, assuming the

water filling capacity of boiler. Using the boiler fill pump the boiler can be


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filled up through Economizer, water wall and SH drains. In order to

prevent entry of impurities from water walls, entering the superheaters,

water wall drain shall be closed at normal level. In the other case when

filling up, using boiler feed pump chemicals shall be fed in feed tank and

circulated with BFP and boiler filled up with200 ppm hydrazine. Addition of

required quantity of chemicals in feed tank shall be done as and when the

values drop.

LINI NG UP THE S YSTE M F OR FILL ING UP THRO UGH BOILER FILL

PUMP (PRESERVATION): -

1. Check that no line clear is pending in boiler, and the boiler is cleaned for

filling up.

2. Check the level is CST and if necessary raise the level to more than 70%.

3. Check that the following vents are open in 52m levels.

a. Drum vents

b. ECO vents

c. SH vents

4. Check that the following drains are in closed condition

a. Emergency drum drain (EBD)

b. Continuous blow down

c. Drain before boiler stop valves

d. Attemperation line drains

e. SH drain header drain to IBD tank

5. Check that the following drains are open

a. SH drain header-filling valve from boiler fill pump

b. SH drain valves (for filling the SH also)

c. Eco drain (for filling through ECO)

d. Down comer and bottom ring header drain (for filling through water walls)

6. Check that the valve in the 2m level from boiler fill pump line to feed tank is

closed.

7. Check that the filling valve at 2.0m level from boiler fill pump line is open.

8. Check that the suction valve to fill pump is open and pump primed.

9. Check that the pump is properly lubricated and oil level available.

10.Check that the supply module for the pump is switched on and local PB


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released.

11. Check that the chemical dozing valve at the filling line is closed.

12. Check that the isolation and regulation valves of start up vent are closed.

13. Check that the shoot blower line isolation valves at 52m level from tenth

header is closed.

14. Check that the boiler stop valves (L) & (R) and their by pass valves (L) & (R)

are closed.

15. Check that the drain line before stop valves are in closed condition.

16. Check that all sampling valves are closed.

17. Check that the hand valves and electrically operated valves of the turbine

side drain lines to IBD tank is closed.

18. Check that the ECO inlet and ECO, RC valves are closed.

After lining up system for filling up, carry out the following: -

1. Arrange for dozing the required chemicals in CST. However it should be ensured

that this chemical added water should not go to the other CST.

2. Start the pump with recirculation valve fully open, and the discharge valve

closed. Al low some t ime, say 10 to 15 mts. For mixing up of the

chemicals.

3. Then open the discharge valve gradually, to maintain the discharge pressure at10 kg/sq.cm. The load current at the module should be checked for about 85 Amps.

It is sufficient to open by about 4 to 5 turns.

4. Check the flow of water to boiler and drop in level to the CST. The level can

be dropped to a minimum of 10%.

5. If it is required to raise the CST level again, check the value of N99110

2H4 and doze chemical as per requirement. Then continue the fil ling up operation.

6. Once normal level is reached in drum, stop filling through water wall drains and

down comer drain.

7. Observe free flow of water in vents at 52m level and close the vents one after

the other, after releasing of air.

8. Allow some time for the pressure to build up after closing of all the vents. Then

close the discharge of boiler fill pump and stop the pump.

9. If the boiler is to be preserved for longer duration, the system should be filled up

atleast twice in a week, so as to keep the system in completely filled up state in

order to prevent entry to atmospheric air.

10. It is preferable to cap the system with nitrogen pressure (inert gas) if provision is


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available and maintain the pressure continuously.

11. The drains and other valve positions need not be disturbed if filling up is to be

done periodically. If not, the system can be made ready for operation as per

requirement.

LINING UP AND FILLING UP THROUGH UP THROUGH BFP FOR

PRESERVATION:

1. Check that the line clears on boiler pressure parts, air preheaters, bottom ash

hopper etc., are returned with men and materials are cleared.

2. Raise the deaerator level using boiler fill pump. For this purpose check that the

isolation valves at the 2m level and at deaerator level are open. ACW make up

pump should not be allowed to overflow if make up is done through boiler fill

pump, by suitably throttling the make up valve. Check that filling valves to boiler are

all kept closed.

3. Line up the boiler feed pump for filling the boiler

4. Check that ECO, SH and drum vents are open at 52m level.

5. Check the SH drains at 16m level are closed

6. Check that EBD, IBD, down comer, water wall and Eco, drain are closed.

7. Check that the isolation and regulation valves of start up vent are closed.

8. Check that the soot blower line isolation valves at 52m level from tenth header

is closed.

9. Check that the boiler stop valves (L) & (R) and their bypass valves (L) & (R) are

closed.

10. Check that the drain line before stop valves are in closed condition.

11. Check that all sampling valves are closed.

12. Check that all feed line drain valves are closed.

13. Check that HP bypass injection hand valve and the PRDS injection hand valve

are closed.

14. Check that the HP heaters are in bypassed condition.

15. Check that the SH & RH attemperation block valves and isolation valves are

closed.

16. Through the chemical feeding funnel available at feed tank add Hydrazine andAmmonia for maintaining 200 ppm N2H4 and pH>9.5

17. Start any of the BFP's which has been line up (for filling the boiler) on recirculation.


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18. Take sample at BFP suction and see that required values are obtained. If. not,

add chemicals to the required level in feed tank. Take care that the isolation

valve of the chemical feeding funnel is kept closed after dozing chemicals.

19. After attaining the required chemical values, feed the boiler through low load

FCV, by opening the BFP discharge valve, isolation valves of low load FCV and

Economizer inlet valve.

20. After observing free flow of water through drum vents, ECO, vent and SH vents,

close the vents one by one. Ensure that no air pockets are formed.

21. After raising the pressure to say 70 Kg/Sq.cm., stop the feed pump and

level they system for preservation.

22. Once in 2 or 3 days, the system may be kept repressurized, if no water flow

through vents is noticed, when opened for inspection.

23. During filling up, if the hydrazine value drops the required chemicals shall be

added to maintain around 200 ppm , throughout filling.

24. Ensure that the chemicals added to the water, particularly ammonia does not

enter the condenser or LP heaters.

LINING UP THE BOILER THROUGH BFP FOR HYDRAULIC TEST:

1. Line up the system for filling up the boiler using BFP as done earlier for

preservation. However, for hydraulic test, it is sufficient that the hydrazine

value is maintained at 10 to 15 ppm and the required ammonia for maintaining

PH 9.5.

2. The required chemicals shall be fed in feed tank as stated earlier.

3. Start the BFP and fill up the entire boiler including SH. Close the vents after

observing the free flow of water.

4. If it is required to conduct hydraulic test at operating pressure, the drum and SH

safety valves need not be gagged or blanked.

5. If the hydraulic test is to conducted at elevated pressures, then drum and SH

safety valves are to be gagged or blanked.

6. Raise the pressure by raising the BFP discharge pressure by increasing

scoop position, such that the rate of rise will not exceed about 10kg/sq.cm

per minute.

7. Keep the test pressure at required level (say 145 kg/sq.cm if hydraulic test

is being conducted at the rated pressure so that there will be margin in not


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exceeding the rated pressure) for about 5 minutes. And then reduce the

pressure to 70 kg/sq.cm gradual ly. Maintain at th is pressure for

maintenance staff to check the boiler for any leak.

8. If the hydraulic test is declared OK, the boiler pressure can be allowed to be

killed by itself, after stopping the BFP.

9. The vents and SH drains can be opened, if the boiler is to lighted up

shortly, after the pressure is killed.

FILLING UP THE BOILER FOR LIGHT UP THROUGH BFP: - LINE UP: -

1. Check than no line clear is pending on boiler, APH, bottom ash hopper etc., so

that lighting up of the boiler will not be held up.

2. Check that the following vents are open in 52m level.

a. Drum

b. ECO Vents

c. SH vents

d. RH vents

3. Check that the following drains are in closed condition

a. Emergency drum drain (EBD)

b. Continuous blow down (CBD)

c. All attemperation line drains

d. Bottom ring header drain

e. Down corner drain

f. Economizer drain

4.Check that the following drains are open condition

a. SH drains and header drain to IBD

b. Drains before boiler stop valves

c. Boiler drains to IBD isolation valve at 2.5m level near IBD tank.

5. Check that the boiler fill up valve from boiler fill pump discharge line is kept

closed.

6.Check that SH filling valve from boiler fill pump discharge line is kept

closed.

7. Check that the start up vent isolation and regulation valves is open.

8. Check that boiler stop valves and their by pass valves are closed. 9.

Check that the soot blower line isolation valve is closed.


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10.Check that ECO, inlet and re-circulation valve are open.

FILLING UP: -

A) CHEMICAL ADDED WATER: -

If the boiler is to be filled with chemically treated water, add the

required chemicals (10 ppm N2H4 and ammonia to the required level) in

feed tank. Start the BFP on RC. After attaining the required chemical values,

feed the boiler through low load FCV (Similar to filling the system for hydraulic

test), upto the drum normal level. Then top feeding and keep BFP on

recirculation, if the boiler is going to be lighted up immediately. If some undue

delay is expected the BFP can be stopped.

B) DEAERATED WATER:

If the boiler is to be filled up with deaerator water, heat the feed tank and raise the

deaerator pressure of about 3 Kg./Sq.cm using foreign steam. The BFP shall be

kept on re-circulation on the above period and when the pressure is achieved,

start feed the boiler. The hydrazine dozing shall be carried out to the running

BFP suction line for scavenging dissolved oxygen. The deaerator level shall be

maintained with any one of the CEP in service, maintaining hot well level by hot

well make up pump. The feeding shall be so controlled that the pressure in

deaerator is maintained almost constant and the make up is sufficient for

maintaining deaerator level. At normal drum level, the feeding to boiler shall be cut

out. The steam admission to deaerator shall also be so controlled that the

pressure will not rise.

Filling the boiler using deaerated water is always better.

DRAINING THE BOILER: -

a) The boiler is required to be drained:

1. If there is going to be sufficiently long shut down, in which case the water

remaining in the boiler may lead to corrosion.

2. If there is any puncture in the boiler tubes.

3. If the boiler is programmed to be filled up for preservation.

b) After shut down of the boiler, open the SH drains at a pressures of 5kg/sq.cm.

c)Open the SH vents, drum vents and ECO. vents when the pressure in the drum is

about 1 to 2kg/sq.cm.


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d) Open water wall drains down comer drains and ECO drains.

e) Care must be taken while draining the boiler that the turbine side drains are kept

closed, in order to prevent back up.

CORROSION IN BOILERS AND ITS PREVENTION:

It is necessary to keep the tubes clean internally and externally free of

deposits that could impair heat transfer and lead to corrosion, ultimately

causing tube failures. Corrosion damage is always experienced inside tubes of

the boi ler, economizer and superheater, when water chemistry is not

maintained properly.

To avoid the corrosion, one should understand the importance of

maintaining the iron oxide coating on the internal surfaces of the boiler tubes.

The oxides Fe304 (Magnetite) a normal corrosion product that forms on steel is

protective to corrosion caused by boiler water. Once it is formed, further

inside corrosion of the tube stops. But if it is destroyed, corrosion will resume

until conditions favorable to oxide formation (magnetite coating) are re-

established in the system.

A) Water Side Problem: -

(1) Hydrogen Induced Brittle Fracture:

This occurs when boiler pH is too low, H 2 atoms are produced between

the deposits and the tube surface and react with cementite (hard iron

compound) at the grain boundaries of the tube material to form methane gas.

The formed methane gas removes carbon from metal, weakening it by

creating fissures in its grains structure. This type of damage is common

where condenser leakage occurs.

(2) Bulk Deposit Corrosion: -

This is caused by concentration of soluble corrosive compounds, as

alkalis (sodium hydroxide). Due to capillary action of the porous deposit

formed on the surface of the tubes, the alkaline liquid is drawn towards the tube

surface and then it attacks on the metal and the metal is eaten. The term

caustic gauging is used for such type of corrosion and tube failure.

(3) Corrosion Fatigue:

Materials that undergo cyclic strains may suffer fatigue failure. The


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strain may be mechanical (vibration) or thermal (corrosion). The reason for the

above mentioned failure is that the corrosion product on the surface cracks and

acts as wedges during boiler cooling down, causing the cracks to extend.

Corrosion attacks the newly exposed surface when the boiler if fired next time,

forming still deeper wedges in the next cooling phase.

(4) Stress Corrosion Cracking: -

The super heater elements containing residual stress are susceptible

to cracking at high temperature, when there exist the presence of corrosion

forming agents in steam such as chloride or hydroxide compounds and oxygen.

(5) Oxidation:

All materials commonly used in high temperature super heater and re -

heater are subject to oxidation. When the oxides scale on the inner surface of

the tubes become sufficiently thick, the differential expansion between the

oxides and the parent metal results in spalling of the oxide from the metal

surface a process called "Exfoliation". The loose flakes are hard and brittle and

generally range from 1mm to 5mm in size. The loose scale can clog the tubes

at bends, causing their failure by over heating and can damage nozzle and

turbine blades if carried along the flow path of the steam.

(B) FIRE SIDE PROBLEM:

Major corrosion problems in coal fired boilers are caused by coal ash.

The fireside deposits are classified as fouling and slagging.

Slagging is the deposition of molten or fused particles on furnace tube

surface. This will occur in the radiant surfaces but slagging also occurs on

the superheater or reheater tubes when molten ash is carried with the hot

flowing gases.

On the other hand, fouling is the condensation of combustible

constituents, such as sodium sulphate, in areas where temperature is such

that, the constituents remain in liquid state. The combustibles, fly ash and

flue gases react chemically to form the deposits. These are generally found

on convection zone of the boiler.

Slagging and fouling on the heat transfer surfaces retard heat flow and

therefore they should be cleaned periodically to maintain the efficiency.

Erosion is another problem faced by the boiler tubes. It is generally


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caused by an excessive amount of abrasive ash in the coal. This is generally

caused in the low temperature section of the sueprheater. Deflection baffles

help to reduce this type of erosion.

SUPERHEATERS AND REHEATERS PROTECTION AND CONTROL:

-

As long as there is a fire in the furnace adequate protection must be

provided for the superheater and reheater elements. This is especially

important during periods when there is no demand for steam, such as starting

up and shutting down. During these periods of no steam flow through the

turbine, adequate flow through the superheater is assured by means of drains

and vents in the headers, links and main steam piping. HP / LP by pass system

shall be kept in service, during hot start of boiler, to maintain steam flow

through RH, in order to avoid starvation.

During al l s tart ups care must be taken not to overheat the

superheater or re-heater elements. The firing rate must be controlled to keep

the maximum exit gas temperature in the furnace from exceeding 538C. To

measure this temperature two furnace temperature probes (L) & (R) are

provided in the furnace outlet zone which can be operated from remote for

extending or retracting. The temperature display will be available in control

room during advanced position. We can stop the probe at any position as per

our requirements between its stroke length.


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Page 20 Thermal Training Institute AIR PRE HEATING SYSTEM

AIR PRE HEATING SYSTEM

INTRODUCTION:

Air heater is a heat transfer surface, in which air temperature is raised by

transferring heat from the flue gas. Since air preheater can be employed

successfully to reclaim heat from flue gas to low temperature levels, than is

possible with economizer, the heat rejected to chimney can be reduced to

higher extent. For every 20C drop in the flue gas exit temperature the boiler

efficiency increases by about 1.0%. In addition to increasing boiler efficiency,

the other advantages are:

Stability of combustion by increased temperature of air admitted for

combustion.

The hot primary air is used for drying coal in the mill for better

grinding and carrying coal powder.

Better combustion of poor quality of coal, having low volatile

content.

Reduction incombustibles in flue gas.

The regenerative Ljungstrom air pre heater is used in the flue gas path

for heating both primary and secondary air. The heat from the f lue gas that is

at higher temperature is transferred to incoming cold air by means of

continuously rotating heating surface elements of specially formed metal plates.

As the rotor slowly revolves, the mass of heating surface elements alternates

through the gas and air passages. Heat is absorbed by the air preheater

elements while passing through flue gas streams, then as these same

elements pass through the air path, they release the stored up heat. This

increases the temperature of the incoming cold air.

CHAPTER - 2


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DESCRIPTION:

The air preheater employed has three sectors, having separate paths for

1. Flue gas

2. Primary air

3. Secondary air

There are two nos trisector APHs for each boiler. Each air

preheater consist of the following components.

a. Connecting ducts which connect the top and bottom ends of APH.

b. Rotor housing is located in between the top and bottom connecting plates and it

encases circumferentially the rotor.

c. Rotor is an open ended drum which contains the heating surface elements.

The rotor is divided into 12 sectors, which are further divided into

compartments for housing heating surface elements.

d. The heating surface elements are formed from numerous undulated metal

sheets and are packed in reversible containers called baskets. These are

arranged in three tiers viz-hot end, intermediate and cold end.

e. The rotor is supported by and rotated on a spherical roller thrust bearing at

bottom called support bearing. At the top there is a guide bearing which is free

to allow axial movement of the rotor, as the rotor expands.

f. There are six nos. sector plates spanning across the open ends of the

rotor, sealing the gas and air from each other.

g. Sealing arrangements: The sealing system consists of radial, axial

and circumferential seal plates, to prevent or minimize leakage of air from

one sector to the other sectors. Rotor post seals are also provided to avoid

leakage through the rotor post.

h. There are two Nos. main drives for the APH for rotating the air preheater

elements. For initial rotation of the air preheater or for inspection of elements,

there is an air motor d riven by service air. This is also used as emergency

stand by while both the main drives fail.

i. Lubrication system: Separately for support bearing and guide bearing there is a

lub oil unit, having 2Nos. pumps, cooler, strainer etc.

j. Soot blowers are provided on both top and bottom end of the APH for

cleaning the heating surface elements using steam.


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k. Three nos. access doors are provided APH one each in gas inlet, primary air

and secondary air outlet duct. Another set of access doors is provided in

the duct, one each in flue gas outlet, secondary air inlet and primary air inlet ducts.

l. For observing the cold end elements, a glass faced observation port is

provided in each APH, with electric lighting bulb for easy inspection, while

the APH is in service.

m. Two numbers stationary APH washing manifolds are provided in the gas

inlet and outlet duct for water washing the heating elements. These can also be

used for flooding the APH with water in case of fire.

n. Rotor stoppage alarm is provided by using a proximity switch which is mounted

in the support bearing area.

o. Fire sensing devices for giving alarm are provided in the APH flue gas path, by

providing thermo couples at various locations across the width of the APH, for

sensing any abnormal rise in temperature due to fire in APH.

STEAM AIR PREHEATER

The SCAPH on each at the outlet of the FD fans are used to control the cold

end temperature of the APH, for corrosion control and also for raising

secondary air temperature during cold start up of the unit. These must be

brought into service, only after starting FD fans, and preferably before light up

of the boiler. The SCAPH uses auxiliary steam from PRDS for controlling the

temperature. The drain from SCAPH can be sent to atmosphere through IBD

expander. The steam lines should be properly drained, before charging, in

order to avoid hammering, leakages, and damage to the system.

PRESTART CHECKS FOR Ljungstorm APHs

1. Check that all work permits and permission are canceled covering APH, its

drives, lub oil pumps, duct and dampers.

2. Check that all access doors are closed.

3. Check that the APH inlet / outlet dampers are open, if the boiler is going to be

lighted up.

4. Line up the lub oil pumps as indicated below:

a. Check lub oil level in the support and guide bearing oil sumps for normal level


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using dipstick.

b. Check that the suction, discharge valves are open.

c. Check that the pump is free to rotate.

d. Check that the cooler is charged on water & oil side.

e. Check that the oil flow meter and the temperature indicators and pressure

gauges are available. The ACW pressure shall be around 2.5 to 3.0kg/sq.cm.

f. Check that the local stop push buttons are released.

g. Switch on the module for the lub oil pumps.

5. Establish the supply for APH rotor stoppage alarm and verify in UCB that the

alarm persists in annunciation window before starting APH.

6. Check that the service air is available and the air supply valves to APHs are kept

closed. (If not the APHs will start rotating once air supply is established) Open

the drain valve in the service airline to remove the water collected in the service

airline.

7. Establish supply to APH solenoids for APH & B

8. Switch on supply modules for APH A & B main drives and stand by drives.

9. Keep the APH soot blower system ready for service, by switching on the respective

modules, and keeping open the steam line drains for charging the steam.

10. Check the availability of inspection light.

Note: APH should be in service, preferably before starting the ID fan, but

necessarily before starting FD fan.

TAKING INTO SERVICE APHs

1. The lub oil pumps of support and guide bearings can be started from UCB or

from local.

2. Start any one of the pump locally and see that there is no oil leakage.

3. Check the lub oil flow and pressure.

Normal Flow: 5 Lit / min for guide bearing and

10 Lit/min for support bearing at pressure of about 1.5 to 2kg/sq.cm.

4. Stop the pump and start the other pump and see that the flow and pressure is normal,

and that there is no oil leakage.

Note: All the 8Nos. pumps shall be started locally and checked before giving clearance

for remote operation from UCB. The stop push buttons shall be kept in released


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condition.

5. Now open the air valve for starting the air motor of APH A or B as the case may be.

As the main drives are not in service, the solenoid will supply air-to-air motor.

6. Check the operation of APH for any abnormal sound or abnormal seal rubbing.

7. If the performance is normal, the main drive can be started locally or from UCB.

8. Check the performance of APH on main drive-the local current and abnormal sound

and vibrations are to be watched.

9. On starting the main drive, the air motor shall stop. This can be verified in UCB by

the indication given from the service air pressure switch, provided in the line.

10. Now stop the main drive from UCB and check that the air motor comes on auto.

11. Start the stand by drive motor from UCB and check the performance.

12. The main drive can be kept in continuous service keeping the stand by drive ready.

13. The air preheater soot blowers are to operated at regular intervals, after lighting up of

the boiler.

14. The DP across air preheaters has to watched closely to see that the APHs are free

from choking.

The oxygen and carbon dioxide levels in flue gas, before and after APH have to be

Abnormal drop in carbondioxide level after APH shall indicate poor

performance of the APH seals. Normal drop: 2%.

SHUT DOWN OF APH DRIVES:

1. In normal course of operation, the APH can be stopped after shutting down

of the boiler and stopping of ID, FD and PA fans and the flue gas

temperature at APH inlet less than 100C.

2. In case of fire in APH, the boiler shall be fired out immediately, and the ID, FD

and PA fans stopped. The APH shall be stopped and its isolation dampers will

be closed. The APH shall be flooded with water using APH wash pump and the

APH washing lines.

LINING UP OF SCAPH:

1. Ensure that there is no work permits are pending in the steam lines, drains etc

of SCAPH.

2. Open the drains in the inlet steam line (L) & (R) before the isolation valves.


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Page 25 Document Title

3. Open the isolation valves of the SCAPH inlet pressure control valves (CV2A &

CV2B).

4. Open the drains after control valves.

5. Open the inlet valves (4Nos) to the SCAPH A & B.

6. Open the drain valves to the SCAPH drain tanks A & B. If the drains from

SCAPH A & B are to be made through to IBD tank open the isolation valves of

the steam trap. Ensure that the by pass valves of the steam traps are kept closed.

7. For taking into service, the SCAPH drain Tanks:

a. Open the isolation valves of the SCAPH drain tank level control valves.

b. Check that the by pass valves of the level control valves A & B are kept closed.

c. Open the inlet isolation valves to the SCAPH drain tanks A & B.

d. Ensure that instrument air supply is available for the level control valves A & B.

TAKING INTO SERVICE:

1. Check that before SCAPHs are taken into service, secondary airflow is available

i.e. FD fans are in service.

2. Open the inlet pressure control valves (CV2A & 2B) by about 5% for warming

up the lines.

3. After sufficiently warming up the lines open the control valves as per

requirement.

Normally, the control valves openings shall be so adjusted to get the total of

flue gas temperature at APH outlet and secondary air temperature at APH

outlet equals about 175C.

The control valve opening / closing is carried out from UCB on auto or manual.

4. The drain valves provided in the steam lines, which were kept open for warming

up should be closed after sufficient warming up.

5. The level in the SCAPH drain tanks are controlled by the self -regulation

pneumatic control valves.

CUTTING OUT OF SERVICE:

1. The SCAPHs can be cut out, as and when the flue gas temperature at

APH outlet reaches about 125 to 130C.

2. For cutting out the SCAPH, it is sufficient that the inlet pressure control valves

are kept closed.


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Page 26 Thermal Training Institute DRAUGHT SYSTEM

DRAUGHT SYSTEM

INTRODUCTION:

The difference of pressure causing flow of air and hot gases through the

boiler furnace, economizer, air heater, flue ducts and chimney to atmosphere is

termed as draft. The draft system in a boiler plant serves the following functions.

1. Supplies required quantity of air for combustion.

2. Removes the product of combustion continuously from the furnace to the

atmosphere for the effective combustion of incoming fuel.

3. Overcomes the pressure losses when the burnt product passes through

the superheaters, reheaters, economizer, airheaters, flue ducts,

ESP and chimney and discharges it to the atmosphere at the required velocity.

4. Maintaining a balanced draft in the furnace.

In power plant boilers, the draft is obtained by means of fans namely forced

draft and induced draft fan.

The FD and ID fans develop enough draft to move the tremendous

volume of air and gases through the flue gas ducts and stack to atmosphere

maintaining a balanced draft in the furnace.

FANS IN THE SYSTEM:

In a balanced draft system the FD fan supplies the secondary air for

proper and complete combustion while the ID fan removes the flue gases

from the furnace maintaining a pressure in the furnace just below the

atmosphere. Balanced draft facilitates opening of inspection windows for the

inspection of fire inside the furnace and avoids the air / gases blowing out.

CHAPTER - 3


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In order to improve the combustion and efficiency of the boiler a portion

of secondary air is preheated in the regenerative air heater before it reaches

the furnace wind box known as hot secondary air.

The cold secondary air is sent after filtering to the suction of the scanner

fan which boosts up the secondary air pressure for cooling the scanner head which

is exposed to the hot furnace zone. Additionally an emergency air supply

connection from atmosphere is provided for supplying the cooling air in case

both FD fan trips.

Two Nos of ignitor air fans with tap off from FD fan outlet inter connecting

duct supplying the necessary air for the eddy plate ignitors.

The air required for transporting the pulverized coal from the mills to the

furnace is known as primary air and supplied by the primary air fan. Part of the

primary air is also preheated in the regenerative air heater to dry the coal that

is fed to the mill. The cold primary air is used as tempering air by which

the temperature of the coal air mixture leaving the mills can be controlled.

The heat carried away with the flue gases is transferred to superheaters,

reheaters, economizer, air heater on the way to the chimney. The gates

mounted in the ducts are used to stop the air / gas supply completely where as

the dampers regulate the air / gas flow according to the requirement.

INDUCED DRAFT FAN:

Two numbers of NDZV31 SIDOR type fans are used as ID fan to handle

the hot gases from the boiler. These fans are of double suction single stage

centrifugal type in which the flue gases enters the impeller axially and after

passing through the impeller leaves radically. A large part of the energy

transferred to flue gases is converted into pressure energy as the gases

pass through the impeller. The spiral casing converts part of the kinetic energy

in the flue gases to pressure energy. These fans are driven by a constant

speed 6.6KV, 1500 KW, 50 Hz induction motor. The output of the fan is

contro lled by the variable speed hydraulic coupling or inlet damper control.

The spiral casing, suction chamber, rotor shaft with fan impeller, bearing

assemblies and shaft seals are major subassemblies of ID fan.

The spiral casing rests on its foundation an with respect to the rotor

during expansion the spiral casing is guided by central, side and control


28/119


frames to maintain the orientation. Suction cone provided on either side help to

achieve the accelerated inlet flow to the impeller. The impeller sealing rings

fixed on the suction zone over lap the impeller rings maintaining a specific

clearance which provides necessary sealing of the impeller inlet flow from its

discharge.

The rotor assembly consists of impeller and shaft supported by spherical

roller bearings located on either side the impeller. The bearings are lubricated

by means of forced oil from hydrant coupling. The blades of the impeller

are common to both suction flows and the blades are so designed that the two

suction flows are allowed to mix within the impeller itself.

Sealing for the shaft where it passes through the spiral casing on both

the sides is achieved by labyrinth seals for axial sealing. The labyrinths are

housed in a seal housing which is mounted on the bearing pedestal. The seal

housing is connected to the spiral casing by flexible asbestos cloth for radial sealing.

HYDRAULIC COUPLING:

The ID fan is driven by constant speed squirrel cage induction motor. The

fan and motor are coupled by a variable speed hydraulic coupling of voith

make, West Germany. The hydraulic coupling serves as a starting device as

well as regulating device, and is a self-supporting unit. The speed control of

the fan is performed by a scoop control unit of the hydraulic coupling and is

operated by an external actuator, hooked up to the fan control system.

A fi ll ing pump, a scoop tube, primary shaft with primary wheel, secondary

shaft with secondary wheel, bearing assemblies and the housing are the main parts

of the hydraulic coupling. The base of the housing forms the oil tank. The

primary shaft and the secondary shaft are supported by antifriction roller and

ball bearings. The primary shaft drives the filling pump through a gear train.

An electrical driven actuator drives the scoop tube and controls the position of

the scoop tube to vary the speed of the driven equipment.

The fil l ing pump supplies the variable speed turbo coupling with oil

whereas the scoop tube takes the working oil out from the turbo coupling back into

the oil tank and determines the oil filling in the variable speed turbo coupling

and thus the output speed.


29/119


OPERATION OF HYDRAULIC COUPLING:

The filling oil draws the working oil from the tank and pumps it through the oil

cooler, the working oil orifice plate and filling pipe into the primary wheel

collecting channel. The centrifugal force of the primary wheel forces the oil

through bores into the working chamber of the coupling between primary and

secondary wheel. This chamber is connected by oil openings of the

scoop chamber formed by shell. Under the action of the centrifugal force an

oil ring builds up in t he working and scoo p chamber. The thickness of this

ring is determined by the position of the scoop tube. The scoop tube supplies the

scooped oil back into the oil tank utilizing the pressure head.

ID FAN PRESTART CHECKS:

1. Ensure that there is no pending work permit in the ID fans, ESP or in the

furnace and APH.

2. Ensure the manholes are closed.

3. Check the water flow to the cooler of the hydraulic coupling.

4.Check the oil flow to the bearing and the oil level in the hydraulic coupling

oil tank.

5.Check the avai labi l i ty of instruments to measure f low, pressure,

temperature etc of the ID fan, HC and drive motor.

6. Check the closing and the opening of the suction and discharge gates.

Keep them closed before starting the ID fan.

7. Ensure operation of the scoop tube of the hydraulic coupling from UCB and

keep the scoop at minimum position.

8. By simulation checks confirm all the protections and inter lock functions

properly.

9. Ensure the operation of inlet guide vane from UCB and keep the vane

at minimum position.

10. Ensure the local push button is in the released position.

11. Check that the discharge dampers of blower motors are open and the

modules are kept ON.

12. Check that the flue gas circuit is made through

i. APH outlet flue gas dampers are open (GD-3, GD-4)


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ii. APH inlet flue gas dampers are open (GD1 & GD-2)

iii. ESP inlet gates are open GG5, 6, 7 & 6)

iv. ESP outlet gates are open (GG9, 10, 11 & 12)

13. Check that the seal through is filled with water and overflow is maintained.

14. Start the ID fan after checking the protections when the fan is started after

long shutdown.

START PERMISSIVE FOR INDUCED DRAFT FAN:

Check that the final start permissive is available in the control desk. The

final start permissive will be coming if the following conditions are satisfied.

1. ID fan stopped.

2. Cooling water to the coolers of the hydraulic coupling adequate.

3. Suction gate closed.

4. Discharge gate closed.

5. Control actuator of hydraulic coupling and IGV minimum.

6. Scoop tube outlet temperature normal for oil.

7. DP across twin filter normal for oil.

8. ID fan tripped indication should not exist lockout relay in the 6.6 KV

breaker should be reset.

To start the first ID fan (say ID fan A) both the ID fans A and B should be in

stopped condition and the 220 V DC control supply for the breaker of other

ID fan switched ON, either in test position or in service position.

Now the first ID fan can be started.

POST START CHECKS:

1. Check the opening of gates on interlock, after 10secs.

2. Check the performance of the motor.

3. After raising the scoop position check for smooth operation of the fan.

4. Check the lub oil pressure.

5. Check the starting of both the discharge gate blower motors on interlock. If

not start manually.

6. Maintain the furnace vacuum at

5mm WC by suitable loading of the fan.


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ID FAN TRIP INTERLOCKS:

1. Motor drive end bearing temperature very high : 90C

2. Motor non-drive end bearing temperature very high : 90C

3. ID fan drive end bearing temperature very high : 82C

4. ID fan non-drive end bearing temperature very high : 82C

5. Oil temperature at cooler inlet (for hydraul ic coupling) : 110C very high

6. Differential pressure of oil across twin filter high : 0.3kg/sq.cm.

7. ID fan motor winding temperature high : not connected

8. ID fan running with lub oil pressure low : > 10 sec

9. Electrical protection of motor.

a. Load current IB

b. Locked rotor I stall

c. Standing time TS

d. Phase unbalance Si

e. Earth fault Ic

f. Differential

g. Winding temp

h. Repeated start trip TSV (1/2 hr > 3 times).

STARTING SECOND ID FAN:

1. Before starting the second ID fan the prestart checks and permissive for the

fan shall be verified.

2. One of the FD fans shall be in service before starting the second ID fan.

3. The isolation gates for ESP paths shall be checked for open condition if any

of the path is closed, while starting the first ID fan.

INTERLOCK OPERATIONS:

a. When any one of the ID fan is started.

1. The suction and discharge gates will open after 10 secs.

2. The discharge gates blower motors of both the ID fans shall start.

3. Permissive shall be available for starting any of the FD fans.

b. When any one of the ID fans trip, while both the fans are in service.


32/119


1. The suction and discharge gates of the fan will close

2. The corresponding FD fan will trip.

3. Leaving three mills at the top, other bottom mills will trip.

c. When both ID fans trip.

1. The boiler will trip.

2. Both FD fans will trip.

3. The suction and discharge gates of both the fans will remain open.

4. Blower motors, will continue to run.

FORCED DRAFT FAN:

The main purpose of forced draft fan is to supply the required amount of air

to the furnace at the required pressure for combustion of fuel. There are two

nos FD fans available for supplying the secondary air or combustion air to the

furnace, through secondary air dampers.

DESCRIPTION:

The fresh air drawn from atmosphere passes through a filter,

suct ion chamber, inlet nozzle, inlet guide vanes, suction nozzle and enters the fan

impeller of this axial fan. A large part of the energy transferred to the air as velocity

energy after the impeller is converted into pressure energy in a set of outlet guide

blades and the diffuser. The fan is driven by a constant speed squirrel cage

induction motor. The fan is connected to the motor by means of a pin type flexible

coupling. The airflow is controlled by changing the direction of air entry to the

impeller blade, which is achieved by the inlet guide vane control unit.

The major subassemblies of forced draft fan are as follows:

a. Stator parts

b. Impeller with shaft assembly and bearings.

c. Flow regulating device.

a. STATOR PARTS:

The stator parts comprise of suction chamber, shaft protecting tube and core

of inlet guide vane, impeller housing, diffuser, stabilizer blades and free / slidable

supports.


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The split type suction chamber and diffuser are of welded sheet metal

construction which are placed on suitable supports to accommodate expansion.

The impeller housing is an undivided construction fabricated from sheet

metal. A conical piece is provided in the housing which is supported by a set of

outlet guide blades. The conical blades support the inner bearing assembly.

The blades are designed for the energy conversion from kinetic energy and to

serve as a support for the inner bearing.

The stabilizer is a set of stationary blades, placed between inlet guide

vane unit and impeller. At low loads there is circulation of flow media

between impeller and stabilizer. The maintains the flow in the impeller in an

optimum level so that starving does not occur while delivering the required quantity of

air.

b. IMPELLER WITH SHAFT ASSEMBLY AND BEARINGS:

The impeller hub is of welded sheet metal construction on which the

non-profiled solid twisted blades are welded. The impeller hub is fitted to the

flange welded to the rotor shaft. The fan rotor is supported in between a fixed

bearing (Inner bearing) near the impeller side and a free bearing (Outer

bearing) near the coupling end. These are self-aligning type antifriction grease

lubricated bearings.

c. FLOW REGULATING DEVICE (IGV UNIT):

The inlet guide vane control assembly of the fan consists of a number of

aerofoil vanes fixed to individual shafts. These guide vane shafts are connected to

a regulating ring which is guided to rotating mechanism by a set of rollers and

spring assemblies called suspension assemblies. A control lever is connected

to the regulating ring which can be operated by the external actuator.

PRE START CHECKS:

1. Ensure No line clears are pending.

2. Ensure the electric supplies to the drives and equipment are available.

3. Ensure the air heater flue gas inlet dampers GD1 and GD2 outlet damper GD3,

GD4 and the gas gates GD5 to GD12 before and after ESP are kept open.

4. Ensure the secondary air dampers SAD5 & SAD6 and outlet dampers SAD7

& SAD8 of air heaters A & B are kept open. Atleast one pair should be kept


34/119


open for starting the first FD fan.

5. Ensure the SADs in wind boxes are in open position as per FSSS logic.

6. Do simulation checks to confirm that all protections and inter locks are

functioning properly.

7. The inlet guidance and discharge damper of the FD fan should be checked

for its operation and kept closed before starting the fan.

8. Ensure at least one ID fan is in service to start the first FD fan. Both ID fan

should be in service to start IInd FD fan.

9. Scanner fan shall be in service with its suction chamber open to atmosphere.

10. Ensure bottom ash hopper seal water trough is filled with water and make

up available.

FD FAN START PERMISSIVES

1. Inlet guide vanes in minimum position fully closed.

2. FD fans discharge dampers in closed position.

3. FD fans in stopped condition.

4. FD fan A stopped with FD fan B running and ID fans A & B running (for FD

fan A)

(or)

FD fan A stopped and ID fan A stopped and ID fan B running (for FD fan B)

(or)

FD fan B stopped and ID fan A running and ID fan B stopped (for ID fan A)

5. Bearing temperature of motor or fan are not very high.

When any FD fan is started impulses shall given for.

1. Opening the outlet damper of the corresponding FD fan after the motor reaches

its rated speed. (After 10secs).

2. Connecting the inlet guide vanes control drive of the fan to auto control, if

auto/manual station is selected on auto.

3. Closing the atmospheric suction damper of scanner fan.

TRIPPING OF FD FAN:

(Typical for FD fan A)

a. FD Fan A shall trip on the following conditions:

1. FD fan A bearing temperature very high at 105C (Prior to this, FD fan A


35/119


bearing temperature high will be annunciated in UCB at 95C.

2. FD fan A motor bearing temperature very high at 90C. (Prior to this, FD fan A

motor bearing temperature high will be annunciated in UCB at 85C.)

3. FD fan A motor winding temperature very high (Prior to this, FD fan A motor

winding temperature high will be annunciated in UCB). This is not connected to

trip at present.

4. Both ID fans trip. For any one of the ID fans tripping condition, if both FD fans

are in service one of the FD fans will trip.

5. Vibration level of fan/motor very high (Prior to this, vibration shall be

annunciated in UCB (Not connected to trip at present).


a. Load current IB


c. Starting time TS


e. Earth fault Ic.

f. Differential

g. Winding temp.

7. When both FD fans are off, with FD fan B already in tripped condition

impulse shall be given for.

1. Disconnecting the inlet guide vane control drive from auto control system

(output signal from A/M station).

2. Bringing the inlet guide vanes of FD fan A to the maximum position.

3. Bringing the inlet guide vanes of FD fan B to the maximum position.

4. Opening the outlet damper of FD fan B.

5. To open the atmospheric damper to the scanner air fan.

6. To trip the boiler.

7. The outlet damper of FD fan A shall remain open.

POST START CHECKS:

1. Check the performance of the motor and fan locally. Any abnormal sound or

v ibrat ion shal l be checked up promptly and the fan should be stopped

immediately, if necessary.

2. The fan should be loaded slightly by opening the IGV immediately after the


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discharge damper is opened fully, so that flow is established for the fan.

3. Check the rise in the bearing temperature. Continuously monitor the same,

by taking readings every hour.

4. Check the pressure indicators, flow recorders etc are working satisfactorily

by varying the load on the fan.

5. Before light up of the boiler, it should be ensured that atleast 30% of the full

load airflow is admitted to the boiler. About 150 T/Hr airflow in each of the flow

indicators (L) & (R).

PRIMARY AIR FAN:

Two number of NDZV 20 radial fans are used for supplying the required

primary air for transporting the pulverized coal from the mills to the furnace.

The quantity of cold primary air taken before the APH, and the hot primary air

at the outlet of the APH are controlled in such a manner, that the flow to the

mills is maintained at 55 T/Hr and the coal air temperature at the outlet of the

mills at about 80C. These fans are of double suction single stage centrifugal

type in which the fresh air from the atmosphere enters the impeller axially

and after passing through the impel ler leaves rad ically. A l arge part of the

energy transferred to the air is converted into pressure energy as the air passes

through the impeller. The spiral casing converts part of the kinetic energy

in the ai r to pressure energy. These fans are driven by constant speed squirrel

cage induction motor. The output of the fan is controlled by an inlet guide

vane damper assembly. Each fan is provided with a shut off gate at the

discharge duct. The fan and the motor shafts are coupled with pin type flexiblecoupling.

The spiral casing, suction chamber, rotor shaft with impeller, bearing

assemblies and shaft seals are major sub-assemblies of PA fan.

The spiral casing rests on its foundation and with respect to the rotor during

expansion the spiral casing is suitably guided by the frames to maintain its

orientation. Suction cone provided in the casing helps to achieve accelerated inlet

flow to the impeller. The impeller sealing rings fixed in the suction cone overlaps

the impeller ring maintaining a specific clearance which provides necessary

sealing of the impeller inlet flow from its discharge.


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The rotor assembly consists of impeller and shaft, supported by spherical

bearings located on either side of the impeller. The bearings are lubricated

by separate lub oil system.

Sealing for the shaft where it passes through the spiral casing on both sides

is achieved by labyrinth seals for axial sealing. The labyrinths are housed in a seal

housing which is mounted on the bearing pedestal. The seal housing is

connected to the spiral casing by flexible asbestos cloth for radial sealing.

Lub oil system for cooling and lubrication of fan bearing consists of one

pair of pumps, filters and coolers, one each being standby. The system is fitted

with thermometers, pressure gauges and pressure switches L.O. system can

be operated in remote. Three pressure switches provided in the lub oil header

before bearing are set at 0.8, 0.6 and 0.4 kg/sq.cm. for alarm, auto-change

over and protection of the fan respectively.

PRE-START CHECKS:

1. Ensure that there is no pending work permit in the PA fans, air heaters, PA

ducts, mills, furnace and flue gas ducts.

2. Ensure the manholes are closed.

3. Check that air preheater inlet & outlet dampers for the particular fan side are

open.

4. It is preferable to have the APH in service before taking any PA fan into

service.

5. Ensure that air circuit is made through atleast in one mill to the furnace, by

opening cold air gate and damper and the mill discharge valves.

6. Check the oil level in the lub oil tank.

7. Ensure the cooling water flow through the cooler.

8. Check the availability of the instruments to measure the flow, pressure,

temperature of the PA fan and its lub oil system.

9. Check the opening and closing of the discharge gate. Keep it closed

before starting the PA fan.

10. Ensure that the local push button is in released position.

11. Check the operation of vane control from UCB.

12. By simulation checks confirm all the protections and interlock functions

properly. Especially when the PA fan is started after long shut down, all the


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protections must be checked.

PA FAN LUB OIL SYSTEM:

Line up and taking into service:

1. Ensure oil level in the tank. Level should be between the maximum

and minimum marks in the oil tank gauge glass.

2. Ensure the suction and discharge valves of lub oil pump are kept fully opened.

3. Commission one of the coolers and ensure water flow through it.

4. One of the L.O. filters may be made through.

5. Check for full opening of the needle valve before bearing in L.O. line.

6. The L.O. pump may be started from local or remote.

7. Ensure the power supply and control supply to the control panel.

8. Ensure local push buttons for the pumps are released.

9. Ensure the pressure switches and temperature gauges are intact.

Start one or two L.O. pump manually from control panel putting the

selector switch in manual position and observe the L.O. pressure low annunciation

in the control panel gets reset. Check that there is no oil leak in the system.

After resetting the L.O. pressure low annunciation the selector switch

may be turned to auto.

Now trip the running L.O. pump and observe that stand by pump starts

on auto. The L.O. pump starts on auto when the L.O. pressure drops to 0.6-kg/

sq.cm annunciation will appear in the panel window. Thus any one of the

pumps may be kept in service.

Ensure adequate oil flow to the bearing by seeing the return oil flow in

the view glass.

a. Normal working pressure of lub oil inlet is 0.8 to 1.2 kg/sq.cm.

b. Normal working pressure of cooling water inlet is 3 to 6 bar.

c. Differential pressure across filter should be less than 0.3 kg/sq.cm normally the

DP will be 0.1 kg/sq.cm.

The lub oil pressure can be varied by adjusting the recirculation valve in the oil line.

PA FAN START PERMISSIVES:

1. Primary air fan discharge gate closed.

2. Inlet guide vane control actuator minimum.


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3. Lub oil pressure to the fan bearings adequate, should be more than 0.8kg/sq.cm.

4. No master fuel trip relay is actuated.

5. PA fan 'stopped' condition is established.

6. PA fan tripped indication should not exist lock out relay in the 6.6 KV

breaker should be reset.

7. Seal air fan in service.

8. All the cold air dampers to the mills should be less than 5% open.

9. Both ID fans running.

Any ID fan ON and PA fan A stopped. Then PA fan B can be started.

If all the above permissives are available, start permissive for both PA fans

will be present, if atleast one ID fan is running. So any of the PA fans can be

started, with one ID fan in service. Once one of the PA fans is started the start

permissive for the other fan will vanish. The start permissive for the other fan will

appear, after taking into service the other ID fan.

POST START CHECKS:

1. Note the starting current of the motor.

2. Check the opening of the discharge gate on interlock or open the gate by

switching on from UCB control desk. Approximately 3 min duration for full open

of gate.

3. Immediately after the discharge gate is opened load the fan slightly by opening

the inlet guide vane by about 5 to 10%.

4. Check the fan and the motor for smooth running.

5. Check the bearings for proper lubrication, and monitor regularly the bearing

temperatures.

6. Maintain the PA header pressure at 850mm WC by suitable loading of the

PA fan, for taking mills into service.

PA FAN TRIP INTERLOCKS:

1. Motor drive end bearing temperature very high 90C

2. Motor non drive end bearing temperature very high 90C

3. PA fan drive end bearing temperature very high 82C

4. PA fan non drive end bearing temperature very high 82C


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5. PA fan running with lub oil pressure low (with t ime delay of 2 min)

0.4kg/sq.cm.

6. If boiler master fuel trip relay acts both PA fans will trip.


a. Load current IB


c. Starting time TS


e. Earth fault Ic.

INTERLOCK OPERATIONS:

1. When one PA fan trips, the running mills will trip, leaving top three mills in

service.

2. When both PA fans trip, all the mills will trip on header pressure very low, less

than 490mm WC. Alarm 620mm WC.

3. When the boiler trips, both PA fans will trip on MFT protection.

4. When any one of the PA fans trip, the vane control will come to minimum

and the discharge gate will close automatically.

5. The discharge gates of both PA fans will close, when both PA fans trip.


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Page 41 Thermal Training Institute FUEL FIRING SYSTEM

FUEL FIRING SYSTEM

INTRODUCTION:

The selection of fuel firing equipment for a boiler furnace is governed

by the factors such as required heat input, characteristics and availability of fuel

etc., Most of the utility boilers burn coal as the prime fuel for steam

generation. Fuel oils are also burnt in these furnaces to light use the unit, to

bring the unit up to temperature and pressure up adjacent elevation of coal

nozzle and for stability purposes.

Boiler burning, fuels are either pulverized into fine powder or

finely atomized as the case may be and burned under suspension in boiler

furnace. The functions of fuel firing system are:

1. Preparing and supplying fuel and air.

2. Introducing the correct fuel air mixture into furnace and ignite the same.

3. Creating turbulence to ensure even distribution and high degree.

Of mixing of air with fuel for complete combustion in a short period and also to

sweep away the products of combustion to expose fresh fuel particles for

successive Combustion.

4. Making the furnace to maintain a heat supply to prepare and ignite the

fuel particles for sustaining combustion.

5. Firing the fuel / coal with wide range of quality.

The single drum natural circulation reheat 210MW BHEL boiler has

direct firing tangential burner system for its furnace. The fuel and air from

the fuel preparation plant is being directly fed into the furnace through the

burners housed in the four wind box assemblies located in the furnace corners.

These burners discharge the fuel air mixture tangentially to an

CHAPTER - 4


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imaginary circle at the center of the furnace. The swirling/rotating action produces

sufficient turbulence. In this tangential firing system the furnace itself acts as

a burner ensuring good turbulence and complete combustion in a short period

at a fair ly low flame temperature level.

WIND BOX ASSEMBLIES AND BURNER ARRANGEMENTS

The fuel firing system equipment consists of four-wind box assemblies

located in the furnace corners, one at each corner of the furnaces. The wind

box assembly is a tall structure divided in its height into thirteen compartments

which houses 6Nos coal nozzles and 5 auxiliary air nozzles

alternatively at different elevations from top to bottom. End air nozzles are

placed at the top most and bottom most compartments. The secondary air

admitted in the coal nozzle compartment is called as fuel air, the air

admitted in the other compartments is auxiliary air and end air. The coal nozzle

elevations are designated a A,B,C,D,E,F elevation from bottom to top, the

bottom end air and top end air elevations as AA and FF respectively. The

auxiliary air nozzles in between coal elevations are designated as

elevations AB, BC, CD, DE, EF. The furnace corners are designated as

corner 1,2,3 and 4 in clockwise direction looking form top and counting front

water wall left corner as 1.

The same elevation of the coal nozzles at four corners are fed from a single

coal mill. Thus 6Nos. bowl mills supply pulverized fuel / coal for the coal burners at

different elevations of the wind boxes in the furnace corners. The bowl mills

are named after the elevations to which they supply the pulverized coal.

Each pair of coal nozzle elevations is served by one elevation of oil burners

(4Nos / elevation) located in the auxiliary air nozzle. Thus 16 oil guns, non-

retractable type are accommodated in the four elevations AB, CD, DE and EF.

Heavy fuel oil can be fired at the oil guns of all these four elevations, whereas

light fuel oil (High speed diesel oil) can also be fired at AB elevation. Each oil

gun is associated with ionic flame monitor ignitor, arranged at the side of the

oil gun in the auxiliary air compartment and they directly light up the oil guns.

Flame sensing scanners are installed in flame scanner guide pipe

assemblies in all the 20 auxiliary air compartments. The safe scan II TM is


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used to detect flame from two different sources using one scanner heat. It will

discriminate an individual oil gun flame and a background fireball flame. If

discriminates between the two flames by sensing characteristics (frequency

and intensity) of visible light they emit. As the scanner heads are exposed to

visible light they emit. As the scanner heads are exposed to high temperature of

the furnace, cooling air is supplied to them by means of 2Nos AC scanner fans.

The coal and air nozzles are tiltable by + 01 - 30 degrees about horizontal, in

unison at all elevations and corners. This shifts the flame zone across the

furnace height for the purpose of steam temperature control, especially for

reheat steam temperature.

SECONDARY AIR DAMPER CONTROLS:

The operation of the auxiliary and fuel air dampers in the wind box

assemblies are effected by the FSSS.

AUXILIARY AIR DAMPERS:

During the furnace purge period and initial operations upto 30% boiler

loading all elevations of auxiliary and end air dampers modulate a

predetermined (approx 40mm WC) set point differential pressure between wind box

to furnace 30 to 40% of total air flow to have an air rich furnace will be

supplied during the above period to avoid unhealthy furnace conditions.

a. AUXILIARY AIR DAMPERS:

During the furnace purge period and initial operations upto 30% boiler

loading all elevations of auxiliary and end air dampers modulate a

predetermined (approx 40mm WC) set point differential pressure between wind box

to furnace 30 to 40% of total air flow to have an air rich furnace will be

supplied during the above period to avoid unhealthy furnace conditions.

When the unit load exceeds 30% MCR, the differential pressure set point is

changed and ramps to a higher setting (approx 100mm WC). Simultaneously,

the aux. Air dampers associated with coal or oil elevations not in service

c

Basics of Thermal Power Plant

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