IG/

For restricted Circulation Only

500 MW FAMILIARISATION

Power Management Institute Noida

CONTENTS

S. No. Subject Page No.

1. Salient Features of Boiler 1

2. Boiler Pressure Parts 8

3. Once through Boiler 25

4. Fuel Firing System 35

5. Air/Draught System 60

6. Furnace Safeguard and Supervisory System 80

7. Soot Blowing System 99

8. Data Sheet of 500 MW Boiler and Auxiliaries. 106

9. Salient Features and Constructional details of KWU Steam

Turbine. 113

10. Turbine Oil System 127

11. Turbine Control Fluid System 135

12. Constructional Features of Turbine Governing System and

H.P./ L.P. Bypass System 143

13. Turbine Tripping Devices and Turbine Metal Temp. limit

Curves 158

S. No. Subject Page No.

14. Automatic Turbine RUN UP System-(ATRS) 164

15. Data Acquisitions System (DAS) 177

16. Feed Regenerative System 193

17. Boiler Feed Pump and Condensate Pump 200

18. Data-Sheet of 500 MW Turbine and its Auxiliaries Turbine

Metal Temp. Limit curves. 217

19. Design and Constructional Feature of 176 500 MW Generator 227

20. Excitation System and Auto Voltage Regulator 247

21. Protections of Generator 268

22. Generator Auxiliaries 273

23. Data Sheet of Generator 284

24. Unit Start up and Shut-Down Procedures 285

25. Major Differences between 210 MW and 500 MW Units 332

PMI, NTPC 1

1. Salient Features Of Boiler

BOILER UNITS

The boiler is a radiant reheat, controlled, circulation. Single drum, dry-bottom type unit.

The general arrangement of boiler and its auxiliaries is shown in the Figure no 1. The

boiler units are designed for the following terminal conditions (MCR):

Evaporation a) SH Outlet : 1.725 t/hr

b) RH Outlet : 1.530 t/hr

Working Pressure after stop valve : 178 kg/cm (g)

Steam Temperature at SH

Outlet : 540oC

Steam Temperature at RH inlet : 344.1o C

Steam Temperature at RH Outlet : 540o C

Steam pressure at RH inlet : 42.85 kg/cm (g)

Steam Pressure at RH Outlet : 43.46 kg/cm (g)

Feed water Temperature at ECO : 256o C

Furnace Design Pressure : + 660 mmwc (g)

The boilers are of single furnace design, circulating pumps to provide assisted

circulation.

PMI, NTPC 2

PMI, NTPC 3

Each boiler corner is fitted with tilting tangential burner boxes comprising four high

energy arc igniters, four light-up heavy oil fired burners and eight pulverised coal

burners. The angle of tilt from the horizontal is about-30 to +30.

Feed water to the boiler passes through HP feed heaters into the economiser and then

to the steam drum from where it flows into the suction manifold and furnace wall circuits

via the three boiler circulating pumps, returning to the steam drum as a water/steam

mixture. This mixture is separated in three stages, the first two stages are incorporated

into the turbo separators and the final stage takes place at the top of the drum just

before the steam enters the connecting tubes comprising of first stage superheating,

Within the steam circuit there are a further four stages of (superheating, making five in

total. There are also three stages of reheat.

Superheater temperature control is provided by spray attemperation situated in the

connecting link between the superheater low temp. pendant outlet header and the

superheater division panel inlet headers.

Reheat temperature control is provided by titling burners or spray attemperators

installed prior to the first stage reheater.

PULVERISED COAL SYSTEM

The system for direct firing of pulverised coal utilises bowl mills to pulverise the coal and

a tilling tangential firing system to admit the pulverised coal together with the air

required for combustion (secondary air) to the furnace.

AS crushed coal is fed to each pulveriser by its feeder, primary air is supplied from the

primary air fans which dries the coal as it is being pulverised and transports the

pulverised coal through the coal piping system to the coal nozzles in the wind box

assemblies.

PMI, NTPC 4

The pulverised coal and air discharged from the nozzles is directed towards the center

of the furnace to form firing circle.

Fully preheated secondary air for combustion enters the furnace around the pulverised

coal nozzles and through the auxiliary air compartments directly adjacent to the coal

nozzle compartments. The pulverised coal and air streams entering the furnace are

initially ignited by suitable ignition source at the nozzle exit. Above a predictable

minimum loading condition the ignition becomes self sustaining. Combustion is

completed as the gases spiral up in the furnace.

PRIMARY AIR SYSTEM

The primary air (P.A.) draught plant supplies hot air to the coal mills to dry and convey

pulverised coal to the burners. Cold air ducts, however, are included in the system to

regulate mill temperatures and seat mill components against any ingress of coal dust.

The P.A. system comprises two P.A. fans, two Steam Coil Air Preheater (SCAPH) and

two regenerative type primary air preheaters. Each fan, which is of sufficient rating to

support 60% MCR load, discharges through a SCAPH into a common bus duct that has

four outlets, two directing air into the primary air preheater for heating, two direct cold air

straight to the pulverising mills. On the other side of the primary air preheaters, the

outlet ducts combine to form a hot air crossover duct which outlets to the mills at the

L.H.S. and R.H.S. of the boiler furnace, This arrangement of bus duct and cross over

duct ensures continued plant operation even if one fan and/or one primary air preheater

is out of service. The SCAPHs located in the fan discharge ducts, ensure that the

primary air preheaters combined cold end temperature (gas leaving temperature plus air

entering temperature) does not fall below the specified minimum to avoid "Cold End

Corrosion'.

Seal air fans boost up the primary air pressure and are provided for supplying sealing

air to each mill to maintain sufficient differential between primary air and seal air thereby

PMI, NTPC 5

safeguarding the lub oil from being contaminated by coal dust.

SECONDARY AIR SYSTEM

The secondary air draught plant supplies the balance of air required for pulverised coal

combustion, air for fuel oil combustion, and overfire air to minimise the production of

nitrous oxides(NOx).

The Secondary air system comprises two forced draft (F.D.) fans, two steam coil air

preheaters (SCAPH) and two regenerative type secondary air preheaters. Each fan.

which is of sufficient rating to support 60% boiler MCR load, discharges through a

SCAPH into a common bus duct that has two outlets each directing air through a

secondary air preheater. Hot air from secondary air preheaters is sent to wind boxes at

each side of the boiler furnace for proper combustion as secondary and overfire air.

Overfire air can be admitted to the furnace through the upper levels of furnace wind

boxe nozzles to assist in reducing the amount of NOx formed in the furnace. Control of

unit air flow is obtained by positioning the FD fans blades while the distribution of

secondary air from wind box compartment to furnace is controlled by secondary air

dampers.

The SCAPHs are located in the FD fan discharge ducts to ensure that the secondary

airpreheaters combined cold end temperature (gas leaving temperatures plus air

entering temperature) does not fall below the specified minimum to protect against cold

end corrosion.

FLUE GAS HANDLING SYSTEM

The flue gas handling plant draws hot flue gases from the furnace and discharges them

to atmosphere through the chimney. During its passage to the chimney, flue gas is

passed through a feed water economiser and four regenerative airpreheaters to

PMI, NTPC 6

improve boiler efficiency, and through four electrostatic precipitators to keep dust

emission from chimney within prescribed limits.

Flue gases travel upward in the furnace and downward through the rear gas pass to the

boiler outlet (boiler rear gas pass below the economiser). It then passes through the

primary and secondary air preheaters, the electrostatic precipitators and induced

draught (I.D.) fans to the chimney. Since primary and secondary streams are provided

with separate bisector regenerative air heaters, control dampers at the outlet of the air

preheaters are provided to regulate the gas flow through these streams to get same gas

outlet temperature.

Three I.D. Fans, each of which is of sufficient rating to support 60% boiler MCR load,

are served by a common inlet bus duct to ensure that plant operation continues even

when two fans are out of service. During normal usage, two ID fans will be operational

and one available as standby.

SOOT BLOWING SYSTEM

On load, gas side cleaning of boiler tubes and regenerative air heaters is achieved

using 126 electronically controlled soot blowers which are disposed around the plant as

follows.

88 - Furnace Wall Blower - Steam

34 - Long Retractable Soot Blower - Steam

4 - Air heater Soot Blowers for - Steam

Primary and Secondary Air

heaters

The boiler water wall panels are provided with suitable wall boxes for future

accommodation of an extra sixteen furnace wall blowers and twenty-four long-

retractable soot blowers for upper furnace, arch and rear pass zone, if necessary.

PMI, NTPC 7

Steam for soot blowing is taken from division panels superheater outlet header. Steam

is then passed through a pressure control valves where the steam pressure is reduced

to the required limit of soot blowing. However, to soot blow the regenerative air

preheaters during boiler start up, a separate connection is also provided from the

auxiliary Steam System.

FURNACE SAFEGUARD SUPERVISORY SYSTEM

The Furnace Safeguard Supervisory System (F.S.S.S) is a major component of station

safety monitoring equipment. It permits the remote (Control Equipment Room) and

partly local (adjacent to the boiler) light-up and shut down of all oil burners and igniters

together with continuous monitoring, fault detection and associated shut down of any or

all burners upon fault disclosure.

The system also incorporates the logic sequences required for enforcing proper purging

of the steam generator and for tripping the master fuel relay system.

The pulverised coal burners and their associated mills are controlled by a separate mill

control sequencing system which is provided with essential information regarding milling

plant status from loc. instrumentation as well as start and run permissives for each mill

system from the F.S.S.S.

Both systems integrate with the Analogue Control System (A.C.S.) to provide full on-line

firing safety, optimum operational control and in-depth system awareness.

PMI, NTPC 8

2. Boiler Pressure Parts INTRODUCTION

The boiler units are of the balanced draught single drum radiant furnace type that

include an arch between the furnace and the rear gas pass. The water circuit is of

controlled circulation design incorporating boiler circulating pumps in unheated down

comers at the front of the boiler and utilising refill bore tubing in sections of the furnace

wall panels. Boiler units of 500 MW units are identical in design and comprise a single

furnace, three superheater stages, three reheater stages and a feed water economiser,

BARE TUBE ECONOMISER

The Function of the economiser is to preheat the boiler feed water before it is

introduced into the Steam drum by recovering some of the heat of the flue gas leaving

the boiler.

The economiser is located in the boiler back pass. It is composed of two banks of 156

parallel tube elements arranged in horizontal rows in such a manner that each row is in

line with the row above and below. All tube circuits originate from the inlet header and

terminate at outlet headers which are connected with the economiser outlet headers

through three rows of hanger tubes.

Feed water is supplied to the economiser inlet head via feed stop and check valves.

The feed water flow is upward through the economiser, that is, counter flow to the hot

flue gases. Most efficient heat transfer is, thereby, accomplished, while the possibility of

steam generation within the economiser is minimised by the upward water flow. From

the outlet header the feed water is lead to the steam drum through the economiser

outlet links.

PMI, NTPC 9

The economiser recirculalting line, which connects the economiser inlet header with the

furnace lower rear drum, provide a means-of ensuring a water flow through the

economiser during startups. This helps prevent steaming. The valves in these lines

must be open during unit startup until continuous feed water flow is established.

WATER COOLED FURNACE Welded Wall Construction

The furnace walls are composed of tubes. The space between the tubes are fusion

welded to form a complete gas tight seal. Some of the tube ends are swaged to a

smaller diameter while other tubes are bifurcated where they are welded to the outlet

headers and lower drum nipples.

The furnace arch is composed of fusion welded tubes.

The back pass walls and roof are composed of, fin welded tubes.

The furnace extended side walls are composed of fin welded tubes.

The back pass front (furnace) roof is composed of tubes peg fin welded.

All peg finned tubes are normally backed with a plastic refractory and skin casing which

is seal welded to form a gas tight envelope.

Where tubes are spread out to permit passage of superheater elements, hanger tubes,

observation ports, soot blowers, etc., the spaces between the tubes and openings are

closed with fin material so a completely metallic surface is exposed to the hot furnace

gases.

PMI, NTPC 10

Poured insulation is used at each horizontal buckstay to form a continuous band around

the furnace thereby preventing flue action of gases between the casing and water walls.

Bottom Construction

Bottom designs used in these coal fired units are of the open hopper type, often

referred to as the dry bottom type. In this type of bottom construction two furnace water

walls, the front and rear walls, slope down toward the centre of the furnace to form the

inclined sides of the bottom. Ash and/or slag from the furnace is discharged through the

bottom opening into an ash hopper directly below it. A seal is used between the furnace

and hopper to prevent ambient air being drawn into the furnace and disturbing

combustion fuel/air ratios. The seal is effected by dipping seal plates, which are

attached around the bottom opening of boiler furnace, into a water trough around the

top of the ash hopper. The depth of the trough and seal plates will accommodate

maximum downward expansion of the boiler (predicated 320.3 mms).

WATER AND SATURATED STEAM CIRCUITS

In a controlled circulation Boiler, circulating pumps placed in the downcomer circuits

ensure proper circulation of water through the waterwalls. Orifices installed in the inlet of

each water circuit maintain an appropriate flow of water through the circuit. Feed water

enters the unit through the economizer elements and is mixed with boiler water in the

steam drum. Water flows from the drum through the downcomers to the pumps suction

manifold. The boiler circulating pumps take water from the suction manifold and

discharge it. via the pump discharge lines, into the furnace lower front inlet header.

Furnace lower waterwall right and left side headers assure proper distribution to the rear

header.

In the waterwall inlet headers the boiler water passes through strainers and then

through orifices which feed the furnace wall tubes, the economiser recirculating lines.

PMI, NTPC 11

The water rises through furnace wall tubes where it absorbs heat. The front wall tubes,

rear tubes, rear wall hanger tubes, rear arch tubes, rear screen tubes, extended side

wall tubes and side wall tubes from parallel flow paths.

The resulting mixture of water and steam collects in the waterwall outlet headers and is

discharged into the steam drum through the riser tubes. In the steam drum the steam

and water are separated, the steam goes to the superheater, and the water is returned

to the water side of the steam drum to be recirculated.

BOILER CIRCULATION SYSTEM

Boiler water circulates from the steam drum into unheated down comer pipes, then from

the down comers into heated furnace wall tubes back into the drum. The furnace walls

will absorb radiant heat from the furnace and then discharge a saturated steam /water

mixture into the drum. Inside the drum, saturated steam is separated from the water,

then directed into superheater tubing for further temperature increase. Water separates

from the steam will combine with incoming boiler feed water, then re-enter the down

comers to repeat the cycle.

PMI, NTPC 12

Fig No 2. FLOW PATH IN DRUM

The boilers are designed with a controlled circulation system which incorporates boiler

water circulation pumps, smooth and rifled bore furnace wall tubing, and orifice plates

at the inlet to furnace wall tubing.

Water flows from the bottom of the steam drum via six large bore downcomers into a

suction manifold common to three parallel mounted boiler water circulation pumps. The

manifold has connections at both ends to the chemical clean pipework, and at three

points along its length to feed individual circulation pump suctions. Water will flow from

the pumps through two discharge pipes into the front leg of the water wall inlet headers

at the bottom of the furnace. Each discharge pipe is fitted with a circulating pump

Discharge Stop/Check Valves which are controlled via sequence equipment to open

PMI, NTPC 13

and close as the pump is taken in and out of service. If, however all three pumps are

out of service all of the valves will open to enable thermosyphonic circulation to take

place. Initiating any pump to restart will cause them all to close again then continue with

the in and out of service regime. Controls for the pumps are located in the U.C.B. and

comprise a SEQUENCE pushbutton, ammeter and a DUTY/ STANDBY selector. Pump

status is indicated on RUN/STOP lamps on Panel. The operating regime for the boiler

water circulation pumps is two duty/one standby.

From the Waterwall inlet headers, water travels upward through furnace wall tubing via

furnace upper front rear and side headers into riser tubes which direct a saturated

steam/water mixture into the steam drum. Furnace wall tubing is manufactured from a

combination of both smooth and rifled bore tubing which permits the use of lower tube

flow rate whilst still retaining full tube protection. The required distribution of water to

give the correct flow rates through the various furnace wall circuits is achieved and

maintained by the use of suitably sized orifices installed inside the water wall inlet

headers at the inlet to each furnace wall tube. Orifice size varies for different circuits or

groups of circuits depending on the circuits length, arrangement and heat absorption.

Perforated panel strainers are also located inside the water wall inlet headers to prevent

the orifices blocking and to ensure an even distribution of water around the other inlet

headers. Refer to fig no.2.

The saturated steam/water mixture enter the steam drum on both sides behind a water

tight inner plate baffle which directs the mixture around the inside surface of the drum to

provide uniform heating of the drum shell. This eliminates thermal stresses from

temperature differences through the thick wall of the drum, between the submerged and

unsubmerged portions. Having travelled around this baffle the mixture enters two rows

of steam separators where a spin is imparted. This forces the water to enter the outer

edge of the separator where it is separated from the steam. Nearly dried, the steam

leaves the separators and passes through four rows of corrugated plate baskets where

by low velocity surface contact, the remaining moisture is removed by wetting action on

the plates. From the baskets, steam flows out of the drum into superheater pipework.

PMI, NTPC 14

Water which separates from the saturated steam

drains back to combine with incoming boiler feed

water from the economiser then re-enters the

downcomers to repeat the cycle.

Boiler Water Circulation Pumps

Each boiler-Water Circulation pump consists of a

single stage centrifugal pump on a wet stator

induction motor mounted within a common

pressure vessel. The vessel consists of three

main parts, a pump casing, motor housing and

motor cover as shown in Fig No. 3.

The motor is suspended beneath the pump

casing and is filled with boiler water at full system

pressure. No seal exists between the pump and

motor, but provisions is made to thermally isolate

the pump from the motor in the following respect:

a) Thermal Conduction: To minimize heat conduction a simple restriction in

the form of thermal neck is provided

b) Hot Water Diffusion: To minimize diffusion of boiler water, a narrow

annulus surrounds the rotor shaft, between the hot

and cold regions. A baffle ring restricts solids entering

the annulus

c) Motor Cooling: The motor cavity is maintained at a low temperature

by a heat exchanger and a closed loop water

circulation system, thus extracting the heat conducted

from the pump.

PMI, NTPC 15

In addition this water circulates through the stator and bearing extracting the heat

generated in the windings and also provides bearing lubrication. An internal filter

is incorporated in the circulation system.

d) In emergency conditions if low-pressure coolant to the heat exchanger fails, or is

inadequate to cope with heat flow from the pump case, a cold purge can be

applied to the bottom of the motor to limit the temperature rise

The pump comprises a single suction and dual discharge branch casing. The case is

welded into the boiler system pipe work at the suction and discharge branches with the

suction upper most.

Within the pump cavity rotates a key driven, fully shrouded, mixed flow type impeller,

mounted on the end of the extended -motor shaft. Renewable wear rings are fitted to

both the impellers and pump case. The impeller wear ring is the harder component to

prevent galling.

The motor is a squirrel cage, wet stator. Induction motor, the stator, wound with a

special water-tight insulated cable. The phase joints and lead connections are also

moulded in an insulated material. The motor is joined to the pump casing by a pressure-

tight flange joint and a motor cover completes the pressure tight shell.

The motor shell contains all the moving parts, except for the impeller. Below the impeller

is situated an integral heat baffle which reduces the heat flow, a combination of

convection and conduction, down the unit. A baffle wear ring cum-sleeve above the

baffle forms a labyrinth with the underside of the impeller to limit sediment penetration

into the motor. Should foreign matter manage to pass the labyrinth device into the motor

enclosure, it is strained out by a filter located at the base of the cover-end bearing

housing.

PMI, NTPC 16

PMI, NTPC 17

The motor design is such that for ease of maintenance, the stator shell, complete with

the stator pack, the rotor assembly, can be withdrawn from the motor in sequence, after

removal of the motor from the pump case. Removal lifting lugs are supplied for

attachment to permanent lugs on the side of the motor case for securing hoists for the

raising and lowering of the motor.

SUPERHEATER AND REHEATER

The arrangement, tube size and spacing of the Superheater and Reheater elements are

shown in the Figure No. 4.

Superheaters

The superheater is composed of three basic stages of sections; a finishing Pendant

section (34), a Division Panel Section (30) and a Low Temperature Section including

LTSH (23), the Backpass Wall and Roof Sections (12)(13)(14)(19)(21)(17)(7)(8).

The finishing Section (34) is located in the horizontal gas path above the furnace rear

arch tubes.

The Division panel Section (30) is located in the furnace between the front wall and the

Pendant Platen Section. It consists of six front and six rear panel.

The Low Temperature Section (23) and (24) are located in the furnace rear backpass

above the Economiser Section.

The Backpass wall and Roof Section forms the side (7) (8) front (12) and rear (19) walls

and roof (14) of the vertical gas pass.

PMI, NTPC 18

Reheater

The reheater is composed of 3 stages or sections, the Finishing Section (46) the Front

Platen Section (47) and the Radiant Wall Section (40)(41).

The Finishing Section (46) is located above the furnace arch between the furnace

screen tubes and the Superheater Finish (34).

The Reheater Front and side Radiant Wall (40) & (41) is composed of tangent tubes on

the furnace width.

Steam Flow

The course taken by steam from the steam drum to the superheater finishing outlet

header can be seen in Fig. No. 4. The elements, which make up the flow path, are

essentially numbered consecutively. Where parallel paths exist, first one and then the

other circuit is numbered. The main steam flow is:

Steam drum - SH connecting tubes (1)- Radiant roof inlet header (2) - First pass roof

front (3) - Rear (4) Radiant tube outlet header (5)-SH SCW inlet header side (G)-

Backpass sidewall tubes (7) & (8)-Backpass bottom headers (9), (10) & (11)- backpass

Front, and rear (12) (21)-Backpass screen (13) Backpass roof (14)-Backpass SH &

Eco.. supports(15) SH & Eco support headers(16)-LTSH support tubes (17)-SH Rear

Roof tubes (18)-SHSC Rear wall tubes (19)-LTSH inlet header (22)-LTSH banks

(23)(24)-LTSH outlet header(25)-SH DESH link (26). SH DESH (27)-Division panel (30)-

Division panel (30)-Division panel outlet header (31)-SH Pendent assembly (34)-SH

outlet header (35).

After passing through the high pressure stages of the turbine, steam is returned to the

PMI, NTPC 19

reheater via the cold reheat lines. The reheater desuperheaters are located in the cold

reheat lines. The reheat flow is.

Reheater radiant wall inlet header (38) (39)- radiant wall tubes (40) (41) reheater

assemblies (46) (47)-reheater outlet header (48)-Reheater load (49).

After being reheated to the design temperature, the reheated steam is returned to the

intermediate pressure section of the turbine via the hot reheat line.

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 when starting up and when 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. Reheater drains and vents provide means to boil off residual water in the

reheater elements during initial firing of the boiler.

Safety valves on the superheater main steam lines set below the low set drum safety

valve provide another means of protection by assuring adequate flow through the

superheater if the steam demand should suddenly and unexpectedly drop Reheater

safety valves, located on the hot and cold reheat piping serve to protect the reheater if

steam flow through the reheater is suddenly interrupted.

A power control valve on the superheater main steam line set below the low ser super

heater safety valve is provided as a working valve to give an initial indication of

excessive steam pressure. This valve is equipped with a shut off valve to permit

isolation for maintenance. The relieving capacity of the Power Control Valve is not

included in the total relieving capacity of the safety valves required by the Boiler Code.

During all start-ups, care must be taken not to overheat the superheater or reheater

PMI, NTPC 20

elements. The firing rate must be controlled to keep the furnace exit gas temperature

from exceeding 540 C. A thermocouple probe normally located the upper furnace side

wall should be used to measure the furnace exit gas temperatures.

NOTE

1. Gas temperature measurements will be accurate only if a shielded, aspirated

probe is used. If the probe consists of a simple bare thermocouple, there will be

an error, due to radiation, resulting in a low temperature indication. At 588 C

actual gas temperature, the thermocouple reading will be approximately 10

degrees low. Unless very careful traverses are made to locate the point of

maximum temperature, it is advisable to allow another 10 degrees tolerance,

regardless what type of thermocoupie probe is used.

2. The 540 C gas temperature limitation is based on normal start-up conditions,

when steam is admitted to the turbine at the minimum allowable pressure

prescribed by the turbine manufacturer. Should turbine rolling be delayed and

the steam pressure to permitted to build up the gas temperature limitation should

be reduced to 510 C when the steam pressure exceeds two-thirds of the design

pressure before steam flow through the turbine is established.

Thermocouples are installed on various Superheaters and Reheater terminal

tubes, above the furnace roof, serve to give a continuous indication of element

metal temperatures during start-ups (Superheater) and when the unit is carrying

load (Superheater and Reheater). In addition to the permanent thermocouples,

on some units temporary thermocouples provide supplementary means of

establishing temperature characteristics during initial operation.

Steam temperature control for Superheater and Reheater outlet is provided by

means of windbox nozzle tilts and desuperheaters.

PMI, NTPC 21

DESUPERHEATERS

General

Desuperheaters are provided in the superheater connecting link and the reheater inlet

leads to permit reduction of steam temperature when necessary and to maintain the

temperatures at design values within the limits of the nozzle capacity. Temperature

reduction is accomplished by spraying water into the path of the steam through a nozzle

at the entering end of the desuperheater. The spray water comes from the boiler feed

water system. It is essential that the spray water be chemically pure and free of

suspended and dissolved solids, containing only approved volatile organic treatment

material, in order to prevent chemical deposition in the desuperheaters and reheater

and carry-over of solids to the turbine.

CAUTION

During start-up of the unit. if desuperheating is used to match the outlet steam

temperature to the turbine metal temperatures, care must be exercised so as not to

spray down below a minimum of 10 above the saturation temperature at the existing

operating pressure. Desuperheating spray is not particularly effective at the low steam

flows of start-up. Spray water may not be completely evaporated but be carried through

the heat adsorbing sections to the turbine where it can be the source of considerable

damage. During start-up, alternate methods of steam temperature control should be

considered.

The location of the desuperheater helps to ensure against water carry-over to the

turbine. It also eliminates the necessity for high temperature resisting materials in the

desuperheaters construction.

PMI, NTPC 22

Superneater Desuperheaters

Two spray desuperheaters are installed in the connecting link between the superheater

low temperature pendant outlet header and the superheater division panel inlet

headers.

Reheater Desuperheaters

Two spray type desuperheaters are installed in the reheater inlet leads near the

reheater radiant wall front inlet header.

STEAM DRUM INTERNALS

The function of the steam drum internals is to separate the water from the steam

generated in the furnace walls and to reduce the dissolved solids contents of the steam

to below the prescribed limit. Separation is generally performed in three stages, the first

two stages are incorporated into the turbo-separators, the final stage takes place at the

top of the drum just before the steam enters the connecting tubes.

The steam-water mixture entering the top of the drum from the furnace riser tubes as

shown in Fig No. 5 sweeps down along both sides of the drum through the narrow

annulus formed by a baffle extending over the length of the drum. The baffle is

concentric with the drum shell and effects adequate velocity and uniform heat transfer,

thereby maintaining the entire drum surface at a uniform temperature. At the lower end

of the baffle, the steam-water mixture is forced upward through two rows of turbo

separators.

Each turbo separator consists of a primary stage and a secondary stage. The primary

stage is formed by two concentric cans. Spinner blades impart a centrifugal motion to

PMI, NTPC 23

the mixture of steam and water flowing upward through the inner can, thereby throwing

the water to the outside and forcing the steam to the inside. The water is arrested by a

skim-off lip above the spinner blades and returned to the lower part of the drum through

the annulus between the two cans. The steam proceeds up to the secondary separator

stage.

The secondary stage consists of two opposed banks of closely spaced thin, corrugated

matel plates which direct the steam through tortuous path and force entrained water

against the corrugated plates. Since the velocity is relatively low, this water does not get

picked up again, but runs down the plates and off the second stage lips at the two

steam outlets.

From the secondary separators, the steam flow is upward to the third and final stage of

separators. These consists of rows of corrugated plate dryers extending the length of

the drum with a drain through between me rows. The steam flows with relatively low

velocity through the tortuous path formed by the closely spaced layers of corrugated

plates, the remaining entertained water is deposited on the corrugated plates, the water

is not picked up again but runs down the plates into the drain through Suitably located

drain pipes return this water to the water side of the drum.

PMI, NTPC 24

STEAM DRUM INTERNALS ARRANGEMENT

Figure No. 5

RECOMMENDED

OPERATING RANGE

PMI, NTPC 25

3. Once Through Boiler GENERAL PRINCIPLE

In a drum boiler, the flow rate of water passing through the steam generating tube walls

is different from the flow rate of steam produced. It passes through a loop consisting of

the drum, the downcomers and the steam generating tube walls, and it is far greater

than the flow rate of steam produced because of the recirculating of the circuit: In the

drum type circulation: the recirculating coefficient is high (generally) between 3 and 10):

the tubes and piping have relatively large diameters and the water velocities are low.

The drum constitutes a fixed point the thermodynamically speaking, the steam

generating tube walls are at the saturation temperature and all the steam superheat

must be performed in the heat exchangers independent of the steam generating tube

walls.

In a once through boiler, the steam generating tube walls are in series with the

economiser and the superheaters, and the same flow rate of water passes successively

through the economiser, the evaporator and the super heaters.

In the once through circulation; there is no recirculating, the tubes are of small diameter

and the water velocities are high.

Different Types Of Once Through Boilers

There are three types of once through boilers as shown in Fig. No. 6 and the difference

between them lies in the principle of circulation in the evaporator. Let us examine them

successively.

In the first type. the same flow rate of water passes through the economiser, the

evaporator and the superheaters at normal ratings, but at low loads a minimum flow of

PMI, NTPC 26

water is maintained constant in the economiser and evaporators by the use of

recirculalting pumps installed beneath the separator.

In the second type we have the same operating principle, but the minimum water flow

rate of the low ratings is obtained by the use of the feed water pumps themselves; the

non-vaporized excess water, i.e. the drains from the separator being sent to the

deaerator via a heat exchanger called the "Starting exchanger".

In the third type, the circulation in the evaporator is performed at all loads by the boiler

circulating pumps which are installed at the evaporator inlet after the water coming from

the economiser has been mixed with the saturated water from the separator.

Thus, in the evaporators of these three types of boilers, the proportion of steam in the

emulsion is very high (up to 100% for the first and second types, up to 80-90% for the

PMI, NTPC 27

third type) and it is impossible to aviod calefaction or D.N.B (departure from nucleate

boiling). We have to live with this, and it is therefore necessary to have a large speed

per unit mass in the evaporator tubes (3.50 m/sec. for the water at the inlet) and

consequently a high-pressure drop in this apparatus (15 to 20 bars).

Furthermore, the diagrams of the first two types eliminate the (thermodynamically

speaking) fixed point created by the drum. Which makes it possible to carry out the start

of superheat in the final part of the steam generating tube walls. The superheaters are

reduced in consequence, and this has a very beneficial effect on the cost of the boiler,

especially in high-pressure cycles where the evaporation part is reduced and the

superheat part amply increased due to the rapid decrease in the enthalpy of the

saturated steam for pressures greater than 140 bars.

Design Of Once Through Boiler

Let us examine, especially with regard to operation with low loads and during startups.

The Figure No. 7 represents the boiler circulation system during these special types of

operation.

At full load, the final part of the evaporator corresponds to a first superheat stage with a

final temperature of 395, i.e. about 30C above the saturation temperature. Between 35

and 100% of load, the separator plays no role at all since only dry steam passes

through it.

The separator operates as an emulsion separator only for ratings where the steam

flowrate is below 35%. as the water flowrate passing through the evaporator is then

maintained constant, and the difference between the feedwater flowrate and the

flowrate of steam produced must be recirculated via the feedwater pumps (which then

function as controlled circulation boiler pumps).

PMI, NTPC 28

Fig No. 7

FLOW DIAGRAM THROUGH STEAM GENERATOR The starting heat exchanger is sized so as to obtain a water temperature the inlet of the

feedwater tank which is very close to-the saturation temperature present there

(difference of about 12C.

The figure shows the control valves designated as AA, AN and ANB:

Valve AA is sized solely for cold start-ups and low-pressure start-ups; it is closed and blocked in the off position at a pressure of 60 bars.

Valves AN and ANB are sized for high- and medium-pressure Start-ups and for low ratings (below 35% of full load).

ANB is sized so that it can cope alone with the low ratings between 35 and 11 % (technical minimum with fuel oil alone):

AN is used for all start-ups (cold and hot) so as to send to the condenser the flowrate of water coming from the expansion of the evaporator water at the start

PMI, NTPC 29

of evaporation: this flowrate is high for 5 to 6 minutes.

For this same reason AA is used for cold start-ups. AN is also used for cleaning the boiler circuit water in the condensate treatment

station at start-ups and at low loads, when the quality of the feedwater is

inadequate.

This type of one through circulation has the following advantages:

Elimination of the boiler circulating pumps, which tends to reduce the maintenance expenses.

No risk of leakage into the starting heat exchanger which operates with a very slight pressure difference between the two circuits (25 bars maximum).

The terminal part of the evaporator is in fact a superheater, which represents an economical solution since it avoids covering the upper part of the tube walls of a

superheater or a wall reheater (bringing a very expensive double

PMI, NTPC 30

Operation On Sliding Pressure

One of the characteristic features of the once-through boiler is its small storage

capacity, which permits sliding pressure operation (i.e. with constant opening of the

turbine inlet safety valves) by maintaining possibilities of sufficiently rapid load

variations. In addition, this characteristic is further improved by limiting the opening of

the turbine inlet safety valves to 90%.

Sliding pressure operation has the following advantages:

The turbine operating conditions are better since the HP body operates in the same way as the IP or LP body. with much lower mechanical stresses:

At partial loads, the mechanical stresses in the boiler are also lower since the pressure is lower:

At partial loads, the steam consumption per kW of the turbine is slightly smaller. It is easy to obtain the reheated steam temperature a the intermediate loads

because of the virtually constant steam temperature at the HP body outlet:

It is easy to obtain the nominal superheated steam temperature at the intermediate loads because of the lower service pressure.

START UP

One of the main advantages of once through boiler lies in the possibility of performing

rapid and frequent start-ups and rapid load variations. This is particularly useful for

disturbed or small electric networks.

The start-up of a once through boiler can be much more rapid than that of a natural or

controlled circulation boiler because of the elimination of the large, thick drum which are

the origin of unacceptable heat stresses during the rapid pressure variations which

occur when rapid start-ups are desired.

PMI, NTPC 31

In the once through boilers, the pressure parts have been sized so that the thicknesses

are at all points less than 70 mm. The temperature gradients can then rise by up to 5 C

per minute for a total of 5000 cycles.

Moreover, these requirements for rapid start-ups have led the boiler constructor to

review certain more traditional parts of the equipment.

Boiler Supports

To withstand a temperature gradiant of 5C per minute during the planned 5000 cycles,

the support of the furnace consist of a large number of vertical flats of small size fixed

against the tubes of the steam generating tube walls by a large number of welded flats

so as to improve contact between the vertical flats and the tubes and to permit the

vertical flats to follow without delay the temperature of the tubes. Laboratory fatigue

tests have confirmed the appropriateness of the design chosen.

Steam Piping

Large thick nesses must be avioded by paying attention either to the number of pipes in

parallel or to the quality of the steel.

From the economical point of view, the ideal arrangement would be to have a single

connecting pipe between the boiler and the turbine. But in a 500 MW unit, this would

lead to thicknesses greater than 80 mm, which is not acceptable.

We therefore installed two pipes and we determined a trace which gives the assurance

of having the some temperature in both of these pipes in all cases of operation.

PMI, NTPC 32

Hp By-Pass

During hot start-ups, the first steam supplied by the boiler is at a temperature much

lower than that of the pipes.

To avoid problems of heat stress in the pipes, the HP by-pass must be installed close to

the boiler and before the piping connecting pieces.

Start-Up Times

The start-up times which are summarized in the table below can thus be achieved with

this type of boiler.

Time after first lgnition First coupling Full load

Cold start-up 50 min 3 to 4 hours

After 36 hours week-end shut-down 30 min 1 to 2 hours

After 8 hours overnight shut-down 25 min 45 to 75 minutes

CONSTRUCTION OF BOILER AND HEAT EXCHANGERS

With regard to boiler construction, the major differences between natural or controlled

circulation boiler and once through boiler lie in the spiral design of the tube walls.

The spiral tube zone between the end of the ash pit hopper and the start of the superheater zone : it consists of 404 tubes of 33.7 mm diameter which each

encircle the furnace twice. So that there is a good homogeneity of the steam

temperature at the outlet of these tubes. From this point of view, the temperature

homogeneity thus achieved is certainly better than that which would be obtained

with vertical tubes which take account of the differences in heat flow at the

PMI, NTPC 33

various points of the furnace; homogeneity of the steam temperatures is esential

with once through boiler when the steam is superheated at the furnace outlet.

two parts consisting of vertical tubes - one in the lower part of the furnace and the other in the upper part of the boiler - to form the box around the

superheaters, reheater and economiser.

The number of vertical tubes is three times the number of spiral tubes. The junction is

performed with forged and drilled pieces forming a three-legged pipe. as shown in the figure No, 8.

The arrangement of the furnace supports consists of:

- The system of structures which maintain the furnace tube walls and the fixation

of these structures onto the comer pieces by rods permitting horizontal

expansion.

- The system of hangers for the spiral tube walls consisting of vertical flats which

PMI, NTPC 34

remain at the same temperature as the tubes.

The vertical tubes in the upper part of the boiler are designed to support the weight of

the spiral parts of the tube walls via junction pieces.

Generally speaking, the superheaters and reheaters are sized in the same way as in a

drum boiler.

Nevertheless, there is a slight difference for the superheaters which are calculated here

with an additional steam temperature margin of 20 degree to take account of the

regulation of the steam temperature at the separator outlet in the feedwater control

system.

CONCLUSION

The once through system and the type of operation of the control systems and the

precautions taken in tie construction of the boiler enable high temperature gradients to

be achieved in full safety for a large number of cycles.

Finally this type of boiler, which operates with a high-characteristic water-steam cycle,

thus enables peak electricity to be produced with low fuel consumption.

PMI, NTPC 35

4. Fuel Firing System INTRODUCTION

This chapter relates to the fuel (oil & Coal) systems and fuel/combustion equipment

under supply of BHEL for 500 MW boiler.

Fuel Oil System

The Fuel Oil System prepares any of the two designated fuel oil for use in oil burners

(16 per boiler, 4 per elevation) to establish initial boiler light up of the main fuel

(pulverized Coal) and far sustaining boiler low load requirements up to 15% MCR load.

To achieve this, the system incorporate fuel oil pumps, oil heaters, filters, steam tracing

lines which together ensure that the fuel oil in progressively filtered, raised in

temperature, raised in pressure and delivered to the oil burners at the requisite

atomising viscosity for optimum combustion efficiency in the furnace.

Coal System

The coal system prepares the main fuel (pulverised coal) for main boiler furnace firing.

To achieve this the raw coal from overhead hopper is fed through pressurised coal

valve, SECOAL nuclear monitor, gravimetric feeder and into mills where it in crushed

and reduced to a pulverised state for optimum combustion efficiency. The pulverised

coal is mixed with a primary air flow which carries the coal air mixture with a primary air

flow which carries the Coal Air mixture to each of 4 corners of the furnace burner

nozzles and into furnace.

Burner Nozzles

PMI, NTPC 36

Both the oil and coal burner nozzles fire at a tangent to an imaginary circle at the

furnace centre. The turbulent swirling action this produces, promotes the necessary

mixing of the fuels and air to ensure complete combustion of the fuel. A vertical tilt

facility of the burner nozzles, which in controlled by the automatic control system of

boiler ensures a constant reheat outlet steam temperature at varying boiler loads.

TILTING TANGENTIAL FIRING SYSTEM

General

In the tangential firing system the furnace itself constitutes the burner. Fuel and air

introduced to the furnace through four windbox assemblies located in the furnace

comers. The fuel and air streams from the wind box nozzles are directed to a firing

circle in the centre of the furnace. The rotative or cyclonic action that is characteristic of

this type of firing is most effective in turbulently mixing the burning fuel in a constantly

changing air and gas atmosphere.

Air And Fuel Nozzle Tilts

The air and fuel streams are vertically adjustable by means, of movable air deflectors

and nozzles tips, which can be tilted upward or downward through a total of approx. 60

degrees. These movements effected through connecting rods and tilting mechanism in

each windbox compartment, all of which are connected to a drive unit at each corner

operated by automatic control. Provision is given in UCB to know the position of nozzle

tips during operation. The tilt drive units in all four corners operate in unison so that all

nozzles have identical tilt positions,

Windbox Assembly

The fuel firing equipment consist of four windbox assemblies located in the furnace

corners.

PMI, NTPC 37

Each windbox assembly is divided in its height into number of sections or compartment.

The coal compartments (fuel air compartment) contain air (intermediate air

compartments). Combustion air (secondary air) is admitted to the intermediate air

compartments and each fuel compartment (around the fuel nozzle) through sets of

lower dampers. Each set of dampers is operated by a damper drive cylinder located at

the side of the windbox. The drive cylinders at each elevation are operated either

remote manually or automatically by the Secondary Air Damper Control System in

conjunction with the Furnace Safeguard Supervisory System.

Some of the (auxiliary) intermediate air compartments between coal nozzles contains oil

gun.

Retractable High energy Arc (HEA) ignitors are located adjacent to the retractable oil

guns. These ignitors directly light up the oil guns.

Optical flame scanners are installed in flame scanner guide pipe assemblies in the

auxiliary air compartments. The scanners sense the ultraviolet (UV) radiation given off

by the flame and thereby prove the flame. They are used by Furnace safeguard

Supervisory System to initiate a master fuel top upon detection of flame failure in the

furnace,

AIR FLOW CONTROL AND DISTRIBUTION

Total air (tow control is accomplished by regulating fan dampers or fan speed. Air

distribution is accomplished by means of the individual compartment dampers. The

airflow to the air boxes can be equalised by observing-and equalising the reading of the

flow meters located in the hot air duct to windbox.

TOTAL AIR FLOW

PMI, NTPC 38

In order to ensure safe light-off conditions, the pre-optional purge air flow (at least 30%

of full load volumetric air flow) is maintained during the entire warm-up period until the

unit is on the line and the unit load has reached the point where the air flow must be

increased to accommodate further load increase. To provide proper air distribution for

purging and suitable air velocities for lighting off, all auxiliary air dampers should be

open during the purge period, the lighting-off and the warm-up period.

After the unit is on the line, the total required amount of air (total air flow) is a function of

the unit load. Proper air flow at a given load depends on the characteristics of the fuel

fired and the mount of excess air required to satisfactory burn the fuel. Excess air can

best be determined through flue gas analysis (Orsat measurements).

The optimum excess air is normally defined as the 02 at the economiser outlet produces

the minimum capacity. Operation below the optimum excess air will result in high

capacity due to unburned carbon where as operation above the optimum excess air will

result in high capacity due to excessive H2 S04 condensation. Operation below

recommended range will result in excessive black smoke and operation above this

range will result in excessive white smoke.

NOTE

The most suitable amount of excess air for a particular unit, at a given load and with a

given fuel must be determined by experience. This is best done from observation of

furnace slagging conditions. Slagging tendency of a particular fuel may dicatate an

increase of operating excess air.

AIR FLOW DISTRIBUTION

The function of the windbox compartment dampers is to proportion the amount of

secondary air admitted to an elevation of fuel compartments in relationship to that

admitted to adjacent elevation of auxiliary air compartments.

PMI, NTPC 39

Windbox compartment damper positioning affects the air distribution as follows:

Opening up the fuel-air dampers or closing down the auxiliary air dampers increases the

air flow around the fuel nozzle. Closing down the fuel air dampers and opening the

auxiliary air dampers decreases the air flow directly around the fuel stream.

Proper distribution of secondary air is important for furnace stability when lighting off

individual fuel nozzle, when firing at low rates and for achieving optimum combustion

conditions in the furnace at all loads.

Proper distribution of secondary air also has an effect on the emission of pollutants from

coal tired units. As the unit load increases the quantity of Nitrogen Oxide (NO) Produces

in a furnace (due to the oxidation of nitrogen in the fuel) increases and the upper

elevations of fuel nozzles are placed in service. The quantity of NO produced can be

reduced by limiting the amount of air admitted to the furnace adjacent to the fuel and

increasing the quantity of air admitted above the fire (over fire air). When the unit has

reached a predetermined load (app.50%) the over-fire air dampers should open and

modulate as a function of unit load until, at maximum continuous rating (MCR) when up

to 15% of the total air is admitted to the furnace as over fire air. The optimum ratio of

over fire air to fuel and auxiliary air, as well as the optimum tilt position of the over-fire

air nozzles, to produce a minimum NO emission consistent with satisfactory furnace

performance must be determined through flue gas testing (i.e measurement of NO)

during intial operation of the unit.

The correct proportion of air between fuel compartment and auxiliary air compartments

depends primarily on the burning characteristics of the fuel. It influences the degree of

mixing, the rapidity of combustion and the flame pattern within the furnace. The

optimum distribution of air for each individual installation and for the fuel used must be

determined by experience

The wind-box compartments are normally provided with drive (except end air

compartments) so they may be operated by a secondary air damper and overfire air

PMI, NTPC 40

control system in conjunction with the furnace safeguard supervisory system. When on

automatic control the system should provide modulation of the auxiliary air dampers as

required to maintain a pre-set windbox-to-furnace differential pressure. The fuel air

dampers should be closed prior to and during light off. When the fuel elevation is proven

in service, the associated fuel air dampers should open and be positioned in proportion

to the elevation firing rate. Normally the end air compartments are provided with manual

adjustment which can be kept in the required position during commissioning of the unit.

FUEL OIL FIRING SYSTEM

Fuels

A coal tired unit incorporates oil burners also to a minimum oil firing capacity of 15% of

boiler load for the reasons of:

a. To provide necessary ignition energy to light-off coal burner

b. To stabilise the coal flame at low boiler/burner loads

c. As a safe startup fuel and for controlled heat input during light-off.

Auxiliary steam is utilised in boiler for following purposes:

a. For atomising the HFO at the cil gun

b. For tank heating, main heating and heat tracing of HFO

c. To preheat the combustion air at the steam coil air heater and to warm up the

main air heater (this reduces Sulphuroxide condensation and thus cold end

corrosion of main air heater)

With above provisions and with proper oil steam and combustion air parameters at the

burner, HFO is safely fired in a cold furnace.

PMI, NTPC 41

Burner Arrangement

In a tangentially fired boiler, four tall windboxes (combustion air boxes) are arranged,

one at each corner of the furnace as shown in Figure No. 9. The coal burners or coal

nozzles are located at different .levels on elevations of the windboxes. The number of

coal nozzle elevations are equivalent to the number of coal mills. The same elevation of

coal nozzle at 4 corners .are led from a single coal mill.

The coal nozzles are sandwitched between air nozzles or air compartments. That is, air

nozzles are arranged between coal nozzles, one below the bottom coal nozzle and one

above the top coal nozzle. If there are 'Q' numbers of coal nozzles per corner there will

be (n+1) numbers of air nozzle per corner.

Air Nozzles - 9 Lower 7 + 2

3 AA EA

The coal fuel and combustion air streams from these nozzles or compartments are

directed tangential to an imaginary circle at the centre of the furnace. This creates a

turbulent vortex motion of the fuel air and hot gases which promotes mixing ignition

energy availability, combustion rate and combustion efficiency.

The coal and air nozzles are tillable 30 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.

The air nozzles in between coal nozzles are termed as Auxiliary Air Nozzles, and the

top most and bottom most air nozzles as End Air Nozzles.

The coal nozzle elevations are designated as A.B.C.D etc., from bottom to top, the

bottom end air nozzle as AA and the top end air nozzles as XX. The auxiliary air

PMI, NTPC 42

nozzles are designated by the adjacent coal nozzles, like AB. BC.CD.DE. etc. from

bottom to top.

The four furnace corners are designated as 1,2.3 and 4 in clockwise dilection looking

from top and counting front water wall left corner as '1'.

Each par of coal nozzle elevations is served by one elevation of oil burners located in

between. For example in a boiler with 8 mills or 8 elevations of coal nozzles, there are

16 oil guns arranged in 4 elevations, at auxiliary air nozzles AB.CD.EF, & GH.

Heavy fuel oil can be fired at the oil guns of all elevations.

Each oil gun is associated with an high energy are ignitor.

Combustion Air Distribution

Of the total combustion air a portion is supplied by primary air fans that goes to coal mill

for drying and pulverising the coal and carrying it to the coal nozzles. This 'Primary Air'

flow quantity is decided by the coal mill load and the number of coal mills in service. The

primary air flow rate is controlled at the air inlet to the individual mills by dampers

The balance of the combustion air, referred as 'Secondary Air" is provided by FD fans.

A portion of secondary air (normally 30% to 40%) called 'Fuel Air' is admitted

immediately around the coal fuel nozzles (annular space around the casting insert) into

the furnace. The rest of the secondary air called 'Auxiliary Air' is admitted through the

auxiliary air nozzles and end air nozzles. The quantity of secondary air (fuel air+auxiliary

air) is dictated by boiler load and controlled by FD fan blade pitch.

The proportioning of air flow between the various coal fuel nozzles and auxiliary air

nozzles is done based on boiler load, individual burner load, and the coal/oil burners in

service, by a series of air damper. Each of the coal fuel nozzles and auxiliary and end

PMI, NTPC 43

air nozzles is provided with a knock knee type regulating dampers, at the air entry to

individual nozzle or compartment. On a unit with 8 mills there will be 8 fuel air dampers,

7 auxiliary air dampers, 2 end air dampers and 2 over fire air dampers per corner.

Each damper is driven by an air cylinder positioner set, which receives signal from

'Secondary Air Control System'. The dampers regulate on elevation basis, in unison, at

all corners.

Furnace Purge

Traces of unburnt fuel air mixture might have been left behind inside the furnace of

some fuel or might have entered the furnace through passing valves during shutdown of

the boiler.

Lighting up a furnace with such fuel air accumulation leads to high rate of combustion,

furnace pressurisation and to explosions at the worst. This is avoided by the "Furnace

Purge' operation during which 30% of total air flow is maintained for above 6 minutes to

clear off such fuel accumulations and fill the furnace with clean air, before lighting up.

During furnace purge, all the elevations of auxiliary and end air dampers are opened to

have a uniform and thorough purging across the furnace volume.

PMI, NTPC 44

PMI, NTPC 45

Fuel Air Dampers

Its operation is independent of boiler load.

All fuel air dampers are normally closed. They open fifty seconds after the associated

feeder is started and a particular speed reached; that it modulates as function of feeder

speed.

Fifty seconds after the feeder is removed from service, the associated fuel air dampers

close.

The fuel air dampers will open fully when both FD fans are off.

Fuel Oil Atomisation

Atomisation is a process of spraying the fuel oil into fine mist, for better mixing of the

fuel with the combustion air. While passing through the spray nozzles of the oil gun, the

pressure energy of the atomising steam breaks up the oil stream into fine particles.

Poorly atomised fuel oil would mean bigger spray particles, which takes longer burining

time results in carryovers and make the flame unstable due to low rate of heat liberation

and incomplete combustion.

Viscosity of the oil is another major parameter which decides the atomisation level. For

satisfactory atomisation the viscosity shall be less than 28 centistokes.

External mix type steam atomised oil guns suitable for both LFO and HFO have been

provided. Atomisers of this type are widely known as J-tips. The atomiser assembly

consists of nozzle body welded on to the gun body, back plate, spray plate and cap nut.

PMI, NTPC 46

SYSTEM REQUIREMENT

The maximum total output of oil burners is 30% of the boiler MCR. This meets the

turbine synchronisation needs before firing coal burners.

Each oil burner capacity is about 2% of boiler MCR.

For coal burner ignition and coal flame stablisation a minimum oil burner output,

equivalent to 10-20% of maximum coal burner capacity is required. This roughly

corresponds to 40 to 50 % rating of an oil burner.

The oil burner output is a function of oil pressure at the oil gun and the normal turndown

range of the oil burner is 3:1.

For steam atomised oil burner, the oil pressure at the oil gun shall not fall below 2.5

kg/sq cm (g) to ensure good atomisation and stable flames.

The oil burners have to be operated at loads, lower than the maximum rating for

reasons mentioned below:

a) during cold startups of the boiler, to have a controlled and gradually increasing

heat loading, to avoid temperature stresses on pressure part materials, as

dictated by boiler startup curves.

b) To conserve fuel. oil by operating the oil burners just at the "Coal flame

stabalisation" requirements.

Igniters

High Energy Arc type electrical igniters are provided, which can directly ignite the heavy

fuel oil. The main features of this system are :

PMI, NTPC 47

a. An exciter unit which stores up the electrical energy and releases the energy at a

high voltage and short duration.

b. A spark rod tip which is designed to convert the electrical energy into an

intensive spark.

c. A pneumatically operated retract mechanism which is used to position the spark

rod in the firing position and retract to the non-firing position.

Each discrete spark provides a large burst of ignition energy as the current reaches a

peak value of the order of 2000 amps. These sparks are effective in lighting of a well

atomised oil spray and also capable of blasting off any coke particle or oil muck on the

surface of the spark rod.

For a reliable ignition of oil spray by the HEA ignitorjt is very much necessary to

maintain '.he following conditions:

a. The atomisation is maintained at an optimum level. All the atomising parameters

such as oil temperature, steam pressure, clean oil gun tips etc., are maintained

without fail. The atomising steam shall be with 20C superheat minimum.

b The cold legs are minimum. The burner fittings are well traced and insulated

c The spark rod tip is located correctly at the optimum location.

d The oil gun location with respect to the diffuser and the diffuser location with

respect to the air nozzle, is maintained properly.

e. The control system is properly tuned wit ignitor operation. The time of

commencing of all the operational sequences is properly matched.

PMI, NTPC 48

f. It may become necessary to close the air behind the ignitors, during the light of

period for reliable ignition. This must be established during the commissioning of

the equipment and proper sequences must be followed.

The following facts must be borne in mind to understand the igniters and the system

clearly:

a. The spark rod life will be drastically reduced if left for long duration in the

advanced condition when the furnace is hot.

b. Too much retraction of spark rod inside the guide tube will interfere with nozzle

tilts and may spoil the guide tube.

c. A minimum discharge of 300 kg/hr of oil is essential for a reliable ignition.

d. A plugged oil gun tip may result in an unsuccessful start.

e. A cold oil gun and hoses cause quenching of oil temperatures and may lead to

an unsuccessful start. In such cases warming up by Scavenging prior to start is

necessary.

Fuel Oil Gun Advance/Retract Mechanism

The atomiser assembly of an operating oil gun is protected for the hot furnace radiation

by the flowing fuel oil/steam which keeps it relatively cool. Once the burner is stopped

there is no further flow of oil/steam. Under such situation it is required to withdraw the

gun from firing position to save it from possible damage due to over healing.

In the system provided, the oil gun is auto advance, auto retraceable. It is driven by a

pneumatic cylinder and a 4 way dual coil solenoid pilot control valve, with a stroke

PMI, NTPC 49

length of 330 mm. There are three position limit switches, one for, "gun engaged"

position, another for "gun advanced" and the third for "gun retracted" position, which

have been suitably interlocked into furnace safeguard supervisory System logics for

safe and sequenced operation.

Steam Scavenging Of Fuel Oil Guns

Before stopping the oil burner, the oil gun is scavenged with steam to keep the small

intricate passages of the atomiser parts clean.

* In the auto programmed burner stop sequence, a planned shut down is followed

by steam scavenging the oil side for quite sometime, to achieve this requirement.

* During emergency tripping of the burners or boiler, the oil gun is neither

scavenged nor retracted automatically. Normally such emergency trips may last

only for a short while and the fuel oil guns shall be re-started or local manually

scavenged immediately on resuming boiler operation.

HFO Lumping System

The screw pump is a constant quantity pump and when only a small quantity of oil is

fired, the excess oil from the constant quantity pump should be by-passed. This is done

automatically by pneumatic operated, pressure maintaining cum regulating valve by by-

passing the excess quantity through the return oil line to storage tank. The delivery

pressure of oil is maintained constant at the pump outlet, whatever be the quantity of oil

fired,

Set the pressure control valve for maintaining adequate and constant pressure at the

upstream of the HFO flow control valve at maximum firing rate.

PMI, NTPC 50

The flow control valve upstream pressure required is the sum of the following at

maximum firing rate:

a. Oil pressure at the gun inlet.

b. Static head between flow control valve and top level of burners, and frictional

pressure drop in these lines.

c. Flow control valve pressure drop, for best turndown.

HEAVY FUEL OIL HEATING SYSTEM

Three 150% duty steam-oil heat exchangers and three duplex strainers are provided for

operation in combination.

The HFO temperature control valve and the trap station for heaters, steam jacket of

strainers and line tracers are provided in the system.

All these equipment are laid out on the floor The drain points are to be suitably piped up

to the drain pit from the drain trays.

Steam Heaters and Strainers

The steam heaters are of fixed tube sheet, U tube type, with oil on shell side and steam

on the tube side. The oil space is protected against exceeding of allowable pressure by

low lifting spring loaded safety valve. The exchanger is equipped with the valves

needed for air release and draining.

The duplex basket type discharge strainers are at the heater outlet, with fine mesh of

250 micron filtration. The fine filtering prevents chocking of lines, valves and burner

PMI, NTPC 51

atomisers. The burner tip wearing rate is also reduced. When the pressure drop across

the strainer exceeds about 0.5 kg/sq. cm. (corresponding to 60% clogged status), the

standby strainer section is put into service and it is taken for cleaning.

SCANNER AIR SYSTEM

The scanner viewing heads are located in the burners and they are exposed to furnace

radiation continuously. The scanner heads cannot withstand high temperatures that will

arise due to this exposure. A constant cooling air is required around the scanner head

to cool it to a safe working temperature to ensure a reliable operation and long life. The

scanner head cannot be exposed to a continuous temperature of 175C without cooling

air.

PULVERIZED COAL SYSTEM GENERAL

The system for direct firing of pulverised coal utilizes Pulverisers to pulverize the coal

and a Tiling Tangential Firing System to admit the pulverized coal together with the air

required for combustion (secondary air) to the furnance.

As crushed coal is fed to each pulverizer by its feeder (at rate to suit the load demand)

primary air is supplied from the primary air fans. The primary air dries the coal as it is

being pulverized and transports the pulverized coal through the coal piping system to

the coal nozzles in the windbox assemblies.

A portion of the primary air is pre-heated in the bisector air heater. The hot and cold

primary air are proportionally J-nixed, prior to admission to the pulveriser, to provide the

required drying as indicated by the pulveriser outlet temperature. The total primary air

flow is measured in the inlet duct and controlled to maintain the velocities required to

PMI, NTPC 52

transport the coal through the pulveriser and coal piping. The total primary air flow may

constitute from approximately 15% to 25% of the total unit combustion air requirement.

The pulverised coal and air discharged from the coal nozzles is directed toward the

centre of the furnace to form a firing circle. Fully preheated secondary air for

combustion enters the furnace around the pulverised coal nozzles and through the

auxiliary air compartments directly adjacent to the coal nozzle compartments. The

pulverized coal and air streams entering the furnace are initially ignited by a suitable

ignition source at the nozzle exit. Above a predictable minimum loading condition the

ignition becomes self sustaining. Combustion is completed "as the gases spiral up in the

furnace.

A large portion of the ash is carried out of the furnace with the flue gas; the remainder is

discharged through the furnace bottom into the ash pit.

COMBUSTION OF PULVAREED COAL IN TANGENTSALLY RRED FURNACES

The velocity of the primary air and coal mixture within the fuel nozzle tip exceeds the

speed of flame propagation. Upon the nozzle tip the stream of coal and air rapidly

spreads out with a corresponding decrease in velocity, especially at the outer fringes

where eddies form as mixture occurs with the secondary air. Here flame propagation

and fuel speeds equalize, resulting in ignition. As the stream advances in the furnace,

ignition spreads until the entire mass is burning completely.

The speed which the air and coal mixture ignites after leaving the windbox nozzles

depends largely on the amount of volatile matter in the fuel. Heat released by oxidizing

the volatile components in the coal accelerates of the fixed carbon to its ignition

temperature.

The key to complete combustion consists of bringing a successive stream of oxygen

PMI, NTPC 53

molecules into contact with carbon particles, the smallest of which are relatively large by

comparison with he oxygen molecules. As combustion of the carbon progresses it

becomes increasingly difficult to bring about contact with the diminishing oxygen supply

in the limited time available, which for this type of firing is in effect greater due to the

longer travel taken by the gases.

The cyclonic mixing action that is characteristic of this type of firing is most effective in

turbulently mixing the burning coal particles in a constantly changing air and gas

atmosphere. As the main part of the gases spiral upward in the furnace, the relatively

dense solid particles are subjected to a sustained turbulence which is effective in

removing the products of combustion from the particles and in assisting the natural

diffusion of oxygen through the gas film that surrounds the particles.

PULVERIZERS

The pulverizer, exclusive of its feeder, consists essentially of a grinding chamber with a

classifier mounted above it. The pulverizing takes place in a rotating bowl in which

centrifugal force is utilized to move the coal. delivered by the feeder, outward against

the grinding ring (buil ring) as shown in fig no. 10. Rolls revolving on journals that are

attached to the mill housing pulverize the coal sufficiently to enable the air stream

through the pulverizer to pick it up. Heavy springs, acting through the journal saddles,

provide the necessary pressure between the grinding surfaces and the coal. The rolls

do not touch the grinding rings, even when the pulveriser is empty. Tramp iron and

other foreign material is discharged through a suitable spout.

PMI, NTPC 54

R P PULVERIZER GENERAL ARRANGEMENT Figure No. 10

PMI, NTPC 55

LEGEND 1. HOT AIR CONTROL DAMPER

2. COLD AIR CONTROL DAMPER

3. HOT AIR SHUTOFF GATE

4. COLD AIR SHUTOFF GATE

5. PULVERIZER DISCHARGE VALVES

8. PULVERIZER DISCHARGE SEAL AIR VALVE

7. FEEDER SEAL AIR SHUTOFF VALVE

8. PULVERIZER SEAL AIR SHUTOFF VALVE

9. FAN - ISOLATING VALVES

SEAL AIR A. TO FEEDER

B. TO PULVERIZER DISCHARGE VALVES (COAL PIPES)

C. TO HOT AIR SHUT OFF GATE ft CONTROL DAMPER

PMI, NTPC 56

The air and coal mixture passes upward the classifier with its deflector blades where the

direction of the flow is changed abruptly, causing the coarse particles to be returned to

the bowl for further grinding. The fine particles, remaining in suspension, leave the

classifier and pass on through the coal piping to the windbox nozzles.

FEEDERS

The raw crushed coal is delivered from the bunkers to the individual feeders, which, in

turn feed the coal at a controlled rate to the pulverisers.

In order to avoid overloading the pulveriser motor due to overfeeding, an interrupting

circuit should be used to reduce the coal feed if the motor should become overloaded

and to start the coal feed again when the motor load becomes normal.

PULVERISED COAL DRYING

For satisfactory performance, the temperature of the primary air and coal mixture

leaving the classifier should be kept at approximately 77C for our coals. To low a

temperature may not dry the coal sufficiently; too high temperature may lead to fires in

the pulveriser. The outlet temperature must not exceed 90C in any case. The moisture

content of coals varies considerably. Therefore the best operating conditions for an

particular installation must be determined by experience.

Figure No. 11 shows the location of dampers, shutoff gates and valves generally

utilised. The hot air control damper (No.1) and the cold air control damper (No.2)

regulate the temperature entering the pulveriser, by proportioning the air flow from the

hot air and cold air supply ducts. These dampers also regulate the total primary air flow

to the pulveriser.

The hot air shutoff gate (No.3) is used to shutoff the hot air to the pulveriser. The hot air

PMI, NTPC 57

gate drive must be interlocked with the pulveriser motor circuit so that the gate will be

closed any time the pulveriser is not in service. It must also be interlocked with the

temperature controller to effect closing of the hot air gate when the pulveriser outlet

temperature exceeds 90C.

The pulverise discharge valves (No.5), the cold air shutoff gate (No.4) and the seal air

shutoff valves (No.8) are always kept wide open. They are closed only when isolation of

a pulveriser or feeder is required for maintenance. Pulveriser discharge valves are also

closed on loaded, idle pulverisers when other pulverisers are being restarted after an

emergency fuel trip.

An adequate supply of clean seal air for the pulveriser trunion shaft bearing, etc.,

normally is assured by installing two booster fans and a filter in the seal air system. One

fan normally runs continuously, however it may be isolated for maintenance by closing

its inlet shutoff damper (No.9). The filter in this system is an inertial separator type

which discharges approximately 90% of its input as clean air. A bleed off system, with a

control valve, will control the amount of air being bled from the filters, so that the

differential pressure between the filter air outlet and the filter bleed air outlet is zero.

The control valve should be installed so the valve fails open with a loss of instrument

air.

The coal pipe seal air valve (No.6) is utilized to admit seal air to the coal pipes for

cooling when the pulveriser is isolated. The seal air valve is open whenever the

pulveriser discharge valve air closed an vice versa.

Primary air velocity requirements in the pulveriser and coal piping preclude wide

variations in system air flows. Therefore a constant air flow is maintained over the entire

pulveriser load range. The air flow should be low enough to avoid ignition instability and

high enough at avoid settling and drifting in the pulverised coal piping or excessive

spillage of coal from the pulveriser through the tramp iron spout.

PMI, NTPC 58

Coal spillage may also be caused by overfeeding, insufficient heat Inputs for

drying, too low a hydraulic pressure on the rolls or excessive wear of the grinding

elements.

GRAVIMETRIC FEEDERS

The STOCK Model 7736 gravimetric feeder is designed-to supply 4366 to 76,408 Kgs.

of coal to the pulveriser per hour while operating on 415 volt, 3-phase, 50 Hertz power

supply.

BELT AND DRIVE SYSTEM

The feeder belt is supported by a machined drive pulley near the outlet, a slotted take-

up pulley at the inlet end, six support rollers beneath* the feeder inlet, and a weighted

idler in the middle of the feeder. A counterweighted scraper with replaceable rubber

blade continuously cleans the carrying surface of the belt after the coal is delivered to

the outlet. Proper belt tracking is accomplished by crowning the take-up pulley: in

addition, all three pulley faces are grooved to accept the molded V-guide in the belt. The

pulleys are easily removable for belt changing and bearing maintenance.

Belt tension is applied through downward pressure exerted by the tensioning idler on

the return strand of the belt. Proper tension is obtained when the round protrusion at the

center of the tension roll is in line with the center indicator mark on the tension indicator

plate. The tension roll indicator is found on the drive motor side of the feeder and is visible through the viewing port in the tension roll access door. Tension adjustments can

be made with the feeder operating or at rest by turning the two belt take-up screws

which protrude through the inlet and access door.

CHANGES IN HUMIDITY OR TEMPERATURE MAY CAUSE VARIATIONS IN BELT

LENGTH. BELT TENSION SHOULD ALWAYS BE MAINTAINED WITHIN THE TWO

EXTREME MARKS ON THE TENSION INDICATOR PLATE.

PMI, NTPC 59

The belt drive system consists of Louis-Allis 5 HP variable speed DC shunt wound

motor with a speed range of 100-1750 rpm. The motor is housed in a totally-enclosed,

non-ventilated enclosure with Class II epoxy coated insulation with tropical protection,

severe duty house down provisions, and a 150 watt space healer wired for 240 VAC

operation.

The motor operates through a multiple-reduction gearbox to a total reduction of 149.6:1.

A reluctance-type magnetic sensor is provided on the motor drive to detect motor

speed. This data is used for motor speed control feed back information, for zero speed