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

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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.

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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.

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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

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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

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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.

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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.

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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.

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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.

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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.

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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.

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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

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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 circuit’s 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.

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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.

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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.

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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.

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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

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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

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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.

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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.

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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

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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 30°C 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 12°C.

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<heat exchange

surface), as is the case in natural or controlled circulation boilers for the high-

pressure steam cycles.

• Saving of demineralized water during the start-ups and low-load operation during

which no water is discharged to the sewers.

• Saving of heat during the periods of operation of the condensate treatment

station at boiler start-ups.

ADVANTAGES OF SLIDING PRESSURE OPERATION

- Improved overall efficiency

- Lower thermal stresses in turbine

- Lower pressure during start-up results in lower thermal stresses

- Larger control range for reheat temperature

- Reduced pressure at low loads increases the life of heavy components

- Overall reduction in power consumption

© 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 5°C 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 ½ 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 20°C 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.

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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

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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 175°C 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

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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

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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.

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R P PULVERIZER GENERAL ARRANGEMENT Figure No. 10

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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

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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 77°C 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 90°C 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

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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 90°C.

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.

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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.

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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

detection (i.e. motor speed less than 60 rpm). for derivation of a pulse signal for data

logging, and for feeder weighing control information. One revolution of the feeder belt

drive pulley delivers a predetermined weight of coal. regardless of its density, to the

outlet. A signal from the combustion control system, operating through the speed

control, 'regulates the belt drive motor speed, and thereby regulates the coal feedrate.

A paddle-type alarm is mounted above the center of the belt to detect the presence or

absence of coal on the belt. The alarm consists of a stainless steel paddle mounted on

one end of a horizontal shaft and a dust-tight switch housing on the other end. Multiple

single-pole switches, depending upon the number of functions requiring control, are

mounted in the switch housing. The switches are actuated by adjustable cams mounted

on the end of the shaft inside the switch housing. Loss of coal on the belt results in a

contact closure of limit switch LSFB. This switch can be used to stop the belt drive

motor, start a bunker Vibrator, or simply to indicate a loss of coal to the control room.

This contact closure also prevents weight correction and operation of the total coal

integrator when there is no coal on the belt and prevents calibration when there is coal

on the belt.

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5. Air/Draught System GENERAL

Air flow for the main steam -generators are handled by two forced draught fans and two

primary air fans. The gas produced in the furnace is evacuated by three I.D. Fans of

which one ID fan is stand by

This chapter contains descriptions of draught and air systems associated with the main

steam generators.

PRIMARY AIR/MILL SEAL AIR SYSTEM

The primary air draught plant supplies heated air to the coal mills to dry and convey

pulverised al to the furnace.

Ambient air is drawn into the primary air ducting by two 50% duty, motor driven axial

reaction fans, each capable of providing sufficient air to support 60% BMCR:

The inlet to each tap is silences and includes pneumatically operated guide vanes to

control fan output. The position of guide-vanes is controlled by the 'P.A header pressure

control loop' to main air pressure in the P.A bus duct at a preset level. Air discharging

from each fan passes first through a steam coil air pre-heater then through a motor

operated guillotine gate into the P.A bus duct. The motor operated isolating gates are

manually operated from separate OPEN/CLOSE push button stations in the unit control

room (UCB).

The P.A bus duct has four outlets, two direct cold air through the primary air heaters into

the hot-air CFOSS over duct; two take cold air to the seal air fans and the hot air duct

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prior to the mill air flow venture.

The primary air heater air inlet and outlet ducts are fitted with motor operated, biplane

dampers, which are operated from, push buttons in UCB. Both inlet and outlet dampers

are operated from UCB through separate push buttons.

The hot air cross over duct extends around to each side of the boiler to from the hot air

to mills ducts, both of which are branched to supply hot air to four coal mills.

Each branch is fitted with identical equipment. Hot air first passes through pneumatically

operated isolating gate, then on through a motor operated regulating damper, and a

flow venture into the coal mill. The hot air isolating gates are operated through the

FSSS interlocks.

During tripping of a mill, hot air gate and damper will close automatically and cold air

damper will open fully to provide cooling air to mill.

The hot air regulating dampers modulate under the automatic Control loops to maintain

the required air flow to the mill under varying load conditions. Flow transmitters located

about the venture provide a measured mill P.A flow signal to automatic control loops.

Cold air taken direct from the P.A. bus duct is routed to each side of the boiler to form

the cold airbus. A branch of cold air bus connects to the hot air dueling upstream of the

flow venture and includes a motor operated regulating damper which modulates in

response to the mill outlet temperature control loop to maintain the mill outlet

temperature at a preset level.

Two branches from the cold air bus deliver air to the mill for sealing purposes. Each

branch has two 100% duly parallel mounted strainers (duly/standby) further connected

to two mill seal air fans which boost the air pressure to maintain sufficient differential

between P.A. and seal air. Each strainer is fitted with hand operated inlet and outlet

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dampers. Seal air fans have hand operated inlet dampers pneumatically operated outlet

damper. One for is standby in each branch.

SECONDARY AIR SYSTEM

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

combustion air for fuel oil combustion and over flow air to minimise Nox production.

Ambient air is drawn into the secondary air system by two 50% duty, motor driven axial

reaction forced draft fans with variable pitch control, each capable of providing sufficient

air to support 60% BMCR. Silencer is provided at the suction.

Air discharging from each fan passes first through a steam coal air preheated then

through a motor operated isolating damper into the secondary air bust duct. The

isolating dampers are operated from separate push button stations in UCB.

Flow is measured across the venture provided in the discharge ducts, by two

transmitters which feed their signal to the automatic total air control loop. This signal is

added to the Coal mill P.A. flow signals then compared with the air flow demanded by

the boiler load control loop. Any difference will cause the pitch angle to modulate

towards the demanded flow. The F.D. bus ducts direct air through the two secondary air

heaters into the cross over duct.

The secondary air heater inlet and outlet ducts are fitted with motor operated biplane

dampers which are controlled from separate push button stations in the UCB. One other

outlet from F.D. bus duct directs air into the scanner air fans.

The cross over duct extends around to each side of the boiler furnace to form two

secondary air to burner ducts. At the sides of the furnace, the ducts split to supply air to

two corners. Then split again to supply air to each of nineteen burner/air nozzle

elevations in the burner box. Each elevation is fitted with a pneumatically operated

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regulating damper which is controlled by the Secondary Air Damper Control System to

maintain optimum secondary air distribution for combustion with varying fuels and firing

conditions.

Five basic types of burner box dampers are used:

a) Coal/air dampers which admit air immediately around the pulverised fuel nozzle

and hence are constituent in the primary stages of combustion.

b) Secondary air dampers, which admit air around the coal/air and P.F. nozzles and

hence are involved in the latter stages of combustion. These dampers will be

controlled to maintain the desired differential pressure between the secondary air

to burners and the furnace.

c) Oil/secondary air dampers, which generally fulfill the same requirements as but

with additional requirement of providing air for oil burning. When oil burning is in

progress, the associated damper will modulate according to oil header pressure.

d) Bottom tier secondary air dampers, which form part of the secondary air system,

but utilised to maintain clear conditions in the lower furnace.

e) Over fire air damper,which direct air over the coal flame to minimise NOx

production.

FUEL GAS SYSTEM

The flue gas draught 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 an economiser and four air heaters to improve thermal efficiency, and

through four electrostatic precipitators to keep dust emission from the chimney within

prescribed limits as shown in Fig no. 12.

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The flue gas dueling starts from boiler down stream of the economiser and directs flow

towards the primary and secondary air heaters. The primary end secondary air heaters

gas inlet ducts are fitted with Biplane isolating dampers. The gas outlet ducts of all four

air heaters are fitted with lower type regulating dampers. The outlet ducts of

corresponding primary and secondary air heaters combine then discharge through a

regulating damper, into the electrostatic precipitator common inlet duct which directs

flue gas through four electrostatic precipitors into the ID fan common inlet duct. The inlet

outlet ducts of each precipitator have motor operated guillotine gates.

From the ID fan common duct, flue gas flows through two of three 50% duty I.D. fans

(one standby), each capable of supporting 60% BMCR, into a common duct to the

chimney. Each fan has a motor operated guillotine gate for isolation at the inlet. The

outlet of two extreme fans has a similar gate whereas the outlet of middle fan which

bifurcates into two branches is fitted with two guillotine gates.

The fans are equipped with pneumatically operated inlet guide vanes and a variable

speed control that are controlled by boiler furnace draught control loop to maintain

furnace draught at a preset level.

SCANNER AIR SYSTEM

The function of scanner air system is to provide a continuous supply of clean air to

purge and cool the flame scanners. Air for the system is drawn from the FD fan

discharge ducts through a filter by one of the two scanner fans then discharged through

distribution pipe work to each flame scanner. Since a continuous supply of air is

necessary for safe operation of flame scanners, duplicate fans are provided for

duty/standby service. Fan A is driven by A.C. motor and will operate as the duty fan.

Fan B is the standby and is driven by a D.C. motor. During failure conditions the system

will continue to operate with the standby fan being driven from a secure supply.

An emergency damper is provided in the suction duct to facilitate suction from

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atmosphere. The discharge of each fan includes a pneumatically operated isolating

damper which will open and close in response to signals frm FSSS.

A pressure switch is provided to initiate the automatic start up of the standby fan if

scanner duct to furnace differential becomes less than 6 inches of W.C.

STEAM COIL AIR PREHEATER SYSTEM

The duty of steam air heaters is to maintain the primary and secondary air heater

average combined gas outlet and air inlet temperatures at preset values.

To achieve average primary air heater gas outlet and air inlet temperatures, the quantity

of steam entering the steam air heater is regulated by a temperature control valve. For

isolation purposes, tour manual operated isolating valves along with steam traps are

provided at inlet hand outlet of each SCAPH. Condensate leaving the SCAPH passes

through a isolating valve before entering the SCAPH drain vessel.

PRIMARY AIR FANS

There are two primary air fans per boiler. The fan consists of the following components:

a) Suction bend, with an inlet and an outlet side pipe for volume measurements.

b) Fan housing with guide vanes (stage 1)

c) Main bearings (anti-friction bearings)

d) Rotor, consisting of shaft, two impellers with adjustable blades and pitch control

mechanism.

e) Guide vane housing with guide vanes (stage 2)

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f) Diffuser with an outlet-side pipe for pressure measurements.

Suction bend, fan housing and diffuser are welded structural steel fabrications,

reinforced by flanges and gusets, resting on the foundation on supporting feet. The

supporting feet are fixed on the foundation in such a way that they slide and without

clearance at the sliding supports of suction bend and diffuser. On its impeller side, the

suction bend is designed as an inlet nozzle. Guide vanes of axial flow type are installed

in the fan and guide vane housings, in order to guide the flow. Further more, the guide

vanes are connecting the core and jacketing of the housing,

Suction bend and diffuser are flexibly connected to the fan housing via expansion joints.

Fan and guide vane housing are horizontally split, so that the rotor can be removed

without having to dismount the servomotor.

Those parts of the pitch control unit which are arranged in the guide vane and diffuser

cores are accessible through assembly openings.

The fan is driven from the inlet side. The shaft runs in antifriction bearings. The main

bearings are accommodated in the core of the fan housing. The impellers are fitted to

the shaft in overhung position.

The centrifugal and the setting forces of the impeller blades are absorbed by the blade

bearings. For this purpose the blade shaft is held in a combination, of radial and axial

antifriction bearings. Each blade bearing is sealed off by means of several seals, in both

directions (towards the inside and the outside).

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Bearings

The rotor is accommodated in cylindrical roller bearings. In addition, two angular contact

ball bearings are arranged on the driving side in order to absorb the axial thrust.

Double contact tele-thermometers and double resistance thermometers are fitted to

monitor the bearing temperature.

Pitch Control Unit

An oil-hydraulic servomotor flanged to the impeller and rotating with it adjusts the blades

during operation. This results in a closed flux of force between adjusting forces and oil

pressure, so that no forces are released to the outside (bearings, housing, foundation).

The servomotor consists of piston, cylinder and control parts.

At pitch control, the translational movement of the servomotor piston is converted into a

rotational movement of the blade shafts via adjusting levers, so that the blade angles

are variable.

Oil System

The main bearings and the hydraulic servomotor are supplied with oil from a common

oil reservoir. This has the advantage that for both the units the same oil can be used. It

is recommended to use turbine oil with a viscosity of 61.2-74.8mm2/s at 40°C.

Two oil pumps are mounted on the reservoir. One is operated as the main pump,

whereas the other one is used as standby machine. The latter is started via the

pressure switch, in the event the control oil pressure declines.

FORCE DRAUGHT FANS

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There are two forced draft fans per boiler. Each fan consists of the following

components:

a) Suction bend, with an inlet-side pipe for volume measurements

b) Inlet housing

c) Fan housing, with a pipe for volume measurements

d) Main bearings (antifriction bearings)

e) Impeller with adjustable blades and pitch control mechanism.

f) Guide vane housing with guide vanes

g) Diffuser, with an inlet- side pipe for volume measurements.

Suction bend, inlet housing and diffuser are of one-part, fan and guide vane housings of

two-part design.

Suction bend, fan housing and diffuser are structural steel fabrications, reinforced by

flanges and gussets, resting on the foundation on supporting feet. Fan and inlet guide

vane housings are split horizontally in such a way that the rotor can be removed while

the servomotor remains in place. The impeller-side end of the suction bend is designed

as inlet nozzle.

The fan is driven from the inlet side. The shaft runs in antifriction bearings. The main

bearings are accommodated in the core of the fan housing. The impeller is fitted to the

shaft in overhung position. The fan shaft is designed in such way that the maximum

operating speed is below the critical speed.

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The centrifugal and the setting forces of the impeller blades are absorbed by the blade

bearings. For this purpose, the blade shaft is held in a combination of radial and axial

antifriction bearings. Each blade bearing is sealed off by means of several seals, in

both directions (towards the inside and the outside).

Those parts of the pitch control unit which are arranged in the guide vane and diffuser

cores are accessible through assembly openings.

The sliding supports of the feet of suction bend and diffuser are fixed on the foundation

in such a way that they slide without clearance.

Bearings

The rotor is accommodated in cylindrical roller bearings. In addition, an angular contact

ball bearing is arranged at the driving side in order to absorb the axial thrust.

Double contact tele-thermometers and double resistance thermometers are fitted to

monitor the bearing temperature. These thermometers must be connected to signalling

instruments on the site.

Pitch Control Unit

An oil-hydraulic servomotor flanged to the impeller and rotating with it adjusts the blades

during operation. This results in a closed flux of force between adjusting forces and oil

pressure, so that no forces are released to the outside (bearings, housing, foundation).

The servomotor consists of piston, cylinder and control parts.

At pitch control, the translational movement of the servomotor piston is converted into a

rotational movement of the blade shafts via adjusting levers, so that blade angles are

variable.

© PMI, NTPC 71

Oil System

The main bearings and the hydraulic servomotor are supplied with oil from a common

oil reservoir. This has the advantage that for both the units, the same oil can be used.

Turbine oil with a viscosity of 61.2 - 74.8 mm2/S at 40°C is recommended for this

purpose.

Two oil pumps are mounted on the reservoir. One is operated as the main pump,

whereas the other one is used as standby machine. The latter is started via the

pressure switch, in the event the control oil pressure declines.Non-Return Valves

prevent the pressure oil from flowing back to the reservoir through the pump being out

of operation.

INDUCED DRAUGHT FANS

There are three induced draught fans per boiler, two operating and one standby. The

induced draught fans are NDVZ type.

NDVZ fans are single-stage, double-inlet centrifugal fans. Principal fan elements:

housing, inlet dampers, rotor with bearings and shaft seal.

The box-section scroll with the inlets and the outlet is of two-part welded design. The

supporting structure of the housing is formed by parallel lateral walls that are welded to

the coating surfaces of the scroll and of the inlets. Special supporting bolts, ribs and

reinforcements stiffen the welded structure. The inserts welded into the box-section

scroll and into the inlets guide the flow and moreover reinforce the components. Scroll

housing, inlets and outlet consist of rectangular sections and are equipped with man-

holes. The bottom part of the housing rests on claws on the foundation.

The scroll skin is equipped with a wear protecting coating on the inside.

© PMI, NTPC 72

The inlet dampers are accommodated in the inlet damper housing, they are commonly

adjustable externally.

The rotor consists of shaft and assembled impeller and runs in two sleeve bearings that

are arranged outside of the housing.

The impeller consists of a centre disc and two cover discs that are reinforced by forged

rings. The bent blades are welded into position between the impeller discs.

The blades are protected by screwed - on wear plates. The plates are screwed on

according to the number order.

The shaft is of hollow design. The fan shaft has been rated so that max. operating

speed is below the critical speed. Impeller and shaft are connected by means of a

flange. This screwing is protected by wear plates.

The fan housing is sealed at the shaft passage to the outside by means of two-part

labyrinth seals.

Bearings

The rotor is placed between oil-lubricated sleeve bearings. The drive-side bearing is

designed as thrust bearing which absorbs the axial thrust of the rotor.

The bearing housing is sealed towards the outside at the shaft passage by means of

auxiliary seal kit.

The bearings are lubricated with oil Thermometers are fitted to monitor the bearings

temperature.

© PMI, NTPC 73

Shaft Sealing

The shaft seals are fitted to the bearing pedestals and connected with box section scroll

by means of flexible coverings. The individual labyrinth sealing rings and the distance

rings are held together by screws in the sealing casing.

Regulation

The fan is adapted 10 changing operating conditions by means of varying the peed of

fan and also by adjustable inlet dampers arranged in front of impeller on either side

According to the required capacity, the speed of the fan can be varied and/or the inlet

damper position can be adjusted. For achieving speed changes a hydraulic coupling is

provided.

Variable speed Turbo Coupling

The turbo coupling is an infinetely variable fluid coupling with plain bearings and silumin

rotating parts.

The oil sump is flanged on below the housing of the turbo coupling. During operation the

filling pump in the housing delivers the required quantity of working oil and lube oil. An

auxiliary lube pump also installed in the housing, ensures that lube oil is delivered when

the couplings starts up or runs down. As standby, another aux. lube pump is installed.

The primary runner, comprising primary shaft, primary wheel and shell is supported in

the bearing housing and the coupling housing.

The secondary runner comprising secondary shaft and the secondary wheel is

supported in the scoop tube housing.

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PRIMARY AND SECONDARY AIRHEATERS

The Rotary Regenerative Air Preheaters are-designed for use on plant where hot air is

required for combustion or for fuel saving. The Air Preheaters contain in a small space,

heating elements of a large surface area.

In regenerative heat exchangers, the heat transfer surface is alternately heated by the

flue gases passing through it and cooled by the air passing through it. The flue gases

and the air flow through the same passages at different times so that unlike the

recuperative heat

Exchanger where heat (tows through the passage walls from the flue gases to the air,

the heat is absorbed by the regenerative mass from the hot flue gases and then

released to the cold air. This process can be periodic or if the regenerative mass

rotates, as in this project, the process is continuous.

In the Air Preheaters, flue gas flows through one side of the regenerator and through

the other side flows the incoming air prior to entering the furnace. The regenerator is

slowly revolved so that the heating elements pass alternately through the stream of hot

flue gases and through the stream of cold air. A portion of the heat in the flue gas side is

transferred to the air when the elements pass through the air side so heating the flow of

air and thereby cooling the elements. Thus the heat in the flue gases is partly recovered

and returned to the furnace via the air flow.

The two streams, flue gas and air. which (tow through diametrically opposite segments

of the rotor, are separated from each other by a small blanking section with sealing

plates to form a division between them. The two streams flow in opposite directions, i.e.

in contra flow. In this particular plant the flow arrangement is gas down and air up.

Basic Construction

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The rotor is the central part containing the heat transfer matrix. The rotor is radially

divided into twelve sectors. The heating elements are arranged in these twelve sectors

in two or more layers. The housing surrounding the rotor is provided with duct

connections at both ends, and is adequately sealed by radial and circumferential sealing

members - forming an air passage through one half of the preheater and a gas passage

through the other. The weight of the rotor is carried on the underside by a supherical

roller thrust bearing whilst at the top a spherical roller guide bearing is provided to resist

radial loads. The rotor revolves continuously absorbing heat from the flue gases and

transfering it to the air for combustion.

Each airheater is provided with a electric motor drive for normal operation and an air

motor drive for emergency and also for use during off load water washing.

Rotor Seals

Seals are provided at both ends of the airheater to minimise leakage between the air

side and the gas side of the preheater. The hot and cold end raidal seals are attached

to each diaphragm of the rotor and are set at a specified clearance from the sector

plates which separate the air and gas streams. The hot end sector plates are

automatically deflectabte to provide leakage area reduction during transient as well as

full load operation.

The seals provided at rotor post are set to operate with minimum clearance with respect

to the horizontal sealing surface of the sector plate centre, section. The bypass seals

provide sealing between the periphery of the rotor and sealing surface of the connecting

plate and/ or the preheater housing.

Axial seals are provided vertically in the rotor shell in line with radial seals. Various seal

gap to be maintained

Heating Surface Elements

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The heating surface elements in the cold end are manufactured from thin steel sheet

adjacently, one being undulated and the other being thin sheet steel. The notches run

parallel to the rotor axis and space the plates the correct distance apart.

As the cold end, i.e. gas leaving - air entering end of the preheater. is most susceptible

to wosion due to temperature and fuel conditions, the elements are arranged in tiers.

The lower or cold end tier of elements are manufactured from corten steel to combat

corrosion and are termed "cold end elements". The middle tier is termed the

"intermediate elements" and the upper tier termed the "hot end elements" and are both

made from carbon steel.

All elements are packed into containers to facilitate removal and handling. The cold end

packs are arranged such that they can be withdrawn from the rotor in a radial direction

without disturbing the hot end and intermediate packs.

The 'Hot' and 'Intermediate' ends are provided with Double undulated type heating

elements. The undulations provide high turbulance to the gases and air passing through

the preheater.

Rotor Drive Assembly

The driving force for turning the rotor is applied at its periphery. A pin rack mounted on

the rotor shell is engaged by a opinion attached to the low speed shaft of a power driven

speed reducer. An air motor is provided as an auxiliary drive for the air heater. This

drive ensures the continued operation of the preheater, even if power to the electric

motor is interrupted.

The air motor may also be used to control the speed of the rotor during water washing

of the heating surfaces.

Rotor Bearings

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The complete rotor is supported by a spherical roller thrust bearing. The load is

transmitted to the thrust bearing by a trunnion, bolted to the tower end of the rotor post.

To guide the upper end of the rotor a guide trunnion is bolted to the face of the rotor.

Oil Circulating Sytems

Separate oil circulating systems are provided to supply support bearing and guide

bearing with a bath of continuously cleaned oil at the proper viscosity. The bearing oil

supply is circulated by means of a motor driven pump through an external filtering

systems. A thermostat is used to limit the operation of the system to temperatures which

will ensure against overloading the pump or motor as a result of high oil viscosities.

Soot Blowers

Both primary and secondary airpreheaters are provided with twin nozzle swivelling arm

type electric driven soot blowers for on load cleaning at gas outlet end only.

ELECTROSTATIC PRECIPITATORS

The gas cleaning plant consists of four BHEL make Electrostatic Precipitators type

4xFAA-7x36-4x48125-2. The units are designed to operate on the exhaust gases from

each of the 500 MW Steam Generators.

The exhaust gases to be treated pass along the inlet duct and enter the steel

precipitator casing via a inlet funnel. To ensure the gases are evenly distributed across

the full sectional area of the treatment zones, spliterplates within the inlet funnel and two

rows of distribution screen at the inlet of stream are positioned.

After treatment by successive zones within the precipitators the clean gases pass

through the outlet funnel and flow along the outlet duct work connected to the I.D. fans

and are hence discharged to atmosphere via the chimney.

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In order to maintain the required standard of gas distribution within the precipitator,

vertical outlet baffles are located immediately after the final treatment zone.

Each precipitator is designed for two horizontal streams of gas flow. Each stream is

having six treatment zones or fields. Each treatment zone consists of parallel rows of

sheet type collecting electrodes suspended from the precipitator casing with wire type

discharge electrodes arranged mid way between them, fixed to upper and lower frame

assemblies.

Each separate electrical zone. comprising of the discharge electrodes, is suspended

from a discharge suspension arrangement mounted on the casing top plate.

The transformer-rectifier sets, one per zone - seven total per stream, are arranged at

top house access platform level adjacent to each relative zone. The respective control

panels and L -T distribution equipments are located within the control room built at

ground level immediate to each precipitator.

Seven rapping gear motors for collecting electrodes and fourteen rapping gear motors

for emittiog electrodes are provided for each stream. The rapping gear operates

continuously to dislodge the precipitated dust which falls under gravity into the pyramid

type hoppers. located directly beneath each treatment zone, for removal by the ash

handling system.

To assist the dust to remain in a free flowing state, electric heaters are provided

externally at the bottom portions of each hopper.

Each pair of precipitators is served by an arrangement of access platform and stairways

from ground level for the top housing level.

To facilitate removal and replacement of the transformer rectifier sets and other

© PMI, NTPC 79

maintenance, a lifting beam arrangement is provided at the top house roof level on each

casing of precipitator. A single hoist and geared trolley is provided to serve each lifting

beam arrangement. For the safe operation of these precipitators a full safety interlock

system is provided.

© PMI, NTPC 80

6. Furnace Safeguard And Supervisory System

FSSS means Furnace Safeguard & Supervisory system, the system which is used in

almost all types of boiler. The system has been designed to offer maximum protection,

minimum nuisance trips, minimum power consumption and maximum life of the

components used. It is designed to ensure the execution of a safe, orderly operating

sequence in the start-up and shutdown of fuel firing equipments in a boiler and to

prevent errors which may be committed by an operator due to ignorance. The system is

a complex one and has got multiple steps which cannot be left at the discretion of

operators to follow it in a sequential manner. Adequacy of ignition energy is a crucial

part in a boiler before cutting in any fuel input into the furnace. Because it is possible

that unburnt particles must have got accumulated inside the furnace and sudden ignition

of it may lead to major explosions in a boiler.

In our earlier projects, FSSS was implemented in relay based/solid state hardware.

From Ramagundam stage II (3 x 500 MW units) onwards. FSSS is being implemented

in microprocessor ased hardware. In all these projects, control was achieved through

PB stations mounted ir the control Desk. However in all our new projects like Farakka

Stage III (1 x500 MW). VSTPP stage II (2 x 500 MW) FSSS shall be implemented in

state of the art microprocessor based systems with CRT/KBD operation.

Basically FSSS is designed to perform the following functions :

a. Prevent any fuel firing unless a satisfactory furnace purge sequence has first

been completed.

b. Prevent start-up of individual fuel firing equipment unless certain permissive

interlocks have first been satisfied.

c. Monitor and proper components sequencing during start-up and shut-down of

© PMI, NTPC 81

fuel firing equipment.

d. Subject continued operation of fuel firing equipment to certain safety interlocks

remaining satisfied.

e. Provide component status feedback to the operator and, in some cases, to the

unit control system and/or data logger.

f. Provide flame supervision when fuel firing equipment is in service and effect an

elevation fuel trip or Master Fuel Trip upon certain conditions of unacceptable

firing/ load combinations.

g. Effect a Master Fuel Trip upon certain adverse Unit operating conditions.

There is a Logic Cabinet Assembly which contains control hardware in separate

sections of the cabinet. Each section controls the following functions in the unit.

a. Power Distribution section.

b. Two section for the unit.

c. Each of the four oil elevations (AB, CD, EF and GH) has a section for the oil

elevation and associated Safe Scan equipment.

d. There are separate sections for each coal elevation ("A" thru "H")

e. A "fireball" flame scanner section.

There is also a Unit Simulator panel assembly located on a swing panel in the Unit

section.

There are also four Oil Elevation Simulator panel assemblies. Each Simulator is located

© PMI, NTPC 82

on a swing panel located in the associated oil elevation section.

Each of the eight coal elevation sections has a Coal Elevation Simulator which is

located on a swing panel in the associated coal elevation section. The Unit contains four

(4) elevation of HWA igniters, for (4) warm-up elevation and eighteen (8) coal elevation.

Flame monitoring is accomplished with Jour (4) elevations of "Safe Scan" equipment

and five (5) elevations of "Fireball" (Series 510) (lame scanners.

The "Safe Scan" equipment is used as discriminating flame scanner. This equipment is

the Safe Scan 1-"Rear Access" model.

OIL RECIRCULATION

When all heavy oil nozzle valves are closed (PURGE PERMITS (white) "NOZL VALVES

CLOSED" light is on), the heavy oil recirculation valve can be opened to heat the heavy

oil to obtain the proper viscosity for efficient combustion.

This is accomplished by depressing the H.O. RECRIC.VLV "OPEN" push button

When the heavy fuel oil recirculation valve is moved from the closed position, the

associated (red) "OPEN" push button is illuminated. The recirculation valve is proven

fully open when the associated (green) "CLOSE" light goes off. When the recirculation

valve is proven fully open. the heavy oil trio valve can be opened to heat the oil to the

"proper" temperature.

The heavy oil recirculation valve can be closed by depressing the H.O.RECIRC. VLV

"CLOSE" push button. The heavy oil recirculation valve is closed automatically when

any heavy oil nozzle valve is not closed (PURGE PERMITS (WHITE) "NOZL VALVES

CLOSED" light goes off).

© PMI, NTPC 83

FURNACE-PURGE

Before boiler is lighted up furnace purging is done for which following interlocks should

be satisfied see Fig. 13. Furnace purging is done to remove all unburnt particles which

may have been inside the furnace during earlier unit operation.

1) Loss of AC is not there i.e. 110VAC/220VAC.

2) Loss of 220VDC is not there

3) Both drum level V. high condition is not there from Hydra step instrument.

4) Both drum level V. low condition is not there from Hydra step instrument.

5) One set of ID/FD fans should be in service.

6) No. P.A. fan should be in service

7) Air flow should be more than 30%

© PMI, NTPC 84

8) Auxiliary air dampers should be in modulating condition.

9) WB/Furnace AP should be 40mm WC.

10) Furnace pressure V. high condition should not be-there.

11) Furnace pressure V. low condition should not be there.

12) HOTV should be closed.

13) Heavy oil/light oil trip valves/hydromotor valves to be closed.

i) All the secondary air heater are not off.

ii) The reheat protection signal is established

iii) Adequate water wall circulation exists

14) Pulverisers off condition should be there.

15) Feeders off condition should be there.

16) Hot air gate should be closed.

17) No flame condition should be there.

18) No boiler trip condition should be there. (Manual pressing of pushbuttons)

When all above conditions are satisfied "PURGE READY" bulb will glow on the console

insert. After pressing 'Push to Purge" button purge will start for 5 minutes. After 5

© PMI, NTPC 85

minutes 'Purge complete' indication wilt come. Within this time it is ensured that all

unburnt particles have been removed from the furnace.

If light off firing provision is there LOTV can be opened now. Otherwise HOTV should be

opened and as well as heavy oil return valve should also be opened. For doing this we

have push button on FSSS console insert. By pressing OPEN push button the valve will

open and RED indication will glow. Initially the HORV is opened and when it is proved

open. the HOTV is opened. Healed heavy oil will be under recirculation. This is done to

avoid condensation of high viscosity oil. HOTV will open only when temperature of

heavy oil has already reached more than 90°C and as well as heavy oil and atomising

steam pressure condition must also be satisfied.

HEAVY OIL ELEVATION ‘AB' START

Ho elevation AB start permit will be there when the following conditions are satisfied.

1) Air flow is greater than 30% and less than 40%.

2) The burner tilt is horizontal.

When any feeder is proven and in service for more than 50 seconds, above condition is

no longer required to satisfy oil elevation start permit.

There are two modes of firing oil in one elevation i.e. (1) on pair basis (2) on elevation

basis.

The 'pairs' firing mode is selected when no feeder is in service. Elevation firing mode is

selected when at least one feeder is proven.

Pairs Firing Mode

Pairs are made for opposite corners i.e. pair 1 & 3 and 2 & 4. When pair 1 & 3 push

© PMI, NTPC 86

button is pressed the following events will occur.

i) The thirty second counting period is started.

ii) The seventy second counting period is started.

The oil gun No. 1 will advance from the retracted position if the following conditions are

there.

i) The gun No. 1 is engaged.

ii) The corresponding scavenge valve is closed

iii) Heavy fuel oil isolation valve is proven open and as well as atomising steam

isolation valve is also proven open in the local control station.

Local gun maintenance switch in remote position.

The oil gun will advance and the 'Gun advance indication will appear on the console

insert.

After the gun has advanced, command will go to the HEA ignitor spark rod. Spark rod

will also advance. 'Advance' indication will come on the console insert for gun and as

well as for ignitor. The following things will occur.

i) If atomising steam valve is not opened, the oil gun will be retracted.

ii) Ignitor will start sparking, once steam atomising valve is fully opened.

iii) Heavy oil Nozzle valve will also start opening. Indication will come on the console

insert if oil value is fully opened

© PMI, NTPC 87

The HORV is closed automatically. However if MFT occurs, HOTV will close

automatically. Similarly on atomising steam or. low or fuel oil header pressure low also.

HOTV will be closed automatically.

By this time oil gun should catch fire and scanner will sense the flame and give an

indication on the console insert "FLAME". After 25 seconds ignitor will retract to its

original position and after 30 seconds command will go to corner No. 3. Gun of comer 3

for AB elevation will get established in the same manner as explained for gun No, 1

when the seventy second counting period expire corner trip signal is established if any

of the following conditions is there.

i) The corner oil gun does not prove flame.

ii) The atomising steam valve is not open.

iii) Heavy oil nozzle valve is not open.

Pairs 2 & 4 can also be taken in service in the same manner as explained for pair 1 & 3.

Afterwards associated, secondary air dampers will change from "Auxiliary air" to "Oil

Firing' mode. For this 3 out of 4 guns should at least be in service to fulfill "oil elevation

established' condition.

Elevation Firing Mode

Elevation firing mode is selected when any feeder is proven. By pressing elevation firing

push button guns are proven on pair basis only i.e. first 1 & 3 guns and then 2 & 4 guns

in the same manner as explained in pair mode basis. But out of 4 guns 3 have to be in

service to satisfy 'HFO elevation' in service condition. Otherwise "UNSUCCESSFUL

START" indication will come and this will not be a permissive for taking mills in service.

© PMI, NTPC 88

Oil Elevation Shutdown

If oil guns are to be removed, pair 1-3 'stop' push button can be pressed and following

events will occur.

i) The heavy oil nozzle valve at corner No. 1 will get closed.

ii) Steam scavenge valve will receive a open command. As well as steam atomising

valve will also receive a redundant open command. HEA ignitor receives an

advance signal for 15 seconds.

iii) The moment scavenge valve gets fully opened a five minute counting period is

started

iv) After five minutes steam scavenge valve & steam valve will get closed.

v) Oil gun will get retracted to its home position.

vi) After PO seconds command will go to corner No. 3 and same procedure will be

follow /ed as explained for gun No. 1 likewise pair 2 & 4 can also be stopped.

Pulverize Operation

Prior to starting any pulverizer ignition energy must be adequate to light off coal, Mill

ignition energy condition is satisfied for all the mills when the following conditions are

satisfied. This is called ignition permit.

i) Any associated oil elevation in service i.e. AB/CD/EF/GH [A & B mills can be

taken in service if "AB oil elevation established" condition is there. Similarly

C.D.E.F.G & H mills can be taken in service when corresponding oil elevation are

© PMI, NTPC 89

there in service.]

OR

ii) Adjacent feeder is in service and the feeder speed is more than 50% and boiler

load is greater than 30%.

The above conditions can be applied to any pulverizer to fulfill pulverizer ignition energy

condition.

Pulverizer Ready

Prior to starting a mill a "Pulverizer Ready" signal for the selected pulverizer must be

established. This signal is made ready when the following condition are satisfied.

1) Pulverizer start permit* condition should be there. For this following conditions

should be satisfied.

a) The air flow more than 30% and should be 50%.

b) The burner tilt is horizontal.

c) ‘No Master Fuel Trip' condition.

2) ‘Primary air permit' signal should be established. There are two separate

"Pulverizer primary air permit" signals. One is for A.B.C & D and other is for

E.F.G and H mills. Any one signal is established when the following conditions

are satisfied.

a) Both F.D fans and both P.A. fans ON

OR

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At least one F.D. and one P.A. fan ON conditions there.

b) Hot PA duct pressure of A.B.C and D is not vary low for A, B.C and D and

Hot PA duct pressure of E, F.G and H is not very low for E, F, G and H

3) Pulverizer discharge valves should be opened.

4) Pulverizer outlet temperature is less than 95°C.,

5) Corresponding feeder is selected for remote control.

6) Cold air gate should be opened.

7) The Tramp iron hopper valve is proven open,

8) Feeder inlet gate is proven open,

9) Pulverizer Lub oil pressure satisfactory signal should be there. This is there for

500 MW Units where tub oil pump is there in the pulverizer.

10) No pulverizer trip signal exists.

11) Satisfactory lub oil level and temp' condition should be there.

Now pulverizer can be placed in service after the “Pulverizer start permit". 'IGNITION

Permsi” and "Pulverizer Ready” lights are on.

When we press the push button of the selected pulverizer, seal air valve will get opened

and differential pressure from seal air header to pulverizer under bowl should be greater

than 200 MMWC. Afterwards pulverizer will get switched ON. Now following things will

be valid for this pulverizer.

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a) Pulverizer discharge valve cannot be closed.

b) Pulverizer seal air valve cannot be closed.

c) The associated auxiliary air damper cannot be closed.

d) Hot air gate can be opened.

e) Feeder can be started

f) Cold air damper are opened to 100% position.

g) Hot air dampers can be operated.

After starting the pulverizer hot air gate can be opened and air temperature can be

increased or decreased to the required valve by operating cold and hot air damper.

Afterwards feeder can be started. After starting the feeder following signals will be made

through.

1) Within 5 seconds coal flow should be established through the mill. Otherwise

"Low current Relay" set for 30% mill current will trip the feeder.

2) Within 180 seconds fire ball scanners should indicate "Flame" for the mill ignition

energy to get stablised. Otherwise feeder will trip.

When pulverizer is in service following checks will be in operation.

1) Feeder speed will come to minimum when pulverizer bowl differential pressure is

high.

© PMI, NTPC 92

2) No coal flow will trip the feeder.

3) A high pulverizer temp. will close the hot air gate and open the cold air damper to

100% position.

4) Seal air underbowl differential pressure is less than 125 MMWC for more than

sixty seconds will trip the pulverizer.

5) Pulverizer ignition energy not available will trip the pulverizer.

6) Primary air trip signal will trip the pulverizer. .

Boiler Trip

A "boiler trip" signal is established if any of the following conditions exist :

1) Loss of ACS power for more than two (2) seconds.

2) Loss of 220 VDC Battery power for more than two (2) seconds.

3) Loss of unit critical power for more than two (2) seconds.

4) A "simulator trip" signal exists.

5) The water drum level is "low for more than ten (10) seconds.

6) The water drum level is "high" for more than ten (10) seconds,

7) All boiler feed pumps are off.

8) "Inadequate waterwall circulation" signal exists.

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9) All I.D. (Induced Draft) fans are off.

10) All F.D. (Forced Draft) fans are off.

11) The air flow is less than 30% before the boiler load exceeds 30%.

12) The deaerator level is "tow-low".

13) At least two of the three pressure switches indicate a "high furnace pressure

trip" condition.

14) At least two of the three pressure switches indicate a "low furnace pressure trip"

condition.

15) Both BOILER TRIP (emergency) push buttons are depressed simultaneously

16) All feeders are off and toss of power exists at the elevation that is in service.

17) A toss of reheat protection occurs.

18) Loss of fuel trip

19) Unit Flame Failure

The "toss of fuel trip" signal becomes "armed" when the first oil elevation that is placed

in service has at least three of the four heavy oil nozzle valves proven fully open (OIL

VALVE-COR (green) "CLOSED" lights are off). After the "toss of fuel trip" signal is

"armed", a "losses of fuel trip" signal will be established if all of the following conditions

occur simultaneously:

A. At all elevations, the feeder is off -or- loss of elevation power exists. This

condition exists for more than two (2) seconds.

© PMI, NTPC 94

AND

At all four oil elevations, all oil nozzle valves are closed or an "elevation trip"

signal is established at the associated oil elevation.

B. The "toss of fuel trip" signal is proven established when the Data logger receives

a "LOSS OF ALL FUEL TRIP" signal.

A. "toss of fuel trip" signal is also established when all of the following conditions exist

simultaneously :

B. At all coal elevations, the feeder is off -or- toss of elevation power exists. This

condition exists for more than two (2) seconds.

AND

The heavy fuel oil trip valve is not fully open

AND

The heavy fuel oil trip valve is not fully open

C. At all four oil elevations, all oil nozzle valves are closed.

When the "master fuel trip" memory signal exists for more than five (5) seconds, the

"loss of fuel trip arming" memory signal is removed.

The Data Logger's "LOSS OF ALL FUEL TRIP" signal will remain established if item B

above is satisfied. When the heavy oil trip valve is proven fully open (for oil

recirculation), this signal is removed.

© PMI, NTPC 95

Cause of Trip System

The first boiler trip command that causes a MFT will illuminate the appropriate indicator

in the cause of Trip Section on the console insert. Any successive boiler trip commands

to the other indicators are blocked. There will be only one indicator which will be

glowing.

PRIMARY, SCANNER AND SEAL AIR FANS OPERATION Primary Air Fan Control Primary air fans are used to transport the

pulverized coal to the respective coal burners

at each coal elevation.

When MFT signal is established, the PA fans

are tripped,

When the furnace purge cycle is completed

and a No .MFT signal is there. PA fans can be

placed in service. When all coal air dampers

are positioned to less than 5% open, permit to

start PA fans will be established.

A "primary air permit' signal is required before

a pulverizer can be started.

Scanner Air Fan Control There are two scanner fans in one boiler. One

is AC scanner fan and other is DC scanner fan.

When both FD fans are off, the scanner air

emergency dampers are opened. Otherwise

scanner fan inlet is there from F.D. fans

© PMI, NTPC 96

discharge. Once the fan is started scanner

outlet damper is opened. In case of AC failure

DC scanner fan gets started. Scanner air fans

are provided. Because scanners are to be kept

cool otherwise scanner lens will get damaged.

Seal Air Fan Control There are two seal air fans for the unit, one

seal air fan is used for supplying seal air to

mills and other is standby.

By pressing the seal Air fan start push button

seal air fan will get started and its discharge

damper will also open. Seal air fan will get

tripped when both P.A. fans are off.

FLAME SCANNERS

In our earlier projects, we were using UV type flame scanners. However the UV type

flame scanners have not given satisfactory performance. The reasons for unsatisfactory

performance of these flame scanners are that the intensity of UV rays is very low

resulting in poor sensitivity and the deposition of ash on the shutter/detector tube blocks

the sensing of actual flame.

© PMI, NTPC 97

In view of this it was decided to use flame scanners working on the principle of visible

light detection known as safe scan. These scanners use a photodiode to detect the

flame and a fiber optic light guide.

OPERATION

The visible-light scanner is designed to monitor the characteristic frequencies and

intensify levels of visible light emitted, from the combustion of fossil fuels. Light is

© PMI, NTPC 98

transmitted from the windbox by a fiber optic light guide and converted to an electrical

signal by a photodiode. Both the flame intensify and the frequency of the (lame

pulsations are amplified by electronic circuitry at the boiler side and converted to a

current signal suitable for transmission to remote equipment.

In the remote equipment, three parallel circuits receive the flame signal from the boiler-

mounted equipment. These circuits simultaneously and independently examine the

flame signal for absolute magnitude, intensify, and frequency. The existence of all three

components in a specified range is required before acknowledging .hat it "sees" flame.

A simplified system block diagram, which shows signal paths from the light guide to its

various outputs, is shown in Fig. 14.

The fault detection circuit constantly compares the flame signal magnitude against fixed

high and low limits. If a fault, such as broken cable, occurs anywhere between

the.photodiods and the remote chassis, the flame signal will deviate from an established

range of good signals. A comparator circuit will detect this deviation, initiate a fault

alarm, and disable the flame permissive signal.

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7. Soot Blowing System

INTRODUCTION

On load, gas side cleaning of boiler tubes and regenerative airheaters is achieved using

126 microprocessor controlled sootblowers which are disposed around the plant as

shown in Fig No. 15.

88 - Furnace Wall Blowers - Steam

34 - Long Retractable Soot Blowers - Steam

4 - Airheater Soot Blowers for Primary and Secondary

Airheaters - Steam

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

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

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

SOOT BLOWER PIPING

Steam for Sootblowing is taken from the division panel outlet header. To sootblow the

regenerative airpreheaters during boiler startup, however, a separate connection is also

provided from the auxiliary steam system.

The supply pipework from superheater of steam source (division panel outlet header) is

fitted with a hand operated and motor operated isolating valves followed by a pressure

control valve and a spring loaded safety valve as protection against steam over

pressure. The safety valve vents via expansion chamber direct to atmosphere. The

isolating valves will be opened and closed as determined by the operator in relation to

boiler load via the sootblower control system. From the steam source after the pressure

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reduction the main line is split into six sootblowing sections.

Steam is fed through various Sections at the steam main pressure of 30 kg/cm2 /g).

Further reductions to the blowing pressure is achieved by adjusting the set screws of

the individual soot blower valve head at the time of soot blows system commissioning.

Branches of the section pipelines supply steam to individual soot blowers. At various

points on each section, pipeline connections are made via motor operated drain valves

Intermittent blowdown tank. These drain valves are all operated by the sootblower

control system.

On request to soot blow. the control system will open the sections drain valves and

crack open the inlet steam main isolating valve in order to warm up the soot blower

pipings and drain any condensate to the intermittent blowdown tank. Once the piping is

proved to be warmed up resulting in no condensate being produced, the control system

will close the drain valve and fully open the inlet isolating valve thus bringing the

pressure control valve into operating and signally commencement of soot blowing

sequence operation. Steam temperature in each section is maintained by the

temperature control valves/drain valves automatically as per the setting.

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SOOT BLOWERS Wall Blowers

The blower assembly consists of a stationary body and rack gear housing and a rotary

gear box assembly to which the swivel tube assembly is attached. The Swivel tube

assembly is supported <y bushings at each end of the body casting. The horizontal

guide rods are used to assure proper alignment of the rotary gear box assembly.

A stationary electric motor is situated on the right side of the blower, this motor, through

a rack gear housing assembly operates a pinion which drives a horizontal rack

assembly, the outer end of which is fastened to the rotary gear box assembly.

When the rotary gear box approaches the fully extended position a ramp cam attached

to the free end of the rack contacts a bearing surface, which is a part of the clevis

bracket assembly and bushes the valve steam assembly admitting steam to the swivel

tube. When the blower is started the rack pinion moves the rack and rotary gear box

towards the boiler. Operation of the rack gear housing causes rotation of a shaft

extending out from the rack gear housing into a switch box. Located in this switch box

are two cam actuated limit switches. One can holds limit switch LSTE in open position

when the blower is fully retracted. Extending of the blower moves the cam allowing

LSTE to close. The blower is then under its own control.

Near the fully-extended position, the ramp cam strikes the lever that open the SBV head

valve. The second limit switch cam strikes the LSTS limit switch, which opens the circuit

to the traverse motor and closes the circuit to the rotary motor.

The rotary motor is attached to the gear box assembly. When LSTS closes, the motor

rotates the swivel tube through a gear train.

When the blowing sweep is finished, the Cam assembly, on the swivel tube contacts

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and rotates the arm on the limit switch LSTR. The traverse motor begins to retract the

blower. Near the fully retracted position the cam again opens the switch LSTE to halt

the blower.

Long Retractable Sootblower

The LRD-IIE model Soot Blower is a boiler cleaning device in which a rotating lance

extends into and detracts from the boiler to make sure that the cleaning medium steam-

directed through the nozzles, removes the deposits from tube surfaces.

The lance is attached to a carriage housing which runs on tracks inside the blower

housing. The carriage and lance are moved by means of a traversing chain operated by

a electric power pack. Rotary motion is applied to the lance through the travelling

carriage by a second chain driven by a separate electric power pack. Control movement

is by a stop limit switch and a reverse limit switch.

The unit can be supplied with different traversing and rotating speeds. Standard

traversing speed: are available in various increments from 1.25 to 3.65 m. per minute.

Standard rotating speed' are available in various increments from 4.25 rpm to /.75 rpm.

These speed variations are accomplished by changing the power pack and jack shaft

drive sprockets, other speeds are possible for special application by the use of special

sprockets.

Flow of blowing medium though the retractable soot blower is controlled by the valve

mounted at the rear end of the blower. The feed pipe is attached to the outlet of this

valve head. This feed pipe passes through packing gland in the traveling carriage and

lies inside the lance tube extending to almost the entire length of the blower.

The wheels on the travelling carriage run on tracks welded to the inside of the blower

housing. Sideways motion is limited by a roller on each side of the carriage which use

the housing sides as guides.

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The ends of traversing chain are connected to each end of the carriage. The rotary

chain is continuous. It passes over sprockets on the carriage and causes rotation

through a gear train.

The lance is flanged to the carriage and supported on the boiler end by bearing and

yoke plate.

The electric gear box on the right side is for traversing and the electric gear box on the

left side is for rotation when viewed from the rear end of the blower.

Motion is transmitted from the gear boxes to jack shafts on each side of the blower.

Tension on the internal chains are adjusted by adjusting the screws on chain tighteners

which hold the idler sprockets on the outboard end of the unit.

The housing completely covers the blower except the traversing and rotating gear

boxes.

The housing is open at the bottom except for tie bars at intervals. A section of the top of

the housing near the rear end of the blower is cut away to allow access to the traveling

carriage. The access areas have removable cover. A short section of the tract at the

rear is removable to permit removing the traveling carriage for major maintenance.

The soot blower valve head is operated by a trip pin on the top of the traveling carriage

which engage a trip cam and through the trip rod linkage and valve lever causes the

head valve to open or close.

The stroke of the head valve is governed by the length of the trip rod. To change the

valve stroke, loosen the join nuts where it screws into the rod connection and turn the

rod. One end of the rod has a right hand thread, the other end in left hand. When the

desired length is attained, tighten the join nuts.

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Airheater Sootblower

The cleaning device consists of an electric motor coupled to a gear driven crank

mechanism which oscillates the swivel header carrying the twin nozzle pipes.

The cleaning medium Is conveyed through the swivel heads and respective nozzle pipe,

to the nozzle at the end.

A rotary point in the supply line permits free motion of the swivel header while

connected to the source of supply.

The arc traversed by the nozzle and the rotation of the rotor subjects the entire area of

the rotor to the action of the cleaning jet.

To maintain the desired steam pressure at the nozzle an orifice plate is provided in the

supply line.

Drain connections are provided in the steam piping at suitable locations for removing

condensate from the piping system while the device is idle. and just before it is

operation.

The steam supply line to cleaning device has to be supported in such a way to avoid

axial and side thrust being applied on to the rotary swivel joint. If care is not taken in this

regard. heavy leak may occur in this joint

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8. Data Sheet For 500 MW Boiler

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9. Salient Features And Constructional Details Of KWU Turbine

GENERAL DESCRIPTION The turbine is a reaction, condensing type. tandam compound with throttle governing

and regenerative system of feed water heating. It is coupled directly to the generator as

shown in Fig No. 16. The turbine is suitable for sliding pressure operation to aviod

throttling losses at partial loads. The turbine is a single shaft machine with separate HP.

IP and LP turbines. HP-turbine is a single flow cylinder where as IP and LP are double

flow cylinders. The individual turbine rotors and the Generator rotor are connected by a

rigid couplings. Steam flow to HP-turbine is controlled by four combined main stop and

control valves by a simple throttle governing system. On the 2 exhaust lines of HP-

turbine, swing check valves are provided which prevent hot steam from the reheater

flowing back into the HP-turbine. The Hot reheat steam is admitted to the IP-turbine

through four combined stop and control valves. IP-exhaust is connected to the LP-

turbine by cross over pipes without valve sat diametrically opposite points.

HP-TURBINE

The outer casing of the HP-Turbine is of the barrel type and has neither axial nor a

radial flange. The guide blade carrier is axially split and kinematically supported. The

space between the outer casing and the inner casing is fed from admission steam to

HP-Turbine. This steam is drained through HP- casing drain during start up which

promotes quicker heating of inner casing which results in lesser problems of differential

expansion. The inner casing is attached in the horizontal and vertical planes in the

barrel casing so that it can freely expand radially in all directions and axially from a fixed

point (HP-inlet side). The HP-turbine is provided with c. balance piston in the admission

side to counter act the axial thrust caused by steam forces. HP-turbine is provided with

18 stages of reaction blades.

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IP-TURBINE

It is of double flow construction and consists of 2 casinos. Both are axially split and the

inner casing kinamatically supported and carries the guide blades. The inner casing is

attached to the outer casing in such a manner as to be free to expand axially from a

fixed point and radially in all directions. IP turbine is having 14 reaction stages per flow.

f xti.u-tion steam to high pr. heater no.5 and Deaerator are taken from IP turbine and the

SP-exhaust respectively.

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LP TURBINE

The casing of the double-flow LP cylinder is of three-shell design. The shells are axially

split and of rigid welded construction. The inner shell taking the first rows of guide

blades, is attached kinematically in the middle shell. Independent of the outer shell, the

middle shell, is supported at four points on longitudinal beams. Two rings carrying the

last guide blade rows are also attached to the middle shell. LP-turbine is provided with 6

reaction stages/flow.

Blading

The entire turbine is provided with reaction blading. The guide blades and moving

blades of the HP and IP parts and the front rows of the LP part with inverted T-roots and

shrouding are milled from one piece. Thu last stages of the LP part consist of twisted,

drop-forged moving blades with fir-tree roots inserted in Corresponding grooves of the

rotor and guide blade rows made of sheet steel.

Bearings

The HP rotor is supported by two bearings, a journal bearing at the front end of the

turbine and a combined journal and thrust bearing directly adjacent to the coupling with

the IP rotor. The IP and LP rotors have a journal bearing each at the end of the shaft.

The combined journal and thrust bearing incorporates a journal bearing and. a thrust

bearing which takes up residual thrust from both directions. The bearing temperatures

are measured by thermocouples in the lower shell directly under the white metal lining.

The temperature of the thrust bearing is measured in two opposite thrust pads.

The front and rear bearing padestals of the HP turbine are placed on baseplates. The

pedestals of the LP part are fixed in position the front pedestal and the pedestal

between the HP and IP parts are able to move in axial direction.

The brackets at the sides of the HP and IP parts are supported by the pedestals at the

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Lovel of the machine axis. In the axial direction the HP and IP parts are firmly

connected with the pedestals by means of casing guides, without restricting radial

expansion. Since the casing guides do not yield in response to axial displacement, the

HP and IP casings as well as the associated bearing pedestals move forward from the

front LP bearing pedestal on thermal expansion.

Shaft Glands and Blade Sealing Strips

All shaft glands sealing the steam in the cylinders against atmosphere are axial-flow

labyrinths. They consist of a large number of thin sealing strips which in the HP and IP

parts are alternately caulked into grooves in the shafts and surrounding sealing rings.

The sealing strips in the LP part are only caulked into the sealing rings. These rings are

split into segments which are forced radially against a projection by helical springs and

are able to yield in the event of rubbing. Sealing strips of similar design are also used to

seal the radial blade tip clearances.

Steam and Control Valves

The HP turbine is fitted with four initial steam stop and control valves. A Si0p and

control valve with stems arranged at right anqles to each other are combined in a

common body. The stop valves are spring-operated single-seat valves; the control

valves also of single-seat design, have dif (users to reduce pressure losses.

The IP turbine has four combined reheat stop and control valves. The reheat stop

valves are sprhg loaded single-seat valves. The control valves, also spring loaded, have

uffusers. The control valves operate in parallel and are fully open in the upper load

range. In the lower load range, they control the steam flow to the IP turbine and ensure

stable operation even when the turboset is supplying only the station load.

Both the main and reheat stop and control valves are supported kinematically on the

foundation ceiling below the machine floor before the turboset. All valves are actuated

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by individual hydraulic servomotors.

Turbine Governing System

The turbine has an electro-hydraulic governing system. An electric system measures

and controls speed and output, and operates the control valves hydraulically in

conjunction with an electro-hydraulic converter. The electro-hydraulic governing system

permits run-up control of the turbine up to rated speed and keeps speed swings

following sudden load shedding low. The linear output-frequency characteristic can be

very closely set even during operation.

Turbine Monitoring System

In addition to the measuring instruments and instruments indicating pressures,

temperatures, valve positions and speed, the monitoring system also includes

measuring instruments and indicators for the following values

Absolute expansion, measured at the iront and rear bearing pedestal of the HP turbine

Differential expansion between the shafting and turbine casing, measured at several

points Bearing pedestal vibrations, measured at all turbine bearings.

Relative shaft vibration (bearing pedestal shaft) measured at all turbine bearings.

Absolute shaft vibrations, obtained from bearing pedestal vibration and relative shaft

vibration by calculation.

Oil Supply System and Control Fluid System

An oil supply system lubricates and cools the bearings and drives the hydraulic turning

gear. The main pump driven off the turbine shaft draws oil from the main oil tank.

Auxiliary oil pumps maintain the oil supply on start-up and shutdown, during turning

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gear operation and when me main oil pump is faulted. When the turning gear is started,

a jacking oil pump forces high-pressure oil under the shaft journals to prevent boundary

lubrication. The lubricating and cooling oil is passed through oil coolers before being

supplied to the bearings.

The control fluid pumps situated on a control fluid tank supply the hydraulic turbine and

bypass governing system as well as protection devices and valve actuators with HP and

LP control fluid, respectively.

ANCHOR POINT OF TURBINE

In designing the supports for the turbine on the foundation, attention must be given to

the expansion and contraction of the machine during thermal cycling. Excessive

stresses would be caused in the components if the thermal expansion or contraction

were restricted in any way. The method of attachment of the machine components is

also a decisive factor in determining the magnitude of the relative axial expansion

between the rotor system and turbine casings, which must be given careful attention

when determining the internal clearances.

The fixed points of the turbine are as follows:

- The bearing housing between the IP and LP turbines.

- The rear bearing housing of the IP turbine.

- The longitudinal beam of the I.P turbine.

- The thrust bearing in rear bearing casing of H.P turbine.

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Casing Expansion

The front and rear bearing housings of the HP turbine can slide on their baseplates in

an axial direction. Any lateral movement perpendicular to the machine axis is prevented

by fitted keys.

The bearing housings are connected to the HP and IP turbine casings by guides which

ensure that the turbine casings maintain their central position while at the same time

allowing axial movement. Thus the origin of the cumulative expansion of the casings is

at the front bearinq housing of the LP turbine.

The casing of the IP turbine is located axially in the front area of the longitudinal beam

by fitted keys cast in the foundation. Free lateral expansion is allowed. The center

guides for this casing are recessed in the foundation crossbeams. There is no restriction

on axial movement of the casings.

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Hence when there is a temperature rise the outer casing of the IP turbine expands from

its fixed point, towards the generator. Differences in expansion between the outer

casing and the fixed bearing housings to which the housings for the shaft glands are

attached are taken up by bellows expansion joints.

Rotor Expansion

The thrust bearing is incorporated in the rear bearing housing of the HP turbine. Since

this bearing housing is free to slide on the base plate the shafting system moves with it.

Seen from this point, both the rotor and casing of the HP turbine expand towards the

front bearing housing of the HP turbine. The rotor and casing of the IP turbine expend

towards the generator in a similar manner.

The 1.P. turbine rotor is displaced towards the generator by the expansion of the

shafting system from the thrust bearing. The magnitude of this displacement, however,

is reduced by the amount by which the thrust bearing is moved in the opposite direction

by the casing expansion of the IP turbine.

Differential expansion

Differential expansion between the rotors and casings results from the difference

between the expansion originating from the bearing housing behind the HP turbine and

that from the thrust bearing. This means that the maximum differential expansion of the

HP and IP turbine occurs at the end furthest from the thrust bearing.

Differential expansion between the rotor and casing of the IP turbine results from the

difference between the expansion of the shafting system, originating from the thrust

bearing, and the casing expansion originating from the fixed point of the I.P casing on

the longitudinal beam members.

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HP-TURBINE CASING Barrel-type Casing

The HP casing is designed as a barrel-type casing without axial joint. An axially split

guide blade carrier (4) is arranged in the barrel-type casing (3) as shown in Fig. No. 18.

Because of its rotational symmetry, the barrel-type casing also remains constant in

shape and leak proof during quick changes in temperature (e.g on start-up and shut-

down, on load change and under high pressures). The guide blade carrier, too, is

almost cylindrical in shape as the horizontal joint flanges are relieved by the higher

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pressure arising outside and can thus be kept small. For this reason, turbines with

barrel-type casings are especially suitable for quick start-up and loading.

HP-TURBINE HAFT SEALS AND BALANCE PISTON FUNCTION

The HP turbine has two shaft seals. The function of these is to seal the interior of the

casing from the atmosphere at the passages in the shaft on the admission and exhaust

sides. The difference in pressure before and behind the raised part of the shaft seal on

the admission side serves to counteract the axial thrust caused by steam forces. The

effective seal diameter is sufted to the requirements for balancino the axial thrust.

IP TURBINE CASING Double Shell Construction

The casing of the IP turbine is split horizontally and is of double-shell construction as

shown in Fig No. -19. A double-flow inner casing (4,5) is supported in the outer casing

(2,3) Fig.4). Steam from the HP turbine enters the inner casing from above and below

through two inlet nozzles (7) flanged to the mid section of the outer casing. This

arrangement provides opposed double flow in the two blade sections and compensates

axial thrust. The center flow prevents the steam inlet temperature from affecting the

support brackets and bearing sections.

This inner casing arrangement means that the steam inlet conditions are limited to the

inlet section of the inner casing, whereas the joint of the outer casing is only subjected

to the lower pressure and lower temperature prevailing at the outlet of the inner casing.

The joint flange can thus be kept small and material accumulations reduced to a

minimum in the area of the flange. In this way, difficulties arising from deformation of a

casing with flange joint due to non-uniform temperature rises, e.g. on start-up and shut-

down, are avoided.

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The joint of the inner casing is relieved by the pressure in the outer casing so that

this joint only has to be sealed against the resulting differential pressure.

LP TURBINE

Casing Construction

The LP turbine casing consists of a double flow unit and has a triple shell welded casinq

as shown in Fig = 20. The outer casing consists of the front and rear walls, the two

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lateral longitudinal support beams and the upper part. The front and rear walls, as well

as the connection areas of the upper part are reinforced by means of circular box

beams. The outer casing is supported by the ends of the longitudinal beams on the

base plates of the foundation.

Inlet Connections

Steam admitted to the LP turbine from the IP turbine flows into the inner casing (4,5)

from both sides through steam inlet nozzles before the LP blading. Expansion joints are

installed in the steam piping to prevent any undesirable deformation of the casings due

to thermal expansion of the steam piping.

ATMOSPHERIC RELIEF DIAPHRAGM Function and Operation:

Atmospheric relief diaphragms are provided in the upper half of each exhaust end

section to protect the turbine against excessive pressure. In the event of failure of low

vacuum trips, the pressure in the condenser rises to an excessively high level until the

force acting on the rupturing disc, ruptures the breakable diaphragm thus providing a

discharge path for the steam. The diaphragm consists of a thin rolled lead plate.

Hydraulic Turning gear

The function ,of the hydraulic turning gear is to rotate the shaft system at sufficient

speed before start up and after shut down. This avoids irregular heating up or cooling

down and thus the associated distortion of the turbine shaft. The blade ventilation during

turning operation provides good heat transfer at the inner wall of the casing which is

conductive to temperature equalization between upper and lower casing parts. During

turning gear operation, the shaft system is rotated by a double row blade wheel which is

driven by oil provided by the auxiliary oil pump. After passing the blading, the oil drains

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into the bearing pedestal and flows with the bearing oil into the return flow.

The turbine is equipped with a mechanical barring gear which enables the shaft to be

rotated in the event of a failure of normal hydraulic turning gear. The barring gear may

only be operated after the shaft system has been lifted with high pressure lift oil. If it is

hard to start turning by means of the mechanical barring gear, this may be due to

incorrect adjustment of the shaft lift oil system or to a rubbing shaft. Before steam is

admitted to the turbine, corrective action must be taken.

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10. Turbine Oil System

INTRODUCTION

The turbine oil system fulfills the following functions:

a. Lubricating and cooling the bearings.

b. Driving the hydraulic turning gear during interruptions to operation, on start-up

and shutdown.

c. Jacking up the shaft at low speeds (turning gear operation, start-up and shut-

down).

OIL SYSTEM

When the machine is running, the main oil pump situated in the bearing pedestal draws

oil from the main oil tank by injectors and conveys it to the pressure oil system for

lubrication put s. The return oil is drained into the tank. During "the start up and shut

down condition, one of the two full load auxiliary oil pumps circulates the oil. Turbine oil

system is shown in fig No. 21.

When main and full load auxiliary oil pumps fails, the lubrication oil is maintained by a

DC-driven emergency oil pump.

The jacking oil pr. required for supporting the shaft system is supplied by one of the 2

jacking oil pumps, which takes its suction from the main oil tank 2 nos oil vapour

extractors are mounted on the MOT to produce a slight vacuum in the main oil tank and

the bearing pedestals to draw off any oil vapour. There are 2x100% oil coolers and a

duplex filter on the oil line to thrust bearing. Main oil tank is provided with a Basket type

filter.

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MAIN OIL TANK

The main, oil tank contains the oil necessary for the lubricating and cooling nf the

bearings and of the lifting device. It not only serves as a storage tank but also for

detrainment the oil

The capacity of the tank is such that the full quantity of oil is circulated not more than 8

times per hour. This results in a retention time of approx. 7 to 8 minutes from entry into

the tank to suction by the pumps. This time allows sedimentation and detrainment of the

oil.

Oil returning to the tank from the oil supply system first flows through a submerged inlet

into the riser section of the tank where the first stage deaeration takes place as the oil

rises to the top of the tank. Oil overflows from the riser section through the oil strainer

into the adjacent section of the tank where it is then drawn off on the opposite side by

the suction pipe of the oil pumps. Main oil tank has the following mountings:

1. AC auxiliary oil pump - 2nos.

2. DC emergency oil pump - 1 no.

3. Shaft lift oil pump - 3 nos.

4. Oil Injector - 1 no.

5. Oil Vapour extractor - 2 nos.

6. Oil level indicator

7. Sonar level-limit switch

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MAIN OIL PUMP WITH HYDR. SPEED TRANSMITTER

The main oil pump is situated in the front bearing pedestal and supplies the entire

turbine with oil that is used for bearing lubrication, cooling the shaft journals and as

primary and test oil. The main oil pump is driven direct from the turbine shaft via the

coupling. These pumps also convey oil in the suction branches of the main oil pump for

oil injector, which maintains a steady suction flow to main oil pump.

Hydraulic speed transmitter operates on the same principle as a centrifugal pump

impeller. The variation of the pressure in the primary oil circuit due to a speed variation

serves as a control impulse for the Hydraulic speed governor. The Hydraulic speed

transmitter is supplied with control oil supplied from the control equipment rack. The

suction of the pump is always flooded and hence maintains an uniform suction

pressure.

ELECTRICAL SPEED PICK UP

A non-magnetic disc of the electrical speed transmitter in which small magnets are

inserted around the circumference gives impulses to the electrical speed pick up.

When the disc rotates with the pump running, an electric current arises due to the

alternating effect between the magnets and the Hall generators. This current is

forwarded as a signal to the electrical speed pick up.

AUXILIARY OIL PUMP

The auxiliary oil pump is a vertical one stage rotary pump with a radial impeller and

spiral casing. It is fixed to the cover of the oil tank and submerges into the oil with the

pump body It is driven by an electric motor that is bolted to the cover plate of the main

oil tank. The pump shaft has a sleeve bearing in the pump casing and a grooved ball

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bearing in (he bearing yoke. The bearings are lubricated from the pressure chamber of

the pump; the sleeve bearing via a bore in the casing; the grooved ball bearing via lube

line.

D.C.BEARING OIL PUMP

This is a vertical, centrifugal submerged type and serves for lubrication and cooling of

the bearings during emergency conditions when one of the other pump faMs. This is

driven by a D.C.motor.

SHAFT LIFT OIL PUMP

The lift oil pump is a self-priming screw-spindle pump with three spindles and internal

bearings. The pump supplies the oil to lift the turbine rotor at low speeds.

FUNCTION

When the turbine is started up or shut down, the hydraulic lifting device is used to

increase or maintain the oil film between rotor and bearings. The necessary torque from

the hydraulic turning device or from the manual turning device is reduced in this way.

MODE OF OPERATION

The bearings are relieved by high pressure oil that is forced under the individual bearing

pins, thus raising the rotor. In order to avoid damage to the bearings, the lifting oil pump

must be switched on below a certain speed.

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LIFTING OIL PUMP, OIL SUPPLY

The lifting oil pumps, which are jack-screw immersion pumps situated on the tank,

supply the high pressure oil for the lifting device. The oil is drawn off directly by one of

two three phase a.c. motor driven 100% pumps. The d.c. motor driven emergency lifting

oil pumps ensure the lifting oil supply if the A.C. lifting oil pumps fail. The pressure oil

piping of the lifting oil pump that is not in operation is closed by the check valves. In

order to protect the lifting oil system from damage due to improper switching on-off the

lifting oil pump when the check valve is closed, spring-loaded safety valves are situated

in the piping between the lifting oil pumps and the check valves.

The necessary pressure in the system is kept constant by means of the pressure

limiting valve. The pressure limiting valve can be relieved by the bypass valve. The

superfluous flow from the pump is conducted into the main oil tank.

The necessary lifting oil pressures are set for each bearing by the fine control valves in

the oil pi |.—f'1 eh llUi valves in the lifting oil pipes prevent oil from flowing out of the

bearings into the header during turbine operation when the lifting device is naturally

switched off.

OIL VAPOUR EXHAUSTER

The function of oil vapour exhauster is to produce a slight negative pressure in the main

oil tank and in the bearing casing and thus draw off the oil vapour.

The exhauster and the motor attached to it with flanges are a closed unit. The casing is

constructed as a spiral with aerodynamic features and is provided with supports for the

exhauster. The motor is bolted to the cover of the casing.

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DUPLEX OIL FILTER (FOR JOURNAL AND THRUST BEARING)

It is provided to filter the oil before supply. The duplex filter consists of two filter bodies

and is fitted with a changeover device which enables the filters to be switched as

desired.

OIL COOLER

Function of oil cooler is to cool the lubricating oil supplied to the bearings of turbine.

Oil cooler consists of the tube nest the inner, outer shell and water boxes. The tube nest

through which the cooling water flows is surrounded by the oil space formed by the

outer shell. The oil to be cooled enters the oil cooler and flows to the inner shell. This

shell support0 the large affle plates which are provided with an" opening in the centre.

Between every two large plates there is a small intermediate plate which is held by the

short tubes placed into the steel rods. The small intermediate plate is smaller in

diameter than the inner shell and leaves an annular gap. This arrangement serves to

achieve a cross-flow pattern forcing the oil flowing to the outlet branch to flow through

the middle of the large plates, while passing round the edge of the short ones. The inner

oil shell with the large plates is attached lo the lower tube plate into which the finned

cooling tubes are expanded. The water box with a cooling water inlet branch is bolted to

the lower tube plate. The tube nest is free to expand upwards in response to any

thermal effect.

THREE-WAY CONTROL VALVE

The three-way control valve is electrically driven and has the function of regulating the

lubricating oil temperature. Possible oil flow paths for regulating the oil temperature:

(i) All lubricating oil flows through oil cooler

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(ii) Lubricating oil flows through oil cooler and by-pass piping.

(iii) All lubricating oil flows through the by-pass piping.

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11. Turbine Control Fluid System

GENERAL DESCRIPTION

A number of turbines use high pressure lubricating oil (termed 'power oil') as the

hydraulic fluid to operate turbine steam admission valves. This power oil is supplied

from a turbine-shaft-driven oil pump. For start-up and emergency conditions, an

electrically-driven power oil pump is provided. Large modern units tend to use (ire-

resistant fluids (FRF) instead of lubricating oil. since the risk of fire due to high pressure

lubricating oil in proximity to hot components is then eliminated.

The power oil supply to the steam admission valves of the turbine described is derived

from a separate hydraulic package, using fire-resistant fluid as the operating medium.

The package and connecting pipework are made of stainless steel.

CONTROL FLUID SYSTEM

The turbine governing system is supplied with fire resistant control fluid and the control

fluid system is independent of turbine oil system.

In 500 MW. Fire Resistant Fluid (FRF) is having a separate tank called as control fluid

tank. Control fluid pumps are mounted on this tank itself. These pumps have four

stages. A tapping is taken after the 1st stage developing 8 bar ( referred to as LP

stage). The discharge of these pumps is at 32 bar (HP stage). The HP-control fluid is

mainly used as power fluid for the servomotor piston operation and the IP-control fluid is

used for control junctions. The control fluid is passed through very fine filters of 3 to 5

micron mesh size. It is also provided with a regenerative system which filters and

eliminates acidity and mechanical impurities.

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HP CONTROL FLUID PUMP WITH LP EXTRACTION

The extraction or dual pressure pump is a vertical rotary pump in multiple stages. It is

attached to the cover of the fluid tank and submerges in the control fluid. Driven by a

electrical motor located on the cover plate of the tank. After one stage oil is delivered to

LP control fluid circuit and the HP control fluid is taken after 4 stages of the pump. The

pump shaft is guided by a sleeve bearing in the suction casing and by ball bearings in

the bearing support. The bearings and the bevel gear coupling are lubricated from the

first stage pressure chamber.

CONTROL FLUID TANK FUNCTION

The control fluid tank contains the fire resistant control fluid necessary for the governing

system of the turboset. Apart from its function of storing, it also deaerates the control

fluid.

The tank is designed so that entire contents can be circulated a maximum of 8 times per

hour. This results in the control fluid remaining approximately 7 to 8 minutes" in the tank

in which time any air in the fluid can be separated and any ageing materials deposited.

FLOW OF CONTROL FLUID IN TANK

The control fluid returning from the governing system enters the tank through the control

fluid inlet and flows into the riser chamber of the tank where the first deaeration takes

place The control fluid then drops t trough the strainer into the adjacent chamber and

flows to the control fluid pumps which conduct the fluid to the governing system.

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CONTROL FLUID PUMP

The control fluid pumps are situated on the control fluid tank and immerse with the

pump bodies into the control fluid in the tank. They draw from the deepest point in order

to conduct control fluid that is as free of air as possible. The driving elements of the

pumps are fixed to plates on the cover of the tank.

CONTROL FLUID STRAINER

The control fluid strainer is a basket strainer installed in the tank. It is 0.28 mm wire

mesh and can be exchanged by opening the hatch.

CONTROL FLUID INDICATOR

The control fluid tank is provided with a local liquid level indicator and level switches

with which the maximum and minimum levels of the control fluid can be transmitted. A

storage space is provided between the operating level of the control fluid, which

corresponds to the nominal contents of the tank and the tank cover. This can

accommodate the fluid in the entire control fluid system when the turbine is shut down.

The floor of the tank is sloping with discharge facilities at the deepest points.

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Fig. No.- 22 REGENERATING PLANT

1. Control fluid tank 2. Control fluid puny 32/8 bar 3. Saftyvotv 4. Circulating pump 5. Shut-off value 6. Fuller’s earth filter 7. Mechanical fine filter 8. Strainer a. Control fluid approx, 8 bar a1. Control fluid approx 32 bar c. Return flow C.1 Reser room drainage C.2 Main room drainage

REGENERATING PALANT

GENERAL

Of the various types of fire resistant fluid, the only ones suitable for use with KWU

turbines are phosphoric esters of the group HS-D which have a lower water and

chlorine content.

Their chemical composition and structure necessitate certain measures and alterations

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compared with an oil system.

Fire resistant fluid system for KWU turbines are provided with a bypass regenerating

plant as shown Fig No. 22. The design of this plant is made to the specifications of the

fluid manufacturer. Any acids and ageing products are removed during operation by the

continuous filtering through Fuller's earth and mechanical fitters.

The mode of operation of this natural earth treatment is based on a ion-exchange

reaction. In addition to the precautions against acidifying of the fluid, continuous care is

taken that any solid particles are separated by the fine filter so that they-can. not speed

up the reaction. The fine filter of this plant retains particles of Fuller's earth as well as

providing the essential cleanliness of the whole system and increasing the life of the

filters.

The Fuller's earth needed for regenerating the fire resistant fluid must be dry (at 150°C

the amount of expellable water must only be 1% of the weight).

The strainer number 30/60 mesh is the granular size to be used (or this must

correspond to the details from the fluid manufacturer). The dust proportion of the

granulate must not be used. The amount of earth must not be too tittle and must be

stamped or shaken to avoid the formation of gaps and channels which would reduce the

effectiveness of the Fuller's earth.

The efficiency of the regenerating plant is to be controlled by an exact record of the

neutralization value and the degree of purity.

CONSTRUCTION OF REGENERATING PLANT

The filter group consists of 2 Puller's earth filters and a mechanical Filter. The cleaning

and deacidifying takes place in a separate circuit. A pump conducts a constant amount

of fluid through the filter group into the tank. When the filter is contaminated there is an

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increase in the fluid pressure. A spring safety valve is installed to protect the system

against an excessively high increase in pressure.

FULLER'S EARTH FITTER

The Fuller's earth filter contains three sections with a special granulate which binds the

acid present. Two filters work in parallel and cannot be switched over.

MECHANICAL FINE FITTER

Following the Fuller's earth filters is a fine filter with textile inserts of finest mesh. These

inserts retain the finest particles of dirt, both metallic and non-metallic impurities. In this

way the fine filter also serves the safety of the control fluid system by trapping any

particles of granulate that may be circulating. The fine filter also separates water

particles and other ageing materials which would make it necessary to renew the control

fluid too soon.

SOME USEFUL HINTS ABOUT FRF

It is a phosphoric ester (mild chemical) and causes itching to eyes, nose. ears etc. If

comes in direct contact. It has got high affinity towards oxidation thus care must be

taken to prevent entry of moisture into this system.

1. Possibilities of mixing of FRF with lub. oil The chances of mixing up of these two oils is at following junctions:

i) Hydraulic Governor: In this governor the lube oil is present as signal of

speed in the form of primary oil. To prevent mixing up of oils here a bellow

has been provided.

ii) Overspeed Trip Device:- At this place also FRF and lube oil are separated

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with the help of bellows.

iii) Thrust bearing trip device:- Proper drainage system and introduction of

bellows have prevented any possible mixing up.

2. Trip Fluid This fluid is generated in the main trip valves. The pressure of trip fluid will

appear only after the Main trip valves are reset, (cocked up) with the aux. start up

fluid.

3. Aux Trip Fluid This is also generated in the main tripe valves and keeps the main trip valves in

cocked up position, once the aux. start up fluid is withdrawn.

4. Start Up Fluid It is generated in the start up device. It is function is to supply or drain trip oil

through test valve from the HP/IP stop valves servomotors. When start up fluid is

developed it helps in resetting the stop valves which starts opening when this oil

is being denied in the start up device.

5. Aux Start Up Fluid It is generated in the start up device. It is used in resetting the protection device

i.e. local trip valve, main trip valve, thrust bearing trip device and overspeed trip

device. Once these device are reset and aux.trip fluid generated then aux. start

up fluid can be drained and the function is governed by the aux. trip fluid.

6. Primary Oil It is generated in the primary oil pump coupled with MOP and operates with

flooded suction so as to correctly reflect the speed. This oil signal is used in

hydraulic governor and low vacuum trip device, as a speed component.

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7. Aux. Secondary Fluid This is generated in the aux. followup pistons (mounted with the hydraulic

governor) and operates hydraulic amplifier. One solenoid valve is also provided

in this line to drain this fluid in case load shedding relay gets energised

8. Secondary Fluid: Connected to the hydraulic amplifier/ Electrohydraulic converter is a battery of

follow up pistons. These follow up pistons generate secondary oil which is in turn

responsible for the control valve opening. Each battery of follow up pistons

contain six follow up pistons. Three follow up pistons are for HP control valves

and three for IP control valves.

9. Test Fluid Supplied through a solenoid valve.(8 bar control fluid filtered) to the thrust

bearing trip device for testing this protection through ATT.

10. Test Oil Overspeed

It is generated in overspeed trip test device. This oil is supplied under the

overspeed bolts for testing of overspeed protection.

11. Singal Fluid LP Bypass This oil is generated in the follow up pistons connected with converter of LP

bypass governor. It is directly used for the operation of water injection valve and

through various protections to LP bypass stop and control valve operation.

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12. Constructional Features Of Turbine Governing System And HP/LP By Pass System

HYDRAULIC SPEED GOVERNOR Function

The function of the hydraulic speed governor is to operate the control valves to give the

appropriate turbine steam for the particular load condition.

Construction

The principal components of the speed governor are the bellows(8). the link (11), the

speed setting spring(13), the sleeve (5) and the follow-up piston(4) as shown in Fig No.

23. The primary oil supply from the hydraulic speed transmitter is available at

connection a1. A fire resistant fluid is used as the hydraulic fluid in the governing

system. An additional bellow (9) prevents primary oil getting into the control fluid circuit

should there be a leakage in the governor bellows (8). In this case the leakage oil can

be drained off via connection c1. Should a leak in the bellows (9) occur, the control fluid

that has leaked in will also be drained off via connection c1.

The primary oil pressure (connection a1) is dependent on the speed and determines the

.position of the link (11) via the bellows (8) and the pushrod (10). The speed setting

spring (13) opposes the primary oil pressure. Its precompression can be varied either by

hand or remotely by the motor (16). The sleeve (5) which can slide on the bottom end of

the follow-up piston (4) is attached to the link (11). The follow-up piston is held against

the auxiliary secondary fluid pressure (connection b) by the tension spring (3). The

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follow-up piston and sleeve have ports which at normal overlap allow sufficient fluid to

escape to produce equilibrium between the auxiliary secondary fluid pressure and the

force of the tension spring (3).

Each steady-state position of the link (11) and hence of the sleeve (5) corresponds to a

specific force from the tension spring (3) and hence to a specific secondary fluid

pressure which in turn determines the position of the control valves.

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MODE OF OPERATION

If the primary oil pressure falls (as a result of increasing load and the resulting drop in

speed), the link (11) and the sleeve (5) sliding on the follow-up piston (4) are moved

downwards by the speed setting spring (13) so that the overlap of the ports in the

sleeve and the follow-up piston is reduced. This causes the pressure in the secondary

fluid circuit to rise and the follow-up piston follows the movement of the sleeve against

the increasing force of the tension spring (3) until normal overlap of the ports and

equilibrium are restored. The lift of the control valves is increased in this manner by the

increased secondary fluid pressure.

Conversely, a rise in primary oil pressure causes the lift of the control valves to be

reduced.

When the precompression of the speed setting spring (13) is varied with the reference

speed setter it changes the relationship between the primary oil pressure and the

secondary fluid pressure and hence the relationship between speed and power output.

Lever (12) allows the link (11) to be depressed by hand to give a lift signal to the

governor, e.g. to provide a second means of overspeeding the machine for testing the

overspeed trips in addition to the overspeed trip tester.

Starting and load limiting device

Before start-up, the pilot valve (21) is brought to its bottom limit position either by hand

or remotely by the motor (20). This causes the bellows to be compressed via the lever

(6) and the pin (7) until the governor assumes the position "Control valves closed". With

the valve (21) in the bottom limit position control fluid from connection can flow

simultaneously to the auxiliary starting fluid circuit (connection u1) and as starting fluid

via connection u to the stop valves to prepare these for opening. When the valve (21) is

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moved back the auxiliary starting fluid circuit is depressurized and subsequently the

starting fluid connection u is opened to the return c. This opens the stop valves. Further

upward movement of the valve (21) causes the pin (7) to release the bellows as with

falling primary oil pressure and the control valves are opened. The release of the

bellows can be limited by the pin (7) so that the control valves do not open any further

despite a reduction in primary oil pressure.

COMBINED MAIN STOP VALVE AND CONTROL VALVE Function

One stop and one control valve are combined in a common body The main stop valve

provides a means of isolating the turbine from the main steam line and can rapidly

interrupt the supply of steam to the turbine. The function of the control valve is to

regulate the flow of steam to the turbine according to the prevailing load.

SERVOMOTOR FOR MAIN STOP VALVES AND REHEAT STOP VALVE

The operative part of the servomotor consists of a two part piston the lower disc-shaped

part of which is connected via piston rod to the valve stem as shown Fig No 24. The

other part of the piston is bell-shaped and moves within the housing which is in the form

of a cylinder, Two spiral springs are placed between two halves of the piston; at the

lower end a spring plate is interposed between the springs and the piston disc. When

trip fluid is admitted to the space above the bell-shaped part of the piston, it moves this

half of the piston downwards, compressing the springs, until it seats against the piston

disc.

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After the main stop valves have been opened, the turbine is started by the control

valves. Before the main stop valves can be opened, however, they must be

"pressurized ", i.e. prepared for opening, by admitting trip fluid from the trip fluid circuit

to the space above the piston to press it down against the piston disc after overcoming

the resistance of the springs. The edge of the bell-shaped half of the piston is designed

to produce an fluid-tight seal with the piston disc.

To open the valve, fluid from the trip circuit is admitted to the space below the piston

disc and, simultaneously, the space above the bell-shaped half of the piston is opened

to drain. This causes both halves of the piston to move together in the direction which

opens the valve. In order to reduce fluid leakage past the bell-shaped part of the piston

when the valve is open, a back seat is provided in the housing against which the collar

of the piston can seat.

When the valve is tripped, the pressure in the trip fluid circuit, and hence in the space

below the piston disc, falls, with the result that the springs separate the two halves of

the piston and the piston disc connected to the valve stem moves to close the valve.

Just before the valve disc seats, the piston disc enters a part of the cylinder where the

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diametral clearance is reduced. This arrangement restricts the flow of fluid past the

piston disc and so produces a braking action which causes the valve disc to seat gently.

All fluid connections are routed through a test valve. All operations can be controlled by

means of the test valve and the starting and main trip valve.

AUXILIARY VALVE

The auxiliary valve controls the fluid supply to the extraction check valve actuators and

its function is to give the check valves a signal to close in the case of a drop in load or

tripout so that steam can not flow out of the bleeder lines back to the turbine. The

auxiliary valve several check valves.

EXTRACTION SWING CHECK VALVE

Extraction swing check valves are provided to prevent the backflow of steam into the

turbine from the extraction lines and feedwater heaters.

Two free-swinging check valves are installed in each of the extraction lines. In the event

of flow reversal in the extraction line, the valves close automatically, whereby actuator

assists the closing movement of the first check valve.

The mechanical design of the swing check valves is such that they are brought into the

free-swinging position by means of the trip medium pressure via actuator and the disc

level, and are then opened by the differential pressure present. If trip medium pressure

falls, the swing check valve is moved into the steam flow by means of spring force

acting via the lever, hinye pin and disc lever, and closes when differential pressure is

either lowered or reversed.

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CONVERTER FOR ELECTROHYDRAULIC BYPASS GOVERNOR Function

The converter, which is a jet pipe hydraulic regulator as shown in Fig No. 25 has a

plunger coil system (2) that detects a direct current from the electrical LP-bypass

governor. This electrical measured variable is converted into a. force and transferred to

the jet pipe.(3).

Mode of Operation

The control fluid supplied to the jet pipe(3) via connection a leaves at high speed and

travels to the nozzles of the pressure distributor.

The energy due to the velocity of the jet is converted into compression energy in the

pressure distributor which is connected to both sides of the associated control piston by

two pipes.

1. Electric LP bypass governor 2. Plunger coil measuring system 3. Jet pipe 4. Adjusting spring 5. Adjusting screw 6. Jet pipe regulator

a Control fluid

a1 Control fluid under control piston of differential pressure relay a2 Control fluid above control piston of differential pressure relay

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The position of the jet pipe(3) depends on the voltage of the direct current to which the

plunger coil system is subjected and on the adjusting springs (4). If the variable to be

controlled is at its nominal value, the forces of the plunger coil system (2) and the

adjusting system (springs 4) are in equilibrium. The jet pipe is then in the centre

position, the pressure on both sides of the associated control cylinder-is equal and the

piston is at rest. By using the adjusting screws (5), the initial stress of the adjusting

spring (4) can be changed and thus the required nominal value set according to the

design data.

If the variable to be controlled deviates from its nominal value, the jet pipe (3) moves

away from its centre position via the plunger coil system and conducts fluid above or

below the associated control piston which is thus moved upwards or downwards (1)

ELECTRO-HYDRAULIC L.P. BYPASS CONTROL SYSTEM FUNCTION

The function of the I.P.bypass control system is to monitor the pressure in the reheat

system and to control it under certain operating conditions. During start-up and

shutdown, and at operation below minimum boiler load, the volume of steam not utilized

by the i.p. and 1 p. cylinders of the turbine must be passed to the condenser via the I.P.

by pass valves as shown Fig No. 26. This requires the bypass control system to

maintain the pressure in the reheater constant in accordance with the pre-set reference

value. In the event of disturbance, e.g. load shedding or trip-out, the amount of excess

reheat steam passed to th^ ondenser depends on the capacity of the condenser.

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Mode of operation

In the electro-hydraulic 1.p. bypass control system, the electric controller governors a

number of hydraulic actuators. The line between the electric controller and the hydraulic

part of the control system is provided by the electro hydraulic converter in the form of a

jet-pipe amplifier controlled by a plunger coit.

In order to monitor the flow-dependent reheat pressure, irrespective of whether fixed-

pressure or variable-pressure operation is used. the pressure before the h.p. drum

blading. which is also flow-dependent, is used as the reference input for the electric

controller. This variable -reference value is replaced by a fixed reference value for

certain operating sequences, such as start-up and shutdown. The controlled variable is

the reheat pressure after the boiler outlet.

The electric controller has a characteristic tailored to the pressure distribution within the

turbine and monitors the reheat pressure. Monitoring is either a function of the reference

input "Pressure before h.p. reaction blading" or a function of the fixed reference value

under certain operating conditions. If the reheat pressure exceeds the reference value,

the electric controller will act on the plunger coil of the electro-hydraulic converter and

initiate bypass operation.

The bypass control system operates the combined bypass emergency stop and control

valves via various intermediate elements. This double shut-off arrangement separates

the condenser from the reheater both during normal operation and when it cannot

accept any more bypass steam.

The hydraulic part of the control system includes the necessary protective and safety

devices for the condenser as well as the interlocks for the water side.

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BYPASS LIMITING REGULATOR Function

The function of the bypass limiting regulator, which is a jet pipe hydraulic regulator, is to

prevent steam being blown into the condenser when the limit values condenser

pressure too high; Bypass flow too much and injection water pressure too low are

exceeded. This is done either by closing or not releasing the bypass stop and control

valves (1) as shown in Rg No. 27.

The jet pipe regulator (2) has an adjusting and a measuring system. The adjusting

system allows the nominal value of the controlled variable to be achieved by effecting a

force which counteracts the force of the measuring system. This counter force of the

adjusting system is produced by a spring loading which can be manually set within the

required limits by means of an adjusting screw. The measured variable is transformed

into a force and transferred to the jet pipe (1)

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Fig. No. - 27 BYPASS LIMITING REGULATOR

Mode of Operation

The control fluid supplied to the jet pipe(1) via connection a leaves at high speed and

travels to the nozzles of the pressure distributor. The energy due to the velocity of the

jet is converted into compression energy in the pressure distributor which is connected

to both sides of the associated control piston by two pipes.

The position of the jet pipe (1) depends oh the forces of the corrugated tube measuring

system acting on both sides of the jet pipe and on the forces of the adjusting system,

which also act on both sides. The adjusting system consists of adjusting springs and

adjusting screws for setting the nominal value. If the variable to be controlled is at its

nominal value, the torces of the measuring system and the adjusting system are in

equilibrium. The jet pipe is then

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in the centre position, the pressure on both sides of the associated control cylinder is

equal and the control cylinder piston is at rest. By using the adjusting screws (4), the

pre-tension .of the adjusting springs (3.9) can be changed and thus the required

nominal value set according to the design data.

If the variable to be controlled deviates from its nominal value, the jet pipe(1) moves

away from its centre position via the corrugated tube measuring system and conducts

fluid above the below the associated control piston which is thus moved upwards and

downwards.

Turbine gland sealing system:

HP-Turbine, IP-Turbine and LP-Turbine gland leak off are connected to seal steam

header and vapour exhauster system as shown in Fig No. 28. Initially gland steam

requirement for all the three cylinders is met by supplying auxiliary steam to the seal

steam header and the header pressure is maintained by the seal steam control valve.

When unit load is raised above 30 to 35% HP & IP-glands start supplying gland leak off

steam to the header to feed the requirement of LP glands. Beyor d 40% load. no aux.

steam is required. Once the self sealing takes place, then the seal steam header

pressure is maintained by opening the leak of steam control valve to condenser.

Leak off from the stop and control valves spindles, HP-inlet pipe coupling leak off are

normally connected to seal steam header only. The.end pockets of the Turbine gland is

connected to the suction of the gland steam vapour exhauster through the gland steam

condenser. This system presents hot gland steam escaping into atmosphere/preventing

the air from getting sucked into the turbine.

Gland steam header is provided with a motor driven drain valve which helps to raise the

gland steam temperature during start up. This drain valves normally remains open till

the gland steam temperature to LP gland increases beyond 150°.

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13. Turbine Tripping Devices TURBINE TRIPPING DEVICE

Function

The function of the tripping device is to open the trip fluid circuit in the event of abnormal

conditions, thereby closing the valves and thus shutting off admission of steam to the

turbine.

CONSTRUCTION

The tripping device consists in the main of the two valves (12) that slide in the casing

(11) and are loaded by the springs (5,6) as shown in Fig No. 29. The valves (12) are

designed as different at pistons being forced tightly against the body assemblies (10) by

the rising pressure of ie fluid. Control fluid flows into the body (11) via connection a and,

with a tripping device latched in'(in the position shown), into the trip fluid circuit via

connection x. The trip fluid circuit leads to the stop valves and the secondary fluid

circuits. Via passages drilled in the body(11) (section A-A) fluid flows to the auxiliary trip

fluid circuit which leads to the hydraulic protection devices.

Operation

When starting the unit. the valves (12) are lifted by the start up fluid (connection ul)

against the force of the springs (5,6). and forced tightly against the assemblies (10). In

this way pressure is built up in the trip fluid circuit (x) and the auxiliary trip fluid circuit

(x1). The pressure in the auxiliary trip fluid circuit keeps the valve in the position shown

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while the start up fluid drains through the start up device.

Should the fluid pressure in the auxiliary trip fluid circuit drop below a specific value for

any reason (e.g. by tripping of a protection device) the valves (12) move downwards

due to the spring force and their own weight, thus connecting connections x and x1 with

the fluid back flow c. This depressurizes the trip fluid circuit which causes the main and

reheat stop valves to close. The fluid supply to the secondary fluid circuits is also shut

off, thus causing the control valves to close.

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The two valves (12) work independently of each other so that even if one valve fails the

function of the tripping device is not impaired. The limit switches (1) transmit electrical

signals to the control room.

Emergency Trip Valve

The emergency trip valve enables the turbine to be manually tripped from the governing

rack. This valve blocks the draining of auxiliary trip fluid when it is in it's normal position.

But when this valve (Ball head) is pushed down, it drains the aux. trip fluid and actuates

the emergency tripping thereby tripping unit.

Solenoid valve for Remote Trip Out

The solenoid valve is installed in the aux. trip fluid line to the automatic trip gear and

when operated electrically causes the auxiliary trip gear and when operated electrically

causes the auxiliary trip fluid circuit to the opened and the turbine stopped the solenoid

valve is remote controlled. All remote tripping commands actuates this solenoid valve

only.

Overspeed trip

The function of the overspeed trip releasing device is to open the auxiliary trip fluid

circuit and thereby shutdown the turbine when an overspeed is reached which would

subject the turbine to a high centrifugal force.

When the overspeed trip operates, the eccentric bolts fly out radially and strike the

pawls which in turn causes the aux. trip fluid to drain and hence the tripping of the

turbine. Aux. trip fluid drains are connected back to control fluid tank and is prevented

form getting mixed up with bearing luboil.

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Thrust Bearing Trip (Axial Shift)

The thrust bearing trip opens the auxiliary tripping circuit in the event of axial

displacement of the rotor which can be caused by excessive wear of the thrust bearing

pads. This draining of the aux. trip fluid causes the unit to trip.

Low vacuum Trip

The purpose of the low vacuum trip is to operate the trip valve by draining the aux. trip

fluid whenever the condenser back pressure increases beyond the permissible limits.

The range in which the vacuum safety device operates can be varied by adjusting the

initial tension of the spring provided in the safety device. In order to prevent aux. trip

fluid from getting drained during starting and testing, when vacuum is not there an

auxiliary pilot valve cuts off the draining of the aux. pilot valve is released as soon as the

speed of the machine attains certain value by connecting primary oil lines to the aux,

pilot valve and thus enabling the vacuum trip device to come into operation.

VACUUM BREAKER FOR REDUCING THE RUNNING DOWN OF THE TURBINE FUNCTIONS

With normal shut down or tripping of the machine, the function of the vacuum breakers

is to cause an increase in condenser pressure by conducting atmospheric air into the

condenser together with bypass steam flowing into the condenser from the bypass

station. When the pressure in the condenser increases, the ventilation of the turbine

blading is increased, which causes the turboset to slow down so that the running down

time of the turboset and the time needed for passing through critical speeds are

shortened.

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Total Vacuum Breakers

In special cases requiring a rapid shut down of the turboset, the total vacuum breaker is

employed.

Shaft Position Measuring Device

The function of the shaft position measuring device is to measure the differences

between the axial expansions of the turbine casing and rotor. It is mainly used during

the commissioning stages and inspections. It is not a continuous measuring device. The

shaft position measuring devices can be engaged and disengaged by means of a lever

provided in it.

Low Lubricating oil Pressure Tripping

Whenever the lube oil pressure at the turbine axis reduces to less 1.3 atg. (slight

variation may be there), the unit is tripped and at a pressure of 1.1 atg, D.C. emergency

oil pump will cut in.

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14. Automatic Turbine Run Up System INTRODUCTION

To have the higher unit availability and the optimum utilization of the fuel and capacity of

the power generating units, keeping in view the high capital involved, high-fuel cost and

ever increasing demand in the grid, it is always been a point to have minimum outage of

the power generating unit reduce the extent of damage in case of faults and the facility

to detect the faults and their causes, facility to have and overview of the system and its

performance and the facility for start up and shut down of the power generating unit in

the minimum operating time. it is always desirable to have proven and reliable system to

perform all the above mentioned functions and have high availability of the system. In

view of the above, the turbine is equipped with Automatic Turbine Run Up System

(ATRS), Turbine Supervisory Instruments (TSI), Turbine stress evaluator (TSE), Electro

hydraulic Governing system (EHG) etc.

The ATRS works in conjunction with other turbine related controls like TSE, TSI, EHG

etc to have a centralised control of turbine and related auxiliaries. The signals,

commands and feedback status of various above mentioned turbine control system are

to be acquired, validated and executed so as to achieve proper operation and control of

Turbine generator set.

For start up, acquisition and analysis of a wide variety of information pertaining to

various parameters of steam turbine, demand quick decisions and numerous operations

from the operating personnel. In order to reduce the arduous task of monitoring various

parameters and effect sequential start up, minimise the possible human errors and to

achieve start up in minimum time in optimum way, Automatic Run Up System (ATRS) is

introduced.

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For all our previous projects like Singrauli, Korba, Ramagundam, Farakka Stage I & II

etc. We have ATRS implemented in solid state hardware i.e. electronic part of the ATRS

wore realised in printed circuit boards using transistors, resistors etc. and for operator

intervention and observation and plant control & monitoring, conventional pushbutton,

hand/auto stations indicating lights, lamp indications etc. were being provided on unit

control Desk, (UCD) and Unit Control Panel (UCP). However for NTPC's future projects

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due to the advent of microprocessor based technology and CRT/KBD based plant

control and monitoring philosophy, it has been decided to specify ATRS based on state-

of-the-art micro-processor technology having CRT/KBD operational features, so as to

have uniform and operating hardware philosophy for plant C & I as well as for turbine C

and I. Accordingly from Farakka Stage-Ill onwards micro-processor based ATRS with

CRT/KBD control and monitoring facility is being specified for turbine operation and

control.

CONTROL AND MONITORING PHILOSOPHY

The ATRS is based on functional group philosophy i.e. the main plant is divided into

clearly defined sections called functional groups such as oil system, vacuum system,

turbine system (Fig No. 30 Fig No. 31). Each functional group is organised and

arranged in sub group control (SGC), sub loop control (SLC) and control interface (Cl).

Each functional group continues to function automatically all the time demanding enable

criteria based on process requirements and from neighboring functional groups if

required. In the absence of desired criteria. the system will act in such a manner as to

ensure the safety of the main equipment.

The task of the ATRS control system is to control, monitor and protect all devices/drives

for:-

- Start up and shut down sequence performed in a reliable way.

- Protect drives and related auxiliaries.

- Uniform and sequential information- to operator about the process.

- Distinct information about the nature and location of faults.

The ATRS is based on functionally decentralised Hierarchical structure. Functionally

decentralised means that each and every drive/actuator has its own dedicated set of

hardware including controllers, interface modules etc. The "Hierarchical structure" refers

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to the control system divided into three control levels to achieve higher degree of

automation. Each control level has its own specific task and depends on the

subordinate lower control levels. If the higher control level fails, the next control level is

not affected and allows the plant to run safely.

Levels of control available in ATRS are as follows:

Functional group control The functional group control is the automatic control

of a part of the system, this part being mostly

independent (functional group) or of a part process.

These controls obtain: - group controls (GC)

- sub group controls (SGC)

- sub loop controls (SLC)

Group controls Group controls contain the operational logic circuits of

the underlying sub group controls. They are designed

with logic technique and not with sequential

technique.

The group control has the task of deciding when. how

many and which of the underlying sub-group controls

shall be operating or stopped.

In all cases when a functional group is only composed

of one single subgroup, the group control only

decides when the functional group shall be operating

or stopped. For a unified concept and a unified

operational technique for all functional group controls

the group control shall also be used for functional

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groups with one subgroup control only.

Subgroup controls The subgroup controls contain the sequential logic for

switching machines on and off, including all auxiliary

equipment (subgroups). They are preferentially

designed in sequential technique, except for cases

when technical and economic reasons permit a

renouncement on the sequential technique. In these

cases a pure logic circuit is used.

Sub loop Control Sub loop controls are different from other group

controls, as they can be switched on and off

manually, but they can only receive directional control

commands (servo HIGHER/LOWER, motor ON/OFF,

etc.) from criteria (such as auxiliary oil pump

automatic controls, pressure and level monitoring

controls).

System Operation Modes

The following three operating modes are possible under sequence start up modes :

Automatic Mode

In automatic mode the sub-group control co-ordinator is first switched to "Automatic

ON". The automatic control becomes effective and induces the desired operating status

only when a program has been selected ("Startup or Shutdown").

The step is set if the criteria for it are fulfilled and a command is sent to the sub-ordinate

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control interface. Preparation are simultaneously made for setting the next step. The

program continues in this manner from step to step. The step which outputs the control

and the criteria for the next step can be displayed in the CRT' and control station.

Manual interventions are not necessary when the program runs withouts faults.

Semi-Automatic Mode

In the semi-automatic mode enables continuation of the program step by step despite

the fact that the step criteria are missing. The presence of all criteria necessary for a

step are simulated by manual operation of a keyswitch; this enables continuation of the

program. This is useful if the program stops because e.g.. fulfilled plant criteria cannot

be detected because of a defective transmitter. Exact knowledge of the process events

is a prerequisite (or this, however.

Operator Guide Mode

The automatic control only processes information (criteria from the plant but does not

output any commands. All commands must be input to the control interface level

manually the selected step sequence now continues with the process independent of

the fulfillment of criteria.

Control Interface (Cl)

The control interface module forms the link between the individual commends and the

power plant. Each remote controlled drive has a control interface module. The module

consists of command section monitoring section, power supply and alarm section. The

command section proves the control commands according to their priority and validity

and passes actuation signals to the interposing relays in the switch gear. Solenoid

valves can be actuated directly up to certain capacity (36 W). The monitoring section

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normally checks the command functions. the position of the drive and the check back

signals and transmitters to the desk, protection logic and FGC.

System Description

Automatic turbine run up system consists of three sub group controls viz. Oil supply

system, condensate and evaluation system and turbine system. These groups in

conjunction with Electrohydraulic speed control and Turbine stress evaluator achieves

the task of synchronising and block loading of the machine or orderly shut down as

required.

SGC Oil Supply and Fire Protection System

The sub-group control for oil system performs its tasks comprising starting relevant oil

pumps, ensuring Turning of the turbine during start up and hot rolling to ensure even

distribution o? heat, this task is accomplished through jacking oil pumps & shaft turning

gear pump and ensuring the Lubricating oil at pre-defined temperature under various

circumstances. The programme is accomplished through a number of steps and seven

SLCs, these include, controls of turning gear system, AC/DC Lub oil pumps, jacking oil

pumps, oil temperature controller etc.

Shut Down Programme

The shut down programme essentially switches OFF all the equipments in the oil

system. The shut down programme would take OFF if the SGC is "ON" and operator

presses the button for shut down programme and the release criterion for shut down

programme is available. The release criterion would be available only if the HP turbine

casing has cooled down and the metal temperature is below 100°C at the top and

bottom of the casing. This interlock is necessary as an inadvertant initiating of shut

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down programme, could lead to starving of the oil supply to the bearings as well as

depriving the turbine of turning operation.

SGC Condensate and Evaluation System

The Sub-group control for condensate and evacuation system accomplishes its task

which comprises of keeping at least one of the two condensate pumps in operation,

evacuating the non-condensate gases from the system, maintaining the desired level of

condenser pressure when turbine is in operation and breaking the vacuum as and when

required by the mechanical process. The SGC acts directly on condensate pumps,

discharge valves, air isolation valves, vacuum pumps, air ejectors and air ejector

bypasses etc.

Shut Down Programme

The shut down programme can also be initiated by the operator provided the following

release criteria are available:

i. The speed of turbine is less than 200 rpm and

ii. LP bypass system has closed,

SGC Turbine

The sub-group control turbine, during start-up executes the tasks which comprise of

"Warming-up the Turbine" "Speed increase" synchronization and subsequent block

loading. This sub-group also shuts down the turbine which comprises unloading closure

of steam supply to turbine, isolation from grid and bringing the turbine drains to desired

positions. This SGC acts directly on the following systems drains, warm up controller,

starting device of Turbine Governing System, Speed and load set print devices of

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Turbine Governing System, Auto Synchroniser etc.

Shut Down Programme

If the operator switches ON SGC and press the manual push button for shut down

programme, the shut down programme would commence. No release criteria is defined

for initiating the shut down programme manually. In other words, the shut down

programme can be initiated by the operator as per the convenience of the power

system.

Unloading and Shut Down

During the planned shutdown, prior to beginning of unloading, if possible, the main

steam temperature is to be reduced, in order to keep the metal Temperature at a lower

value when the turbine is re-started after short shut down. The load and main steam

temperature are not to be reduced simultaneously. The rate of unloading is governed by

the margins shown on turbine stress evaluator.

The unloading of the set is accomplished with the help of electro-hydraulic governing

system. The generator gets isolated from the grid as a result of the action of low forward

power relay.

In the event of emergency, the turbine can be shut down under any load condition, by

operating the automatic trip gear, The turbine can be shut down either by remote

tripping via the solenoid valve or by manual tripping.

However, the shut down programme commences automatically under protection

channel if the "Condition 1 AND condition 3 "exist simultaneously" "OR condition 2 AND

condition 3 exist simultaneously". The condition "1" and "2" and "3" are described

below.

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Condition - 1 Emergency stop valve 1 and "OR" Interceptor valve 1 AND 2 are

closed and the pressure of the fluid in the governing system is less

than 5Kg/cm2.

Condition – 2 "HP control valve 1 or 2 not closed "AND" the start-up programme

is in "AND" starting device is at a position less than 56%.

Condition - 3 Generator breaker is "OFF".

Auto synchroniser (type-siemens 7 VE 2): This device precisely adjusts the generator

voltage and frequency with the grid voltage and frequency and closing command is

given before 'the phase coincidence point taking into account circuit breaker closing

time.

TURBINE STRESS EVALUATOR Function

The recent advances in steam turbine design caused power outputs and main steam

conditions to climb steadily higher. This has also involved a higher degree of material

utilization and as a result it has become necessary to pay special attention to the

additional thermal stresses which result from temperature changes. Each time a turbine

is started m from the hot or cold .condition,, each change in load and each time it is shut

down involves free thermal expansion and restricted thermal expansion which produces

the extra stress.

For economic reasons the question most important to the operator is how quickly the

turbine can be started up and loaded without causing damage or premature ageing of

the components. Under certain conditions a rapid application of load may even be

necessary in order to safeguard the turbine. Furthermore, the permissible rate of toad

© PMI, NTPC 175

change is of importance for the performance of the turbine generator in the power

system.

Whereas temperature differences within the individual turbine components are

responsible for thermal stresses, it is the mean temperature of the components which

determines the free thermal expansion. Free thermal expansion is monitored by the

turbine monitoring system.

To prevent damage due to excessive thermal stresses, recommended values for

permissible speed an bad changes are quoted by the turbine builder. Naturally, these

estimated values cannot be comprehensive enough to execute all operational changes

utilizing the permissible margins for the particular turbine to the fullest extent and taking

into account the instantaneous thermal condition of the machine.

An instrument which, from the turbine wall temperatures, determines the permissible

operational changes under all operating conditions, also taking into account the recent

operating history. The measuring points are located in the body of the first mainstream,

combined emergency stop valve and control valve in the line and the H.P. and 1.P.

turbine cylinders. the shaft of the H.P. turbine is partly responsible, and the shaft of the

1.P. turbine primarily responsible for the performance restrictions.

This instruments, the turbine stress evaluator, allows turbines to be driven in an

optimum manner, i.e. effecting changes as rapidly as possible while incurring minimum

stresses. It comprises three principal parts : a measuring section, electrical computing

circuits and a display instrument.

LATEST TREND IN TURBINE STRESS EVAIUATORS

With the advent of state-of-the-art microprocessor based technology having CRT/KBD

control and monitoring facilities, the TSE has also got a face change. The improved

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version of TSE now available is known as Turbine Stress Control System (TSCS). This

TSCS works in conjunction with ATRS and EHG to achieve all the functional

requirements

This new TSCS not only performs the general functions of TSE like computation and

display of stress margins available, continuous on-line monitoring of thermal stress

levels, limits of speed and load changes allowable, but also carry out the fatigue

analyses and provide at least three modes of turbine operation i.e. slow, normal and

fast. That is to say, depending upon the urgency of unit start up, the operator shall be

able to select any of the three modes of turbine run-up and loading.

The TSCS has its own dedicated CRT/KBD and one suitable conventional display

instrument for indication to operator. The stress margin and speed/load gradient

displays changes colour so as to attract the operators attention whenever the

permissible limits are reached/exceeded.

ADDITIONAL ADVANTAGES OF TSCS

1. Computation of residual life of Turbine.

2. Three rates of Turbine run up and loading.

© PMI, NTPC 177

15. Data Acquisitions System (DAS) INTRODUCTION

Power plant operation is a very specialised field & requires close & simultaneous

monitoring of various plant equipment like boiler, turbine, generator, feed pump etc. and

associated auxiliaries to maintain the continuous availability of the unit as well as to

ensure safe & efficient operation of these equipments. For such monitoring purposes, a

tot of plant data (analog inputs) in the form of physical measurements and status of

equipment (digital inputs) are needed (around 4500).

In olden days this was achieved in a central control room where all these

measurements were displayed in the form of indicators, recorders etp. and some local

panels in the field where some specific monitoring was carried out.

But these indicators had to following limitations:

1. Limitation of Physical Space

An indicator showing one measurement consumed lot of space.

2. Limitation of Simultaneity in Monitoring

An indicator showing a feed water/condensate measurement was far away from

an indicator showing turbine measurement & simultaneous monitoring was

difficult. More number of operators were required and co-ordination between over

burdened and tensed operators under emergency condition was difficult.

3. Limitation of Historical Storage

The only device capable of historical storing for future reference was recorder

which could only store very limited no. of analog points and for a limited time.

Most recordings of values & events had to be done'*manually.

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4. Lack of Flexibility

It was very difficult to change assignments of plant input to various indicators/

recorders.

5. Unavailability of Processed/Calculated Data

Efficiency of the Plant and equipment performance calculations had to be done

manually.

All the more these indicators suffered from tack of accuracy because of scaling

factor, limitations of range & parallax error.

Hence a need was felt for computerized intelligent system presenting data from

the entire plant in desired and on as required basis to facilitate easy monitoring,

recording and to enable operator to take quick decisions regarding operation.

This could only be possible with the help of a digital computer and this lead to the

concept of computerized data acquisition system (DAS) for power plant

monitoring and smooth operation.

COMPUTER BASED DAS

This system has the following features : (A) in the form of displays on CRTs (with a

facility of display & hard copy print out) through simple key strokes/dedicated key.

1. Variety of plant data in the form of analog measurements displayed on CRTs

(Display units) in the form of group reviews (organised in plant functional groups).

2. Graphics

Graphics showing the replica of entire plant & its subsystems & individual

equipments embedded with live plant data-current values of temperature,

pressure and the status of different pumps, motors etc. through alphanumeric or

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color codes (e.g. red for ON and green for OFF) gives the feel of the entire plant

in its latest status to the operator. This is also helpful for new operators to learn

plant operation quickly.

3. Bar Charts

Bars (both vertical & horizontal) showing like analog measurements e.g. reheater

tube metal temperatures helps the operator to compare measurements to find

the hottest spot etc.

4. Alarm Displays

- These displays helps in annunciation of a lot of abnormal conditions not

covered in the annunciation window.

- These displays helps in pin pointing the individual alarm which led to a

group alarm in the annunciation window. Further exact measurement

values & the rate of change thereof can be seen on CRTs.

5. Historical Storage

X-t plot shows plot of a physical variable like main steam pressure. Generator

Load with time nearing different intervals like 10 sec. 1 mts, 10 mts. 1 hrs, 24 hrs

even 1 day etc. Group trends shows values of different inputs collected at

different intervals over a period of time.

6. Plant Start up Guidance Messages (PSGM) & Operator guidance Messages

(OGM) Plant start up guidance messages shows the different steps/ criterias to be taken/

fulfilled before starting an equipment in the form of flow charts with the current

status of the step/criteria being completed/fulfilled or not. Operator guidance

messages shows the various steps to be taken by the operator in the event of a

fault etc.

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These displays are particularly very useful in starting/shut down of the unit.

In the Form of Printouts (Logs)

Three (3) types of logs are recorded in the system. Even & Time activated & Operator

demanded types.

a. Event Activated Logs

Data for these types of logs are collected & printed on event change(s)

1. Sequence of Events Reports

This reports contains a list of plant events in the chranological order of

occurrence (with a capability of distinguishing two events with a time difference

as low as one millisecond) following a major event/break down such as unit

trip/auxiliary trip. This report helps in quickly pin pointing the reason for the unit

trip/ auxiliary trip by tracing the sequence of events & in the restoring the unit/

auxiliary.

2. Post Trip Log

In the event of unit trip/auxiiiary trip. values of certain important parameters

before & after the trip are printed in this report which also helps the operator to

analyse the trip and finding the reason for the fault.

3. Start-up/Run up Logs

There are two such logs: Boiler start-up & Turbine Run up log.

Start-up Log

The system includes the capability to generate Boiler and Turbine Start-up Logs.

Each start-up Log (Boiler and Turbine) consists of 100 essential to start-up points

subdivided into 10 groups. 10 point each. Upon Control Room Operator

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command, the appropriate start-up Log stores on bulk memory (drum or disk)

data at a ore-assigned period of time apart for each points assigned to the

specific Start-up Log. The period for data acquisition is operator selectable (1, 2.

3. or 5 minute intervals for turbine run-up tog and at either 3.5 or 10 minute

intervals for boiler Start-up tog. Intervals are selectable by operator. Log is

initiated by turbine roll-off for turbine run up tog and at the start of the boiler

purge sequence for boiler start-up log and is stopped by the operator. The

system outputs the start-up log automatically upon completion of collection of

each 30 sets of data. There is also the capability to generate on-demand printout

of all stored in the memory but not printed out previously sets of data.

TURBINE RECALL LOG

The turbine recall log is initiated by the occurrence of any one of several alarms and

consists of all data defined for the turbine diagnostics log. The system stores

information whenever the turbine generator is on-line.

The data is stored in the computer memory at the one minute intervals. A total of 30

minutes of latest data is stored in the memory. Following the initial 30 minutes of

operation, the oldest data in the memory is discarded and replaced with new data. The

log printout is initiated by preselected alarm conditions and continues at 1-minute

intervals after initiation of the log until the alarm condition has disappeared or until

stopped by the operator.

TURBINE SHUTDOWN ANALYSIS LOG

A log of up to 100 turbine data points is printed whenever the generator is taken off line.

Data, except turbine speed, is collected at 2-minute intervals. Turbine speed is collected

at 1 second intervals for the first 5 seconds after the shutdown is initiated and at 2

minute intervals thereafter, the log continues until terminated by the operator,

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Time Activated Logs

Data for these logs are collected & printed periodically at regular intervals.

Hourly Log

An hourly log of up to 150 points is provided to furnish data for routine analysis of plant

performance. This log is printed automatically and on demand in the form of shift log

and daily tog.

Shift Log

Shift log consists of 150 points subdivided into 15 groups of 10 points each. The values

are stored each hour on the hour and are cumulative, average or instantaneous values;

all values outputed to the log which are unreliable, or where the value is calculated from

an unreliable points is indicated as unreliable with an asterisk, or similar symbol.

The report is automatically output at the end of each shift and upon' Control Room

Operator request, and is available for inspection of the incoming shift supervisor to

acquaint him with performance of the generating unit during previous shift.

The shift log. When generated automatically at the end of the each shift, is output to the

log printer and to the magnetic tape units.

Daily Log

Daily Log is a plant management and accounting oriented log, and like hourly log & shift

log consists of '50 points subdivided into 15 groups of 10 points each. The values are

automatically stored each hours on the hour, and is cumulative, average or

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instantaneous values as selected. All values outputed to the log which are unreliable, or

where the value is calculated from an unreliable, point shall be indicated as

unreasonable with an asterisk or a similar symbol. The system permits assignment of

any analog input, calculated value, transformation, or digital input to the daily log.

The log output automatically, at midnight, to the log printer and to the magnetic tape

unit. The operator may request a review of the data collected for the daily log at any

lime.

This includes incomplete average, integration and totals. The printout includes a

summary of all hourly data saved since midnight and the last line is totals,, average

integrations of all data up to the last hour to the time of the request. However, this

control Room Operator demanded printout is output to the utility printer only and does

not eliminate the requirement to produce Daily Log at midnight.

Turbine and Generator Diagnostic Log

The turbine diagnostic log shall contain up to 100 turbine and generator data points to

be used in analyzing possible turbine and generator trouble. The data shall be printed

out on demand by the operator. Once the log printoiitis initiated, data shall be collected

and printed out every minute until four sets of data have been recorded.

Summary Log

The system shall permit the operator to specify up to 5 summary logs of up to 25 points

each to be printed out on demand. The summary logs shall consist of a report of the

processed results of data accumulated for the previous 24 hour period. The logs shall

include but not be limited to; daily maximum, daily minimums, hourly values, duration of

a point in high and low alarm.

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Operator demanded logs/special logs Performance Log

Provision shall be made for logging up to 100 calculated points (Class II) performance

calculation results/The log shall be printed automatically once a day, or on demand at

any time and shall consist of hourly values of the calculated points using the most

recent averaged data every hour.

Test log

The log consists of 50 parameters obtained to be monitored in a very short time span.

Vibration log

This log consists of all vibration inputs

Motor Start limit log

This provides a record & warning of 6.6 KV motor start limits.

Maintenance data log

This log records operating/maintenance information about certain plant equipment for

the purpose of scheduling preventive maintenance & routine equipment inspection.

A sample log displays taken from DAS/V.S.T.P.P. is given in figure No. 32.

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Summaries These displays/printout enable the operator to review a specific class of points.

Alarm summary Pint/display all points (analog, calculated, and digital) that are off normal at the time the

request is made by the operator.

Off-scan summary Print/display all points (analog and digital) that are off-scan at the time the request is

made by the operator.

Constants summary Print/display a list of all plug-in constants (those normally used as constants and those

used as substitutes for inputs points used in calculations).

Scan period summary Print/display all scanned inputs by point ID number listing point English Description and

assigned scan period.

As an example: Singrauli of 2x500 MW DAS supplied by Hitachi, Japan is described

bellower.

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© PMI, NTPC 187

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This system is based on dual CPU-one master & other standby. (PI. refer Fig No. 33).

Each CPU (H-80M) has 512 KW of working memory, 4MW of bulk memory (1C file) a

floppy disk & console CRT. Both CPU's can access 35 MW (2x17.5 MW) Fixed Disk

through multiple access controller (MAC). There are two buses a set of devices

(Operator & utility CRTs, Alarm Printer. Analog Input & Digital inputs. Trend recorders,

& one Ivlag. tape) are connected to one bus and the other bus connects another set of

devices - (supervisor CRTs, supervisor Printer, programmer CRTs, Log Printer,

programmer Printers, Alarm CRT, other Mag. Tape).

The DAS has a communication processor (CLC-/ /(i H) through which communication

can be achieved with other computer systems.

HARDWARE FEATURES

CPU Model HIDIC 80 M Word 6 bits instruction Set 151 Bus width Data 16 bits Address

32 bits control 6 bits Execution Time Addition 0.48 micro sec

Multiplication 1.36 micro sec Floating point 1.84 micro sec Floating multiply 2.5 micro sec

Main Memory Word 16 bites + 6 bits Size Cycle time 512 KW 480 neno sec

Disk Model H-71 42C

Storage 88MW

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Data Rate 188 KW/sec. Access. Time seek 25 m sec. Rotational 8.5 m sec.

Mega Tape Model H-715/C

Tracks 9 Density 1600 bpi Recording PE Transfer rate 36 kw/sec. Speed 45 ips

IC File Storage 2mB non volatile

Cycle 600 m. sec. Access width 16 bits Access time 100 micro sec. Transfer Rate 600 kb/sec. Error check ECC

CRT’s Model H787 C-3

Screen 20” color High Resolution

Formal 42 lines x 96 chars 5x7 dot char. Size

High speed printers Speed 180 cps

Char. Set 96 Width 132 chars Code ASCII

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SOFTWARE System Software

HIDIC 80 DAS is having process monitor system (PMS) operating system. This is on-

line, Real-Time System with multi programming and multi-tasking functions. The basic

unit of task controlled by PMS is called task. A programme may comprise of no. of tasks

with 10 different priority levels, processing of task is done asynchronously. AH the tasks

have to complete the system measures like allocation of CPU, memory or input/output

devices, whenever a task has to suspend its processing (e.g. wait for completion of I/O),

task scheduling takes place and the next ready task with highest priority is taken up for

processing.

Real time needs can be taken care by giving an external interrupt (could be timer

interrupt) to the current task and starting another task for real time processing. PMS

provides memory protection against use of a task's memory by another task, however it

also provides an intertask communication system for transfer of data. PMS allows 2

grounds of operation: foreground and background. The DAS application program runs in

foreground. Programmes in development and testing can be done in back-ground. PMS

separates programmes into. 2 types: Resident (always resident in main memory) and

non-resident (swapping to auxiliary memory). Memory allocation to a program is done in

terms of pages (1024 words). 64 pages comprise a logical space, which is assigned to a

task. There are total 16 logical spaces numbered 0 to 15 logical space no. 0 is provided

for exclusive use by the PMS. Pages of logical space is mapped to the physical

memory. Programmes in a logical space can share information with other logical space

by having a shared memory area. This is achieved by mapping corresponding pages in

two logical spaces to same pages in the physical memory.

DAS has assembler, FORTRAN compilers.

© PMI, NTPC 191

Utilities

- Compilation & Editing

- Job submission, debugging

- CRT format modification

- I/O data base change

- Calculation Data base change

- Log data format generation.

Application Software

Apart from core hardware and systems software DAS needs application software for

end usage. A list of functions for which application software is written follows: -

1. Input Scanning & Processing

- Analog, Digital, SOE Remote input.

2. Calculations:

Basic Calculations:

- Periodic calculations

- Transformation

Performance Calculations

Equipment & Unit performance

3. Alarm Monitoring

- High, Low, High-Low, Low-Low, High-High

- Display/Printing of alarm message.

© PMI, NTPC 192

4. CRT Display Functions

- Alarm message display every 2 sec.

- Graphic diagram every 5 sec.

- Video trend every 10 sec. bar chart every 5 sec.

- Plant group display every 2 sec.

- Group alarm display every 2 sec.

- Periodic display every 2 sec.

- Operation guidance message.

5. Operator Request Functions

- Display/print/entire a point value.

- Get/display/print time & date.

- Acknowledgement of alarm messages.

- Analog Trending

- Summary Display.

6. Operator Report & Logging

- Alarm printing

- SOE log

- Pre & Post occurrence digital trend.

- Data Group Print.

- Performance deviation summary log.

- Periodic log.

- Shift/daily log.

© PMI, NTPC 193

16. Feed Regenerative System CONDENSATE WATER SYSTEM DESCRIPTION OF SYSTEM

The condensate extraction pumps deliver the condensate through the three low

pressure feedwater heaters, the deaerating feedwater heater to the deaerating storage

tank, which is the beginning of the feedwater system. Fig. No. 34 gives a symbolic

representation of the arrangement of Various Turbine auxiliaries The low pressure

feedwater heaters receive extraction steam from the turbine. The condensate absorbs

heat from the extraction steam as it passes through the feedwater heater. The

deaerating feedwater heater further preheats the condensate prior to its entry into the

deaerating storage tank. The condensate in the deaerating feedwater heater is warmed

by extraction steam during normal operation andauxiliary steam & cold reheat steam

are utilized as the heat sources during start-up & turbine shut down condition.

Fig. No. - 34 ARRANGEMENTS OF TURBINE AUXILIARIES

© PMI, NTPC 194

FEED WATER SYSTEM

The purpose of the Feedwater System is to provide an adequate flow of properly heated

and conditioned water to the boiler and maintain boiler drum level compatible with the

boiler load. This system also conveys water for the boiler reheater attemperators,

superheater attemperators, auxiliary steam desuperheaters the high pressure bypass

desuperheater and HP fill & purge SGCW pump cooler. Feedwater is heated to achieve

an efficient thermodynamic cycle.

Under normal operating conditions, feedwater flows from the outlet nozzles of the

deaerator storage tank through the Boiler Feed Booster Pumps (BF BP), to the Boiler

Feed Pumps (BFP), From the discharge of the boiler feed pump, the flow continues

through both high pressure feedwater heater strings to the boiler economizer inlets. A

bypass line around the heaters is provided for removal of either or both heater strings

from service.

The high pressure feedwater heaters receive extraction steam from the cold reheat line

and the IP turbine. The feedwater absorbs heat from the extraction steam as it passes

through the heaters.

FEED REGENERATIVE SYSTEM INTRODUCT10N

A feedwater heater is a special form of a shell and tube heat exchanger designed for

the unique application of recovering the heat from the turbine extraction steam by

preheating the boiler feedwater. Its principal parts are a channel and tube sheet, tubes,

and a shell. The tubes may be either bent tubes or straight tubes. Feedwater heaters

are defined as high pressure heaters when they are located in the feedwater circuit

upstream from the high pressure feedwater pump. Low pressure feedwater heaters are

located upstream from the condensate pump which takes its suction from the condenser

hot well. Because the discharge pressure from these pumps differs greatly, the physical

and thermal characteristics of high and low pressure feedwater heaters are vastly

© PMI, NTPC 195

different. Typically low pressure feedwater heaters are designed for feedwater pressure,

between 27 Kg/cm2 and 56 Kg/cm2 High pressure feedwater heaters range from 112

Kg/cm2 for nuclear heat sources to 335 kg/crn2 for supper critical boilers. Regardless of

the actual design pressure, the classification depends upon the cycle location relative to

the feedwater pumps. The design pressure is specified sufficiently high so as to not

over-pressure the channel side of the heaters under any of the various operating

conditions, particularly at pump shutoff.

Each feedwater heater bundle will contain from one to three separate heat transfer

areas or zones. These are condensing, desuperheating and sub-cooling zones

Economics of design will determine what combination of the three is provided in each

heater.

A condensing zone is present in all feed water heaters. Large volumes of steam are

condensed in this zone and most of the heat is transferred here

The desuperheating zone is a separate heat exchanger contained within the heater

she!!. This zone's purpose is to remove superheat present in the steam. Because of the

high steam velocities employed, condensation within the desuperheating zone is

undesirable.

The sub-cooling zone, like the desuperheating zone. is another separate counter flow

heat exchanger whose purpose is to sub cool incoming drains and steam condensate.

HEATER OPERATION

The following are precautions that should be adopted when operating these feedwater

heaters.

Start Up

Feedwater heater operation should not be undertaken if any of 'he protective devices

© PMI, NTPC 196

are known to be faulty.

Fee ater heaters are not to be operated at fluid temperatures higher than those shown

on the specification sheet. Feedwater heaters must not be subjected to abrupt

temperature fluctuations. Hot fluid must not be introduced rapidly when the heater is

cold, nor cold fluid when the heater is hot.

Prior to opening the feedwater valve, the channel start-up vents are to be opened and

remain open until all passages have been purged and feedwater begins to discharge.

To remove air from the shellside of a heater which does not operate under vacuum, the

shell start-up vent valves should be opened prior to the admission of steam to the

feedwater heater. The extraction lines must be free of all condensate to prevent

damage to the heater internals by slug flow. When the drains outlet valve is opened, the

shell start-up vent valves are to be closed and the operating air vent valves are to be

opened. Continuous venting of air and other non-condensibles is assured by keeping

the shell operating vent valves open.

On initial plant start-up of horizontal feedwaier heaters, having integral drain coolers, the

liquid level is to kept just below the high level alarm point. This will avoid the possibility

of flashing at the sub-cooler inlet and the possible tube damage that can result. During

initial start-up phases, the drains approach temperature (difference between drain

cooler outlet and feedwater inlet temperatures ) should be monitored. Approach

temperatures in excess of 8oC indicate the probability of flashing at the sub-cooler inlet.

In this case, the liquid level should be raised until the drains approach temperature,

approached the specified value.

The various turbine extractions are charged as follows :

LPH-1 - Always in service

© PMI, NTPC 197

LPH-2 - These extractions are charged when the unit load is around 50

MW.

Desecrator - Extraction is charged when IP exhaust pressure is 3.5 kg/cm2.

(around 40% of unit capacity)

HPH - 5 & 6 - These extractions are charged when unit load is around 50 to 60%

of unit capacity.

General Performance

Feedwater heater operating conditions and performance should be checked regularly

against the values stated on the specification sheet provided for each heater.

Any significant deviation of heater performance from that specified value should be

investigated.

Failure in feed water heaters

The failure of a feedwater heater to perform satisfactorily may be caused by one or

more factors, such as:

1) Air or non-condensible gas blanketing resulting from improper piping installation

or lack of suitable venting.

2) Flooding resulting from inadequate drainage of condensate.

3) Operating conditions differing from design conditions.

4) Tubing failure.

© PMI, NTPC 198

5) Maldistribution of flow.

Abrupt flooding, unusal noise or loss of feedwater temperature rise can indicate tubing

failure. If such a condition occurs, the heater must be removed from service as quickly

as possible. Tube failures tend to have a chain reaction effect; impingement on adjacent

tubes can cause additional failures.

HEATER DRIP AND VENT OPERATION

Description of System

The purpose of the Heater Vents and Drains System is to remove condensate that has

accumulated in the shell side of the closed feedwater heaters from their heat source the

extraction steam, and cascade the condensate to the next lower pressure heater. This

system also removes any non-condensable gases from the feedwater heater.

The heater drain system transfers the shell drains from each closed feedwater heater to

the next lower stage of heating and ultimately to the condenser through HP/LP Flash

Tank. The normal operating flow path is from Heater Nos. 6A/B to No. 5A/B and on to

the deaerator wherein the drains are incorporated in the feedwater flow. From Heater

No. 3 the drains cascade to Heater Nos. 2, 1. through the Drain Cooler and into .the

condenser, through LP Flash Tanks, The drain line from'the drain cooler section of each

heater, except Heater No. 1. divides into two branches, one leading to the next lower

pressure heater and the other HP/LP Flash tanks. Each branch contains a modulating

type control valve located near the inlet of the receiving vessel. "Pie drain from Heater

No. 1 has a separate drain line to the LP flash tank. through a modulating control valve.

Vents and drains of HP/LP Flash Tanks are finally connected to the condenser. Normal

drain from each closed feedwater heater except LP heater 1 has also been provided

with a local manually operated level control bypass valve, which can be used in the

event of controller failure.

© PMI, NTPC 199

All HP feedwater heaters 6A-6B & 5A-5B are provided with shell operating vents and

shell start-up vents, which are routed to the HP Flash Tank manifold, wherefrom drain

and vent are connected to the Condenser. Likewise, LP feedwater heaters 2 & 3 are

provided with shell operating vents and shell start-up vents, which are directly routed to

the condenser along with LP Flash Tank vent line. LP feedwater heater no.1, however,

is provided with shell operating vent alone, which is also routed to the Condenser. The

operating vent lines from all feedwater heaters are fitted with orifice plates (or flow

control. The start-up vent lines from feedwater heater nos. 6A-6B, 5A-5B, 3 & 2 and

deaerating feedwater heater are provided with manually operated modulating valves.

The deaerating feedwater heater vents, both start-up and operating, are piped to

atmosphere.

© PMI, NTPC 200

17. Boiler Feed Pump And Condensate Pump

BOILER FEED PUMP BOOSTER PUMP Each pump set consists of a weir type FAIE 64 booster stage pump and a Weir type

FK4E36 pressure stage pump.

The Weir type FAIE64 booster stage pump is a single stage, horizontal, axial sptit

casing type. having the suction and discharge branches integrally cast in the casing

lower half, thus allowing the pump internals to be removed without disturbing the suction

and discharge pipework or the alignment between the pump and discharge.

The pump shaft is sealed at the drive end and non-drive end' by Crane mechanical

seals. The rotating assembly is supported by plain white metal lined journal bearings

and axially located by a Glacier double tilting pad thrust bearing.

BOILER FEED PUMP Description The Weir type FK4E36 pressure stage pump is a (our stage horizontal centrifugal pump

of the barrel casing design as shown in Figure No. 35.

The pump internals are designed as a cartridge which can be easily .removed for

maintenance without disturbing the suction and discharge pipework, or the alignment of

the pump and the turbo coupling.

The pump shaft is sealed at the drive end and non-drive end by labyrinth glands, each

gland being sealed by the condensate injection sealing system. The rotating assembly

© PMI, NTPC 201

is supported by plain white metal lined journal bearing and axially located by a glacier

double tilting pad thrust bearing.

© PMI, NTPC 202

Pump Casing

The pump casing consists of a forged steel barrel with welded suction, discharge

branches, inter stage tapping and mounting feet. The drive end of the casing is closed

by a suction guide which is entered from the non drive-end of the casing and is located

by a spigot against the outer face of the casing. A metaflex joint is located between the

suction guide spigot and the casing outer face to prevent leakage between the barrel

casing and suction guide.

The discharge cover closes the non-drive end of the pump casing and also forms the

balance chamber which, in turn is closed by the gland housing. The discharge cover is a

close fit in the casing bore and is held in place by a ring of studs and nuts. A spring disc

is located between the last stage diffuser and the discharge cover, balance drum bush

to provide the force required to hold the ring section assembly in place against the drive

end of the barrel before start up. Once running, the discharge pressure assists the

spring disc in holding the ring sections in place. The last stage diffuser is free to slide

over the balance drum bush which is shrunk into the discharge cover bore to minimise

the flow of liquid to the balance chamber

Two holes are drilled radially through the periphery of the discharge cover to provide

outlet connection through which the liquid from the balance chamber is returned to the

pump suction piping and two similarly drilled holes are also provided in the discharge

cover for the welded connection of the kicker stage deliveries. The nondrive end bearing

housing is attached to the gland housing secured to the outer face of the discharge

cover by socket head screws and dowel pins.

To assist in removing the cover, two tapped holes are provided on the flange for the use

of starting screws and a tapped hole is provided on top of the cover for an eye-bolt.

The drive end bearing housing is secured to the outer face of the suction guide by cap

screws and dowel pins.

© PMI, NTPC 203

Rotating Assembly

The dynamically balanced rotating assembly consists of the shaft, impellers, abutment

rings, keys, gland sleeves, shaft nuts, balance drum, thrust collar and the pump half

coupling.

The impellers are of the single entry shrouded inlet type and are keyed and shrunk onto

the shaft, the keys, one per impeller, being alternately fitted on diametrically opposite

sides of the shaft to maintain rotational balance.

The balance drum is keyed and shrunk on the shaft and held in place against the shaft

locating shoulder by the balance drum nut and lock-washer. The inner end of the

balance drum is recessed and the bore of the recess is a Close fit over the kicker stage

impeller hub

Journal and Thrust Bearings

The rotating assembly is supported at each end of the shaft by a white metal lined

journal bearing and the residual thrust is carried by a tilting pad double thrust bearing

mounted at the non-drive end of the pump.

The thrust bearing has eight white metal lined tilting pads held in a split carrier ring

positioned on each side of the thrust collar.

The split floating oil sealing ring is located in a groove in the thrust bearing housing to

restrict the escape of lubricating oil from the thrust bearing chamber. To ensure that the

thrust bearing remains flooded, an orifice is fitted at the oil outlet.

Hydraulic Balance

The rotating assembly is subject to varying forces due to the differential pressure forces

© PMI, NTPC 204

acting on the impellers. The pump has therefore been designed so that the shaft is kept

in tension by the location of a balance drum at the non-drive end, and is hydraulically

balanced so that only a small residual thrust remains, which is carried by the thrust

bearing.

The main components of the hydraulic balancing arrangement are the balance chamber

machined in the discharge cover, the balance drum bush fitted in the bore of the

discharge cover. The thrust caused by the discharge pressure acting on the area

outside of each impeller wear ring on the inlet side of the impeller is balanced by the

same pressure acting on equal area on the outlet side of each impeller. The thrust

caused by the suction pressure acting on the impeller is overcome by the much greater

thrust caused by the discharge pressure acting on an equivalent area on the outlet side

of each impeller. The resultant thrust force, due to the different pressures acting on

these equal areas, tends to move the rotating assembly towards the drive end of the

pump.

The thrust force will vary with the load on the pump but the hydraulic balance

arrangement will reduce its effect, enabling the residual thrust to be taken by the tilting

pad thrust bearing. This bearing has a double face so that the surges in opposite

directions which occur during the start-up period and during transient conditions will be

accommodated.

The hydraulic balance arrangement operates as follows:

The pump product passes from the kickers stage of the pump between the balance

drum and the bush and enters the balance chamber at a pressure approximately equal

to the suction pressure. Two ports in the discharge cover allow the product to be piped

back to the pump suction side. The pressure differential across the balance drum is

therefore equal to that across the impellers. The cross-sectional area of the balance

drum is sized to give a small residual thrust towards the drive end of the pump

© PMI, NTPC 205

STEAM DRIVEN FEED PUMP

Plant Description

The single cylinder turbine is of the axial flow type. The live steam flows through the

Emergency stop valve and then through the Main Control Valves (5 Nos.) (Nozzle

governing).

These valves regulate the steam supply through the turbine in accordance with load

requirements. The control valves are actuated by a lift bar which is raised or lowered via

a lever system by the relay cylinder mounted on the turbine casing.

© PMI, NTPC 206

The journal bearings supporting the turbine shaft are arranged in the two bearing blocks

as shown in fig No = 36. The front end bearing block also houses the thrust bearing

which locates the turbine shaft and takes up the axial forces.

There are 14 stages of reaction blading. The balancing piston is provided at the steam

admission side to compensate the axial thrust to the maximum extent. Since the axial

thrust varies with the load the residual thrust is taken up by the thrust bearing. The leak

off from the balancing piston is connected back to the Turbine after 9th stage.

The turbine is provided with Hydraulic and Electro hydraulic governing system. A

primary oil pump is used as a speed sensor for Hydraulic governing and Hall probes are

used as a speed sensor for Electro Hyd. governing.

Whenever steam is drawn from the cold reheat line or auxiliary supply, steam flow is

controlled by auxiliary control valve. During this period the main control valves (4 Nos.)

will remain fully opened and the by pass valve across it will remain closed. (Bypass

remain closed for a short period when changeover from IP steam to CRH takes place).

The steam exhaust from the BFP - Turbine is connected to the main condenser and the

Turbine glands are sealed by gland steam.

Rotor Barring

Turbine is provided with a hand barring facility. The Turbine Rotor is connected to the

pressure pump through detachable coupling and to the booster pump through a set of

reduction gears.

Oil System

Adequate supply of oil to the turbine for lubrication and control purposes is provided by

2x100%. Auxiliary oil pumps which are motor driven and independent of turbine speed.

© PMI, NTPC 207

An oil tank of requisite capacity is situated at 'O' met level. The design and dimensions

of the tank provide for the required retention time for the oil. The two aux. oil pumps

draw their suction from the side of the tank. One of the two pumps is kept as standby.

An emergency oil pump which is driven by a D.C. Motor is provided to take care of

lubrication of the turbine in case the motor driven aux. oil pumps have failed, and the

turbine is tripped.

A jacking oil pump is provided to supply shaft lift oil at sufficient pressure to facilitate

easy rotor barring. 2x100% duty oil coolers are provided for cooling the oil. The oil

temperature is controlled by varying the quantity of cooling water flow, Rotary type -

three way valve is provided to change over the coolers.

Oil filter

A duplex type oil filter is provided and either one can be isolated during the running of

the turbine. The control oil is further filtered by plate type filters.

Pressure accumulator

To avoid pressure drops in the control oil circuit which might cause an emergency

tripping of the turbine, pressure accumulators are provided in the respective, circuits.

Such a condition might be caused during the tripping of the running oil pump which is

not coupled to the turbine rotor.

Oil pressure

The control oil pr. is around 5 to 8 atg and the lubricating oil pressure is 0.8 to 1.7 atg.

Oil temperature

The oil temp. after the coolers is to be maintained at 45 to 48°C.

© PMI, NTPC 208

SOVERNING SYSTEM INTRODUCTION

Under normal circumstances the boiler feed pump drive turbine is driven by

electrohydraulic control system, which is backed by hydraulic governor.

The electrohydraulic turbine controller maintains the speed of turbine corresponding to

the set point signal from feed water controller. In the event of failure of electronic

controller, hydraulic governor comes into action and maintains the control valve

position. Thereafter speed of turbine can'be set manually.

Steam to BFP drive turbine during normal operation is supplied from one of the bled

steam line of 500 MW turbine. During start, shutdown and low load operation of 500

MW turbine, steam to BFP drive turbine is supplied from cold reheat line or auxiliary

steam header. The governing system provides the facility to start the BFP-drive turbine

through steam from cold reheat line and then automatically switching over to bled steam

supply as soon as it is available. The system is so designed that the changeover from

cold reheat line to bled system line and vice versa is smooth.

The other features of BFP turbine governing system are a quick closing stop valve,

overspeed protection, rotor axial shift protection, manual trip, remote solenoid trip and

remote engagement of tripping device.

HYDRAULIC SPEED CONTROL Speed Acquisition

The turbinespeed is sensed by a governor impeller, which is driven by turbine shaft

through gear wheels. The governor impeller is supplied with a small quantity of oil from

oil pump through an adjustable needle valve. Depending on the speed of turbine, the

© PMI, NTPC 209

governor impeller builds up a pressure on its periphery. This oil pressure called as

primary oil pressure serves as speed signal for hydraulic governor.

Hydraulic speed governor

Primary oil pressure from governor impeller acts on below of speed governor. The force

exerted on the bellow by primary oil pressure is transmitted to the lever through a pin. A

compression spring (also called as speeder spring) is mounted on the top of the lever.

Spring is pre compressed by speeder gear motor or hand wheel. The spring balances

the primary oil pressure acting on the bellow. The travel of the bellow is transmitted to

the lever which is pivoted at one end. At its free the pivoted lever is connected to control

sleeves of hydraulic amplifier.

Hydraulic Amplifier

Hydraulic amplifier consists mainly of 2 sets of control sleeve, follow-up piston, tension

spring and a set of lever system. Control sleeves and follow-up pistons are provided

with control ports. The follow-up pistons are held in their respective control sleeves with

the help of tension springs. The overlap of ports between control sleeves and follow up

piston depends upon primary oil pressure, speeder gear position, and tension in the

springs.

Oil from the trip oil circuit is admitted into the follow-up piston through an orifice. The

pressure inside follow-up pistons depends on overlap of control ports. This pressure is

called as secondary oil pressure.

Any variation of primary oil pressure due to speed change, or change in speeder spring

position due to change in speed reference signal, causes the overlaps of ports between

control sleeve and follow up piston to readjust. This results in either increase or

decrease of secondary oil pressures. These secondary oil pressures are transmitted to

main and Auxiliary control valve actuators respectively.

© PMI, NTPC 210

Control Valve actuators (Servomotors)

Servomotors, the main & Aux. Control valves corresponding to the main and auxiliary

secondary oil pressure generated by hydraulic amplifier. Servomotor mainly consists of

a pilot valve, a power cylinder and a feed back system. Upon change is secondary oil

pressure, pilot valve provides oil passages to and from power cylinder. The movement

of power piston is transmitted to the control valve through the levers. The feed back

lever resets the pilot valve to its neutral position, once the power piston occupies new

position corresponding to changed secondary oil pressure. The system is such that the

increase in secondary oil pressure causes the control valves to open. Hence

interruption of secondary oil pressure causes the control valve to close.

EMERGENCY STOP VALVE AND STARTING DEVICE

The emergency stop valve is of quick closing type. The valve is actuated by means of a

starting device. The stop valve consists of a spring loaded piston and piston disc. which

is connected to the valve cone through a spindle. For opening the stop valve start-up oil

from starting device is admitted to the space above the spring loaded piston, by

operating the starting device. Due to start up oil pressure, the piston moves towards the

piston disc and they form a tight seal against each other. Oil from trip oil circuit is then

admitted to the space under the piston disc and the space above the piston is

connected to oil drain. The trip oil now forces both the piston disc. and piston to the

outer position thereby opening the stop valve. As long as the trip oil pressure is

maintained the piston and the piston disc, can not be separated by spring force. The

stop valve is closed only when the trip oil pressure drops substantially. On a loss of trip

oil pressure, the pressure of secondary oil tapped from trip oil circuit drops to zero, thus

causing the control valves to close. This arrangement provides a two fold protection

against steam entering the turbine.-

Once emergency stop valve is opened, starting device acts as load limiting device.

Further operation of starting device builds up the secondary oil pressure. At a preset,

© PMI, NTPC 211

valve of secondary oil pressure control valve actuators shall open the control valves,

thereby admitting the steam into the turbine. Maximum value of secondary oil pressure

can be limited at any point of operation by starting device.

Electric Hydraulic Converter

The electro hydraulic converter is the connecting link between electrical and hydraulic

parts of governing system. The output signal of electronic governor is given to plunger

coil system of electro hydraulic convertor (EHC). Plunger coil positions a control slide

which in turn provides oil passages to & from the power piston of EHC. A linear variable

differential transformer mounted on power piston gives feed back signal to electronic

governor. Power piston through levers drives control sleeves of two hydraulic amplifiers.

The output of these hydraulic amplifiers i.e. secondary oil pressure is connected in

parallel with the output of hydraulic governor amplifiers. Thus the secondary oil

pressures loading to servomotors can be regulated either by electronic governor or by

hydraulic governor.

Tester for Emergency Stop Valve

Testing device is provided to check the proper functioning of emergency stop valve

during normal operation of turbine if the testing device is operated, it will admit the

pressure oil to the space behind the test piston of emergency stop valve, which will then

be pushed against the piston and move the piston and the valve spindle towards the

closed position.

Tripping Device

Whenever the turbine is to be tripped, the governing oil pressure (trip oil line-after tripping

device) is being drained by tripping device Thus pressure in front of stop valve piston disc and

secondary oil pressure falls resulting in closure of stop valve and control valves.

© PMI, NTPC 212

Over Speed Governor

Over speed governor protects the turbine against speeds higher than the safe value for

turbine operation. Over speed governor consists of an eccentric pin located inside the

turbine shaft and held in position with a spring. At a preset speed (trip speed) centrifugal

force of eccentric pin will overcome the spring force and the pin will move out of shaft.

The outward movement of the pin actuates a lever of tripping device and thereby

tripping the turbine.

Axial Shift Protection

Whenever the turbine rotor movement in axial direction is more than permissible, a

projection on the rotor comes underneath the lever of tripping device and actuates the

lever, thereby tripping the turbine.

Overspeed Governor Tester

Overspeed governor tester facilitates the testing of overspeed governor pin when

turbine is operating at maximum continuous speed. When overspeed governor tester is

operated, it will hydraulically bypass the tripping device ensuring uninterrupted oil

supply to governing system irrespective of the position of tripping device. On, further

operation of overspeed governor tester it gives an impulse to directional valve.

Directional valve provides oil passage from oil pump to over speed governor pin through

adjustable needle valve. The pressurised oil causes the pin to move out of shaft and

actuates the tripping device lever. This ascertains the free movement of overspeed

governor pin.

Remote Engagement of Tripping Device

Tripping device can be put into operation remotely with the help of a control switch, a

© PMI, NTPC 213

pressure switch and a solenoid valve.

As soon as control switch is pressed, solenoid valve gets energised and oil from main

oil pump flows into tripping device bringing the tripping device in action. Soon after trip

oil pressure is built up, solenoid valve gels de-energised through the pressure switch.

Pressure Regulator

Pressure regulator maintains a constant pressure in the control oil line to the electro

hydraulic converters.

Accelerator

Accelerator is incorporated in the speed governor to raise the turbine speed

instantaneously. This can be used to test the overspeed governor by actual

overspeeding.

Solenoid Valve and Pressure Regulator

Whenever steam is drawn from cold reheat line or auxiliary supply, steam flow is

controlled by auxiliary control valve and the main control valve are full open.' Under this

condition it is required to close the bypass valve (5th valve) of main control valves (For

a short period) otherwise steam will flow through by pass valve only. For this whenever

the auxiliary control valve opens, impulse is given to energise the solenoid valve which

then connects the constant pressure regulator to secondary oil line. Pressure regulator

limits the secondary oil pressure corresponding to four valve opening.

Accumulator

Oil accumulator supplies oil for a short time during change over of pumps or during

quick action of servomotor.

© PMI, NTPC 214

Damping Device

Damping device damps out the quick oscillations in secondary oil pressure generated

by hydraulic amplifier. This helps in improving the stability of the governing system.

Solenoid Valve for Remote Tripping

The turbine can be tripped from remote place e.g. control panel by energising the

solenoid valve. When solenoid valve is energised, it interrupts oil supply to governing

system and at the same time depressurises the governing system. Turbine shall trip

immediately on loss of governing oil pressure.

Hand Trip Valve

If required turbine can be tripped locally by hand trip valve. Hand trip valve, when

operated, depressurises the trip oil circuit and thus causing the turbine to trip.

Tracking of Electronic Governor by Hydraulic Governor

Under normal circumstances, the turbine is driven by the electro hydraulic control

system, and the hydraulic governor is made to track it continuously. The task is

performed by a control circuit whose set point is the position of the power piston in the

electro hydraulic converter.

The control circuit generates the set point for start up and load limiting device of

hydraulic governor. The set point generated by the control circuit is approx. 5% higher

than the position of the EHC. Since the controller, which issues the lower set point will

have control. electro hydraulic system will regulate the turbine. For load limiting device

to be effective in full range of operation, it is necessary to set the speed reference of the

hydraulic governor at max. value. When electro hydraulic control system fails, output of

electro hydraulic converter shall reach maximum and hydraulic governor through load

© PMI, NTPC 215

limiting device shall maintain the control valve position but at a slightly higher value

corresponding to tracking deviation. Now hydraulic speed governor reference can be

adjusted at desired level.

Disturbing Process Signal Unit (DPSU)

When main 500 MW turbine trips, the bled steam to drive the boiler feed pump turbine

shall not be available and it has to derive the steam from cold reheat line. As soon as

the bled steam supply is stopped, the BFP drive turbine governing system will open the

control valves more and more. Thus main control valves immediately open fully and

then aux. control valves start open till the required steam quantity is met through cold

reheat line.

In process, when main control valves open, very fast response from electronic governor

is expected and will interfere with the hydraulic governor which is comparatively slow.

To avoid this interference a disturbing process signal unit is incorporated to set the

hydraulic governor to maximum value opening position as soon as 500 MW turbine

trips. 500 MW turbine trip signal is given to a solenoid valve of D.P.S.U. As a result,

pressure oil enters inside the power cylinder of D.P.S.U. The upward movement of

piston tensions the amplifier spring to a position corresponding to required auxiliary

valve opening. Thus interference between electronic governor and hydraulic governor is

avoided.

CONDENSATE EXTRACTION PUMP Description

Each condensate extraction pump, which is driven by a 1120 KW induction motor,

delivers 810,000 kg/hr of condensate water against 307m. of total dynamic head at the

rated condition.

© PMI, NTPC 216

Construction of C.E. Pumps

The pumps are of the direct driven by a constant speed motor through a rigid coupling,

vertical barrel, double suction, multi stage, diffuser type.

The pump consists of internal assembly, discharge assembly and suction barrel. The

internal assembly comprises of 5 stage casing, a guide van, five impellers, 2 column

pipes, a suction bell and shaft and is submerged in water in the suction barrel.

The discharge assembly comprises a discharge head with a stuffing box to seal the

pump shaft and is installed on the suction barrel. The suction barrel is installed on the

pump floor.

Water is admitted into the casing from the suction barrel through the suction bell and

first stage casing and discharged through the column pipes by the energy imparted by

the impellers.

Internal bearings (Leaded bronze bearings) installed in a column pipe and the top

casing are provided for supporting the pump shaft against the radial load. Upper and

lower bearings (leaded bronze) are installed in the stuffing box and suction bell.

The weight of the pump rotor and the hydraulic thrust acting on the rotor in the axial

directions are supported by the thrust bearing in the motor.

The impellers are driven by a 1120 kw vertical shaft induction motor mounted on the

discharge head. The coupling spacer is furnished between the pump and motor in order

to remove the gland and seal ring without removing the motor.

Adjustment nut is provided at the top of the drive shaft to facilitate adjustment of the

axial location of the rotating part.

Grand packings are used for shaft sealing.

© PMI, NTPC 217

18. Data Sheet of 500 MW Turbine and Its Auxiliaries and Turbine Metal Temp. Limit Curves

© PMI, NTPC 218

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© PMI, NTPC 220

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Turbine-Metal temperatures - limit curves

As the thickness involved in the Turbine stop valve, control valve and casing is quite

high, these should be heated or cooled gradually otherwise severe thermal stresses will

be induced across the thickness. Hence in KWU design all these casings are monitored

for thermal stresses by measuring the inner layer and middle layer of the casings.

Designer has given as set of curves known as limit curves which gives the relationship

between the middle layer temperature and the permissible difference in inner layer to

middle layer. Refer fig. 37, 38, 39 AND 40.

The permissible difference should never be exceeded for a given middle layer

temperature,

When the unit is being rolled, the inner layer and middle layer temperatures of stop

valve, control valve, are measured. The following terms are defined which are used for

roiling the turbine.

If V1 = Temperature of inner layer.

Vm = Temperature of middle layer.

Then the permissible temperature difference Vi-Vm is obtained from the limit curves.

Then Vi-Vm is defined as "Permissible Margin" for a given Vm.

The actual difference Vi-Vm is measured. Then Vi-Vm is defined as "Actual Margin " for

a given Vm.

Then the difference between the Permissible Margin and actual margin is known as

available margin.

© PMI, NTPC 222

This available Margin will guide the Turbine Operator to control the Turbine rolling and

steam parameters to avoid thermal stresses being induced in the thick casings. The

available margin should be always +ve to avoid thermal stresses If it goes into - ve, it

indicates that turbine casings are subjected to severe thermal stresses and turbine

should be tripped.

During turbine heating, inner well temp. will be always greater than the middle layer and

the margins obtained by the difference of Vi-Vm is designated as, Upper margin.

During turbine cooling or load reduction inner wall temperature will be less than the

middle layer temperature and the margin obtained by difference of Vi-Vm will be

negative and denoted as lower margin.

Turbine Start-up curves

These curves relates the middle layer temperature of the casings to permissible steam

admission temperatures. These curves are very useful as it guides the operator in

selecting the main steam and Hot reheat steam temperature for a given middle layer

temperature. These curves gives the necessary relationships between the middle layer

temperature and the steam temperature for various operations like, stop valve opening,

turbine rolling, turbine speeding up and Turbine loading.

A good understanding of the limit curves and start-up curves will help a great deal in

rolling and operating the turbine safel;

© PMI, NTPC 223

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19. Design And Constructional Feature Of 500 MW Generator

INTRODUCTION

Turbo generator for 500MW units are two pole water and hydrogen gas cooled

generator. The primary water flows through the stator winding and picks up heat losses

arising in generator directly, which are transferred to the secondary cooling water. Core

losses, windage losses and rotor winding losses are picked up by circulating hydrogen

and transferred to Hydrogen cooling water in gas coolers. The stator frame is designed

to withstand the impact due to possible explosions inside. A brushless excitation system

with rotating diodes is provided to excite the machine.

Generator

a. Type : THDF 115/59

B. Cooling Stator Winding : Directly Water Cooled

c. Stator core & Rotor : Directly Hydrogen Cooled

d. Apparent Power : 588 MVA

e. Active Power : 500 MW

f. Power Factor : 0.85 (lag)

g. Terminal Voltage : 21 KV

h. Stator Current : 16200 Amps.

i. Hydrogen Pressure : 4 Kg/cm 2

j. Short Circuit Ratio : 0.48

k. Class and Type of Insulation : Miscalastic

The two-pole generator uses direct water cooling for the stator winding, phase

connectors and bushings and direct hydrogen cooling for the rotor winding. The losses

© PMI, NTPC 228

in the remaining generator components, such as iron losses, windage losses and stray

losses, are also dissipated through hydrogen.

The generator frame is pressure-resistant and gas tight equipped with one stator end

shield on each side. The hydrogen coolers are arranged vertically inside the turbine end

stator end shield.

The generator consists of the following components

* Stator

Stator frame

End shields

Stator core Stator winding

* Rotor

Rotor shaft Rotor winding

Rotor retaining rings

Field -connections

* Hydrogen Coolers

* Bearings

* Shaft Seals

The following additional auxiliaries are required for generator operation

* Oil system

* Gas system

* Primary water system

* Excitation system

© PMI, NTPC 229

STATOR

The 3Ø Armature winding of the generator is housed inside the stator which comprises

of a stator frame, end shields and the bushing compartment. Since an explosive gas

Hydrogen is used, the frame is made rigid enough to withstand a pressure of 10 bar.

The lower part of the frame near the exciter end houses the terminal box. The phase

and neutral leads of the 3Ø stator winding are brought out of the generator through six

bushings located in the terminal box. The hydrogen coolers are arranged vertically in

the turbine side stator end shield. The stator end shields contain the shaft seal and

bearing components.

STATOR FRAME

The stator frame consist of a cylindrical center section and two end shields which are

gas-tight and pressure resistant.

The stator end shields are joined and sealed to the stator frame with an 0-ring and

bolted flange connections. The stator frame accommodates the electrically active parts

of the stator. i.e., the stator core and stator windings. Both the gas ducts and a large

number of welded circular ribs provide for the rigidity of the stator frame. Ring-shaped

supports for resilient core suspension are arranged between the circular ribs. The

generator cooler is sub-divided into cooler sections arranged vertically in the turbine

side stator end shield. In addition, the stator end shields contain the shaft seal and

bearing components. Feet are welded to the stator frame end shields to support the

stator on the foundation. The stator is firmly connected to the foundation with anchor

bolts through the feet.

© PMI, NTPC 230

© PMI, NTPC 231

STATOR CORE

The staler core is stacked from insulated electrical sheet steel laminations and mounted

in supporting rings over insulated dovetailed guide bars. Axial compression of the stator

core obtained by clamping fingers, pressure plates and non-magnetic through-type

clamping boits, which are insulated from the core. The supporting rings form part of an

inner frame cage. This cage is suspended in the outer frame by a large number of

separate flat springs distributed over the entire core length. The flat springs are

tangentially arranged on the circumference in sets with three springs each, i.e. two

vertical supporting springs on both sides of the core and one horizontal stabilizing

spring below me core. The springs are so arranged and tuned that forced vibrations of

the core resulting from the magnetic field will not be transmitted to the frame and

foundation.

© PMI, NTPC 232

© PMI, NTPC 233

The pressure plates and end portions of the stator core are effectively shielded against

© PMI, NTPC 234

stray magnetic fields. The flux shields are cooled by a flow of hydrogen gas directly over

no assembly.

STATOR END SHIELDS

The ends of the stator frame are closed by pressure containing end shields. The end

shields feature a high stiffness and accommodate the generator bearings, shaft seals

and hydrogen coolers. The end shields are horizontally split to allow for assembly.

The end shields contain the generator bearings. This results in a minimum distance

between bearings and permits the overall axial length of the TE end shield to be utilized

{or accommodation of the hydrogen cooler sections. Cooler walls are provided in shield

on both sides of the bearing compartment for this purpose. One manhole in both the

upper and lower half end shield provides access to the end winding compartments of

the completely assembled machine.

Inside the bearing compartment, the bearing saddle is mounted and insulated from the

lower half end shield. The bearing saddle supports the spherical bearing sleeve and

insulates it from ground to prevent the flow of shaft currents.

The bearing oil is supplied to the bearing saddle via a pipe permanently installed in the

end shield and is then passed into the lubricating gap via ducts in the lower bearing

sleeve. The bearing drain oil is collected in the bearing compartment and discharged

from the lower half of the end shield via a pipe.

The bearing compartment is sealed on the air side with labyrinth rings. On the hydrogen

side, the bearing compartment is closed by the shaft seal and labyrinth rings. The oil for

the shaft seal is admitted via integrally welded pipes. The seal oil drained towards the

air side is drained together with the bearing oil. The seal oil drained towards the

hydrogen side is first collected in a gas and oil tight chamber below me Bearing

© PMI, NTPC 235

compartment for defoaming and then passed via a siphon to the seal oil tank of the

hydrogen side seal oil circuit.

The static and dynamic bearing forces are directly transmitted to the foundation via

lateral feet attached to the lower half end shield. The feet can be detached from the end

shield. -since the end shields must be lowered into the foundation opening for rotor

insertion.

Stator Winding

Stator bars, phase connectors and bushings are designed for direct water cooling. In

order to minimize the stator losses, the bars are composed of separately

insulated/strands which are transposed by 540°C in the slot portion and bonded

together with epoxy resins in heated molds. After bending the end turns are likewise

bonded together with baked synthetic resin fillers.

The bars consist of hollow and solid strands distributed over the entire bar cross-section

so that good heat dissipation is ensured. At the bar ends, all the solid strands are jointly

brazed into a connecting sleeve and the hollow strands into a water box from which the

cooling water enters and exits via teflon insulating hoses connected to the annular

manifolds. The electrical CQ Cnection between top and bottom bars is made by a bolted

connection at the connecting sleeve.

The water manifolds are insulated from the stator frame permitting the insulation

resistance of the water-filled winding to be measured. During operation, the water

manifolds are grounded.

© PMI, NTPC 236

Miscalastic High-voltage Insulation

High-voltage insulation is provided according to the proven Miscalastic system. With this

insulating system several halt-overlapped continuous layers of mica tape are applied to

the bars. The mica tape is built up from large area mica splitting which are sandwiched

between two polyester backed fabric layers with epoxy as an adhesive. The number of

layers, i.e., the thickness of the insulation depends on the machine voltage. The bars

are dried under vacuum and impregnated with epoxy resin which has very good

penetration properties due to its low viscosity. After impregnation under vacuum, the

bars are subjected to pressure, with nitrogen being used as pressurizing medium (VPI

process). The impregnated bars are formed to the required shape in MOULDS and

cured in an oven at high temperature. The high-voltage insulation obtained is nearly

void-free and is characterized by its excellent electrical, mechanical and thermal

properties in addition to being fully waterproof and oil-resistant. To minimize corona

discharges between the insulation and the slot wall, a final coat of semi conducting

varnish is applied to the surfaces of all bars within the slot range. In addition, all bars

are provided with an end corona protection to control the electric field at the transition

from the slot to the end winding and to prevent tie formation of creepage spark

concentrations.

Bar Support System

To protect the stator winding against the effects of magnetic forces due to load and to

ensure permanent firm seating of the bars in the slots during operation, the bars are

inserted with a side ripple springs, a hog-curing slot, bottom equalizing strip, and a top

ripple spring located beneath the slot wedge. The gaps between the bars. in the stator

end windings are completely filled with insulating material and cured after installation.

For radial support, the end windings are clamped to a rigid supporting of insulating

material which in turn is fully supported by 'the frame. Hot-curing conforming fillers

arranged between the stator bars and the support ring ensure a firm support of each

© PMI, NTPC 237

individual bar against the support ring. The bars are clamped to the support ring with

pressure plates held by clamping bolts made form a high-strength insulating material.

The support ring is free to move axially within the stator frame so that movements of the

windings due to thermal expansions are not restricted.

The stator winding connections are brought out to six bushings located in a

compartment of welded r end. Current transformers or metering and relaying purposes

can be mounted on the bushings.

STATOR WINDING (Corona Protection)

To prevent potential differences and possible corona discharges between 'the insulation

and the slot wall, the slot sections of the bars are provided with an outer corona

protection. This protection consists of a wear-resistant, highly flexible coating of

conductive alkyd varnish containing graphite.

1 Stator bar (slot end)

2 High-voltage insulatation

3 Outer corona protection

4 Transition costing

5 End corona protection

6 Glass tape-epoxy protective layer

7 Stator her (end winding)

Fig. No. - 44 TYPICAL BUILDUP OF CORONA PROTECTION

At the transition from the slot to the end winding portion of the stator bars. a

© PMI, NTPC 238

semiconductive coating is applied. On. top of this, several layers of semi-conductive end

corona protection coating are applied in varying lengths. This ensures uniform control of

the electric field and prevents the formation of corona discharge during operation and

during performance of high voltage tests.

A final wrapping of glass fabric tapes impregnated with epoxy resin serves as surface

protection.

ROTOR SHAFT

The high mechanical stresses resulting from the centrifugal forces and short-circuit call

for a high quality heat-treated steel. Therefore, the rotor shaft is forged from a vacuum

cast steel ingot. Comprehensive tests ensure adherence to the specified mechanical

and magnetic properties as well as a homogeneous forging.

The rotor shaft consists of an electrically active portion, the so-called rotor body, and the

two shaft journals. Integrally forged flange couplings to connect the rotor to the turbine

and exciter are located outboard of the bearings. Approximately two-thirds of the rotor

body circumference is provided with longitudinal slots which hold the field winding. Slot

pitch is selected so that the two solid poles are displaced by 180°.

Due to the non-uniform slot distribution on the circumference, different moments of

intertia are obtained in the main axis of rotor. This in turn causes oscillating shaft

deflections at twice the system frequency. To reduce these vibrations, the deflections in

the direction of the pole axis and the neutral axis are compensated by transverse

slotting of the pole.

The solid poles are also provided with additional longitudinal slots to hold the copper

bars of the damper winding. The rotor wedges act as a damper windings in the area of

the winding slots.

© PMI, NTPC 239

© PMI, NTPC 240

Rotor Winding

The rotor winding consists of several coils which are inserted into the slots and series-

connected such that two coil groups form one pole. Each coil consists of several series-

connected turns, eachof which consists of two half turns which are connected by

brazing in the end section.

The rotor winding consists of silver-bearing de-oxidized copper hollow conductors with

two lateral cooling ducts. L-shaped strips of laminated expoxy glass fiber fabric with

Nomex filler are used for slot insulation. The slot wedges are made of high-conductivity

material and extend below the shrink seat of the retaining ring. The seat of the retaining

ring is silver-plated to ensure a good electrical contact between the slot wedges and

rotor retaining rings. This system has long proved to be a good damper winding.

Retaining Rings

The centrifugal forces of the rotor end windings are contained by single-piece rotor

retaining rings. The retaining rings are made of non-magnetic high-strength steel in

order to reduce stray losses. Each retaining ring with its shrink-fitted insert ring is shrunk

into the rotor body in an overhung position. The retaining ring is secured in the axial

position by a snap ring.

Field Connection

The field current is supplied to the rotor winding through redial terminal bolts and two

semicircular conductors located in the hollow bores of the exciter and rotor shafts. The

field current leads are connected to the exciter leads at the exciter coupling with

Multikontakt plug-in contacts, which allow for unobstructed thermal expansion of the

field current leads.

© PMI, NTPC 241

Hydrogen Cooler

The hydrogen cooler is a shell and tube type heat exchanger, which cools the hydrogen

gas in the generator. The heat removed from the hydrogen is dissipated through the

cooling water. The cooling water flows through the tubes, while the hydrogen is passed

around the finned tubes.

The hydrogen cooler is subdivided into«identical sections, which are vertically mounted

in the turbine-end stator end shield. The cooler sections are solidly bolted to the upper

half stator-end shield, while the attachment at the tower water channel permits them to

move freely to allow for expansion.

The cooler sections are parallel connected on their watersides. Shut off valves are

installed in the lines before and after the cooler sections. The required cooling water

flow depends on the generator output and is adjusted by control valves on the hot water

side. Controlling the cooling water flow on the outlet side ensures an uninterrupted

water flow through the cooler sections so that proper cooler performance will not be

impaired.

The sleeve bearings are provided with hydraulic shaft lift oil during startup and turning

gear operation. To eliminate shaft currents, alt bearings are insulated from the stator

and base plate respectively. The temperature of the bearings is monitored with

thermocouples embedded in the lower bearings sleeve so that the measuring points are

located directly below the babbitt. Measurement and any required recording of the

temperature are performed in conjunction with the turbine supervision. The bearings

have provisions for fitting vibration pickups to monitor bearing vibrations.

Shaft Seals

The points where the rotor shaft passes through the stator casing are provided with a

© PMI, NTPC 242

radial seal ring. The seal ring is guided in the seal ring carrier which is bolted to the seal

ring carrier flange Hid insulated to prevent the flow of shaft currents. The seal ring is

lined with babbitt on the shaft journal side. The gap between the seal ring and the shaft

is sealed with hydrogen side and air side seal oil. The hydrogen side seal oil is supplied

to the seal ring via an annular groove in the seal guide. Inside the seal ring, this seal oil

is fed to the hydrogen side annular groove in the seal ring and from there to the sealing

gap via several bores uniformly distributed on the circumference. The air side seal oil is

supplied to the sealing gap from the seal ring chamber via radial bores and the air side

annular groove in the seal ring. To ensure effective sealing, the seal oil pressures in the

annular gap are maintained at a higher level than the gas pressure within the generator

casing, the air side seal oil pressure being set to approximately the same level as the

hydrogen side seal oil pressure. The oil drained on the hydrogen side of the seat rings

is returned to the seal oil system through ducts below the bearing compartments. The

oil drained on the air side is returned to the seal oil storage tank together with the

bearing oil.

On the air side, pressure oil is' supplied laterally to the seal ring via an annular groove.

This ensure free movement of the seal ring in the radial direction.

© PMI, NTPC 243

Cooling System

The heat losses arising in the generator interior are dissipated to the secondary coolant

(raw water, condensate. etc.) through hydrogen and primary water.

Direct cooling essentially eliminates hot spots and differential temperatures between

adjacent components which could result in mechanical stresses; particularly to the

copper conductors, insulation, rotor body and stator core.

Hydrogen Cooling Circuit

The hydrogen is circulated in the generator interior in a closed circuit by one multi-stage

axial-flow fan arranged on the rotor at the turbine end. Hot gas is drawn by the fan from

the air gap and delivered to the coolers, where it is recooled and then divided into three

flow paths after each cooler.

Flow Path - I

Flow path-1 is directed into the rotor at the turbine end below the fan hub for cooling of

the turbine end half of the rotor.

Flow Path - II

Flow path-11 is directed from the coolers to the individual frame compartments for

cooling of the stator core.

Flow Path-Ill

Flow path-Ill is directed to the stator end winding space at exciter end through guide

ducts in the frame for cooling of the exciter end half of the rotor and of the core end

© PMI, NTPC 244

portions.

The three (lows mix in the air gap. The gas is then returned to the coolers via the axial-

flow fan.

The cooling water flow through the hydrogen coolers should be automatically controlled

to maintain a uniform generator temperature level for various toads and cold water

temperatures.

Cooling of Rotor

For direct cooling of the rotor winding, cold gas is directed to the rotor end windings at

the turbine and exciter ends. The rotor winding is symmetrical relative to the generator

center line and pole axis. Each coil quarter is divided into two cooling zones. The first

cooling zone consists of the rotor end winding and the second one of the winding

portion between the rotor body end and the mid-point of the rotor. Cold gas is directed

to each cooling zone through separate openings directly before the rotor body end. The

hydrogen flows through each individual conductor in closed cooling ducts. The heat

removal capacity is selected such that approximately identical temperatures are

obtained for all conductors. The gas of the first cooling zone is discharged from the coils

at the pole center into a collecting compartment within the pole area below the end

winding. From there, the hot gas passes into the air gap through pole face slots at the

end of the rotor-body. The hot gas of the second cooling zone is discharged into the air

gap at mid-length of the rotor body through radial openings- in the hollow conductors

and wedges.

Cooling of Stator Core

For cooling of the stator core, cold gas is admitted to the individual frame compartments

via separate cooling gas ducts.

© PMI, NTPC 245

From these frame compartrnents. gas then flows into the air gap through slots in the

core where it absorbs the heat from the core. To dissipate the higher losses in the core

ends, the cooling gas slots are closely spaced in the core end sections to ensure

effective cooling. These ventilating ducts are supplied with cooling gas directly from the

end winding space. Another flow path is directed from the stator end winding space past

the clamping fingers between the pressure plate and core end section into the air gap. A

further flow path passes into the air gap along either side of the flux shield.

All the flows mix in the air gap and cool the rotor body and stator bore surfaces. The gas

is then returned to the coolers via the axial-flow fan. To ensure that the cold gas

directed to the exciter end cannot be directly discharged into the air gap. an air gap

choke is arranged within the range of the stator end winding cover and the rotor

retaining ring at the exciter end. ,

Primary Cooling Water Circuit In The Generator

The treated water used for cooling of the stator winding, phase connectors and

bushings is designed as primary water in order to distinguish it from the secondary

coolant (raw water, condensate, etc.). The primary water is circulated in a closed circuit

and dissipates the absorbed heat to the secondary cooling water in the primary water

cooler. The pump is supplied with hot primary water from the primary water tank and

delivers the water to the generator via the coolers. The cooled water flow is divided into

two flow paths as described in the following paragraphs.

Flow Path-1

Flow path-1 cools the stator windings. This flow path first passes to a water manifold on

the exciter end of the generator and from there to the stator bars via insulated hoses.

Each individual bar is connected to the manifold by a separate hose. Inside the bars,

the cooling water flow through hollow strands. At the turbine end, the water is passed

© PMI, NTPC 246

through similar hoses to another water manifold and then returned to the primary water

tank Since a single pass water flow through the stator is used, only a minimum

temperature rise is obtained for both the coolant and the bars. Relative movements due

to different thermal expansions between the top and bottom bars are thus minimised.

Flow Path-11

Flow path-11 cools the phase connectors and the bushings. The bushings and phase

connectors consist of thick walled copper tubes through which the cooling water is

circulated. The six bushings and the phase connectors arranged in a circle around the

stator end winding are hydraulically interconnected so that three parallel flow paths are

obtained. The primary water enters three bushings and exits from the three remaining

bushings.

The secondary water flow through the primary water cooler should be controlled

automatically to maintain a uniform generator temperature level for various loads and

coldwater temperature.

© PMI, NTPC 247

20. Excitation System 500 MW

INTRODUCTION

In 500 MW Turbo-generator, brushless excitation system is provided. Brushless exciter

consists of a 3 phase permanent magnet pilot exciter, the output of which is rectified

and controlled by Thyristor Voltage Regulator to provide a variable d.c. current for the

main exciter. The 3 phase are induced in the rotor of the main exciter and is rectified by

the rotating diodes and to the tiled winding p» generator rotor through the d.c. leads fed

in the rotor shaft. Since the rotating rectifier bridge is mounted on the rotor, the slip rings

are not required and the output of the rectifier is connected directly to the field winding

through the generator rotor shaft. A common shaft carries the rectifier wheels, the rotor

of the main exciter and permanent magnet rotor of the pilot exciter.

The voltage regulation is effected by using thyrism 04.2. an automatic voltage regulator.

There are two independent control systems right up to the final thyristor element - an

auto control and a manual control. The control is effected on the 3 phase output of the

pilot exciter and provides a variable d.c. input to the main exciter. The feedback of

voltage and current output of the generator is fed to the AVR where it is compared with

the set point generator volts set from the control room. The current feedback is utilised

(or active and reactive power compensation and for the limiters. There are 3 limiters,

under excitation limiter, over excitation limiter and U/Hz limiter which act on the AVR. A

power system stabiliser is also envisaged for damping oscillations in the power system.

The manual control system consists of an excitation controller which control the

excitation as set on the manual set point from the control room.

A field forcing limiter allows field forcing during emergency up to the capability of the

main exciter. In cases of defects in the automatic control system, the excitation

automatically changes over the manual regulation through protective relays. In order to

© PMI, NTPC 248

ensure a bumpless transfer, a follow up circuit controls the manual channel so that it

follows the auto channel continuously.

De-excitation of the machine is effected by driving the thyrister to inverter mode of

operation causing the thyrister to supply maximum reverse voltage to the field winding

of the main exciter. Approximately 0.5 Sees. after de-excitation command is received

two field suppressions contactors connect field suppression resistors in parallel to main

exciter field winding and following this a trip command is transmitted to the field circuit

breaker via its trip coil. In the event of a failure of the electronic de-excitation through

inverter operation. De-excitation would be effected with a delay of 0.5 Seconds by the

field suppression resistors.

The main advantage of rotating diode excitation system is that it eliminate the use of slip

rings and carbon brushes which pose constant maintenance problems.

EXCITATION SYSTEM

500 MW turbo generator set being procured from BHEL based on KWU design is

provided with Brushless excitation system simplified diagram of the system is given in

Figure-47.

A common shaft carries the rotating rectifier set. the rotor of the main exciter and the

permanent megnet rotor of the pilot exciter. The three phase pilot exciter is a 16 pole

revolving magnet field unit. the frame of which accommodates the laminated core with

the three phase winding. The rotor consists of a hub with mounted poles. While rotatinq

at 3000 r.p.m. 220v at 4nnHz is generated at pilot exciter which is fed to the field of

main excitar after rectification in AYR cubicle.

© PMI, NTPC 249

The main exciter which is a six pole revolving armature machine has three phase

winding is placed in the rotor slots and generates 600V at i50Hz. The output of main

© PMI, NTPC 250

exciter is fed to rotating rectifier wheel which is mounted on the rotor shaft. Rectifier

wheel consisting of silicon diodes and RC network rectifies the AC output of main

exciter and feeds the D.C. output to the field of turbo generator through the copper lead

mounted in the bore of the generator shaft thus eliminating completely the slip ring and

bush assembly. As there is no direct access to the measurement of exciter current the

generator field current is measured indirectly through a quadrature axis coil which is

placed between two poles of main exciter. Voltage induced in this coil is proportional to

the exciter current thus enabling a determination of excitation current. The relation-ship

between the exciter output current and the quadrature axis current/ voltage is

established during the exciter works tests. Salient operational parameter of the system

is given below :

Main Exciter

Rated Parameters :- 600V, 6300A. 8780 KW,

Under field forcing condition :-

820V, 8600. 7050KW for 10 sec.

Pilot Exciter

220V ± 10%. 195A, p.f. =06

frequency = 400 Hz, 65 KVA:

Fig 47 shows the basic arrangement of the exciter. The three-phase pilot exciter has a

revolving field with permanent magnet poles. The three-phase AC generated by the

permanent magnet exciter is rectified and contributed by the TVR to provide a variable

dc current for exciting the main exciter. The three phase ac induced in the rotor of the

main exciter is rectified by the rotating rectifier bridge and led to the field winding of the

generator rotor through the dc leads in the rotor shaft.

A common shaft carries the rectifier wheels, the rotor of the main exciter and the

© PMI, NTPC 251

permanent magnet rotor of the pilot exciter. The shaft is rigidly coupled to the generator

rotor. The exciter shaft is supported on a bearing between the main and pilot exciters

The generator and exciter rotors are thus supported on total of three bearings.

Mechanical coupling of the two shaft assemblies results in simultaneous coupling of the

dc leads in the central shaft bore through the Multikontakt electrical contact system

consisting of plug-in bolts and sockets. This contact system is also designed to

compensate for length variations of the leads due to thermal expansion.

RECTIFIER WHEELS

The main components of the rectifier wheels are the silicon diodes which are arranged

in the rectifier wheels in a three phase bridge circuit. The contact pressure for the silicon

wafer is produced by a plate spring assembly. The arrangement of the diode is such

that this contact pressure is increased by the centrifugal force during rotation.

Two diodes each are mounted in each aluminium alloy heat sink and thus connected in

parallel. Associated with each heat sink is a fuse which serves to switch off the two

diodes if one diode fails (loss of reverse blocking capability).

For suppression of the momentary voltage peaks arising from commutation. Each wheel

is provided with six RC networks consisting of one capacitor and one damping resistor

each which are combined in a single resin-encapsulated unit.

The insulated and shrunken rectifier wheels serve as dc buses for the negative and

positive side of the rectifier bridge. This arrangement ensures good accessibility to all

components and a minimum of circuit connections. The two wheels are identical in their

mechanical design and differ only in the forward directions of the diodes.

The direct current from the rectifier wheels is fed to the dc leads arranged in the center

bore of the shaft via radial bolts.

© PMI, NTPC 252

The three-phase alternating current is obtained via copper conductors arranged on the

shaft circumference between the rectifier wheels and the three-phase main exciter. The

conductors are attached by means of banding clips and equipped^ with screw-on lugs

for the internal diode connections. One three-phase conductor each is provided for the

four diodes of a heat sink set.

THREE-PHASE MAIN EXCITER

The three-phase main exciter is a six-pole revolving-armature unit. Arranged in the

stator frame are the poles with the field and damper winding. The field winding is

arranged on the laminated magnetic poles. At the pole shoe bars are provided their

ends being connected so as to form a damper winding. Between two poles a

quadrature-axis coil is fitted for inductive measurement of the exciter current.

The rotor consists of stacked laminations which are compressed by through bolts over

compression rings. The three-phase winding is inserted in the slots of the laminated

rotor. The winding conductors are transposed within the core -length, and the end turns

of the rotor winding are secured with steel bands. The connections are made on the

side facing the rectifier wheels. The winding ends are run to a bus ring system to which

the three-phase leads to the rectifier wheels are also connected. After full impregnation

with synthetic resin and curing, the complete rotor is shrunk into the shaft. A journal

bearing is arranged between main exciter and pilot exciter and has forced oil lubrication

from the turbine oil supply.

THREE-PHASE PILOT EXCITER

The three-phase pilot exciter is a 16 pole revolving-field unit. The frame accommodates

the laminated core with the three-phase winding. The rotor consists of.a hub with

mounted poles. Each pole consists of 10 separate permanent magnets which are

housed in a non-magnetic metallic enclosure. The magnets are braced between the hub

and the external pole shoe with bolts. The rotor hub is shrunk into the free Shaft end.

© PMI, NTPC 253

COOLING OF EXCITER

The exciter is air cooled. The cooling air is circulated in a closed circuit and recooled in

two cooler sections arranged along side the exciter.

The complete exciter is housed in an enclosure through which the cooling air circulates.

The rectifier Wheels, housed in their own enclosure draw the cool air in at both ends

and expel the warmed air to the compartment beneath the base plate.

The man exciter enclosure receives cool air from the fan after it passes over the pilot

exciter. The air enters the main exciter from both ends and is passed into ducts below

the rotor body and discharged through radial slots in the rotor core to the lower

compartment. The warm air is then returned to the main enclosure via the cooler

sections.

© PMI, NTPC 254

VOLTAGE REGULATOR

BASIC MODE OF OPERATION

The THYRISIEM 04-2 voltage regulator is designed for excitation and control of

brushless generators. The block diagram Fig. 48 shows the circuit configuration. The

machine set consists of the generator and a direct coupled exciter unit with a three

phase main exciter, rotating rectifiers and a permanent magnet auxiliary exciter. The

© PMI, NTPC 255

main components of the voltage regulator are two closed-loop control systems each

followed by a separate gate control unit and thyristor set and a de-excitation equipment.

In addition to this, a open-loop control system for the signal exchange between the

regulator and the power station control room and other plant components is provided as

well as power supply equipment.

Control system 1 for automatic generator voltage control (AUTO) comprises the

following:

- Generator voltage control: the output quantity of this control is the set-point for a

following

- excitation current regulator, controlling the field current of the main exciter

(= output current of the coordinated thyristor set).

- circuits for automatic excitation build-up during start-up and field suppression

during shut-down; this equipment acts into the output of the generator voltage

control, limiting the set-point for the above excitation current regulator. The

stationary value of this limitation determines the maximum possible excitation

current set-point (field forcing limitation);

- limiter for the under-excited range (under excitation limiter).

- delayed - limiter for the over excited range (over excitation limiter).

The field forcing limitation limits - practically undelayed - the output current of the

thyristor sets to the maximum permissible value, when the voltage regulation calls for

maximum excitation. Normally, this maximum permissible value is 1.5 times the rated

excitation The over excitation limiter ensures delayed reduction of the excitation current

to the rated value in the over excited range, i.e. between rated excitation and maximum

excitation. The delay time depends on the amount by which the rated value has been

© PMI, NTPC 256

exceeded. These limiters protect thyristor sets and machines against over excitation

with too high values or too long duration.

In the under-excited range, the under excitation limiter ensures that the minimum

excitation required for stable parallel operation of the generator with the system is

available and that the under-excited reactive power is limited accordingly. The response

characteristic is formed on the basis of the generator reactive current, active current and

terminal voltage and can be matched to the generator and system data.

Control system 2 (MANUAL) mainly comprises a second excitation current regulator

with separate sensing for the actual value. This control system is also called Manual

control system, because for constant generator voltage manual re-adjusting of the

excitation current set-point is required when changing the generator load. The excitation

current regulator permits plotting of generator characteristics and setting of protective

relays during no-load and short-circuit runs of the generator during commissioning and

maintenance work. The system can also be used for setting the generator excitation

during normal operation when the automatic voltage is defective. Normally, the

automatic voltage regulator is in service even during startup and shut down of the

generator set.

The set-point adjuster of the excitation current regulator for MANUAL is tracked

automatically (follow up control) so that in the event of faults, changeover to the

MANUAL control system is possible without delay. Automatic changeover is initiated by

some special fault conditions. Correct operation of the follow-up control circuit is

monitored and can be observed on a matching instrument in the control room. This

instrument can also be used for manual matching.

Either control system is coordinated with a separate gate-control "and thyristor set.

Separate equipment is also provided for supplying power to either control system.

The two separate thyristor sets for automatic voltage regulation (AUTO) and excitation

© PMI, NTPC 257

current control (MANUAL) have the same ample dimensioning regarding rated current

and blocking voltage. Each thyristor is fused separately. The thyristor set for automatic

voltage regulation can be switched off by means of an isolator with contacts in the gate-

control, power supply and output sides.

This isolator in conjunction with corresponding arrangement and design of the thyristor

set enables an exchange of thyristors and fuses during operation if necessary whilst

operation is continued by means of the excitation current regulator (MANUAL). In

addition, the thyristor set for automatic voltage regulation is equipped with a current-flow

monitoring system for detecting failure of firing pulses or fuses. Automatic changeover

to the current regulator (MANUAL) is initiated by this system.

On the input side, the thyristor sets are fed with auxiliary power from a 220 V, 400 HZ

permanent magnet auxiliary exciter. The output side of the thyristor sets feeds the field

winding of the main exciter with variable d.c. current.

To de-excite the generator during shutdown or when the generator protection system

has picked up, a command is transmitted to the outputs of both control systems, driving

the thyristor set being in service to maximum negative output voltage. The negative

voltage (inverter operation) de-excites the main exciter in less than 1/2 sec. The

generator de-excitation following is a function of the relevant effective generator time

constant.

Approximately 1/2 sec. after receiving the de-excite command, two field suppression

contactors (one being redundant) switch a field discharge resistor in parallel to the main

exciter field winding. Subsequently an Off command is issued to the field breaker via its

tripping coil. In the event of failure of the electronic field suppression by inverter

operation, de-excitation would be achieved with a delay of 1/2 sec. via the field

discharge resistors.

The THYRISIEM 04-2 voltage regulator equipment is arranged within the cubicle group

© PMI, NTPC 258

selected according to the power circuits and the 24 V d.c. or 15 V d.c. open and closed-

loop control circuits. The signal exchange between the power circuits and the electronic

circuits is via voltage isolating transducers, transformers and coupling relays.

The closed-loop control systems are made up of modules of the Simadyn C system

whereas modules of the Simatic C1 system are used for the electronic open-loop

control and the alarm system.

EXCITATION CONTROL DURING START-UP AND SHUTDOWN, FIELD BREAKER CONTROL, DE-EXCITATION

Excitation and voltage closed-loop control are not necessary for speeds under approx.

0.95 times rated speed. Furthermore closed-loop control of the generator voltage to the

rated voltage would not be permissible at low speeds since the generator and unit

transformer would become saturated. For this reason, functions are provided for

enabling excitation during start-up and. for blocking excitation during shutdown of the

generator.

The speed is detected via the largely speed-proportional voltage of the auxiliary exciter.

In addition to this, redundant speed criteria n < and n > are used from a speed limit

monitor, if available.

The field breaker is to be switched on after reaching the speed limit required by a

manual command or from a functional group control system. In both cases the

command passes, as welt as the check-back signals "Field breaker Off/On" and

"Control voltage fault" through a Iscamatic control module AS-11; See Fig, No. 49.

© PMI, NTPC 259

Closing of the field breaker is interlocked with the criterion "Ramp function generator

© PMI, NTPC 260

lower limit" to ensure that the generator voltage builds up slowly without overshooting.

During excitation current control (MANUAL), the lower limit of the set point adjuster is

interlocked instead to ensure that zero excitation is obtained after closing. In addition to

this, the power supply of both tripping channels must be available and the key operated

switch for blocking the excitation during commissioning and maintenance work

(arranged in the voltage regulator cubicle) must be set to the position "Excitation not

blocked".

With the field breaker being closed and the speed limits exceeded, the pulse blocking

signal to the gate control set disappears, the ramp function generator runs up thus

building up the generator excitation provided that automatic voltage control (AUTO) has

been selected. The run-up command is, stored by a memory with permanent relay.

When the speed drops below the limit values during shutdown, this initiates together

with the status "Generator not loaded".

- field breaker OFF command

- pulse blocking signal to the gate control set

- run-down of ramp function generator.

Run-down of the ramp function generator may also be initiated during rated speed by

the OFF state of the field breaker, when the breaker is tripped from the generator

protective system.

The speed criteria are monitored with respect to their importance. Presence of the

criterion n < or absence of the criterion n > while the generator is loaded will be

alarmed.

Under excitation current control (MANUAL), no automatic excitation build-up is effected

© PMI, NTPC 261

during start-up. When the field breaker is closed, the excitation current is at its lowest

possible value = zero value approx. The desired excitation can be set on the set point

adjuster (LOWER/ RAISE pushbutton in the control room). During shutdown of the

generator, the field current set-point adjuster receives a continuous LOWER command

on tripping of the field breaker so that the set-point adjuster is set to the kwer limit

position.

The tripping circuits for the de-excitation are provided twice for redundancy reasons.

This should be complemented by corresponding safety in the power supply for the trip

circuits.

A de-excitation command from the generator protection system or a "Field breaker

OFF" command from the control room energizes relays K12 (system 1)/K22 (system 2)

which seal in and start the time relays K13/K23, set to 0.5 s. Via relays K12/K22 the

thyristor set operating is driven to inverter operation, thereby reversing the main exciter

field winding voltages and thus reducing the thyristor set output current to zero in less

than half a second. The field discharge contactors K14/K24, energized by time relays

K13 or K23 respectively, switch a field discharge resistor in parallel to the field winding

of the main exciter and trip field breaker Q 1 via its tripping coil.

The field discharge resistor ensures that proper de-excitation is achieved even in the

event of failure of the electronic de-excitation circuit.

The ON command for the field breaker interrupts the self-holding state of relays

K12/K22 (provided that the above-mentioned conditions for closing are fulfilled) and

then starts the motor drive of the field breaker- interruption of the ON command on

completion of breaker closing is effected via a field breaker auxiliary contact in the 220

V control circuits.

The field breaker is automatically tripped during generator shutdown by speed criteria

as described above if not tripped earlier by the reverse power protection system. In

© PMI, NTPC 262

emergencies, the field breaker can also be tripped manually via the generator protection

system by actuating the emergency pushbutton on the control desk. In this case. also a

turbine trip command is remitted to the turbine control equipment.

The OFF pushbutton for the field breaker is normally only connected for reset of the

Iscamatic control module AS11 in the above cases. Should the OFF pushbutton be

required to really trip the field breaker, interlocks must be provided with the generator

breaker and possibly with the station service supply breaker(s) to prevent de-excitation

of loaded generator.

Local non-electrical (mechanical) tripping of the field breaker is not permissible as the

other essential s. field suppression by inverter operation) are not tripped in this case.

For emergency de-excitation a push-button or switch is locally provided (in the cubicle).

Emergency de-excitation is possible also by tripping the MCB's for the pulse power

supply of the thyristor sets.

Mechanically closing of the field breaker is to be avoided also, as the sealed in relays in

the tripping circuits would not drop out in this case.

During short-circuit operation of the generator for setting of the generator protective

equipment, the degree of excitation is adjusted by means of the excitation current

regulator (MANUAL). During this mode of operation, a "MANUAL faulted" criterion

available in the alarm system of the regulator can provisionally be used for tripping the

field breaker.

A comparison of rotating diode excitation system for 200 MW and 500 MW is given

below:-

© PMI, NTPC 263

COMPARISON OF 500 MW & 200 MW EXCITATION SYSTEM

DESCRIPTION 500 MW 200 MW

1. Type of system

2. Dependency on

external supply

3. Response of

Excitation system

4. Requirement of

additional Brg.

and increase of

T.G. shaft length.

5. Maintenance

Brushless activities system

with Pilot exciter, main

exciter and Rotating

Diodes.

No external source

requirement since pilot

exciter has permanent

magnet field.

Slower than static type

since control is indirect (on

the field of main exciter

and megnetic components

Less since sliprings and

brushes are avoided

One additional brg. and

shaft length is increased

Static Excitation system using

Thyristors & taking supply

from output of generator.

Field flashing supply required

for excitation build up.

Very fast response of the

order of 40ms. control direct

and solid state devices are

employed.

No additional brg. and no

increase in shaft lenght.

More since slip rings, brushes

are required. Also over hang

vibrations very high resulting

in faster wear and tear.

© PMI, NTPC 264

SYNCHRONISATION

Basically during synchronisation, synchroniser checks the state of voltage and

frequency at generator terminals with that of the grid and gives a command for closing

of generator circuit breaker at a suitable instant. Going to do so two parameters in the

generator output viz voltage and frequency are required to be controlled to get a

matching value with that of the system.

© PMI, NTPC 265

© PMI, NTPC 266

A speed controller is used to control the speed of turbo set to attain a particular

frequency. Speed control loops pick up signals from bus duct PT and frequency of these

voltages corresponds to the rotational speed of the turbo set. The speed reference

value is set by means of potentio meter which can be operated either remotely or

manually. In the vicinity of rated speed (47Hz to 54Hz) a reducing gear box lowers the

setting speed of the potentio meter to facilitate smooth control over the speed. Remote

adjustment of speed reference value takes place from UCB and the same is displayed

by two indicators, (i) (0-60Hz) and (ii) 47 to 55 Hz).

On the other hand. AVR for brushless excitation system comprises of two channel (i)

Auto channel and (ii) Manual channel.

Auto channel of AVR comprises a regulator for the generator voltage, its associated set

point adjuster, under and over excitation limiter and a field forcing limiter where as

manual channel comprises a controller for the field current in the field winding of the

main exciter and a set point adjuster.

Excitation can be controlled by either of the above channel during synchronisation. But

this control is not necessary for speeds below 0.95% times the rated speed.

Furthermore closed-loop control of the generator voltage to the rated voltage would not

be permissible at low speed since this may lead to a critical V/F ratio (saturation point).

The field breaker is closed during the start up cycle by a manual command after 95% at

rated speed is reached Under this condition blocking signal to the thyristor gate

disappears and depending on the limit set in the gate control unit excitation starts

building up and reaches its maximum within 20 sec.

Under manual mode of operation, no automatic excitation build up is effected during

start up. When field breaker is closed, the excitation current is set to its lowest possible

value which corresponds to the lower limit setting of the field current set point adjuster.

The desired excitation can be obtained by adjusting the set point adjuster. (Lower/Raise

Push button in UCB).

© PMI, NTPC 267

Taking the above two signal (speed/voltage) the auto-synchronises checks the same

with the system voltage and frequency and gives closing command to generator circuit

breaker.

© PMI, NTPC 268

21. Protections Of Generator

INTERNAL FAULTS

The generator being very costly and critical item in a power plant and therefore has

been protected by duplicated protections. Phase to phase faults are covered by

differential relays 87G & 87 GT. Stator earth faults has been covered by 64 G1 & 64 G2.

The voltage operated relay 64 G1 is capable of detecting earth faults anywhere in the

stator winding but 64 G2 cover only 95% of the stator winding.

The first stator earth fault relay (64 G1) is comprised of three elements. One element

would operate after a time delay if fundamental voltage across the grounding resistor

exceeds above 5 volts and thereby protecting 95% of the stator winding. This is

connected to trip the generator. Second element detects the collapse of third harmonic

voltage across the grounding resistor with generator terminal voltage being healthy, as

detected by third element and will give an alarm in UCB. This would detect earth faults

in the 0-5% of stator winding towards an 1 including neutral and monitors the opening

and shorting of the stator grounding circuit.

The earth fault relay 64 G2 is a voltage operated inverse time relay connected to open

delta PT connections and serve as a standby to the relay 64 G1.

The generator bus ducts have an isolated phase construction throughout and therefore

cannot have a direct phase to phase fault. Their coverage by only 87GT for. short

circuits is therefore considered sufficient. Earth faults in gen. bus ducts are covered by

64 G1 & 64 G2 For earth protection of generator rotor circuit, relay 64 F has been

provided.

© PMI, NTPC 269

External faults and Abnormal operating conditions

The large turbo generators have a very limited capacity to withstand negative sequence

currents a precise negative sequence protection (46G) has been provided. While the

generator has a certain overload capacity, continued operation at a toad higher than its

rated current can lead to overheating and consequent insulation failure therefore an

over current relay 51 G has been provided for giving an overload alarm. The ultimate

protection against stator overheating would be provided by the generator temp.

monitoring system (covered under C&l). The gen. overload protection will also protect

the Generator bus ducts from overheating. No further protection or monitoring for bus

ducts has been provided since they would have a naturally cooled rating slightly higher

than the, gen. rating.

A three phase single zone impedance relay (210) has been provided for the back up

protection of generator against external three phase and phase to phase faults in the

400 Kv system. The zone of 21 G should extend beyond the 400 KV switchyard and

have a time delay of around 2 sec. so that generator is tripped only if the 400 Kv time

protection has not cleared the fault even in second zone. Local breaker back up (or

breaker failure) protection 502 has been provided for the 400 KV. Generator breaker. A

loss of excitation protection 40G has been provided and pole slipping protection 98G

has also been provided for tripping the generator in case of pole slipping due to

instability in the comparatively week long distance 400 KV system.

The generator can develop dangerously high over voltage in the event of maloperation

of AVR or load throw off while generator excitation is under manual control. An over

voltage relay 59G set at 110% has been provided to detect this condition and give an

alarm in case it persists for 2-3 seconds. For protection of GT & UAT's against the serve

over voltages an over fluxing relay 99T has been provided and generator has to be

tripped if v/f remains above 1.2 p.u. for about 2.3 seconds.

Monitoring of the turbine generator can occur due to stoppage of steam supply. While it

© PMI, NTPC 270

does not a act the generator, it can cause gradual overheating of turbine blades. The

low forward powe. relay 37 GA has been used for detecting this condition and

annunciating it for corrective action by the operator.

The underfrequency relay 81 G has been connected for giving an alarm to the UCB

operator and not to initiate any tripping. The operator then has to watch the situation

and take appropriate action. The duration of operation at a low frequency should

however be recorded in Das.

Monitoring of generator VT fuses has also been provided to give an alarm in UCB. As

VT fuse failure may mat-operate loss of excitation, pole slipping, internal fault

protection, back up impedance, stator earth fault and low forward relays, these

protections.would be blocked on VT fuse-failure

Logic Diagram : Protection logic diagram and protections setting adopted for

500 MW as Shown, in Fig - 51 and 52''

© PMI, NTPC 271

© PMI, NTPC 272

© PMI, NTPC 273

22. Generator Auxiliaries

INTEGRAL SYSTEM Like other 200MW machines, generator integral system comprises of three system viz. (i)

seal oil system (ii) Primary water system (iii) H2 Cooling system.

Seal Oil System

Generator shaft seals are supplied with pressurised seat oil to prevent hydrogen losses at

the shaft and ingress of air into the generator. During normal operation, shaft seals are

supplied with seal oil by a separate system, consisting of hydrogen side seal oil circuit

and air side seal oil circuit. The oil used in seal oil circuit is same as that used in turbine

generator journal bearings.

During normal operation, in the air side AC pump draws seal oil from the seal oil storage

tank and feeds it to the seals via cooler and filters which drains towards the air side and

return back to the seal oil storage tank.

Similarly H2 side oil pump supplies oil to seal through a separate strainer and cooler. By

dividing the seal oil system into two separate circuits, hydrogen losses at the seals are

kept to a minimum and good hydrogen purity is also maintained. For the air side seal oil

circuit. three seal oil pumps are provided. In the event of failure of one AC pump. the

second AC pumps automatically takes over. If both pumps fail. the seal oil supply is taken

over by the standby D.C. pump. On the other hand one seal oil pump is used for oil

circulation in H2 side seal oil circuit.

In the event of failure of this pump, seal oil in hydrogen side is derived from air side seal

oil circuit. Prolonged operation in this fashion may result in deterioration of hydrogen

purity. Oil pressure maintained at different stages are as follows.

© PMI, NTPC 274

H2 Casing Pressure H2 Seal oil pressure Air side seal oil

(4 bar) 10 m bar more than Air side ( 5 bar)

The simplified schematic diagram of the system is shown in Figure-53. Salient operational

parameters of the system as given below :

© PMI, NTPC 275

© PMI, NTPC 276

i) H2 side seal oil pump operating pressure - 10bar

ii) Axis side seal oil pump 1/2/3 operating pressure - 9bar

iii) Seal oil temperature after air side cooler (Operating value) - 30-400C

(500C (Alarm)

550C (Trip).

iv) Air side seal oil pressure - 5.7 bar

v) Air side/H2 side seal oil temperature - 40-690 C

PRIMARY WATER SYSTEM

When the turbine generator is on load. heat produced in the stator winding is removed by

continuously circulating low conductivity demineralised water through hollow conductors

of the stator windings and pass age ways within the terminals.

The losses occurring in the generator stator winding components e.g. 3tator winding,

terminal bushings and phase connectors are dissipated directly through water. The

cooling water being in contact with the high voltage winding, must have an electrical

conductivity of the order of 0.5 to 1 micro mho/cm. Primary water supply system

comprises of the following components.

i) Primary water pump & filter unit. (ii) Primary water cooler unit (iii) Primary water tank (iv)

primary water measuring instrument rack (v) Ion exchange.

Primary water is circulated by one of the 2X100% duty pump through a strainer type filter

with magnet bar and cooler. The primary water is drawn from the primary water tank and

passes to a primary water manifold (inlet) via coolers and filters and from there to stator

bars via teflon hoses. The primary water leaving the stator winding is passed through

similar teflon hoses to anther primary water manifold (outlet) and is then returned to the

primary water tank. A separate flow path from a point before the stator winding inlet cools

the bushings and phase connectors. The primary water pump and filter unit is equipped

with 1% bypass capacity polishing unit. One percent of total primary water continuously

© PMI, NTPC 277

gets polished in this mixed bed ion exchange. A simplified sketch of primary water system

is shown in Fig.-54. Salient operational parameters of the system are given below.

i) Primary water temperature (water manifold) - 650C.

(Alarm - 700C)

ii) Conductivity in main circuit - 0.5 mho/cm

(Alarm at 1.5 mho/cm)

iii) Primary water pump ½ (discharge) - 10 bar

iv) Primary water temperature - 30-450C

(550C (Alram)

60°C (Trip)

v) Primary water flow at bushing - 16.6dm3/sec.

15.0 dm3/sec. (Alram)

13.3 dm3/sec. (Trip)

vi) Primary water flow at bushing - 0.42 dm3/sec.

0.39 dm3/sec. (Alram)

0.37 dm3/sec. (Trip)

vii) Primary water pressure (stator winding) - 3 bar

2.8 bar (Alram)

viii) Primary water temp. after stator wdg - 600C

700C (Alram)

ix) Primary water temp. after bushing - 600C

700C (Alram)

© PMI, NTPC 278

x) Pressure in primary water tank - 0-0.2 bar

0.3 bar (Alram)

GAS SYSTEM

© PMI, NTPC 279

The hydrogen is circulated in the generator in a closed circuit by a multi stage axial fan

located at the turbine end. The fan draws hot gas from the air gap and delivers it to the

coolers where it is cooled and recirculated. Gas system comprises of the following

components

i) Hydrogen distributor and cylinders rack

ii) Carbon dioxide distributor and cylinders rack

iii) Gas drier

vi) Nitrogen distributor and cylinder rack.

Hydrogen supply to the generator is obtained from a hydrogen distributor where the

hydrogen cylinders are placed. The hydrogen gas available in the header as cylinder

pressure is passed into two parallely connected pressure reducer for expansion to the

intermediate pressure of 8 Kg/sq. cm. This pressure is further reduced to generator

coupling means of h/o parallel connected pressure reducers and then cir

As a precaution against explosion hazard, the air must neither be directly replaced with

hydrogen during generate- filling nor the hydrogen directly replaced with air driving

emptying operation. In both the cases Co2 is used for scavenging.

© PMI, NTPC 280

Carbon dioxide which is available in liquid state in the cylinder is evaporated and

© PMI, NTPC 281

expanded in Co2 vapouriser by electrical heating elements. An expansion orifice after

vapouriser is provided for controlling Co2 pressure being fed during Co2 filling operation.

For removing Co2 from generator compressed air supply with dampness filter is

connected to compressed air supply system. Under all conditions of operation, except

Co2 purging with air, the compressed air hose should be disconnected to ensure that no

air can be admitted into the generator filled with hydrogen.

A small amount of hydrogen circulating in the generator for cooling is passed through a

drier for removing the moisture and maintaining the gas in dry condition. The gas drier

chamber is pressure resistant and filled with absorbing material. The' absorbing material

can be reactivated by hot air supplied through a blower and heater circuit;

Nitrogen bottle rack is also provided in the system for supplying the nitrogen to the

primary water tank to create an inert atmosphere in the p.w.tank lo reduce the; possibility

of air getting mixed with hydrogen carried through primary water. A simplified diagram of

gas system is shown in Fig.55. Salient operational parameters are as given below :

i) H2 bottle pressure - 150 bar

ii) H2 operating pressure - 4 bar

iii) Normal H2 purity - 98% (alarm - 95%)

iv) H2 & Co2 filling rate - 150 m3/hr (STP)

v) Cold gas temperature . - 25 - 40°C

50°C (alarm)

vi) Hot gas temperature - 40 - 60°C

65°C (Alarm)

© PMI, NTPC 282

EMERGENCIES IN A 500 MW MACHINE

In a 500 MW machines, emergencies arising during operation should be handled carefully

to avoid any unwanted tripping and also damage to the machine. Apart from the electrical

protections which safeguards the machine from external and internal electrical fault, the

following conditions may give rise to an emergency situation for the generator which may

result into the tripping of the generator.

Primary water flow

Through two protective circuit quantity of primary water i.e. water flow in the generator

winding and bushing is being monitored thus preventing overheating of the stator winding

and the bushing incase of inadequate flow of primary water.

Primary Water Temperature

This protective circuit protects the water cooled components of the generator against

inadequate cooling and thus overheating as a result of higher inlet temperature of primary

water.

Cold Gas Temperature

This protective circuit protects the hydrogen cooled components of the generator against

inadequate cooling due to increases inlet temp. of the hydrogen into the generator.

Liquid in the bushing box

It a leak occurs in the components located within the generator, the water/ oil would leak

out and this liquid may be collected in die terminal box which may rise the liquid level in

the terminal box rapidly. Due to this liquid level, a short circuit or an earth fault may occur.

To prevent the same, tripping is envisaged in case of dangerous liquid level, tapping the

© PMI, NTPC 283

signal from level switch in terminal box.

Seal oil temp

If the seal oil temperature is too high, as may occur in the event of fault in the seal oil

coolers, the proper sealing of the generator is no longer ensured.

Due to the increased temperature of the seal oil, the viscosity would become less and the

hydrogen would leak out of the generator casing. Further, the losses in the seals which

result in additional heat which would increase the temperature of the seal oil. The

protection initiates a turbine trip in such cases. /

Hot air temp. of the Main Exciter

In the event of failure of the exciter air coolers, the inlet cooling air temp. increases which

endangers the operation of the exciter. This protection would initiate the opening of an

emergency ventilation door on the exciter casing in such conditions.

Except the emergency cooling of exciter in all the above cases turbine tripping is initiated

in case of any of the above faults. The generator itself is then isolated from the grid and

de-excited.

© PMI, NTPC 284

23. Data Sheet for Generator

© PMI, NTPC 285

24. Unit Start Up And Shutdown Procedure

START UP OF BOILER / Line-up of the Boiler Prior to Start-Up

1. It is assumed that all work in the boiler furnace and air & gas path ducts have been

completed.

2. It is also assumed that the unit has been chemically cleaned, the steam lines have

been blown, and safety valves on the drum, SH & RH headers have been set.

3. Ensure that boiler furnace, air preheaters, economizer, electrostatic precipitators,

ID.FD & PA fans, gas & air ducts are all cleared of men and foreign material.

4. Close all boiler penthouse heat removal doors and access doors.

5. Close all boiler access doors and observation doors and ash hopper doors"

6. Verify cooling water is supplied to boiler access doors and ash hopper doors.

7. Fill the bottom ash hopper and hopper seal through. Open the continuous make up

to these & ensure proper overflow.

8. Ensure that boiler drum manhole covers are properly seated.

9. Verify that gags, have been removed from safety valves installed on boiier drum,

SH & RH headers,

© PMI, NTPC 286

10. Local pressure controllers are connected with SH & RH electromatic relief valves.

11. Open following dampers:

a Secondary RAPH air inlet/outlet

b Secondary RAPH gas inlet/outlet

c Primary RAPH air inlet/outlet

d. Primary RAPH gas inlet/outlet

e. EP inlet/outlet

f. Seal Air Fan suction filter isolating dampers

g. Seal air Fan suction dampers

h. Cold primary air gates (to mill)

i. Cold air control dampers (5%)

j. Pulverizer discharge valves

12. Close following dampers :

a. Over-fire dampers

b. Hot air shut-off gates

c. Hot air control dampers

© PMI, NTPC 287

13. Verify secondary air dampers are open or modulating.

14. Open boiler drum motor operated vent valves

15. Open SH start-up vent valves partially and various SH header manual vent valves

& drain valves full.

16. Open RH startup vent valves partially and various RH header manual vent valves

& drain valves full.

17. Close boiler drum, SH and RH nitrogen blanket isolation valves.

18. Close SH desuper heater spray water supply block and control valves.

19. Close RH desuper heater spray water supply block and control valves

20. Verify that boiler drum pressure & level transmitter hydrastep are in operatable

condition.

All three Motor cavity of C.C. pumps should be in tilted condition. Discharge valves

of C.C. pumps should be in close position for Boiler filling.

21. Open all boiler drum level gauge glass, level transmitters & hydrastep isolation

valves & close blow down valves.

22. Verify boiler drum gauge glass illumination lights are energised.

23. Open steam pressure gauge isolation valves.

24. Open feed water, boiler drum water, saturated steam and superheated steam

sample (me isolation valves.

© PMI, NTPC 288

25. Open chemical feed system drum inlet isolation valves.

26. Open the continuous blow down line manual isolation valves & close emergency

blow down line motor operated valves.

27. Open the continuous blow down tank vent valve to deaerator. (CBD Vent to be

opened to deaerator only after ensuring that Boiler water silica is within limits.

Normally less than 0.2 ppm).

28. Close water watt drain header drain valves to IBD tank & waste and bottom ring

header drain valves.

29. Verify that CBD Tank drain (he level control valve is operatable and its isolation

valves are open.

30. Open economizer recirculation valves.

31. Close economizer inlet header drain valves.

32. Close drain valves at FW inlet to economizer inlet header.

33. Open FW inlet valve to economizer inlet header and close its equaliser.

34. Open the soot blowing system steam supply valves as required.

35. Ensure all soot blowers are retracted. Before light up, availability of APH soot

blowers to be ensured.

36. Retract Furnace Temperature probes. 'Ensure cooling air is available to probes.

II Following Instrumentation & controls an to be checked and made available.

© PMI, NTPC 289

i) Alarm annunciation system and DAS.

ii) Remote operation of following :

a) ID Fan - inlet/outlet dampers, control vanes and hydraulic

coupling scoop tube.

b) FD Fan - outlet dampers & variable blade pitch control.

c) PA Fan - outlet dampers and variable blade pitch control.

d) Controls connected with FSSS.

iii) Following local & remote indicators & recorders:

a. F.D. Pressure

b. P.O. Temperature

c. P.O. Flow

d. Atomising Steam Pressure

e. Furnace Draft

f. Windbox to Furnace Diff. Pressure

g. Air path - Press & Temp.

h. Gas path - Press & Temp.

i, Air flow

j. Drum level

k. Drum pressure

I. S.H. Outlet Pressure

© PMI, NTPC 290

m. Furnace Temperature

n. Drum Metal Temperature

o. SH/RH Metal temperature

p. Feed Water Press at Economizer Inlet

q. Feed Water Temp. at Economizer Inlet.

r. Main Steam temperature

s. Reheat Steam temp.

t. Reheat Steam pressure

u. Steam Flow

v. Feed flow

w. Condensate flow

x. All BFP Instruments

y. C.C. pump ∆P Recorder and C.C. pump cavity Temperature recorder.

Suction Manifold to pump casing ∆T indicators.

z. Instruments tied-up with Feed Water System and condensate system.

III Line-up Condensate System

1. Ensure normal and emergency makeup to condenser hotwell and maintain its

level.

2. Open following valves:

a. LPH 1.2 & 3 condensate inlet/outlet valves.

b. Deaerator level control station inlet/outlet valves

© PMI, NTPC 291

c. Condensate supply to LP Bypass & Exhaust hood spray

d. Condensate supply to LP & HP Dosing

e. GSC minimum flow recirculation control inlet/outlet valves.

f. Condensate spill control inlet/outlet valves.

g. CEP seal injection valves

h. CEP suction valves

i. CEP suction vent valves.

j. CEP recirculation control outlet valves

3. Close following valves:

a. LPH 1,2 & 3 condensate bypass valves.

b. GSC minimum flow recirculation control bypass valve

c. Condensate spill control bypass velvet.

d. CEP discharge valve.

4. Start condensate extraction pump with discharge valve shut. Slowly open

discharge valve as the pump speeds up.

5. Adjust sealing water to all valve glands provided with sealing arrangement.

6. Maintain Deaerator Feedwater Storage Tank level.

© PMI, NTPC 292

IV Start the Boiler Feed System and Fill Drum as described below

1. Ensure normal and emergency makeup to condenser hotwell and maintain its level

(NWL: 430 mm above O.OM).

2. Ensure condensate extraction pumps running and maintain deaerator feedwater

storage tank level (NEWL: 61G mm above tank C.L.).

3. Admit steam to deaerator from auxiliary steam header and slowly build-up a

pressure of 1.5 ata in the deaerator. This arrangement would obviate any

possibility of oxygen corrosion in the boiler.

4. Line-up feedwater system as under:

a. Open low load feed control station isolation valves and close its bypass

valve.

b. Close HP Heater FW inlet/outlet valves and open HP Heater bypass valve.

c. Open BFP discharge valves.

d. Open BFP suction valves

e. Open 3FP inter stage valves

t Open BFP kicker stage valve

g. Ensure BFP recirculation system is in service.

h. Ensure boiler Feed Pumps are properly linked up and their auxiliary oil

pumps are running.

© PMI, NTPC 293

5. Start Motor Driven BFP and engage the hydraulic coupling to the minimum speed

required. If MD BFP is not available. Turbine Driven BFP can also be started

provided condenser is ready and vacuum has been established.

6. Gradually open the low toad feed control valve and control the feed water flow to

the boiler. Take water to the boiler till the drum level becomes normal (NWL:

225mm below C.L. of drum).

V. Line-Up Fuel Oil System

1. Fill F.O. Day Tank

2. Start HFO pump after lining-up the heating & straining unit. Keep the pump

running on recirculation.

3. Open manual valves on atomizing steam supply line and fuel oil supply line to

each burner.

4. Verify that oil guns are inserted and coupled.

5. Verify that instrument air is available to each oil gun advance-retract cylinder.

6. Verify that cooling air is available to each burner flame scanner.

7. Verify all atomizing steam header traps are in service.

8. Verily that atomizing steam pressure control valves ASPRV, is in service. Open

ASPRV local pressure controller pressure sensing the line isolation valve.

9. Verify also that fuel oil flow control valves HOFCV is in service.

10. Close the fuel oil header trip valve, HOTV, and open the fuel oil short recirculation

© PMI, NTPC 294

valve.

11. Admit steam to HFO heaters and gradually bring up P.O. temp. to 130°C. Circulate

oil through the short recirculation valve.

12. Adjust HFO pump recirculation pressure control valve so as to maintain 24 kg/

cm2 g. F.C. pressure at the boiler front (before HOFCV).

13. Open P.O. recirculation valve, HORV, and then trip valve, HOTV.

14. Gradually open HOFCV and warm-up P.O. line upto the burner front.

VI Start-up

The 500 MW unit steam generator is equipped with a comprehensive FSSS which allows

the furnace purging and starting of fuel firing system in a predetermined sequence when

the set preconditions, as outlined in FSSS logic diagrams, are satisfied.

Sequence operation and interlock & protection of ID/FD/PA fans & RAPH and their

associated control/lube oil pumps & dampers, Boiler Water circulating pumps and

Reheater Protection have been outlined in logic diagrams.

The sequence of Start-up is as under:

1. Start two boiler water circulating pumps. Before starting C.C. pumps, ensure-that

Drum Vents are closed and all the pumps discharge valves are kept full open.

2. Start both secondary regenerative air preheaters.

3. Place steam coil air preheaters in service.

4. Start two ID fans. Verify that their inlet & outlet dampers open.

© PMI, NTPC 295

5. Start one PA fan. Verify that its outlet damper open.

6. Start two FD fans. Verify that their outlet dampers open.

7. Start A.C. scanner air fan. Verify that its outlet damper opens.

8. Adjust flow through I.D. Fan & F.D. fan and position of windbox dampers to permit

a purge air flow of at least 30% of total air flow and a furnace pressure of

approximately - 12 mm wg.

9. Adjust the auxiliary air dampers to obtain approximately 35 mmwg windbox to

furnace diff. pressure.

10. Check all purge permissives are satisfied.

11. When "Push to Purge" Green indicating lamp appears on FSSS Console Insert,

initiate a furnace purge. "Purging" Amber indicating lamp appears on FSSS

console Insert.

12. After the completion of purge cycle (5 mins.) "Purge Complete" Yellow indicating

lamp appears on FSSS Con-sole Insert. Reset MFR (A) & MFR (B)-Green lamps

appear on FSSS Console Insert.

13. Verify that none of the boiler trip conditions exist.

14. Close F.O. Recirculation valve, HORV. Ensure that Heavy Fuel Oil Trip Valve

remains open.

15. Start one pair/elevation of oil burners. HFO Elev. control. HFO Corner Control. Oil

Elev. Monitoring.

© PMI, NTPC 296

16. Regulate oil flow to increase the boiler water temperature at HO°C/hr. and

maintain furnace exit gas temperature below 540°C.

17. Place the furnace temperature probe to monitor furnace exit gas temperature.

18. During the warming up period the economizer recirculation valves remain open.

Blow down the unit as required by opening the valves to maintain the drum water

level in the visibility range of the gauge.

19. While the unit is heating up frequent checks should be made of the boiler

expansion movements.

VII Pressure Raising

1. Start with minimum number of oil guns. Gradually the number of oil guns may be

increased to commensurate with the rate of pressure rise.

2. Check the oil flame & stack. In a cold boiler, the furnace tend to be smoky in the

beginning which will clear up as the furnace warms up. But the flame should be

clear & bright. The stack also should be clear. In no case white smoke should be

allowed from the chimney.

The smoke from stack should neither be too black or too white. Air flow, windbox

pressure, respective SAD's position to be checked in addition to the oil pressure

and atomising steam pressure to get clear and bright flame.

3. Close the Superheater Vents when copius amount of steam comes out of them.

Only SH vents, drains to be closed at 2.5 Kg/cm2 Drum pressure. Drum Vents to

be kept closed before starting the C.C. pumps. The starting vent shall be! kept

partially open.

© PMI, NTPC 297

4. Monitor drum metal temperature.

5. Monitor furnace exit gas temp. and in no case it should be more than 540°C to

protect the Reheater tubes which have no cooling steam flow.

6. Maintain drum level at normal and check boiler water concentration.

7. Observe the expansion of the lower part of the boiler and log the readings of the

markers provided to compare with the previous readings.

8. Keep watch on the bottom ash hopper for proper overflow for removal of the

accumulated unburnt oil.

9. With the rise of steam pressure and flow it will be necessary to feed water to boiler

continuously. Open the low load feed control valve as required. Economizer

recirculation valves may now be closed.

10. When steady flow of feed water is established through the Economizer, place

water side of HP heaters in service as described below:

a) Open HPH 5A/5B/6A/6B inlet and outlet box vent valves.

b) Gradually open HPH 5A & SB FW inlet valve equalisers.

c) Close above vent valves when water comes through them.

d) Open main valves, observe that their equalisers get closed.

e) Gradually open HPH 6A & 6B FW outlet valve equalisers.

f) Open main valve, observe that their equalisers get closed.

g) Close HP Heater bypass valve.

© PMI, NTPC 298

VIII Starting the Coal Fire

1. Start second PA Fan. Verify that its outlet damper opens.

2. Open seal air valves to pulverizers and coal feeders.

3. Start seal air fan, verify that its outlet damper opens.

4. Open coal bunker outlet gates - both manual and motor operated.

5. Start the pulverizer serving a lower elevation of coal nozzles. Open the hot air shut

off gate and bring the pulverizer upto the required operating temperature (77°C)

without coal.

6. Start the coal feeder at minimum rating.

7. When the pulverizer is proven in service, the fuel air dampers should open

automatically. Maintain the fuel feed at minimum consistent with stable ignition.

8. Maintain proper coal/air temperature leaving the pulverizers. Regulate the hot and

cold air dampers to hold pulverizer outlet temp at 77°C.

9. With increased firing rate. it may be required to open attemperator spray water.

Open the isolating valves of CVs and put the system in operation at around 50%

MCR.

10. The F.O. guns and igniters must remain in service until the feeder rating of each of

the two adjacent pulverizers exceeds 50% MCR load of each feeder.

11. Verify that auxiliary air dampers adjacent to idle coal nozzles get closed when

boiler load exceeds 30% MCR Load.

© PMI, NTPC 299

12. Verify also that windbox to furnace differential pressure increases to 100 mm wg.

13. Ensure that the air flow to the furnace commensurates with the firing rate at all

times.

14. Place additional pulverizers in service as the unit load demands.

15. Remove F.O. guns and igniters from service when the unit firing conditions are

stablised. Ignitors are only to give high energy arc during start-up of oil gun.

16. Adjust the feedwater supply to the boiler as required to maintain normal drum

water level.

INTEGRATED BOILER - TURBINE START-UP I Line-up Condenser Circulating water System

1. Open CW valves at condenser inlet.

2. Ensure that condenser CW outlet pipe seal pit is established.

3. Start water box priming pump so as to evacuate air in the condenser water boxes

& C.W. piping and establish initial vacuum in the water boxes to achieve desired

syphon.

4. Start circulating water pump

5. Establish cooling water flow through the condenser.

6. Place the condenser on-load tube cleaning system in service.

© PMI, NTPC 300

II Line-Up Oil System

1. Ensure turbine main oil tank level. Check oil tank level annunciation High/ Low: -

900 mm/-950mm from top of tank.

2. Start oil vapour extractors and generator bearing chamber exhaust fans,

3. Place one turbine oil cooler oil side in service and both turbine oil cooler water side

in service

4. Place oil temperature control valve in service.

5 Ensure that air side and gas side seal oil pumps are running.

6. Verify that hydrogen purity in generator is more than 94%.

7. Start Primary water Pumps.

8. Verify that fire protection channels 1 & 2 are not in operated condition.

9. Prepare the Auxiliary oil pumps (AC) & emergency oil pump (D.C.) by opening the

suction and discharge valves. Prime the pumps by opening air vent cocks.

10. Start one Auxiliary oil pump and gradually fill up the bearing oil system. When the

system is filled, check the following:

a. Oil level in the oil tank.

b. Pressure in lub oil system ( 1.2 kg/g)

c. Sufficient flow through all bearings.

© PMI, NTPC 301

d. Oil temperature downstream of turbine oil coolers is within permissible limits

(38° -47°C)

11. Switch off Auxiliary oil pump.

12. Check automatic starting of the second auxiliary oil pump and Emergency oil

pump.

13. Open turning gear oil valve. Check auto - closing of this valve at low lube oil

pressure ( 1.1. kg/cm'g).

14. Establish tub oil pressure and reopen turning gear oil valve.

15. Start one jacking oil pump. Verify that Uurbine starts running on turning gear

III Line-Up Control Fluid System

1. Fill up the control fluid tank with fire resistant fluid and maintain its level.

2. Check control fluid tank level annunciations. (High/low 700mm/-750mm from top of

tank).

3. Verify that none of the fire protection channels are in operated condition.

4. Start one oil vapour extractor of control fluid tank.

5. Check control fluid temperature in the tank. If the temp. isless than 20°C. then

switch on the control fluid tank heating rod.

6. Place the control fluid tank heating controller on Auto. Verify that the tank temp. is

maintained between 50°C and 60°C.

© PMI, NTPC 302

7. Start one control fluid circulating pump for regeneration of control fluid so as to

remove any acids, ageing products, any solid particles, etc. by passing the control

fluid through Fuller's earth filter and mechanical filter.

8. Place both control fluid cooler water side in service and any one control fluid cooler

fluid side in service.'

9. Start one HP control fluid pump. Check control fluid pressure is more than 30

Kg/Cm2g.

10. Fill-up the governing oil system. While filling up maintain the control fluid tank level.

11. Place the control fluid cooler water outlet temp. control valve on Auto. Maintain

cooler outlet oil temp. at 52.6°C.

12. Switch off running control fluid pump. Verify that the second control fluid pump

starts automatically.

13. Keep one control fluid pump and one control fluid circulating pump running.

14. Check the protection and governing system, as out lined below :

a. Open emergency stop valves (ESV) of HPT. Interceptor valves (IV) of IPT

and control valves (CV) of both HPT & IPT. check that their opening is

smooth.

b. Operate manual trip lever of turbine and check that all above valves close

fully.

c. Reset Manual trip lever. See that no valves open automatically.

© PMI, NTPC 303

d. Repeat steps (a).(b) & (c) for the following protection system one by one:

i) Turbine over speed.

ii) Axial displacement high.

iii) Condenser vacuum too low.

iv) Bearing Lube oil Press too low.

v) Emergency PB operated.

15. Ensure that all ESVs, IVs and CVs are closed.

IV Line-Up Condenser Evacuation System

1. Verify that vacuum breaker is closed.

2. Verify also that seal steam condenser level is not high.

3. Switch on one seal steam condenser exhauster.

4. Open warming - up drain valve.

5. Open auxiliary steam supply valve to main turbine gland sealing system.

6. Open sealing steam station inlet isolation valve.

7. Close Warming up drain valves when sound of steam is heard through this valve

Reopen the valve so as to drain any condensed steam. Repeat closing & opening

until tine becomes warm up.

8. Open gland sealing steam header drain valve.

© PMI, NTPC 304

9. Crack open seal steam control bypass valve.

10. Keep open, gland sealing header drain valve till sound of steam is heard through

this valve. Close the valve.

11. Repeat steps 8 & 11 until line become warm-up.

12. Regulate seal steam control bypass valve to maintain seal steam header pressure,

and to supply steam to turbine gland seals.

13. Open seal water supply valves and supply seal water to the vacuum pump system

separator. Verify that the level in the separator is maintained by maximum &

minimum level control valves.

14. Verify that the vacuum pump is also filled simultaneously, via the heat exchanger.

to seal off the gaps & clearances.

15. Ensure cooling water supply to the heat exchanger.

16. Open condenser air supply valves from condenser to vacuum pumps.

17. Close the condenser air inlet valve to the vacuum pump system.

18. Open the bypass valve and close the motor air valve.

19. Start one vacuum pump.

20. When diff. pressure across the valve as 17 above exceeds 30 m bar then the valve

as 17 above opens automatically.

21. Similarly start the second vacuum pump for hogging opn.

© PMI, NTPC 305

22. Verify also that the stand by pump starts automatically whenever condenser

pressure is more than 200 mbar.

23. When condenser pressure falls below 120 mbar, the standby pump shall stop

automatically.

24. For holding operation, keep running only one vacuum pump.

25. At reduced heat toad and constant cooling water supply, the pressure within the

condenser drops and may become close to the vapour pressure of the operating

liquid-thus reducing the capacity of the pump. The capacity of the pump, thereafter

is maintained by opening the motor air valve and closing the bypass valve.

26. Verify that the bypass valve closes automatically when vacuum pump cavity temp.

limit is exceeded and closure of bypass valve shall cause opening of the motor air

valve.

27. Verify also that when the vacuum pump cavity temp. limit becomes normal, motor

air valve closes first and then bypass valve opens.

28. Raise condenser vacuum.

V. Start-Up After 72 Hours Shut-down.

1. Verify that following valves are closed. :

a. Emergency stop valves (ESVs) at HPT inlet.

b. Interceptor valves (ESVs) at IPT inlet..

c. Control valves (CVs) of both HPT and IPT.

© PMI, NTPC 306

d. Non return Valves (NRVs) at HPT exhaust.

e. All extraction NRVs

f. Drains before HP control valves

2. Open following valves

a. Drain before seat ESVs

b. Drain before seat IVs

c. MS strainer drain valves

d. HR Strainer drain valves

e. MS Header drain valve

f. HR Header drain valve

g. CR Header drain valve

h. Drain before extraction NRVs and HPT exhaust NRV

i. Drains after HP control valves.

j. HP casing drain

k. Drains before and after IP control valves

I. Root valves on impulse lines of all pressure gauges.

3. Ensure that minimum stable fire is established in the boiler

4. Open main steam header manual drain valves and manual vent valves before HP

bypass valves.

© PMI, NTPC 307

5. Open bypass isolating valves of MS shut-off valves at boiler outlet.

6. Gradually open bypass jogging valve of MS shut off valve

7. When the sound Of steam is heard through drain valves, close these valves and

plug them.

8. When steam comes out through vent valves, close them and plug them.

9. Ensure that while heating, differential temperature between parallel steam lines

remains below 28°c.

10. Open MS shut off valves when pressure has built-up in MS piping.

11. Gradually open HP bypass valves so as to establish 15% steam flow through them,

fully. taking care to see that temperature control loop is functioning normally. Also

ensure that L.P. bypass system is functioning normally.

Steam flow thus established shall assist in heating main steam, cold reheat and

hot reheat lines.

HP bypass Valves can further be opened to assist in increasing steam parameters.

12. Put the Automatic Turbine Run-up System (ATRS) in service.

13. Verify conditions, as under, are satisfied

a. Turbine on turning gear (Speed 15 rpm.)

b. Condenser pressure is less than 0.5 Kg/cnr2a.

© PMI, NTPC 308

c. At least one condensate extraction pump is ON

d. Trip fluid pressure is more than 5 Kg/cm2.

e. Diff. Temp. between HP casing mid section and top is less than 30°C.

f. Diff. Temp. between IP casing mid section and top (both front and rear) is

less than 30°C.

g. HP control fluid temp. is more than 50°c.

h. Lub oil temp. after cooler is more than 350C(380-470C)

i. Degree of superheat of MS before HP bypass is more than 50°C.

j. MS temp. before HP bypass 1s less than 400°C.

k. MS temp. before HP Bypass is more than mid-wall temp. of HP control

valves.

I. MS press, before turbine is a function of mid-wall temp. of HP control

valves.

14. Raise starting device above 42%. Verify that all ESVs have opened. Verify also

that drains before HP control valves are in Auto and these valves have opened.

15. Raise starting device above 56%.

Verify that all IVs have opened;

16. Place "Seal steam controller1' and "oil temperature controller" in AUTO.

17. Verify that following generator conditions are fulfilled.

© PMI, NTPC 309

a. One Gen. bearing chamber exhaust fan is .ON

b. H2 temp. controller is in AUTO

c. H2 purity is more than 94%

d. H2 pressure is more than 3 Kg/cm2

e. One air side seal oil pump is ON.

f. H2 side seal oil pump is ON.

g. Diff. pressure between seal oil air side and H2 side (both turbine end and

exciter end) is more than 0.7 Kg/Cm2.

h. Seal oil prechamber level (both turbine end and exciter end) is low.

i. Generator bushing box liquid level is low. (Less than 90 mm)

j. One primary water pump is ON.

k. Primary water temperature is in AUTO.

l. Primary water conductivity is less than 1.5 uS/Cm.

m. Primary water flow is adequate (more than 13.3DM3/Sec.)

18. Before opening the control valves, verify also the following:

a. The degree of superheat of MS before turbine is more than 50°C.

b. MS temp. before turbine is more than either mid-wall temp. of HP casing or

© PMI, NTPC 310

simulated mid-section temp of shaft.

c. HR temp. before turbine is more than simulated mid section temp o( IP

shaft.

19. During the warm-up and start-up of the turbine observe the following parameters

a. Turbine stress evaluator

b. Differential expansions

c. Axial shift.

d. Temperature difference between upper and lower halves of the casings.

e. Bearing temperature & vibration

f. Steam parameters.

g. Operating parameters of condensing system.

h. Operating parameters of turbine oil and control fluid systems.

20. Maintain MS temp. and pressure at 350°C and 50 Kg/Cnr^g, respectively, at

turbine inlet as per the start-up curve.

21. Raise turbine speed set point so as to admit steam to the turbine and start rolling.

22. Raise turbine speed set point (300-360 rpm) for warming up the casing.

23. Verify that gate valve gearing has closed (stop turning gear).

© PMI, NTPC 311

24. Close all the drain valves of step 2<a),(b),(c),(d),(e) and (f).

25. While admitting steam to the turbine, ensure that permissible wall temp. of limit

curves are not exceeded. –

26. Hold the set at 300-360 rpm for soaking purposes.

27. Verify that MS temp. ahead of turbine is less than .either mid-wall temp. of HP

casing or simulated mid-section temp. of HP shaft.

28. Close drains before HP control valves

29. Close drains after HP control valves if diff. temp between HP control valves and

saturated steam is more than 50°C.

30. Raise the speed to 3000 rpm at the rate of 34 rpm/min provided following parameters

are within permissible limits:

a. The differential expansion of the

casings. HP : + 5 mm IP : + 8mm LP : +30mm

- 3 mm - 2mm - 3mm

b. The differential temperature between the top and the bottom of the casing

(less than ± 45°C).

c. Bearing temperature and vibration (less than 90°c and 50 uM).

d. HPT exhausthood temperature,(less than 500°C)

31. Verify that following conditions are fulfilled

a. Both auxiliary oil pumps are off.

© PMI, NTPC 312

b. HR temp. before LP bypass is less than simulated mid-section temp. of IP

shaft.

c. Cold H2 gas temp. is less than 45°C

d. Main exciter air temp. is less than 45°C

e. Cold primary water temp. is less than 50°C

f. Cold primary flow through bushings is more than 0.37 DM3/Sec.

g. Diff. temperature between primary water and H2 is more than 1°C.

32. Place AVR on AUTO and switch on Field Breaker. Verify that generator voltage is

more than 95%.

33. Switch on Synchronizer and synchronise the generator with the grid. Verify that

generator breaker is on.

34. Raise the starting device and take a block load of 10%

35. Close HP bypass, verity that LP bypass also closes.

36. Raise turbine inlet M.S. press, at the rate of 0.8Kg/Cm2 per minute upto 75

Kg/Cm2a at 25% turbine load and turbine inlet MS temp. at the rate of 1 6 °C/min

upto 485°C at 50% turbine load.

37 Close also HP casing drain valve provided HP casing top temp is more than 320°C.

38. Verity that drains before HPT exhaust NRVs have closed with the opening of

NRVs.

39. Close drains before and after IP contro! valves when diff. ternp. between !P control

valves and CR steam is more than 50°C.

© PMI, NTPC 313

40. Open extraction steam supply block valves to LPH -2 & 3 and deaerator.

41. Raise turbine load at the rate of 4 MW/min upto 125MV\,.

42. Verify that drain before L.PH-2/3 NRV closes when opening of respective

extraction NRV is more than 15%.

43. Verify also that drain before deaerator extraction line NRV closes when opening of

extraction NRV is more than 15% & diff, pressure across the NRV is more than

300 mm we.

44. Put hotwell level control and deaerator feed water storage tank level control on

Auto.

45. Raise turbine inlet MS pressure at the rate of 1.5 Kg/Cm2 per minute to 170 Kg/crr2a

(at 54% turbine load).

46. Above 20% Boiler MCR. the drum level 3-Element control can be put on Auto.

47. Raise turbine load at the rate of ^ MW/min upto 485 MW.

48. At 27% turbine load cut in the first mill, and at 40% turbine load cut in the second

mill.

49. At 40% boiler MCR combustion control can be put on Auto.

50. At 40% turbine toad cut in HP heaters as under :

a. Open drain after extraction 5 -NRV

b. Open extraction steam supply valves to HPH Heaters 5 A & 5B and 6 A and

© PMI, NTPC 314

6B.

c. Verify that extraction steam NRVs to HPH 6A & 6B are open.

d. Verify that drain before extraction 5 NRV closes when opening of this NRV

is more than 15%.

e. Close drain after extraction 5 NRV when sound of steam is heard through

this valve.

51. At 50% load cut in the second condensate extraction pump and the second boiler

feed pump.

52. Cut in mills as under.

No. of MM Turblne Load

Third 50%

Fourth 60%

Fifth 70%

Sixth 74%

53. Above 54.43% boiler MCR, SH & RH steam temp. controls can be put on Auto.

54. Between 50% and 97% turbine load maintain, turbine inlet MS temp. at 485°C.

55. From 97% to 100% turbine load. raise the turbine inlet MS temp. at the rate of

0.6°C/min to 535°c.

VI. Start-up after 48 Hours Shut-down

1. Auxiliary equipment and systems shall be started in the same manner and

sequence as in case of start-up after 72 hours shut-down.

© PMI, NTPC 315

2. Check all auxiliary equipment and systems are working satisfactorily.

3. Also check the healthiness of alt protections and interlocks.

4. Ensure that the boiler has been lighted-up and steam flow established through HP

and LP bypass systems. Verify that all the conditions regarding MS and RH steam

temp are satisfied.

5. Raise HPT inlet MS press, and temp. to 60 Kg/Cm2g and 380°C, respectively,

prior to steam rolling.

6. Admit steam to the turbine and start rolling.

7. While admitting steam to the turbine, ensure that per-missible wall temp. of limit

curves are not exceeded.

8. After holding the set at warming - up speed (300 -360 rpm) for soaking purposes,

verify that criteria for MS temp. is satisfied.

9. Close all the drains.

10. Raise the speed to 3000 rpm.

11. Verify that criteria for RH steam temp. is satisfied.

12. Put AVR on auto and switch on the Field Breaker, Verify that Generator voltage is

more than 95%.

13. Switch on synchronizer and synchronize the generator with grid and take a block

load of 10%.

© PMI, NTPC 316

14. Close HP bypass system. Verify that LP bypass system also closes.

15. Close alt the drains.

16. From 60 Kg^ gradually raise HPT inlet MS press at the rate of 1.65 Kg/Cm'7 per

minute such that 170 Kg/Cm2a. press is reached at 55% turbine load.

17. Raise HPT inlet MS-temp at the rate of 1.6°C/minute from 380° to 485°C.

Maintain MS temp. at 485°C upto turbine load 475 MW. Thereafter raise MS temp.

at the rate of 0.6°C/min, such that 535°C is reached when the turbine load has

reached 500 MW.

18. Gradually load turbine at an average rate of 4 MW/min upto 475 MW.

19. At 40% turbine load cut in HP Heaters.

20. Follow Mill cutting - in sequence as under:

No. of Mill Turbine Load (%)

First : 20

Second : 33

Third : 45

Fourth : 55

Fifth : 67

Sixth : 74

VII Start-up After 8 Hours Shut Down

Procedure for this start up is identical to the procedure for startup after 48 hours

shutdown. are outlined with following deviations.

© PMI, NTPC 317

1. Prior to rolling, ensure that HPT inlet MS press and temp. are 84 Kg/Cm2g and

450°C -460°C, respectively.

2. Raise HPT inlet MS press at the rate of 1.8 Kg/Cm2 per minute from 84 Kg/ Cm2g

to 102 Kg/Cm2'g. Maintain this pressure, between 20% and 60% turbine load.

Maintain HPT inlet MS temp. at 485°C between 30% and 60% turbine load.

3. At 60% turbine load cut-in H.P. Heaters.

4. From 60% to 94% turbine load, raise HPT inlet MS press, from 102 Kg/Cm2'a at

the rate of 3.2Kg/Cm2 per minute.

5. At full load, raise HPT inlet MS temp. from 485°C to 535°C at a rate of 0.75°C/ min

upto 500°C and there after at a rate of 2.5°C/min.

6. Mill cutting in sequence is as under :

No. of Mill Turbine Load (%)

First 14

Second 28

Third 39

Fourth 50

Fifth 58

Sixth 67

I NORMAL SHUT- DOWN OF TURBINE ;

1. Inform the boiler house that the turbine is being shut down.

2. Switch on the load controller and adjust the toad gradient.

3. Ensure that both HP bypass and LP bypass controllers are on.

© PMI, NTPC 318

4. Check that Turbine Stress Evaluator (TSE) is in service.

5. Verify that following sub loop controls are ON and all drives are available and are

ready for operation.

a. Auxiliary Oil Pumps

b. Emergency Oil Pump

c. Jacking Oil Pump

d. Turbine Gear (Gate Valve Gearing)

e. Drains

6. Ensure that there is no fault in the electro-hydraulic Turbine controls.

7. Check for non-seizure of ESVs and IVs.

8. Unload the turbine by gradual closing of control valves through remote or manual

operation of speeder gear. The unloading should be carried out at a rate governed

the margins shown on TSE.

9. During unloading always-keep watch on following para meters, such that theyare

always within permissible limits as under:

a. Differential expansion:

HP Casing : + 5 mm

- 3 mm

I.P. Casing : + 8 mm

- 2 mm

© PMI, NTPC 319

L.P. Casing : + 30 mm

- 3 mm

b. Bearing Vibration 50 uM

c. Wall temperature of ESV, HP. casing. HP & IP turbine shaft.

d. Differential temperature between parallel steam lines 28°C.

10. Having unloaded the turbine to no load. trip the turbine manually. Ensure the

following:

a. ESVs; IVs and control valves get closed.

b. All extraction steam line valves to HPH 5. Deaerator, LPH 3 & 2 get closed.

c. Initiate load run-back in ACS.

d. Initiate fast opening of HP bypass system.

e. Generator is isolated through low forward power relay.

f. Synchronizer is OFF.

g. Field Breaker is OFF.

h. Drains before HP control valves are open.

11. When turbine speed falls below to 790 rpm. Verify the following :

a. One Auxiliary Oil Pump is ON.

© PMI, NTPC 320

b. Oil temp. control valve is on Auto.

c. Gate valve gearing has opened.

12. When turbine speed falls below 510 rpm, verify that one Jacking oil pump is ON

and the machine is running on turning gear.

13. Operate HP-LP bypass systems to stabilize boiler conditions and maintain steam

flow through reheater till the boiler is shut-down.

14. Maintain condenser vacuum as long as firing in boiler is ON.

II NORMAL SHUT-DOWN OF BOILER

In shutting down the boiler the reduction of firing rate is determined by the

requirement of the turbine.

1. Gradually reduce load on the unit, reduce the firing rate in line with the decreasing

steam flow.

2. Take "Combustion control" and "SH/RH steam temperature control on Manual

when the load drops to 50% approx.).

3. When the feeder rating on all pulverizers is reduced to 40% of MCR, Start the F.O.

guns associated with the upper most pulverizer in service.

4. Place the uppermost service pulverizer on manual control. Gradually reduce the

feeder rating. When minimum feeder rating (about 25% load) is reached, close the

hot air shut-off gate.

5. When the pulverizer outlet coal - air temp. falls to about 50°C, stop the feeder.

© PMI, NTPC 321

6. Keep the pulverizer running until it is completely empty. Then stop the pulverizer.

7. Verify that the associated windbox dampers get closed and remaining dampers

remain modulating by the Secondary Air Damper control system

8. When the feeder rating on all remaining pulverizers reaches 40%, take the second

pulverizer, supplying the next higher elevation, out of service

9. In the sante manner continue taking out pulverizers at consecutive lower

elevations. When down to 2 pulverizers the adjacent F.O. guns shall be placed in

service, prior to reducing the feeder rating of either pulverizer to lower than 50%

MCR.

10. Operate all sootblowers. Soot-blowers operation normally done when the load is

above 60% and with 5 mills.

11. Reduce the air flow in \\n6 with fuel reduction until 30% of maximum air flow is

reached. Further lowering of air flow shall only be done after the fire-out of boiler.

Open all auxiliary air dampers and reduce the windbox to furnace diff pressure to

35 mm wg.

12 Check the expansion movement of the boiler as the load is reduced.

13. After the last pulverizer has been shut down. remove the F.O. guns in service.

Check & ensure all fuel to the furnace is cut-off

14. Ensure that SH & RH desuper heater spray water supply block valves are closed.

15. Purge the furnace.

16. When RAPH inlet gas temperature falls below 205°C shutdown the draft fans and

© PMI, NTPC 322

air heaters.

17. Keep one boiler circulating water pump running

18. Close atomising & heating steam supply. Stop P.O. pumps.

19. Close MS shut - off valves along with their integral bypass valves.

20. Open alt reheater vents and drains.

21. Close CBD & stop chemical dosing to the drum.

23. Break condenser vacuum. When vacuum falls to zero stop gland seal steam

supply.

24. Verify the operation of steam line and turbine drain valves, as under:

a. Open drains after HP control valves when diff. temp between HP control

valves and HPT inlet saturated steam falls below 20°C.

b. Open HP casing drain when HP OUTER CASING TEMP. (top/bottom)

temp. becomes less than 300°C.

c. Closing of HPT exhaust NRVs causes opening of drains before NRVs.

d. Open drains before and after I FT control valves when diff. temp. between

IPT control valves and- cold reheat steam becomes less that 20°C.

e. Open drains before extraction NRVs, when the opening of respective

extraction NRV becomes less than 5%.

25. Maintain drum water level near normal (225 mm below centre line of drum), add

© PMI, NTPC 323

make-up as required.

26 Thereafter boiler feed pumps and condensate extraction pumps may be taken out

of service.

27. Open drum vents and superheater vents & drains when drum pressure falls to 0.2

Kg/Cm2 g. (Drum Vents to be opened after stopping C.C. pumps).

28. Boiler may be emptied, when the boiler water temperature falls below 95°C,

III EMERGENCY SHUTDOWN :

A variety of conditions may arise with either the boiler and the turbine that can be

considered as an emergency nature necessitating a shutdown. For purpose of

clarification, this section details only an immediate, emergency,

Trouble Is with the boiler

1. Stop the pulverizers one by one and then oil burners, as detailed above. If

necessary, actuate MFR emergency push button on FSSS Console Insert.

2. MFT should be operated without hesitation in case of any doubt about the flame

stability.

3. Purge the furnace in the normal way, if possible. If not, purge the furnace at the

earliest opportunity.

4. Keep one ID fan running so as to maintain furnace in suction in order to prevent

any pressurization due to tube rupture, etc.

5. Maintain drum level at normal.

© PMI, NTPC 324

6. Cool boiler as quickly as conditions permit.

7. Proceed to shutdown the turbine and auxiliaries in the normal way.

Trouble Is with the turbine

1. Operate emergency push buttons from either the ATRS control Insert or the local

turbine front.

2. Ensure that ESVs, IVs and control valves of both HPT & IPT have closed.

3. Isolate the generator and switch off field breaker and synchronizer.

4. Break the vacuum. (When the Vacuum in condenser is killed. Boiler is to be tripped

immediately as there will be no flow through re-heaters.

5. Proceed further as per the step detailed in turbine shut down.

6. Check HP -LP bypass system have opened.

7. Shutdown the boiler and keep it boxed & bottled up. as detailed below, so that it

can be taken in service at short notice.

a. Do not reduce drum pressure in line with unit toad reduction. The rate of

pressure drop should be much smaller than the rate followed during normal

shut down.

b. Close all superheater drains and vents. Reheater vents and drains are left

open.

c. Close each hot air shut - off gate to pulverizers When coal - air temp. falls to

© PMI, NTPC 325

50°C, stop the feeder

d. Stop the pulverizer when it is empty.

e. Keep two boiler circulating water pumps running so as to prevent local

overheating due to hot slog deposits.

f. Verify that all fires are extinguished.

g. Keep ID & FD fans and air heaters running until the airheater gas inlet temp.

falls below 205°C.

h. As the unit cools down and the water shrinks, add make-up intermittently to

the boiler so as to prevent drum level from dropping below the visibility limit

of the gauge glass.

DO'S General

1. Have constant watch over the proper operating parameters viz, pressure, diff.

press., temperature, level, flow, etc. of various flowing mediums and set right

deviations timely.

I Boiler - General Do's

a. Check that position of all valves are as per the Valve Operating Schedule

for Boiler.

© PMI, NTPC 326

b. Check for proper functioning of fuel oil pumping unit.

c. Check for vibration, bearing temperature and abnormal noise.

d. Check for proper lubrication of bearings. Lubricate with proper grade of

lubricants.

e. Check the temperature rise of lubricants in the bearings.

f. Check flow of cooling mediums.

g. Move dampers from their set position atleast once a week, so as to ensure

that damper spindles do not get seized in their bearings.

h. The correct working of drum level gauges should be tested once every

eight hours or more, if considered necessary.

i. The drum level gauges should be well illuminated under all conditions.

j. Drum level high and low alarms should be tested at least once a week.

k. Should the water level in drum drops out of sight in the gauge glass, all

firings must be stopped immediately.

l. Be alert to detect the symptoms of water carry over from boiler.

m. Operate all janitors once in every day to keep them in good working

condition.

n. Flue gas O2 analyser, which is tied-up with total air flow control, should be

hecked periodically to ensure continuous and dependable operation.

o. Maintain air flow through mills whenever the unit is being fired.

p. Check that fire is never lost in the furnace while mill is in operation.

q. Check (or mill rejects periodically.

r. If fires in the bunker or in the mill are noticed, immediately take appropriate

© PMI, NTPC 327

action for putting out the fire.

s. Operate soot blowers once in every shift initially until a suitable frequency of

soot blowing is determined as per experience.

t. Blow compressed air once in every day to clear off coal dust over piping,

cable trays, equipment, etc. Keep environments clean. Fuel leaks and

accumulation can lead to fire hazard.

Don’ts (Bolter)

a) Do not take any equipment for maintenance without service manuals and cross

sectional drawings.

b) Do hot overload any equipment

c) Do not mix oils or lubricants of different quality.

d) Do not run any rotating equipment when heavy vibrations are present,

e) Do not run any equipment without checking interlocks.

f) Do not run any rotating equipment with high bearing temperature.

g) Do not run any equipment when foreign matter is left inside.

h) Do not run any equipment without ensuring proper lubrications.

i) Do not attempt repair beyond your capabilities.

j) Do not increase flow through a fan unless the motor current becomes steady

© PMI, NTPC 328

k) Do not start an ID fan unless clear flow path is established.

l) Do not take out air heaters and draftfans from service until the air heater gas inlet

temperature drops below 205°C.

m) Do not start the feeder without mill running. Do not start the feeder without

ensuring proper ignition energy in the furnace

o) Do not vary the speed control of feeder when the feeder is idle.

p) Do not keep open hot air valve while the feeder is in idle condition

q) Do not attend any maintenance work when power and/or steam supply is ON.

r) Do not subject the satisfy relief valve to any sharp impact while handling.

s) Do not reduce the air flow below 30% MCR air flow. until all fires are out and the

unit is off the line.

t) Do not increase the firing rate such that furnace exit gas temperature exceeds

540°C, until the steam flow is established in the reheater.

u) Do not remove warm up oil guns and igniters from service until two adjacent coal

elevation are in service with feeder ratings on each associated pulverizer greater

than 50%.

v) Do not fire coal at separated coal nozzle elevations without the support of warm up

oil.

© PMI, NTPC 329

II For Turbine

Do's

1. Check the level of oil in the main oil tank.

2. Check bearing and pedestals for cleanliness.

3. Check the differential pressure at the lub oil and control fluid filters.

4. Check the oil pressure in the bearings.

5. Check the vibration of bearings. It should be with in the prescribed limits. In case of

any rising tendency, watch and trip the set if the readings exceeds the permissible

values. During speed raising of the turbine from 0-3000 rpm, hold the turbine at

same speed where there is a change in the level of vibration. If necessary, reduce

the speed and re-start only after the vibration is stablised.

6. Check the differential expansion and if the values exceed permissible limits, trip

the set.

7. Check the control fluid pressure and the suction pressure or MOP.

8. Check auto operation of auxiliary oil pumps and emergency oil pump.

9. In case of total power failure see that machine is tripped.

i) Check the closing of ESVs, IVs and HP & IP control valves.

ii) Check that the emergency oil pump has cut in and oil to bearings is

available.

© PMI, NTPC 330

iii) Check that DC seal oil pump has cut in.

10. In case of sudden heavy load throw off due to grid disturbances and if there is no

possibility of immediate loading of the ynit.

i) Trip the turbine by hand.

ii) Check that the AOP has cut in. If not, start AOP manually. If AOP does not

start, emergency oil pump should start automatically, if that also fails to start

automatically, start it manually.

11. Speed raising should be carried out at uniform rate to minimise shaft vibration.

However, the critical speeds of the turbine should be passed rapidly at a steady

rate.

12. Before and after shut downs check the tightness on closing of ESVs, IVs, HP & IP

control valves.

13. Constantly check the operation of the gland sealing system.

14. Regularly note the readings of all the turbovisory instruments, take measures for

abnormalities. At least once in a year check all the turbovisory instruments

mounted on the turbo set.

Don’ts (Turbine)

1. Do not operate the set if the condenser pressure is more than 0.3 Kg/Cm2a.

2. Do not start the turbine if the condenser pressure is more than 0.5 Kg/cm2a.

3. Do not run the turbine if the bearing drain oil temperature exceeds 72 °C

© PMI, NTPC 331

4. Do not run the set continuously at low load or no load. Do not run the turbine for a

long time at no load or low loads immediately after it has run for long duration at

highel- loads, to avoid quenching of the internal parts.

5. Do not allow the turbine to run if the LP exhaust temperature goes beyond 100

degree C.

6. Do not drain oil saturated with hydrogen vapours in the oil tank.

7. Do not rely solely on the lamp indications for the operation of pumps, valves and

fans. Physical check has also to be resorted to.

8. Do not synchronise the machine with the grid without checking the opn. of solenoid

valves for load shedding relay.

9. Do not start the turbine without ensuring that standby oil pumps are healthy to

operate.

10. Do not start the turbine with faulty instruments. Get them rectified before starting.

© PMI, NTPC 332

25. Major Differences between 210MW and 500 MW Units

© PMI, NTPC 333

© PMI, NTPC 334

© PMI, NTPC 335