3. HYDROGEN COOLING AND SEAL OIL SYSTEM/STAGE – I

3.1 HYDROGEN COOLING SYSTEM OF GENERATOR

The 210 MW Generator is cooled by Hydrogen in stator. H2 is cooled by

cooling condensate flowing in the gas coolers.

3.2 ADVANTAGES OF HYDROGEN COOLING

Windage and ventilating losses are reduced due to the low gas density. An increased output per unit volume of active material because of the

high thermal conductivity and high heat transfer co-efficient of hydrogen. The life of the insulation on the stator winding is increased because of

absence of oxygen, moisture and corona discharge. The reduction of windage noise, dirt and moisture because of lesser

density of gas and closed recirculation Because of the hydrogen atmosphere inside the generator the chances of

fire is reduced, as the hydrogen cannot support combustion or oxidization.3.3 HYDROGEN GAS SYSTEM

Gas system is provided for the following reasons. To provide means for putting hydrogen in or taking hydrogen out of the

generator using CO2 as a scavenging medium. To maintain the gas pressure. To indicate the conditions of gas pressure, temperature, purity and the

presence of liquid by alarms. To dry the gas and remove vapour which might get into the generator

from the seal oil.3.4 GAS SUPPLY (Fig.1)

The gas either H2 or CO2 is distributed uniformly to the various

compartments of the generator by means of perforated pipe of the manifolds

located in the top and bottom of the housing.

3.5 GAS DRIER (Fig. 2)

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A gas drier with activated alumina absorbent material filled is

connected across the generator blower. Gas is circulated through the drier.

The absorbent material will take up about 1kg of water, after that

disconnecting the drier from the machine and then heating with a built in

electric heater can dry it out. Before and during the drying process air is

forced through the drier by a small blower to remove the moisture. A

thermostat protects the drier against over heating. The dryness of active

material can be determined by the colour of absorbent material as seen

through the window in the bottom of the drier. The colour will be light blue,

when dry and grayish pink, when saturated with moisture.

3.6 LIQUID DETECTOR

Float-operated switches in small housing are provided under the

generator frame and under the main lead box to indicate the presence of

any liquid, which might be due to leakage from H2 coolers openings, are

provided in each frame ring at the bottom of the frames to drain the liquid

collected to these water detectors. Each detector is provided with a vent

return line to the frames so that drain line from the frames will not become

gas bound. Isolating valves are provided in both the vent and drain lines to

inspect the float switches. A drain valve is provided for removal of

accumulated liquid.

3.7 GENERATOR HYDROGEN PURITY INDICATING TRANSMITTER

The purity of the gas in the generator is determined by use of the

hydrogen purity indicating transmitter and the purity meter blower. The

purity-indicating transmitter is a differential pressure instrument which

measure the pressure developed by the purity meter blower. An induction

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motor loaded very lightly so as to run at constant speed, drives the purity

meter blower and circulates the gas drawn from the generator. Thus the

pressure developed by the purity meter blower varies with pressure,

temperature and purity of the gas. The purity-indicating transmitter is

provided with automatic compensation for pressure variations.

The purity meter has two scales, one at upper and another at lower.

The lower scale is calibrated in terms of purity and is associated with the

pressure compensated pointer. The upper scale indicates gas density

(relative to air at 100%) and is associated with the uncompensated pointer.

Only the lower scale is visible. Upper scale is used for calibration purpose.

The lower scale is divided into three sections. In the approximate centre of

the scale there is a point marked air-100. This pointer is used for calibrating

the gauge with out removing the gas from the generator. The left hand end

of dial shows percentage of CO2 in the mixture of CO2 and air. This scale is

used during scavenging operation when CO2 is being admitted into the

generator. The right hand side of the dial shows percentage of H2 present in

the mixture of H2 and air. This scale is used to determine the purity of H2.

The hydrogen purity-indicating transmitter produces a pneumatic

output signal. This output signal is then converted in to electric signal. It is

carried to a remotely located receiver provided with a dial on the Hydrogen

control panel. Two switch assemblies are provided with purity-indicating

transmitter. These are used for “Hydrogen purity High or Low” alarms when

the differential pressure varies.

3.8 GENERATOR BLOWER PRESSURE GUAGE AND HYDROGEN PRESSURE INDICATING TRANSMITTER

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A dual pressure gauge is provided on the hydrogen control panel,

which indicates both the pressure developed by the blower on the generator

rotor and the hydrogen pressure in the generator housing. The range of

blower pressure gauge scale is 0-400cm of water. This portion of the

instrument is connected directly to the generator housing and reads the

differential pressure across the blowers on the rotor. This pressure can be

used as a check on the purity meter or it can be used as a purity indicator

when the purity meter is not available. The hydrogen pressure portion of the

instrument has a range of 0-7 Kg/cm2 and is connected directly to the

generator housing to indicate the pressure of hydrogen. The transmitter

produces a pneumatic output signal and then it is converted in to an electric

signal. This is carried into a remotely located receiver, which is provided with

dials. High and Low pressure alarm switches are provided in the hydrogen

control panel.

3.8.1 HYDROGEN TEMPRATURE ALARMS

Hydrogen cold gas thermostats and the resistance temperature

detectors are located in a generator to provide an alarm if hydrogen

temperature raises above normal.

3.8.2 PRESSURE REGULATORS AND GAUGES

The generator is equipped with a hydrogen pressure control, which has

a pressure regulator and a pressure gauge for the gas going to the

generator. The top gauges indicate the machine gas pressure and also

setting of the regulator on the hydrogen control panel. The bottom gauge

indicates the amount of pressure available from the hydrogen supply

system.

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3.8.3 HYDROGEN GAS COOLERS

The hydrogen is cooled by passing it through coolers where the gas

gives up its heat to the cooling water in the finned tubes of the coolers.

3.9 SEAL OIL SYSTEM (Fig.3)3.9.1 GLAND SEALS (Fig. 4)

Gland seals, which are supplied with oil under pressure, are used to

prevent the escape of the gas along the rotor shaft. Oil is supplied in two

annular grooves in the gland-sealing ring. From these grooves the oil flows

both ways along the shaft through the clearance space between the shaft

and the inner diameter of the gland ring. As long as the oil pressure in the

circumferential groove exceeds the gas pressure in the machine, oil will

flow towards the hydrogen side of the seal and prevent the escape of

hydrogen from the generator. The purpose of having two seal grooves in

the gland rings is to provide separate hydrogen side and airside seal oil

system. When the feed pressure is in these two system are properly

balanced there will be no flow of oil in the clearance space between the two

feed grooves. Oil which is supplied from the hydrogen side of the seal oil

system will flow inwards along the shaft towards the inside of the generator

and oil which is supplied by the air side of the system will flow outwards

along the shaft towards the bearing. The oil in the space between the two

feed grooves will remain relatively stationary due to pressure balance

between the two systems.

The gland oil is fed to the feed grooves through passages in the

supporting brackets. The gland ring is provided to restrict the flow of oil

through the seal. This gland ring can move radically with the shaft, but is

restrained from rotating by a pin to the supporting structure. Oil leaving

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the gland seal rings is caught in chambers of each side of the seal from

which it is drained back to the gland seal oil system.

3.9.2 OIL SUPPLY

The function of the seal oil system is to lubricate the seal and prevent

hydrogen escaping from generator, with out introducing an excessive

amount of air and moisture into the generator. This oil which is in contact

with air or hydrogen absorbs appreciable volume of gas and will absorb

moisture, if water vapour is present. If oil with air and water absorbed in it

is pumped into the hydrogen compartment, some of the air and moisture

will come out of the oil and contaminate hydrogen in the generator.

Therefore it is necessary to add fresh gas to the generator to maintain the

purity. Contaminating air and moisture are kept out of the generator by

separating the airside of the seal oil system from the hydrogen side of the

seal oil system. When this is done, hydrogen side oil is returns to the

hydrogen side of the seal ring in the generator, thus preventing the escape

of absorbed hydrogen to the outside atmosphere. The airside seal oil

returns to the airside of the seal ring, thus preventing the release of absorb

air or moisture with the hydrogen compartment at the generator.

The seal oil is supplied to the airside of the gland seal rings at a

pressure of 0.84 Kg/cm2 above the generator gas pressure. The hydrogen

side seal oil is maintained at the same pressure by means of pressure

equalizing valves. As a result, the interchange of air side and hydrogen

side oil at the gland rings is held to a minimum, the air side seal oil flowing

only to the air side and the hydrogen side seal oil flowing only to the

hydrogen side. While the interchange of seal oil at the gland seal rings is

held to a minimum, the variation in pressure over a long period of time may

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result in a gradual increase or decrease in amount of oil in the two sides of

the seal oil system. A hydrogen side drain (Fig .5) regulator is provided for

adding or removing of oil from the hydrogen side of the seal oil system.

This chamber has two float valves; one will introduce oil into the chamber

from the airside of the system. If the oil level gets low, the other float valve

allows the oil from the chamber, to flow to the airside of the system if the

level gets high. The quantity of oil in the hydrogen side of the gland seal

system is kept constant and the oil levels are properly maintained.

3.9.3 DEFOAMING TANKS

Oil returning from the hydrogen side of the gland seal ring goes to two

de-foaming tanks, where most of the gas comes out of the oil. These de-

foaming tanks are located in the bearing brackets of the generator. The oil

levels in the de-foaming tanks are maintained by overflow connection, one

de-foaming tank is provided for each seal. Trap is provided in the drain

line between the two tanks so that the difference in the pressure at the two

ends of the generator will not cause circulation of oil vapour through the

generator

3.9.4 SEAL OIL PUMPS

The airside seal oil pump receives its oil supply from the combined

bearing and airside seal oil drain. It pumps part of this through a seal oil

cooler to the air side of the seal ring and returns part of it back to the suction

side of the pump through a differential pressure regulator. This pressure

regulator maintains the differential pressure at 0.84 Kg/cm2 between the

airside seal oil pressure and hydrogen pressure. (Fig.6) The back up pump is

also provided. The hydrogen side seal oil pump receives its supply from the

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hydrogen side seal oil flow chamber. It pumps part of this through seal oil

cooler to the hydrogen side of the seal ring. A pressure-equalizing valve is

provided in the hydrogen side to feed the line at each end, which maintains

the (Fig.7) hydrogen side seal oil pressure at the same value as the airside

seal oil pressure. A bypass line is provided around the pump which allows

that portion of the oil not required by the pressure equalizing valve to return

to the suction side of the pump.

3.9.4 SEAL OIL BACK UP

The seal oil back up from the main bearing oil feed system is normally

closed. If the air side seal oil pump fails or if the seal oil pressure at the seal

decreases to 0.56 Kg/cm2 above the H2 pressure, the back up regulator valve

will open automatically and provide oil pressure for the seals. The back up

pressure may be supplied from several sources. When bearing oil flows to

the seals through the seal oil back up, the excess oil will overflow through

the seal oil return line into the main bearing oil frame.

The main oil pump on the turbine pump shaft is the primary source of

seal oil back up when the turbine speed is 75% or full speed. When the

turbine oil reservoir is equipped with a turbine governor auxiliary oil pump,

this pump will be actuated by turbine controls, and it will supply seal oil back

up when the turbine speed is less than 75%. When the airside seal oil

pressure at the seal drops to 0.35 Kg/cm2 above gas pressure, a switch will

close and automatically starts the airside seal oil back up pump. The pump

will continue to operate as it is held in by an interlock in the control and can

be stopped only by the push button. When this pump starts, “seal oil

pressure low ” and “seal oil back up pump Running” signal will appear.

When this appears, the H2 pressure in the machine should be decreased to

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0.14 Kg/cm2 gauge or less. Amount of gas pressure that can be maintained in

the generator depends not upon pressure being developed by the source of

seal oil pressure then operating, but depends upon the pressure available

from the next backup source of seal oil pressure.

The generator may also be operated with H2 with out the H2 seal oil

pump. Under this condition the seal oil from the airside feed groove will flow

in both direction along the shaft. Charging of hydrogen is required to

maintain the H2 purity since airside oil flowing into the H2 side of the sealing

will bring air and moisture into contact with the H2 inside the machine and

will remove some of the H2 from the generator by absorption into the oil.

3.9.5 GENERATOR BEARING DRAIN LOOP SEAL

A loop seal is provided in the bearing drain line before it enters the

turbine bearing oil drain system .It will prevent the entry of hydrogen into

the main oil reservoir, if seal fails. A vent to the atmosphere is provided on

the upstream or inlet side of the loop so that only hydrogen flowing through

the bearing drain will be carried out of the system before sufficient pressure

can be built up to blow the oil out of the loop seal and allow the hydrogen to

reach the drain oil reservoir. Since this loop seals presents an obstruction to

oil flow in the bearing drain system, the vapour extractor in the main oil

reservoir is not able to ventilate that part of the generator bearing oil drain

system on the upstream side of the loop seal. Therefore, an additional

vapour extractor assembly consisting of extractor, control bypass and check

valve are provided as a part of the loop seal assembly to provide the

negative pressure in the generator drain system on the upstream of the loop

seal required for normal operation.

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3.10 OPERATION OF THE SEAL OIL SYSTEM

The operation of the seal oil system involves initial commissioning

operation, adjustment of parameters after commissioning, normal operation

when the unit is running and some of the abnormal operations. It also

involves the routine checkups and inspection of the systems.

3.10.1 INITIAL OPERATION OF OIL SYSTEM

The seal oil system will be filled automatically from the turbine oil

system as described before when the bearing oil is being circulated. When

the seal oil system is put into operation for the first time it may be necessary

to add additional oil to the turbine oil system in order to supply the oil

required to fill the defoaming tanks, the hydrogen side seal oil drain

regulator to the current levels.

1. Set the relief valve No.258 on the airside seal oil pump so that it will open on low pressure by screwing out on the adjustment until it is almost all the way out. This will prevent damaging the pressure gauges by high pressure in the event that the other valves in the pump discharge are incorrectly set when the pump is started.

2. Set the differential seal oil pressure regulator No.256 to open with minimum differential pressure between the oil and gas pressure. Releasing the spring pressure on the regulator may do this. Loosen the vent in the bellows to allow oil to drip out during preliminary operation at the seal oil unit. This will get rid of any entrapped air and will ensure stable operation of the regulator. After the pump has been started, tighten the plug after approximately one litre of oil has leaked out from the plug.

3. Close valves Nos.263, 265 & 266 to isolate the main turbine oil system back up from the rest of the seal oil system during the part of preliminary operation.

4. Start the airside seal oil pump. This provides oil pressure at the shaft seals.

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5. Oil will now flow through valve No.232 into the hydrogen side drain regulator.

6. The de-foaming tanks will now fill up to the level of overflow connections.

7. Excess oil in the deforming tanks flows to the hydrogen side drain regulator. The hydrogen side drain regulator maintains a steady quantity of oil in the gas side seal oil system. Valve Nos.231 & 232 will maintain the normal level. If the level becomes high, float valve No.231 opens and allows the excess oil to flow out. If the level becomes low float valve No.232 opens and the oil flows from the airside seal system into the chamber. Jacks are provided in the valves No.231 & 232 so that they can be opened or closed manually in case of emergency.

8. Close valve No.254, and close the differential pressure regulator No. 256.

9. Screw down on the adjustment of the relief valve No.258 until the pressure on the discharge of the airside seal oil pump is 7 Kg/cm2.

10. Open valve No.254.

11. Valve No.238 should be partially closed to prevent hunting of the differential pressure regulator No.256 and approximately one-half turn open is the proper setting for this valve.

12. Adjust the differential pressure regulator No.256 to maintain the airside seal oil pressure at the seals at 0.84 Kg/cm2 above the hydrogen pressure (with cold oil, setting will be approximately 1.05 Kg/cm2).

13. Set the relief valve No.243 and the hydrogen side seal oil pump similar to relief valve No.258.

14. Start the hydrogen side seal oil pump. This will provide oil to the hydrogen side of the shaft seals.

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15. Close the valves 211, 218 & 242 and screw down on the adjustment of the relief valve 243 until the hydrogen pressure at the hydrogen side seal oil pump discharge is 7 Kg/cm2.

16. Open valves 211, 218 & 242.

17. Adjust pressure-equalizing valves 210 & 217 to give +50mm (or) 50mm of water on the pressure gauge associated with each equalizing valve and this adjustment is made by means of the adjusting screw on the bottom of the valve.

18. Pressure equalizing valve 210 & 217 will now maintain the hydrogen side seal oil pressure at the shaft seals at the same pressure as the airside seal oil pressure.

19. If a pressure reducing valve 291 is provided to reduce the pressure of the back up oil from the main oil reservoir, it should be adjusted as follows .Set the pressure reducing valve 291 to the closed position and start the high pressure auxiliary pump. Adjust the relief valve 280 to hold the value at 10.5 Kg/cm2 when the pressure-reducing valve is set for maximum pressure. When the shaft pump is operating at a speed greater than 75% of normal operating speed, it will provide the necessary seal back up. Adjust the pressure-reducing valve 291 to the value 8.7 Kg/cm2.

20. Open valves 263 & 265.

21. Stop the airside seal oil pump. Start the high-pressure auxiliary pump.

22. Adjust the back up regulator 264 to maintain the airside seal oil pressure at the seals at 0.56 Kg/cm2 above the hydrogen pressure.

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23. Check the operation of back up pressure switch PSA/542/414 which should close when the turbine oil back up pressure drops to a value equivalent to 6 Kg/cm2 (gauge) at the seals. Close valve 265 Block regulator 264 in open positions and the throttle valve 265 until the pressure at the seals is 6 Kg/cm2. Adjust pressure switch PSA/542/414 to close at this point. Release regulator 264. Open above valve 265. Stop the high pressure auxiliary oil pump.

24. Start the airside seal oil pump.

25. The seal oil back up supply from the pumps should be checked individually to assure their availability and correct functioning in the case of emergency.

26. Turning gear oil pumps supply only 0.35 Kg/cm2 pressures of the seals. This is insufficient to maintain 0.56 Kg/cm2 differential pressures. Therefore when these pumps are operating such that turbine is at standstill or on turning gear and high-pressure auxiliary oil pumps is out of service. When this differential pressure drops to0.35 Kg/cm2, switch PSA/542/424 will automatically close and start the airside seal oil back up pump. Check the proper functioning of this differential pressure switch and back up pump by opening valve 273, close valve 248 temporarily and then read pressure gauge. Slowly crack open valve 274. The DPSA/542/424 should operate when the pressure gauge decreases to 0.40 Kg/cm2 from the initial reading. Close valve 274 and open valve 248. Stop the airside seal oil back up pump.

27. Check the operation of differential pressure switch DPSA/542/414. This switch closes when pressure differential across the air side seal oil pump decreases to 0.35 Kg/cm2 (gauge) and actuate the “air side seal oil pump off” alarm.

28. Stop the H2 side seal oil pump. Pressure switch DPSA/542/434 should close.

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This switch should close when the difference between discharge and suction pressure of the H2 side seal oil pump decreases to 0.35 Kg/cm2

(gauge) and actuate the “H2 side seal oil pump off” alarm.29. Start H2

side seal oil pump. The seal oil supply system is now in normal operation.

3.10.2 ADJUSTMENTS WHILE RUNNING

Final adjustments should be made after the generator is synchronized

and the seal oil temperature after the seal oil coolers is between 27C and

50C.

1. Re adjust the differential seal oil pressure regulator 256 if necessary to maintain differential pressure 0.84 Kg/cm2. This adjustment should be made at low hydrogen pressure. At high hydrogen pressure, blower may develop as much as 0.15 Kg/cm2 pressure. The gas and oil sensing lines to the seal oil pressure regulator come from high pressure zone of the generator. Therefore the operation of the regulator is not affected by the gas pressure, however the gas pressure line from the generator to the gas pressure gauges come from a low pressure zone of the generator. At low pressure, the observed differential between the air pressure gauge and oil pressure gauge may be 0.84 Kg/cm2. While at high pressure the apparent differential pressure will be 0.99 Kg/cm2. This is not the true difference since the gas pressure in the high-pressure zone of the generator has gone up to 0.15 Kg/cm2 more than the gas pressure in the low-pressure zone of the generator.

2. Stop the air side seal oil pump and check the setting of the back up pressure regulator 264 which should open when the seal oil pressure at the seals decreases to 0.56 Kg/cm2 above the hydrogen pressure. This regulator should maintain the differential pressure. Check the airside seal oil back up pump to make sure that it starts automatically when the differential pressure switch DPSA/542/424 closes.

3. When the generator is operating at maximum pressure and maximum oil temperature, throttle valve 242 until seal oil differential pressure gauges no longer give steady readings. If seal oil differential pressure

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(oil to gas) is not with in plus or minus 50mm of water, readjust regulator 210 &217 to obtain this value.

3.10.3 NORMAL OPERATION

Seal oil pressure must be maintained at the seals when hydrogen is in the generator or when the shaft is running.

The hydrogen pressure may be varied upto3.12 Kg/cm2. The seal oil pressure is maintained at 0.84 Kg/cm2 above the hydrogen pressure when air side oil pump and back up pump is in operation, and at o.56 Kg/cm2 above the hydrogen pressure when the seal oil pressure is being supplied by the turbine oil back up system is 0.21 Kg/cm2

differential is required to maintain a seal when running. The temperature of the seal oil leaving seal oil coolers should be

maintained between 27˚C and 50˚C. The vapour extractors should be operated continuously when the

generator is filled with hydrogen as these vapour extractors ventilate the bearing brackets and bearing oil drain system.

The filter should be turned until free every eight hours. At least once in every 12 months filters should be drained through the pipe plug in the bottom when the system is shutdown.

When the air side back up pump starts, the auxiliary in the main oil reservoir should be started or the pressure in the generator should be decreased to less as the next back up pressure available is from the low pressure pumps.

The generator can be operated without hydrogen side oil pump. The hydrogen should be purged from the generator before the main oil

reservoir is drained.3.10.4 OPERATION WITHOUT HYDROGEN SIDE SEAL OIL

PUMP

The hydrogen side seal oil pump supplies oil to the hydrogen side of

the shaft seal at pressure equal to the oil on the airside of the shaft seals.

Therefore interchange of airside and hydrogen side gland seal oil is

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minimized. The air brought into the machine due to this interchange will not

adversely affect the purity of hydrogen since hydrogen is added to make up

normal gas leakage. The hydrogen side seal oil pump can be bypassed any

time for maintenance. When the pump is not in service, the flow of airside

seal oil to the hydrogen side seal oil is greatly increased. So, a certain

amount of hydrogen is removed from the generator by absorption. The purity

may be decreased to 90% to conserve hydrogen. This will increase the

density of the gas slightly. With the generator running at the rated speed,

approximately 20M3 of hydrogen per day will maintain the purity at 90%

during bypassing operation. At standstill the flow of seal oil will be less,

therefore the hydrogen required to maintain the purity will be decreased

when the seal oil system is operated without hydrogen side seal oil pump,

the air side seal oil will flow through the seal in to the defoaming tank and

into the hydrogen side drain regulator. When this operation occurs at low

gas pressure the oil level in the hydrogen side drain regulator may rise.

During operation at higher gas pressures, the gas pressure forces the oil

down to the normal level. The oil fluctuates to provide sufficient head for the

oil to flow through the valve 231.

3.10.5 OPERATION WITH GENERATOR SHUTDOWN

Hydrogen may be left in the generator during shutdown if the seal oil

pressure is maintained and the vapour extractors are in operation. If it is

necessary to shutdown the turbine oil pumps, close valves 263 & 265 to

prevent the seal oil from leaking back into the turbine oil reservoir through

the back up regulator and there by causing loss of oil pressure to the seals.

The generator bearing feed and drain lines must be free from oil flow from

the bearing oil pump, through the generator bearing and into the loop seal.

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If the back up seal oil pump starts under this condition, valves 263 and 265

should be opened and the H2 pressure should be reduced to 0.14 Kg/cm2 or

less as the next back up pressure available from the low pressure turbine

pumps. If the turbine pumps cannot be started, the hydrogen should be

purged out.

3.10.6 STARTING SEAL OIL PUMPS

Before starting any of the seal oil pumps, be sure that each has an

adequate supply of oil so that pumps will not run dry. This would result in

destructive wear between the various rotating stationary parts. The

sequence of starting is (a) air side seal oil pump (b) H2 side seal oil pump.

3.11 INSPECTION AND MAINTANANCE OF SEAL OIL SYSTEM3.11.1 SEAL OIL PUMPS (AIR SIDE & HYDROGEN SIDE)

The seal oil pumps have a special double helical rotor, which

eliminates end thrust and trapped liquid. The large surfaces of helical rotors

provide high efficiency and long life. The sealing of the drive shaft on the

seal oil is achieved by means of a bellow type mechanical seal subject to the

suction pressure on the pump only, which results in leakage. The mechanical

seal consists of a spring, spring holder, synthetic rubber bellows holding ring,

carbon seal and floating seal. The synthetic rubber bellows is secured to the

shaft by the shoulder of the holding ring, which also secures the bellows and

carbon seal in such a way that the seal is spring loaded against sealing

surface of the stationary iron seat. If any leakage of oil is detected the

mechanical seal should be replaced.

3.11.2 SEAL OIL BACK UP REGULATOR

Since the back up regulator 264 is normally required it is advisable to

check it once a month to ensure that it will operate correctly when needed.

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Read the seal oil pressure gauge at valve 236. Slowly throttle valve 254. When the pressure gauge readings decrease

to 0.28 Kg/cm2 from its initial reading, the back up regulator 264 should start to open.

Continue to throttle valve 254 slowly. If the seal oil pressure continues to drop, it indicates that the back up regulator needs to be readjusted.

With the backup regulator 264 set to hold the seal oil pressure at 0.56 Kg/cm2 above gas pressure, slowly close 254. The seal oil pressure should not decrease; when the valve 254 is fully closed the seal oil pressure is being supplied by the turbine oil system.

Stop the airside seal oil pump. Open valve 254. Check for proper operation of the airside seal oil back up pump. Start the airside seal oil pump.

All pressure switches should be checked for correct operation and for

mechanical defects. Each self-cleaning filter should be cleaned at regular

intervals. A handle wheel controls the cleaning operation and it does not

require any housing. The dirt removed is collected at the bottom of the filter

housing, from where it can be drained by means of a valve.

3.11.3 PRESSURE EQUALIZING VALVES

Pressure equalizing valves 210 & 217 should be checked to see that

they are holding the differential seal oil pressure (air side Vs hydrogen side)

to within plus or minus 50mm of H2O.

3.11.4 HYDROGEN SIDE DRAIN REGULATOR

Hydrogen side drain regulator should be drained and the cover

removed, so that any accumulation of sludge or other foreign materials can

be cleaned out.

3.11.5 SEAL OIL COOLERS

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The cooler tube bundles should be removed and inspected for cleanliness. The tube should be cleaned if required, vent the coolers on the oil side by loosing the pipe plug provided for this purposes.

3.12 OPERATION OF HYDROGEN GAS SYSTEMS

When hydrogen mixes with oxygen, an explosive mixture is formed

which will explode violently when ignited. So the hydrogen should not be

allowed to mix with oxygen or air in the generator when filling up. So, CO2 is

used as a medium for charging the generator with hydrogen or scavenging.

3.13 GAS CHANGE OVER OPERATION (Fig. 8 & Fig. 9)

Gas changing operations are preferred with the generator at stand still

or turning gear. In case of emergency these operations can be performed

while the machine is accelerating or decelerating. The generator should not

be allowed to operate at normal operating speed in CO2. Atmospheric air

contains approximately 21% oxygen by volume. At the upper end of the

explosive range (70% hydrogen & 30% air) the oxygen content of the

hydrogen and air mixture is 21% × 30% = 6.3% oxygen. Therefore before

introducing hydrogen in to the generator oxygen content should be reduced

to less than 6.3% oxygen. Therefore before introducing hydrogen into the

generator, oxygen content should be reduced to less than 6.3%. Air content

will be reduced to 14% having equivalent oxygen of 21% × 14% = 3%

oxygen will introduce two volumes of gas. When CO2 is introduced at the

bottom of a frame with the rotor at standstill about 11/2 volumes will be

sufficient to remove the air since there is no much mixing of air and CO2

under this conditions. Purity meter connections are provided to take gas

samples from either the top or bottom of the machine. During the addition of

CO2 gas sample should be taken from the top of the machine. After 11/2

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volumes of CO2 is put in the generator, the purity meter should read about

95% CO2.

A hydrogen cooled turbine generator is operated with the gas in the

machine 95% H2 or above by volume. If 3½ volumes are introduced into the

generator when it is running the resulting mixture will be 95% H2 and 2 ½

volumes with the rotor at stand still. Connecting the purity meter sampling

lines to the bottom of the frame should check purity of gas. During the

normal operation of the H2 cooled generator the H2 purity is maintained 95%

or above. When the H2 side seal oil pump is shut down, gas must be added to

maintain the pressure and to maintain the purity. Inflow of 1M3 of air will

require the addition of 24M3 of H2 to maintain the purity at 95% and 10M3 of

H2 to maintain the purity at 90%. For removing H2 from the generator, CO2 is

introduced at the bottom of the housing as a scavenging gas and H2 is driven

at the top. Sufficient CO2 is introduced to reduce the H2 content in the

mixture to 5%. Two volumes of CO2 being required for scavenging at

standstill condition. CO2 purity should be 95%. After the H2 has been

scavenged from the generator housing, it may be opened and the oil

pressure may be turned off. Hand holes in each end of the generator end

should be opened and a fan directed into the opening at one end to drive out

the CO2. This precaution is suggested to prevent inhaling of CO2, which might

occur if an immediate visual inspection is made through hand hole.

3.14 REPLACING AIR WITH CO2

Make certain that normal seal oil pressure is established at the seals. Make sure that the purity meter system is operating correctly. Make sure that the purity meter blower is running and that power and

or air supplies to the H2 panel have been turned on. Check that sufficient quantities of H2 and CO2 are available.

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Connect the purity meter sampling lines to the top manifold, open valves 34 & 36, close valves 33&35.

Connect vent to the top manifold by opening valve39 & closing 38. Isolate the H2 supply by closing valve 30. Connect the CO2 supply to the bottom manifold by opening valve 37. Recheck valves 40,41&42 in the H2 panel to be sure valve 40 is closed.

Valves 41 & 42 are open. Admit CO2 to the generator through the valve 48.Do not allow machine

pressure to exceed 0.2 Kg/cm2 gauge during the time CO2 is being admitted. The valves in the CO2 supply including valve 58 should be opened wide to prevent the valve from freezing. Observe the frost line on the CO2 feed line to see that it disappears at least 3m from the point at which the CO2 feed line enters in to the generator. Observe the purity meter indications as the CO2 is being admitted. When 1½ machine volumes have been put in to the generator, the purity meter should read approximately 93% of the CO2. Continue to admit CO2 until the purity meter indicates 95% CO2.

Close valves in CO2 supply and then close valve 58. 3.15 REPLACING CO 2 WITH H2

Connect vent to bottom manifold by opening valve 38 and closing valve 39.

Connect purity indicator sampling lines to the bottom by opening valve 33 and 35 and by closing 34 and 36.

Open valves 30 and 2. Admit hydrogen either by opening valve 53 or by using the regulator

on the hydrogen pressure control manifold. The generator gas pressure should not be allowed to exceed 0.2 Kg/cm2 to 0.35 Kg/cm2.

Continue to admit hydrogen until purity meter indicates at least 95% hydrogen purity. This will require approximately 21/2 times machine volumes.

Open valve 24 for approximately two minutes for purging out of the CO2 from the lead box.

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When the desired purity level has been reached close valve 38. The machine pressure will increase for one Kg/cm2 increase in gas pressure; one machine volume of gas will be required.

The pressure regulator on the hydrogen pressure control manifold may be set for the desired machine gas pressure as follow.

1. Close valve 2, 53 and 1 and turn the ‘T’ handle of the regulator in anti clockwise direction.

2. Open valve 54 to allow the manifold pressure to go to zero.3. Close valve 54.4. Open valve 1 and set the desired pressure by tuning the ‘T’

handle of the regulator clockwise and observe the pressure gauge on the generator side.

5. Open valve 2, the regulator will now allows hydrogen to flow into the generator until the set pressure has been reached.

3.16 REPLACING H2 WITH CO2

1. Connect the purity meter sampling lines to the top manifold by opening valves 34 and 36 closing valves 33 and 35.

2. Close hydrogen charging by closing valve 30 and 2 and removing the removable link in the hydrogen feed line.

3. Connect the CO2 feed to the bottom of the generator by opening valve 37 and closing valve 38.

4. Connect the vent to the top manifold by opening valve 39. Hydrogen will now escape to the vent through the line and valve 39.

3.17 REPLACING CO2 WITH AIR

The seal oil system may be stopped provided the shaft is not running. Remove the manhole cover at each end of the generator. As soon as

the covers have been removed, take a gas sample with a gas analyzer. After the gas covers are removed, wait at least for one hour before blowing dry air to one end of the generator to drive out the CO2. Open all water detector drains.

Ventilate for several hours after removing manhole cover, before entering into the generator.

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When replacing the manhole covers, do not forget to close the water detector lines.

3.18 DETERMINATION OF GAS PURITY 3.18.1 FROM THE PURITY INDICATING TRANSMITTERS

Direct reading purity meter determines machine gas purity. Valves in

the purity meter circulating lines permit sampling from the top or bottom of

generator. During normal operation, the sample should be taken from

bottom. The pressure head developed across the purity meter blower is

directly proportional to the density of the gas passing through the blower.

Connection from the high and low pressure sides of the blower are taken to

the purity meter. The purity meter is a differential pressure instrument,

which converts the pressure head to reading on a scale. A pressure

compensating device incorporated in the purity meter allows the calibration

of the scale directly in terms of the machine gas purity.

3.18.2 FROM THE BLOWER PRESSURE GUAGE

Generator blower pressure is indicated by a differential pressure gauge

in the hydrogen control cubicle. The blower pressure gauge may be used to

check the purity meter since both gauges operate in the same principle. If

the purity meter is out of service and generator is running at rated speed the

machine gas purity may be obtained as described below. The readings of the

blower pressure gauge and the upper scale of the purity meter are both

proportional to machine gas density. They are therefore proportional to each

other for the same hot gas temperature. The constant may be obtained by

direct measurement at a number of typical hot gas temperatures. There

after for a given hot gas temperature either gauge reading may be

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transferred to the other gauge reading by multiplying or dividing by the

constant.

Density = Blower pressure × constant

(or)

Blower pressure = Density / Constant.

Since a straight-line relationship exists between upper scale (density)

of the purity meter and blower pressure readings, a chart may readily be

prepared. A series of straight line will go through the origin and some other

point for a given hot gas temperature. These curves may be obtained when

the purity meter and the blower pressure instrument both are in operation.

3.18.3 USING PORTABLE INSTRUMENT

Gas purity may be checked with portable instruments. Oxygen and CO2

content may be determined by means of the standard apparatus.

3.19 NORMAL OPERATING PURITY

During normal operation of the generators with both airside and

hydrogen side seal oil pumps in operation, the hydrogen purity can be

maintained at 95% or greater. If the hydrogen side seal oil is not in service,

hydrogen purity should be maintained at 90% to avoid excessive use of

hydrogen.

3.20 POSSIBLE CAUSES OF LOW PURITY

(1)Incorrect setting of the pressure equalizing valves.(2)Malfunctioning of the hydrogen side drain regulator.

The pressure equalizing valves 210 & 217 should maintain at a

differential pressure with in plus or minus 50mm water column between

airside and hydrogen side. If one or both of these valves are not operating

correctly, air may enter in to the generator through the airside seal oil. Float

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valves 231 &232 maintain the oil level in the hydrogen side drain regulator

with in certain limits. If valve 232 does not function properly when the oil

level rises, airside oil is forced into the tank. The level is maintained since

valve 231 is open when the oil level is high and allows the extra oil to be

forced into the loop seal.

3.21 TESTING GENERATOR AND GAS SYSTEM FOR LEAKS

The Generator and gas system should be tested at 1.05 and 3.12

Kg/cm2 with air to check for any leaks. This test should be made with oil

circulating through the seals of hydrogen side seal oil pumps in service. The

allowable leakage is 0.5M3 per day at 1.05 Kg/cm2 and 1.06M3 per day at 3.12

Kg/cm2. Dry air should be used for the pressure test.

Leak Detection:

If the leakage rate is not with in the specified limits in systematic

search for leaks should be made either with soap or either liquid solutions or

by means of leak solutions or by means of leak detectors with “From 12”.

The use of odorants such as either is not permitted. Liquid soap solution

provided a quick and simple method of leak detection and evolution. They

are not suitable for inaccessible parts or for very small leaks. Solutions of

industrial or domestic liquid soap with the addition of glycerin as a thickening

agent are satisfactory.

The location of leak may be determined by injecting “From 12”gas into

the generator at the rate of approximately 1Kg / 28M3 of machine volume.

“From 12” is inert, nontoxic and readily available. Very small quantities

“From 12” at leakage points can be detected with the aid of suitable halide

detectors.

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3.22 LEAKAGE SOURCES SEARCH PROCEDURE AND CORRECTIVE ACTION (a) GENERATOR FRAME AND LEAD BOX

Check bearing brackets and lead box joints, lead bushings, gas cooler

flexible end, piping connections, and instrument boards. Tighten bolts in

sequence such that loads on gasket or other joints are evenly applied.

(b) VALVES AND PIPE LINES

Check all gas valves and piping associated with the gas supply

including water detectors and the hydrogen and carbon dioxide manifold.

(c) VENT VALVES AND VAPOUR EXTRACTORS

Leakages or losses through vent lines from the generator or vapour

extractors can be checked at the vent outlets on the powerhouse roof.

When checking vents, use an activated charcoal filter in the suction side of

the detector to remove oil vapour. There should be no leakage from the

main oil reservoir and generator vents, but there may be some from the

generator bearing loop seal vent. If leakage is detected at the loop seal

vent, investigate the bearing drains.

(d) BEARING DRAINS

Access to each drain is made through a pipe plug in the drain line or

slightly below floor level. Use an activated charcoal filter and stop the

vapour extractor during the search. Leakage into the bearing cavities is due

to faulty joints between the gland seal and bearing brackets. It may also be

due to faulty oil seal or due to an excess of hydrogen side seal oil pressure

over airside oil pressure. Tighten the gland seal brackets. Check the

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differential pressure at seals and check the difference between hydrogen

side and airside seal oil pressure.

(e) GAS DRIER

Check all flanges and openings in gas drier tank including the entry

points for heater leads and thermostat. Check also the three way valves

which are a frequent source of leakage.

(f) HYDROGEN GAS CONTROL PANEL

Check all joints instrument canes and purity meter blower for leaks. Do

not use liquid solution on electrical leads.

3.23 CALCULATION OF GAS LEAKAGE

To obtain accurate leakage tests, it is necessary to correct the

observed pressure drop for changes in temperature and atmospheric

pressure.

L = 6552 × V / H [(P1 + B1) / (273 + T1) - ( P2 + B2) / (273 + T2) ]

L = Leakage in m3 per day at standard conditions for the entire test period.P1 = System pressure at the beginning of the test.B1 = Barometric pressure at the beginning of the test. T1 = System temperature at the beginning of the test.P2, T2, B2 = Pressure, Temperature and Barometric readings at the end of the test.V = Static gas volume of the system in m3.H = Test period in hours. (Temperature in degrees centigrade and pressure in Kg/cm2).

3.24 OPERATION OF THE SIGNAL SYSTEM3.24.1 HYDROGEN PURITY – HIGH OR LOW

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The low hydrogen purity signal is given by an alarm contact system in

the purity indicating transmitter and indicates that the hydrogen purity in the

generator is lower than 90%. This signal indicates low hydrogen purity, which

may be a result of improper operation of the pressure equalizing valves, or

the hydrogen side drain regulator. The high hydrogen purity signal is given

by an alarm contact in the purity indicating transmitter and indicates that

the purity meter pointer is above 100% mark. If the meter indicates a higher

purity, either the pointer or the meter has struck or the purity meter blower

has stopped.

3.24.2 HYDROGEN PRESSURE – HIGH OR LOW

The high hydrogen pressure signal is given by a signal contact on a

pressure switch mounted in the generator panel. It indicates the pressure of

hydrogen in the generator is higher than the operating pressure by 0.2

Kg/cm2. The low hydrogen pressure switch located adjacent to the first. It

indicates that the hydrogen pressure has dropped to 0.15 Kg/cm2 below the

operating pressure. The alarm may also be given if there is a sudden large

drop in load and the water supply to the coolers is not decreased. The

resultant rapid drop in temperature will cause the hydrogen to contract and

reduce the pressure in the machine. If this low-pressure alarm is sounded,

the gas supply system should be checked.

3.24.3 WATER DETECTORS - HIGH

Two liquid detectors are located at the bottom of the generator and

one at the bottom of the generator lead box. The water detector high signal

indicates that one of these three detectors has become filled with water or

oil. A float-operated switch on the detector gives the signal. The detector at

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the hydrogen cooler would be the first to be filled if water were leaking from

the coolers. A secondary cause for an alarm would be an overflow of the oil

from the gland seal oil system. At the exciter end of the generator, the water

detector high signal indicates that the detector at the bottom of the main

lead box was full of oil, which had overflowed from the seal. At the water

detector, at the centre of the generator, and alarm would indicate that an

overflow of liquid from either end of the machine had reached that point. The

three detectors may given pressure signal by making contact on a second

alarm separately or may be paralleled to give one alarm. Opening the valve

at the bottom and draining each detector will show whether the liquid is

water, or oil or a mixture.

3.24.4 HYDROGEN TEMPERATURE – HIGH

It is given by making of contacts on the hydrogen cold gas thermostats

and by resistance temperature detectors in the generator. These contacts

should be set at a few degrees above the maximum temperature when the

generator carrying full load and the cooling water is at its maximum

expected temperature. This signal may be due to overload, low hydrogen

purity or high water temperature in the coolers.

3.24.5 HYDROGEN SUPPLY PRESSURE - LOW

This signal indicates that the pressure of hydrogen at the header is

low. It is given by making a contact on a pressure switch located on the

hydrogen pressure control manifold. This signal will appear when the

pressure decreases to 5.25 Kg/cm2 gauge.

3.24.6 DEFOAMING TANK LEVEL HIGH

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This signal indicates that the oil level in one of the defoaming tanks is

too high and is given by signal contact on the float switches located in both

the collector end and the turbine end-defoaming tank. This alarm will sound

when the oil level rises above the overflow connection, a condition which

might be due to an excessive oil flow from the seals, a clogged overflow or

drain line or the improper functioning of the float valve 231 in the hydrogen

side drain regulator. If this alarm is sounded, the oil level in the defoaming

tanks should be checked immediately, since if the level gets too high the oil

will back up through the seals in the generator. Float valve 231 & 232 are

provided with jacks by which they can be opened manually in case of

emergency. The defoaming tanks are provided with drain valves through

which excess oil can be drained. Care should be taken in the operation of

these valve as the defoaming tanks contain hydrogen at the same pressure

as the gas in the generator.

3.25 SEAL OIL PRESSURE LOW

This signal indicates that the seal oil pressure at the seals has dropped

to 0.35 Kg/cm2 above the gas pressure. It is given by making of a contact on

a differential pressure switch located on the seal oil supply unit. If this alarm

appears while the turbine is in service, the turbine oil back up system should

be checked, as it may not be functioning properly. Either the turbine oil back

up pumps are not producing their normal pressure or regulator 264 is not

operating correctly as this regulator should maintain 0.56 Kg/cm2 differential

pressure. This regulator can be bypassed by opening valve 266, if it is found

that the trouble is in the regulator.

This differential pressure switch also activates the seal oil back up

pump at the same time as the alarm is given. This pump restores the

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differential pressure of the seal oil to 0.84 Kg/cm2 above the gas pressure. If

it is found that the cause of the trouble lies in the turbine oil back up pumps

and at least one of these pumps cannot be taken into normal service

immediately, the hydrogen pressure in the generator should be reduced to

0.14 Kg/cm2 or less as the only back up for this seal back up pump are the

motor driven low pressure oil pumps which can deliver only a limited

pressure.

3.26 AIR SIDE SEAL OIL PUMP OFF

“The air side seal oil pump-off” signal indicates that the power supply

to the motor is cut off. The signal is given by a contact on a differential

pressure switch across the airside seal oil pump on the seal oil supply unit.

This contact closes and the alarm is given when the pressure across the

pump decreases to the 0.35 Kg/cm2. When this alarm is given, the contactor

of the seal oil pump motor circuit should be inspected.

3.27 SEAL OIL TURBINE BACK UP PRESSURE LOW

This signal indicates that the turbine oil back up pressure has dropped

below the setting of pressure switch (PSA/542/414), which should close at 4.9

Kg/cm2. This signal indicates that high pressure oil is not available from the

pump on the turbine shaft or from a turbine auxiliary oil pump.

The alarm should occur only when a turning gear oil pump is

lubricating the turbine generator unit. If the air side seal oil pump fails and

the air side seal oil back up pump controlled by pressure switch

(dpsa/542/424) starts while alarm is on, the turbine auxiliary oil pump (or the

back up pump on the main oil reservoir) should be started. This will provide

the necessary back up pressure for the airside seal oil back up pump. If the

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turbine auxiliary oil pump (or back up pump on the oil reservoir) cannot be

started immediately and the turbine back up pressure in the turning gear oil

pump, the hydrogen pressure in the generator should be reduced to 0.14

Kg/cm2 gauge or less.

3.27 HYDROGEN SIDE LEVEL LOW

This signal indicates that the oil level in the hydrogen side seal oil

drain float chamber is too low and is given by making of a contact of a float

switch located in the chamber. This low level may be due to improper

functioning of float valve 231. If this alarm appears, the action of this valve

should be checked immediately since if it struck in the open position, the oil

will drain from this chamber and the seal would be lost, allowing the

hydrogen in the generator to escape into the bearing and seal oil system,

there by causing a dangerous condition. This float valve 231 is provided with

jack by which it can be closed manually in case of emergency.

3.28 HYDROGEN SIDE SEAL OIL PUMP OFF

This signal indicates that the hydrogen side seal oil pump is not

operating. A differential pressure switch connected across the inlet and

outlet of the pump, when the differential pressure drops to 0.35 Kg/cm2

gives the signal. The generator can be operated without this pump, but the

hydrogen consumption of the unit will be increased due to the contaminated

airside seal oil flowing in the hydrogen side of the seal rings.

3.29 AIR SIDE SEAL OIL BACK UP PUMP RUNNING

This signal indicates that the seal oil back up pump is running. This

signal is given either by an interlock switch in the contactor for the motor

drive for this pump, or by a differential pressure switch connected across the

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inlet and outlet of the pump. The switch closes when the pump starts, the

turbine auxiliary pump (or the seal oil back up pump on the main oil

reservoir) should be started immediately to serve as a source of high

pressure seal oil backup. If high-pressure backup cannot be provided and if

the airside seal oil pump cannot be restored, the hydrogen pressure should

be reduced to 0.14Kg/cm2.

3.30 GLAND SEAL OIL AIR SIDE HIGH OR LOW

This signal indicates that the differential pressure between the airside

and hydrogen side of the seal oil at the generator bearing is beyond the

limits. In case that both the differential pressure switches (DPSA/542/434 and

DPSA/542/444) should alarm, and the valve 242 should be re adjusted. So

that, through the pressure equalizing valves 210, 217, the difference in

pressure between the airside and hydrogen side of the seal oil at the

generator bearings is restored to the normal limits. In case only one of the

two differential pressure switches should alarm, its corresponding pressure

equalizing valve should be checked.

3.31 AIR SIDE (HYDROGEN SIDE) FILTER DIFFERENTIAL

PRESSURE HIGH

The filters on airside and hydrogen side of the seal oil system are

equipped with differential pressure switches, which will alarm and the filters

should become chocked. In case of alarm the filter should be taken out of

service and cleaned.

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