210 Mw Lmw Turbine Emergencies
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210 MW LMWTURBINE EMERGENCIES
HANDLING OF TURBINE DURING EMERGENCY
• 1) Partial load throw off (due to high frequency).• 2) Partial load throw off due to Aux. Failure.• 3) Sudden increase of load from full load• 4) Vibration high• 5) Differential Expansion high/low• 6) Axial shift high• 7) Loss of vacuum• 8) High level in condenser• 9) Loss of Barring gear or barring gear fails to start
1) Partial load throw off (due to high frequency).
• Probable Reasons and Action to be taken:-
• The steam pressure at ESV will increase:• FSSS operator will control the boiler
pressure.• Governor will control the load.• If it fails, then throttle the control valves
• Maintain steam pressure to normal till the frequency comes down.• As the turbine load will decrease :
There should be a constant watch on the supervisory instruments.• Disturbance in differential
expansion, vibration, axial shift, eccentricity
• Disturbance in condenser level : Adjust and maintain the hot well level• Disturbance in vacuum• Check and adjust ejector steam
pressure and gland steam pressure to normal
• Turbine lub oil temp will change.
• Adjust coolers water flow as necessary.
• Disturbance in Deaerator pressure.
• Deaerator pressure tends to fall,
• Maintain it through extraction on 16 ata. Header.
• 2) Partial load throw off due to Aux. Failure.
• Probable Reasons and Action to be taken:-
• There is a provision for initial load throw off by help of a relay without time delay which reduces turbine load to 100 MW in case of tripping of following equipment’s .
• One C.W. pump tripped in case both are running : If the partial load relay fails or not in circuit• Reduce the turbine load.• Inform the boiler operator to
control steam parameter in case one C.W. pump or one Condensate pump trip.
• Careful watch may be kept on vacuum.• Inform the local operator to check
the cause of tripping.• Maintain the deaerator and hotwell
• One BFP in case two are running and standby does not come on auto remote /not available.
• One condensate pump in case two are running and standby dose not come on auto/remote not available:
• Tripping of any one of I.D. fan.• Tripping of any one of F.D. fan.• Tripping of any one of P.A. fan.
• In case of full load throw off (due to opening generator breaker)
• • The turbine may trip due to over speeding : Close the control gear hand wheel
• If control valve can not control the speed :• Immediately trip the Turbine by emergency
• Check that ESV and control valves are closed.• Close the valves in the M.S. line. Close the
Extr. Steam valves.
• 3) Sudden increase of load from full load
Probable Reasons and Action to be taken:-• Low frequency : Since the turbine is on
governor control ,the load will increase due to fall in speed.
• Reduce the load by the control gear and bring the boiler parameter and load to normal.
• Due to sudden rise of boiler drum pressure.: • Bring back the load by governing
wheel and maintain the pressure to normal by control gear and inform the boiler operator.
• 4) Vibration high
• Probable Reasons and Action to be taken:-• • Normally the vibration in bearings
should be within 40 microns. If it exceeds the limit, turbine should be shut down:
• Check the turbine cylinder drain should not be cold (if necessary open the drain valve for few minutes if turbine is on load).• : Check bearing oil press ,flow and
temp. to be normal .• Check for any rubbing sound etc.
• :• • Turbine bearing oil press low : Reduce the load
/speed. Check the oil line pressure and maintain it.
• • Bearing oil inlet temp. High :Check the cooling water valves are open in oil coolers and vent the coolers. Put the standby cooler in service, ensure C.W.P. is running normal. Check bearing oil outlet temp. (less then 65° c ).
• 5) Differential Expansion high/low• Probable Reasons and Action to be
taken:-• • Normally the rotor gets long during running
and gets short during shut down. • The expansion is to be controlled by
admitting steam or shutting off supply to flange and stud heating, chamber heating accordingly.
• In case if it goes beyond control, the machine to be tripped.
• Check the steam temp. Reduce load / speed.
• Check the valve for flange heating is in open condition if rotor is long or close if rotor is short. Check and adjust gland steam temp.
• • Check the vacuum to be normal
• 6) Axial shift high• Probable Reasons and Action to be
taken:-• • In case of excessive shift the
turbine will trip through the protection• • Sudden change in load : Control
the load and maintain the steam parameter.
• • Abrupt change of steam parameter: Control the load and maintain the steam parameter
• • Sudden closure of FCNRV of heater (HP/LP) :
• Control the load and maintain the governing pressure,
• If heaters are healthy, open FCNRV.
• • Variation in lub. Oil temp .pressure and quality :• Maintain lub. Oil pressure and
temp. for quality• Get the oil tested from laboratory
and if necessary oil is to be changed.
• 7) Loss of vacuum
• • If there be any loss of vacuum , start the standby ejector at the earliest. Still if it does not improve start starting ejector. • With no further improvement of
• Reduce the load as required to maintain the vacuum at 700 MM. • Meanwhile try to detect the specific
reason which may be as follows :• Steam ejector not properly
functioning : • Check that steam pressure in 6
• EMERGENCY OPERATIONS OF TURBINE• 1 Turbine Trip• 2 Boiler Feed pump Trip• 3 Condensate Extraction Pump Trips• 4 Deaerator Level High • 5 Deaerator level Low
• 6 H P Heater level High
• 7 LP Heaters 2,3, 4 Levels High• • 8 Turbine Lubricating Oil Pressure Low
• 9 Lubricating Oil Temperature Low
• 10 Lubricating Oil Temperature High
• 11.C. W. Pump/Pumps Trips/Trip
• 12. Fire in Turbine Oil System
• 13. Loss of Vacuum (Partial or Completely)
• 14. Exhaust Hood Temperature High
• 15. High Turbine Bearing Temperature
• 16 Axial Shift High • 17 Eccentricity High • 18 Differential Expantion High (Posit
ive) • 19 Differential Expantion (Negative)
• 20 Bearing Vibration High
. Deflection of turbine rotor
• 1)Machine could not be put on barring gear after tripping.
• 2). Improper heating during start up.• 3). Improper draining of steam lines and casing.• 4). Abrupt drop in M. S. temperature due to• carry over of moisture in gland, deformation of'
cylinder.• 6. Mal function of eccentricity pick up.
• EFFET .1. Turbine vibration will @,increase.2. Unusual noise from the turbine.3. Bearing oil and metal temperature
will rise.4. If, machine is on barring gear, it will
draw more power.5. Eccentricity high alarm will appear.
• ACTION :-
1. Strictly follow the start UP procedure while starting.
Maintain the steam parameterswhile starting. Give proper heating and soaking time at required
speed while rolling.Drain the steam lines and casings as per
• If eccentricity is high when machine is on barring gear it is due to rotor bent., continue the machine on barring gear for a longer time. Soak the m/c at 500 rpm., for a longer time. So that bent rotor may get even out.• .
• When eccentricity increases with vibration, reduce the unit load and allow it to comeback to the original value and stop the machine for checking for any abnormality.
Differential Expansion High (Positive) HP:4mm,IP:3mm,LP:4.5mm
• 1. High steam temperature.• 2. Gland steam temperature high.• 3. Fast rolling or fast pick up of the load.
• Inadequate soaking period.
• High condenser vacuum• • Which effect on LP differential
• EFFECT• 1)Seal rub can be expected.• 2)Metalic rub sound from fhe
turbine when interfearance exists.• 3)Turbine vibration may
• ACTION• 1)Use flange and stud heating in Hp and Ip
turbines. (When Hp, Ip differntial expansions are higher (+ ve direction.)• 2)Soak the turbine properly.• 3)It should not incease more than +
3.5 mm. • Maintain the steam temperature
accordingly. (as per starting diagram).• 4)Load the turbine gradually.
• If it is beyond control try to control the boiler parameters and if turbine is tripped,drop the vacuum immediately.
• It LP differential expansion is increasing drop the vacuum to a certain extent. (to an acceptable limit)
Differential Expantion (Negative)HP:(-1.2mm),IP:(-3.0mm),LP:(-4.5)
• CAUSES• Time taken during rolling and loading is
more than required.• Increment in exhaust hood temperature.• Low condenser vacuum.• Flange heating valves are passing (HP and
• EFFECT• Seal rubs can be expected.
• Metalic rubbing sound from the turbine when interfearance exists.
• Increment in vibration
• ACTION• 1)During hot start rolling and loading of
the machine should be as per the starting curve.,
• 2)Start 2nd ejector to improve the vacuum.
• 3. Try to bring down the exhaust hood temperature.
• 4.If turbine is tripped due to-ve maximum differential expansions open, thevacuum bkr. to avoid the possible damage,
• 5.Increase the boiler steam temperature and load the machine little faster.
• 6.If differential expansion is extremly -ve and machine has been rolled then do not trip the@ turbine. • Control the boiler parameters.• 7.Check the passing on the flange
heating valves if any and arrest.
• One Side Quick Closing Valve Or Interceptor Valve Close
• CAUSE:• 1.Mechanical fault in quick closing
valve or in the interceptor valve.• 2.Drainage of Oil from the connected
• EFFECT :• 1.Will reduce the generator load.
• 2.Uneven flow on LHS/RHS, Reheater/Superheater.• 3.May damage the superheater or
• 4.Safety valve of superheater/Reheater blows.
• 5.Unit may trip due to disturbances created in the boiler.
• 6.Thrust on turbine.
• ACTION• 1.Try to reopen the valve.
• 2.Reduce the load on the machine.
• 3.Reduce the boiler firing rate.
• 4.While rolling the machine the proper opening of all the above valves. (Before rolling the machine see that the HP/LP and CV are open or not
• Water Induction In Steam Turbine• CAUSE• 1.Due to carry over of wet particles from the
boiler.• May be due to high drum level• or. sudden @ drop in steam temperature, (if
considerable).• 2.If reheater attemperation is in service. Water
particles may carry over throughCRH lines. (during shut-down or low load operaticn), result in reheater temperature to fall.
• 3.Defective gland steam temperature controller.
• 4.GSC tube leakage and GSC drain system fails.
• 5.Feed water heater tube lefkage and the heater level controller is defective, failure
• of 'heater level hi-hi' protection. (Heater should be bypassed on operating the
• • EFFECT• 1.Thermal shocks and leakage through joints• 2.Chilling of IP cylinder and damage to the IP rotor.
(Due to cold reheat spray in service.)• 3.Low gland steam temperature will chill the glands
and leads to distortion of glands, vibration and increase in Eccentricity,.
• 4.Damage to the rotor blades, diaphrams and leads to -the vibration.
• ACTION :• 1.Machine should be shut-down if it is rolled
and open the steam drains fully.
• 2.If the machine is running on load, stop the ingress of water. Increase the steam temperature by openning the drains.
• 3.If unit is on load and if Joints of the turbines are found heavily leaky. (due to water induction), Stop turbine.
HANDLING OF TURBINE DURING EMERGENCY
• IMPORTANT DO’S• 1) IF THE TEMP. OF MAIN STEAM DROPS BELOW 520
0C,• UNLOAD THE THE SET AT RATE OF 2MW/degC OF
STEAM TEMP. DROP. THE SET SHOULD HAVE BEEN APPROXIMATELY
• UNLOADED AT 100MW AS MAIN STEAM TEMP. APPROACHES 4650C
• 2) IF TEMP. GRADUALLY FALLS BELOW 4650C, REDUCE THE LOAD AT THE RATE OF 1MW/degC TO COMPLETELY UNLOAD AT 3650C
TURBINE EMERGENCIES• TURBINE MUST BE STOPPED MANUALLY
INDEPENDENT OF THE ACTION OF RELEVANT PROTECTION UNDER FOLLOWING SITUATIONS.
• 1) SUDDEN APPEARANCE OF EXCESSIVE VIBRATIONS.
• 2) WATER HAMMER.
• 3) OIL CATCHING FIRE.
• 4)EMERGENCY GOVERNOR OUT OF ORDER.
• 5) OIL TEMP. RAISING AT 750C AT THE DRAIN FROM BRG.
6)DROP OF OIL LEVEL IN THE TANK BELOW THE LOWEST PERMISSIBLE VALUE.
• 7)APPEARANCE OF METALLIC NOISE IN THE TURBINE STEAM FLOW PATH.
8) AXIAL SHIFT OF THE ROTOR AT THE THRUST BRG. REACHING 1.2 MM IN THE DIRECTION OF GEN. OR 1.7MM IN THE DIRECTION OF THE FRONT BRG.
• 9)ACCIDENTAL VACUUM DROP TO NON PERMISSIBLE VALUES.
• 10) INCREASE TO LIVE OR REHEAT STEAM TEMP. TO 5650C PRESSURE HAS DROPPED TO 0.5Kg/cm2
• 11) LUBRICATION OIL GAUGE AND LUBRICATION PUMP HAS FAILED TO START.
• 12) ) LUBRICATION OIL PRESSURE HAS • DROPPED TO 0.3Kg/cm2 GAUGE
• 13) INCREASE IN SPEED BEYOND 3360 RPM.
• 14)SHARP FALL IN THE MAIN STEAM TEMP. FROM NOMINAL VALUE TO 450degC
• 15) ESV , IV OR CONTROL VALVES SEIZED.
• 16)VERY HIGH BOILER DRUM LEVEL AND • PROTECTION NOT APPEARED.
• 17)VERY VERY HIGH LEVEL IN HP HEATERS AT CONDENSATE SIDE.
• 18) IF OIL FLOW THROUGH ANY BRG. OUTLET STPOOED OR REDUCED TOO MUCH.
• 19)IF THE CHEMICAL VALUES OF FEED , CONDENSATE OR STEAM ARE CHANGED AND NOT UNDER CONTROL.
• 20)IF GS WATER FLOW NOT AVAILABLE AT UNIT SIDE.
• 21) IF INSTRUMENT AIR PRESS. REDUCED TOO MUCH AND NOT IMPROVED IMMEDIATELY.
• 22) IF DIFF. EXP. OF HPT, IPT & LPT ROTORS ARE NOT UNDER PERMISSIBLE VALUES.
PROTECTION OF TURBINE• The following protections are provided for 210
MW turbine set• 1) Tripping on Axial shift High . -1.7 to +1.2 mm.
• 2) Tripping on Very Low Vacuum • in condenser 540 mm of Hg.
• 3) Tripping on Low Lub oil Pressure . < 0.3 Kg/cm2
• 4) Tripping on Low main steam • temperature 4500C
• 5) Tripping on HPT Exhaust Temp. 4500C• 6) Tripping on HP Heaters level • very very high 4250 mm• 7) Tripping on Boiler drum level • very high + 300 mm.• 8) Tripping on Boiler Tripping.
• 9) Tripping on Generator Tripping.• 10) Tripping on Damper tank very low (Emergency)• 11) Tripping on Stator Water conductivity High 18 micro
ohm/cc.• 12) Tripping on Stator water flow low (Emergency) < = 13 m3 /hr.
• 13) Emergency tripping from Local/Mechanical.• 14) Tripping from control desk.• 15) Elecro-Hydaulic Transducer
protection• 16) Overspeed Protection . 16 %.
Axial Shift Protection :
This protection acts in the event of “High Axial Shift” of rotor caused by the unacceptable wear of the axial bearing pads which results in inadmissible displacement of the rotor with respect to the thrust bearings. This condition is very dangerous which results in rubbing of rotating parts against stationary parts of theturbine. The trip points correspond to 1.2 mm thrust--bearing wear from either side. Thetrip signal is initiated from axial shift measuring unit.
Very Low Vacuum in CondenserThis protection acts in the event of vacuum fall in the condenser to low set point no. 2 (540 mm Hgc). There is a pretrip alarm at vacuum fall to set point no. 1 (650 mm Hgc). Both trip and pretrip signals are initiated from a metallic below type vacuum switch. Very low vacuum in the condenser (does not allow full steam from turbine to enter the condenser when turbine trips). Also this cause temp rise in the steam
at the end of expansion stages and this may cause misalignment and damage at the exhaust end of the turbine and also possibly buckling of condenser tubes.Loss of vacuum is usually caused due to failure in the supply of condenser cooling water or malfunctioning of condenser air ejector system. Main reasons are inefficient cooling tower, air ingress through glands, scaling on condenser tubes from inside and outside.
Lub Oil Pressure Very Low• This protection acts in case the lub oil pressure • fall to (0.3 Kg/cm2). There is a pretrip alarm at • pressure fall to low set (0.6 Kg/cm2). Both trip and• pretrip signals initiated from two metallic bellow • type pressure switches connected in parallel. • Low pressure of the lub. oil results in damaging• the rotating parts in the turbine as well as outside• the turbine like pedestal bearing at• slip ring end of the generator. Lubrication with • sufficient pressure and purity of oil is the• initial requirement of the turbo generator set. • The DC motor driven emergency pump is• used as a final back up to supply oil.
Main steam temp. Low Protection• This protection acts in case the main steam• temp. in any of the emergency stop valves
decreases to low set point (4500 C) from• (5200 C) suddenly. The sudden drop of temp. causes
mismatch between turbine casing and rotor temp. resulting in high eccentricity and diff. expansion.
• The high diff. expansion of the turbine rotor relative to the turbine cylinder can damage the
• turbine seals due to rubbing.
HPT Exhaust Temp. High protection
• This protection acts in case the exhaust • steam temp. reaches to 4500 C. Sensing • of the temp. is taken through the temp. • indicator in case of failure of NRV in • CRH line the HP turbine temp. may • shoot up beyond permissible working • limit because of reverse steam flow.
Hp Heaters Level Very Very High Protection
• For initiating alarm and protection actions in the event of • abnormal levels in any of the hp heaters• This protection acts in case the level goes to high set point • which isolates HP heaters and further goes to isolate the • HP heaters from service, in addition to tripping • of the turbine.
• Heater tube leakage causes HP heater level high, results in admission of water into turbine through bleed side from where the steam tapping are taken to HP heaters. High set point - 750 mm of water col.,
• Very high set point - 4250 of water column.
Boiler Drum Level Very High Protection• This protection acts in case the level of water in • boiler drum reaches high. In absence of this• protection, water may enter into the turbine • with steam, while it in rolling condition.
• The provision of boiler drum level indication is • made in control room itself to maintain the • level.
Turbine Tripping on Boiler Tripping
• This protection acts in case the boiler • trips out. 30 sec. delayed tripping is • achieved through timer. It is well • understood that tripping of the boiler • means stopping steam production and • steam supply to the turbine.
Reverse Power Protection• A steam turbine requires protection against • overheating when its steam supply is cut off and • generator runs as motor. Such overheating occurs
• because insufficient steam is passing through the • turbine to carry away the heat that is produced by
• windage loss. When a generator operating in
• parallel with other machines looses its driving force, it remains in synchronism with the system and continues to run as a synchronous motor drawing sufficient power to drive the prime mover.
Damper Tank Level Low Protection
• This protection acts (with 10 sec.-delayed action) in case the oil level in damper tank decreases below low set point.
• The oil from this tank serves for sealing, low level of which is not permitted. Sealing with low pressure of oil results in leakage of hydrogen from seals
Stator Water Conductivity High Protection
• This protection acts in case the• conductivity of stator water • required for its cooling goes high.• In this case the stator• windings may be earthed through • water due to low resistance.
Stator Water Flow Low Protection :• This protection acts in case the flow • of stator water used for cooling of • the stator goes low. The insufficient • flow of water does not serve the • purpose of cooling water of stator• winding (13 m3/h r.) insulation • gets damaged.
Emergency Tripping from Local
• This protection acts in case the push button of local for tripping is pressed by hand.
• This is the emergency mechanical protection provided at local only. In abnormal condition ,irrespective of electrical protections ,turbine may be
• tripped without delay.
Emergency Tripping from Remote
• This facility is provided on control desk itself.
• By pressing this push button in • emergency the “Turbine Trip • Solenoid”gets energized and• turbine trips with ESV & IV closing.
Electro Hydraulic Transducers
• Electro hydraulic transducer is intended for • anticipatory closing of control valves in case the• total loss of external load and thus reduces the• transit overspeed peak. GCB may get open due to• its own fault, results in overspeeding of the • turbine.• This interlock is automatically introduced• when ESV & IV are open and GCB is closed
Overspeed Protection• This totally mechanical protection provided at 10 • % & 16%. Overspeed of the turbine with • corresponding striker is ON. The inadmissible • overspeed can result in rapture of the rotor as a • result of stress due to centrifugal force.
• Trip initiation takes place at set value of overspeed • with the assistance of mechanical overspeed • governors. • This is a built in feature in governing system.
• Turbine protection means isolation of steam • supply to the turbine. On each and every• elect. protection command goes to energies • turbine trip solenoid,which after energisation• gives command to close ESV,IV and control valves.
Also the “Turbine Lock Out Relay” (TLR)• which picks upon tripping gives command to • close main steam stop valves, short time• opening and again closing of turbine • evacuation valves and closing of a temp. valves etc.
•EMERGENCIES IN TURBINE OPERATION
• 1) VACUUM STARTS DROPPING.
• 2)DEREARATOR LEVEL VERY LOW.
• 3) DEREARATOR LEVEL VERY VERY LOW.
• 4) HOTWELL LEVEL VERY LOW.
• 5) HOTWELL LEVEL VERY VERY LOW.
• 6) ONE CEP TRIPPED & ST/BY DID NOT PICKUP.
• 7) BOTH CEP TRIPPED.
• 8) MC -36 VALVE FAILED TO OPERATE.
• 9) GLAND SEAL STEAM HDR CONTROL VALVE (AS-55) SUDDENLY CLOSED.
• 10) AUX. PRDS/TAS CONROL VALVE FAILED TO OPERATE.
• 11) ONE CW PUMP TRIPPED.
• 12) RUNNING DRIP PUMP TRIPPED & ST/BY DID NOT PICKUP.
• 13) SPEEDER GEAR NOT OPERATING FROM PCR.
• 14) SUDDENLY OPENED ONE OF THE LP BYPASS VALVE WHILE T/G SET ON LOAD.
GENERTOR PROTOCTION• 1. Generator differential protection C.T.• 2. Gen. Stator earth fault protection bus duct C.T.• 3. Gen. Intertern protection CB CT neutralaguide.• 4. Gen. Negative phase sequence protection –C.T.
bus duct.• 5. Gen. Overload protection C.T. in bus duct.• 6. Gen. Over voltage protection – voltage P.T.• 7. Gen. Field failure protection (CT and PT bus
duct)• 8. Gen. Minimum inpedance back-up protection
(CT&PT) bus duct.
• 9. Gen. Reverse power protection – C.T. & PT (LV side)
• 10. Gen. Low forward power protection –CT&PT (LV side)
• 11. Local breaker back-up protection - HV side CT• 12. Gen. Rotor earth fault protection• 13. Gen under frequency protection – voltage relay.• 14. Gen. Over frequency protection - voltage relay.
Generator transformer protection
• 1. G.T. overall differential protection.
• 2. G.T. overfluxing protection –V/f = voltage
• relay bus duct LV & GT
• 3. G.T. restricted earth fault protection –HV side CT• 4. G.T. back-up over current protection – HV side.
Unit auxiliary transformer protection
• 1. UAT differential protection.
• 2. UAT back-up overcurrent protection.
• 3. UAT earth fault only annunciation.
Faults involving stator Winding (Protection of Stator)
• A) Phase to phase short circuit.• B) Phase to Ground short circuit.• C) Short circuits between turns.• D) Open circuits in stator winding.• E) Overheating of the stator.
Faults involving Gen. Rotors (Protection of rotor)
• A) Short circuit in rotor Windings.• B) Open circuit in Rotor winding.• C) Grounds on the Filed circuit.• D) Over-heating.
• 1) STATOR WATER PUMP TRIPPED AND • ST/BY PUMP DID NOT PICK UP.• 2) H2 BOOSTER PUMP TRIPPED AND • ST/BY PUMP DID NOT PICK UP.• 3) H2/SEAL OIL DP HIGH/ LOW.• 4) DAMPER TANK LEVEL HIGH/LOW
• 5) HADRAULIC SEAL OIL TANK LEVEL • HIGH/LOW• 6) GENERATOR H2 LEAKAGE• 7)DM/GS COOLING WATER PUMP TRIPPED• 8) EXPANSION TANK LEVEL HIGH/LOW.• 9)STATOR WATER CONDUCTIVITY HIGH• 10) RECTIFIER TRANSFORMER TEMP HIGH.
• 11) BRG. OIL TEMPERATURE MORE THAN 650 C
• 12SEAL BABBIT TEMP. MORE THAN 75 0C• 13) UNDER FREQUENCY ALARM• 14)GEN. OVERVOLTAGE ALARM.• 15)FAULT IN THYRISTER BLOCK.• 16)GEN. VOLTAGE AUTO CHANNEL
• 17) UAT WINDING TEMP.HIGH.• 18)GEN. WINDING TEMP.HIGH.• 19) UAT OIL TEMP.HIGH• 20)ROTOR EARTH FAULT ALARM• 21)COLD GAS TEMP.HIGH/LOW.• 22) HOT GAS TEMP.HIGH/LOW.
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