KNOWLEDGE MANAGEMENT SERIES:
“LOW VACUUM IN STEAM TURBINE” 1.
INTRODUCTION
ction 2.1.1. A ction
Improvement in Power Plants operating performance is the continuing goal of Electricity generating companies to achieve lowest cost of generation. Survey of Indian Thermal Power Stations revealed that “Low Vacuum Vacuum”” is one of the major major reason for poor heat rate of the machine. Maintaining the design vacuum in the condenser is, therefore, vital for achieving an optimum heat rate/ efciency in a Thermal Power Plant. If existing Indian Thermal Power Stations run with designed vacuum, not only results in saving of money for power generation companies but also results in signicant reduction in CO2 emission in the environment and thus provides better future for our coming generation.
2.
MAIN REASONS OF POOR VACUUM AND ACTION TO BE TAKEN BY O&M ENGINEERS
2.1.
Low CW Flow in Condenser Tubes
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Poor priming boxes
Partial opening of inlet/outlet of valves of condenser CW system
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Passing of CW inlet/outlet water box partition plate
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Problem related with CW system
Low CW discharge pressure
High resistance in CW inlet/ outlet pipelines due to dirtiness
Erosion of CW pump impeller
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Proper commissioning of priming system to ensure that there is no air lock in CW water box.
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Ensure all valves in CW line are fully open.
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Thermal scanning of CW inlet/outlet water box to rule out passing of CW inlet/outlet water box through partition plate.
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Ensure CW motor running at rated current and voltage.
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Inspection of Sacricial anode during overhauling.
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Ensure proper cleaning of CW inlet/ outlet pipelines and CW inlet ducts during initial commissioning.
2.2.
Deposits in the Condenser Tubes
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Chocking of condenser tubes
of Cooling Coolin g Water
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Improper functioning of Traveling water screens
Non-commissioning of Condenser On load Tube Cleaning System (COLTS)
Poor Quality of Cooling Water (mud/slime/algae/sewage (mud/slime/algae/sewa ge etc.)
In closed cycle it will results in salt deposits in condenser tubes in short duration of operation (if proper care is not taken wrt Chemical Dozing, Blow down in Cooling Tower) 27
In open cycle, Improper functioning of Traveling water screens,
High salt level/turbidity in cooling water (Normally observed in closed cooling water circuit)
LP front and rear gland expansion bellows
LP casing Parting Plane
Bursting diaphragms area
LP front & rear diffuser vertical gasket joints
Improper functioning of debris lters
Oil/Grease deposit in condenser tubes
Gland Steam leak off valve and its surrounding area
Extraction-1/2 pipe line from condenser to LP heater-1/2
Condenser neck welding area
LP by pass (both side) control valve and its downstream pipeline up to condenser
Turbine/HP/LP ash boxes and its bellows
Periodic maintenance of traveling water screen/debris lters.
CEP suction line area
Hot well/LP heaters stand pipes
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Ensure adequate blow down, chemical dozing in cooling tower system.
Condenser man hole joints
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Ensure commissioning of COLTS prior to rst synchronization.
Vacuum pump suction line valves ange joints
Boiler Feed Pump Drive Turbine (BFPDT) exhaust ange (applicable only for 500 MW, KWU machines, where BFPDT is provided)
Pressure/temperature tapping points provided for PG test in vacuum system
Plug point for PG test thermo well/ spare points
2.2.1 A ction l
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Clean the condenser tubes by “High Pressure Bullet Cleaning/Chemical Cleaning” prior to conductance of PG Test in new machines.
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Periodic cleaning of the condenser tubes/water boxes.
2.3.
High CW inlet temperature
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Improper Tower
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High CW inlet temperature due to summer season.
functioning
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Cooling
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2.3.1 A ction l
2.4. l
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Periodic maintenance of cooling tower for its proper functioning.
2.4.1. A ction
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LP front and rear glands
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Measure air ingress with the help of installed rotameter.
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Conduct Condenser Water Fill Test for points below LP blade tip and ensure that there is no fall in water level in 24 hrs.
High Air Ingress in Condenser Probable locations for air ingress in vacuum system:
Air ingress from turbine glands due to low gland steam header pressure
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Steam Pressurization Test for points which are not covered by condenser water ll test.
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Helium Leak Detection test for all points during operation of the machine.
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2.5.
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Ensure proper functioning of Gland Seal Steam System. Ensure water is lled in vacuum breaker, GSC siphon, and Main ejector siphons. Ensure glands of the valves operating in vacuum system are properly sealed by CEP water.
High
temperature of inlet air coming from condenser
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Ensure shut off vacuum of each ejector/ vacuum pump is as per design value.
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Ensure proper functioning of seal water heat exchanger, in case of vacuum pump.
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Periodic maintenance of vacuum pump/main ejector during planned overhaul.
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Incase of high temperature of inlet air to vacuum pump/main ejector, inspect air suction line coming from sub cooling zone of condenser for any puncture/leakage.
2.6.
Excessive Thermal Condenser
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Passing of high temperature drains
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Non availability of CEP spray to Flash box/tank
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Insufcient CEP spray during steam dumping operation
Ensure hotwell level is normal and hotwell level controller is on Auto.
Poor Prformance of the Vacuum Pulling Device (Poor shut off Vacuum)
to be
In case of Vacuum pumps:
Loading
of
High seal water inlet temperature
Low seal water inlet pressure
Low seal water inlet ow
Improper functioning of heat exchanger provided for cooling of seal water
2.6.1. A ction
Leaking of Vacuum pump glands
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High temperature of inlet air coming from condenser
Passing of high pressure drains can easily be detected by measuring temperature across drain valves with the help of laser thermometer.
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Maintenance of LP bypass/CEP spray system and Inspection of Steam throw device during overhauling.
3.
CONCLUSION
In case of Main Ejectors:
Erosion of nozzle/diffuser
Failure of gaskets
inter-stage
sealing
Deviation of inlet parameters to ejectors
Insufcient condensate water
High
condensate temperature
inlet
steam
water
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Improvement of vacuum in thermal power station will increase the prot of the power generating companies and hence will help in nation’s growth in a big way. It has been experienced that most of the low vacuum 29
problems at site are related to High air ingress, Condenser tube chocking, and Poor performance of vacuum pulling device. Hence, if all the care mentioned in this write up is taken care of, the performance of the machine can be enhanced with improvement in vacuum.
KWU Steam Turbine by Steam Pressurization by Sh. N.K.Bansal, Sh. A.K.Khan, Sh. N.S.Tomar (Issue : September 2000) l
Descaling of Stainless steel tubes in condensers by sulphamic acid – An experience by Sh. W.R.Aswar (Issue : September 2000)
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On load acid cleaning of Condenser – at Khaperkheda TPS by Sh. Anil Dattaray Palamwar , Bhalchandra Vinayak Gokhale, Hemchandra Dharmadas Wasnik (Issue: July 2004)
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Vacuum leak detection in KWU Steam Turbine by Steam Pressurization of Condenser – A Feedback by Sh. M.K. Sharma, H.R.Singh (Issue: September 2000)
REFERENCE Articles published in FEEDBACK Journal: l
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Innovative use of a new technology to solve low vacuum problem in a 210 MW set by Sh. N.K.Bansal & Sh. N.S.Tomar (Issue : March 2005) Sacricial Anode in Condenser Cooling Water box – A Case Study by Sh. N.K.Bansal, Sh. N.S.Tomar (Issue : January 2007) Procedure of Vacuum leak detection in
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BHEL Haridwar supplied 210/250/500 MW KWU Design Steam Turbine O&M Manual: l
Chapter on “Fault Tracing” for Low Vacuum
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