Industrial Automation

Steam Turbine Interlocks and Associated Protection Systems

This document outlines critical turbine trip interlocks and protection systems, ensuring turbine and generator safety by initiating immediate shutdowns during severe faults or unsafe conditions.

Turbine Trip Interlocks Overview
  • Description: Critical protection signals sent from the generator to the turbine control system.
  • Function: Both Class A and Class B trips indicate severe or critical faults that require immediate turbine shutdown to prevent damage.
  • Action: Initiates an immediate turbine shutdown to protect the generator and connected systems.
  • Description: A protective measure that cuts off the fuel supply to the turbine.
  • Function: Activated during critical failures or unsafe conditions to prevent continued operation without controlled fuel input.
  • Action: Immediately stops fuel flow, causing the turbine to shut down.
  • Description: A safety feature that activates when the turbine’s speed exceeds predefined limits.
  • Function: Prevents mechanical damage or catastrophic failure due to excessive rotational speed.
  • Thresholds:
    1. FEG (Frequency Governor): Trips at 10% overspeed.
    2. REG (Regulation Governor): Trips at 12% overspeed.
    3. Primary Oil Pressure High: Trips at 15% overspeed when the pressure is 3.05 kg/cm².
  • Action: Rapidly closes steam valves and initiates turbine shutdown.
  • Description: Monitors the pressure of the lubricating oil.
  • Function: Protects bearings and other components from insufficient lubrication.
  • Action: Shuts down the turbine to avoid damage due to low lubrication pressure.
  • Description: Monitors the distributing oil pressure critical for turbine operation.
  • Function: Ensures the turbine is shut down if distributing oil pressure falls below safe levels, protecting critical turbine parts.
  • Action: Initiates turbine shutdown to prevent damage.
  • Description: Monitors the vacuum level in the turbine condenser.
  • Function: Trips the turbine to prevent damage from inadequate vacuum conditions, which can affect turbine efficiency and safety.
  • Action: Shuts down the turbine to maintain operational safety and efficiency.
  • Description: Monitors the axial position of the turbine rotor.
  • Function: Protects the turbine from excessive axial movement that can cause mechanical stress and damage.
  • Thresholds:
    • Hydro Mechanical: Trips at ±0.85 mm.
    • Hydro Mechanical: Trips at ±0.65 mm.
  • Action: Shuts down the turbine if axial movement exceeds safe limits.
  • Description: Monitors the electrical grid frequency.
  • Function: Trips the turbine if the frequency falls below safe operational levels, indicating a potential instability in the power grid.
  • Action: Shuts down the turbine to protect it and the grid from potential damage.
  • Description: A manual emergency shutdown mechanism.
  • Function: Allows operators to quickly shut down the turbine in case of an emergency that automatic systems have not addressed.
  • Action: Instantly closes steam supply and shuts down the turbine.
  • Description: Monitors the level in Low-Pressure Heaters (LPH).
  • Function: Prevents turbine damage from high water levels in the LPH, which could lead to water induction into the turbine.
  • Action: Shuts down the turbine to avoid mechanical damage.
  • Description: Monitors the metal temperature of turbine bearings.
  • Function: Protects bearings from overheating, which can lead to bearing failure.
  • Action: Shuts down the turbine to prevent bearing damage due to high temperatures.
  • Description: Monitors the boiler drum water level to prevent excessive water carryover.
  • Function: Protects the turbine from water induction, which can cause severe mechanical damage.
  • Action: Shuts down the turbine to prevent water from entering the steam path, protecting turbine blades and other components from damage.
Steam Turbine Interlock Scenarios and Management 

In a thermal power plant, turbine interlocks are crucial safety mechanisms designed to protect the turbine and its associated systems from damage due to abnormal operating conditions. They ensure the reliable and safe operation of the turbine by monitoring critical parameters and initiating corrective actions when necessary. Understanding the causes, effects, and appropriate actions for various interlock scenarios is essential for maintaining optimal performance and preventing equipment failure. 

  • Motor protection relay operates.
  • Working oil temperature high.
  • Discharge flow high (480 t/hr).
  • Discharge temperature high (175ºC).
  • Suction pressure low/deaerator level very low.
  • Lubricating oil pressure very low.
  • Motor bearing temperature high.
  • Drum level will start decreasing.
  • Another pump may start automatically if in auto mode.
  • Start another pump manually.
  • Check the operation of recirculation, warm-up, and cooling water valve of the newly started pump.
  • If the previous feed pump tripped due to high feed flow, manually decrease the drum level control valve.
  • Analyze the cause of the pump trip and restore the pump to service.
  • Motor protection activates.
  • Hotwell level very low.
  • Condensate extraction pump discharge pressure very low (not on discharge header pressure).
  • Deaerator level will start falling.
  • Hotwell level will start rising.
  • Condenser vacuum will begin to drop.
  • If no CEP is running, the unit might need to be tripped.
  • Start standby condensate extraction pump manually if it didn’t start automatically.
  • If only one pump is operational, reduce the load accordingly to manage the situation.
  • Simplify the operation to stabilize the system until the primary CEP is restored or the issue is resolved.
  • Expander level very high or very low.
  • Trouble in electrical supply (module trips).
  • If maintaining full load, hot well level will rise.
  • Deaerator level will fall.
  • Take expander level controller on manual if there’s trouble.
  • Attempt to maintain expander level manually.
  • If expander level control is problematic and CBP trips frequently, reduce unit load to maintain deaerator level.
  • Ensure expander drain to hot well and steam to condenser valves are opened whenever CBP trips.
  • Excess makeup to hot well.
  • Faulty deaerator level regulator.
  • Risk of water entering LP turbine gland through steam piping.
  • Risk of water entering PRDS steam piping, potentially lowering steam temperature.
  • Overflow valve may open if deaerator system is on auto.
  • Check operation of deaerator level controller; switch to manual if necessary.
  • Open overflow valve if level is too high.
  • Change over LP gland steam to PRDS (Pressure Reducing and Desuperheating System) if needed.
  • Drain PRDS header and LP gland steam drain.
  • Malfunction of makeup valve.
  • Unauthorized closure of valve between CEP (Condensate Extraction Pump) and deaerator.
  • Boiler tube leakage.
  • CBP not running at load >85 MW.
  • Risk of loss of suction pressure for boiler feed pump.
  • Drum level will decrease.
  • To maintain drum level, unit load must be reduced.
  • Check and adjust makeup valve settings.
  • Open valve between CEP and deaerator if closed improperly.
  • Address boiler tube leakage promptly.
  • If suction pressure remains low despite adjustments, switch off feed pumps and consider tripping the unit.
  • If CBP trips at full load or is not running, reduce load to stabilize deaerator level.
  • Faulty HP heater drain to deaerator level controller or stuck control valve.
  • HP heater drain to LPH valve not opening or operating properly at low load.
  • Deaerator pressure higher than normal.
  • Inadequate extraction pressure to HP heater.
  • Tube failure in HP heater.
  • HP heater drain to HPDFT (High Pressure Drain Feed Tank) will open on high high level (if interlocks are active).
  • HP heater bypass from water and steam sides may activate automatically.
  • Risk of water entering the turbine if HP heater level builds up rapidly.
  • Take HP heater level controller on manual and maintain the level within acceptable limits.
  • Check and manually open HP heater drain to LPH valve if necessary.
  • Adjust deaerator pressure if it’s too high.
  • Adjust steam extraction pressure by opening throttled valves.
  • Manually bypass HP heaters if level is very high and they are not bypassing automatically. 
  • Difficulty in expander level controller leading to high level in LP heater.
  • Level controller valve issues in LP heaters
  • Tube failure in LP heaters.
  • Low extraction pressure in LP heaters.
  • Possibility of water entering turbines if levels are high and uncontrollable.
  • Check and adjust level controllers manually if needed.
  • Adjust extraction pressure by opening throttled valves.
  • Manually bypass LP heaters if levels are high and uncontrollable.
  • Air lock in oil cooler.
  • Motor-driven oil pump (MOP) suction failure.
  • Duplex oil filter clogged.
  • Oil leaks in lines, flanges, or bearings.
  • Excessive oil consumption in generator seal oil system.
  • MOP failure.
  • Rise in bearing metal temperature.
  • Turbine trips on low-low lubricating oil pressure.
  • Automatic start of Standby Oil Pump A (SOP A), Standby Oil Pump B (SOP B), Auxiliary Coolant Engine Oil Pump (ACEOP), and Diesel Coolant Engine Oil Pump (DCEOP) at respective pressures.
  • Check oil pressure and adjust by manipulating valves if possible.
  • Inspect for oil leaks throughout the oil circuit.
  • Start standby oil pumps (SOP) if necessary.
  • Consider tripping the turbine if oil pressure remains low.
  • Check and switch over to the standby oil cooler if the current one is defective. Remove any air locks from the cooler.
  • Regularly inspect and change over to standby oil filters. Clean filters if pressure difference increases. Isolate line filters during shutdowns for cleaning if necessary.
  • Water side valves in oil coolers are open more than required, especially at low load. Control room operator needs to inform temperature controller about load variations.
  • Cold season during startup, where temperature doesn’t increase even if cooling water (CW) valves of the cooler are closed.
  • Unstable oil film in bearings.
  • Potential damage to bearings.
  • Increase in bearing vibration.
  • Throttle the cooling water valves of the oil coolers if necessary.
  • If the machine is off-load, do not roll it until oil temperature reaches an acceptable working value.
  • If the machine is on-load, rapidly raise the oil temperature to an acceptable level.
  • Use the oil centrifuge heater to increase the oil temperature.
  • Failure of cooling water (CW) system.
  • Cooling tower induced draft (CTID) fan/fans trip.
  • High CW inlet temperature.
  • Dirty oil coolers with low heat transfer efficiency.
  • Air lock in oil coolers.
  • Increase in bearing metal temperature.
  • Increase in bearing vibration.
  • Rise in seal oil temperature.
  • Restore the CW system or restart the CTID fan/fans.
  • Reduce the CW inlet temperature. Open CW valves of the coolers if they are throttled.
  • Remove any air locks from the cooler.
  • Put a standby oil cooler in service if available.
  • Isolate and clean the dirty oil cooler if possible.
  • Trip the unit if lubricating oil temperature continues to rise and is uncontrollable.
  • Consider external cooling of the oil cooler to reduce the oil temperature.
  • Motor protection relay operation.
  • auxiliary supply failure.
  • Fall in turbine vacuum.
  • Rise in lubricating oil, seal oil, and hydrogen temperatures.
  • Unit will trip on exhaust pressure high-high if all CW pumps trip.
  • Restore CW pump(s) after checking.
  • Restore auxiliary supply; start pump  if station bus supply and pump are available.
  • Start standby CW pump immediately if available.
  • Reduce load on the machine until the second pump is in service.
  • Trip the unit if vacuum drops significantly.
  • Monitor exhaust load temperature and LP differential expansion, try to control.
  • If no CW pump is in service for a long time, prime the CW tunnel and start a CW pump.
  • Oil leakage from oil lines.
  • Bearing oil collected on laggings reaching the ignition point.
  • Extensive damage to the turbine and surroundings.
  • Address oil leakage promptly.
  • Use dry CO2 powder at the first sign of smoke; inform the fire section.
  • Use CO2 powder cylinders for extensive fire.
  • Trip the turbine if the fire is beyond control.
  • Expel hydrogen if the fire is near the hydrogen zone using CO2.
  • Prevent oil from lodging on laggings; address even small leaks to prevent fire.
  • CW pump trip/CTID fan trip.
  • Loss of gland steam pressure.
  • Defective gland steam pressure regulator.
  • Loss of ejector steam pressure.
  • Improper sealing of valves/glands in the vacuum system.
  • Air leakage in vacuum system or puncture.
  • Malfunction of vacuum breaker valve.
  • Damaged or leaking bursting plate.
  • Stuck deaerator level control valve.
  • Empty surge tank.
  • Slow or rapid vacuum drop.
  • Unit trip on exhaust pressure high-high if vacuum drops rapidly.
  • Overheating of LP turbine casing and exhaust hood.
  • Start standby CW pump if available; rectify and make the tripped pump available.
  • Investigate and start CTID fan(s) if tripped.
  • Check and maintain  header pressure for gland steam.
  • Check deaerator pressure if LP gland steam is fed from it.
  • Maintain ejector steam pressure by adjusting PRDS header pressure.
  • Manually control or bypass the defective gland steam pressure regulator.
  • Ensure proper sealing water to valves/glands in the vacuum system.
  • Thoroughly check for leaks or punctures if vacuum falls suddenly.
  • Ensure vacuum breaker valve is closed during normal operation.
  • Inspect and replace damaged bursting plate; shut down the unit if necessary.
  • Start standby CEP if tripped; rectify issues and maintain load.
  • Address stuck deaerator level control valve; bypass if needed.
  • Change over ejector if the primary one fails to maintain vacuum.
  • Activate second ejector if vacuum falls slowly to maintain it.
  • Trip the unit if vacuum falls rapidly and cannot be maintained.
  • Fill the surge tank if empty.
  • Vacuum falls.
  • CW pump trip.
  • High steam temperature during cold startup.
  • Turbine running at 3000 RPM without synchronizing or at low load for an extended period.
  • LP differential expansion will become negative.
  • Prolonged high exhaust hood temperature may cause vibration.
  • Investigate and address the cause of vacuum fall.
  • Start standby CW pump if available; ensure CW supply is restored.
  • Start a second CW pump if the unit is running at higher loads (>60 MW).
  • Reduce HP drains to condenser hot well.
  • Control steam temperature and pressure during cold start.
  • Synchronize and load the machine quickly; charge LP heaters at 25 MW load.
  • Control gland steam pressure and temperature.
  • Start makeup water spray in the condenser; initiate makeup from TOP.
  • Keep vacuum as high as possible with both ejectors in service.
  • Maintain steam temperature and pressure within limits.
  • High lubricating oil inlet temperature.
  • Excessive bearing vibration.
  • Bearing failure.
  • Contaminated lubricating oil.
  • Inadequate oil flow or low pressure.
  • Bearing failure.
  • Increased bearing vibration.
  • Maintain oil temperature, pressure, and flow.
  • Check and change duplex and line filters if necessary.
  • Periodically check for oil contamination and moisture.
  • Regularly drain collected water from the main oil tank (MOT).
  • Monitor oil cooler outlet temperature.
  • Abrupt load changes.
  • Sudden drop in steam temperature or vacuum.
  • Lubricating oil failure to thrust bearing.
  • Salt/silica deposition in turbine.
  • Worn out thrust pads.
  • Overloading of thrust pads.
  • Increased turbine vibration.
  • Abnormal differential expansion.
  • Control boiler parameters and sudden changes.
  • Maintain turbine vacuum.
  • Reduce unit load.
  • Check flow, pressure, and temperature through bearings.
  • Shutdown unit for turbine washing if salt deposits are present.
  • If axial shift is uncontrollable, trip the machine and bring it to standstill condition.
  • Rotor deflection.
  • Failure to use bearing gear after tripping.
  • Improper heating or draining during startup.
  • Abrupt changes in main steam temperature.
  • Malfunction of eccentricity pickup.
  • Increased turbine vibration.
  • Unusual noise from turbine.
  • Increased bearing oil and metal temperature.
  • Increased power draw if on barring gear.
  • Follow startup procedures strictly.
  • Maintain steam parameters and proper heating/soaking times.
  • Drain steam lines and casing properly.
  • Check and calibrate eccentricity pickup.
  • If eccentricity is high on barring gear, run the machine at 500 RPM for longer to even out rotor.
  • Reduce unit load if vibration increases, and stop machine for checks if necessary.
  • Maintain turbine vacuum and check bearing flow parameters.
  • High steam temperature.
  • Poor insulation.
  • Salt deposition in casing.
  • High gland steam temperature.
  • Fast rolling or loading.
  • Inadequate soaking period.
  • High condenser vacuum.
  • Seal rub and metallic rubbing sounds.
  • Increased turbine vibration.
  • Use flange heating in HP and MP turbines.
  • Ensure proper soaking of turbine.
  • Maintain steam temperature as per startup diagram.
  • Load turbine gradually.
  • Check insulation and attend to it if poor.
  • Remove salt deposits by washing if needed.
  • Control boiler parameters; if tripped, drop vacuum immediately.
  • Adjust vacuum to acceptable limits if LP differential expansion increases.
  • Prolonged rolling or loading time.
  • High exhaust hood temperature.
  • Low condenser vacuum.
  • Passing flange heating valves (HP and MP).
  • Seal rub and metallic rubbing sounds.
  • Increased vibration.
  • Follow startup curve during hot startup.
  • Start second ejector to improve vacuum.
  • Reduce exhaust hood temperature.
  • Open vacuum breaker if tripped due to excessive negative differential expansion.
  • Increase boiler steam temperature and load machine faster.
  • Control boiler parameters if rolling has started.
  • Check and fix passing flange heating valves.
  • Turbine passing through critical speed.
  • Rapid changes in turbine inlet steam temperature.
  • Low lubricating oil temperature.
  • Vacuum below desired value.
  • Unbalanced steam flow.
  • Incorrect shaft alignment.
  • Improper gland steam pressure and temperature.
  • Rotor mechanical imbalance.
  • High eccentricity.
  • Uneven expansion.
  • Overspeed.
  • Increased eccentricity.
  • Potential machine damage if vibration exceeds safe limits.
  • Roll machine smoothly, avoiding critical speed.
  • Maintain steady steam parameters.
  • Ensure proper lubricating oil pressure (>1 kg/cm²) and temperature (42ºC).
  • Maintain turbine vacuum.
  • Monitor gland steam temperature (130-150ºC) and pressure.
  • Ensure balanced steam flow through HP/LP valves.
  • Regularly check HP and MP pedestal expansion.
  • Control differential expansion; use flange heating if needed.
  • Control high eccentricity.
  • Reduce unit load if bearing vibrations remain high.
  • Balance the rotor if necessary.
  • Blade deposition.
  • High load with HP heaters out of service.
  • Machine overload.
  • Inability to reach full load.
  • Increased axial shift.
  • Potential thrust pad damage.
  • Clean rotor by water washing.
  • Adjust unit load based on curtish stage pressure.
  • Put HP heater in service or reduce unit load if unavailable.
  • Barring gear motor trip or fuse blow.
  • Unsatisfied start permissives.
  • Barring gear motor shaft failure.
  • Shaft may start to hog.
  • Restore barring gear motor supply quickly.
  • Manually rotate the shaft as specified.
  • Continue barring for a minimum of 16 hours.
  • Rotate the shaft by 180° every half hour for 8 hours, then every hour until the MP top casing temperature reaches 190ºC.
  • Tube corrosion or erosion.
  • High condenser inlet CW pressure.
  • Low CW flow through condenser tubes.
  • Increased condensate conductivity.
  • Lower boiler pH and increased water conductivity.
  • Isolate one side of the condenser to check conductivity.
  • Reduce turbine load by 50%.
  • Repair leakage and check conductivity after restoration.
  • Mechanical fault in valve.
  • Oil leakage from servomotor piston.
  • Reduced generator load.
  • Uneven flow in reheater/superheater.
  • Potential damage to superheater/reheater tubes.
  • Unit trip due to disturbance.
  • Uneven turbine thrust.
  • Try to reopen valve by pressing solenoid plunger after reducing load.
  • Ensure HP/LP QCA valve openings while rolling.
  • High drum level or sudden drop in steam temperature.
  • Reheater spray commissioning.
  • High levels or tube leakage in GSC/LPH/HPH.
  • Extraction pump loss.
  • Valve chest distortion, thermal shocks, and joint leaks.
  • MP cylinder chilling and MP rotor damage.
  • Increased vibration, differential expansion, and axial shift.
  • Rotor blade and diaphragm damage.
  • Maintain normal drum level.
  • Gradually increase steam temperature; open drains.
  • Trip machine if temperature variation is uncontrollable.
  • High H2 leakage.
  • Improper H2 seal oil system.
  • Lower H2 temperature.
  • Defective instrument.
  • Low H2 pressure alarm.
  • Potential for fire if heavy H2 leakage is neglected.
  • Increase H2 pressure to 2.1 kg/cm².
  • Check and attend to gas leaks and seal oil system.
  • Switch to low range and reduce load if necessary.
  • If H2 pressure remains low, expel H2 with CO2 and switch to air cooling.
  • Stop machine if H2 pressure drops below 150 mm WC.
  • Excessive H2 injection.
  • Internal generator fault.
  • H2 CBP not in service.
  • High-pressure alarm.
  • Potential H2 leakage.
  • Lower pressure until alarm resets.
  • Trip machine if internal generator fault causes H2 heating.
  • High H2 leakage.
  • Poor-quality H2 injection.
  • Risk of H2 explosion if purity falls below acceptable limit (92%).
  • Low H2 purity alarm.
  • Improve H2 purity and pressure.
  • Replace H2 with pure H2.
  • Shut down unit and expel H2 with CO2 if purity continues to fall.
  • Improper H2 dryer regeneration.
  • H2 cooler leakage.
  • Permitted moisture condensation.
  • Water/oil in generator alarm.
  • Risk of electrical fault or explosion if water/oil level is unsafe.
  • Regularly drain water/oil from generator.
  • Ensure satisfactory H2 dryer/heater operation.
  • Check H2 coolers for leakage.
  • Verify proper vacuum pump operation.
  • BCW pump trip.
  • Heavy BCW system leakage.
  • Auxiliary supply system failure.
  • Low BCW flow or total failure.
  • BFP motor winding temperature increase.
  • A.C. plant trip.
  • Possible shutdown of auxiliaries fed by BCW system.
  • Start second BCW pump if available.
  • Activate emergency BCW line if both pumps fail.
  • Trip unit if BCW cannot be restored quickly.
  • Shut down unnecessary auxiliaries immediately.
  • Sudden increase in air consumption.
  • Line leakage.
  • Control compressor trip.
  • 415V auxiliary supply failure.
  • Auto controls using air will become inoperative.
  • Burner tilt slips down to minimum position.
  • BFP recirculating valves open, potentially tripping BFP on feed flow high.
  • Ash handling inoperable.
  • Manual maintenance of boiler drum, deaerator, and heater levels.
  • Standby feed pump warm-up valve closes.
  • Potential unit trip due to disturbances.
  • Lock burner tilt at horizontal position.
  • Restore control compressors or use station air if available.
  • Isolate and repair leaky air lines.
  • Restart unit after corrective actions for instrument air issues.

Sundareswaran Iyalunaidu

With over 24 years of dedicated experience, I am a seasoned professional specializing in the commissioning, maintenance, and installation of Electrical, Instrumentation and Control systems. My expertise extends across a spectrum of industries, including Power stations, Oil and Gas, Aluminium, Utilities, Steel and Continuous process industries. Tweet me @sundareshinfohe

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