Turbine Protections & Turbine Cycle Emergencies

Operation Philosophy

• Achieving and maintaining the required output of the unit

• Maintaining plant conditions at their optimum efficiency

• Inspection for plant deterioration by studying the operation trends of main equipments and their auxiliaries (condition monitoring). The maintenance logs can be generated from this information.

• Testing of stand-by plant auxiliaries, protective devices, alarms and automatic cut-in feature.

• Changing over of auxiliaries depending on their running hours.

• Generation of shift logs, event logs, post trip logs etc.

• Readiness to respond to abnormal conditions.

When shutting down hot machine or before turbine start up the turbine must be put on turning gear. When a hot turbine is shut down; due to its heavy weight, high temperature and uneven cooling the turbine rotor will have permanent deformation. The main function of turning gear is to promote even cooling of the rotor and cylinders when turbine is shut down. The turning speed may vary from 3 to 200 rpm. The high speed turning gear also enables to bring the turbine off TG with minimum steam admission while rolling the turbine.

Barring (Turning) Gear

When shaft or rotor, revolving in bearings, speeds up to where centrifugal force tending to whip it sideways just balances the elastic stiffness tending to keep it straight, the slightest mechanical builds up a whirling motion, which may be of destructive amplitude. The rotor then is revolving at critical speed. This phenomenon is due to resonance frequency when the rotation speed corresponds to the natural frequencies of lateral vibration of the rotor. The operation engineer should be aware of the critical speed of the machine and ensure that the speed is not held near critical speed and speed rise should be kept uniform without any interruption. A typical critical speed range may be from 900 to 2600 rpm.

Critical Speed

Basic principle: Establishment of normal temperature gradients from inlet to outlet and the free development of all the thermal expansions. To limit the stresses in thick metal sections, the starting process requires controlled heating of main steam leads,valves,steam admission areas, and turbine components. The rate of rise of turbine speed and load is controlled by the heating process to a degree dependent on the temperature of the metal parts. Thermal stresses in the thickest of the metal sections currently in use will be acceptable if the rate change of surface metal temperature are below 150-200 deg C/hour. Turbine stress evaluator checks the wall temperature differentials and controls steam admission.

Soaking of turbine : Depending on the type of the start turbine is soaked at some particular speed for predetermined time interval. During soaking turbine speed is held constant and steam parameters kept steady care being taken to see that soaking speed is not near critical speed. Soaking helps in bringing differential expansion within specified limits and also gives time to check other turbovisory parameters.

Thermal constraints

Turbine protections1. LOW VACUUM

( Hyd trip > 0.3 bar, electrical trip > 0.3bar)

2. HIGH AXIAL SHIFT

( 1.0 mm)

3. OVERSPEEDING OF TURBINE

( Mechanical Trip3330 rpm, electrical trip >3330 rpm)

4. MAIN OIL TANK LEVEL LL( fire protection 2)

( 1030 mm from top)

5. C. F. TANK LEVEL LL( fire protection 1)

( -115 mm )

6. LOW LUBE PRESSURE( A 4.8 bar,T 2.2 bar)

7. CONTROL OIL PRESSURE( T 3.5 bar)

8.TURBINE REMOTE PUSH BUTTON TRIP

9.FIRE PROTECTION 1 & 2(PUSH BUTTON)

10.BOILER TRIP LEADING TO TURBINE TRIP

11.GENERATOR TRIP LEADING TO TURBINE TRIP

Turbine protections

TURBINE ADVISORY TRIP

1. HIGH SHAFT VIBRATION / ECCENTRICITY

( A 120mic,T 200 mic)

2. HIGH BEARING VIBRATION

( A 35 mic,T 45mic)

3. HIGH BEARING METAL TEMPERATURE

( A 90C,T 120C)

4. HIGH DIFFERENTIAL EXPANSION

(HP -3/5mm,IP -2/7mm,LP -3/30mm)

5. HIGH/LOW MAIN STEAM TEMPERATURE

6.HIGH DRUM LEVEL

Turbine Critical Parameters

• Absolute bearing vibrations• Absolute shaft vibrations• Axial shaft position of the rotor• Differential expansion• Absolute expansion• Control valve position

Turbine Protection Logic

Governing & Protection Oils

Oils.JPG

Turbine Protection System

Governing Rack Front View

Turbine Trip Gear

Remote Trip Solenoid Valves

Main Trip Valves

Turbine Control Desk

Turbine Emergencies• Any deviation from specified normal operation

defines the emergency operation condition for a turbine. Emergency occurs due to abrupt change of important parameters to unacceptable values, which may lead to temporary or permanent damage to the equipment.

In case of severe emergency, generally the turbine is tripped abruptly either through lockout relay or manually.  

Following are the emergency conditions under which the set should be tripped irrespective of the protection systems to act.

Emergency Conditions

• Under Frequency (47.5 Hz) Since the turbine design is based on constant

speed operation, there are some limits on low frequency that the set can be operated without damage to some of the components. The operation of low frequency below 6% from the rated frequency (50 Hz) for certain sustained period is not allowed as the natural frequency of LP turbine block falls within this range and may cause high vibration level particularly in the last stage blading. This will cause higher stresses at the root of blades, which may fail.

Emergency Conditions

• Overspeed (10%)The turbine governing system is designed to control the

speed on loss of bulk load (through load rejection relay). However, in case of total isolation of the machine from the grid or failure of speed governor/emergency governor may cause speed rise to dangerous levels of 112%. The maximum speed limit is 3360 rpm. The machine should not be allowed to cross this limit in any case. This can be achieved by manually tripping the turbine by emergency turbine trip push button.

Frequent overspeeding of turbine may lead to failure of LP turbine last stage blading

Emergency Conditions

• Sudden Drop in Steam Temperature Thermal Shock

Sometimes it happens that the boiler output does not match with the turbine output, which causes fall in steam temperature. This temperature mismatch, particularly at the 1st stage of HP turbine, occurs due to too fast load change at the control valves causing decrease in steam temperature at the greatly reduced flow. This sharp drop in steam temperature gives serious thermal shocks. The surface stresses may surpass the yield strength, thereby reducing the life of turbine (fatigue and creep).

Emergency Conditions

• Measures to be taken :• So in case of sharp temperature drop from 535oC to

520oC, start unloading the set by 3 MW/min. and the set should be unloaded to 100 MW corresponding to 480oC. Below this temperature turbine should be tripped immediately.

• To avoid thermal shocks, load or unload the set as per the recommended curves of the manufacturer.

Emergency Conditions

• Avoid priming of superheater in boiler as it sharply drops the temperature.

• Avoid mismatch of temperature beyond recommended limits during rolling the turbine.

• In case of boiler trip, turbine should be tripped immediately.

Increase the excess air percentage to increase the final superheater outlet temperature.

Emergency Conditions

• Total A.C. failureIn case of total power failure of the grid or the station

getting isolated from the grid, the following emergency operations should be carried out on turbine side.

Start DC lube oil pump (EOP) to save the bearings if it is not started on auto.

ii)    Remove main ejectors and gland steam cooler from service and kill the condenser vacuum by opening vacuum breaker valves or air valve of starting ejector. Stop gland sealing when machine comes to rest.

Emergency Conditions

• If the turbine emergency switchgear is charged through the starting of DG set, start AOP and put the machine on turning gear. If hydraulic turning gear cannot be established, rotate the rotor by manual barring.

• See that all the extraction NRVs have closed.• Monitor bearings babbit metal temperature and

bearings drain oil temperature.

Emergency Conditions

• High Axial Shift (+/-1 mm)

The increase in axial shift will lead to overloading of the thrust pads of the thrust bearing and eventual failure. It also contributes to high bearing vibration and abnormal differential expansion. Protection has been incorporated to trip the turbine and break vacuum at a limiting value of + 1 mm of axial shift.

Emergency Conditions

• Abrupt change in load• Sudden drop of steam temperature• Sudden drop of vacuum

• Sudden closure of extraction NRVs or IVS / IPCVs.

• Sudden closure of HP heaters• Lube oil failure to thrust bearing Turbine overload or scaling in the blades

Emergency Conditions• Condenser Vacuum Very Low (0.75bar)

In case of condenser vacuum going low limit 0.75 kg/cm2, hand trip the turbine if not tripped through protection, low vacuum in condenser may be due to following reasons.

Partial or complete loss of cooling water High condenser level Heavy air ingress in condenser• Loss of gland seal steam• Malfunction of air ejectors Fouled condenser tubes i.e. condenser DP high ( >

0.5 kg/cm2 )

Emergency Conditions

• Exhaust Hood Temperature High • High Bearing & Shaft Vibration • High Differential Expansion • Water Hammering • Turbine Trip

Turbine Cycle EmergenciesHot Well level Hi/Lo.

Condensate Extraction Pump suction strainers chocking.

Boiler Feed Pumps suction strainers chocking.

CEP/BFP tripping.

Air Compressors tripping.

Failure of instrumentation air.