Overspeed : Oil Injection & Actual

PROCEDURE FOR PERFORMING OVERSPEED TRIP OIL INJECTION TEST FROM LOCATION:

STEP-1: Press the RHS knob and ensure build up of startup oil pressure (>8.5 ksc), Note down and release the lever.

STEP-2: Press the LHS Knob and hold it in the pressed condition till completion of test. Not down Aux trip fluid pressure (>5.0 ksc).

STEP-3: Rotate the testing device hand wheel in clock wise direction to build test oil pressure.

STEP-4: Note down the test oil pressure at which the Over speed trip device strikers act, Aux trip fluid pressure (0 ksc) and primary oil pressure.

STEP-5: Rotate the testing device hand wheel in anti clock wise direction to reduce test oil pressure till 0 ksc.

STEP-6: Press the RHS knob to buildup startup oil pressure and reset the trip device and then release it after ensure resetting of both strikers.

STEP-7: Release the LHS knob and note down Aux. trip fluid pressure (>8.5 ksc). Resp: UCE

PROCEDURE FOR ACTUAL OVERSPEED TEST-MAIN TURBINE

6.1: PRECAUTIONS

6.1.1 The persons carrying out the test must be aware of the maximum speed at which the prime mover may be run and the need to limit over speeding to the minimum duration.

6.1.2 Before carrying out an over speed test on a turbine generator, precautions must be taken to ensure that the field breaker is open and the machine is in de-excited condition. Resp: UCE

6.1.3 Check that the healthiness of the overspeed trip device has been ensured earlier by performing oil injection test within a period of 30 days. Resp: UCE

6.1.4 Ensure that oil injection test (onload testing equipment) was not carried out immediately before the actual overspeed test to avoid freeing of the gear giving an artificial result. Resp: UCE

6.1.5 Before carrying out any overspeed test, precautions must be taken to limit the supply of steam or fuel to the minimum necessary machine should be warmed up sufficiently so that casing expansion are at least 80% of full load expansion. Expansion readings are taken and logged. Resp: UCE

6.1.6 During an overspeed test, two independent speed indicators are desirable and both must be watched for any sign of malfunction. Resp: C&I

6.1.7 The turbine desk should be manned and the turbine should be hand tripped above 3330 RPM. Resp: UCE

6.1.8 The manual emergency trip must be manned by a person who can see clearly the speed achieved and who will trip the prime mover if the maximum permissible speed is exceeded or it the prime mover shows signs of distress. This person must be in direct communication with the person controlling the speed of the prime mover. Resp: UCE

6.2: PROCEDURE FOR DOING ACTUAL OVERSPEED TEST DURING SHUTDOWN:

6.2.1: During load reduction take the turbine control to hydraulic by reducing speeder gear at about 50MW in Stage-I and 150 MW in Stage-II and close the EHC isolation valves. (Applicable to stage-I only: Changeover 6.6 KV Unit bus supply to Station Supply). Resp: UCE

6.2.2: Reduce boiler firing and load gradually. Resp: UCE

6.2.3: Charge HP/LP bypass and reduce throttle pressure to 50 ksc, while avoiding steep drop in temperature. Resp: UCE

6.2.4: Reduce load further after bringing the boiler load to minimum with 02 consecutive mills in service preferably C & D with 01 elevation of oil guns. Resp: UCE

6.2.5: Reduce the load to zero MW and then desynchronize the machine, open field breaker. Resp: UCE

6.2.6: Cross check the speed measurement with an offline digital instrument. Resp: MTP

6.2.7: Increase electrical over speed protection set point to 3330 rpm. Resp: C&I

6.2.8: Bring turbine speed to 3000 RPM by operating speeder gear / Starting device. Further raise the speed and ensure maximum speed reached with both Starting device and Speeder gear at 100% is around 3210 rpm. Resp: UCE

6.2.9: After ensuring that all the precautions are met, raise the turbine speed from local by rotating the over speeding knob. Resp: UCE

6.2.10: Note the speed at which the turbine trips, along with the corresponding primary oil pressure and record it. Resp: UCE

6.2.11: Normalise Electrical overspeed setpoint. Resp: UCE/C&I

NOTE: The maximum permissible speed at which the test is to be carried out should be decided before doing the test, and informed to all persons involved in the test.

6.3: PROCEDURE FOR DOING ACTUAL OVERSPEED TEST DURING STARTUP AFTER OVERHAUL:

6.3.1 Before the turbine is rolled, its Stop valves operation, Governing characteristics, CRH NRV operation and Extraction lines FCNRVs operation must be checked for smooth operation over the maximum travel practicable in such circumstances. Resp: Re-commissioning Incharge / TMD

6.3.2 Before the turbine is rolled its governor should be set to commence controlling the speed at the lowest speed possible. Operation of the governor, control gear and all valves must be observed for signs of sticking or other malfunction. Resp: Re-commissioning Incharge / C&I / TMD

6.3.3: Before rolling the turbine Turbine tripping (Closure of Stop valves) must be ensured through Remote HMI trip, Local Governing rack trip lever and also through electrical over speed protection by simulation as part of Protection & Interlock checking, Protocol must be signed. Resp: Re-commissioning Incharge

6.3.4: When the turbine generator is being returned to service, the steam tightness of the Steam Control Valves must be proved by ensuring the turbine does not accelerate from the barring gear speed when the Turbine stop valves are opened and control valves are in closed condition. Resp: Re-commissioning Incharge /TMD

6.3.5: During first run-up after overhaul, Shaft speed should be measured with a digital Tachometer and speed indicator readings compared with Tachometer reading. Resp: Re-commissioning Incharge / MTP

6.3.6: After the turbine has been rolled to 3000 rpm, perform oil injection test (preferably before electrical testing of generator) and ensure both strikers act, record Primary oil pressure and Test oil pressure at which the strikers act. Resp: Re-commissioning Incharge Resp: Re-commissioning Incharge

6.3.7: Allow the overall expansion to reach 80% of full load expansion. During the course of electrical testing, the steam temperatures may be increased gradually to achieve the expansions. Resp: Re-commissioning Incharge

6.3.8: Cross check the speed measurement with an offline digital instrument. Resp: MTP

6.3.9: Increase electrical over speed protection set point to 3330 rpm. Resp: Re-commissioning Incharge / C&I

6.3.10: Bring turbine speed to 3000 RPM by operating speeder gear / Starting device. Further raise the speed and ensure maximum speed reached with both Starting device and Speeder gear at 100% is around 3210 rpm. Resp: Re-commissioning Incharge / TMD

6.3.11: After ensuring that all the precautions are met, raise the turbine speed from local by rotating the over speeding knob. Resp: Re-commissioning Incharge / TMD

6.3.12: Note the speed at which the turbine trips, along with the corresponding primary oil pressure and record it. Resp: Re-commissioning Incharge

NOTE: The maximum permissible speed at which the test is to be carried out should be decided before doing the test, and informed to all persons involved in the test.

7.0: PROCEDURE FOR ACTUAL OVERSPEED TEST OF TDBFP DRIVE TURBINE

7.1 Oil injection test to be done before stopping TDBFP as per the procedure given below:

Step-1: Open control oil supply v/v to test oil generating device.

Step-2: Push & hold lever of over speed test device downwards completely. This will ensure stop v/v in open condition.

Step-3: Holding lever downward, rotate hand wheel slowly in clockwise direction to generate/increase test oil. Note test oil pressure at which over speed protection device operates.

Step-4: Rotate hand wheel anticlockwise completely to reduce test oil pressure.

Step-5: Energise Remote trip engage to reset the device. Release the lever and De-energise Remote trip engage after device resets.

Step-6: Close control oil supply v/v to test oil generating device.

7.2 Record test oil & speed in protect check register. If oil injection test is not satisfactory, reason is to be investigated. If speed cannot be raised to 5400 rpm (approximately) due to low load conditions, then at least try to ensure buildup of test oil pressure. Resp: UCE/ TMD

Preparation for Overspeed test:

7.3 TDBFP is to be stopped as per shutdown procedure & kept on barring. TDBFP to be isolated from steam side. Both vacuum p/p can be taken in service if required. Resp: UCE

7.4 When turbine casing temperature reaches less than 120 degree, stop barring of TDBFP .TDBFP is to be isolated from oil side and permit to be issued for decoupling of turbine from main pump & gearbox (BP) side. Resp: UCE /TMD

7.5 After decoupling of turbine, charge oil system & put TDBFP on barring. Adjust turning gear valve to maintain barring speed 500 rpm (approx). Resp: UCE /TMD

7.6 TDBFP governing characteristic to be checked. Resp: TG desk engr/TMD/C&I

7.7 Ensure closing of control valves before opening ESV. Resp: TG desk engr

Procedure for Overspeed test:

7.8 Ensure tripping of turbine (closure of stop valves) through Local trip gear, Remote HMI trip and electrical overspeed protection (by reducing setpoint below barring speed). Resp: TG desk engr/C&I

7.9 Charge TDBFP from steam side with aux PRDS v/v as per procedure. Charge Extraction source also if available (i.e If Main turbine is in service).Resp: TG desk engr.

7.10 During speed rising, close monitoring is required for control valve opening, if control valve is not opening as per speed control output, trip BFP and investigate reasons. Resp: UCE/TMD

7.11 Roll TDBFP turbine as per the allowed rate and soak at 2000, 4000 and 5400 rpm for 30 min each. Offline speed to be measured by MTP at above speeds. Turbovisory parameters also to be observed. Soak the turbine at this speed till casing temperature reaches 200 deg C. Resp: TG desk engr /MTP

7.12 Carry out Oil Injection test three times to ensure the healthiness of over speed trip device and consistency in test oil pressure at which bolt acts is to be established. Procedure for oil injection test given in 7.1 to be followed. Resp: UCE/MTP/TMD

7.13 Turbine electrical protection set point to be raised 6100 from 6000 rpm & Speed reference blocking to be increased to 6100 rpm from 5800rpm. “OS Test” Push button to be pressed (Turns green) to remove blocking of Nrtd at 5800 rpm. Resp:UCE/ C&I

7.14 Before going for actual over speeding ensure Remote trip engage is in De-energised condition as this will not let the device act. Resp: UCE/C&I

7.15 Turbine speed to be increased in steps of 100rpm till 5900 rpm & later in step of 50 rpm. Operation executive/TMD executive should be present in local to trip TDBFP if speed goes beyond 6100 rpm. MTP engineer should be present in local to measure speed offline. Turbovisory parameters to be kept under observation. Resp: HOD operation /HOD TMD /HOD(C&I)MTP

7.16 Note the speed at which turbine trips. Maximum TSI parameter to be noted. Resp: UCE/TMD

7.17 Restore OS protection setpoint to 6000 rpm and Speed reference blocking to 5800 rpm. Press the “OS test” button in mimic (Turns Red) to restore 5800 rpm limit on Nrtd. Resp: UCE/C&I

8.0 FREQUENCY OF ACTUAL OVERSPEED TEST

A Turbine on which on load oil injection test being carried out successfully on monthly basis, shall be given an actual overspeed test at intervals of 24 months or as per manufacturer's recommendation whichever is less.

If the on load overspeed test mechanism by oil injection is not functioning, then the actual overspeed should be carried out while stopping the machine for planned shut down or before taking the machine again in to service

If, for any reason, it is not possible or practicable to carry out an actual overspeed test at least once every 12 month, then this should be reported to the Director of Operation, and formal dispensation sought on each and every occasion.