Barring Gear Causes and Prevention

Barring Gear Types, Reasons of Stal ling and Preventi ve actions

AK Sehgal Sr. Mgr. COS-ST [email protected]

Functionality • In hot state, to Rotate the Turbine Rotor at Slow speed for uniform cooling. • To prevent rotor from Sagging due to gravity. System: Motive force is provided by Either Hydraulic wheel or Electric Motor. At lower rpm hydrodynamic action is not there, so turbine rotor is lifted Hydrostatically by supplying with High pressure jacking (Lifting) oil pressure .

LMZ design, 3.54 rpm Motorized Turning Gear

HPT

IPT PT

IPT

LPT

Gen.

Old 210/500 MW Barring Gear in a KWU design Hydraulic Wheel

HPT

IPT

LPT

Gen.

Front Pedestal –new 500 MW BHEL turbine -Hydraulic Wheel

HPT

IPT

LPT

Gen.

Failure to keep Hot machine on Barring Gear • Because of positive drive and higher torque margin in case of Motorized ones, Barring gear stalling is hardly a issue. Even if minor touching of seals is there, it is rubbed off. • Never the less, in old fleet i.e. Pre-Vindhyachal stage-II turbines there is no issue of stalling (Except appropriate auxiliary steam temperature free of condensate). • Post vindhyachal, reduced gland clearances, Turbine is extremely sensitive to Auxiliary steam temperature supply.

• If Turbine fails to rotate freely with one man then it should not be forced upon. Interstage Seal Fin and Gland seal fin Wear will result due to Temporary Rotor Bowing and Casing Hogging due to Top-Bottom temperature difference. If forced rolling, Permanent Rotor Bow may also result. • On the other hand One instance of Barring stalling eats up to 3 days of outage loss, so preventive actions are must.

Main Reasons of i nability of turbine to sustai n on barring gear and associ ated checks In case of KWU design 210/500 MW there are number of reasons established on the basis of cases; • Resistance to Rotation • Inadequate Lift • Motive Power During coasting Down, When Turbine Speed reaches 220 rpm Turning valve Opens. After how much time of tripping of turbine, Barring gear is stalling, is very important and helpful in finding out its reason.

Rubbing

•due to condensed moisture/liquid. •due to air chilling of glands • Deviated mechanical Clearances. •Due to presence of foreign material •Also possible due to Generator seal ring rubbing on shaft.

Rubbing provides resistance to rotation.

Entry of Wet/ condensed st eam into Turbine glands

Entry of Wet/ condensed st eam into Turbine glands •

Case: 21.09.2003, 0043 hrs both Units tripped on low condenser vacuum due to tripping of CW pumps. after attending the CW pumps problem, boiler lighted up and at the time of charging of aux seal steam for vacuum pulling, turbine speed started reducing and got stopped at 0900 hrs barring gear stopped. Reason: Wet Steam/Condensate. Such type of cases including Post trip can be avoided by checking:

•

Check-1 Open Motorized Warm up valve MAW-10 and see if temperature rises more than 50C, if So it indicates the presence of Wet Steam/Condnesate.

•

Check warm Orifice is there in Warm up drain line or not? If yes check size. If No, Get it installed. Check if the location of continuous warm up line/drain is at height than or distant from seal steam supply temperature monitoring point, if yes then modify and bring it nearest and to a lower elevation than main seal steam control valve. check whether steam traps provided in aux PRDS are working ? Check source aux. steam temperature (270 Degree+ for New and 220 Deg+ For old one.? Check if unit PRDS is charged commonly from farthest unit?

• • • • •

Also Check CRH NRVs for proper seat contact area and full closure (If passing, during initial HPBP opening, low temperature steam with spray is likely to deaccelerate the turbine when on Barring).

• After unit trip: do not keep Gland seal steam charged for more than 1-2 hours. Unnecessarily possibilities of change in auxiliary seal steam temperature . • During Controlled cooling: Ensure seal steam supply isolating valve is tightly closed and W arm up drain valve i.e MAW-10 and MAL-81 drain valve is opened to rule out possibility of condensate remaining entrapped. • Also ensure implementation of X-98.

Air Chilling of Glands Case: Talcher # 4, 22.03.2004 ,0016 hrs Turbine trip, Came on Barri ng gear, Full vacuum maintained with gland seal steam supply charged. 0615 hrs, Seal steam isolated, turbine speed subsequently started dropping. 0640 vacuum breaker opened. (but s peed continued to drop and c ame to standstill at about 0705 hrs. Turbine was left to cool and time was utilized for PG test preparations. Reason: Air Chilling

• • • •

Check: Post Trip vacuum killing Sequence of operation : Stop Vacuum pump Kill Vacuum by opening Vacuum breaker Isolate Gland seal steam at Vacuum of 0.2-0.3 Kg/cm2 or Back pressure of 0.7 -0.8 kg/cm2 Stop Gland steam Exhauster i mmediately af ter isolation of gland seal steam

Disturbed Seal Fin Clearances •Case: FSTPS # 5, 500 MW, 2001-02 Barring gear problem and High Vibration . During inspection rubbing marks on shaft under LPT front gland. Clearances restored. No problem subsequently. Reason : Abnormal LPT gland clea rance . LPT Gland Clearanc e: Check that Gland box is not skewed and keys are properly set. Ensure at least 0.7 mm radia l clearance all around the front and Rear glands by inserting long feeler (500 mm long). •

Case: Talcher # 5, 13.05.04 0659 hours Turbine manually tripped. Everything normal. Various checks were done by BHEL and NTPC by finally shifting of HP casing by 0.30 mm on LHS (Lo oking from front pedestal towards generator), turbine came on Barring speed. Check: Disturbance in Casing position/c entering due to pipe push/pull in Turbine area MS ,CRH, HRH, Cross around piping Hangers and Supports. Walk down survey and Healthiness. Checking in Hot (Pre-shut down), Cold (Shut down) and setting and final checking in subsequent hot (after shut down) condition .

Disturbed Seal Fin Clearances

• Case: 02.01.2005, 0100 hrs -Unit shut down for boiler tube leakage. Af ter one hour Condenser Pass -B was drained to attend debris f ilter choking, and one CW pump was also stopped. At 1100 -1300 hrs Other CW pump stopped. At 1455 hrs i.e. about 14 hours of unit trip, Barring gear speed came down f rom 75 rpm to 0 rpm. No other intervention was made. Reason: CW draining of Condenser Check: Till HPT temp