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:
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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.
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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?
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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
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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 <290 deg C, do not empty condenser water box. Else it is likely to disturb LPT Rotor/Casing radial and or LPT gland clearances.
Rubbing due to Foreign Material
Case:
Korba # 3, on four occasions of Unit trip on 06/05, 30/07, 29/11/2003 &10/1/2004, T urbine did came on Barring Gear but after 42.7, 39.5,33 & 32.6 hours respectively.During O/H axial bump check valure of HPT not OK. HPT opened. A metallic piece of shape (15* 12mm size, 4 mm thick) was f ound stuck in 22nd stage fixed blades of HPT which was rubbing agai nst rotating blade whenever machi ne speed reduced.
In sufficient Lift of Rotor Journals •
Case : Dadri# 4, During annual shutdown in the year 2000 Turbine
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Check that no extra pocket is formed near jacking oil recess and proper jacking oil recess is there inside the bearing. Lift Pedestal Covers and see if any abnormal ity or leakage is there in hose, joints, pressure gauge etc. Check J.O. Hose of Proper length, routi ng and flexible clamping with adjacent oil piping and clamping of metal pipe in each pedestal. Check and set requi red bearing lift by adjusting jacking oil pressure with one Jack ing oil pump and Mai ntain protocol for Jacking oil header pres sure, bearing lift and jacking oil pressure at each bearing. Checking of J.O. Pipe c lamps/supports from MOT room to Beari ng Pedestal for tightness/adequacy by actual shaking of piping. (To prevent pipe failure due to shaking/vibration during start up and shut down)
came to standstill after the machine was desynchronized. Jacking oil ho se found b urst at mid section . Probably vigorous rubbing during trips etc. caused hose to fail. Later site has properly routed the hose and clamped it with adjacent piping inside the pedestal and no failure observed since then. • There are number of cases because of jacking oil hose failure/ Jacking oil Piping. Checks: • • • •
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Jacking oil Hose replacement to be done whenever bearing is removed for inspection. Jacking oil hoses pressure rating and quality to be ensured. Trend monitoring of Jacking oil back pr essure (provided inside in bearing no. 1,2,3,4) at all brgs. Indicates health of Hoses and NRV’s. Check that there are no grooves on journal near the jacking oil pocket 9especially Brg. 6). This will lead to leakage of lifting oil past the pocket and affect lifting. Take action for Rotor replacement/correction. Prevention: J.O. Flushing of the system to be done during overhauls (whenever bearings are opened) (To prevent scoring marks in journal and bearing which leads to leakage of lifting pressure).1. Close oil supply to all bearings2. Remove brg. end hose of brg. 6/7 (whichever applicable) and route to a Drum .3. Run JOP with brg. 6/7 supply valve partially open for few minutes. 4. Flush through Jacking oil header end.
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Ensure that Jacking oil header pressure is maintaining for a given lif t settings. (There are no. of cases which show in that header pressure is not consistentBHEL is developing alternative valve supplier.) After unit synchronization clean J.O. f ilter. Install Jacking oil header f ilter, wherever not existing Make arrangement for locking/cover ing of J.O. pressure setting val ve for Brg. 5,6, 7 (if applicable)- to prevent accidental disturbance. Check J.O. NRV’s (provided inside the pedestal ( brg. 1,2,3,4) (to prevent sticking/passing) During overhauling check there are no grooves on rotor journal nar jacking oil pocket . Take action for rotor replacement or journal machining .
Precautions • Record and trend coasti ng down time during each uni t tripping-3000 rpm to 200 rpm. • After each uni t tripping, when turbi ne on Barring gear, Check whether an y abnormal sound is comi ng near Glands and bearings or any abnormal vibration.
Motive Power
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Visually Check the condition of Barring Gear Oil supply isolation val ve for actual opening. Ensure that Pipe f or Manual Barring is al ways available at f ixed location near the Barring ge ar. Check condition of Barring gear nozzle and wheel blades and clearances during Overhauling. There has been issue of inherentl y lesser barring speed which is giving ver y-2 less torque margin to tolerate practical deviations .
Developments • The reason of l esser speed/torque has been found due to submergence of hydraulic wheel in Pedestal Oil. BHEL has suggested modi fication vide X-99, which has yielded minor benefit. Further COS -ST has suggested modi fication for completely coming out of issue.
Miscellaneous
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Covering of Turbine floor Oil piping trench with Grilled plate. Covering of oil piping in Pedestal approach area wi th removable grilled plate platform (wherever not provi ded). In case of Probl em Please preserve 1 second trend i nterval data for Turbo visory, CW, jacking oil header pressure….
Thanks for listening patiently.
