Chandrapur # 7 – 500 MW Problem: Boiler trips On “Boiler Load Index” protection On tripping of Turbine Back to Technical Services

SOE/Trends Report Turbine trip0.00 Sec. Flow HPBPL0.45 Sec. Flow HPBPR1.45 Sec. LPBP Askania output > 40%1.50 Sec. LPBPCVs Open(Not Closed)~20.0 Sec. Boiler Trip On “BLI”3.00 Sec. Back to Technical Services

What is BLI ? (Only Applicable for Tube Mills ) To trip boiler in unsafe firing mode (only in coal firing) Ignition support Logic as below: 1.Mill Air flow > 60% and 2.Steam flow < 30% of BMCR (Original logic instantaneous) Back to Technical Services

Possible reasons for Boiler trip on BLI  MS Flow measurement troubles during turbine tripping (Ist Stage Pressure vanishing,HPBP(L&R) flows summation errors, Software Problem in MVN Block in Procontrol).  HPBPCVs opening delay.  Delay in opening of LPBPCVs. Back to Technical Services

HP-Bypass Trend taken on actual Trip Controller Output Actual Valve position

LP bypass opening delayed by ~ 20 Secs HP-Bypass & LP-Bypass valves behavior during Turbine Trip

Sequence of work done to avoid Boiler Trip The Steam flow < 30% contact was delayed by 3 secs. (Original Instantaneous ) The HP Bypass valve will retain at 60% and will be on auto after quick opening instead of falling to the Auto value which was causing steam flow to fall below 30%. LPBP Spray water pressure signaling system was optimized to get 2.0 Kg/cm2 with limiter in bottom position. Contd….….. Back to Technical Services

Sequence of work done to avoid Boiler Trip FRF Leakages in system checked. Inter- relationship between LPBP controller output v/s valve lifts checked. Cold trials of LPBP by, *simulation taken & opening time of 2 to 3 Sec achieved. The Timing of BLI trip has now been increased to 10 Sec. As per MOM with Trichy unit. Contd….….. Back to Technical Services

*simulation 1.Set on load. 2.LPBPSVs locked closed by keeping signal FRF v/vs to respective Slide valve in closed condition. 3.LPBPWIs locked closed by keeping FRF v/vs to respective WI valve in closed condition. 4.Keeping Limiter in top position. 5.Simulating Injection Water Switch to “permit” position. 6.Giving a kick command from PCR panel for LPBP set point>Actual pressure. Only EHC output versus LPBPCVs opening time analysis was done. Back to Technical Services

Engineer’s Views 1.Till recently,the problem of delayed opening of LPBP had not come to surface in other projects due to: Absence of BLI protection ( Introduction of Tube mills) Dilution of RH protection ( Efficient auto-loops,CMC ) 2.In WR only DTPP 1& 2 and CHT 7 are equipped with Tube mills.”PPPs optimize number of protections”.So, BLI has been modified with “&-gate-gen load > set) in case of DTPP.RH protection also has never operated.SO NO FOCUS ON LPBP OPENING TIME. 3.At KKT 3&4 cold trials of LPBP were taken & system has been tuned to achieve 9-10 sec consistently. 4.It is an important observation that in 210 MW machines the minimum time of opening (not closing) of LPBPCVs is 9 sec.-without steam.With steam,partially chocked filters & opening event taking after several days, it may take slightly more time Contd….…..

Engineer’s Views 5.It has been analysed & concluded that in the opening process of LPBPCVs,maximum time is consumed by the limiter to travel to the “permit” position.To have an idea of relative time taken by each element of the system following break-up of time may be considered: a)Askania out-put going to 100%-1.00 Sec (1.0st sec.) b)Limiter traveling to top “permit position”10.00 Sec. (11th sec.) c)LPBPSVs start opening (12th sec.) d)LPBPCVs start opening (13th sec.) 6.This research work was informed to HWR. Back to Technical Services

Engineer’s Views 5.HWR has reported: “Dear , This has reference to your mail message dated to shri ,SDGM-STE/FES regarding DELAYED OPENING of LP bypass control valves. As desired, you may have some positive pressure maximum upto 5kg/cm2 in condensate water injection impulse lines to limiter and condensate pressure switch by optimizing the orifice.It is also requested that condensate pressure setting in LP bypass limit controller and spray water pressure switch of LP bypass rack should be made as 8.5kg/cm2 as this will help in early opening of LP bypass control valves.Above changes can be done at site. Contd….. Back to Technical Services

Contd……… as desired opening time of LP bypass valves is given below LP bypass 500MW STOP VALVE 3.00 SECONDS CONTROL VALVE 3.00 SECOND LP BYPASS 210MW STOP VALVE 0.8 SECOND CONTROL VALVE O.8 SECOND THE ABOVE VALUES ARE CALCULATED VALUES FOR OPENING OF INDIVIDUAL VALVE ONLY AND NOT OF THE SYSTEM.WE DO NOT HAVE FEED BACK OF ANY SITE ABOUT THE TIME TAKEN BY THESE VALVES FOR OPENING AFTER GETTING OPEN COMMAND THROUGH LP BYPASS CONTROLLER.REGARDS DGM-STE/FES Back to Technical Services

CONCLUSION 1.Designers from HWR & TRY to review in a coordinated way to ensure safety of Boiler (BLI) keeping in view of the limitations posed by LPBP system ( “t”..opn>20sec). 2.After a persuasion period of more than 3 years,designers have agreed to provide a timer of 10 Sec. in BLI,which may not yield the desired results in saving the boiler from tripping on BLI. 3.In the running sets (having no tube mills) either RH protection has been kept out of service or LIMITER is kept locked in “permit” position.This aspect must be kept in mind while deliberating on BLI. Back to Technical Services

Contd…. 4.Now CHT U#7 will have protections with timers : BLI=10 Sec, RH= 9 Sec. 5.Issue of Chandrapur # 7 (now 4 year old) cannot be resolved until we opt for one of the following alternatives in this forum: A.Modifying permissive philosophy of Limiter : Condensate pressure at rack can be set to 8.0 Kg/Cm2 with Askania at zero.Under this condition limiter can be adjusted to attain “permit” position.Limiter may be adjusted to act when spray water pressure falls below 6 Kg/Cm2. Back to Technical Services

Contd…. With this,system will be protected when CEPs fail but it will not be protected against non-opening of WI valves.However, back up spray switch will protect system under both the conditions. As for as vacuum protection is concerned,it will be taken care by back up vacuum switch. Limiter will have same settings for vacuum permissive. Protection against RH pressurization acting on Limiter will remain unchanged. Contd… Back to Technical Services

B.Review of RH-BLI timers for 500 MW sets: Review these timings keeping in view that repetitive & consistent representative opening time for LPBPCVs in 500 MW with partially clogged FRF filters,having steam & valve operation after considerable time is about: 20 seconds with limiter in bottom position &, More than 10 seconds with limiter in top position Back to Technical Services Contd….

C.Introduction of original philosophy prescribed by KWU: KWU had anticipated the problems in protecting Reheater against starvation due to anticipated delay in opening LPBP valves. Their recommendations for this are reproduced below: “To protect the reheater from thermal damage as a result of heat accumulation, it must always have a minimum amount flowing through it. For this reason the electro hydraulic LP bypass controller at a preset deviation actuates the reheater safety valves. In normal bypass operation this signal is interlocked via limit switch MAN 11 AND 12 CG001D & MAN 11 & 12 CG002B unless quick- closing LP bypass valves MAN11 AND 12 AA001 are completely open and Lpbypass control only one valve fails to meet these condition, the signal goes through and the safety valves open (thus they open at precisely the existing reheater pressure (below design pressure)) So that safety valves do not respond unnecessarily as a result of delays present in the hydraulic control elements, the signal to them is passed via two parallel time delay relay which delay contact by approx. 5 sec.” Back to Technical Services

Course of action As suggested & deliberated by the specialists during Seminar on 7 th & 8 th Aug 2002 we are going ahead with implementation of the suggested logic of hooking up RH ERV’s in LPBP system at Chandrapur #7 to protect boiler tripping on BLI/RH protection when turbine trips. MSEB has informed that a capital shut down will be taken on unit 7 shortly. We intend to carryout this modification in this shut down itself to facilitate liquidation of this long pending issue. Back to Technical Services