HP bypass valve starts opening : Emergency

Emergency Handling by Opn Gr-II:Unit in CMC

Unit Status before 

Unit Load : 350 MW 
Coal Flow : 260 TPH
Mills I/S : BDEFG 
Oil Guns I/S : Nil

INCIDENT:

• 12:59:00 HP bypass valve BP-2 started opening & got opened up to 98% & at 12:59:31 BP-1 also started opening & opened up to 98%.

ACTION TAKEN:

• Machine came in CMC- pressure control mode (earlier in CMC-load control mode) due to pressure deviation and load came down to 174MW. Machine taken in TF mode pressure control after reducing throttle pressure
• HP bypass OSU pump buttons in vertical panel acknowledged, OSU pump-2 took start, but still the valves stayed in open condition.
• AB elevation oil gun taken in service and feeder F unloaded to min followed by oil support at CD elevation for flame stability.
• MDBFP kept in service and TDBFP- A hand tripped to maintain drum level.
• Further as Drum Level was still increasing TDBFP-B also hand tripped due to which runback acted and tripped mill G.
• MDBFP suction flow maintaining high and drum level also increasing rapidly so drum level control taken in manual.
• Air flow master taken in manual and total air flow increased from 950 to 1050 tph
• HP bypass BP-1 and BP-2 closed from local and unit stabilized. Drum level kept in auto at 280MW with MDBFP and one TDBFP in auto.

ANALYSIS:

• At 12:59 HP bypass valve BP-2 opened up to 98% while demand was 0 followed by opening of BP-1 valve also up to 98% . 
• Load came down to 174 MW from 350 MW and drum level was fluctuating. For controlling the drum level MDBFP taken in service and later both TDBFPs hand tripped one by one.
• It was observed that HP bypass OSU pump-2 took start and run time exceeded alarm came followed by opening of HP bypass valves. HP bypass OSU pump-1 also took start but from MMI close command for BP valves not executing. OSU pump faults acknowledged from vertical panel and OSU pump-2 took start but still valve were full open.. Local checked no oil leakage observed .Both bypass valves closed from local

OBSERVATIONS :

• HP bypass valves did not trip on downstream temperature high.
• OSU pumps operation was normal till 12:56.
• Drum level variation: +ve 138mm & -ve 320mm.
• TDBFP A ESV not getting opened after resetting.
• It is learnt that while doing cleaning by C&I mtc in the HP Bypass area (No PTW/LWC issued) inadvertently BP2 fast opening accumulator valve was opened which was earlier kept closed due to suspected drifting in BP2 valve

SUGGESTION :

• Any PM or Mtc work in HP Bypass valves area should be properly intimated to Operation by taking PTW/LWC.
• HP Bypass valve area being the most vulnerable, any maintenance/operation work should be carried out under proper supervision by respective executives.

NOTE:

• No PTW/LWC was issued from operation related to HP bypass on dtd: 06.09.2016 i.e.; the day