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RE: Coke Drum Relief Capacity

#7615

Mitchell Moloney
Participant

A risk analysis needs to be performed.  If the blowdown level is normally under control and not high, one would argue that simultaneous opening of the coke drum PRV and having a high level would constitute double jeopardy.  It is standard design practice to not design for double jeopardy.  In that case it is perfectly acceptable to have coke drum PRV’s lined up to the BD tower.  HOWEVER, if BD contactor level is unreliable and often high enough to impede a PRV release, causing pressure in the coke drum to rise over the allowed over pressure (10% for normal contingency, 21% fire, etc), then you need to mitigate the risk.  Changing PRV destination is an extreme option.  It probably makes sense to improve level reliability by installing a second independent level device, of a different design, to ensure reliability. For example, one type would be DP cell with tap blowbacks and the other could be a gamma device or a simple pressure gauge on the pump suction converted to level .
 
Regards – Mitch Moloney

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