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Coke Drum Relief Capacity

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This topic contains 2 replies, has 3 voices, and was last updated by  Mitchell Moloney 14 years, 2 months ago.

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  • #4235

    coker squirrel
    Participant

    Coke Drum relief lines are routed to the Blowdown Drum but this poses a problem.  If the Blowdown Drum level is high (above the entry nozzle) or the Blowdown Condensers are partially restricted with paraffin, this could restrict relief capacity.
     
    Are there any other better design possibilities?  

  • #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

  • #7489

    Anonymous

    The blowdown condensors are part of the relief path for drum PSVs (if drum PSVs are routed to BD system).  When the condensors foul (and can you think of any coker heat exchanger more prone to fouling?), they partially restrict the relief path.  Very much like partially closing a block valve on the PSV discharge.  In the case of block valves on PSV discharge, we always LOCK THEM OPEN because we need to be sure there is NO RESTRICTION.  But in the case of BD condensors, we just let them foul until some arbitrary measure (usually when drum cool time is impact) is used to call for cleaning.  I submit that the BD condensors need to be looked at as a block valve on the PSV discharge.  Unlike a block valve where you can see if the stem is partially closed, the degree of fouling (restriction) in the condensors is hard to quantify.  But understanding the degree of fouling in the BD condensors is essential to know your true PSV relief capacity.  Obviously with severly fouled BD condensors, the PSV capacity will be reduced, and coker feed should be reduced to stay within the capacity of the PSV.
     
    For new cokers, it would seem wise to design for at least 20% overcapacity so that when one drum is at maximum cooling (i.e. maximum load on the BD condensors), and an adjacent drum has vapor line blocked (PSV controlling case), the BD condensors will still provide adequate relief path EVEN WITH 20% FOULING.  But you still need good monitoring to know when you’re at 20% fouling so you can schedule cleaning.  Alternative relief paths include main fractionator (everybody groan) or a separate quench drum (my preference for best process safety).

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