Block Outlet Case

My friend, regarding the overpressure protection system design, one of important responsibility for process engineer is determining the cause of overpressure. In this posting, we will discuss whether a kind of case is possible occurred.

There are many causes of overpressure. Every possible overpressure condition shall be reviewed to ensure overpressure protection device capacity is adequate to protect the system. The following picture presents the causes of overpressure.

My friend, let discuss how block outlet is applicable to be considered.
(other cases in next posting.)


For review whether this case applicable or not, check : is there any valve that possible closed? Or any control valves which is ‘fail to close’ position? is there any object that possible block the fluid flow ? If YES, it may be blocked and lead to pressure increase.

The required load capacity of PSV can be determined based on the mass balance in the system after block outlet occurred.

Here is just simple case.

For system which comprises of many vessel / separator with same design pressure, block outlet case is only considered for PSV in the most upstream location. When block outlet occurred at downstream the system pressure increase lead to open PSV at the upstream.

I think, it is better to explain by this picture

Therefore, for system with same design pressure, the block outlet only applicable for one PSV only.

My friend, that’s all, I can share to you today. Hopefully, this simple article useful for you.
Tomorrow is 6 April 2012, that is holiday for all of us;
I just want to say, happy holiday my friend, God bless you all.,,


  1. I've seen a case that a vessel in series with the same design pressure, has the psv with blocked discharge in both of them.

    This case is happened due to the 1st vessel, has by pass line which very possible to operate manually.

    Then,they design PSV for the 2nd vessel with blocked deischarge case as well.

    What do you think?

  2. Gandi, if both 1st and 2nd stage has similar design pressure. only One(1) PSV block outlet case is considered.That is , PSV in most upstream of the system.

    When the valve at outlet vapor line of 1st vessel blocked. The PSV at 1st vessel will protect the 1st vessel.
    When the valve at outlet vapor line of 2nd vessel blocked, where the fluid from 1st vessel is keep flowing, The PSV at 1st vessel already provide adequate protection both for 1st and 2nd vessel.
    So that, the block outlet case for PSV at 2nd vessel is no longer needed.

    Please, See again 3rd drawing in above article.

    Hope this short description is clear enough

  3. Pak Triyanto,

    Could you inform the reference that describe above PSV blocked discharge case of many vessel / separator with same design pressure?

    Can "Safety Analysis Checklist" in the API 14C para A.4.2.c.2 and A.4.2.c.3 be interpreted like that?

    Thank You.


  4. Pak tri why dont we just erase the psv 02 and 03 ?
    I mean, we use 1 psv for3 vessel

  5. @babaruz
    the other psv on the subsequent vessel is used for fire case, but the psv 01 is use for blocked outlet and fire case as well

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