Dear Ankur Sir,
As you told “Many times layout constraints do not permit what is the most desirable configuration”. As this is the new plant I will figure out if there is any such constraint for this unit.
But one more thing I could not understand is, Logic behind providing PSV protection for this Closed Drain Drum?
As this CDD is open communication (floating) with Flare KO Drum. There is no isolation (automatic) in between line of CDD & Flare KO Drum. So we can take the credit of this line for relieving during over pressurization of CDD during Fire or Possible Gas Blow By scenario. Only thing is, I require to size this line for the controlling case of CDD. So with this I can eliminate the requirement of PSV for CDD. Please let me know your view on this.
Rajiv,
Your P&ID shows the vent connected from the CDS drum to the flare header and also shows PSVs connected to the flare header which I had not noticed earlier.
In case if you have an open circuit to the flare header from the CDS drum you may avoid providing PSVs for the CDS drum.
The advantage of a buried drum is that the fire case is not applicable at all and thus a PSV for fire case doesn't apply. PSV may still be applicable for a gas blow-by case after due analysis of the gas blow-by scenario.
The argument that has been given for not providing a PSV to a buried CDS drum is that since it is buried no fire-case is applicable and even if a gas blow-by case is applicable and the vent from the CDS drum is a open circuit to the flare system, a PSV is not required. Most advocates of a PSV on a open circuit closed drain drum have only one argument that since the vessel is being designed as per ASME Section VIII, Div. 1 it requires a PSV as per code. This also is not true, since the latest version of ASME Section VIII, Div. 1 allows exceptions for not providing PSVs on vessels designed as per the code.
One interesting case that I had argued for not providing a HP steam separator designed as per ASME Section VIII, Div. 1 and receiving wet steam from a HRSG was that the separator was fully rated which means that the design pressure of the upstream equipment and the separator was the same and the upstream equipment was protected by dual PSVs.
Looking at your P&ID where the CDS drum vent is connected to the flare with a LO valve (open circuit) and is probably floating with the flare, you probably don't even need a PSV if your vent is properly sized for the maximum vapor flow resulting either out of an external fire or from a gas-blow by.
Another example of a vessel designed as per ASME Section VIII. Div. 1 but not provided with a PSV is the Flare KOD which is connected to the flare with an open circuit and floats with the flare system
Hope this helps.
Regards,
Ankur.