Help on calculating required relieving capacity 3

[ME231]

I have found various sources (including manufacturer's websites) that tell me how to determine relieving capacity of a burst disk given a relieving pressure and disk size. I have read articles on Coefficient of Discharge and Resistance to flow methods.
However, how do I determine the REQUIRED relieving capacity?
Is this typically something that a customer purchasing pressure equipment would determine based on his application and require the vessel manufacturer to meet?
Any helpful pointers from the forum would be much appreciated!

 

[zdas04]

That is actually a very good question. In the 1980's API and ASME jointly defined a term called "Credible Scenario". Their thinking (and I find it to be excellent) was that while the human mind can come up with an infinite number of "Possible" overpressure scenarios, which of them are actually "Credible". For example, say a wellhead choke can provide 20 MMCF/d of flow at 2000 psig if fully opened into a vessel with a 1440 psig MAWP--if the reservoir pressure on the well is 300 psig, then the 20 MMCF/d is not credible, in fact the reservoir pressure is not capable of creating an overpressure condition on the vessel at all and the failed-choke scenario is not a credible overpressure scenario.

API 520 has a list of categories of credible scenarios. Some are very specialized (e.g., one of them is runaway chemical reaction which is rarely credible for a 2-phase gas/liquid separator on a wellsite), some must be considered nearly always (e.g., fire case and blocked outlet case).

The process for finding required flow rate is to run through the API 520 (or 521) list and determine which scenarios are credible. For the ones that are credible you need to assess the flow rate during that scenario. Size your device for the largest flow credible flow rate.

David

 

[ME231]

Thank you David for your response.
Would you agree though that the customer is in a better position to determine the required relieving capacity than the manufacturer?
Is this responsibility placed on the customer anywhere within the ASME code? Since the API is not really a "code" I would expect it would not be specified therein.
Thanks!

 

[Duwe6]

Yes, typically the Vessel Data report lists the PRV as "supplied by others". You, the mfr, have no idea what the customer is going to put into a vessel [especially the 'runaway reaction' scenario].

Also, for our litigenous business environment, DO NOT take on the PRV / overpressure liability. Put "Pressure Relief Device(s) to be Supplied by Customer" in ALL your contracts.

 

[ME231]

Thanks a lot to you both for your feedbacks...

 

[TD2K]

Ditto to the above, the customer has to identify the relieving requirements.

 

[zdas04]

I work with a lot of vessel fabricators that have all gotten into a crack over this. Historically, someone picks a size valve and an orifice size (usually not based on much) and just uses that size forever.

I don't agree with the concept of just marking "supplied by others". I think it should be a collaboration. For example, I wrote a vessel RFQ just this afternoon where I evaluated the credible scenarios, picked the one with the highest required rate, and provided that flow rate to the vessel fabricator to size an over pressure protection plan. I don't expect the fabricator to know what my process can give the vessel, but I don't think it is reasonable for the end-user to develop PSV sizing expertise.

David