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API 520: PSV allowable overpressures

API 520: PSV allowable overpressures

API 520: PSV allowable overpressures

Does anyone have any info. on the following:
1.  What is the basis for the 10% allowable overpressure specified by API520? Is it to do with the 0.9 derating factor used by vendors to determine capacity?
2.  What is the basis for 21% allowable overpressure for 'fire case' relief valves.
3.  Why is a higher overpressure allowed for fire case relief valves?

I would appreciate anyone's comments on the above.   

RE: API 520: PSV allowable overpressures

As an obvious novice, you may find it more beneficial to find an experienced associate with which you can address your relief system design questions.  There will be many yet to come.

As with most instructions, it is generally best to read the entire selection before proceeding.  In the Foreword of RP-520 you will find that it is intended to supplement the information contained in Section VIII, “Pressure Vessels,” of the ASME Boiler and Pressure Vessel Code.  If you don't already have access to a copy, then some relief valve vendors include excerpts in handbooks that they offer.  For example:

The key point to remember is you need to have an understanding of the construction codes for the equipment you are trying to protect from overpressure.  There are many codes to which equipment may be constructed and the recommended practices of API RP-520 will not be suitable for all equipment.

More specifically, to address your questions, careful that you don't confuse overpressure for relief valve sizing with the pressure increase allowed for the equipment (accumulation).  As it happens, the 10% and 21% allowable overpressures are limitations only when the relief device is set at the equipment MAWP.  Vessels built to ASME Section VIII are limited to 10% accumulation for non-fire relief cases and 21% accumulation for fire relief cases.  

I'm afraid I can't say how the allowances for accumulation have come to be but likelihood of occurrence may have been a factor and they are less than the test pressure as called for by ASME Section VIII.

RE: API 520: PSV allowable overpressures


1.  Apologies, code does not prohibit one from sizing relief valves based on overpressure higher than 10% (21% for fire)if set pressure < MAWP.

2.  My original question however was more one of curiousity as to the basis of these values (10% and 21% ACCUMULATION)which is a question that more experienced associates around my office were not able to answer...

RE: API 520: PSV allowable overpressures

Well, that's a relief! (pun intended)

Apparently you are more knowledgeable concerning the subject than I understood from your question.

Since the basis of the allowable pressure excursions comes from ASME you might want to think about rewording your question and posting it in the ASME Code forum.  It would be interesting to see if anyone knows where the 10% and 21% came from.

As additional comment, at one time, it seems I've seen allowable accumulation for fire to be only 20%.  In fact API 620 follows the 10% and 20% allowables.  I think it has been just in recent years that ASME went from 20% to 21%.

Good luck in finding your answer.

RE: API 520: PSV allowable overpressures

I've always took the 21% increase as simply an additional 10% increase on the original 10% increase.  For example 1.10 x 1.10 = 1.21.

Steve Braune
Tank Industry Consultants

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