PED Certification on multiple valves

QUESTION
While reading the PED requirements, I have noticed the following thing:
In the case of multiple relief valves, the 1 pressure relief device can be set to MAP, while others can be set +5 percent above it. However, at the same time, PED doesn’t allow us to go beyond the 10 % from MAWP for the full relieving capacity. If 1 of 2 valves is in 100 barg set, then the another has to be in 105 barg set. To reach full capacity, the second one requires 10 % overpressure, so it will be in 116 barg. 16 % overpressure??? Can someone explain to me please at which point I am making a mistake?

REPLIES

don1980
You’re not mistaken. That’s what it says. This is one of the subtle differences between PED and ASME Sec VIII. Although they both allow a set pressure of 1.05xMAWP, when multiple relief devices are used, PED limits the relieving pressure to 1.1MAWP in such cases, while ASME allows it to reach 1.16MAWP. By not allowing the pressure to rise to 1.16MAWP, PED effectively negates the feasibility of setting a valve at 1.05MAWP because this requires one to size the second PSV for a condition in which the PSV isn’t fully open. The secondary PSV’s relieving capacity (at 5% overpressure) can’t be accurately predicted, or calculated.

SOURCE
https://www.eng-tips.com/viewthread.cfm?qid=457391
Above is a snippet.

@don1980, Thank you for pointing out this “subtlety” which is not so “subtle”.