PED Certification on multiple valves

Chemical Engineering general discussion Safety Relief Valve (PSV)
#1

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.

#2

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

#3

Hi guys,
It’my first message on this incredible forum.
I’m a technician of Psv and I can only say to @don1980 that he has a very deep knowledge on the matter.
About Ped, sure, accumulation beyond mawp it’always 10%, also in case of multiple psv installation. That’s means that if you want to use more than 1 psv to protect your system and you want to set the first one at mawp, the second one shall have 5% of certified overpressure.
That’s why some superior european psv manufacturer has some valves, usually Posv, certified at 5%.

Sincerely,

Alex

#4

Welcome aboard, Alex! Yes, @don1980 is the undisputed PRD master! I have been sizing PRDs for 44 years. I know more about how the US does it, than PED. I’m looking forward to hearing from you.

#5

Compliments from Latexman are a high honor - thanks!

Alex, I recently learned that Leser makes a spring-loaded PRV (Series 441) that’s designed to reach full lift at 5% overpressure. These Series 441 valves are not built according to the API 526 PRVs (face-face dimensions, D-T orifices, etc.) but they’re otherwise perfectly acceptable. So, for multi-PRV installations in Europe, this is a product that can satisfy the PED’s strict pressure accumulation limit of 10%.