PED Certification on multiple valves 2

[gguliye]

Dear Team,
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?
regards,

 

[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.

 

[gguliye]

Don1980
Plus ped also allows the asme VIII valves to be used with the boilers. I think engineers who accept ped may have problems in their boilers. Seems to me asme VIII is safer to follow

 

[The Obturator]

Taking don1980's last point further - 5% overpressure full lift designs exist for spring operated types (DIN/TUV design mainly where 5% OP was standard for compressible fluids). Here you would need to ensure that those designs are actually certified and can be CE marked. (you could not ASME VIII "UV/NB" stamp these - I mention this since some users do want to see both UV and CE stamps).

The PED overpressure above 10% issue comes up now and again with no ruling. I'll continue to look further. I hope some others can enlighten us here. Reading some earlier postings on this forum, I have read where 21% was used as it was fire case. Other times the local authority didn't bat an eyelid etc.



Per ISO, only the term Safety Valve is used regardless of application or design.

 

[gguliye]

avalveman,
21 % fire case? Something wrong in here.
21 % is allowed in ASME Section VIII, however, in PED, 10 % is allowed however, and if higher percentage is desired, no problem. That's what I have read in the book ( not from PED, but from PRV book)
regards,

 

[The Obturator]

That's what I meant. ASME 21% has been used for fire in a PED installation (doesn't make it right). Just as you have noted from a book but not from PED. Lets see what else turns up.

Per ISO, only the term Safety Valve is used regardless of application or design.

 

[don1980]

It's acceptable to use 21% accumulations. PED Guideline 5/2 directly addresses this question. This official PED response states that the 10% accumulation limit does not apply to fire scenarios. Interestingly, that's all it says - it just says that fire cases aren't limited to 10% but it doesn't state a limit. Therefore, it's left to the discretion of the user, based on their risk tolerance. For global consistency, I use 21% in PED jurisdiction.

 

[gguliye]

Don1980
Thank you very much

 

[The Obturator]

Thanks for that Don1980 Needed that.

Per ISO, only the term Safety Valve is used regardless of application or design.