10 replies to this topic

[Kryz]

Dear All,

Can set pressure be larger than Maximum Allowable Working Pressure?

I am working safeguarding memorandum for gas dehydration plant using TEG glycol. I found on contactor vessels for fire relief psv that set pressure is 12514 kPag, while design pressure (MAWP) is only 11377 kPag. Is it OK with ASME Sec VIII???

 

Under ASME Sec VIII Division 1 in UG-134 (page 97)  it is written as follow: "( For pressure relief devices permitted in UG-125©(2) as protection against excessive pressure caused by exposure to fire or other sources of external heat, the device marked set pressure shall not exceed 110% of the maximum allowable working pressure of the vessel. If such a pressure relief device is used to meet the requirements of both UG-125© and UG-125©(2), the device marked set pressure shall not be over the maximum allowable working pressure."

 

Hence I deducted that I have 12514/11377 = 1.099938, it is 110%, OK.

 

However elsewhere in this ASME, plus in API RP 520, it is only allowed to be 105%, and for second psv only. MY EXAMPLE HAS ONE AND ONE ONLY PSV!

I run also software like Farris Size Master, or Pentair PRV size (as relief valve is Crosby 900 series Omni Trim). Both programs respond clearly that set pressure can be only 105% max. Programs display warning message!!!

 

Is my interpretation of ASME correct, as someone set this psv as 10% over MAWP. Or something is wrong here? This is important decision as it refers to safety in the plant. - Violation of ASME code either!

 

What is your opinion? Regards Kryz.

 

[Bobby Strain]

You already answered your question. With the ASME code reference.

 

Bobby

[lokeshmiddha]

Hi Kryz

 

Set pressure can not be greater than maximum allowable working pressure for a single PSV system.

 

you can take set pressure as 105 % of maximum allowable working pressure in case of multiple pressure relief devices and set pressure as 110% of maximum allowable working pressure in case of supplemental pressure relief devices.

 

Regards

Lokesh

[Kryz]

Hi Bobby,

 

thanks for your reply, are you able to say: YES or NO?  Can I make set pressure 110% of MAWP for fire relief single psv only?

 

Regards,

Kryz.

[Kryz]

Hi Lokesh, thanks for your reply. What do you mean by supplemental?  Can you please confirm the paragraph I quote from ASME - i.e., UG-134 ( is talking about supplemental.  I am not sure if I understand this part very well, but I can see explicitly the number of "110%" there for fire relief.  What exactly that number means for us - relief engineers?

 

Regards,

Kryz.

[lokeshmiddha]

 

Hi Kryz

 

Set pressure can not be greater than maximum allowable working pressure for a single PSV system.

 

you can take set pressure as 105 % of maximum allowable working pressure in case of multiple pressure relief devices and set pressure as 110% of maximum allowable working pressure in case of supplemental pressure relief devices.

 

Regards

Lokesh

 

 

Hi Kryz

 

In your query you quoted  para from ASME  UG-134, which talks about ASME  UG-125. Just read ASME  UG-125 for better understanding. 

 

Regards

Lokesh

[fallah]

Kryz,

 

Yes, the set point of a PSV can be higher than the MAWP of the relevant vessel. Read the second post of the discussions included in the link of the attached file....

 

 

Attached Files

•  API 520 Supplemental PSV.docx   9.82KB   312 downloads

[lokeshmiddha]

Hi Fallah

 

Set point of a PSV can be higher than the MAWP of the relevant vessel if that PSV is supplimental PSV for fire case (Other psv is avaiable for other relief cases). If single PSV is used and fire is the only relief scenerio than i believe Set point of a PSV can not be higher than the MAWP of the relevant vessel.

 

Regards

Lokesh

[fallah]

Lokesh,

 

It appears that, before submitting your post, you didn't read the explanation in the link i attached to my previous post; because you did repeat almost what is included in the mentioned explanation...

[Kryz]

Lokesh,

 

It appears that, before submitting your post, you didn't read the explanation in the link i attached to my previous post; because you did repeat almost what is included in the mentioned explanation...

 

 

Kryz,

 

Yes, the set point of a PSV can be higher than the MAWP of the relevant vessel. Read the second post of the discussions included in the link of the attached file....

 

Kryz,

 

Yes, the set point of a PSV can be higher than the MAWP of the relevant vessel. Read the second post of the discussions included in the link of the attached file....

Dear All,

 

Thanks to all of you  who replied to my post. Finally after investigation, I found that the PSV I am talking about are suplemental.

 

It was proposed initially the PSV-XXX set pressure to be 11,377 kPag (same as MAWP). However, original designer raised the concern regarding interaction this PSV with other PSV’s in plant, particularly with PSV-YYY for 4th stage discharge compressor relief. This particular PSV-YYY is set also 11,377 kPag and it is sized for blocked outlet of compressor discharge scenario. It is undesirable to have PSV-XXX on TEG contactor to pop up during blocked outlet scenario, as PSV-YYY serve for this purpose. It will cause relief valve to chatter as some load could be taken by fire relief PSV on TEG contactor. That statement was documented in the original design basis, and decision was made to elevate set pressure of PSV-XXX to 110% of MAWP, which is 12,514 kPag.

 

The concern raised by original designer in opinion of author of this post is too conservative. It can be demonstrated that PSV-YYY for compressor blocked outlet is located closer to compressor discharge than Contactor fire relief PSV-XXX. When overpressure scenario (blocked outlet) will occur the pressure gradient (pressure losses) between PSV-XXX inlet and PSV-YYY inlet will prevent second valve to pop up (if both would be set at 11,377 kPag). When already first PSV is relieving, actual pressure at second PSV will be always lower than its set pressure (due to flow frictional losses). Hence it is possible to bring back set pressure of PSV-XXX to originally proposed 11,377 kPag.

 

I am thinking of recommending that our client do this eventually. (Change set pressure back to 11,377 kPg)

 

However, as this PSV is sized properly, any alteration/interruption might not be desired by plant operation unless it is safety related problem. The PSV setting complies with the design code (ASME, API), hence this is not a safety issue.  Moreover, consideration is required for other relief valves in the plant - i.e., like for TEG coalescer and Lube Oil Coalescing Filter where the PSVs also need to be reset to the same pressure as TEG Contactor (all those three PSV are fire relief cases with small D orifice size and set @ 12514 pig), as inconsistency in set pressure may affect when each valve pop-up during relief scenario, as described in the beginning of this section.  The purpose of selecting set pressure is to ensure the relief valve will open for scenario which PSV is determined, fire PSV and block outlet PSV are different in the plant, and they must only open for those contingencies which they are designed.

 

So this is my concern to prevent me to do this recommendation. (Change set pressure back to 11,377 kPg)

[Bhavinkumar]

@ Kryz

 

It was really interesting explaination in your last post. Here I just wanted to quate your last lines in last para 

 

"The purpose of selecting set pressure is to ensure the relief valve will open for scenario which PSV is determined, fire PSV

and block outlet PSV are different in the plant, and they must only open for those contingencies which they are designed"

 

I feel  that there are chances to misinterprit above line for readers. What I mean is .......PSV is designed for protection against overpressure  contigencies may be fire or blocked outlet or else.......while the governing case may  be any one of them.....so they must not open only for the governing case but should be open for any overpressure scenaio considered while sizing. Here in above discussion the governing case many be fire case for PSV-XXX and block outlet case for PSV-YYY. But PSV-XXX should also open at its  set pressure other then governing case (e.g. fire case) and vice versa for PSV-YYY.