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Omitting PSV on pressure vessel
6

Omitting PSV on pressure vessel

Omitting PSV on pressure vessel

(OP)
It is well known that a PSV does not provide any protection against failure due to fire on an unwetted vessel. Many recognised companies practice not to install PSVs on unwetted vessel if fire is the only contingency.

Does anyone know of any officel reference, ASME clause or other codes that support the above practice?
 

RE: Omitting PSV on pressure vessel

When ASME says that you need a PSV as protection on every vessel, I'm quite certain that they don't mean you need to install a PSV- they mean you need secure overpressure protection that does not rely on operator intervention to function.  

It's in areas like this where ASME and many other such codes sometimes fall down- they should contain design philosophy and guidance for professionals to use rather than making a prescriptive edict to apply one particular solution which may, as in this case, offer an unskilled person a false sense of security.

While it may be the right tool to protect against operational overpressure, a re-seating device such as a PSV is the wrong type of protection against overpressure due to fire for a vessel containing gas or other non-vaporizing contents.  A rupture disc, passive automatic depressurization valve (ie. with fusible link) or thermal relief (fusible plug) combined with fire suppression etc. are the right tools for the job.

RE: Omitting PSV on pressure vessel

2
Take a look at ASME Code Case 2211.  It lists conditions where a relief device would not be required.  The basis is overpressure protection is provided by system design.

Good luck,
Latexman

RE: Omitting PSV on pressure vessel

Latexman, I seriously doubt you can use Code Case 2211 to eliminate a fire case relief valve, and I base this on Interpretation VIII-1-04-38 below:

Question (6): Is it required by the rules of Section VIII, Division 1 that overpressure protection be
provided for an unexpected source of external heat if engineering analysis shows that the vessel will fail
due to excessive temperature before the pressure relief device(s) would be activated?

Reply (6): Yes.

RE: Omitting PSV on pressure vessel

(OP)
Thanks a lot for all your valuable inputs. Unfortunately the conclusion is not as clear as I hoped for.

It seems that ASME require a PSV til be installed even though fire is not real source of overpressure.

I dont have a problem with installing a depressuring valve. However it really hurst my technical pride to install a PSV when everyone knows that it does no good.

Any new input on this thread is appreciated.
 

RE: Omitting PSV on pressure vessel

"If the pressure vessel if designed to ASME, then a PSV is required..."

i think to say a PSV is required is not accurate.  if the intent is a relief device is required, i would not argue.

"....A rupture disc, passive automatic depressurization valve (ie. with fusible link) or thermal relief (fusible plug) combined with fire suppression etc. are the right tools for the job. "

i think that moltenmetal gave some sound advice here but it appears to have been over-looked.

what is wrong with a rupture disc or fusible plug for this application?

 

RE: Omitting PSV on pressure vessel

The 2008 addendum to ASME Section VIII incorporated Code case 2211 as paragraph UG-140.  This essentially eliminates the need for a "courtesy" RV provided you have adequate documentation.

RE: Omitting PSV on pressure vessel

I like that phrase "courtesy RV".  I've done that several times to avoid the onerous Code case 2211 documentation.

Good luck,
Latexman

RE: Omitting PSV on pressure vessel

It was my understanding, if Code Case 2211 is declared it must be prior to building the vessel.  

Although, I will admit I have not reviewed the 2008 revision.

RE: Omitting PSV on pressure vessel

psafety,

That's the way it was prior to the 2008 addendum.  Not sure now, but it probably still must be declared prior to building the vessel.  
 

Good luck,
Latexman

RE: Omitting PSV on pressure vessel

@BenThayer & moltenmetal
Thanks for knocking my head...I agree with you. It should be relief device i.e PSV, RD, etc instead of PSV only.

As for code case 2211, i believe it can be applied for process contingency. However, i doubt it can be applied for fire contingency.

@Latexman
When you said "I've done that several times to avoid the onerous Code case 2211 documentation.". Is this applicable to fire contingency as well ? I have seen application HIPS for process contingency plus a relief device and depressruring valve for fire contingency. Do you mean High Integrity Protection system (HIPS) is also initiate the depressuring valve ?

For fire contingency, we always experience same argument in every projects. Everyone aware that relief device will not able to protect the vesssel under fire contingency, it only "buy" time for operator to take early action. However, everytime we can not avoid to put a relief device for vessel designed to ASME VIII (code requirement), like the answer given in CJKruger's response. Having a "courtesy RV", we focus on other protection measures i.e. depressusring, RD instead of PSV, external cooling, fire proofing, etc.

@dennisr,
"It seems that ASME require a PSV til be installed even though fire is not real source of overpressure."

Well... it is not easy to eliminate fire contingency. I really have no one experience that i can eliminate fire contingency. Anybody has ? How this is eliminated ?

JoeWong
Chemical & Process Technology

RE: Omitting PSV on pressure vessel

JoeWong88, the only way (I know of) to avoid a fire case is to elevate the vessel to above 25ft or 35ft.

However, I am trying to get hold of WRC Bulletin 498 to see if it allows us to use UG-140 (old Code Case 2211, thanks Lizking) to eliminate a PSV for a fire case.  

RE: Omitting PSV on pressure vessel

CJKruger,

Thanks for your idea. Beside, i am looking for this document also.

Elevate the vessel to above 25ft per API Std 521(why 35ft ?) is one the "measures" as it considered only pool fire. For some applications in gas plant, LNG plant, petrochemical plant, etc which possibly has jet fire scenarios, elevate vessel may becoming impractical.


 

JoeWong
Chemical & Process Technology

RE: Omitting PSV on pressure vessel

JoeWong,

Not applicable to a credible fire case.  The cases I recall used a vessel with enough MAWP to contain all credible scenarios; fire was not credible.  They also did not use HIPS.

Good luck,
Latexman

RE: Omitting PSV on pressure vessel

1) PSV is not PRV
2) Jet fire can sometime 30-80 meter from the source) but can be diluted by PFP ( paasive fire protection coated/lining at vessel surface) use CFD study, then you can save  a lot of money.
3) Vessel can happen under fire pool is for liquid vessel contains C4 and heavier, which normally not so high design pressure vessel.
6) Mechanical protection was normally by vessel thickness + PSV,BDV, while instrumntation by PSHH, HIPPS , it depend you use philosophy  2 barrier or 3 barrier( PSHH+BDV+HIPPS or PSV/Flare), to save flare sizing people will use HIPPS but need regular maintenance and redundant valve
7)All vessel if use need ASME stamp- you must has over pressure protection ( PRV or PSV), when design pressure>3.5 barg/ Diameter >6",etc.

RE: Omitting PSV on pressure vessel


Guys.....let me set the record straight.

Code Case 2211 [or now UG-140(a)] can indeed be used even if the vessel is exposed to the risk of fire. CC2211 originated from just such a case.....a vapor-filled pressure vessel that would fail before the vapor expanded enough to lift the PSV.

UG-140 has two parts, 140(a) and 140(b). Part 140(a) is essentially the same as the old CC2211. 140(b) is new, going a little beyond the old CC2211.
 

RE: Omitting PSV on pressure vessel

(OP)
don1080,

Very interesting! Where do you have this information from. I would really like to know if UG-140 have been used to exclude fire case on a vapour filled vessel.

RE: Omitting PSV on pressure vessel

Latexman,
Thanks for your information.

Your case "Not applicable to a credible fire case." still maintain my understanding...

JoeWong
Chemical & Process Technology

RE: Omitting PSV on pressure vessel


CC 2211 (now UG-140(a))can indeed be used in a fire case. In fact it was a fire case for a vapor filed vessel that originated CC 2211.

This provision is applicable when there are no credible scenarios that can cause the vessel pressure to exceed it's MAWP. For vapor filled vessel, exposed to fire, the pressure will rise as the gas heats and expands, but that pressure increase will usually not reach the MAWP before the vessel fails. Even if the pressure does reach the MAWP, or set pressure of the PSV, the vessel will not be adequately protected because the heat is not being carried away by a boiling liquid.
 

RE: Omitting PSV on pressure vessel


dennisr.....my information comes from the company that made the original inquiry that initiated CC 2211.

Regards

RE: Omitting PSV on pressure vessel

don1980,
Thanks for your information.

I am still wondering how a HIPS system can help in a vessel expose to fire...

JoeWong
Chemical & Process Technology

RE: Omitting PSV on pressure vessel


Joewong....We're discussing UG-140(a) (used to be CC 2211) rather than HIPS. Your right in saying that HIPS can't effectively defend against a fire scenario, but that's another subject. Vapor filled vessels, exposed to fire risks, are candidates for UG-140(a).

From your question, I'm guessing that you might be referring to UG-140(b) which allows the use of instrumentation to build a case for "overpressure by system design." That instrumentation can be HIPS but it doesn't have to be. I've heard others that interpreted it that way too, but that's an overly restrictive interpretation. It can be HIPS or it can be non-HIPS.

Personally, I think the wording could have been a little clearer. I think a lot of people are going to misinterpret UG-140(b). I'm also very surprised to find practically zero discussion on UG-140 when I search the internet. This was published by ASME last July, but I don't think a lot of folks have noticed it yet.

Regards.

 

RE: Omitting PSV on pressure vessel

(OP)
I have just received the new revision of API-521 (2007). In the new revision it is clearly written that, if accepted by local regulation, relief valves on unwetted vessels may be replaced by other means of protection such as water spray or depressurization.

Also the new revision distinct between pool fire and gas jet fire. In case of gas jet fire only (gas plants) the relief valve does not provided any protection and should be replaced by other means of prtection.

Also ASME VIII UG-140 allows relief valves to be omitted if allowable overpressure cannot be exceeded. This clause also refer to API-521.

Conclusion: In our synthesis gas plants we will consider to delete relief valves on unwetted vessels (reactors) and install depressuring valves instead.

RE: Omitting PSV on pressure vessel

When there is no basis for sizing, it becomes difficult to justify selection.  Some vendors don't want to supply a valve without a basis and without a calculation.  I have seen process engineers make up an imaginary liquid for fire sizing when no other case exists and still use the properties for the gas!  This is not a valid calculation.

Or another basis that does not work is "thermal expansion" for gas.  In this case, Carbon Steel will often melt before thermal expansion of the gas can reach relief temperature.  Thermal expansion of gas is almost never a viable case.  Using it when it is not viable raises questions.

. . .and so the question still arises; "what is the basis?". The basis is "Nominal valve selected without sizing because no viable relief case exists.  Valve supplied because ASME Section VIII requires it."  Maybe this will work.  

I know that made up cases and calculations don't work when they are scrutinized by approval agencies, et al.

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