relief valve set pressure Q
relief valve set pressure Q
(OP)
No one answered my relief valve question, I have got another one, hope you can help
If the back pressure is zero, does the set pressure have to be 10% lower than the design pressure (Max. pressure)? If not, what's the rule when determining set pressure? (If client doesn't specify)
Thanks
If the back pressure is zero, does the set pressure have to be 10% lower than the design pressure (Max. pressure)? If not, what's the rule when determining set pressure? (If client doesn't specify)
Thanks
Cutie





RE: relief valve set pressure Q
RE: relief valve set pressure Q
Cutie
RE: relief valve set pressure Q
Good luck,
Latexman
RE: relief valve set pressure Q
i have seen folks try to be "safer" by having a relief device lift earlier than MAWP but it typically results in events that should have never happened.
not quite the same thing but read up on these PSV's that were set below the MAWP of the device.
http://www.csb.gov/newsroom/detail.aspx?nid=167
RE: relief valve set pressure Q
Does it mean if the MAWP is less than 70psi the set pressure is MAWP+/- 2psi, if the MAWP is over 70psi, the set pressure has to be MAWP +/- 3%ofMAWP?
Cutie
RE: relief valve set pressure Q
If you, as the design engineer, say the valve should be set at 100 psig, the tolerance comes into play when you buy the valve. the valve should be stamped with a set pressure of the 100 psig.
BUT, when it is tested on the bench and is "popped" then the actual activation pressure has to be +/- 3%.
so if you say you want a valve set at 100 psig and i (as a mechanic) install the proper spring and adjust the valve, i have to have it pop at 97 to 103 psig. if it pops at 103 psig, i can stamp it at 100 psig and ship it.
if it pops at 96 or at 104 psig, it is outside the tolerance and i have to readjust until it falls between 97-103 psig.
make more sense?
RE: relief valve set pressure Q
Cutie
RE: relief valve set pressure Q
The Safety-Relief Valve shall be set at or below MAWP, Ref. ASME Sec. VIII, Div. 1, Para. UG-134(a).
The allowable accumulation is 3 psi or 110% of MAWP, whichever is greater, Ref. ASME Sec. VIII, Div. 1, Para. UG-125(c).
The Set Pressure Tolerance for Safety-Relief Valves is +/- 2 psi up to 70 psi and 3% of Set Pressure above 70 psi, Ref. ASME Sec. VIII, Div. 1, Para. UG-126(d).
JAC
RE: relief valve set pressure Q
RE: relief valve set pressure Q
JAC
RE: relief valve set pressure Q
When deciding on the PRV set pressure, at least two points you may have to look at.
Benthayer, JAC, Latexman, etc has given you the first point where set pressure can same as or lower than MAWP.
Second point you have to looks at is premature opening of PRV by considering
- maximum operating pressure
- type of PRV
- superimpose back pressure
- blowdown
- net spring setting
- tolerance,
- etc
For a conventional type PRV, the maximum operating pressure may needs to be approx. 10% lower than the set pressure to avoid unnecessary premature opening. Whilst for pilot operated PRV, it may be approx. 3%. Please check with your PRV vendor. They probably can give you a better idea.
JoeWong
Chemical & Process Technology
RE: relief valve set pressure Q
When deciding on the PRV set pressure, at least two points you may have to look at.
Benthayer, JAC, Latexman, etc has given you the first point where set pressure can same as or lower than MAWP.
Second point you have to look at is premature opening of PRV by considering
- maximum operating pressure
- type of PRV
- superimpose back pressure
- blowdown
- net spring setting
- tolerance,
- etc
For a conventional type PRV, the maximum operating pressure may needs to be approx. 10% lower than the set pressure to avoid unnecessary premature opening. Whilst for pilot operated PRV, it may be approx. 3%. Please check with your PRV vendor. They probably can give you a better idea.
JoeWong
Chemical & Process Technology
RE: relief valve set pressure Q
Cutie
Thank you, EngTips
RE: relief valve set pressure Q
A rupture disc is installed parallel to a relief valve, relieving pressure of the relief valve has been given: 25Barg. Then what should the burst pressure of the rupture disc be?
Should I use a reverse acting rupture disc in this case?
Thanks
Cutie
Thank you, EngTips
RE: relief valve set pressure Q
RE: relief valve set pressure Q
If the question is non related to original post, it is always better to open a post.
PRV and Rupture disc will have different manufacturer tolerance. RD may be about 10% (confirm with manufacturer) and PRV probably much lower. RD may need to set lower pressure. But under positive tolerance RD will premature open, PRV will never open. Under negative tolerance, PRV + RD may open simultaneously. PRV will reseat quickly and do nothing. Then what is the purpose of PRV ?
What is the design load for PRV and rupture disc ? What is the design pressure and MAWP of the vessel ? What is the purpose of having PRV + RD in parallel.
JoeWong
Chemical & Process Technology
RE: relief valve set pressure Q
Generally anything less than 110% will leak, although it does really depend on what process your talking about?
RE: relief valve set pressure Q
Say you have Vessel A with a 450 psig MAWP and Vessel B downstream with a 600 psig MAWP. Assume no rotating equipment in between them. Generally in this case, you would want each of their PSV's set to the lower of the MAWPs.
RE: relief valve set pressure Q
Say you have a pressure drop at normal operation of 200 psi through your system, and the MAWP for all equipment is 600 psig, your relief valve may be set at 400 psig to protect the equipment at the front end.
RE: relief valve set pressure Q
Generally I want a relief valve to be set at the MAWP of the system it is protecting, and to have plenty of other controls on the system to reduce the need to rely on it. It is a final, passive and reliable layer of defense, NOT a routine defense.
RE: relief valve set pressure Q
What you're saying is correct- you do want a PSV set pressure to take into account inlet line losses if they're significant (> 3% to 5%).
However I would still be very hesitant to place a new relief valve at the end of a chain of equipment like that. Pressure drop is not always a reliable value in gas plants due to the multiple variables that go into it and there are also lots of potential complications (methane hydrate formation, liquid slugs, etc) that could make reliving difficult. Almost without exception I would recommend placing the relief valve at the first equipment item.
RE: relief valve set pressure Q
I agree. It makes more sense to place the relief valve at the front end, closer to the source of the overpressure in the example I gave. I was just giving an example of when you would set the PSV set pressure below the MAWP. If the relief valve is placed at the front end, then you can set the pressure at the MAWP.