relief valve set pressure Q 6

[EngCutie]

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


Cutie

 

[BenThayer]

i pretty much set PSV's at MAWP unless there is a specific process reason not to.

 

[EngCutie]

When I prepared datasheets, I did use the maximum pressure, but after my manager looking at, he said if there was an accident, the package will be running at the maximum pressure, he said it might not be good, he wanted the pressure relief valve to start relieving before reaching maximum pressure. I'm not sure if it's a good idea?

Cutie

 

[Latexman]

Pressure vessel code limits the relief valve set pressure to the design pressure (MAWP for ASME Code). It can be lower, but not higher.

Good luck,
Latexman

 

[BenThayer]

set it at whatever pressure your manager wants/decides. just make sure that there is no credible scenario where an operational upset can cause the PSV to lift prematurely.

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

 

[EngCutie]

I just read ASME VIII, the section about relief valves, it says: ‘set pressure tolerances, plus or minus, of pressure relief valve shall not exceed 2 psi for pressure up to and include 70 psi and 3% for pressure above 70psi’

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

 

[BenThayer]

the set pressure tolerance you are referencing is in the setting of the valve and not the selection of the set-point.

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?

 

[EngCutie]

oh, I see, thank you it does make more sense now

Cutie

 

[JAlton]

Perhaps these ASME Code references will be helpful.

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

 

[olcrazy1]

set pressure has to be at or below MAWP, it can not be greater than MAWP. Now accumulation pressure can be greater than MAWP. Just because a valve is set to pop at a given pressure doesn't mean that it is fully open at a set pressure. If a valve is set to open at 100 psig it is not fully open and flowing until it reaches over pressure, which in most cases is 10% over the set pressure the difference in over pressre and set presure is called accumulation pressure. In a fire case it is 21% when a valve is fully open which is why the same valve with the same set pressure will have a greater capacity in a fire sizing than a blocked flow. So just because you set a valve at or below MAWP it does not mean the system will not exceed MAWP when it is relieving.

 

[JAlton]

I agree with Ben & olcrazy1. Some high pressure, high performance, Sec. I, Power Boiler Safety Valves and some Sec. VIII, Snap acting pilot valves will go full open on initial pop, but they are the exception rather than the rule. These designs are more expensive because you get what you pay for. The majority of Sec. VIII, direct spring PRVs will not go fully open on initial pop. More likely, they go 50 to 70% open and then gradually reach full lift at or before 110% of Set Pressure (Accumulation Pressure). This overpressure scenario is common in Pressure Vessels, but if you have a situation where some component in your system is sensitive to the overpressure, you should probably reconsider your operating parameters or choose other components. ASME Sub-Committee Safety Valve Requirements has been considering rules for PRVs in applications below 15 psi for some time. The Set Pressure Tolerance and Overpressure Scenarios come in conflict with Hydrostatic testing requirements when you get to those low pressures. It makes the issues you are facing even more mind boggling.

JAC

 

[JoeWong88]

EngCutie,
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

 

[JoeWong88]

EngCutie,
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

 

[EngCutie]

very helpful! Thanks everyone.

Cutie


Thank you, EngTips

 

[EngCutie]

Another Q: Rupture disc burst pressure:

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

 

[foxymophandlpapa]

Set the burst pressure the same as the PSV set pressure: 25 barg.

 

[JoeWong88]

Cutie,
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

 

[cannondale]

As a rule I always go for 115% of the max duty pressure unless this is within 10% of the pipework pressure.

Generally anything less than 110% will leak, although it does really depend on what process your talking about?

 

[Jefka]

Another situation where you would deviate from the equipment MAWP as a PSV set is when a client is cobbling together a plant with used equipment from all over the place.

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.

 

[Casimo5]

To expand on Jefka's example, if you have a string of equipment with the same MAWP (no rotating equipment or block valves in between them) and a single relief valve protecting all of them after the last piece of equipment (as is the case in some gas plants), you may want to set the relief valve at a pressure that takes into consideration the pressure drop through the system.

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.