PSV Set Pressure 1

[Sharik]

Is there a limit on how much higher a PSV Set Pressure can be above the normal operating pressure of the equipment it is protecting? For example, an ASME Section VIII, Div. 1 pressure vessel has a MAWP of 14.4 Bar, normal operating pressure is 5 bar and the PSV set pressure is 13.9 bar. Is this acceptable? I know its acceptable as far as the MAWP and PSV Set Pressure but it seems like a large difference between operating pressure and the PSV set pressure. I could have sworn in the older ASME Codes there was a requirement regarding the difference between the normal operating pressure and the PSV Set Pressure but I can't find anything like that in the later Code editions.

 

[Shmulik]

Please see at the following discussion:
thread794-197791

 

[Sharik]

Shmulik, thanks but the discussion doesn't really answer my question. I'm not concerned regarding the PSV Set Pressure and overpressuring the equipment, the PSV's have been sized with enough capacity to prevent the pressure from exceeding 10% above the MAWP during the causes of the overpressure.

I am concerned about the large difference between the Operating Pressure (5 bar) on the equipment and the PSV Set Pressure (13.9 bar).

 

[vpl]

Sharik

If the system was designed properly, then there shouldn't be any concern. If you want something operationally, put in a power operated relief valve.

I'd rather have margin between operating and design pressures. You don't want a small pressure transient turning itself into a really large one becuase the safety valve setpoint was reached.

Patricia Lougheed

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[sheiko]

Moreover, the higher the set pressure, the smaller (thus the cheaper) the SRV.

"We don't believe things because they are true, things are true because we believe them."

 

[Sharik]

I understand the PSV Set Pressure should not be so close to the Operating Pressure so the PSV could be relieving frequently. I just want to know if there is a standard or recommended practiced regarding the maximum difference between the normal Operating Pressure and the PSV Set Pressure.

 

[sheiko]

I presume your design pressure is 13.9 barg, right? And that your application is a heat exchanger, right? If so, it is normal to have both tube and shell sides rated to the same (design) pressure, whatever the operating pressures on each side.
As for standards, each company have one. I don't remember if API STD 521 proposes such margins...

"We don't believe things because they are true, things are true because we believe them."

 

[sheiko]

Oups... i did not see your example. MAWP of pressure vessel is 14.4 barg. Sorry.

"We don't believe things because they are true, things are true because we believe them."

 

[Sharik]

Sheiko: It's a pressure vessel, MAWP 14.4 barg, PSV Set Pressure 13.9 barg and a normal Operating Pressure of 5 barg. Is it normal to have such a large difference between the Operating Pressure (5 barg) and the PSV Set Pressure (13.9 barg)?

I do realize it is normal to have the shell side and tubeside MAWP's for a heat exchanger to be the same, but that's not the question.

 

[sheiko]

Sorry for not reading your original post carefully. To answer you i would say it is normal as long as your system operates on spec and safely. Seems to be the case...

"We don't believe things because they are true, things are true because we believe them."

 

[EngAddict]

Normally vessels are not over designed by a factor of 2, hence you will usually find much smaller differences between the MAWP and the OP. There is nothing wrong with such a large difference, provided an increase in pressure 100% above OP does not cause any process issues. On the vessel side it should be fine IMO.

 

[sheiko]

Sharik, see attached.

"We don't believe things because they are true, things are true because we believe them."

 

[subhen73]

As per ASME SEC VIII DIV.1 UG-125 (C), there is no upper limit but the mimimum set pr. is MAWP+10% OR MAWP+3 psi, Which ever is greater. for more detail, refer Code Book.

subhen praharaj

 

[psafety]

subhen,
I would read ASME SEC VIII DIV.1 UG-134.

 

[Sharik]

subhen73 & psafety,

I don't understand your responses. UG-125(c)refers to the PSV's having sufficient relieving capacity to prevent the pressure in the equipment from rising more than 10% or 3 psi, which ever is greater, above the MAWP. UG-134 refers to one PSV not having a set pressure above the equipment MAWP and any additional PSV can have set presures up to 105% of the MAWP.

Neither of these are relevant to my question of what would be the normal difference between the normal Operating Pressure of the equipment and the PSV set pressures. Is there a maximum difference between these two pressures?

 

[pleckner]

The difference between the maximum expected operating pressure and the PSV set pressure is not dictated by any Code or standard. The minimum difference should be the greater of, 10% of the maximum operating pressure or 25 psi to ensure you won't get simmering. But there is nothing that says you can't make this difference greater.

 

[gr2vessels]

Sharik,
My two cents;- it appears that your concern is really why the MAWP is so high, so that you need to set the PSV around that high allowable pressure. That's simply because of the vessel over-design. A good economical design would see the MAWP at the design pressure level or slightly higher. However, there might be other considerations which required additional strength to the vessel, thus increasing the MAWP. Either way, the high MAWP drove your PSV setting higher than you would expect, but as said above, that worked well by sizing a smaller (and cheaper) SRV.
Cheers,
gr2vessels

 

[111Monster]

AFAIK there is no limit on how much higher the set pressure can be over the operating pressure. On the other side, I would not set a conventional valve within 10% of operating pressure unless you like to cycle PSV often.

 

[Duwe6]

to more closely address Sharik's question: yes, it is indeed 'usual & customary' to have the PRV set-pressure at or only slightly below the MAWP. In the example Sharik gives, this allows a MUCH smaller device that still provides the necessary volume [see Sheiko's top post].

Any setpoint between 5 and 14.4 barg is "legal" and meets Code, and is thus deemed "good engineering practice". And the higher the PRV setting is, the smaller [cheaper] the PRV can be to provide the mandatory volume. If it was my vessel, I would have picked a setpoint very similar to 13.9