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Operating pressure/MAWP/PSV setpoint

Operating pressure/MAWP/PSV setpoint

(OP)
Hello all

I am looking at the following scenario. Gas from a well outlet enters a horizontal separator vessel. The vessel has an MAWP of 720 PSIg. A relief valve attached to the vessel has a set point of 720PSIg. The operating pressure is set at 720 PSIg as well.

I am reviewing this process/design and I have rejected it, based on the following (and other reasons):

1) With the operating pressure so close to the PSV set point, I would expect excessive venting of product and wear out of the PSV.
2) Once the PSV opens, there is a possibility that it won't close again, if the operating pressure stays at a value close to the PSV's set point, even if that value is below the PSV'set point. (PSV cannot reach reseating pressure).

I have read and my own experience tells me that the operating pressure should be 10% less than the PSV set point, so in this case, the maximum operating pressure would be 648 PSIg. However, the senior engineer/designer, who designed this setup, claims my reasons are invalid, so I am looking for some "written" proof/code etc. Plus, since I am the one who is supposed to stamp this as built

I hope someone in here can give me a pointer in the right direction.

Thank you

RE: Operating pressure/MAWP/PSV setpoint

(OP)
Actually, I should post what my stand on this application is:

Normally a PSV is set equal to the MAWP of the equipment it protects, but the max normal operating pressure should never be in excess of 90% of the set pressure of the relief valve, otherwise excessive leaking can occur.

RE: Operating pressure/MAWP/PSV setpoint

Every PSV has a blowdown, usually 10%, but can be 3% with pilot PSVs.

If your operating pressure is above the blowdown pressure, then after you lift the PSV, the PSV will not seat again unless the operating pressure drops below the blowdown pressure.

What your senior engineer may be implying is that he knows the about the blowdown pressure, and on restart, he lowers the pressure below the blowdown pressure before repressuring. If he does this, then the PSV should reseat.

As an aside, having an operating pressure at the PSV set pressure probably means you are simmering constantly. This will wear the seats and probably cause good leakage. In general, simmering is not good for the valve, and hence, not good for the process. I am guessing that he is trying to run his process as close to the edge as he can. Pushing your system this hard has it's consequences.

"Do not worry about your problems with mathematics, I assure you mine are far greater."   
Albert Einstein
Have you read FAQ731-376 to make the best use of Eng-Tips Forums?

RE: Operating pressure/MAWP/PSV setpoint

(OP)
Ashereng, you are right about the pushing. I should have mentioned that the separator is part of a skid. From the discussions I had with the design team I gathered that they want to have as high an operating pressure as possible, so they can move around the skid to different wells with possible different operating pressures.

From your answer, and from what I have read, I think there is no code/regulation that would cover this case.

I will advise the design team that I won't stamp this design if the operating pressure is not lowered to 10% of the PSV set point (the PSV in question is spring operated)or if the PSV is not resized.

RE: Operating pressure/MAWP/PSV setpoint

>>
I will advise the design team that I won't stamp this design if the operating pressure is not lowered to 10% of the PSV set point (the PSV in question is spring operated)or if the PSV is not resized. <<

Make that 90% of the psv setpressure, or 10% LESS THAN the psv setpressure.  

ALmost all ASME Sec VIII spring-loaded valves have 10% blowdown, and thus will reseat at 90% of set pressure.  

RE: Operating pressure/MAWP/PSV setpoint

Besides the issue of blowdown, there is a tolerance on setpoint, and the valve will begin to open as it APPROACHES the PSV setpoint (simmer). Operating too close means that there will be a continual leakage. Besides the above, I do think there is a code requirement that mandates a separation between the operating pressure and the PSV setpoint. Unfortunately, I am not at a place where I could look it up for you and quote chapter and verse.
Doug

RE: Operating pressure/MAWP/PSV setpoint

djack,

I'd also be interested in the code reference that you mentioned also. I don't believe I have ever heard of that code.

"Do not worry about your problems with mathematics, I assure you mine are far greater."   
Albert Einstein
Have you read FAQ731-376 to make the best use of Eng-Tips Forums?

RE: Operating pressure/MAWP/PSV setpoint

(OP)
I think we all agree that running the process so close to the set point of the PSV is bad in terms of engineering, economics (who would want to constantly waste their product through a PSV) and safety of operating personnel.

Doug, it would be great if you could find that reference.

When looking at PSV's, all the code seems to be concerned about is MAWP, design pressure and relief rate/capacity (I am kind of oversimplifying here).

RE: Operating pressure/MAWP/PSV setpoint

Hmm. I found in API521 it says, "A pressure vessel is normally designed for a MAWP that will provide a suitable margin above the operating pressure in order to prevent any undesiarable operation of the relieving device." In API520, Part 1, Figure 26 shows a maximum expected operating pressure of 90% of the design pressure, though it is clearly indicates that this is an example the dependson the relieving device. This is also shown in API521, Figure 4 and labelled "Usual maximum normal operating pressure". That seems like a pretty strong statement to me, and you deviate from it at your own risk.

Still, I know I've skirted the problem. Unfortunately, I do not have ASME, Section 8, which I would think would address this issue. Sorry I can't do better.
Doug

RE: Operating pressure/MAWP/PSV setpoint

I would not classify your example as "code". It may be a general rule of thumb or example, but not code.

If you are using a pilot PSV, you can get to 5% relatively worry free.

The 10% I think is just a number in an example.

If you find otherwise in ASME Sect. VIII, please do let me/us know.

Thanks.

"Do not worry about your problems with mathematics, I assure you mine are far greater."   
Albert Einstein
Have you read FAQ731-376 to make the best use of Eng-Tips Forums?

RE: Operating pressure/MAWP/PSV setpoint

(OP)
I should also be sharing a response I got from pleckner on the Cheresources forum for basically the same question. He provides valuable input:

Quote start:

First, you are correct in rejecting this design and your evaluation of the maximum operating pressure and PSV set pressure. They cannot be the same as you have reasoned.

Looking at it slightly different, the design pressure should be about 10% or 25 psi greater than the maximum operating pressure, which ever is greater. In this case, the 10% will do. And the reason for this is as you suspect. Much closer to the PSV set pressure risks having the PSV "simmer" or start opening and allowing the contents to relieve prematurely. The design pressure may or may not be the same as the MAWP. I've written on this and you can find my article on PSV set pressure on this website.

There is no code that defines the maximum operating pressure. It is strictly whatever the process designer decides to set it at. It is just a matter of good engineering judgement.

No, you cannot by code set the PSV any higher than it already is. It is limited by the MAWP.

Quote end:

RE: Operating pressure/MAWP/PSV setpoint

eguentsch,

Yes, it was my memory that there is no "code" that defines how close to the set pressure of a PSV you can run, and by association, where the operating pressure is relative to design pressure/MAWP.

10% is typical for spring PSV (or conventional). Pilots allow you to get to 5%, some say 3%.

The core problem here seems to be that the owner is pushing his system very hard. I think the system is reaching its limits. The PSV is not the only problem here I think.

"Do not worry about your problems with mathematics, I assure you mine are far greater."   
Albert Einstein
Have you read FAQ731-376 to make the best use of Eng-Tips Forums?

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