3 replies to this topic

Hello all

I have posted this question in another part of the forum, until I realized that this section is much better suited for discussion.

I have a question regarding operating pressure and MAWP. What I am looking for is whethere there is some sort of Code or regulations or even recommendations (ASME or API) for this sort of scenario. I did some research myself, but all I have found so far are "good engineering experience" recommendations. Of course there is nothing wrong with that, I guess I just want to make sure that I am not overlooking any code/regulation on that matter.

I am looking at the following scenario. Gas from a well outlet enters a horizontal separator vessel. The vessel has MAWP of 720 PSIg. The relief valve attached to the vessel has a set point of 720PSIg and is spring operated. 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:
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 engineer, who designed this setup, claims my reasons are invalid, so I am looking for some "written" proof/code etc. The other thing the designer could do, is to check and see whether the PSV set point can be changed to a higher value. I believe there is still some room for that, but the relief rate would need to be checked.

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

Thank you

[pleckner]

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.

If this designer still fights you on this, have him read my response and if he wishes, have him contact me. I am very curious as to how this designer figures your reasoning to be invalid.

(By the way, I answered this in your post on the "Professional" section as well.)

Thanks for the response, Phil, and sorry for the double post. I realized too late that there was a relief device forum. I notified the moderator that the other thread can be closed.

To get back to the question I had. I believe, the reason for setting operating so close to the setpoint of the PSV is because this configuration sits on a skid that the owner would like to move around between different wells. So in their eyes, it would be great if the skid would work under any working pressure less or equal the MAWP of the vessel which is also the set point of the PSV.

I have told the engineer in charge that this is really shorthanded thinking, and that I have stamped the drawings with the max operating pressure set to 648 PSIg, which is 10% less than the PSV set point. He will have to take it, or look for someone else to review their drawings/designs.

I will also share this piece of information from API, not a code, but recommendation.

In API521 it says, "A pressure vessel is normally designed for 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 indicated that this is an example that depends on the relieving device. This is also shown in API521, Figure 4 and labelled "Usual maximum normal operating pressure".

Great articles you have posted on that subject, Phil. I read most of them and found them very helpful.

[pleckner]

Thanks for those kind words. That's why the articles are there, hoping they are of use.

I saw your post on eng-tips as well and see we are all in agreement. Again, the 10% margin works well for higher pressures but that 25 psi delta should be considered for lower pressure systems.

You did the correct thing!!