6 replies to this topic

[Lowflo]

API 521 (7.3.1.3.9b) states that PSVs should not be located below the elevation of the flare header. The purpose is to ensure that liquid in the tailpipes can freely drain to the header.

In the introductory paragraph API softens this statement by saying, "The following points are not to be taken as definitive or restrictive."

Based on your experience, how rigidly do companies adhere to this recommendation?

In my experience in the chemical industry, there is very little adherence. For PSVs that are located above the flare header, yes, we ensure that there are no pockets in the tailpipe. But, we rarely elevate the PSV solely for the purpose of satisfying this recommendation. The PSVs on ground-level equipment are usually below the elevation of the flare. The tailpipe rises sufficiently to connect to the top of the flare header, to prevent continuous accumulation of liquid in the tailpipe, but we don't ordinarily extend the PSV inlet pipe just to locate the PSV at a higher elevation than the flare header.

I understand the risks that this poses. The liquid head in the tailpipe can change the effective set pressure of a conventional PSV. For bellows and pilot PSVs, the liquid head creates flow restriction, although normally this is just a momentary effect. The greatest risk, in my view, is that of plugging due to devolatilization or polymer formation. If that risk is recognized up front, and managed by inspecting for plugging during the routine PM of the PSVs, then I don't think that the cost of elevating the PSVs is justified.

What is you experience? Is this API recommendation practiced for your company, or by most of your clients?

[fallah]

API 521 (7.3.1.3.9b) states that PSVs should not be located below the elevation of the flare header. The purpose is to ensure that liquid in the tailpipes can freely drain to the header.

In the introductory paragraph API softens this statement by saying, "The following points are not to be taken as definitive or restrictive."

Based on your experience, how rigidly do companies adhere to this recommendation?

In my experience in the chemical industry, there is very little adherence. For PSVs that are located above the flare header, yes, we ensure that there are no pockets in the tailpipe. But, we rarely elevate the PSV solely for the purpose of satisfying this recommendation. The PSVs on ground-level equipment are usually below the elevation of the flare. The tailpipe rises sufficiently to connect to the top of the flare header, to prevent continuous accumulation of liquid in the tailpipe, but we don't ordinarily extend the PSV inlet pipe just to locate the PSV at a higher elevation than the flare header.

I understand the risks that this poses. The liquid head in the tailpipe can change the effective set pressure of a conventional PSV. For bellows and pilot PSVs, the liquid head creates flow restriction, although normally this is just a momentary effect. The greatest risk, in my view, is that of plugging due to devolatilization or polymer formation. If that risk is recognized up front, and managed by inspecting for plugging during the routine PM of the PSVs, then I don't think that the cost of elevating the PSVs is justified.

What is you experience? Is this API recommendation practiced for your company, or by most of your clients?

In cases where it is impossible/impractical keeping PSV outlets self-draining to the flare header,outlet line goes into the top of the flare header and a manual drain valve is considered in PSV outlet for regular draining of any condensate.
The valve must be accessible from grade/platform,and in cold climates downstream dead leg must be steam traced to prevent freezing.

[Lowflo]

What is you experience? Is this API recommendation practiced for your company, or by most of your clients?

[fallah]

What is you experience? Is this API recommendation practiced for your company, or by most of your clients?

It is a good engineering practice.As a sample please see the "process piping design"Volume 1 by: Rip Weaver.

Edited by fallah, 18 October 2009 - 12:40 PM.

[rxnarang]

I have never seen this requirement not being met. The PSV is always elevated above the flare header.

Regards

[Lowflo]

I have never seen this requirement not being met. The PSV is always elevated above the flare header.

Regards

In your experience all PSVs are mounted at an elevation higher than the flare header, including those PSVs that are protecting ground-level equipment? For ground-level equipment, the inlet pipe will be about 6m or more in length.

Do others out there agree that this is standard practice?

Thanks.

[thakur]

sometimes its not feasible to locate the psv above the flare header level due to cost,location and other factor.in my plant i have not seen such practice followed for the psv which are protecting the ground level equipment.