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Apologies if this has been posted sometime in the past, but can anyone help me with interpreting the API 3% guideline for inlet lines to relief devices?
API RP-520 Part 2 recommends that the pressure drop in the inlet line to a relief device should not exceed 3% of the guage set pressure of the device. It goes on to say that if the PSV is installed on a process line (as opposed to directly on the vessel), the 3% should be applied to the sum of the loss in the inlet line and the incremental pressure loss in the process line caused by the flow through the PSV. I have always read this incremental pressure loss as pressure drop @ rated PSV flow rate minus pressure drop @ normal flow rate. My question is, what should you use for "normal" flow rate?
The application I'm looking at is in a utility system, therefore the "normal" flow rate is variable. There is a nominal "average" flow and there is a max flow. The max flow is much higher than the average and the rated PSV flow is obviously larger than the max flow. Therefore the incremental pressure drop increase between average and rated PSV is very large. This leads me to question, why use the average? Can I use the max? But even this seems fairly arbitrary because the PSV could operate at any point in time.
Any advice?? thanks
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Api 3% Guideline
Started by Guest_Simple Plummer_*, Apr 11 2005 01:06 AM
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djack77494
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Posted 15 April 2005 - 05:11 AM
Dear Plumber,
My guess as to the intention of this guideline is that this pressure drop limitation avoids the possibility of the valve chattering. If you picture the situation, imagine that your pressure builds to the PSV setpoint. Now the PSV snaps open and relieves at its rated capacity. This increment in the system's flowrate causes pressure drop, the only significant component of which is often the pressure loss in the PSV inlet line. The pressure drop means that the pressure that the PSV "sees" is reduced. At some point, the PSV would see a low enough pressure that it would close. Since some of these valves can seal pretty near to their setpoints, I think the conservative approach of allowing a mere 3% loss in the PSV inlet was taken.
To get into this more deeply, and specifically to address the possibililty of the PSV in a normally flowing line, I would view the situation as follows:
Picture the fluid source to be an infinitely large reservoir/container (just so we're not going to worry about pressure variations). Same for the fluid sink or destination. Now what scenario(s) do you envision that would result in a relief event? The "normal" flowrate would be the highest of any flowrates surrounding an event that would not reach the PSV setpoint.
Hope that was useful,
Doug
My guess as to the intention of this guideline is that this pressure drop limitation avoids the possibility of the valve chattering. If you picture the situation, imagine that your pressure builds to the PSV setpoint. Now the PSV snaps open and relieves at its rated capacity. This increment in the system's flowrate causes pressure drop, the only significant component of which is often the pressure loss in the PSV inlet line. The pressure drop means that the pressure that the PSV "sees" is reduced. At some point, the PSV would see a low enough pressure that it would close. Since some of these valves can seal pretty near to their setpoints, I think the conservative approach of allowing a mere 3% loss in the PSV inlet was taken.
To get into this more deeply, and specifically to address the possibililty of the PSV in a normally flowing line, I would view the situation as follows:
Picture the fluid source to be an infinitely large reservoir/container (just so we're not going to worry about pressure variations). Same for the fluid sink or destination. Now what scenario(s) do you envision that would result in a relief event? The "normal" flowrate would be the highest of any flowrates surrounding an event that would not reach the PSV setpoint.
Hope that was useful,
Doug
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