I have a PSV with a set pressure of 229kg/cm2g. What could be the back pressure I consider while designing the flare header so that it would be cost effective as well as safe?
Answers
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21/10/2009
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Ralph Ragsdale, Ragsdale Refining Courses, ralph.ragsdale@att.net
You may find some reluctance from consultants to respond to this issue due to liability, the absence of carefully worded disclaimers, etc. My opinion, simply based on my experience is as follows:
Your 50% plan is valid, so long as any other PSV that could conceivably relieve at the same time into the same HP relief system is designed to handle its calculated backpressure during that period.
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21/10/2009
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Original Questioner, Follow Up,
Thanks, Bob and Keith, for posting your responses, but my query is specifically: is it safe to consider back pressure of 50-70 kg/cm2g when my PSV set pressure is at 229 kg/cm2g?
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16/10/2009
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keith bowers, B and B Consulting, kebowers47@gmail.com
API RP-510 is the reference procedure you must use to perform the calculation. Each PSV design will have unique limitations, which are an integral part of the PSV characteristics. For example, pilot operated valves are much less sensitive to back pressure effects than simple spring operated ones.
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15/10/2009
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Bob Kalaria, KBR, bhavdip.kalaria@kbr.com
Usually during a normal operation (No major relief or flaring) flare header would be at a pressure slightly above the atmospheric pressure. This pressure is determined after a detail flare header analysis. However, flare header analysis cannot be performed without completing relief valve analysis so usually a back pressure in the range of 0.14-0.35 kg/cm²g is assumed for relief valve analysis. Ideally after completing flare header analysis, this number should be adjusted and relief valve will need be rechecked. However, if you assume a conservative number such as 0.35 kg/cm²g for analysis, you can minimize this rework.
If you are performing this analysis for a global scenario such as power failure the back pressure will be much higher as many PSVs will be relieving at the same time, this pressure will be in the range of 1-3 kg/cm²g and what backpressure is to be considered depends on refiners philosophy and it also depends on numbers of flare system in the refinery.
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