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[s'more]

Hi Everyone,

I would like to get some of your opinions regarding the use of conventional PSVs used in a cascaded arrangement, where the outlet of the PSV is relieved upsteam around an isolation valve back into the line.

Being exposed to the industry, I found it common practise for conventional PSVs at tank inlets and outlets to be cascaded back into the tank. From API520, the allowable back pressure in these scenarios should be less than or equal to the overpressure set on the PSV.

For a single valve arrangment around a tank outlet the overpressure would be 110% and the PSV would have a back pressure equal to the tank head. So for a large tank with a head of 175kPa the CSP should be at least 1750kPa. However, this setting would make it difficult to have multiple PSVs cascading in a series.

Have you ever had any bad experiences using PSVs with excess backpressure while still being able to relieve theoretically under MAWP, such as using a conventional PSV with a CSP of 175kPa in the example above?