Thermal PSV Sizing

[Pmatherne]

I am sizing a psv for thermal relief according to API-521. The problem I have is the vessel operates at 1 PSIG, but is rated for 125. With the operating temperature at 100 degrees, this gives the reliving temperature greater than the wall temperature. Which really skews the F factor. So I default to the largest size and I keep coming up with a large orrifice than should be needed. The vessel is 2' x 10' orientated vertically and would be filled with 0.74 SG Natural Gas
Should I use the minimum F factor of 0.01? Or is there another standard or some sort of exepction I am missing?

Future PE Engineer
Pet project I am working on to help other engineers, not much yet hoping to get it grow as I learn more

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[MikeClay]

The bottom line is that the vessel may fail before the PSV lifts. The PSV size is irrelevant. Install a blowdown system to vent the vapors in the event of a fire.

But if you really need to size the relief valve, you can run thermal expansion calculations at various temperatures to determine how much gas needs to be released for each temperature increment. Then the API heat equation can be used to determine the time needed to relieve that amount of gas. The API vapor PSV equation sizing equations can be used to calculate the relief valve size for each temperature increment. Choose the largest relief area for specifying the PSV. Again remember, the vessel may not be intact by the time the PSV lifts.

--Mike--