PSV Builtup Backpressure Question

[polyroly]

Hello Everyone,

I am currently in a disagreement with our process design consultant on a PSV that is relieving butane from a vessel. He has sized the PSV without accounting for any builtup backpressure on the PSV outlet. It is a conventional PSV with no bellows.

The PSV is releiving 4500 SCFM of butane through a 3" vent to atmoshpere (where our plant is we are allowed to vent butane). My concern that the there is a reasonable pressure drop in the vent piping (20 ft) that has not been accounted for.

Am I out to lunch here?

Thanks.

 

[Armen75]

Good to check - but your concern may be misplaced.

A quick calculation (in hysis) shows that relieving to atmosphere you need roughly 30 psi pressure drop in the piping

For the piping this can't be more than 10% of the set pressure (according to API) or else you risk having chattering or other problems in the valve.

For the PSV itself, as long as this pressure (30 psig) is below the critical relieving pressure (based on set pressure and k), the relief flow is considered to be choking in the valve and therefore is not affected by this back pressure.
If 30 psis is above the critical pressure, than you are affecting the performance of the valve and need to be more careful (as it is a sub-critical flow)

 

[JohnGP]

polyroly,
I don't know your system, but intuitively I would want to check the back pressure value - especially if the PSV is protecting a vessel and set for MAWP. Pressure accumulation in the vessel will be a function of valve capacity which will be affected by the back pressure it sees.

API RP 520 recommends that the check be done:

....the magnitude of the built-up back pressure should be evaluated for all systems, regardless of the outlet piping configuration.

Just my thoughts,
Cheers,
John