Article: PSV sizing for gas blowby case

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Assumption : There is an upstream vessel and through a liquid or L/V control valve, fluid is transferred to a lower pressure Downstream vessel which needs to be protected for GBB.
Easy Step : Consider upstream vessel design pressure - Downstream vessel design pressure = DP across the control valve.
Estimate the gas flow through the control valve at that DP, SIxe the PSV for that gas flow.

Rigorous step : Since the above step indicates many failures at the same time (more than double jeopardy), we can fine tune the situation to a single jeopardy situation and estimate some what less amount of gas flow requirement during GBB.

A dynamic study is involved to do this calc; steps are below

Step 1: consider normal liquid level in the upstream vessel, at design pressure. This liquid needs to be drained off to DS vessel before you start the GBB; Assumption : feed stopped due to Pressure / level switches actuation. Otherwise it is difficult to have GBB as you have liquid and gases flowing in.

Step 2: Estimate the actual pressure now in the US vessel after draining off complete liquid from the vessel.
Step-3: Now the upstream vessel pressure - DS vessel pressure is the system DP. This has to be consumed by all fittings between the both vessels. Calculate the fraction needs to be conumed across the control valve.
Step -4: calculate the Gas flow at this DP, which will be some what less to the other easy step.

Size the PSV for this gas load.

Hope this help.