PSV Bypass Valves (in Cryogenic Service)

[RRE]

In my plant design, we have full flow bypass valves around PSVs in a cryogenic hydrocarbon stream. Each PSV has two CSO valves that are closed when servicing the valve (inlet and outlet). Full flow gate bypass valve has been designed around every PSV valve. The release from these valves (PSV and bypass valves) flow to flare.

In a recent incident when a leak caused part of the cryo plant to ESD, the only way to depressurize the fluid was to open to the PSV bypass valve to flare manually (these are all manual valves). In so doing, the flare header piping exceeded its design temp for low temp CS (of -50F).

The question is around the use of manual bypass valves in cryo service. I know that industry specs do not take these manual bypass valves into relieving account, but what has been your experience in this design, especially in using these bypass PSV valves to depressure cryo fluid??

 

[dcasto]

Not to worry. Because there is no stress on the flare header from pressure (thermal contraction maybe if not designed correctly). All my ethylen relief valves go to plain CS, so they hit -124F. We use the bypass to blowdown regually. Now, we do limit the amout we blowdown to under 25,000 pounds without a special use permit.

 

[JoeWong88]

In many plants that i have seen, automatic depressuring system (BDV) are provided for almost all of the cryogenic system UNLESS it meet criteria where BDV is not required.

The operator is not expected to open the bypass valve manual for depressuring purpose during emergency. Manual operation is only expected during maintenance purpose.

dcastor is right. Althought there is low temperature limit for CS, this temperature is at pressure limit. For flare system, you may take the credit of low back pressure. However there is limit of the back pressure. The limit of the back pressure at low temperature may be obtained from piping code i.e ASME B31.3.

JoeWong
Chemical & Process Technology