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Psv Relief Capacity In Control Valve Failure Mode

psv control vlave failure rate relief capacity

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#1 Ali-Perth


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Posted 30 September 2020 - 08:29 PM

Dear All;


Referring to attached sketch , there are two regulator valves in parallel which feed start-up/fuel gas supply for a reciprocating compressor as well as its microturbine. 


PCV-1 is for the fuel gas, so max flow is compressor fuel gas + microturbine fuel gas. PCV-2 Is for the start gas, so max flow is compressor start gas. Because the PCV-2 set point is lower it will remain close during normal operation, and only open when someone is trying to start the compressor (the higher flow won’t get through PCV-1, so the downstream pressure will drop causing PCV-2 to open).


To calculate PSV rate , an idea is as follows:


since during startup mode PCV-1 can be 100% open, with PCV-2 in normal operating position. Blowby case considers PCV-2 failing open.


Inflow rate at worse case: is both PCV-1 & 2 are failed and calculate max flow rate based on wide open.


Outflow rate at start up: Startup gas is required by compressor. To meet the demand, PCV-2 opens to Normal position. PCV-1 will be open 100% as it tries to maintain 1050kPag downstream condition.

The normal outflow in this scenario is normal flowrate of PCV-2 + flowrate of PCV-1 (at 100% open with 8000kPag upstream and 950kPag downstream)


Relief capacity = Inflow at worse case– Outflow at startup


To be conservative, outflow should not be calculated at PSV relieving condition (on upstream side) as we assume the downstream system cannot handle any increased flow (based on  API 521- clause


Any idea?

Attached Files

Edited by Ali-Perth, 01 October 2020 - 12:18 AM.

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