Is a Safety Relief Valve required if protecting against a Fail-Open Valve failure?

[SimStil]

I am working on the design of a test rig for a screw compressor and I have been looking into the use of a safety relief valve in our system. At the discharge side we are planning to have 2 control globe valves :
[ul]
[li]1 for controlling the bypass flow back into the suction side of the compressor[/li]
[li]1 for controlling the back pressure at the compressor discharge[/li]
[/ul]

The idea is to locate the PRV immediately downstream of the compressor discharge and upstream of the branch-off for these two valves. Now the question is, if the control valves are fail-open, is a PRV still required? Besides the valve controlling the compressor back pressure, there is nothing else that can fail and block the flow, and the PRV line would join the discharge line downstream of this valve anyway. I suppose the failure of the valves which could cause an excessive build up of pressure could happen either by mechanical failure or electrical signal loss. If it's a FO valve then the electrical signal program is not present, but would a mechanical failure cause a blockage?

Excuse my ignorance in this field as I am new to engineering. I have read through API 521 and 520 to get some insight into this, but maybe I missed something?

 

[Orsiz]

Although these valves are FO, I think you can't exclude that these valves get stuck in the closed position. In that case, the pressure should be relieved via the PRV.

 

[e43u8]

SimStil,

Failure position, either FC or FO, is referred to failure of incoming instrument air or electrical signal and doesn't cover the case in which the valve may get stuck in the closed or open position. Hence upstream of the valve should be protected against overpressure due to stuck close situation if the upstream design pressure is lower than the source pressure.

 

[LittleInch]

Yes it is still required.

If whatever is controlling these valves gets incorrect data or simply fails to adjust then the valves can close or close more than they should. They have not failed.

A PD compressor can see very rapid rises in presure if flow is restricted even by a small amount.

A Pressure Relief Valve is essential in this arrangement.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[SimStil]

Thank you all for your advice.