Downstream isolation valve must be kept open for spare PSVs.. Why 1

[Nasim12345]

Dear All

As per Saudi Aramco procedure, downstream(outlet) isolation valve must keep open for spare PSVs? What is the reason behind this recommendation? If there are two PSVs installed in one drum, operation team will keep close the spare psvs inlet & outlet. However, as per procedure, spare psvs downstream isolation valve keep open. anyone can explain me what is the reason behind this?

Thanks

 

[pennpiper]

Nasim12345,
Your post does not make sense to me, I will try to understand it.
Issue: Saudi Aramco procedures, (for double PSV installations) require the downstream PSV outlet isolation valve must be kept open.
Question: What is the reason behind this requirement?
Discussion: If there are two PSV's installed on one drum, will the operation team keep the valves open and the spare PSV's inlet & outlet valves closed?
Suggested Explanation: Per Saudi Aramco procedure, Situations that include double PSV's, one PSV (and valve set) is treated as the primary set, the second (PSV & Valve) set is treated as a spare.
Reason: Normally the Valves (upstream & downstream) for the Primary PSV's are both open, and both valves (upstream & down stream) for the second (Spare) isolation valve kept closed.
Justification Having two PSV's on selected equipment/systems, allows continues operation with out shutting down critical equipment or systems when maintenance ie required for damaged or malfunctioning PSV.

Sometimes its possible to do all the right things and still get bad results

 

[MortenA]

#Usually the flange rating on the discharge side is only #150 or for some HP PSV's #300 - if the close a downstream isolation valve then if the upstream isolation valve leaks (or is opened by mistake)and the PSV [also] leaks (and it might well do so) if the upstream pressure rating is e.g. #900 you might bust that isolation valve or the flange.

DS isolation valve must be locked open - EXPECT when the valve is out for service of course.

Best regards, Morten

PS: You can of course speify higher pressure rating on the discharge side but it will cost you (also for the DS isolation valve)

 

[Nasim12345]

Dears

Find attached paragraph 6.5.2.3 from SAEP-318. Last week, during the site visit, we have noticed that a few spare relief valves downstream isolation valve in closed position and requested operations to open it. They are asking us, why this requirement mentioned in this procedure and is there any reason behind this?

Waiting for your advice..

 

[LittleInch]

I think MortenA has given you an excellent reason.

Large companies tend to react to isolated occurrences by instigating company wide rules and procedures to prevent another incident without saying why.

I don't really see any downside to this procedure.

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

 

[Nasim12345]

I got below feedback from one PZV expert. I think below reason is more valid..

"Discharge block valves on spare relief valves shall remain open to prevent possible overpressure of downstream lower class piping due to upstream block valve and PZV leakage"

Thanks
Nasim

 

[LittleInch]

Nasim,

That's exactly the same answer as Morten A

In this case the downstream bit refers to anything downstream the valve internals.

As stated, you often get PRVs with a pressure break inside the valve, e.g. a class 900 inlet and class 150 outlet.

Anyway you have your answer to tell the operations why they need to follow written company procedures.

Maybe you need to ask them if they are ignoring any other written procedures.....

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

 

[Nasim12345]

Thanks to all for the kind support and advice..