isolation valves under relief valves

[psvguy1]

thread1203-147792
i have seen block valves (isolation valves)between vessels and PSV's for many years in the GOM used by many large oil companies so if there are leagal issues with that they would have been addressed.however these vessels usually have PSH (pressure safety high) sensors also in place which will shut the inflo or cause blowdown to protect the vessel these are usually set @ or around MAWP

 

[totallockout]

Sometimes there is double relief, i.e. two PSV's on the vessel. Valve interlocks fitted to the upstream block valves can ensure that the spare relief valve is brought onstream before the other is isolated, thereby ensuring a spare path to relief is always maintained.

 

[JLSeagull]

I like the double PSV note. Some PSV's are installed with a shared three-way isolation valve that selects which PSV applies.

Within the USA there are different regulations depending upon the industry. An isolation valve is not generally permitted under the safety valve in fired boiler applications. Even in this case some states permit deviation.

Within many hydrocarbon applications the isolation valve is permitted except that the valve must be car-sealed or locked open. In the event that the valve is serviced, perhaps some requirements apply to operators monitoring the vessel. I have not checked lately and perhaps the safety shutdown applications must apply.

Also, these things change.

 

[hniaboujemaa]

I'm currently working in this issues with our current client. In my previous at the refinery, we didn't have such isolation valves and to be honnest as per my experience, these valves are important basicaly for PSV maintenance. In the past I have seen making unit shutdown for a PSV leaking or PSV requiring recalibration or replacement witch cost a lot of money. Now we mitigate the risque by using the existing PSV isolation valves and we can remove the PSV while the protected equipment is in service and do the maintenance as required .

regards,

 

[dwene]

It's fine in certain applications. I see them all over the place. Makes inline testing and in service replacement easy. The previous response is correct. Car seal or lock open is required.

 

[Duwe6]

and an Operator is REQUIRED to 'man the discharge valve' for the ENTIRE time that the isolation valve is closed. This way, if the plant/system 'upsets', that operator can vent the system and nothing goes KABOOM.

Everybody forgets about keeping the valve 'manned' while closed for maintenance. Massively inportant -- KABOOM's are expensive and tend to maime/kill people.