6 replies to this topic

[pleckner]

This must be a refinery specific requirement because I have never seen any regulatory requirement. Each company is free to set their own rules as long as these rules do not violate the codes. They may be more strict but never less.

I'm at a loss, however, in tyring to picture your system. I have never ever seen any plant put a CSC valve upstream of a relief device. The more common practice is to put in isolation valves on systems where there is more than one relief device and one of them is a spare. This is done so that one can be taken out of service for maintenance while the other is in service. The two isolation valves are mechanically linked so that when one is opened, the other is closed. This prevents accidental simultaneous isolation of both relief devices.

Perhaps your facility is doing something different. Again, I can't picture it.

[pleckner]

It is a MUST that the isolaton valve on the relief valve in service be CSO but there is absoulutely, positively no reason to CSC the "spare"! Who cares if it is opened by accident? It most certainly IS NOT a safety issue. And I would think that the odds of someone opening the spare by "accident" is pretty slim; but again, who cares? Where is the harm and safety issue?

I would love to understand their thinking.

[JoeWong]

I would be more concerned about the use of a gate valve in the inlet piping to the relief valve. If you are using gate valves, make sure the stem orientation is horizontal so that a failure of a broken stem does not let the gate drop down and block the flow.

If there were going to be any X-raying to confirm valve position, I would expect it to be to confirm the valve is indeed open and you do not have a false indication due to someone breaking the stem when the valve had been closed to service the "spare." Valves that are used infrequently for this type of work (annually or every 5 years is not uncommon) seemed to be turned with "cheater" bars or wrenches on the wheels and are prone to breaking.

Dear All,
I am "newbie:" in this forum. Just browse some discussion threads and found interesting discussion going on. Just wanna to contribute some of my thoughts.

a) CSO vs CSC

a1) 2 x 50% PSV : Both PSV inlet block valves are CSO with cascade set pressure as per API 521 recomendation

b2) 2 x 100% PSV : Duty PSV inlet block vale is CSO whilst Spare PSV inlet block valve is CSC. Reason being both PSVs will have identitical set pressure. Identical PSV setpoint will promote chattering and seat damaging.

Gate valve upstream of PSV
Gate valve's stem may be broken without operator knowing and potentially introduce "obstacle" in the PSV inlet line. For continuous operating line, once gate valve is used, preferable to orientate in horizontal downward position to ensure broken stem will not cause any blockage. Nevertheless, not very often use gate for PSV inlet which is meant for final overpressure protection. Stem "stucked" and "broken" may potentially partially block the inlet line and create flow limitation, large inlet pressure drop (>3% from protected section), PSV chatter, etc. From safety point of view, we normally recommend to use FB ball valve for PSV inlets.

Best regards,

JoeWong