Check Valves in Relief Flow Path
Check Valves in Relief Flow Path
(OP)
What is your opinion on allowing a check valve between the protected vessel and the PSV, or between the PSV and the vessel to which the fluid is routed? I didn't see this addressed in API-521 or any other code or standard. If you're opposed to allowing check valves in the flow path, is this a blanket opinion or one that might vary depending on the corrosiveness or fouling tendency of the fluid?





RE: Check Valves in Relief Flow Path
RE: Check Valves in Relief Flow Path
RE: Check Valves in Relief Flow Path
Overall I can't see why the check valve could not be after the relief valve in the line unless this is a retrofit or the original design overlooked the need for a relief valve. If either of two is correct, then pay the money to correct the problem.
RE: Check Valves in Relief Flow Path
A check valve between the tank and PSV dangerous if the check valve fails close for any reason - you now no longer have a PSV.
A check valve between the PSV and vessel to which you are relieving to is also dangerous, if the check valve fails close for any reason - you now no longer have a PSV.
In any case, I do not rely on a check valve to isolate product - I always use the basis that a check valve leads. If I need positive isolation, I used something else.
If the process fluid is corrosive, you can use a rupture disc, or match the PSV material to suit. There are usually alternatives.
As a general blanket statement, I don't put check valves into my relief system.
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RE: Check Valves in Relief Flow Path
RE: Check Valves in Relief Flow Path
Dont just throw check valves in on all line "just in case". They are not that reliable and they will require inspection and maintenance.
Best regards
Morten
RE: Check Valves in Relief Flow Path
OK, let me explain this a little more clearly. In a chemical plant you often have one PSV that protects multiple pressure vessels. For example, a distillation column typically has a single PSV that protects the column, condenser, reflux drum,and the process side of the reboiler. Rarely is there a check valve in a distillation system but in other systems they're common. Here's a specific example: a large centrifugal compressor with interstage cooling and a knock-out drum at each interstage. A check valve is installed in the vapor line from each drum, and fire is the only scenario. If it were not for the check valves we could install a single PSV on the final drum. There are many other examples. Take for example a case where thermal expansion is the only scenario. Suppose you have three vessels in series but with a check valves between each one. A single thermal expansion PSV could be installed on the downstream drum to protect all three. If the process stream had any tendency to foul then I wouldn't consider such an option, but for a fluid that presented little risk of fouling it might make sense to just install a single PSV.
RE: Check Valves in Relief Flow Path
RE: Check Valves in Relief Flow Path
I voted against the item myself, because I agree with Ashereng as well.
JAC