Relief valve above/below demister
Relief valve above/below demister
(OP)
If you have a vessel with a wire mesh demister pad, some companies insist that the relief valve should be below the demister pad. Others put it above the demister pad (top of vessel).
Question:
- Is anyone aware of a standard that gives us any guidance ?
- Is anyone aware of an actual case where a demister pad plugged up so severely that the vessel overpressured while the relief valve remained closed ?
Thanks
Question:
- Is anyone aware of a standard that gives us any guidance ?
- Is anyone aware of an actual case where a demister pad plugged up so severely that the vessel overpressured while the relief valve remained closed ?
Thanks





RE: Relief valve above/below demister
I am inclined to put it below the demister in order to keep the demister out of the flow path to the PSV inlet. Not all demisters are installed the same way; some are sectioned and tied together with junk steel and chicken wire. (At least some of the ones I have seen.) If a section or a piece breaks free (after a while in service things are never "as new") and comes to rest at or near the PSV inlet I am not sure how you would *not* expect higher than 3% x SP pressure drop, for example.
Irrespective of any API or other guideline, I am inclined to think that below the demister is the better place to be.
Regards,
SNORGY.
RE: Relief valve above/below demister
RE: Relief valve above/below demister
RE: Relief valve above/below demister
The 3 posters above have provided sound advice ( and I aggree with the comments above). I would cover yourself by doing the following.
VALIDATE: That the system (equipment) etc Downstream of the demister does not rely on the demister safety relief valves for overpressure protection. It sounds very unlikely (and i am sure it is) its just i've been caughtout by this before on comparable type systems so I'd advise a short check and DOCUMENT in an approprate format/calculation/report that it has been considered - this shows good due dilligance.
Good question .
RE: Relief valve above/below demister
As to standard, if you check Shell DEP 80.45.10.10 on pressure relief, it states clearly that PSV should be located u/s of demister...and the reasoning can be inferred from next line that allows PSV to be located d/s of demister if the PSV flow rate is lower than "operating" flow & that the operating flow will be stopped in case of an emergency. (So the issue is one of flow...in case relief flow is much higher, the increased flow may add some dP through the demister which is designed for lesser flow).
Regards
RE: Relief valve above/below demister
RE: Relief valve above/below demister