Relief valve above/below demister 1

[CJKruger]

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

 

[SNORGY]

Does the demister pad span the entire vessel (horizontal) or is it a vertical vessel (which I assume)?

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.

 

[ggordil]

From everything that i have seen, relief valves have always been installed below the demister for the reasons that Snorgy listed.

 

[JLSeagull]

This is typically addressed during the process hazard analysis. I never heard anything about this topic in the '80's. The preference has been for the PSV upstream of a demister for the last dozen or more years. I have not checked the standards.

 

[jamesbanda]

CJKrugeR,

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 .

 

[Bala0610]

Good advice has been given by previous posters.

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

 

[dcasto]

If you have no way of causing a blockage in the demister, then you could keep the PSV downstream. We have lots of this type of arrangements in our cryogenic plants where there is just no way of getting a blockage in the demister.

 

[djack77494]

I would also worry about the potential for high differential pressure across the demister that could lead to demister failure. Many demisters are designed for a relatively small dP across them. Should a PSV downstream of the demister pop, especially if you have full flow across the demister, the resulting differential might exceed the strength of the demister leading to its failure. I see no disadvantage to having the PSV upstream of the demister and so wouldn't consider putting it otherwise.