Relief valve after a regulator: mandatory? 1

[colemanstoops]

Here's the situation: a new regulator is to be installed on a nitrogen line that feeds several vessels (and no other users!) for pressure transfer and line purging. EACH ONE of the vessels thus fed has a dedicated relief device that is appropriate for the vessel in question. Thus: I don't believe that a relief valve immediately after the regulator is necessary. However, I can't cite a source offhand for this judgment, nor am I absolutely sure this is the case. (I've always heard of a rule of thumb that a pressure reducing regulator should be followed by a relief device under any circumstances.)

Opinions? And appropriate code citiations? Thanks very much.

 

[CMA010]

I think it depends on the design pressure of the piping downstream of the PCV and whether there are isolation vales present downstream of the PCV. If the design pressure downstream of the PCV is the same as upstream a PSV is not required. If it is lower AND there are isolation valves present before the vessels a PSV is required. Note that if there are no isolation valves present but the design pressure of the piping downstream of the PCV is lower then upstream the PSV's of the vessels will protect the piping and set pressure of the(se) PSV('s) schuldig be set accordingly.

 

[Latexman]

A reason a PSV is usually put downstream of a regulator in the chemical industry is so a friendly chemical (in your case nitrogen) can blow to atmosphere. If the vessel(s) PSV is used, the contents of the vessel will be displaced and released to the atmosphere. The vessel contents may pose health, safety, and environmental concerns if it is released to atmosphere. If you are handling "methyl ethyl death", this makes lots of sense!

Good luck,
Latexman

 

[Chance17]

One item to consider is the regulator diaphragm's actual downstream rating.
For older models, it is common for the upstream side to be rated for the body (say 400#)
then the downstream side be rated based on the diaphragm (say 250#).
So the PSV setting has nothing to do with vessels, equipment or piping but instead the PSV protects the regulator diaphragm.

 

[moltenmetal]

There are many instances where a relief valve immediately downstream of a regulator has been installed for no good reason other than intellectual laziness or ignorance on the part of the people who did the hazards review. It's an easy and cheap add that makes people feel that they did something during the review, beyond drinking coffee and eating doughnuts.

There are some legitimate instances where a relief valve is absolutely necessary, as some component is not adequately protected against regulator failure by other relief valves or by design- usually because there is a valve between them which is probably there to be closed from time to time.