relief valve set pressure Q 6

[EngCutie]

No one answered my relief valve question, I have got another one, hope you can help

If the back pressure is zero, does the set pressure have to be 10% lower than the design pressure (Max. pressure)? If not, what’s the rule when determining set pressure? (If client doesn’t specify)

Thanks


Cutie

 

[moltenmetal]

Much of this commentary assumes that popping a relief valve is a "safe" event- one with no consequences of safety, environmental or economic impact. Sometimes that is indeed the case, but in my experience it is RARELY the case.

Generally I want a relief valve to be set at the MAWP of the system it is protecting, and to have plenty of other controls on the system to reduce the need to rely on it. It is a final, passive and reliable layer of defense, NOT a routine defense.

 

[Jefka]

@Casimo5

What you're saying is correct- you do want a PSV set pressure to take into account inlet line losses if they're significant (> 3% to 5%).

However I would still be very hesitant to place a new relief valve at the end of a chain of equipment like that. Pressure drop is not always a reliable value in gas plants due to the multiple variables that go into it and there are also lots of potential complications (methane hydrate formation, liquid slugs, etc) that could make reliving difficult. Almost without exception I would recommend placing the relief valve at the first equipment item.

 

[Casimo5]

@ Jefka

I agree. It makes more sense to place the relief valve at the front end, closer to the source of the overpressure in the example I gave. I was just giving an example of when you would set the PSV set pressure below the MAWP. If the relief valve is placed at the front end, then you can set the pressure at the MAWP.