MAAP function of set pressure or MAWP? 2

[MAWPeter]

Typically my relief valve set pressure is set at my MAWP, but I have a case where my set pressure is below MAWP because of a small, but constant back pressure.

If my vessel is rated for 45 psig, but my set pressure is 42.5 psig, what is my MAAP for a fire scenario? If I'm telling a relief valve vendor my set pressure and using 21% overpressure for fire, I would expect my MAAP would be limited by the valve at 51.4 psig. Thoughts?

 

[The Obturator]

Set pressure should not be affected by this small constant back pressure. The spring setting is adjusted for this (set pressure minus constant back pressure). What do you mean by MAAP?


*** Per ISO-4126, the generic term
'Safety Valve' is used regardless of application or design ***

*** 'Pressure-relief Valve' is the equivalent ASME/API term ***

 

[Latexman]

I think MAAP means "maximum allowable accumulated pressure". Right?

The set pressure does not affect MAAP. Just MAWP affects MAAP. For fire case, MAAP = 45 x 1.21 = 54.45 psig.

Good Luck,
Latexman

 

[georgeverghese]

By saying MAAP = 51.4psig, think you havent taken into account the additional backpressure that will be created in the vent header as a result of this firecase relief scenario. You've got to work out what the new backpressure will be in this case, and then adjust the MAAP accordingly.