pressure relief valve settings per ASME SEC VIII div 1

[pressure equipment engineer]

Just to confirm my beliefs; The maximum set point of a PRV on a vessel with one PSV installed is the MAWP. The MAWP is defined as the maximum pressure calculated on a vessel using nominal thickness, exclusive of corrosion allowance. Therefore let us say that we have a 100" diameter shell with 0.375" thick material with 20,000 psi allowable stress, JE=1. I calculate a MAWP of 149 PSIG and this is the highest i can set the PRV? accounting for static head, of course

 

[RVAmeche]

If the stamped MAWP is 149 psig, then the maximum set pressure for a single relief device is also 149 psig

 

[georgeverghese]

MAWP calc should INCLUDE corrosion allowance. Others may comment on caveats re the JE=1.0.

 

[MJCronin]

A great question to post on the "Safety Relief valve" forum ....

MJCronin
Sr. Process Engineer

 

[pressure equipment engineer]

I see in mandatory appendix 3 where MAWP is defined and it states "exclusive of corrosion allowance", correct?

 

[SnTMan]

Exclusive in this context means "not including".

Regards,

Mike

The problem with sloppy work is that the supply FAR EXCEEDS the demand

 

[pressure equipment engineer]

Correct, therefore one would use the "nominal thickness", in this example using 0.375" to calculate the MAWP.

 

[david339933]

Nom. Thickness includes CA....would be .375" - CA for MAWP

 

[david339933]

I calculate a MAWP of 149 PSIG and this is the highest i can set the PRV? accounting for static head, of course

MAWP is defined as the pressure at the top of the vessel, no need to account for static head.

 

[SnTMan]

"Not including" means CA is deducted from the nominal thickness for calculations. Code dimensions including thicknesses are nearly always less the CA and any applicable tolerances. See U-5, UG-16(e). The term "nominal" is used to distinguish from "required", etc.

Regards,

Mike

The problem with sloppy work is that the supply FAR EXCEEDS the demand

 

[Bahador_eshghi]

Lets check the issue with another point of view!
from HRSG basic design point of view we normally first define the design pressure of the vessel/drum based on the maximum operating pressure, then this design pressure will be the MAWP of the vessel/drum.
after defining the MAWP of the drum thickness and other characteristics will be calculated.