Relief Valve Fire Sizing - Pressure Vessel

[butelja]

In evaluating the pressure relieving requirements for a pressure vessel in the fire sizing case (UG-133(b)), the pressure is allowed to 21% above the vessel MAWP. There is no mention of vessel temperature, although Appendix M-14(a) suggests it should be considered.

If a high boiling point liquid is in the vessel, the case may arrise where the design temperature is exceeded well before the MAWP is reached. In this case, which of the following is correct?

1.) The vessel must stay below the design temperature and below 121% of the MAWP when relieving.

2.) The vessel must stay below 121% of the MAWP, and temperature is not considered.

3.) The vessel MAWP is derated for temperature, and the pressure must stay below 121% of the derated MAWP at the elevated temperature?

 

[DaleE]

I think the correct solution is to derate the MWP for the elevated temperature. If it's a code vessel I think it will require a calculation for your particular vessel.

 

[Shaheryar]

I guess you have to see what is the rupture temprature and pressure for your vessel, since the high temperature at reliving pressure will only exist for a very limited period of time.