Relief Valve Fire Sizing ASME Sec VIII, Div. 1

[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?

 

[scrobe]

Probably number 3, with a variation.

It might depend on the length of time that the vessel was exposed to temperature above design temperature. If there has been damage to the vessel due to exposure to external fire, it might be best to evaluate the condition of the vessel using API RP 579, Fitness For Service. Based on that evaluation, a vessel rerate may be required that would lower the MAWP since a loss of strength could occur, which you referenced in your question. If then it does get rerated at a lower MAWP, then I would guess that relieving pressure rise would be able to go to 121% of MAWP.

My guess is that UG-133(b) doesn't mention temperature since the fire may either be put out before damaging the metal to a point of failure or the vessel is emptied before that point. However, pressure rise can occur much quicker and as such, the need to protect failure due to overpressurization is needed and addressed in the Code.

 

I would definately derate the MAWP of the tank in your PSV calculations to correspond to the pressure at which the liquid boils at the design temp. of the tank. Use 21% above that pressure.

 

[RichardH]

Allowable stresses in the ASME Code for carbon steel at elevated temperatures (above 700 or 800 degrees F) are based on the Creep to Rupture Criteria (100,000 hrs.). Creep is the result of long term exposure to elevated temperatures and would not necessarily be applicable for the fire case.
If you were to attempt to determine the MAWP at the fire case temperature, you would need to determine an allowable stress based on tensile strength or yield strength of the material at that temperature. Appendix P of ASME Section VIII, Div. 1 gives the "Basis for Establishing Allowable Stress Values".

 

[butelja]

Option #3 was used, and the de-rating was done using allowable stresses at the elevated temperature from ASME Sec IID.