Shantal,A fire case is to be considered if it is possible to have a sustainable fire in the area. Also, the surface considered is the first 30 feet above that surface. I see you work for a petroleum company, so fire cases are always important. A surface that can sustain a fire is also important. If you have an overhead receiver for a distillation column in a structure, it frequently is more than 30 feet above ground. So the question is if the deck below it will hold enough liquid to sustain a fire. You also have the question of wetted surface. I assume the high level case even if that level is not the normal level. That is a conservative but the safest approach. Also note that for pressure vessels there is a case for vapor filled vessels in a fire. That scenario is pertinent for gas plants. The other consideration is whether the vessel is directly in a fire or if the primary heat flux is by radiation. (The equations differ.)I hope this helps.
Each system must be analyzed to see which cases apply. For many pieces of equipment, fire applies as a case as well as many other overpressure cases. The different cases are calculated, with their varied accumulated pressures of 110, 121% etc. and the governing case will determine the required relief valve size (orifice area).
Typical cases to be examined: 1. Auto Control Failure
6. Reflux Failure
7. Loss of Feed
9. Cooling Water Failure
10. Power Failure
11. Instrument Air Failure
12. Steam Failure
13. Other: Back Flow
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Many years ago I asked the same question to a member of the ASME Section VIII (pressure vessel) committee. The answer was the accumulation is a function of the frequency of the event. So I found some data on event failures and yes, fires have a lower frequency than the majority of other events. Two codes commonly used by process engineers, ASME Section VIII and ASME B31.3 (process piping), both have frequency based accumulations. Please understand that this does not consider the consequence and the risk due to an event is a function of both consequence and frequency.
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