PRDs Set Pressure and Overpressure per ASME Code

Some system uses one pressure relief device for overpressure protection, but some uses more than one. Some relief contingency is non-fire, but some is caused by fire. ASME Section VIII (ASME code) has some guideline for PRD set pressure and accumulated pressure for different cases, see Table 1. 

Per ASME code, Table 1 non-fire case is for equipment MAWP 30 psig. For equipment MAWP Is between 15 to 30 psig, the allowable accumulated pressure for single PRD case is 3 psi, and for multiple PRD case is 4 psi, 

PRD maximum set pressure per ASME code is based on MAWP, but MAWP is available only aAer the equipment is made. During design stage , many times, process engineer has to use equipment s design pressure as PRD s set pressure since MAWP is not available. Fortunately, in most cases, equipment s design pressure is very close (equal or less) to MAWP. 

Supplement PRD is an additional PRD used to protect system from fire or other external heat source. It is an 

As mentioned in Section IV, because of unavailable of MAWP during earlier design stage of project, most times, PRD set pressure is selected to be the same as equipment design pressure. 

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