The maximum allowable set pressure for a relief device is determined by the pressure rating of the equipment being protected and by the number of relief devices installed. The maximum allowable relieving pressure is equal to the maximum allowed accumulated pressure, which is a function of the MAWP, the causes of overpressure, and the number of relief devices installed. Figure 1200-17 below summarizes these maximum limits as dictated by ASME Boiler and Pressure Vessel Code Sections I and VIII and by ASME Piping Code B31.3.
Figure 2 of API RP 521 shows the relationship between the various pressures involved in pressure relief, and it defines the terminology. Figure 2 shows a 10% margin between the maximum operating pressure (Po) and MAWP. To avoid PSV leakage and excessive maintenance, provide the following margins between Po and MAWP:
25 psi for Po = 0 – 170 psi
15% of Po for Po = 170 – 300 psi
45 psi for Po = 300 – 450 psi
10% of Po for Po = above 450 psi
The above recommendations are for conventional and balanced PSVs. Pilot-operated PSVs allow the vessel to be operated closer to the MAWP.
The minimum set pressure for conventional PSVs is 5 to 10 psig. The minimum set pressure for bellows-type PSVs is approximately 25 psig. Both of these minimums depend on the size of the PSV. Consult the manufacturers’ catalogs.
For low pressure equipment, the specific design code (e.g., API Standard 620, 650, etc.) should be consulted to determine the maximum relief device set pressure and overpressure.