PRESSURE RELIEF VALVES
during relief does not exceed 10 % or 3 psi of the MAWP of the vessel. Thus, for example, a pressure vessel whose MAWP is equal to 100 psig( 7.03 kgf / cm 2) and the pressure relief valve is adjusted to open at 90 psig( 6.3 kgf / cm 2), the maximum overpressure reached by the valve under relief conditions may be up to 20 %, since( 100 x 1.1) – 90 = 20, however, the accumulation limit value will not be exceeded. Note: an overpressure greater than 10 % of the opening pressure is not permitted by ASME Section VIII, but is permitted by API Std. 520 Part I.
Conclusion
When a vessel is protected by two or more pressure relief valves, the pressure increase above the opening pressure of the primary valve is the overpressure, if it is set below the MAWP of the vessel. For the other pressure relief valves being set to open from the MAWP of the vessel, the pressure increase above the opening pressure( and above the MAWP) is called accumulation. The percentage of overpressure is a value dependent only on the value of the opening pressure of the pressure relief valve, the flow required by the process in relation to the maximum flow capacity through the selected area of the nozzle, in addition to the correct positioning of the nozzle ring. While accumulation is a term referring to the causes of overpressure and the number of pressure relief valves installed, acting at a value equal to or above the MAWP of the equipment being protected.
Therefore, overpressure is a term referring to the valve opening pressure, while accumulation is a term referring to the MAWP of the pressure vessel or boiler. The amount of overpressure could be infinite, depending on the cause( s) of the pressure increase, if not limited by a correctly specified, sized, inspected and maintained pressure relief valve.