PRESSURE RELIEF VALVES
Differences between accumulation and overpressure in the application of pressure relief valves
Pressure relief valves are critical safety components in boilers and pressure vessels, preventing excessive pressure build-up. This article explores key concepts like accumulation and overpressure, highlighting their roles in valve sizing, system protection, and compliance with ASME design codes.
By Artur Cardozo Mathias
About the author Arthur Mathias is an industrial mechanical and chemical technician and a Member of ISA. Now a consultant, he has been active since 1985 in the maintenance, inspection, specification and sizing of valves. Arthur provides training and technical courses, and is the author of Válvulas: Industriais, Segurança e Controle published by ARTLIBER EDITORA.
Photo: Dreamstime
Among the terminologies applied in pressure relief valves, we can mention accumulation and overpressure. Accumulation is the maximum pressure allowed to be reached by a pressure relief valve at a value above the Maximum Allowable Working Pressure( MAWP) of the equipment being protected. Therefore, it is the increase in pressure allowed above the MAWP inside a boiler or pressure vessel, with all pressure relief valves completely open and relieving. Overpressure is the maximum pressure reached by the valve above its set pressure and according to the flow rate required by the process. Therefore, the valve selected for a given application must have a maximum flow capacity that is equal to or greater than the worst cause of pressure generation source and flow volume, which prevents the pressure inside the equipment from exceeding the accumulation limits allowed by the design codes.
Accumulation in pressure vessels
The accumulation value allowed within a pressure vessel is a reference for the minimum number of pressure relief valves that can be installed and according to the causes of overpressure. In boilers, the accumulation value is fixed, therefore, independent of the number of valves installed.
Installation with multiple pressure relief valves
When overpressure is caused by an operational loss of control of the process, the accumulation
allowed by ASME Code Section VIII is 10 % or 3 psi( whichever is greater) for vessels protected by a single valve. When multiple pressure relief valves are installed, the maximum accumulation allowed by ASME is 16 % or 4 psi( whichever is greater) above the MAWP. In these cases, if there are two or more valves installed, the second or last one must be adjusted to open at a maximum of 5 % above the MAWP. The opening pressure of the second or last pressure relief valve, if the pressure vessel( or boiler) is protected by two or more valves, can be determined as follows:
SP = MAWP x A 0
Where: SP = opening pressure of the second or last valve MAWP: Maximum Allowable Working Pressure of the vessel or boiler A = maximum accumulation allowed for multiple valves, being 6 %( 1.06) for boilers and 16 %( 1.16) for pressure vessels O = maximum allowable overpressure, being 3 %( 1.03) for boilers and 10 %( 1.10) for pressure vessels
Note: The allowable value of 3 psi accumulation is applied when the MAWP of the vessel is between 15 and 30 psig, and there is only one pressure relief valve installed. An allowable accumulation value of 4 psi is applied when the MAWP of the vessel is between 15 and 30 psig, and there are two or more pressure relief valves installed for its protection.
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