Performance of a safety relief valve under back pressure conditions

Abstract

Abstract By the nature of its design, the performance of a spring-loaded safety relief valve, especially a conventional valve, is influenced by back pressure. A bellows safety relief valve, since its disc is subjected to a much smaller downward force resulting from back pressure, is able to remain stable under much higher back pressure conditions. Due to the wide range of bellows manufacturing tolerances, the bellows safety relief valves can substantially reduce, but cannot totally eliminate, the back pressure effects on its set point and relieving capacity. Set point change in a conventional safety relief valve, due to constant superimposed back pressure, can be easily negated by cold differential set. For a bellows safety relief valve, however, set point change must be corrected by the set point correction factor which is a linear function of the back pressure to the set pressure ratio, R. There exists an allowable back pressure to set pressure ratio, Ra, and a critical back pressure to set pressure ratio, Rc, for each conventional and each bellows safety relief valve. When total back pressure exceeds Ra, the relieving capacity will be reduced, mainly resulting from the valve lift being reduced, and the capacity reduction factor should be applied in valve sizing. Once the Rc is exceeded, the safety relief valve becomes unstable and will totally lose its overpressure protection capability. For a given back pressure to set pressure ratio, R, the capacity reduction factor is a function of system overpressure, but its relationship is non-linear in nature.