Equipment and piping systems that operate under pressure need to be protected from excessive overpressure. This is accomplished with the installation of pressure-relief devices, which must be properly sized and specified for the intended service conditions. To adequately size a pressure-relief device to provide overpressure protection for equipment and piping, several relief event scenarios always should be considered. Ultimately, overpressure protection is provided with the installation of pressure-relief devices that are specifically sized, specified and installed for the postulated governing overpressure contingency. Too often, pressure-relief devices are sized based on the possibly erroneous presumption that fire exposure will be the most likely governing contingency. Historically, the fire exposure contingency has been emphasized to such an extent that pressure-relief devices often are assessed solely on the basis of fire exposure. Substantiation of this presumption, however, is not obvious in the literature. In fact, one can argue that there are many relief contingencies other than fire exposure which may govern the relief device size. Furthermore, neglecting consideration of other relief contingencies can present a potentially dangerous situation. This paper presents results from a study intended to examine which overpressure relief contingency, if any, most often governs the size of relief devices that are used to protect equipment and piping systems. From previous related work, seven relief contingencies are described and emphasized by the author. For this study, relief device sizing data was compiled from a number of chemical and petrochemical project applications to provide a reasonable sample of contingencies that governed the sizes of existing and new safety-relief valves and rupture discs [1,2,3].
