Validated sizing rule against chatter of relief valves during gas service

Abstract

Abstract Spring loaded relief valves are commonly used for protecting pressure loaded vessels, piping, etc. against overpressure. If the relief valve is installed improperly it may chatter while discharging. This behaviour is defined as an extremely rapid full opening and closing of the valve. The associated mechanical loads can damage the piping, equipment and the relief valve with the result of loss of pressure relief protection, undirected process fluid release, danger of fire and/or explosion, etc. In the technical guidelines and in the literature, sizing rules for the proper installation of relief valves are recommended. Most commonly applied is the 3% pressure loss rule for the allowable inlet pipe pressure drop as recommended in the API RP 520 (API RP 520: Sizing, selection and installation of pressure-relieving systems in refineries, 1993) or AD A2 ( AD-Merkblatt A2 : Sicherheitseinrichtungen gegen Druckuberschreitung—Sicherheitsventile, 1993). Another recommendation is based on the transmission time and the amplitude of the pressure waves generated by the abrupt safety valve opening and closing. In addition to these elementary rules, sophisticated computer codes for numerical simulation of the valve behaviour during the discharge process need to include the interrelationship between the flow in the pipe and that in the safety valve. Experiments have been performed with the objective of checking the validity of the rules. Based also on results taken from the literature a modified pressure surge sizing method is recommended. The usefulness of supplementary measures to establish in industrial installations a posteriore valve stability, e.g., by increasing the inlet pipe diameter, reducing the safety valve lift or installation of an oscillation damper are also evaluated.