Transient Reaction Force of Relief Valves Discharging Into a Closed Piping System

Abstract Designing piping systems and supports to withstand the reaction forces from pressure relief valves (PRVs) is a contentious issue in piping design. ASME B31.1 and API 520 clearly define reaction force requirements for open discharge systems due to the inherent unbalanced forces at the discharge point. However, the guidelines for closed systems are less defined. In closed piping systems, the reaction force could balance out within each pipe section, resulting in a smaller or negligible resultant reaction force. Therefore, codes and standards only state the caution regarding unsteady reaction forces but do not explicitly mandate their consideration. This ambiguity leads to varying design approaches: some designers ignore these forces, others consider them only at the discharge valve, and some evaluate the entire closed system. Therefore, a clearer understanding and a simplified method for evaluating dynamic reaction forces in closed systems are needed. This paper reviews the design considerations for reaction forces in relief systems and proposes a simplified design approach. First, the mechanisms causing reaction forces are discussed based on the existing rules are reviewed. Next, examples of dynamic behavior analysis of the pressure relief system are presented. The analysis uses a 1D unsteady compressible flow program to capture transient pressure wave propagation. Conventional methods for estimating reaction forces are discussed. The evaluation of the piping system focuses on the resultant dynamic load and its relation to the dynamic load factor.