Design of Discharge Piping for Pressure Relief Systems

The ASME Boiler and Pressure Vessel Code and publications by many manufacturers caution the design engineer to consider back pressure in discharge piping when sizing and selecting pressure relief valves. Only general guidelines are given, however, because of widely varying installation requirements. This paper addresses manifolded discharge piping system design. The method of analysis provides a simple technique for determining pressure within a discharge piping system. The method is based on adiabatic flow and uses local Mach number to relate expansion of the gas in the pipes to a mass flow function. The paper illustrates that relief valves discharging into lines and headers should not be sized or selected without careful analysis of the entire relief system.