Pressure relief valves play a critical role in protecting piping, vessels, and process equipment from dangerous overpressure conditions. In engineering drawings, these valves are identified using standard P&ID symbols that communicate how the valve operates and how pressure is controlled. This article explains the most common pressure relief valve symbols, including spring-loaded, pressure and vacuum relief, and pilot-operated designs, helping engineers, designers, and maintenance teams quickly understand their function and apply the correct valve for safe, reliable system design.
Pressure Relief Valve Symbol
This symbol represents a spring-loaded pressure relief valve used in P&ID diagrams to protect pressurised systems from overpressure. The spring element indicates a mechanically set opening pressure, while the directional outlet shows controlled discharge when the set pressure is exceeded. This type of valve automatically opens to relieve excess pressure and recloses once normal operating conditions are restored, making it a critical safety component in piping and process systems.
Pressure and Vacuum Relief Valve
This symbol represents a pressure and vacuum relief valve (PVRV) used in P&ID diagrams to protect vessels and tanks from both overpressure and vacuum conditions. The opposing valve elements indicate dual functionality: one side opens to relieve excess internal pressure, while the other admits air or gas to prevent vacuum collapse. The spring symbol shows that both setpoints are mechanically controlled. This clear visual structure reflects recognised engineering conventions and accurately communicates the valve’s safety role in storage and low-pressure process systems.
Pilot Operated Relief Valve
This symbol represents a pilot operated pressure relief valve, where system pressure is controlled indirectly through a pilot valve rather than a direct spring mechanism. The “P” box identifies the pilot control, while the main valve body handles full flow during overpressure events. This design allows for precise pressure control, reduced leakage at normal operating pressure, and stable performance under high-pressure conditions, making pilot operated relief valves widely used in oil & gas, petrochemical, and high-capacity pressure systems.