The Role of Safety and Relief Valves
Overpressure in a piping system or pressure vessel can lead to catastrophic structural failure, equipment damage, and risk to personnel. Safety and relief valves are the ultimate safety devices designed to automatically open and relieve excess pressure when it exceeds the maximum allowable working pressure (MAWP).
Safety Valve vs. Relief Valve
Though the terms are often used interchangeably, they refer to different operations:
- Safety Valves: Typically used for compressible gases and steam. They feature a rapid, full-opening ‘pop’ action to quickly dump large volumes of gas and lower pressure.
- Relief Valves: Used primarily for incompressible liquids. The valve opening is proportional to the increase in system pressure, preventing pressure spikes without causing hydraulic shock in the system.
Critical Selection Criteria
Sizing and selecting a safety valve requires accurate engineering data:
- Set Pressure: The pressure at which the valve is calibrated to open. It must never exceed the MAWP of the protected equipment.
- Relieving Capacity: The valve must be large enough to discharge the maximum possible mass flow generated by the system under failure conditions.
- Backpressure: Constant or variable backpressure in the discharge piping can affect the set pressure. Balanced bellows or pilot-operated safety valves are used if backpressure is high.
Regulatory Standards
In steam applications, safety valves must comply with local and international boiler codes, such as IBR (Indian Boiler Regulations) or ASME Section I/VIII. Regular testing and recalibration are mandatory to guarantee safety system readiness.
