Pressure Relief Valve
A pressure relief valve is an automatic, self-actuated device that opens to discharge fluid when system pressure reaches a set value, protecting vessels, piping and equipment from overpressure. A spring holds the disc against the nozzle; when inlet pressure overcomes the spring force, the valve opens, and it recloses once pressure falls below the set point by the blowdown margin. Safety valves open with a rapid pop action for gases and steam, while relief valves open progressively for liquids. Conventional, balanced-bellows and pilot-operated designs differ in how they respond to back pressure. Sizing depends on required relieving capacity, set pressure, allowable overpressure, back pressure and fluid properties. Key standards include ASME BPVC Section I and Section VIII, API 520 (sizing, selection and installation), API 521 (pressure-relieving systems), API 526 (flanged steel valves), API 527 (seat tightness) and ISO 4126.
Examples
- Safety valve on a steam boiler drum
- Pilot-operated relief valve on a gas separator
- Thermal relief valve on a blocked-in liquid line