Safety & Relief Valves

Safety valve sizing to API 520 Part I requires five parameters: (1) Set pressure = MAWP of the protected vessel or system; (2) Operating temperature at the valve inlet at relieving conditions; (3) Required relieving capacity - in kg/h for vapour/steam, kg/h or m³/h for liquid, or SCFM for gas - based on the most demanding relieving scenario (fire case, blocked outlet, or control valve failure); (4) Fluid type (gas, vapour, steam, or liquid - API 520 has separate sizing equations for each); (5) Back pressure at the outlet connection. Provide all five and our technical team will calculate the required orifice designation (D through T) to API 526.

Safety valves are designed for compressible fluids (steam, gas, vapour): they feature a rapid, full-pop-open action once the set pressure is reached, maintaining full lift until pressure drops below the re-seat point. Relief valves are for incompressible liquids: they open proportionally - gradually increasing flow with increasing overpressure. Safety relief valves (our standard supply) are dual-duty - certified for both compressible and incompressible service. All three terms are often used interchangeably, but the correct term per ASME Section VIII is 'pressure relief valve.'

Conventional (open bonnet) safety valves are affected by back pressure: if outlet back pressure exceeds approximately 10% of set pressure, the effective set point increases and capacity decreases. They are suitable when outlet back pressure is low and constant. Balanced bellows valves use a metallic bellows to isolate the spring chamber from the outlet: set pressure and capacity are independent of back pressure up to 50% of set pressure (for built-up back pressure) or higher (for superimposed back pressure). Specify balanced bellows when connecting to a manifold discharge header with significant back pressure, or when the outlet fluid is toxic or corrosive.

API RP 576 and API 510 recommend inspection intervals of 3–10 years for safety valves in process service, with the specific interval determined by the fluid service, historical performance, and risk assessment. Clean, non-corrosive steam and air services typically allow 5–10 year intervals; corrosive, dirty, or reactive fluid services may require annual inspection. All Vajra safety valves are supplied with a factory test certificate (set pressure stamped and documented). Field recertification (pop testing and reseating) must be performed by an authorised inspection body or valve service centre.