Safety Relief Valve Sizing

Required orifice area and bore from the relieving capacity, set pressure and overpressure.

Unit system

Inputs

FluidRelieving capacity

Set pressure (gauge)

The pressure at which the valve lifts. Gauge — the relieving pressure adds overpressure and atmosphere on top.Overpressure (fraction)

Typically 10 % on steam boilers; 3–10 % on process valves.Back pressure (absolute)

Discharge coefficient Kdr

Use the manufacturer’s CERTIFIED value (typically 0.65–0.85).Temperature (superheated steam / gas)

Required orifice area

0.002013m²

Equivalent bore

50.62mm

Flow regime	Critical (choked) flow — back pressure does not affect capacity
Relieving pressure (absolute)	12.01 bar a
Density at relief	6.132 kg/m³
Back-pressure ratio	0.0843
Critical ratio	0.5774

Kdr is the manufacturer’s certified value. The figure here is for pre-sizing; the final selection follows the valve’s own certificate.
The result is the REQUIRED area. Pick the next standard orifice up from the catalogue (API letter or manufacturer code).

A PDF carrying the inputs, results, assumptions and the source. A one-time code is sent to your address.

Results are for engineering pre-assessment and are not a product selection commitment. Final responsibility rests with the user.