ChemCalc sizes pressure relief valves (PRVs/PSVs) per API 520 Part I and ISO 4126-2 for gas, steam, and liquid service. The calculator computes the required orifice area and selects the nearest standard API 526 letter designation (D through T).
Service Types Supported
Gas — critical and subcritical flow Steam — dry saturated and superheated Liquid — incompressible, non-viscous Two-phase — flash service (approximate)Key Inputs
| Parameter | Notes |
|---|---|
| Set pressure (barg / psig) | Opening pressure; includes 10% accumulation for fire case |
| Back pressure (barg / psig) | Used to check critical flow condition and Kb correction |
| Relieving temperature (°C / °F) | Gas temperature at relief conditions |
| Required flow (kg/h or m³/h) | Mass or volumetric flow to be relieved |
| Molecular weight / Z-factor | For gas sizing; auto-filled for steam using IAPWS-IF97 |
| Discharge coefficient Kd | Default 0.975 for gas, 0.65 for liquid (API 520 Annex C) |
Outputs
- Required orifice area A (mm² and in²)
- Selected API 526 orifice letter and its standard area
- Rated capacity at the selected orifice
- Flow regime check — critical vs subcritical flow indicator
- Kb, Kw, Kv correction factors display
Standards Reference
| Equation | Standard | Application |
|---|---|---|
| Gas critical flow | API 520 Pt.I §5.6.2.1 | P₁ ≥ Pcrit |
| Gas subcritical flow | API 520 Pt.I §5.6.2.2 | P₁ < Pcrit |
| Steam (dry/superheat) | API 520 Pt.I §5.6.3 | Napier + Ksh correction |
| Liquid | API 520 Pt.I §5.7 | Non-compressible, Kw·Kv·Kp |
| Burst disc (gas) | ISO 4126-2 / GB/T 567.2 | Kdr = 0.9·Kd |
Also Available: Burst Disc Sizing
ChemCalc includes a separate burst disc calculator per ISO 4126-2 and GB/T 567.2-2012, covering gas and liquid service with the combined device correction factor Kdr = 0.9 × Kd.