Pressure Relief Valve Sizing for Gas and Liquid Service

Pressure relief valves (PRVs) protect vessels and piping from overpressure by venting excess fluid when set pressure is reached. Correct sizing ensures the valve can pass required relief flow without exceeding maximum allowable pressure.

Relief Valve Sizing Methodology

API 520 Part I provides the definitive method for sizing pressure relief devices. The orifice area required is calculated based on required relief rate, fluid properties, and valve back pressure.

• Determine relieving scenario: Fire case, blocked outlet, thermal expansion, etc.

• Calculate required relief rate: Mass or volumetric flow at relieving conditions

• Select orifice size: Standard API orifice designations from D through T

• Verify back pressure correction: Balanced bellows or pilot operated valves for high back pressure

Gas Service Sizing

For gas and vapor service, the orifice area formula accounts for compressibility, molecular weight, temperature, and back pressure ratio. Critical flow is assumed when inlet pressure exceeds twice the absolute back pressure.

• Critical flow coefficient: Cp/Cv ratio affects flow calculation

• Molecular weight: Higher MW gas requires larger orifice for same mass flow

• Temperature: Higher temperature gas requires larger orifice

• Overpressure allowance: 10% for single valve; 16% for fire case

Liquid Service Sizing

Liquid relief valves are sized differently from gas valves. The flow formula uses liquid density and viscosity. Hydraulic correction factor applies for viscous liquids.

• Required capacity: Volumetric flow rate at relieving pressure

• Liquid specific gravity: Used in area calculation

• Viscosity correction: Kv factor reduces effective orifice area for viscous fluids

• Balanced bellows: Recommended when back pressure exceeds 10% of set pressure

Installation Considerations

PRV installation affects performance. Avoid inlet pressure drop exceeding 3% of set pressure. Discharge piping must be sized to limit back pressure to acceptable levels.

• Inlet line: Short and direct; minimize fittings and elbows

• Block valve: Normally open with car-sealed or locked-open requirement

• Discharge header: Size for worst-case simultaneous relieving scenario

• Drain provision: Liquid pockets must drain to prevent corrosion and vibration