Pressure Relief Valve Sizing for Vessels and Piping Systems

ted wang
Jun 4
2 min read

Purpose and Regulatory Basis

Pressure relief valves (PRVs) protect pressure vessels, heat exchangers, and piping from overpressure caused by blocked outlets, fire exposure, thermal expansion, or control system failure. ASME Section VIII Division 1 and API 520/521 provide comprehensive procedures for sizing and installing relief devices on process equipment.

Determining the Required Relief Rate

Identify all credible overpressure scenarios: blocked outlet, heat input, reaction runaway
Fire case: API 521 provides heat absorption formulas based on wetted surface area
Blocked outlet case: typically sized for the maximum inlet flow at set pressure
Control valve failure: inlet flow at wide-open valve position with relief pressure
Thermal expansion: liquid relief for solar heating of blocked liquid-filled systems

PRV Sizing Calculations

For gas and vapor service, the required nozzle orifice area is calculated using the API 520 gas sizing equation incorporating back pressure correction factor Kb, combination correction factor Kc, and the overpressure allowance above set pressure. Liquid service uses the liquid sizing formula with viscosity correction factor Kv. The calculated area is matched to the next larger standard API orifice designation.

Set Pressure and Accumulation

Set pressure: pressure at which PRV begins to open, equal to or below MAWP
Accumulation: maximum allowable overpressure above MAWP (10% for single valve, 16% for fire case)
Blowdown: pressure reduction below set pressure before valve reseats, typically 5-10%
Differential pressure across valve at relieving conditions determines actual flow capacity
Back pressure from inlet piping pressure drop must not exceed 3% of set pressure for conventional valves

Installation Requirements

PRV inlet piping must be sized to limit pressure drop to less than 3% of set pressure to avoid chattering. Outlet piping must handle the relieving flow at back pressure below the PRV closing limit. Rupture disk installations upstream of PRVs require a tell-tale pressure gauge in the space between disk and valve. Periodic testing and certification per ASME and jurisdictional requirements is mandatory.