Relief Valve Sizing Calculator

Engineering calculator reviewed for preliminary design use · Last updated: March 2026

Calculate required safety relief valve discharge area per API 520 Standard. Uses fluid-specific equations for liquid, gas, and steam service. Compute from set pressure, back pressure, and fluid properties.

Safety Relief Valve Sizing (API 520)

Pressure relief valves are critical safety devices that protect vessels and piping systems from overpressure. Proper sizing per API 520 ensures adequate flow capacity to prevent vessel rupture.

Liquid Service Formula (API 520)

A = Q / (38 × Kd × Kw × Kc × √(ΔP / G))

  • A: Required orifice area [in²]
  • Q: Required relief flow [USgpm]
  • Kd: Discharge coefficient (typically 0.65)
  • ΔP: Effective differential pressure (set pressure × 1.1 − back pressure)
  • G: Specific gravity

Design Considerations

  • Set pressure must not exceed the Maximum Allowable Working Pressure (MAWP)
  • Use balanced-bellows type if back pressure exceeds 10% of set pressure
  • Select from API standard orifice designations after calculation

Related: Wall Thickness Calculator / Gas Flow Calculator

Frequently Asked Questions

What is the difference between set pressure and relieving pressure?

Set pressure is where the valve begins to open. Relieving pressure is set pressure plus overpressure allowance (typically 10% for single valve, 16% for fire case). Sizing is done at relieving conditions.

Do I need to consider back pressure?

Yes. Conventional relief valves are affected by back pressure, which reduces their capacity. Balanced bellows or pilot-operated valves may be needed when back pressure exceeds 10% of set pressure.

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Editorial Responsibility
  • Last reviewed: March 2026
  • Reviewed by: EngiCompute Editorial Team
  • Intended for preliminary engineering use

These results are preliminary estimates for screening-level design use. They do not replace detailed engineering, code compliance verification, or vendor-certified calculations.