PRV Orifice Sizing Calculator
Required relief valve orifice area for liquid, gas, or steam service per API 520, with selection of the standard API 526 orifice letter.
When to use this calculator
Use when sizing a pressure relief valve for a defined relieving case — fire, blocked outlet, control valve failure, thermal expansion. It applies the API 520 Part I equation for the service and relieving state, including the correction factors for back pressure, overpressure, viscosity, and superheat, and then picks the smallest standard API 526 orifice that covers the required area. Gas service is checked for critical against subcritical flow, since the two use different equations.
Required inputs
- Service — liquid, gas or vapour, or steam
- Required relieving flow, set pressure, overpressure, and back pressure
- Fluid properties: specific gravity, or M, Z, and k for a gas; superheat for steam
- Correction factors Kd, Kb, Kc, Kw, Kv, KN, KSH
Expected outputs
- Required effective orifice area
- Selected API 526 orifice letter and its area
- Flow regime for compressible service — critical or subcritical
- The substituted equation for the relief calculation record
Formula overview
US customary, as the API 520 coefficients are defined: W in lb/hr, Q in US gpm, pressures in psia, T in °R, M in lb/lbmol, A in in².
API 520 Part I, effective area basis:
liquid A = Q / (38·Kd·Kw·Kc·Kv) · √( G / (P1 − P2) )
steam A = W / (51.5·P1·Kd·Kb·Kc·KN·KSH)
gas, critical
A = W / (C·Kd·P1·Kb·Kc) · √( T·Z / M )
gas, subcritical
A = W / (735·F2·Kd·Kc) · √( Z·T / (M·P1·(P1 − P2)) )
Then round up to the next API 526 letter: D, E, F, G, H, J, K, L, M,
N, P, Q, R, T.Worked example
Saturated steam relief: W = 5 000 lb/hr, P1 = 150 psia (set plus
overpressure, absolute), Kd = 0.975, Kb = Kc = KN = KSH = 1.0
A = 5 000 / (51.5 × 150 × 0.975)
= 5 000 / 7 531 = 0.664 in²
API 526 orifice areas: G = 0.503 in², H = 0.785 in²
→ select an H orifice, 18% larger than required.Common mistakes
- Sizing on the operating flow instead of the relieving flow. The valve is sized for the credible scenario that produces the largest relief load — fire, blocked outlet, or utility failure — not for normal throughput.
- Using gauge pressure. Every API 520 equation is written in absolute pressure, and P1 is the set pressure plus the allowable overpressure plus atmospheric, not the set pressure alone.
- Ignoring back pressure. Built-up and superimposed back pressure both affect capacity through Kb and can force a change from a conventional valve to a balanced-bellows or pilot-operated one — which in turn changes the factors.
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