Required PSV orifice area for vapor (critical flow) or liquid service per API 520, with the selected API 526 standard orifice.
Vapor: A = W / (C·Kd·P1·Kb·Kc) · √(T·Z/M) (API 520, critical flow)
How it works
For a known relief load, the tool computes the required orifice area from the API 520 equations, then picks the smallest standard API 526 lettered orifice (D–T) that meets it. Vapor sizing assumes choked flow and warns if backpressure is too high.
Worked example
5000 kg/h of MW-29 vapor (k = 1.3) relieving at 1000 kPag set, 10% overpressure. Requires ≈ 549 mm², which rounds up to a standard 'J' orifice.
Inputs
Service
Relief rate — vapor (kg/h) — Vapor service.
Relief rate — liquid (L/min) — Liquid service.
Set pressure (kPa(g))
Overpressure (%) — 10% typical (process); 21% for fire.
Atmospheric pressure (kPa)
Backpressure (kPa(g))
Relieving temperature (°C) — Vapor service.
Molecular weight (g/mol) — Vapor service.
Specific heat ratio k = Cp/Cv — Vapor service.
Compressibility Z — Vapor service.
Specific gravity — Liquid service.
Rupture disk upstream
Frequently asked questions
Is this a substitute for a full relief study?
No. It's preliminary sizing. The governing relief scenario (fire, blocked outlet, control failure), the relief load itself, certified valve coefficients, and two-phase effects must be established by a qualified relief-systems engineer per API 520/521.
What are the API 526 orifice letters?
A standardized set of orifice areas (D, E, F, … T) that relief valves are built to. Sizing finds the required area, then you select the next-larger standard letter.
What is critical (choked) flow?
When the downstream pressure is low enough relative to the relieving pressure, gas flow through the orifice reaches sonic velocity and won't increase further. The vapor equation here assumes this condition and flags when backpressure is too high for it to hold.
Assumptions
Preliminary sizing — must be verified by a qualified relief-systems engineer.
Vapor case assumes critical (choked) flow, single phase, conventional/balanced valve; Kd = 0.975 (use certified Kd).
Liquid case uses certified-valve Kd = 0.65, Kw = Kv = 1.0 (correct for backpressure / viscosity if applicable).
Backpressure correction Kb and viscosity correction Kv are simplified to 1.0 — apply real correction factors for your case.
Two-phase relief (DIERS / Omega method) and fire-case heat input are not covered.
References
API Std 520 Part I — Sizing and Selection of Pressure-relieving Devices.
API Std 526 — Flanged Steel Pressure-relief Valves (standard orifice areas D–T).
API Std 521 — Pressure-relieving and Depressuring Systems (relief load / scenarios).