Preliminary screening only. Core API 520 Part I orifice-area equations for gas/vapor and liquid service — not the full current-edition procedure (no fire case, no two-phase/omega method, correction factors default to 1.0). Never the sole basis for an installed relief device; use manufacturer certified sizing software and a documented relief scenario review.
Verified against published standard
Relief Case
Relief Case
Enter relief case data.
About This Calculation
A pressure relief valve (PSV) is a plant’s last line of defense against overpressure — sized wrong, either direction is dangerous: too small and the vessel can still overpressure even with the valve open; too large and the valve can chatter (rapid open/close cycling) hard enough to damage itself and the piping. API 520 Part I gives the standard sizing equations, and they genuinely differ by fluid phase: gas/vapor sizing uses compressible, often choked-flow equations; liquid sizing uses an incompressible Bernoulli-based form. Using the wrong one — a real documented failure mode — can undersize a valve by a large margin.
Frequently Asked Questions
Why does this calculator call itself "preliminary" rather than a full API 520 sizing tool?
Full relief valve sizing per the current API 520/521 editions covers subcritical (non-choked) gas flow, the two-phase Omega method, iterative viscosity correction for liquids, and backpressure correction charts specific to the valve type — none of which this implements. This covers the core critical-flow gas and non-viscous liquid equations only, as a screening-level estimate.
What is overpressure allowance and why does it matter?
Overpressure allowance (commonly 10% above set pressure, sometimes more for specific scenarios) is how much the vessel is allowed to rise above set pressure while the valve is relieving — it directly sets the relieving pressure used in the sizing equation, so getting this percentage right matters as much as any other input.
Why do gas and liquid relief valves use such different discharge coefficients (0.975 vs 0.65)?
These are standard, published API 520 values reflecting how differently a valve’s orifice geometry performs with a compressible fluid at choked flow versus an incompressible liquid — they are not interchangeable, and using the gas coefficient for a liquid case (or vice versa) meaningfully mis-sizes the valve.
Does this replace manufacturer certified capacity data?
No — real relief device selection uses the specific manufacturer’s certified capacity for the chosen valve model, not a generic theoretical orifice area. This calculator estimates the required area as a starting point for that selection, and as a check, not a substitute for it.