Size pressure safety valves for multiple relieving scenarios per API Standard 520/521 — with real thermodynamic calculations, not shortcuts.
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What It Does
Enter your vessel geometry, fluid composition, and relief conditions. The calculator determines the required orifice area and selects the appropriate standard orifice designation using Peng-Robinson equation of state: the same thermodynamic model used in process simulators like HYSYS and ProMax.
Supports natural gas, rich gas, propane, butane, and custom multi-component compositions up to C10+ with nitrogen, CO₂, H₂S, and water.
Relieving Scenarios Covered
1A — Fire-Wetted (Liquid Boil Up): Pool fire on a vessel with liquid inventory causing vaporization. Calculates wetted surface area, heat input per API 521, and required relief rate based on latent heat at relieving conditions. Supports horizontal vessels and vertical vessels with different heads.
1B — Fire-Unwetted (Vapor): Fire exposure on vapor space only. Evaluates gas expansion relief requirements when the vessel has no liquid inventory or when the unwetted portion governs.
2A — Blocked Outlet (Vapor): Blocked vapor outlet with an upstream source. Relief rate based on maximum normal vapor generation or inlet flow when the outlet is isolated.
3 — Control Valve Failure (Vapor): Control valve fails open in vapor service or subsequent gas-blowby. Calculates flow through the failed-open CV using the valve Cv and upstream pressure. Includes inadvertent bypass opening for multiple sizes of ball or globe valves!
4 — Hydraulic Thermal Expansion: Liquid-full system thermal expansion per API 521. Evaluates relief requirements for blocked-in liquid systems exposed to solar heat input.
How It Works
Free tier: Run unlimited calculations and view results on screen — orifice size, relief rate, required area, and thermodynamic properties. Use it for screening, quick checks, or preliminary sizing.
Paid report: Need documentation for a project file or regulatory submission? Generate a professional PDF report with full calculation details, methodology references, and an engineer-ready format.
Why Use This
Most free PSV calculators use ideal gas assumptions or fixed fluid properties. This tool runs actual flash calculations with composition-dependent properties at your specific relief conditions — giving you results you can trust for real engineering work.
Built and maintained by a licensed Professional Engineer (PE) with experience in process safety, relief systems, and air permitting across upstream and midstream oil & gas.
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Questions or need a PE-reviewed calculation? Contact Franc Engineering