API 520 / API 521 / API 526 · Engineering Tool
Thermal Relief PSV Sizing
Q = β · V · (dT/dt)
Liquid thermal-expansion relief for a blocked-in line — relief rate, orifice area, and the API 526 orifice letter.
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How it works
A liquid-full line or vessel blocked in between closed valves will pressurise as it absorbs heat from ambient or solar input, because liquids expand with temperature but are nearly incompressible. API 521 calls for thermal relief on such systems where the heat input and blocked-in volume make the pressure rise non-trivial.
The relief rate is the volumetric expansion rate Q = β·V·(dT/dt), where β is the liquid volumetric thermal-expansion coefficient, V the blocked-in liquid volume, and dT/dt the rate of temperature rise. The result is usually a very small flow.
That flow sets the required orifice area through the API 520 liquid-relief equation, and the next standard API 526 orifice is selected. In practice most thermal-relief duties are served by a small D-orifice PSV or a ¾×1 thermal-relief valve; this tool confirms whether that holds.
Assumptions & limitations
- Liquid-full, blocked-in system with external heat input (no fire-case relief — that follows API 521 fire sizing).
- Non-viscous liquid; viscosity correction factor taken as unity.
- Discharge coefficient Kd = 0.62 (API 520 liquid default) unless a vendor value is supplied.
- 10 % overpressure and gauge pressures; back pressure applied as constant. Selection of the next standard API 526 orifice is conservative.
Related Reading
Companion Articles
Safety · Simulation
When is a Thermal Relief PSV Needed?
Not every overpressure scenario requires a full-scale relief valve. Understanding when a thermal relief PSV is the right engineering solution — and when it is not — is a fundamental process safety skill.
Jose CampinsSafety · Simulation
How to Size a Thermal Relief Pressure Relief Valve (PSV)?
Thermal relief PSVs protect systems against minor, predictable overpressure scenarios. Sizing them correctly requires understanding the specific relief case — and why an oversized valve can be just as problematic as an undersized one.
Jose CampinsNeed thermal-relief PSV sizing or a relief and flare system study?
A principal will respond within one business day with an initial fit assessment and a practical next step.