API 520 / 521 Liquid Thermal Expansion Relief Valve Calculator
Calculate trapped liquid volumetric expansion flow rate and required relief valve orifice area (API 526 letters D to T) for solar radiation and steam-traced pipelines per API 520 and 521.
Blocked Pipe Geometry & Heating Source
Trapped Liquid & Valve Set Pressure
API 520 Valve Sizing Results
Recommended Tools & Equipment
Tested hardware and components for high reliability
Frequently Asked Questions
Why do blocked liquid pipelines require thermal relief valves (TRVs)?
Liquids are virtually incompressible. When liquid-filled piping is blocked in between two closed isolation valves and exposed to sunlight, ambient warming, or heat tracing, thermal expansion creates extreme hydrostatic pressures (often 50 to 100 psi per °F of temperature rise), rapidly bursting pipe gaskets or flanges.
Why does API 526 specify a "D" orifice for almost all liquid thermal relief valves?
The calculated thermal expansion flow rate of trapped liquid is typically very small—often less than 0.5 to 2.0 GPM. An API "D" orifice (0.110 sq in) is the smallest standard industrial flanged relief valve size manufactured and comfortably handles dozens of times the required solar thermal expansion rate.
What is the allowable overpressure for thermal expansion relief under API 520?
For liquid thermal expansion scenarios without fire exposure, API 520 allows a maximum accumulation (overpressure) of 10% above the design set pressure (or 3 psi, whichever is greater). If the thermal expansion is driven by external pool fire, 21% overpressure is permitted.