Thermal Relief Valves

Wrong standard. RP 520 deals with relief system design for vessels and piping overpressure protection due to process upsets,fires,etc. Thermal relief valves are TINY, not sized for a flow rate, and are used to protect piping from pressure developed from heating up of fluid in closed piping segments that normally operate below ambient temperature, or might get heated up by heat tracing after being filled at ambient and then blocked in. For example, a section of line normally operating at 150F, but provided with 150# steam tracing and insulation as freeze protection, could approach 350F if blocked in with the heat tracing on.

One has to know the thermal expansion characteristics of the fluid contained in the 'blocked' section, the dimensions of the section, and the temperature range to which the section will be exposed.