Relief valve
Relief valve turns up in 2 fields: valves, boilers. Here is what it means to the people in each one.
Relief valve in valves
A spring-loaded valve for liquid service that opens gradually as pressure rises past its set point.
Unlike a safety valve, a relief valve lifts roughly in proportion to how far pressure exceeds set, because liquid does not expand to slam the disc open. It protects pumps, hydraulic systems and blocked-in liquid lines, including small thermal relief valves that vent the expansion of trapped liquid heated by sun or tracing. Valves built to do both jobs are called safety relief valves.
"Add a 3/4 inch thermal relief valve between those two block valves, that line sits in the sun all afternoon."
Relief valve in boilers
The pressure-relief valve on a hot water boiler, rated in Btu per hour and set at or below the boiler's maximum pressure.
Because water does not expand like steam, the valve lifts gradually as pressure climbs past set. Residential hot water boilers almost always carry a 30 psi relief valve, and its Btu rating must equal or exceed the boiler output. The discharge line runs full size to near the floor or a drain with no valve or cap. A dripping relief valve usually points to a waterlogged expansion tank or a fill valve set too high.
"Relief valve's dripping every time the boiler fires, check the expansion tank charge before you blame the valve."