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.
Also heard as
- PRV
- liquid relief valve
Related terms
Safety valve
A spring-loaded pressure relief valve for steam, gas or vapor that opens with a rapid full lift at set pressure.
Cracking pressure
The pressure at which a check valve or relief valve first starts to open and pass a trickle of flow.
Simmer
For a safety valve to leak with an audible hiss or seep just below its set pressure, before it pops.
Pressure locking
A closed gate valve that cannot be opened because trapped pressure in its bonnet cavity jams the wedge into both seats.