A pressure relief valve is the safety escape hatch of a pressurised system: a spring holds it shut until the pressure gets too high, then it opens on its own. Follow the disc, the seat and the spring.
A pressure relief valve is a safety device that protects a pressurised system from itself. During normal operation it stays closed, so all the fluid goes about its usual business. Only when the pressure rises above a set level does the valve open and let fluid escape, bringing the pressure back down before any pipe, vessel or pump is damaged.
From the outside it is just a squat body with pipes and a cap. The whole decision — open or stay shut — is made inside by a very simple balance between fluid pressure pushing a disc one way and a spring pushing it the other.
A coil spring presses the disc down firmly onto its seat. As long as the fluid pressure stays below the setpoint, this is enough to keep the inlet sealed from the outlet, and not a drop escapes.
High-pressure fluid enters through the inlet below the disc. Its pressure acts on the underside of the disc, pushing upward and trying to lift it. Up to the setpoint the spring still wins and the disc stays put.
Once the upward force of the fluid beats the spring, the disc rises off its seat. That opens a path from the inlet, past the seat, and out to the outlet — the valve has now lifted.
Fluid now escapes through the outlet, so the pressure in the system drops. When it falls low enough, the spring pushes the disc back down onto the seat and the valve re-seals, ready to act again next time.
It is a safety device. It opens automatically to let fluid escape whenever the pressure rises above a set level, so the pipework, vessel or pump cannot be over-pressurised and damaged.
The spring. The fluid must push the disc hard enough to beat the spring before the valve opens, so a stiffer spring (or a tighter adjusting screw) raises the opening pressure, called the setpoint.
As pressure approaches the setpoint the disc starts to crack open and lifts further as the pressure rises, so most relief valves open progressively rather than in one jump.
A check valve lets normal flow pass one way and blocks reverse flow. A relief valve stays shut during normal operation and only opens when the pressure gets too high, purely as a safety escape.
No. The disc, seat and seals have to suit the fluid and its temperature and pressure, so relief valves are selected for the specific duty rather than swapped freely.