Safety valve:

A safety valve will stay fully open until the pressure drops below a reset pressure. A reset pressure is lower than the actuating pressure setpoint. The variations among the actuating pressure setpoint and the pressure at that the safety valve resets is known as blowdown. The Blowdown is expressed as a percentage of the actuating pressure setpoint.

As denoted in above figure, system pressure gives a force which is attempting to push the disk of the safety valve off its seat. The spring pressure on the stem is forcing the disk onto the seat.

Since the pressure determined through system pressure overcomes spring pressure spring compression, and the relief valve opens. Since system pressure is relieved then the valve closes while spring pressure again overcomes system pressure. Many relief and safety valves open against the force of a compression spring. A pressure setpoint is adjusted through turning the adjusting nuts on top of the yoke to rise or reduce the spring compression.

Figure: Safety Valve