The Relief valves are safety valves prevent equipment damage by relieving accidental over-pressurization of fluid systems. The relief valves protects instruments such as motors, pumps, and actuators from damaging due to high pressure in the flow line.
On high pressure
A relief valve bypass the excess pressure in the flow channel to the flow line, if the fluid flow through the pipe line flows in excess pressure.
The relief valve gradually opens as the inlet pressure increases above the setpoint. A relief valve opens only as necessary to relieve the over-pressure condition.Relief valves are typically used for incompressible fluids such as water or oil. Safety valves are typically used for compressible fluids such as steam or other gases. Safety valves can often be distinguished by the presence of an external lever at the top of the valve body, which is used as an operational check.
There are two types of Relief valve Direct acting and Pilot operated
Direct acting :
Spring pressure on the stem is forcing the disk onto the seat. At the pressure determined by spring compression, system pressure overcomes spring pressure and the relief valve opens. As system pressure is relieved, the valve closes when spring pressure again overcomes system pressure. Most relief valves open against the force of a compression spring. The pressure setpoint is adjusted by turning the adjusting nuts on top of the yoke to increase or decrease the spring compression
Pilot-operated relief valves are designed to maintain pressure through the use of a small passage to the top of a piston that is connected to the stem such that system pressure closes the main relief valve. When the small pilot valve opens, pressure is relieved from the piston, and system pressure under the disk opens the main relief valve. Such pilot valves are typically solenoid operated, with the energizing signal originating from pressure measuring systems.
Until the pump line pressure exceeds the relieving pressure set on the control knob, the pilot poppet remain closed. And the main poppet. When the pump line pressure increase the pilot moves to open position. And the water flows out to the tank, Know there is a pressure drop across the main poppet. Due to the pressure drop across the main poppet, the main moves up, so the valve opens. Same thing happens when the pump line pressure drops the pressure difference cause the main poppet close.