Pressure relief valve: The pressure relief valve is a type of valve used to control or limit the pressure in a system by allowing the pressure source fluid to flow from an auxiliary passage, away from the main flow path.
It acts as protector of hydraulic circuit. It is always required as the pressure in the hydraulic line should not increase the design limit of component in hydraulic system.
-Fig. Pressure relief valve
The valve consists of two sections; a body section containing a piston that is retained on its seat by a spring and a cover. The adjusting screw adjusts the limiting pressure. This valve is installed so that one part is connected to the pressure line and other part to the reservoir. The ball is held on its seat by thrust of the spring, which can be changed by turning screw.
When the pressure in the line is above the set pressure, the ball is unseated and the valve is opened. Liquid flow from the pressure line through the valve to the reservoir. This diversion of flow reduces the pressure in the pressure line. When pressure reduces to set pressure, the springs reseat the ball and valve is again closed.
— The set pressure at which valve starts opening is called cracking pressure.
— These are basically normally closed, 2 way valves that open when subjected to supply line pressure above valve setting pressure.