Pressure Relief Valve

The pressure relief valve is used for overpressure protection. Its fundamental task is to limit the system pressure to a maximum value. This limits the pressure in the respective hydraulic system to a maximum and thus ensures consistently stable operation of the system.

Functionality of a Pressure Relief Valve

When the set maximum pressure is reached in the hydraulic system, the corresponding pressure relief valve opens, and the excess flow can drain into, for example, a reservoir or the tank.

Pressure relief valves are mostly used for safety reasons. The limited pressure in hydraulic systems serves to protect systems and consumers and prevents leaks.

The maximum pressure in the valve can be set in various pressure stages, allowing for optimal adaptation of the hydraulic system. The adjustment range is, for example, specified in the order of 10 – 50 bar. The pressure relief valve only operates when the maximum pressure in the system is reached. In most applications, such a valve is used to protect the maximum pump pressure of a hydraulic system from being exceeded.

Direct-Acting Pressure Relief Valve

In a direct-acting valve, the pressure of the medium exerts a force on the piston surface of the conical element in front of the valve’s spring. This force from the oil pressure is opposed by the valve’s spring force.

If the spring force is greater than the oil pressure, the valve remains sealed, and the piston stays on its seat. If the oil pressure exceeds the set spring force of the pressure relief valve, the conical valve body compresses the spring, and the path for the hydraulic medium opens from the pump side to the tank side.

Since in this state the pump must work against the set maximum spring pressure, the unnecessarily expended power is only used to heat the oil. As larger oil flows require very large pipe cross-sections, this design is not suitable for large hydraulic systems.

Pilot-Operated Pressure Relief Valve

This more expensive but also economically superior solution consists of a main control stage and a pilot control stage. The permissible maximum oil pressure is determined by the strength of the pilot control stage’s spring. This is achieved through a very small bore in the face of the main piston.

The main piston in the pilot-operated pressure relief valve is pressure-balanced as long as the pilot piston does not open. Without oil flow, the main piston is held on its seat by a weak spring. When the inlet pressure rises sufficiently for the pilot control valve to open, a small oil flow passes through the bore in the face of the main piston. This creates a pressure drop to the right of the main piston, and the force difference between the rear and front sides of the main piston pushes the main piston against the spring. The connection between both ends of the pilot-operated pressure relief valve opens, and the oil can flow to the tank with negligible resistance.