1. According to the working principle and structure, the pressure reducing valve is divided into two types: direct acting type and pilot type.

2 Function and performance requirements: the pressure reducing valve is used to reduce the pressure of a certain circuit in the system. The pressure reducing valve with constant outlet pressure is called constant output pressure reducing valve. The pressure reducing valve with constant difference between outlet pressure and a certain load pressure is called constant differential pressure reducing valve. The pressure reducing valve with constant ratio between inlet pressure and outlet pressure is called constant ratio pressure reducing valve. The requirement of constant value output pressure relief valve is: no matter how the inlet pressure changes, the outlet pressure should be maintained constant and not affected by the flow change through the valve. For constant differential or constant ratio pressure relief valves, it is required that no matter how the inlet pressure or outlet pressure changes, the pressure difference or their ratio should be constant.

3. The performance of the relief valve: besides the characteristics similar to the relief valve, it also requires the following properties.

The outlet pressure change caused by pressure change of inlet is small. The greater the pressure difference between the inlet and outlet, the more stable the outlet pressure is when the inlet pressure changes.

The change of outlet pressure caused by flow through the valve is small.

It also requires a reverse pressure loss for the one-way pressure reducing valve.

4. Selection of pressure relief valve: when choosing pressure relief valve, the change trend of opening and closing characteristics of pressure relief valve should be considered, which is contrary to that of relief valve. Attention must be paid to the relationship between the setting pressure of the relief valve and the working pressure of the actuator. The pilot type pressure relief valve has more oil discharge than other control valves. There is always a flow of oil from the pilot valve, possibly up to 1L/min, which affects the choice of pump capacity. The two pressure range depends on the flow of the spring and the valve. The minimum adjustment pressure should ensure that the difference between the primary pressure and the two pressure is 0.3~1MPa. When the flow of the valve is far less than the rated flow, it is easy to generate vibration or other unstable phenomena. At this point, measures should be taken in the loop.