Hydraulic control valve according to the structure can be divided into two types of diaphragm and piston, the same working principle, are the pressure difference above and downstream â–³ P as the power, pilot controlled by the diaphragm (piston) hydraulic differential operation, completely by the hydraulic Automatic adjustment, so that the main valve disc fully open or fully closed or in regulation. When the pressure water entering the control chamber above the diaphragm (piston) is vented to the atmosphere or downstream low pressure zone, the pressure acting on the bottom of the valve disk and under the diaphragm is greater than the pressure above, pushing the main valve disk fully open (Piston) above the control chamber pressure water can not be discharged to the atmosphere or the lower reaches of the lower pressure zone, the role of pressure on the diaphragm (piston) is greater than the pressure value below, it will be the main valve disc Pressure to the fully closed position; when the pressure value in the control chamber above the diaphragm (piston) is between the inlet pressure and the outlet pressure, the main valve disc is in an adjusted condition, the adjustment of which depends on the needle valve and the adjustable Pilot valve joint control. Adjustable pilot valve through the downstream outlet pressure and with its changes to open or close its own small valve port, thereby changing the diaphragm (piston) above the control room pressure value, control valve plate adjustment position. Hydraulic control valve according to the different structure and control methods, can be divided into the following categories: remote control float valve (control level), pressure reducing valve, slow closing check valve, flow control valve, hold pressure / pressure relief valve, hydraulic electric Control valve, pump control valve, bypass valve, emergency shut-off valve.