Hydraulic control valves are of diaphragm and piston two types in general, with the work principle identical. The valve consists of one main valve (as shown in the figure) and the outside mounted needle valve, pilot valve, guide pipe and pressure gauge etc. by way of combining and, to get different purposes, functions and places of use, a remotely controlled floating valve, pressure reducing valve, slowly-closed check valve, flow control valve, pressure gathering/retaining valve, hydraulic electric control valve, water level placing valve, water pump control valve or emergency closing valve etc. can be derived from it. It works like this: take the pressure differential ¦¤ P between both upstream and downstream as the power and, controlled by the pilot valve, to have the diaphragm (piston) hydraulic differential operation completely and automatically hydraulic adjusted so as to make the valve disk completely closed or in the adjustable state.
• Fully-closed state: the main valve is in this state just at that time when the water pressure at the main valve's inlet gets into the body and the control chamber, respectively, and the ball valve outside of the main one is closed;
• Fully-opened state: the main valve is in this state just at that time when the water pressure in the control chamber is all drained into atmosphere or the downstream low pressure area after the ball valve outside of it is fully opened;
• Floating state: the main valve is in this state just at that time when the opening of the ball valve makes the water flow through between needle and ball valves to get balanced.
