Hydraulic Control Valve is classified into two types: diaphragm type and piston type, to which the working principles are the same: motivated by the pressure difference (△P) between the upstream and the downstream, controlled by pilot valve to enable the diaphragm (piston) to perform a hydraulic push-pull operation, regulated fully by hydraulic force so as to put the plate of the main valve into a status of completely ON or OFF or regulating status.