Parker solenoid valve type
1. Direct acting solenoid valve: Principle: when energized, the solenoid coil generates electromagnetic force to lift the closing part from the valve seat and open the valve; when de energized, the electromagnetic force disappears, and the spring presses the closing part on the valve seat, and the valve closes. 
2. Distributed direct acting solenoid valve: Principle: it is a combination of direct acting and pilot type. When there is no pressure difference between the inlet and the outlet, the electromagnetic force directly lifts the small pilot valve and the main valve closing piece upward one by one and opens the valve after being powered on.
3. Pilot solenoid valve: Principle: when energized, the electromagnetic force opens the pilot hole, and the pressure in the upper chamber drops rapidly. A high and low pressure difference is formed around the closing piece. The fluid pressure pushes the closing piece to move upward, and the valve opens. When the power is cut off, the spring force closes the pilot hole, and the inlet pressure quickly forms a low and high pressure difference around the closing piece through the bypass hole. The fluid pressure Force the closure down to close the valve.