Principle of direct-acting solenoid valve: when the solenoid is energized, the solenoid coil generates electromagnetic force to lift the closing part from the valve seat and open the valve; In case of power failure, the electromagnetic force disappears, the spring presses the closing part on the valve seat and the valve is closed. Characteristics: it can work normally under vacuum, negative pressure and zero pressure, but the diameter is generally not more than 25mm.

Principle of pilot-operated solenoid valve: when the valve is energized, the solenoid force opens the pilot hole, the pressure of the upper chamber drops rapidly, the pressure difference between the upper part and the lower part is formed around the closing part, the fluid pressure pushes the closing part to move upwards, and the valve opens; In case of power failure, the spring force closes the pilot hole, the inlet pressure quickly forms a lower and higher pressure difference around the closing valve through the bypass hole, and the fluid pressure pushes the closing valve down to close the valve. When the action time is very short and the frequency is high, the direct action type is generally selected, and the pilot type is selected for large diameter