Solenoid valve main three categories

Direct-acting solenoid valve: 

Principle: when electrified, the solenoid coil produces solenoid force to lift the shut-off piece from the valve seat, the valve opens; when the power is cut off, the solenoid force disappears, the spring presses the shut-off piece on the valve seat, and the valve closes. 

Features: in vacuum, negative pressure, zero pressure when the normal work, but the diameter is generally not more than 25 mm.

Pilot-type solenoid valve:

 Principle: when electrified, the electromagnetic force to open the pilot hole, the upper chamber pressure drops rapidly, in the close, close around the formation of high pressure difference, the fluid pressure to move up the closed parts, the valve open; When the power is cut off, the spring force closes the pilot hole, and the inlet pressure rapidly forms a low and high pressure difference around the shut-off valve through the bypass hole. 

Features: high upper limit of fluid pressure range, can be arbitrarily installed (custom) but must meet the conditions of fluid pressure difference.

Diaphragm solenoid valve:

 principle: when the diaphragm solenoid valve connected to the power supply, the coil excitation circuit to form a magnetic field and magnetic force. The movable iron core is attracted by the electromagnetic force, overcomes the spring resistance and the self-weight, and rises, under the action of differential force, the diaphragm overcomes the self-weight and the spool spring to open, so that the main valve forms a path and the solenoid valve is in an open state. When the solenoid valve power supply is disconnected, the moving iron core is under the action of spring force to close the auxiliary valve. Normally open solenoid valves work in the opposite way. 

Features: suitable medium, wide temperature range, suitable pressure range, flexible installation position, waterproof plastic-sealed coil, explosion-proof