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I know about indirect operation but not direct operation. To open or close, indirect operated solenoid valves makes use of the differential pressure of a medium over the valve ports. (These valves require a minimum pressure). The inlet and outlet are segregated by a diaphragm. The diaphragm will have a small hole to let the medium to flow to the upper section. The chamber above the diaphragm will be connected by a small channel to the lowest pressure port. However, this connection will be blocked by a solenoid in the closed position. The diameter of the pilot orifice will be larger than the diameter of the hole in the diaphragm. The pilot orifice will open when the solenoid is energised and causes the pressure above the diaphragm to drop. However, with the help of a small solenoid a large flow rate can be controlled as the extra pressure chamber above the diaphragm will act as an amplifier.

Visit https://www.youtube.com/watch?v=a-4vqC8qARY. This video will explain the indirect operation of a solenoid valve with necessary diagrams.

In direct operated solenoid valves, the medium flows through a small orifice. These orifice can be closed with the help of a plunger. An electric coil should be positioned around the plunger and as the coil is electrical energised, a magnetic field will be created, which attracts the plunger towards the centre of the coil. This opens the small orifice and the medium flows through it.

Thank you so much for the answers. It was indeed informative.