I see that many fellow sailors, just like me, only have a superficial understanding of solenoid valve diagrams!

Solenoids can be divided into three main categories based on their principle of operation (i.e., direct-acting and pilot-operated). Furthermore, due to differences in valve disc structure, materials, and the underlying principles, they are further categorized into six subcategories: direct-acting diaphragm structure, step-type diaphragm structure, pilot-diaphragm structure, direct-acting piston structure, step-type direct-acting piston structure, and pilot-piston structure. Direct-acting solenoid valve: Principle: When electricity is applied, the electromagnetic coil generates an electromagnetic force that lifts the closing element away from the valve seat, thereby opening the valve ； When the power is cut off, the electromagnetic force disappears, and the spring presses the closing element against the valve seat, causing the valve to close. Features: It can operate normally under vacuum, negative pressure, or zero pressure, but the diameter generally does not exceed 25 mm. Direct-acting solenoid valve: Principle: It operates on a combination of direct-acting and pilot-operated mechanisms; when there is no pressure difference between the inlet and outlet, upon power application, the electromagnetic force lifts the pilot valve and the main valve’s closing element upward one after another, thereby opening the valve. When the pressure difference between the inlet and outlet reaches the starting value, upon power application, the electromagnetic force acts on the pilot valve; as a result, the pressure in the lower chamber of the main valve rises while the pressure in the upper chamber drops, and this pressure difference is used to push the main valve upward ； During a power outage, the pilot valve uses spring force or medium pressure to push the closing element downward, thereby closing the valve. Features: It can operate at zero pressure difference or in vacuum as well as under high pressure; however, it requires a higher power level and must be installed horizontally. Pilot-operated solenoid valve: Principle: When electricity is applied, electromagnetic force opens the pilot hole, causing the pressure in the upper chamber to drop rapidly. This creates a pressure difference with higher pressure below and lower pressure above the closing element; the fluid pressure then pushes the closing element upward, thereby opening the valve ； When power is lost, the spring force closes the pilot hole; the inlet pressure rapidly creates a pressure difference around the valve closing element, with lower pressure at the bottom and higher pressure at the top. The fluid pressure then pushes the closing element downward, thereby closing the valve. Features: It has a high upper limit for fluid pressure range; it can be installed in any position (customization required), but the fluid pressure difference conditions must be met. In terms of control method, there are two types: dual-electric control and single-electric control, with dual-electric control featuring two coils A. B ； When A is powered, the solenoid valve operates, but when A loses power, the solenoid valve remains in its original state until B is powered, at which point it will operate.