Air-operated ball valve

A 24 V coil lets 6 bar air into a rack-and-pinion actuator. The air turns a ball a quarter turn, and city water flows to the tank. Cut the power or the air, and springs slam it shut.

• Solenoid (3/2-way, normally closed): three ports, two positions. Coil off: a spring presses the plunger onto the inlet seat P, and port A vents through the hollow core to R. Coil on: the magnet pulls the plunger up. It seals the core and opens P → A.
• Rack and pinion: air between the two pistons drives them apart. Each piston carries a rack; both racks turn one pinion 90°.
• Spring return: springs behind the pistons need 3.0 bar to start moving and 4.5 bar to finish. Less air and the valve stalls part-way.
• Ball valve: a ball with a hole through it. Hole in line with the pipe: open. A quarter turn: shut.
• Fail-closed: no power, or no air, and the springs win. The water stops. That is why tank fill lines use this layout.