This valve is used to control the liquid level of water tank or water tower automatically. The valve is separated by disc into two chambers, up and down. When the water level is lower than the preset value, the ball float valve opens, and the pressure of the up chamber drops, so there is a differential pressure and the disc opens to supply water; When the water level rise to the preset height, ball float valve closes, so the pressure of the up chamber rises, and the disc will close to stop water supply.