The float valve is a simple mechanical liquid level controller. Afloat after detecting the changes of elevation in the liquid free surface opens or closes a valve accordingly. Small float valves are hugely used in domestic water systems. Float valve maintains the level in the cold water header tank which regulated the pressure of the hot water system. In toiler cisterns, a much smaller float valve is fitted to measure the flush water.
The float ball inside a toilet tank attaches to ballcock, by way of a thin, horizontal arm. The ballcock is the valve that allows water back into the tank after the toilet flushes. The float ball does as its name implies and floats on the surface of the tank water when the toilet remains unused. When the toilet handle is pressed and the water drops from the tank into the bowl, the float ball drops along with the water level, which signals the ballcock to allow more water into the tank. As the water level rises again, the float ball rises with it until it reaches a certain height and signals the ballcock to shut off.
Domestic valves are cast in brass or bronze or injection molded in plastic. Most valves close on a rising liquid level, but this can be reversed on industrial valves. To allow maintenance without draining the tank, the valve is mounted on the tank side above the liquid level. The liquid level on low-cost valves is adjusted by bending the ball arm. Superior valves have bolted connections in the arm to facilitate the adjustment. If the outlet of the valve is not piped below the liquid level, the incoming liquid can be aerated to a great extent. The aeration can create serious problems with pumps operating very close to the NPIP or NPSH limit. Float valve can also create waves on the liquid surface which in turn can cause surge problems. Waves cause the float valve to modulate and vary the flow rate of the liquid. The flow modulation can make the problem worse. When modulation occurs close to valve closure then the disc and the seat can be damaged and water hammer symptoms induced. The inlet process system can suffer significant disturbance problems due to surge. Pressure gauges vibrate and the needles fall off. Pressure transducers produce erratic readings and flowmeters give incorrect readings. The liquid surface must be stabilized to eradicate the problem. Piping the outlet under the surface and diffusing the flow can work.