Foot valves are a kind of valve that is used with a pump and it is known as check valves. The pump can be pumping a liquid (hydraulic) or a gas (pneumatic). Check valves are one-way valves, that is, they can open in only one direction. As such, they allow only an unidirectional flow, and close off when the direction of flow gets reversed.
Foot valves are found in well pumps, sump pumps, water intake pumps (in rivers and lakes) and in the pneumatic brake lines of big trucks.
When the water pump is running, suction is created, which pulls the water in the upward direction through the pipe. The foot valve is so designed that it opens up easily because of the upward pressure from the incoming water, and thus, normal water flow through the pipe is established.
When the pump is turned off, the upward pulling force on the water is removed. As soon as this happens, the Earth's gravitational pull acts on the water in the pipe, and it tries to flow back into the well with only the foot valve in its way. The entire weight of the water column pushes down onto the foot valve. Since the foot valve is a one-way check valve, it cannot open in the downward direction. As such, under the influence of the downward acting weight of the water column, the foot valve gets shut tightly.
Most foot valves these days are spring-loaded. So when the suction is removed, the springs in the valve pull them shut tightly, ensuring that the pipe is completely sealed off, effectively preventing any reverse flow of water back into the well when the pump is turned off.
Thus, a foot valve prevents the reverse flow of water from the pump back to the sump. This not only stops wastage of energy, but also protects the water pumps from being damaged as a result of dry runs. Therefore, a foot valve is a necessary part of every water pumping system.
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