What function does the foot valve serve?

A foot valve is used to hold a prime in a pumping system which depends on a vacuum to raise intake fluid to the pump’s inlet opening. It is common in rural shallow well water systems, or in private water systems which provide lake water to a summer cottage, where the water level is within perhaps 8 meters vertically of the pump installation location and it is not convenient to place the pump down in the well or below the lake surface, to depend on external air pressure to push water up the intake pipe into the pump’s impeller by evacuating all air from the intake pipe and allow external air pressure pushing on the surface of the water to push water up the pipe.PPH Flange Bottom Valve/Foot Valve

The problem is that water pumps can’t pump air, the impellers don’t turn fast enough to have much effect. Often venturi jets are used ahead of the impeller to recirculate a small amount of water internally in the pump housing because the venturi jets are able to move a proportion of air from the intake to the outlet, bypassing the impeller, but the process of pumping all the air out of the standing intake pipe is very slow.

So the solution is to put a one-way check valve at the bottom of the intake pipe which allows water to freely flow into and up the pipe when the pump is running, but will not let any water flow out of the pipe to be replaced by air when the pump is shut off. But rather than using simply a standard check valve, what is usually used is a foot valve which is designed to operate in the vertical position and which has screens designed to keep unwanted solids or debris out of the intake pipe.