Foot valves are a kind of valve that is used with a pump. The pump can be pumping a liquid (hydraulic) or a gas (pneumatic). 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. To understand what foot valves do, it is necessary to understand a little about check valves and pumps.
Check valves prevent fluids (liquids or gasses) from flowing in the wrong direction. They are typically in a hose or channel and make it hard for the fluid to flow in one direction but easy to flow in the opposite direction. Check valves occur in nature as well as in technology; for instance, there are check valves in the arteries in our legs so that muscle action tends to squeeze the blood up toward the heart instead of down toward the feet. Check valves keep well water flowing up instead of down and the water in water intake systems flowing out of the lake instead of into the lake.
Pumps move fluids around. Typically, pumps move one "chunk" of a fluid across the pump, and the vacuum that this creates in the fluid is replaced by more fluid moving into the pump. The pump then transfers this new chunk, and the process starts all over again. It is kind of like eating--only in reverse. Pumps work by moving the fluid one bite at a time--each new bite pulling in the next.
For this process to work, there must always be a bite in the pump. If the fluid drains out of the pump when it stops, it must be "primed" with a little of the fluid before it can start working again.
Foot valves are check valves that make sure that the pump is always primed. If the pump is shut off, the foot valve makes sure that there is enough fluid in the pump to insure that it starts up again. This is especially important for pumps that are not easy to manually prime. In a well, the foot valve will be between the water surface and the pump. In a water intake system the foot valve will be at the end of the water intake line.
Water Intake Systems
Water intake systems pose a special problem for foot valves. The valve should be at the end of the water intake line but off the bottom of the lake. It should also be far enough below the surface so that it does not interfere with boats or fishermen. There are industrial frames that can be purchased for this purpose but most people attach the line to a drum filled with rocks. This keeps the foot valve off the bottom but well below the surface.