A foot valve is an important device which helps contain the reverse flow of water back into the sump. Here, we find out how it works.
Did You Know?
Most foot valves usually come fitted with an inlet strainer. This strainer filters the incoming water, thereby preventing debris in the well water from entering into the pipe and clogging the valve, jamming it in the open position.
Pumping water from a lower level to a higher level requires a lot of energy. Electric water pumps have to be kept running for hours on end to get this job done. Wouldn’t it literally be criminal then to allow the work done and energy expended by these water pumps to go to waste, by letting the water that has been pulled up flow back down to the source?
Thanks however, to one simple and ingenious mechanical device known as a foot valve, this can be prevented from happening. The basic purpose of a foot valve is to control the direction of flow of water inside of the water pipe. In the following sections, we first find out why a foot valve is needed, and then discuss the details of its working.
Why are Foot Valves Needed?
In applications involving extraction of water from underground water wells, or in pumping water up from the sumps on the ground to the water tanks on top of multi-storied buildings, powerful electric pumps are used. These pumps usually work by creating a suction which pulls the water up through the water pipe to the required height, where it is stored for further distribution.
As long as an electric pump is running, there exists a constant column of water in the water pipe due to the suction that is created. However, once the pump is turned off, this suction is gone. With nothing else to stop the reverse flow, the entire water column would collapse under gravity, as the water flows down the pipe back to the source. Once the water is gone, the empty space is taken up by air.
When the water in the overhead tank runs out and the pump needs to be restarted, the air filled up in the pipe opposes the flow of water, and as such, even though the pump may be turned on and running, there will be no water supply through the pipe. Water pump motors are usually not designed for such dry runs, and may get damaged if this continues for a long time.
A foot valve is an ideal solution to this problem. It works by preventing the backward flow of water through the pipe when the water pump is turned off. Thus, the column of water in the pipe is maintained, and all the related issues are resolved.
How Does a Foot Valve Work?
A foot valve is what is known as a check valve. 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. The following diagram shows a typical application of a foot valve in the pumping out of water from a shallow underground water well. Let’ study it to understand the working of foot valves.
As shown, the foot valve is installed at the pick-up end of the pipe, which is the end that goes into the water well, and through which water is sucked. Thus, it is located at the very bottom (foot) of the well, and so it isn’t easily visible for studying.
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. This is shown in the image below.
Now, 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. This is shown is the diagram below.
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.