Lift Pumps & Force Pumps

The lift pump and the force pump are two very basic forms of pump, the theory of which is pretty straightforward. Both make use of a downpipe, a cylinder, a piston, a couple of valves, and an outlet pipe or spout. The following graphics are taken with permission from http://etc.usf.edu/clipart.
The lift pump (above), also known as a suction pump, operates as follows: on the upstroke of the piston, the lower valve opens, the upper valve (situated on the piston itself) is closed, and the low air pressure produced in the cylinder allows atmospheric pressure on the surface of the water source, down below, to make the water move up the downpipe and eventually fill the cylinder below the piston. On the downstroke, the lower valve closes, the upper one opens, and water is forced into the cylinder above the upper valve. On the next upstroke, the water above the piston is forced out of the spout, located at the top of the cylinder, at the same time as the volume below the cylinder fills up with water again.	The force pump (above), also known as a pressure pump, operates as follows: on the upstroke of the piston, the outlet or delivery valve is closed and the inlet valve opens. The low air pressure produced in the cylinder causes the water below to move up the downpipe and eventually fill the cylinder. On the downstroke, the inlet valve closes, the outlet valve opens, and the water is forced out via the outlet pipe, which is located at the bottom of the cylinder.
So, we just need a mechanical device to help raise and lower the piston and we've got ourselves a village pump. Two types of mechanism are used: the familiar handle, usually pivoted at the top of the cylinder, that you heave up and down, and the less common wheel and crank combination that simply turns rotational movement into the reciprocating action needed to move the piston. And that's all there is to village pumps: just dig a hole 'til you reach the water source, stick the downpipe into the water and pump away. Er, well, actually, no - in practice there's more to it than this. There's a practical limitation as to how far you can lift a column of water: the vacuum you cause in the cylinder by lifting the pump handle certainly causes atmospheric pressure to force the water up the pipe - but only to the point where the weight of that column of water equals the weight of the atmosphere pushing down on the surface of the water source. It may be shown that (oh, the number of times I've used this handy get-out clause...) the maximum height that water can be lifted is about 34 ft/10.3m, at sea level. However, various factors limit this in practice to about 25 ft/7.7m. So if the water source is deeper than this, the simple set-up as described above can't cope. The answer is to put the cylinder deep down into the borehole, or even below the water level. This allows water to be brought up from much greater depths, but the deeper the cylinder, the more difficult it is to get at for maintenance.
It's human nature to complicate things, and if you really want to know more about refinements to pumps, read on...

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