|Lift Pumps & Force Pumps (& Semi-Rotaries Too)|
|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 plunger, the lower valve opens, the upper valve (situated on or in the plunger itself) is closed, and the low air pressure produced in the barrel allows atmospheric pressure on the surface of the water source, down below, to make the water move up the downpipe and eventually fill the barrel below the plunger. On the downstroke, the lower valve closes, the upper one opens, and water is forced into the barrel above the upper valve. On the next upstroke, the water above the plunger is forced out of the spout, located at the top of the barrel, at the same time as the volume below the barrel fills up with water again.||The force pump (above), also known as a pressure pump, operates as follows: on the upstroke of the plunger, the outlet or delivery valve is closed and the inlet valve opens. The low air pressure produced in the barrel causes the water below to move up the downpipe and eventually fill the barrel. 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 barrel.|
So, we just need a mechanical device to help raise and lower the plunger and we've got ourselves a village pump. Two types of mechanism are used: the familiar pivoted handle that you heave up and down, and the less common flywheel and crank combination that simply turns rotational movement into the reciprocating action needed to move the plunger.
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 barrel 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 barrel deep down into the borehole, or even below the water level, and perhaps add an outlet valve - so forming a so-called "lift & force pump". This allows water to be brought up from much greater depths, but the deeper the barrel, the more difficult it is to get at for maintenance.
It's human nature to complicate things, and if you really want to know about Air Vessels, read on...
|And now it's on to Semi-rotary Pumps - but let's start a new page.|