When rain falls, it seeps into the ground and collects in an underground reservoir known as groundwater. The upper limit of this reservoir, the "water-table", may vary in depth, from just below the surface (like in a spring or oasis) to well over 100 metres. The only way to get at this water is to dig down.
The simplest and cheapest method of lifting groundwater remains the rope and bucket in a wide, shallow well. These can operate to a depth of 100 metres, (although they rarely exceed 45 metres) and can last for a very long time without maintenance.
Most types of groundwater pump have a piston that moves back and forth inside a two-valve cylinder (a valve allows water to pass in only one direction - in this case, upwards):
Suction pumps have the cylinder situated above ground or near the surface. This means that they can only be used for shallow wells. It is called a suction pump because pulling up on the piston creates a low pressure ("suction") in the cylinder, causing the atmospheric pressure outside to push the water up to the surface. Because atmospheric pressure is fairly low, the pressure difference between inside and outside the cylinder is only large enough to raise water from a maximum depth of about 7 metres. It should also be noted that if a shallow-well is used too much, the water table may fall as the underground reservoir of water is reduced. If this level falls below 7 metres, the pump will not work.
Four types of shallow-well pumps are shown below: rower, piston, diaphragm and semi-rotary.
The rower pump is a simpler and cheaper version of the traditional piston pump (see below). Its simple design means it can be easily manufactured and maintained using locally available skills and materials. This type of pump may require "priming", which means pouring water into the cylinder so that the seal around the piston is airtight. It is very important that clean water is used, to avoid contamination of the pump and the spread of water-borne diseases.
Piston pumps, based on the same design as shown in Figure 1, are more widely used. There is a similar risk of contamination from dirty priming water. In cases where the water is to be delivered under pressure (such as to a village water mains) or to a point higher than the cylinder (such as a water storage tank), a "force" pump is required. The operation is the same, but the design is slightly altered so that the top is airtight. This is done by putting a valve on the spout and adding a "trap tube" and air chamber which maintains the pressure (and therefore the flow) during the up-stroke.
This design is often used for fuel pumps in cars. The Vergnet pump is an adaptation of this principle for deep-well use, which can be used in crooked wells, where a rod-operated pump would have problems, and which is fairly easy to maintain.
Because leg muscles are stronger than arm muscles, this design is less tiring to use. Most of the parts can be manufactured locally, the exceptions being the cylinders and pulley.
Web site details on pumps
Contact the Technical Research officer at Practical Action, Bourton Hall, Bourton on Dunsmore, Rugby, CV23 9HN for technical briefs on different pumps, how to choose a pump for different purposes.
Alternatively, e-mail firstname.lastname@example.org
Prince Consort House,
27-29 Albert Embankment,
Tel: +44 (0)20 7793 4500
Fax: +44 (0)20 7793 4545
Web Page: http://www.wateraid.org.uk