Deep-well diaphragm pump
Inside a cylindrical pump body at the bottom of the well, a flexible diaphragm shrinks and expands like a tube-shaped balloon, taking the water in through an inlet valve and forcing it out through an outlet valve. The cylindrical pump is connected to a flexible hose which leads the water to the surface. Movement of the diaphragm is effected by a separate hydraulic circuit that consists of a cylinder and piston in the pump stand, and a water-filled pilot pipe, which is also a flexible hose. The piston is moved, usually by pushing down on a foot pedal, although conventional lever handles may also be used. When foot pressure is removed, the elasticity of the diaphragm forces water out of it, back up the pilot pipe, and lifts the foot pedal. Deep-well diaphragm pumps are still being improved, but most imperfections have been corrected.
The principle of the pump is attractive because it allows thin flexible hoses to be used, making the pump easy to install or remove without the need for special tools or equipment. Replacing spare parts is usually easy; only the replacement of the diaphragm may need the assistance of a skilled mechanic. It is possible to install several pumps in a single well or borehole.
Contents
Suitable conditions
Range of depth: 10–70 m.
Yield: 0.50 litres/s at 10 m depth; 0.32 litres/s at 30 m; and 0.24 litres/s at 45 m.
Useful life: Eight years.
Area of use: Burkina Faso, Cameroon, Ghana, Liberia, Mali, Mauritania, Niger.
Construction, operations and maintenance
The pump is operated by pushing down on a pedal, usually by foot, but sometimes with a handle. Depressing the pedal can take a considerable effort, as much as the bodyweight of the user, and the pump must be built to withstand this. Every day, the pump head, platform and surroundings must be cleaned, and the nuts and bolts tightened. Each month, the drive piston, rings and guide bushing need to be checked and replaced if necessary. At least once a year (more often if borehole conditions warrant it), the downhole parts of the pump have to be checked and the entire pump washed with clean water. The pump can be extracted from the well by the village caretaker and reinstalled, all within one-half hour. Only one spanner is needed to service the pump. Also, the plunger seals in the cylinder at the pump stand cost little and can easily be replaced by the pump caretaker.
In contrast, replacing the pump diaphragm is a major O&M activity. This must be done every two to five years, and some diaphragms come with a three-year guarantee. This activity requires a mechanic who has been trained in replacing pump diaphragms (some mechanics have even been able to repair ruptured diaphragms). Deep-well diaphragm pumps are typically communal, and the water committee should appoint someone who lives near to the pump site to be caretaker. This person will need some training in maintenance and hygiene. The committee should be able to get in contact with the area mechanic quickly, and it must have the financial means to pay for repairs in cash. Often, the pump supplier provides maintenance backstopping.
Potential problems
— pedal rod guides and plunger seals need to be replaced frequently, and the plunger guides may wear out quickly;
— drive hoses often need to be re-primed because water leaks past the plunger seals, and the foot pedal then needs to be raised by hand;
— if solid particles enter the downhole pumping element it must be cleaned, since this will cause the diaphragm to stop working or even rupture;
— if a community cannot afford to replace the pump diaphragm, or if no skilled mechanic is available, users may be forced to return to their traditional sources, temporarily;
— moderate skills in steel fabrication and fitting are needed to produce a pump stand, while advanced manufacturing techniques and tight quality control are
needed to produce the pumping element; in many countries, these parts will have to be imported.
Costs
In 1986, a complete pump that operated to a depth of 30 m cost US$ 860. In Burkina Faso and Benin in 1993, a Vergnet model pump cost US$ 1460–1820 (including 10% VAT), depending on the installation depth.
Acknowledgements
- Brikke, François, and Bredero, Maarten. Linking technology choice with operation and maintenance in the context of community water supply and sanitation: A reference document for planners and project staff. World Health Organization and IRC Water and Sanitation Centre. Geneva, Switzerland 2003.
