Hydraulic ram pumps and Sling Pumps

D WATER RESOURCES AND USE: SKINNER• The situations in which a handpump will be used mayvary across the country. In particular the groundwaterlevel, the corrosivity of the groundwater and thewater demand at the water point may at first indicatethat for cost effectiveness and technical reasons anumber of different pumps are needed.• The best choice of handpump may be one which,although it has proved itself in other countries, is notyet widely used in the country wanting to standardise.The problems associated with introducing a newpump may be discouraging. There is obviously a costimplication, particularly when eventually existing nonstandardpumps have to be replaced.• Governments may want to avoid being seen to favourone manufacturer’s or one country’s product overanother. They may also fear that once they havestandardised, a manufacturer of the chosen pump will‘corner’ the handpump market and will artificiallyraise prices to make more profit. (This latter problemdoes not occur for pumps like the Afridev, the Taraand the India Mark II & III which have designs in thepublic domain).The need to consider sustainability notpump costThe cost of the handpumps forms only a very small partof the whole cost of a rural water supply programme.When choosing a particular type of pump it is thereforenot sensible to base the choice only on the capital cost ofthe pump. Rather emphasis should be put on whichpump is likely to be sustainable. Producing a borehole hasa high investment cost (e.g. US$15,000) whereas the capitalcost of a handpump is typically less than US$1,000. Theborehole investment can no longer provide benefits if thepump used on it fails and can not be repaired.How many standard types?As mentioned above the operating conditions forhandpumps often vary across a country and this maysuggest that a number of pumps are needed. However,pumps designed to cope with difficult operating conditionsare usually also suitable for less onerous conditions.In particular this means that:• a deepwell pump (typically able to lift water > 25m)can usually replace a suction pump (able to lift watera maximum of about 7.5m) or replace a direct actionpump (able to lift water by about 12m). That is, adeepwell pump is also suitable for drawing waterfrom shallow or intermediate depths of groundwater.The capital cost of the deepwell pump may be greaterthan for either of the other pumps, but it may be moresustainable to standardise on just the deepwell pumpinstead of choosing three standard pumps, one of eachtype.• Similarly a direct action handpump is suitable forshallow groundwater levels so it can replace anysuction pump. Indeed it has other advantages over thesuction pump, particularly that it does not lose itsprime (so there is no risk of contamination from usingpolluted priming water). If there is a danger that in thefuture groundwater levels will fall to near the limit ofthe suction pump (about 7.5m maximum lift) then it iswiser to choose a direct action or a deepwell pump.• A corrosion resistant pump is of course suitable fornon-aggressive groundwater as well as for aggressivewater.I believe that serious consideration should be given tochoosing just one corrosion resistant deepwell pump tobe used for all installations in a country.Standardising on corrosion resistant pipesand rodsStainless SteelIf a country standardises on the use of stainless steelpump rods and rising main pipes instead of using galvanisedsteel this will make the pump suitable for aggressivegroundwater. It could be argued that money iswasted when the corrosion resistant stainless steel is alsoused in waters which are considered to be non-aggressive,but:• Even in non-aggressive groundwater galvanised mildsteel rods and pipes will eventually corrode, particularlywhere galvanic (or bimetalic) corrosion can takeplace.• Stainless steel pipes are lighter, making it easier to liftthe rising mains (this operation is necessary to reachthe valves and piston seals of traditional designs ofdeepwell pump).Glass-fibre rodsThe use of glass fibre rods may soon also become attractiveas increased international demand leads to a reductionin price.PlasticPlastic rising mains are corrosion resistant and have beensuccessfully used in the design of a number of pumpssuch as the Afridev, Tara and Nira AF85 pumps. They aremuch cheaper than stainless steel pipes. Only high qualitypipes and solvent can produce a good solvent cementedPVCu pipe joint. Thin walled PVCu pipe can notbe successfully threaded but coarse threaded screwedcouplings can be solvent cemented, glued with resin, orfixed in other ways to the pipe to create a joint suitable fordis-assembly during maintenance. These joints usuallyhave rubber ‘O’ -rings to ensure water-tightness. Airfilledplastic pipes are usually used for the operating rodsof direct action pumps.In Sri-Lanka PVCu rising main pipes have been successfullyused to replace the small and large diametergalvanised iron mains normally used with the India MarkII and III respectively.209