Transcript of "New Developments in Hydraulic Ram Pumping"
Hydraulic Ram PumpResearch ProgrammeDevelopment Technology UnitSchool of Engineering, University of WarwickNEW DEVELOPMENTS INHYDRAULIC RAM PUMPINGTechnical Release 131996
TR13 New Developments in Hydraulic Ram Pumping2This technical release has been written more for ram pump enthusiasts, researchers and manufacturers than forinstallers and users. It describes the main current trends in system and pump design.1 GENERAL TRENDSThe ram pump is a mature technology. Over the lasttwo centuries pump designs have stabilised andmany variations to the basic configuration (of drivepipe, pump, pump house and delivery pipe) havebeen tried. One might think that no furthersignificant change was likely in the ram pump itselfor in the system in which it is used. However thereare changes occurring in both pumping needs and inmaterials.Before the invention of petrol engines or the arrivalof electricity on farms, the ram pump was in manylocations the only feasible way of lifting water fromstreams or springs to neighbouring hillsides. Inconsequence a high cost was tolerated; strong butexpensive pumps made from cast steel, gunmetaland brass were used. Today there are morealternatives, so that ram pumping can only hold itsmarket share in water supply for humans and forcattle by becoming cheaper and simpler.All over the world water is getting scarcer anddirtier. In consequence ideal sites for rarn pumping -where a large flow of clean water drops steeply - arebecoming fewer. Quite often the water requirescleaning if it is to be used for domestic purposes.There are various possible responses to this problemof polluted drive flow. One is to filter the deliveryflow. A second is to use an indirect ram pump thatpermits falling dirty water to power the raising ofclean water from a nearby source. A third is toconcentrate on applications like cattle watering andirrigation where water quality is less important.Filtering and disinfection are well understood, andthe technical options for applying them areincreasing in number. The availability of only one ortwo watts of electricity, say from a small photo-voltaic panel, now enables chemical or ultra-violetsterilisation to be performed at a household orvillage scale. Adding such processes to a rampumping system may require other designadjustments, for example those to permit deliveryflow only in day light hours.Indirect pumping is a technique known for ahundred years or more. Indirect pumps are stillmanufactured but they are complex and hencecostly. They have more wearing parts than normalram pumps and they require a source of clean waterclose to the dirtier flow that drives them. One mightargue that to require such elaboration in systeminstallation and maintenance is to head in the wrongdirection. Field experience suggests that the use ofram pump technology is already severely limited bypeople thinking it is too complicated.It is the authors experience, mostly in an Africancontext, that even after a 3 weeks training coursemany water technicians do not have the confidenceto survey, design and install a ram pump system.The design rules seem complex and they fearmaking any mistake that might cause a system tofail. Yet systems do occasionally fail - through wearand corrosion, insufficient drive flow or flooddamage, siltation or blockage, theft or maliciousdamage. It is not possible to build a perfect system.With petrol-engine pumping at its simplest, the usercarries the pump to site, drops a suction hose intothe water source, rolls out the delivery hose andstarts the pump. With electric powered pumpingusing mains, photo-voltaics or transported batteries,the procedure is a little more complex. Rampumping is more complex again. There has thereforebeen a growing interest in simplifying thetechnology, especially in order to serve irrigationoperated by peasant farmers.2 SIMPLER PUMPSA pump normally comprises an adjustable impulsevalve, a (non-return) delivery valve, a pressurevessel to smooth out the pulsating delivery flow andan anchorage or cradle. Where free air is thebuffering medium in the pressure vessel (whichmust be vertical to work properly), a third (snifter)valve is needed to replenish this air.
TR13 New Developments in Hydraulic Ram Pumping3Simplifications can take a number of forms, but themain ones of current interest are• removing the mechanism to adjust the driveflow,• replacing free air by contained air,• simplifying the anchorage of the pump and itsattachment to drive pipe and delivery.Removing the tuning mechanism of courseremoves all the benefits of tuning, namely the abilityto adjust the pump to match the drive flow locallyavailable. Under some circumstances, especiallywhen only a small fraction of stream flow is needed,there is no great merit in being able to tune. Where amanufacturer produces a range of pumps it is normalfor each step up in size to correspond to a two orthree fold increase in maximum drive flow. Anuntuned pump is effectively permanently set to itsmaximum (or rated) drive flow. Thus using suchpumps singly will restrict the drive flow, and hencedelivery, to one of a few widely spaced values. Ifhowever several (say three) identical pumps, or twopumps of different size, are used in parallel, it isusually possible to get within 25% of any ideal driveflow. In fact there are four distinct alternatives to on-site tuning for matching pumps to available flow,two of them applicable when the system is installedand two when it is in use.During installation the drive flow capacity of asystem can be roughly selected by choosing the rightnumber and size of pumps to be run in parallel.Alternatively pump(s) can be used that are preset toa particular drive flow. This lower level ofadjustability not only simplifies pump design (e.g. itcan be provided by having two or three differentweights of impulse valve), but removes thepossibility that the user completely mistunes hispump. Such mistuning through operator ignorance isquite common in high technology systems as well asin the simple ram pump ones we are discussing here.During operation, there may be a need to respond toa fall in available stream flow. If pumps are nottunable this can only be done by reducing thenumber of pumps in operation or by running themall intermittently. Using the three same size pumpsor two different size pumps arrangementrecommended above, it is possible to follow anychanges in stream flow by changing the number ofpumps in use. Intermittent operation by contrast canbe used with only a single pump but requires muchmore operator activity and also a reservoir capableof storing at least 2 hours drive flow. In practiceintermittent operation, where the user turns on thepump when the reservoir is full and off when it isempty, is very rare. It could become more commonwhere small-farm irrigation is the pumpingapplication. Technically it should also be possible touse a self-priming siphon to achieve intermittentoperation without human intervention: the authorsknow of no example of this being done.Given the desirability of having more than one pumprunning in parallel for reliability reasons, the relativerarity of requiring very close matching of systemdrive flow to stream flow (often extremely variable)and the likelihood of mistuning by inexperiencedoperators - we may expect to see more simplepumps that are untunable or are tuned (preset) onlyduring manufacture.Using "contained " air to buffer the pulsations indelivery flow has real advantages over using aconventional air vessel. By "contained" air or "airpacket" we mean air in a bladder or closed-cellfoam. Normal commercial pressure-surge limiters inwater pipelines use diaphragms to separate the airfrom the water, however such diaphragms aredifficult to make and to seal and are thereforeexpensive. By contrast closed-cell foam such asbubble-wrap has already been used in a number ofsmall ram pumps. The advantages of substituting airpackets for the free air of a conventional pressurevessel are several. The containing chamber need nolonger be vertical, air cannot be lost through tinyholes in welds or fittings, the snifter valve is nolonger necessary, the pump can be operated underwater. Disadvantages are the possible fatigue failureof the air-containment materials, slow loss of airthrough the walls of bladders or foam and thesignificant reduction in air volume at start up.Consider a conventional air vessel of volume 10litres in a pump delivering to 90 meters. Initially,before start-up, the air is at atmospheric pressure (1bar). At start-up the absolute pressure rises rapidlyto 10 bar (9 bar gauge) as the air is warmed andcompressed. It then cools until its volume is about 11itre, namely one tenth of its initial value: the airvessel is now nearly full of water. Over a period of
TR13 New Developments in Hydraulic Ram Pumping4hours however the air is replenished via the sniftervalve to its original volume of 10 litres. The pumpmay run rather noisily until this has taken place.If however a closed air packet replaces theconventional free air, there is no replenishmentmechanism, so throughout the run time it remains at1 litre. It therefore is necessary to provide an airpacket whose initial volume is equal to:Vinit = air volume required in operation(Vop) × delivery pressure in bars absolute.(Note that 10 meters delivery head corresponds to 2bars absolute, 20 meters to 3 bars etc.)Recent research and experimentation suggest thatthe air volume in operation (Vop) can safely be aslittle as twice the volume of water delivered percycle. [The pump efficiency does not fallsignificantly compared with when Vop is large, andthe overpressure of about 30% is usually alsotolerable from a fatigue point of view.] An irrigationpump may only lift to 20 meters, so initial airvolume Vinit, is only 3 times Vop, whereas a domesticsupply pump may lift to 80 meters (Vinit = 9 timesVop) or higher. We would therefore expect this aircompression problem to be more severe with high-lift pumps. However as the delivery head isincreased (while the drive head and drive flow arekept constant) the volume delivered per cycle goesdown. The combination of these effects means thatfor a given size of pump, the appropriate initial airpacket size does not vary much with delivery head.In practical terms, the minimum initial packet sizerelates to pump size roughly as shown in Table 1:This table indicates an initial air packet volume, andtherefore vessel size, equivalent to 1 m of drive pipe(or less length of a larger diameter) should besufficient: this is a tolerable size.With an air packet, pump design can be simplified toessentially an impulse valve followed by a packet-enclosing horizontal tube entered via the non-returndelivery valve. This results in a fairly compactdesign of pump that can be placed under water tomaximise drive head and to reduce noise. Althoughthere are some particular problems that can arisewhen operating under water – for example suckingdebris in through the impulse valve, increasedvulnerability of flood damages and difficulty ofaccess for tuning – in many situations theadvantages outweigh the disadvantages.As materials further improve we may expect moreram pumps to incorporate air packets or even adiaphragm instead of traditional air vessels.Simplifying the pump attachment is aparticular requirement for irrigation use where rampumps and even drive pipes may be removed atnight and will certainly need to be removed at theend of the dry season. The shock forces on pumpswhen they are in use are large, so any anchorage hasto be sturdy. Already it is usual to bolt pumps onto apermanent (i.e. concreted-in) cradle. There is nowinterest in providing clip-on arrangements bothbetween pump and cradle and between pump anddrive pipe, rather than using nuts, bolts or wedges.3 NEW MATERIALS, LOWERCOSTS AND HIGHERPERFORMANCEMaterials For long-life pumps, traditionalconstruction materials are largely suitable. Bycontrast, new materials have particularly found theirplace in cheap ram pumps of modest but adequateperformance. During the last twenty years metalpiping has largely been superseded by plastic,especially PVC, ABS and HDPE. It is thereforeTable 1Drive pipe size (ID) mm 25 50 75 100Assumed driveflow litres/min 25 100 250 500Packet size (for drive head of 2 meters) litres 0.15 0.60 1.50 3Packet size (for drive head of 6 meters) litres 0.45 1.80 4.50 9Volume of I meter of drive pipe for comparison litres 0.5 2.0 4.4 8
TR13 New Developments in Hydraulic Ram Pumping5tempting to use these rust-proof and easily workedmaterials for constructing ram pump bodies.Unfortunately the poor stiffness, fatigue strength andsunlight resistance of plastics poses problems.The water-hammer effect that underlies ram pumpoperation is dissipated in very elastic, or worseenergy absorbing, materials. For this reason we tryto avoid accumulation of air in drive pipes and welook for a high level of wall stiffness in them. Themaximum height a ram pump can deliver to isapproximately hmax=ν Cdp/g. Where ν is themaximum water velocity in the drive pipe and Cdp isthe velocity of sound in that water. It can be shownthat for an infinitely stiff pipe, Cdp/g is about 140meters height per meter/second, in a steel pipe it istypically 120 but in a plastic pipe it is only about 30.[The formula normally used is:Cdp = C11 +DGtEWhere C is the velocity of sound in water, D and tthe diameter and thickness of the plastic pipe, G thestiffness of water and E the stiffness of the plastic.]This effect shows itself in a plastic system beingonly able to deliver to about 30% the height of anall-steel system. For really high head deliveries steeldrive pipe is essential. For delivery height under 50meters, plastic drive pipe is adequateAll materials show fatigue in that a loading thatthey can tolerate easily if it is applied only a fewtimes may cause failure if applied millions of times.In a ram pumping system, the pump and drive pipeexperience between 15 million and 100 millionpressure pulses per year, so fatigue failure is a realdanger. For plastic drive pipes it is usually sufficientto select a pipe pressure rating of 3 times thedelivery pressure. For plastic pumps, fatigue failureis so likely that either they are metal reinforced orthey are restricted to use with very low deliveryheads. Apparently no one is making pumps out ofglass reinforced plastics (GRP) despite this materialhaving suitable stiffness and fatigue performance.Injection moulded plastics are used in centrifugalpumps and hand pumps. They could be used for rampumps too, but the small production runs do not atpresent justify the high tooling costs. A fewexperimental pump bodies have even been made ofconcrete whose inertia may act as a substitute forstrength in the face of sudden forces. The material ischeap, though heavy, but the problems of gettingreally high densities and of sealing the jointsbetween concrete sections have apparently defeatedconcrete pump designers.Certainly the use of simple plastics in small or low-lift ram pumps is now well established alongsidethat of metals for higher lifts. It seems unlikely thatmore complex materials or processes will be soonemployed to make these devices.Lower costs come from use of fewer, cheaper oreasier materials, from mass production and fromdesign simplifications. Understanding of ram pumpsis better than in the past and this had led to a fewdesign changes leading to lower costs.Mass production of complete pumps is constrainedby small markets, while attempts to assemble thepumps from mass-produced fittings have notgenerally led to either high performance or to muchlower costs. Fittings are not cheap if used in anynumber: pumps made from them are generallyclumsy and have too many parts.For fabricated steel pumps the now readyavailability of square section tubing offerssimplification of design and assembly comparedwith traditional round tubing. Square tubing is notefficient at containing high pressures but this is notnormally a problem at all but the highest deliveryheads. Fabrication is better suited to some pump-using countries than employing iron casting,machining, forging or threaded connectors. Weldedjoints can be opened again if necessary by cuttingthem out with an angle grinderProbably the greatest need for cost cutting is inirrigation applications. If a siphon drive pipe couldbe developed (requiring the pump to be submerged),the installation costs of irrigation pumps could bereduced substantially. Some effort is being appliedto designing essentially portable systems for usewith dams as low as 500 millimeters, where thepump and its drive pipe can be quickly disconnectedfrom anchorage and dam respectively.High performance takes various forms, such ashigher efficiency, higher delivery head, quieteroperation and greater durability. It seems that littletheory was used in the past when designing either
TR13 New Developments in Hydraulic Ram Pumping6pumps, or complete systems. Today ram pumpshave something of a fascination for analysts so thatthere are several publications that aid high-performance design. For example the main sourcesof inefficiency are well documented and it is nothard to devise an economical system with an overallefficiency as high as 70%.In Nepal, the Andes, Rwanda and elsewhere there issome need for pumps that lift as high as 200 meters,well beyond the limit of normal machines. Theprocedures and materials for achieving very highheads are known, but so far the market for suchpumps has been too small to cover the costs of fullydeveloping them. DCS, Butwal in the Himalayashave reached 180 meters lift with some reliability.Quiet operation has been traditionally achieved byplacing pumps and drive pipes underground. Foryears some pumps (for example the Blakesmachines) have used rubber impulse valves inotherwise metal systems to reduce noise. The movetowards plastic drive pipes may lower efficiency alittle, but it beneficially converts high-frequencyclanging into less intrusive low-frequencythumping.Only in the area of durability can one find nosignificant improvement. Perhaps the lifespan ofcheap pumps has increased a little from its formerlow level, but it is still far below that achievablewith traditional over-designed machines.