Rotary Positive Displacement Pumps

8/6/2019 Rotary Positive Displacement Pumps

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3.6 ROTARY POSITIVE DISPLACEMENT PUMPS

There is a group of devices which utilize the displacement principle for lifting or movingwater, but which achieve this by using a rotating form of displacer. These generallyproduce a continuous, or sometimes a slightly pulsed, water output. The main advantage

of rotary devices is that they lend themselves readily to mechanization and to high speedoperation. The faster a device can be operated the larger the output in relation to its sizeand the better its productivity and cost-effectiveness. Also, steady drive conditions tendto avoid some of the problems of water hammer and cavitation that can affectreciprocating devices.

Centrifugal pumps, which use a different principle and are described later, have in factbecome the most general mechanized form of pump precisely because they can bedirectly driven from internal combustion engines or from electric motors. But rotarypositive displacement pumps have unique advantages over centrifugal pumps in certainspecialized situations, particularly in being able to operate with a much wider range ofspeeds or heads.

Some types of rotary positive displacement pump have their origins among the earliestforms of technology (eg.the Archimedean Screw), and even today lend themselves tolocal improvization. In the past, industrially manufactured rotary pumps were lesssuccessful than centrifugal ones, possibly because they suffered from a number ofconstructional and materials problems. But modern, tougher and more durable plasticsand synthetic rubbers may well be an important factor in encouraging the manufacture ofa number of new types of rotary positive displacement pumps which could beadvantageous in some situations, as will be described.

3.6.1 Flexible Vane Pumps

Here a flexible toothed rotor is used, generally made of rubber, Fig. 45. This is verysimple in concept, being like a revolving door, but it can involve both considerablefriction and significant back leakage. It cannot therefore be considered as an efficienttype of pump. On the positive side, it will readily self-prime and can achieve a high headat low rotational speeds. Much will depend on the quality of the rotor material and thetype of internal surface of the casing so far as both friction and durability are concerned.

Another similar type, developed recently by Permaprop Pumpen in Germany, has anendless rubber toothed belt which is driven around two pulleys; (see Fig. 46). As itcurves around a pulley, the teeth on the belt spread apart and increase the volumebetween them, thereby drawing in water. The diagram shows how both sides of thechamber simultaneously pump in opposite" directions, and suitable channels in the

casing direct the water. The advantages claimed by the manufacturers are, inter alia,that it can run on "snore" indefinitely - (i.e. pumping a mixture of air and water), it willreadily self-prime and suck water up to 8m and lift it a further 45m under the power of asmall portable single cylinder engine. It is therefore a much more versatile pump thanthe equivalent centrifugal pump, but it is more complicated and expensive.

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Fig. 45 A flexible vane pump

Fig. 46 The Permaprop tooth pump

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3.6.2 Progressive Cavity (Mono) Pumps

None of the rotary pumps so far reviewed lend themselves to being lowered downboreholes; in fact their main selling point is as suction pumps. However, the "progressivecavity" alias "progressing cavity" alias "Mono" pump (after its French inventor,Moineaux), (see Fig. 47) is unique in being a commercially available rotary positivedisplacement pump that readily fits down boreholes. This is a great advantage becausepositive displacement pumps can cope much more readily with variations in pumpinghead than centrifugal pumps. Therefore, any situation where the level may changesignificantly with the seasons or due to drawdown, or even where the drawdown isuncertain or unknown, makes the progressive cavity pump an attractive option. It alsohas a reputation for reliability, particularly with corrosive or abrasiveimpurities in thewater. The reasons for this relate to good construction materials combined with amechanically simple mode of operation.

Fig. 47 shows that this pump consists of just a single-helix rotor inserted in a double-helix stator. A single helix is rather like a simple spiral staircase while a double meanstwo intertwined helixes. The stator helix is usually made from chromium plated steel orfrom stainless steel with a polished surface finish, and is circular in cross section and fitsaccurately into one of the two helices of the stator. The stator is usually moulded fromrubber or plastic and the cross-section of its internal helix is oval, similar to two circlessimilar to the rotor abutting each other. A feature of the geometry of this type of pump isthat the empty second start of the stator is divided into a number of separated emptyvoids, blocked from each other by the solid single start rotor. When the rotor is turned,these voids are screwed along the axis of rotation, so that when "he assembly issubmerged, discrete volumes of water will be trapped between the single start rotor helixand the inside of the double start stator in the voids and when the shaft is rotated thesevolumes of water are pushed upwards and discharged into a rising main.

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Fig. 47 Progressive cavity or 'Mono' pumpSchematic cross-section to illustrate principle of Archimedean screw

Pumps of this kind are usually driven at speeds of typically 1000 rpm or more, and wheninstalled down a borehole they require a long drive shaft which is guided in the risingmain by water lubricated "spider bearings" usually made of rubber. Although frictionforces exist between the rotor and stator, they are reduced by the lubricating effect of theflow of water, and they act at a small radius so that they do not cause much loss ofefficiency. Progressive cavity pumps therefore have been shown to be comparablyefficient to multi-stage centrifugal pumps and reciprocating positive displacement pumpsunder appropriate operating conditions. Their main disadvantage is their need forspecialized components which cannot be improvized and their quite high cost; however,high cost is unfortunately a feature common to all types of good quality borehole pumpand is usually justified by the need to minimise the frequency of the expensive procedureof removing and overhauling any pump from a deep borehole.

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The progressive cavity pump can be "sticky" to start - i.e. it needs more starting torquethan running torque (similarly to a piston pump) to unstick the rotor from the stator andget the water that lubricates the rotor flowing. This can cause start-up problems if electricmotors or engines are used, but certain improved versions of this kind of pump includefeatures which reduce or overcome this problem.

3.6.3 Archimedean Screw and Open Screw Pumps

The progressive cavity pump is one of the more recent pump concepts to appear, whilethe Archimedean screw is one of the oldest, yet they have a number of similarities.

Fig. 48 illustrates a typical Archimedean screw pump (and an animal-powered version isshown in Fig. 97). The traditional version of this pump, built since before Roman timesand still used in a similar form in Egypt, is made up of a helix of square cross-sectionwooden strips threaded onto a metal shaft and encased in a tube of wooden staves,bound like a barrel with metal bands.

The Archimedean screw can only operate through low heads, since it is mounted with itsaxis inclined so its lower end picks up water from the water source and the upper enddischarges into a channel. Each design has an optimum angle of inclination, usually inthe region of 301 to 40 , dependingon the pitch and the diameter of the internal helix.

The principle is that water is picked up by the submerged end of the helix each time itdips below the surface, and as it rotates a pool of water gets trapped in the enclosedspace between the casing and the lower part of each turn. As the whole assemblyrotates, so the helix itself screws each trapped pool of water smoothly further up thecasing until it discharges from the opening at its top; the water pools move much as anut will screw itself up a bolt when prevented from rotating with it. This is also analagousto the trapped volumes of water screwed between the rotor and stator of progressive

cavity pumps.

Traditional wooden Archimedean screws of the kind just described have been tested andfound to have efficencies in the region of 30%.

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Fig. 48 An Archimedean screw. Two men are needed if the water head is more than 0.6metres (See also Fig. 97 for an animal-powered version) (after Schioler [24])

The modern version of the Archimedean screw is the screw pump, Pig. 49. This consistsof a helical steel screw welded around a steel tubular shaft, however unlike anArchimedean screw, there is no casing fixed to the screw, but it is mounted instead in a

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close fitting, but not quite touching, semi-circular cross-section inclined channel. Thechannel is usually formed accurately in screeded concrete. Because of the clearancebetween the screw and its channel, some back-leakage is inevitable, but the total flowrate produced by a screw pump is so large that the backflow is but a small percentage.Therefore modern screw pumps can achieve high efficiencies in the region of 60-70%.

Their primary advantage is that the installation and civil workings are relatively simple,compared with those for large axial flow pumps necessary to produce the same volumeof output (which would need a, concrete sump and elaborate large diameter pipework asin Fig. 66). Also, the screw can easily handle muddy or sandy water and any floatingdebris, which is readily pulled up with the water.

Probably the main disadvantage of screw pumps is that an elaborate transmissionsystem is needed to gear down an electric motor or diesel engine drive unit fromtypically 1500 rpm to the 20-40 rpm which is normally needed. Mechanical transmissionsfor such a large reduction in speed are expensive and tend to be no more than 60-70%efficient, thereby reducing the total efficiency of the screw pump, including itstransmission to about 50-60%. Screwpumps also present a safety hazard by having a

potentially dangerous open rotor and should therefore be fitted with mesh guards.Finally, they cannot cater for much change in level of the water source, unless provisionis made to raise or lower the entire unit and also the maximum head that can be handledwill not exceed much more than 6m in most cases, and will normally be no more than 4or 5m for smaller screw pumps.

Fig. 49 Cross-section through an open screw pump

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Fig. 50 Hydrostatic pressure pumps

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3.6.4 Coil and Spiral Pumps

These pumps use a similar principle to the Archimedean screw except that they runhorizontally while the Archimedean screw is tilted at about 30. The coil and spiral pumpfamily, if fitted with a suitable rotating seal, can deliver water to a greater height, typically5-10m, above their discharge opening. Fig 50 A shows a spiral pump and B shows a coilpump.

Both these pumps work on the same principle, involving either a spiral or a coiledpassage (in the latter case a coiled hosepipe serves the purpose) rotating on ahorizontal axis. One end of the passage is open at the periphery and dips into the wateronce per revolution, scooping up a pool of water each time. Due to the shape of thespiral or the coil, sufficient water is picked up to fill completely the lower part of one turn,thereby trapping air in the next turn. The pools of water progressively move along thebase of the coil or of the spirals as the pump turns, exactly like an Archimedean Screw.However, when acting against a positive head, the back-pressure forces the pools ofwater slightly'. back from the lowest position in each coil as they get nearer to thedischarge; so they progressively take up positions further around the coil from the lowestpoint. The maximum discharge head of either type of pump is governed, by the need toavoid water near the discharge from being forced back over the top of a coil by the backpressure, so this is still a low head device.

The spiral pump has to be designed so that the smaller circumferences of the innerloops are compensated for by an increased radial cross section, so it would normally befabricated from sheet metal; the coil pump is of course much easier to build.

This type of pump was originally described in the literature as long ago as 1806 and hasattracted much fresh interest recently, with research projects on it at the universities ofCalifornia (USA), Salford (UK), Los Andes (Colombia) and Dar es Salaam (Tanzania),[16]. Although historically the coil pump was used as a ship's bilge pump, today it isfinding favour for use in river current powered irrigation pumps by, for example, theRoyal Irrigation Department of Thailand (see Fig. 153) and also similarly bySydfynsgruppen and the Danish Boy Scouts for an irrigation project in south Sudan, withsupport from Danida and on the Niger near Bamako in Mali, under a project supportedby the German aid agency BORDA. Chapter 4.9 deals with some of the practicalapplications of this device.

The advantages of these devices are their inherent mechanical simplicity combined withthe fact that, unlike an Archimedean screw, they can deliver into a pipe to a head of upto about 8-10 m, making them more versatile. The only difficult mechanical component isa rotary seal to join a fixed delivery pipe to the rotating output from the coil. They are

ideal for water wheel applications due to the low speed and high torque needed, (whichis where most of the research effort appears to be concentrated).

Their main disadvantage is that their output is small unless rather large diameter hose isused, being proportional to the capacity of the lower part of one turn of hose perrevolution. A simple calculation indicates that a significant and not inexpensive length ofhose is needed to produce an adequate coil pump, (e.g. just 20 coils of only 1.5mdiameter needs nearly 100mm of hose). Supporters of this concept argue that itssimplicity, suitablility for local improvization and reliability should compensate for these

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high costs, but this type of pump has so far not been popularized successfully forgeneral use and it does not exist as a commercial product.

Fig. 51 Paddle-wheel or tread-wheel

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3.6.5 Paddle-wheels, Treadmills and Flash-wheels

These devices are, in effect, rotary versions of the simple scoop; however instead of onescoop being moved back and forth, a number are set around the periphery of a wheel,(Fig. 51). Like the scoop a paddle wheel is only useful for very low lift pumping, such asflooding paddy fields at no more than about 0.5m height above the water source.

The simplest version is the paddle-wheel in which an operator walks directly on the rim,turning it so that it continuously and steadily scoops up water and deposits it over a lowbund, (Fig. 51). In its basic form the paddle wheel is not very efficient since a lot of thewater lifted flows back around its edges. Therefore an improved version involvesencasing the wheel in a closely fitting box which not only reduces the back-leakage ofwater but also slightly increases the head through which the device can operate.

Paddle wheels have been mechanized in the past, although they are unusual as waterlifting devices today. Many of the windmills used in the Netherlands to dewater largeparts of the country drove large paddle wheels, which when mechanized and refined, areusually known as flash wheels. Flash-wheels function best with raked back blades, andthe best had measured efficiencies in the range 40-70%. Small straight-bladed paddle-wheels are probably only 10 or 20% efficient, but have the virtue of being simple to buildand install in situations were a lot of water needs to be lifted through a small head. Theyare occasionally used on traditional windpumps, as shown in Fig. 110.

3.6.6 Water Ladders and Dragon-Spine Pumps

The main disadvantage of the paddle wheel just described is that to lift water through agreater height a bigger wheel is needed. The water ladder was developed to get aroundthis problem by taking the paddles and linking them together in an endless belt whichcan be pulled along an inclined open wooden trough or flume (Fig. 52). The endless belt

is driven by a powered sprocket, at the discharge end, and passes around a free-wheeling sprocket at the lower end. The lower end of the trough or flume is submerged,so that the moving paddles in the belt, which almost fill the cross section of the flume,push water up it. In many ways this method of water lifting is analagous to a screw pumpwhich also pushes water trapped between the blades of a mechanism up a flume. Aswith the screw pump there is some back-leakage, but with a well-built unit, this is but asmall fraction of the high flow that is established.

The water ladder is still very widely used on small farms in S E Asia for flood irrigation ofsmall fields and paddies from open streams and canals or for pumping sea water intoevaporation pans to produce sea salt. In China it is known as a "dragon spine" or dragonwheel" and in Thailand as "rahad". In most cases it is made mainly of wood, and can

consequently easily be repaired on-farm. It is one of the most successful traditional,high-flow, low-lift water pumping devices and is particularly applicable to rice production,where large volumes of water are sometimes needed.

On traditional Chinese water ladders, the upper sprocket is normally driven by a longhorizontal shaft which traditionally is pedalled by from two to eight people workingsimultaneously; (Fig. 52). The treadles are spaced on the drive shaft so that one or moreof the operators applies full foot pressure at any moment, which helps to smooth thetorque output and keep the chain of boards tensioned and running smoothly.

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Fig. 52 Water ladder or Chinese 'Dragon Spine' pump

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Versions of it have been mechanized by using windmills (see Fig. 111), (in Thailand aswell as China), or a buffalo sweep (China) or with small petrol (gasoline) engines.

Water ladders range in length from 3 to 8m and in width from 150 to 250mm; lifts seldomexceed 1.0 to 1.2m, but two or more ladders are sometimes used where higher lifts arerequired. A rough test made in China with a water ladder powered by two teams of four

men (one team working and one resting) showed an average capacity of 23m 3/h througha lift of 0.9m [1]. Further details of Chinese water ladders are given in Table 6.

Table 6 SPECIFICATIONS OF CHINESE "DRAGON-SPINE" WATER LIFTS

Name of

product

Specifications of

products

Length

of trough

(m)

Dimensions

of

intake

(height x

width) (m)

Weight

of

water

lift

(kg)

Volume

of

timber

used

(m3)

Factory

price

(yuan)

Notes

Single man

hand -turning

water lift

1.5 0.18 x 0.14 18 0.2 46

1.8 0.18 x 0.14 20 0.2 50

2.0 0.18 x 0.14 22 0.2 57

2.3 0.18 x 0.14 24 0.3 64

3.0 0.18 x 0.15 30 0.3 76

3.5 0.18 x 0.15 35 0.3 80

Two men

treadling water

lift

2.3 0.25 x 0.20 50 0.3 93

3.0 0.25 x 0.20 55 0.4 106

Four men

treadling water

lift

3.5 0.25 x 0.19 70 0.5 126

4.1 0.25 x 0.19 75 0.5 138

4.7 0.25 x 0.19 85 0.6 151

5.3 0.25 x 0.19 105 0.7 165

Wind powered

water lift of

diagonal web

member

3.5 0.25 x 0.19 335 1.1 609

4.1 0.25 x 0.19 345 1.2 622

4.7 0.25 x 0.19 350 1.2 635

Wind sail

wheel 4-6m

in diameter,

coupled with a

water

lift

Remarks: The products in the table are made by Chengqiao Water Lift and AgriculturalTool Plant, Hangjiang Commune, Putian County.

Tests on a traditional wooden water ladder powered by a 2-3hp engine were carried outin Thailand in 1961 [16]. The trough was 190mm deep by 190mm wide and the paddleswere 180mm high by 150mm wide and spaced 200mm apart; note that the clearancewas quite large, being 20mm each side. The principle findings of this study were:

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i. the flow rate is maximized when the submergence of the lower end of the flumeis 100%

ii. a paddle spacing to paddle depth ratio of approximately 1.0-1.1 minimizes lossesand maximizes output

iii. the sprocket speed has to be kept to less than 80 rpm to avoid| excessive wearand frequent breakage

iv. the average efficiency of this device was 40%

It is possible that if a smaller clearance had beer: used between the paddle edges andthe trough, a higher efficiency 'may have resulted; no doubt the optimum. spacing isquite critical. If it were too small, friction would become excessive and possibly causefrequent breakage; of the links, while if too large, brick-leakage becomes excessive andreduces the overall efficiency.

3.6.7 Chain and Washer or Paternoster Pumps

The origins of this type of pump go back over2000 years, and they work on a similarprinciple to the water ladder just described except that instead of pulling a series oflinked paddles through an open inclined flume or trough, a series of Linked discs orplugs are pulled through a pipe (Figs. 53, 84 and 96). As with the ladder pump, they lendthemselves to human, animal or mechanical prime-movers and are most commonlypowered by either a team of two to four people or by a traditional windmill.

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Fig. 53(a) Chinese Liberation Wheel chain and washer pump - an animal-poweredversion is shown in Fig. 96

Fig. 53 (b)A view of a hand-operated liberation pump

As discussed in more detail in the next sections covering the use of human and animalpower, a major advantage of this kind of pump is that it requires a steady rotary powerinput which suits the use of a crank drive with a flywheel, which is a mechanicallyefficient as well as a comfortable way of applying muscle power. It also readily matcheswith engines and other mechanical prime-movers.

The main advantage of the chain and washer pump is that it can be used over a widerange of pumping heads; in this respect it is almost as versatile as the commonly usedreciprocating bucket pump as it is applicable on heads ranging from 1m to over 100m.For low lifts, loose fitting washers are good enough to lift water efficiently through thepipe, since back-flow will remain a small and acceptable fraction of total flow. At higher

lifts, however, tighter fitting plugs rather than washers are necessary to minimize back-leakage; many materials have been tried, but rubber or leather washers supported bysmaller diameter metal discs are commonly used. Most chain and washer pumps have abell mouth at the base of the riser pipe to guide the washers smoothly into the pipe. Withhigher lift units where a tighter fit is needed, this is only necessary near the lower end ofthe riser pipe; therefore the riser pipe usually tapers to a larger diameter for the uppersections to minimize friction (see Fig. 53 ).

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The capacity of a chain and washer pump is a function of the diameter of the riser pipeand of the upward speed of the chain. For example, four men are necessary to power aunit with 6m lift and a 100mm riser tube, [1].

Chain and washer pumps have been, and still are in very widespread use, especially inChina, where industrially manufactured pumps of this kind are commonly used and are

often known as "Liberation Pumps". They represented in development terms in China amajor improvement over more traditional and primitive water lifting techniques and aninterim step to modernisation using powered centrifugal pumps. Two to three millionLiberation Pumps were used in China at the peak of their use in the 1960s [17]. Thefollowing performance characteristics relate to typical chain and washer pumps used inChina:

motive pumping discharge efficiency

power head rate (pump only)

2 men 6m 5-8m3/h 76%

donkey 12m 7m3/h 68%

3kW(e) motor 15m 40m3

/h 65%

This indicates that the chain and washer pump is not only versatile, but also rather moreefficient than most pumps. It also has an important characteristic for a positivedisplacement pump of generally needing less torque to start it than to run it, whichmakes it relatively easy to match to prime-movers having limited starting torque.

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Rotary Positive Displacement Pumps

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