affordable low-lift pumping forsmall-scale irrigation development : appropriate equipment selection

Affordable low-lift pumping for small-scale irrigation: appropriate equipment selection

Affordable Low-lift Pumping forSmall-scale Irrigation Development:

Appropriate Equipment Selection

S. van ’t Hof, July 2000

i


HIPPO (High-efficiency Irrigation Pumps Procurement & Organization) Foundation

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Summary

This training manual on the selection of appropriate pumping equipment for small-scale irrigation provides the background information for using PumpSelect, an Excel application made by the same author. Combined they provide the necessary tools to design efficient motor pumps, to assess their efficiency, and to evaluate the pumping costs. The emphasis is on mobile pump sets with relatively small diesel engines and mixed-flow pumps for discharges of 25 to 150 litres/second with static heads of 2 to 6 meters. At the time of writing PumpSelect had an initial database of about 100 pumps and 50 engines, mostly from Asia and Europe.

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Preface

This manual is intended to help reduce pumping costs in small-scale irrigation. Without due care, pumping costs can easily turn out to be twice as high as necessary. After initial small-scale irrigation development by governments or donors, the operation of small-scale irrigation schemes is invariably paid for by resource-poor small farmers. Too often, the sustainability of such schemes is jeopardized by excessive operation and depreciation costs. An extra effort to select appropriate equipment is justified on the grounds that subsistence farmers should never be put in a situation where they must pay more than what could be considered a reasonable minimum.

The manual is a practical synthesis of common pump theory. It is new in the sense that it focuses on diesel-powered, horizontal, volute centrifugal and mixed-flow pumps. It provides background information for a specially developed spreadsheet application in Excel called PUMPSELECT, which contains a small data base of common pumps and engines from Asia and elsewhere. PUMPSELECT allows the conjunctive analysis of the pipe system and overall pumping efficiency. The latest versions of this manual and of PUMPSELECT can be downloaded from the web site of HIPPOnet: http://www.hipponet.nl. HIPPOnet was created as a platform for information exchange on pump sets.

The intended readership includes technicians and decision-makers in governments and development organizations, but also independent local consultants. The manual was developed for a roving training course on pump selection in the Sahelian countries of Burkina Faso, Mali, Mauritania and Niger in 2000. It is hoped that it will be of use elsewhere, too.

The corrections and improvements suggested by Mr. F. Gadelle and Mr. A. Nouwen are gratefully acknowledged. All errors and omissions remain my responsibility. Any comments on the text as it stands and any suggestions for potential improvement that could be included in future editions are welcomed, and should be addressed to:

HIPPO FoundationDe Verwondering 273823 HA AmersfoortThe NetherlandsTel/fax: [email protected]

Sjon van ’t HofAmersfoort

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mailto:[email protected]

http://www.hipponet.nl/


Table of Contents

Summary............................................................................................................................... ii

Preface.................................................................................................................................. iii

Abbreviations and acronyms.............................................................................................. vii

Chapter 1 Introduction........................................................................................................ 1

1.1 Small-Scale Irrigation................................................................................................... 1

1.2 Small-scale irrigation development...............................................................................1

1.3 Fuel-efficient and cost-effective irrigation pumps..........................................................2

Chapter 2 Basics of Water Lifting....................................................................................... 3

2.1 Introduction................................................................................................................... 3

2.2 Kinetic Pumps............................................................................................................... 3

2.3 International System of Units (SI).................................................................................. 5

2.4 Some Useful Equations.................................................................................................. 6

Chapter 3 Pumping Efficiency............................................................................................ 7

3.1 Introduction................................................................................................................... 7

3.2 Common causes of low overall efficiency......................................................................8

3.3 Efficiency improvement in shallow-well pumping in India.............................................9

3.4 Efficiency improvement in low-lift pumping in West Africa.........................................10

Chapter 4 Pumps................................................................................................................ 11

4.1 Introduction................................................................................................................. 11

4.2 Parameters involved in pump selection........................................................................11

4.3 Pump sizes................................................................................................................... 12

4.4 Characteristic curves................................................................................................... 12

4.5 Manufacturer’s data.................................................................................................... 13

4.6 System head curve....................................................................................................... 15

4.7 Operating point........................................................................................................... 17

4.8 Control possibilities..................................................................................................... 17

4.9 Matching characteristics to service data......................................................................19

4.10 Pumps in Series and in Parallel.................................................................................21

4.11 Interpolation.............................................................................................................. 21

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4.12 Extrapolation............................................................................................................. 21

4.13 Net Positive Suction Head (NPSH) and cavitation.....................................................23

4.14 Common pumps with lower than optimum efficiency.................................................24Self-priming pumps....................................................................................................... 24Impeller types................................................................................................................ 25

4.15 Dismantling a volute-mounted pump with an overhung impeller...............................25

Chapter 5 Engines.............................................................................................................. 28

5.1 Introduction................................................................................................................. 28

5.2 Parameters involved in engine selection......................................................................28

5.3 Characteristic curves................................................................................................... 28

5.4 Engine efficiency......................................................................................................... 29

5.5 Engine derating........................................................................................................... 30

5.6 Rotational directions of the pump and the engine........................................................30

5.7 Minimum and maximum speed..................................................................................... 30

5.8 Space, weight and position considerations...................................................................31

Chapter 6 Pipes, Appendages and Configurations...........................................................32

6.1 Introduction................................................................................................................. 32

6.2 Pipe and tubing........................................................................................................... 32

6.3 Hose maintenance and repair......................................................................................34

6.4 Appendages................................................................................................................. 34

6.5 Configurations............................................................................................................. 35

6.6 Pump set specification................................................................................................. 37

Chapter 7 Design and Technical Analysis.........................................................................38

7.1 Introduction................................................................................................................. 38

7.2 Design procedure........................................................................................................ 38

7.3 A Simple Energy Audit................................................................................................. 40

7.4 Efficiency Improvements.............................................................................................. 40

7.5 Archimedes versus Rotodynamic Pump Efficiency.......................................................42

Chapter 8 Economic Analysis............................................................................................ 44

8.1 Introduction................................................................................................................ 44

8.2 Water pumping cost.................................................................................................... 44

8.3 A simple comparison of pumping costs.......................................................................45

8.4 A detailed evaluation of pumping costs.......................................................................46

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8.5 Caveat......................................................................................................................... 47

Chapter 9 Procurement and Prices...................................................................................48

9.1 Pump marketing in West Africa..................................................................................48

9.2 Alternative suppliers................................................................................................... 49

9.3 Improving the performance of the African pump market..............................................49

References............................................................................................................................. 51

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Abbreviations and acronyms

ANSI American National Standards Institute

at technical atmosphere

BEP Best Efficiency Point

CIF Cost, Insurance, Freight

ESCAP Economic and Social Comission for Asia and the Pacific (http://www.unescap.org)

FAO United Nations Food and Agriculture Organization (http://www.fao.org)

FOB Free On Board

HI Hydraulic Institute

HIPPO (as in HIPPO Foundation) High-efficiency Irrigation Pumps Procurement and Organization (http://www.inter.nl.net/hcc/HIPPOMP)

hp horsepower

ILRI International Land Reclamation Institute (http://www.ilri.nl)

IPTRID International Programme for Technology and Research in Irrigation and Drainage (http://www.fao.org/iptrid/)

mH20 meters of water

psi pounds per square inch

RNAM Regional Network for Agricultural Machinery (ESCAP) ([email protected])

UNCDF United Nations Capital Development Fund (French acronym: FENU)

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mailto:%[email protected]

http://www.fao.org/iptrid/

http://www.ilri.nl/

http://www.inter.nl.net/hcc/HIPPOMP

http://www.fao.org/

http://www.unescap.org/unis/pub/order.htm


Chapter 1

Introduction

1.1 Small-Scale IrrigationApproximately half of the irrigated area in sub-Saharan Africa is irrigated by small-scale irrigation (FAO, 1992). Given the scarcity of published data for the region, it is difficult to state the exact amount of lift irrigation. A cursory inventory of irrigation (FAO, 1986a, in Purkey and Vermillion, 1995) showed that lift irrigation covers almost 150 000 ha in the 6 countries of Burkina Faso, Mali, Mauritania, Niger, Nigeria and Senegal:

Country Lift irrigation (ha) Gravity irrigation (ha)Burkina Faso 6700 2000Mali 24460 107000Mauritania 6900 --Niger 11802 2719Nigeria 80500 17300Senegal 16800 3200

Total 147162 132219

Basin-irrigated rice is the dominant crop in countries, such as Burkina Faso, Senegal, Mali and Mauritania. The total number of small irrigation pumps in Burkina Faso, Mali, Niger and Senegal is estimated at 5000 units (CIRAD, 1999).

1.2 Small-scale irrigation developmentThere is general agreement that private, small-scale irrigation development is more cost-effective than its public, large-scale counterpart. It is estimated that the development of agency-directed large-scale pump systems costs 20,000-32,000 US$/ha as opposed to only 4000-8000 US$/ha for village-based pump irrigation (Violet et al., 1991 in Purkey and Vermillion, 1995).

There have been instances of village irrigation development, where the only external investment was the procurement of an irrigation pump. An interesting example of this was the Arrondissement of Diré, Mali, where more than 100 village irrigation schemes of 15 ha each were created during 1986-1990. On a per ha basis, the construction of earth canals and levelling of plots required about 150-400 man-days and the provision of a 55 l/sec irrigation pump 1/15th of 2.5 million FCFA (see figure 35). If a man-day is evaluated at 2 US$ and a pre-devaluation FCFA at 1/300th of a US$, the average investment was 1100 US$/ha, of which half in labour by the beneficiaries.

There seems to be no reason why similar self-help initiatives should not be encouraged by improving the availability of affordable, efficient pumping equipment. For the foreseeable future the motor pump will be a cornerstone of farmer-managed lift irrigation development in West Africa (Purkey and Vermillion, 1995). As such, efforts should be made to make its use as profitable as possible.

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1.3 Fuel-efficient and cost-effective irrigation pumpsApart from more efficient irrigation systems and higher crop yields, there is only one method for improving the economics of lift irrigation: by reducing pumping costs. It seems likely that cheaper pumping as a result of lower investment and fuel costs will enhance the sustainability of small-scale irrigation development. Where crop yields are already good, which is often the case, and where it is difficult to improve irrigation efficiency, since it is not much of an issue in small irrigation schemes anyhow, lowering pumping costs is the only way forward.

Along many Sahelian rivers flood irrigation was traditionally practiced, but the droughts of the early 1970s and mid 1980s left the riparian populations without a livelihood. Along most of the middle and lower course of the large Sahelian rivers, such as the Senegal and the Niger, the conditions for low-lift pumping are ideal. In spite of the favourable inherent low energy requirements of low-lift pumping, organizational and economic sustainability continue to be an issue. Part of the solution is in further decreasing pumping costs. It is shown in Section 7.4 and Chapter 8 that there is scope for decreasing pumping costs from the current level of 250-300 US$/ha to 100-150 US$ha.

The present manual provides the necessary tools to design efficient motor pumps, to assess their efficiency, and to evaluate the pumping costs. It is accompanied by an Excel application, called PumpSelect, made by the same author. At the time of writing PumpSelect had an initial database of about 100 pumps and 50 engines, mostly of Asian and European origin. The emphasis is on diesel-powered mixed-flow pumps with discharges of 25 to 150 litres/second for static heads of 2 to 6 meters.

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Chapter 2

Basics of Water Lifting

2.1 IntroductionA pump is a device that expends energy in order to raise, transport, or compress fluids. The earliest pumps were devices for raising water, such as the Persian and Roman waterwheels and the more sophisticated Archimedes screw. Pumps are classified according to the way in which energy is imparted to the fluid. Only kinetic pumps will be dealt with in this manual.

1. Positive displacement 1.1 Rotary 1.1.1. Gear1.1.2. Vane 1.1.3. Screw (Archimedes)1.1.4. Progressing cavity (Moyno)1.1.5. Lobe or cam

1.2 Reciprocating 1.2.1. Piston1.2.2. Plunger1.2.3. Diaphragm

2. Kinetic 2.1 Radial flow (centrifugal)2.2 Axial flow (propeller)2.3 Mixed flow

3. Jet or Ejector typeTable 1: classification of types of pumps

2.2 Kinetic PumpsIn kinetic pumps a velocity is imparted to the fluid. Most of this velocity head is then converted to pressure head. Even though the first centrifugal pump was introduced about 1680, kinetic pumps were little used until the 20th century.

Centrifugal pumps include radial, axial, and mixed flow units. A radial flow pump is commonly referred to as a straight centrifugal pump. The most common type is the volute pump, illustrated in Figure 2. Fluid enters the pump near the axis of an impeller rotating at high speed. The fluid is thrown radially outward into the pump casing. A partial vacuum is created that continuously draws more fluid into the pump. Volute centrifugal pumps are robust and relatively inexpensive, quiet, and dependable, and their performance is relatively unaffected by corrosion and erosion. They are compact, simple in construction, and do not require inlet and outlet check valves. This explains their enormous popularity.

Figure 1: Axial flow pump Figure 2: Volute centrifugal pump

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Fig.3: Parts of a centrifugal pump (Nouwen, 1995)

In axial flow pumps the rotor is a propeller. They are often referred to as propeller pumps. Fluid flows parallel to the axis as illustrated in Figure 1. Diffusion vanes are located in the discharge port of the pump to eliminate the rotational velocity of the fluid imparted by the propellers. In mixed flow pumps, fluid is discharged both radially and axially into a volute-type casing. The pumping characteristics of mixed flow pumps overlap to a considerable extent with those of propeller pumps and centrifugal pumps.

There are both bowl and volute types of axial flow and mixed flow pumps. Bowl centrifugal, mixed flow and axial flow pumps are often used in vertical turbine, mixed-flow and axial flow arrangements. Volute mixed flow pumps have the same constructional advantages as centrifugal pumps.

Fig. 4: Bowl type axial flow and mixed Fig. 5: Sectional view of 150-HW to 350HW mixed flow pumps flow pumps (China)

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Low-cost, mobile axial flow pumps are manufactured in Asia (see figure 6).

Fig. 6: locally made IRRI “Sipa” pump from the Philippines for lifts up to 3 m (20 to 100 l/sec using 4-12 hp)

A parameter that can be useful in selecting the right pump type for a given application is specific speed. In fundamental SI units, it is a dimensionless number. For the non-dimensionless version nq used in this manual, it is defined as the speed n at which a geometrically similar pump would operate in order to deliver a discharge of 1 m3/s at 1 m of head. Or:

(for units, see section 2.3)

Specific speed will be used in Chapter 4 “Pumps” in a method to extrapolate pump characteristic curves when manufacturer’s data are lacking. Specific speed can also be used to specify kinetic pump types.

Figure 7: impeller shapes and specific speed (nq) (adapted from Nouwen, )

2.3 UnitsDifferent measurement systems are used in the world of pumps. Especially confusing are the British and US customary systems. Even the metric system is not applied consistently: for discharge we see often m3/hour, m3/second and litres/second. The following units will be used in this manual:

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Parameter Unitvapour pressure, atmospheric pressure p mbar (1 mbar = 0,0102 m)

Pa (Pascal, N/m2)

pump head H, NPSH, friction loss Hf, Hl, Hv m (of water; 1 m = 98,1 mbar)

Temperature T CPower P kW W

hp (horsepower; 1 hp = 0,7457 kW)

efficiency (in equations a fraction, never %!)

suction, discharge and impeller sizes d mm (sometimes inches: 25,4 or 25 mm) D, diameter mdischarge q m3/h Q m3/s (q=3600Q)angular velocity (engine, pump speed),n rpmdensity (i.e. mass density) kg/m3flow velocity v m/sgravitational acceleration g 9.81 m/s2

2.4 Some Useful Equations

2.4.1 Velocity (units as above)

2.4.2 Velocity Head ; experimental values for energy losses

in elbows, enlargements, contractions, valves etc. are usually reported in terms of a resistance coefficient, K, as follows: Hl = K(v2/2g). Values for K can be found in the pump selection spreadsheet: [PUMPSELECT.xls]Accessoires!$A$29.

2.4.3 Head in mH20

1 mH20 = 0,09784 atm = 9806,65 Pa = 9806,65 N/m2 = 0,1 kgf/cm2 = 0,1 at = 1,403958 psi

2.4.4 Power Required

2.4.5 Hazen-Williams Formula , where R is

the hydraulic radius (=D/4 in circular pipes) and Ch is the Hazen-Williams coefficient. For Ch

values see Section 6.2. The Hazen-Williams formula is one of the most popular formulas for the design and analysis of water systems. Its use is limited to the flow of water in pipes larger than 50 mm and smaller than 2 m in diameter. The velocity of flow should not exceed 3 m/s. Also, it has been developed for water at 15,5C. Use at temperatures much lower or higher would result in some error.

2.3.5 Hazen & Williams Friction Formula Hf=

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Chapter 3

Pumping Efficiency

3.1 IntroductionInternal combustion engines were used to power pumps as early as the 1920s and 1930s. Nowadays, mostly electric motors are used for irrigation pumping where a reliable electricity supply is available. For political rather than environmental reasons, development policies often tax diesel and heavily subsidize electricity, so efficiency in electric pump sets may be less of an issue to many farmers. Diesel and petrol continue to be used as fuel sources where no electric lines exist or where electric power is not reliable (because the subsidies are not sustainable). The main advantages of mobile pump units are easy installation and easy removal. Installation costs often equal transportation costs. This flexibility makes diesel-powered mobile pump sets very suitable for irrigation development programmes with a high self-help content, especially in Africa, where rural electrification is still rare.

The few data that are available on actual field performance indicate that overall efficiencies are very low, in the range of 0.5 to 8%, and that such poor levels are quite common (FAO, 1992). One of the causes is oversizing of the engine. This usually results in higher investment costs, higher power costs, lower efficiencies, and is generally a luxury which most farmers cannot afford (ASAE, 1983). Some of the causes of low efficiency can be corrected at little cost once the problem is identified (see section 7.4 below), but unfortunately it is easy to run an inefficient pumping system without realizing it.

Figure 9: efficiency range of components of a diesel-powered pump unit (adapted from FAO, 1992, picture from Milnars, 1994)

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Conventionally, overall engine powered pumping plant efficiency performance is computed as the product of efficiencies of the following individual components:

Typical values of overall efficiency (overall in %) for representative pumping plants (ASAE, 1983)

Power source Maximum theoretical

Reccomended as acceptable

Average values from field tests

Electric 72-77 65 45-55Diesel 20-25 18 13-15Petrol 18-23 14-16 9-12

It is possible to add the efficiencies of other components, such as fuel efficiency fuel (FAO, 1992) and pipe system efficiency system.(Van ‘t Hof, 1998). Fuel efficiency can be less than unity as a result of fuel spilt or leaking from tanks or from joints in the fuel pipeline. Under unfavourable field conditions, fuel losses can easily attain 10%. In the case of surface irrigation systems, the aim of pumping is to lift water from one plane of potential energy to a higher plane, so friction loss can be perceived as a lack of efficiency. Thus system

=Hstatic/Hdynamic, where Hstatic is the static head or geodetic head and Hdynamic is the dynamic or manometric head. Generally speaking, the lower the lift (or static head) the lower system. In many cases the system will be in the 30-70% range.

When , then:

Worst case 0.40 x 0.80 x 0.15 x 0.90 x 0.3 x 100 = 1.3 %Acceptable situation 0.65 x 0.98 x 0.30 x 0.98 x 0.7 x 100 = 13.1 %Best case 0.80 x 1.00 x 0.35 x 1.00 x 0.9 x 100 = 25.2 %

This implies that in rare cases there may be scope to improve overall efficiency by a factor of 20. It also means that there is often not much scope for improving overall efficiency beyond half of the maximum attainable. Possible improvements to overall efficiency can be identified by decomposing low efficiencies of certain components into products of sub-component efficiencies. By using the spreadsheet program PumpSelect you can model the actual operation of a pump system and change design parameters to optimize the various efficiencies for low and medium lift pumping systems (see Chapter 7 “Technical Analysis”).

3.2 Common causes of low overall efficiencyLow overall efficiency can be caused by any combination of factors in the product overall,2 =

pump.transmission.engine.fuel.system (see section 3.1). The list of causes is not exhaustive. Factor Cause1. pump 1.1 selection of inefficient pump, e.g. self-priming pump with open impeller (see sections

3.3, 3.4)1.2 the performance of the pump at the operating point is well below BEP1.3 old pump (aging can be very fast when cavitation occurs, see section 4.2)

2. transmission 2.1 incorrect alignment of direct coupling (usually also leads to the destruction of the rubber elements in the coupling). Sometimes incorrect alignment is caused by torque on a poorly designed chassis.2.2 high/low tension of v-belt2.3 selection of less efficient transmission, e.g. v-belt instead of direct coupling2.4 poorly designed transmission (e.g. wrong v-belt dimensions)

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3. engine 3.1 oversized engine (see sections 4.1, )

3.2 low operating temperature, e.g. as a result of overcooling (oversized siphon system) 4. fuel 4.1 fuel lost during transport and storage

4.2 fuel lost from joints in the fuel pipeline5. system 5.1 a throttling valve is used instead of variable speed to adjust Q-H to varying conditions

(see section 4.4)5.2 underdimensioned foot valve (friction losses)5.3 underdimensioned pipes or hoses (friction losses)

3.3 Efficiency improvement in shallow-well pumping in IndiaA case study on water lifting efficiency in the Tarai zone of North Bengal, India, shows how equipment can be modified to achieve better efficiencies. The information is abstracted from the article Fuel efficiency and inefficiency in private tubewell development (Bom and Van Steenbergen,1997, accessible at http://www.inter.nl.net/hcc/solartec2.html ).

In the last two decades there has been a proliferation of private groundwater irrigation in India. Estimates put the figure of diesel pumpsets in India at 6.5 million at present. To this figure another 11 million pumps with electric motors can be added, mainly operating in areas with deep aquifers. Similar dramatic increases in private groundwater irrigation have taken place in Pakistan and Bangladesh. This development has been driven by farmers' investment, although a range of public subsidies has accelerated the pace of groundwater exploitation.

Figure 10: old (right) and improved (left) shallow-well pump in North Bengal, India

Most shallow tubewells in the region, but also open wells and ponds, are operated with 5hp diesel pumpsets. During field surveys it was found that these pumps were generally oversized (only about 1-5 hp is effectively absorbed by the pump), overcooled (the operating temperature is generally only 35°C instead of 80°C) and that there was excessive friction loss in the suction pipes due to the installation of a poorly designed check valve.

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Through a series of experiments on existing pumpsets, a set of modifications appropriate for North Bengal was developed. Overcooling was eliminated by fitting thermosyphon drum-cooling. The drum-cooling increased the engine operating temperature from a low 35°C to a normal level of 80°C, reducing fuel consumption by 13%. Next, removing the check valve from the suction pipe and adding a hand priming device (simple farm pump, see figure 14) reduced the hydraulic friction losses, saving another 18% on the fuel consumption. Finally, to mitigate the effects of the engine being oversized, the engine speed was decreased from 1500 to 1100 rpm. This resulted in a further reduction of the fuel consumption by 20%. Total fuel consumption could in this way be reduced from 1 l/h to 0.5 l/h, while the discharge of the pumpsets remained unchanged. The cost of these modifications on shallow tubewell pumpsets is US$ 8. Other improvements in well technology and water distribution are described in the same article.

3.4 Efficiency improvement in low-lift pumping in West AfricaA case study on water lifting efficiency along the Niger river near Timbuktu, Mali, shows the range of variation in equipment efficiencies and costs. The information is abstracted from the article The design of a low-lift irrigation pump pilot project: improving the availability of affordable pumpsets to African farmers (Van ’t Hof, 1998, accessible at http://www.inter.nl.net/hcc/desk_en.htm). The main purpose of the paper is to describe how high-efficiency, low-cost irrigation equipment of Asian origin could be identified and tested for possible subsequent introduction in Africa.

Since the mid-1970s the drought-affected region of Timbuktu, Mali, has been the scene of many efforts of development agencies to develop community-based irrigation, more or less in the image of developments along the middle course of the Senegal River. Each of these agencies introduced its own choice of pumpsets. Equipment selection was always based on the least favorable pumping conditions. In practice, most pumpsets operated under much more favourable conditions, for which they were not designed. It was found that for static heads in the 1-4 meter range, overall water-lifting efficiencies (here: overall,3 = pump.transmission.system) were 20-25% only. Most of these inefficient pumpsets were procured from Europe, where equipment prices are very high compared to those in Asia, the region where most of the world’s output of irrigation pumps is produced.

The study shows that overall efficiency can be doubled and fuel consumption halved, by: (1) reducing friction losses; (2) selecting appropriate pumps for the actual pumping conditions; and (3) using speed variation to adjust Q-H to the pumping conditions. If these measures are combined with procurement of the equipment from Asia, specific pumping costs for surface-irrigated rice can be reduced from US$ 250-300/ha/season to US$ 110-150/ha/season on the basis of a total water requirement per hectare per season of 15 000 m3.i

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Chapter 4

Pumps

4.1 IntroductionAttainable efficiency in kinetic pumps depends on the type and size of pump, rotation speed and specific speed. With reduced specific speed there is above all a considerable increase in the proportion of disk losses, leakage and balancing flow losses. Maximum efficiencies are obtained in the range nq = 40-60. At higher specific speeds efficiency starts to fall off due to an increasing proportion of hydraulic losses (see figure 10)

Figure 10: optimum efficiencies as a function of specific speed and flow rate (ASAE, 1983).

4.2 Parameters involved in pump selectionWhen selecting a pump for a particular application, the following factors must be considered (Mott, 2000):

1. The nature of the liquid to be pumped;2. The required capacity (volume flow rate);3. The conditions on the suction (inlet) side of the pump;4. The conditions on the discharge (outlet) side of the pump;5. The total head on the pump;6. The type of system to which the pump is delivering the fluid;7. The type of power source (electric motor, diesel engine);

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8. Space, weight, maximum speedii and position limitations;9. Cost of pump purchase and installation;10. Cost of pump operation;11. Governing codes and standards.

Most of these factors are taken into account in PumpSelect. Pump catalogues and manufacturer’s representatives will normally supply the necessary information to assist in the selection and specification of pumps.

A warning is in place regarding factor 1. Many pump sales persons tend to opt for open impellers for surface sources, like rivers or ponds. This is not adviseable because of the low efficiency of such impellers (see Section 4.14).

4.3 Pump sizesSimilar to pipes (see Section 6.2), pumps are often described by the internal diameter of the delivery pipe connection in “metric” inches, i.e. multiples of 25 mm. Below table is a guide to selecting suitable pump sizes for different flows. This is only a guide. It can be used in the design process, but only very cautiously.

Pump size Discharge “metric” inches mm l/sec m3/h

2 50 8-17 30-603 75 17-30 60-1004 100 30-40 100-1405 125 40-50 140-1806 150 50-70 180-2508 200 70-100 250-36010 250 100-150 360-540

4.4 Characteristic curvesThe characteristic curves describe the behaviour of a pump under changing operating conditions. The head H, power input P and efficiency at constant speed n are plotted against the flow rate Q (or q). Dependent on the specific speed, the slope of the Q-H curve varies from flat (low specific speed) to steep (high specific speed) (see figure 16).

Radial impellers cover the specific speed range up to about 120. Mixed flow impellers are feasible from 40 to about 200. Axial flow impellers with very steep and sometimes unstable characteristics lie in the specific speed range from 160 to 350. Characteristics are said to be stable when the head curve rises steadily, i.e. there is always a negative slope in relation to the Q axis. Certain pumps may have an instability range within which it is not permissible to operate them.

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Figure 11: influence of specific speed on the shape of the characteristic (Sulzer, 1989)

4.5 Manufacturer’s data

Because they are able to use different impeller diameters and speeds, pump manufacturers can cover a wide range of requirements for capacity and head with a few basic pump sizes. Figure 17 shows a composite rating chart for one line of pumps which allows the quick determination of the pump size at an impeller speed of 3500 rpm.

Figure 12: composite rating chart for a line of centrifugal pumps (Mott, 2000)

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Figure 13 shows how the performance of a given pump varies as a function of the impeller diameter.

In figure 14 the following curves have been added to figure 13:Curves showing the power required to drive the pump. For example, the pump with an 8 inch impeller would deliver 50 m3/hour against a total head of 75 m of water. Under these conditions, the pump would draw 23 hp. The same pump would deliver 64 m3/hour at 60 m of head and would draw 26 hp;Curves of constant efficiency. The maximum efficiency for this pump is about 57%. Of course, it is desirable to operate a given pump near its best efficiency point (BEP).iii

Curves of constant net positive suction head required (NPSHR). NPSHR is an important factor to consider in applying a pump, as will be discussed in section 4.13. Generally, a low NPSHR

is desirable.

Figure 14: pump performance chart the main pump characteristics with various impeller diameters at constant speed (Mott, 2000)

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Figure 15 shows a more common way to represent the main pump characteristics (pump differs from the pump in Figure 14) (Sulzer, 1989)

4.6 System head curveThe system head curve graphically illustrates that more head is required to increase the flow or discharge through the system. The total head is independent of the pumpiv. Figure 16 is a sketch of a typical system head curve illustrating the parameters which contribute to the total head and how they vary as the discharge increases. For some systems not all the illustrated parameters would be applicablev.

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Figure 16: a system head curve illustrating the parameters which contribute to the curve (ASAE).

Static lift is the vertical distance between the centre line of the pump and the elevation of the water source when the pump is not operating. The static discharge head is a measure of the pump or discharge pipe and the eventual point of use. As a well is pumped the water level declines which is commonly referred to as the well drawdown. When water flows through a pipe system there is a loss of head due to friction. All irrigation systems using pumps require some operating pressure or head with the possible exception of the case where water is discharged directly into an open ditch or partially filled pipeline.

Figure 17: time dependence of the system head curve due to increased friction and a lower water table.

The system head curve is time dependent due to variations in well drawdown, pipe friction, operating conditions, and static water level. Figure 17 illustrates two conditions that would cause different system head curves. Note that aluminium, polyvinylchloride (PVC) plastic and transite pipes, which are commonly used since the late 1970s may have little change in friction factors with time.

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We will not consider well drawdown and operating head in this manual. However, for a simple analysis one could probably make assumptions for both well drawdown and operating head.

4.7 Operating pointIn systems where drawdown and operating head are not considered, total head will consist of static head (Hstat or geodetic head, equal to the sum of static lift and static discharge) and dynamic head Hdyn increasing with the square of the flow rate and depending only on pipework layout, pipe diameter and length. The point of intersection of the system head curve with the Q-H curve is termed the duty point or operating point. How this operating point can vary with seasonal changes in the water level is illustrated in Figure 18.

Figure 18: shifting operating point

The curves sloping upward to the right are system head curves for May and September. The May curve is lower due to the higher static water levels prior to the pumping season which then decline throughout the season to a level represented by the September curve. The two curves labelled A and B which slope downward to the right are TDH-Q curves for two different pumps. For pump A the discharge in May would be 60 l/s but this would drop to 50 l/s by September because of the increased head. The discharge for A throughout the pumping season would range between 50 and 60 l/sec. The discharge for B would also be 60 l/s in May but would drop to only 55 l/s in September. Note that in this example speed is kept constant.

For two reasons it is interesting to be able to keep discharge constant: (1) to prevent your canal system from overflowing; and (2) to improve pumping efficiency at high water levels. The simplest method for maintaining a constant discharge under varying water levels is by changing the speed of the pump. The efficient response to speed regulation should be considered in the design phase.

4.8 Control possibilitiesGiven a certain impeller with a certain diameter, pump output may be controlled by the following methods:

1. Throttling;2. Connection or disconnection of pumps in parallel or in series;

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3. Bypass regulation;4. Speed regulation;

Impeller vane adjustment, prerotation control, and cavitation control are not important here and will not be discussed.Throttling: closing a valve in the delivery line increases the resistance in the system and therefore reduces the flow rate of the characteristic. The system head curve becomes steeper and intersects with the pump characteristic at a new duty point. Throttling control is generally employed only where delivery rates deviating from the nominal flow are required for short periods of operation only.

Parallel or series operation: in low-lift pumping only changing the number of pumps in parallel (e.g. from 2 for a medium-lift to 1 for low-lift conditions) is worth considering especially where it can be combined with speed regulation.

Bypass control: part of the discharge is ducted back to the suction nozzle. For centrifugal pumps it is the least satisfactory option compared with speed control and throttling. It can be used sometimes in axial pumps, because the power input drops with increasing flow.

Speed control: speed control is often the best solution, because pump efficiency remains practically constant.

Figure 19: the effect on efficiency of various control systems

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Speed control can be effected with: (1) internal combustion engines; (2) variable speed gears and v-belts; (3) steam turbines; (4) pole reversing electric motors; (5) variable-speed electric motors (slip-ring motors, frequency controlled or thyristor-controlled motors); and (6) electromagnetic, hydraulic and hydrostatic couplings. In PumpSelect only options 1 and 2 can be examined. Generally, option 1 is preferred if a diesel engine is used as the primary mover.

When varying speed, it is important to understand the way in which capacity, head, and power vary with speed. These relationships are called the affinity laws and will be discussed in section 4.9.

4.9 Speed control and reducing impeller diameterProper selection of a pump requires use of pump characteristic curves. The normal procedure is to first determine the system head curve and desired discharge. Then pump manufacturers’ catalogues are used to select pump models for consideration that will operate efficiently at or near the design discharge. Engineers, pump installers and irrigation equipment salesmen have historically determined the design discharge and total dynamic head for that discharge. These values were used to select the pump. Very few irrigation or drainage systems will have fixed operating conditions. By determining the system head curve or curves for a range of discharges above and below the design discharge, sufficient information will be available to evaluate pump performance for all expected operating conditions.

The affinity laws for varying speed are: , and ,

or: (1) capacity varies directly with speed; (2) the total head capability varies with the square of the speed; and (3) the power required by the pump varies with the cube of the speed. In fact, NPSHR also varies with the square (or rather the power of 1.6) of the speed (see section 4.13). With small speed changes (up to 10%) efficiency remains virtually unchanged. With bigger speed changes the efficiency must be slightly decreased at lower speeds.

Since it is mainly designed for pump selection with diesel engines, speed variation is one of the main methods to match pump characteristics to a set of expected operating conditions and extensive use is made of the affinity laws in PumpSelect. The other main method is by varying impeller diameter. This is the main method for fixed-speed electric motors. A set of affinity laws exists for varying impeller diameter, but it will not be used here, because the results do not always seem to be very reliable. Instead, it is much better to interpolate between manufacturers’ data for different standard impeller diameters (see section 4.11).

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Figure 20: alteration of the pump characteristic after reduction of the impeller diameter (Sulzer, 1989)

When reducing the impeller diameter the tips of the impeller blades become blunt. By sharpening the impeller blades on the underside, the outlet angle can be enlarged to obtain up to 5% more head and flow (but 10% more power) near the best efficiency point. At least 2 mm must be left. After reducing the impeller diameter it must be balanced again. The process requires both experience and equipment and is normally only carried out by the manufacturer, who often offers a range of 4 or more diameters.

Figure 21: influence of sharpening of impeller blades on pump characteristics (Sulzer)

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4.10 Pumps in Series and in ParallelTwo or more pumps may be connected and operated in series. Pumps in series are connected so that the discharge from the first pump is piped into the inlet side of the second. This type of operation could be used where the same discharge rate is needed but a larger head is required than can be produced with a single pump. The combined head is equal to the sum of the individual heads for a specific discharge.

Two or more pumps may be operated in parallel. A typical example would be where two or more pumps draw water from a single pump sump and the individual flows are discharged into a single pipeline. If the pumps are not taking water from a common source, which results in different pumping heads generated by each pump, then the analysis becomes difficult and pumps are not properly selected. The consequence is that the pumps oppose each other resulting in fluctuating discharges, one pump provides all the water while the other operates inefficiently or may pump water from one pump back down another.

In many remote places operating pumps in series and parallel can help standardize pumping equipment and reduce maintenance problems. A nice example is described by Hecq, J. and Dugauquier, F. (1990) where five 80 l/sec pump sets are used in parallel to irrigate the 100 ha village irrigation scheme of Toya near Timbuktu, Mali. The irrigation development programme of the UNCDF had adopted a modular approach and would normally use 1 pump to irrigate a single 20 ha irrigation scheme. Thus, the Toya pumpsets can use the same maintenance infrastructure as 50 others. In fact, the 5 pumps, although installed side by side, do not have common pipelines or irrigation channels and can be operated independently, reducing organizational difficulties, that would otherwise occur in a 100 ha village-managed scheme.

4.11 InterpolationA set of affinity laws exists for varying impeller diameter, but it will not be used here, because the results do not always seem to be very reliable. Instead, it is much better to interpolate between manufacturers’ data for different impeller diameters. A self-explanatory example of interpolation is shown in sheet “pumps” of PumpSelect for the case of the Kirloskar 17.5-20. Take care to interpolate for similar discharge flows. Do not interpolate values for P, but calculate the new values for P on the basis of interpolated values of H and .

4.12 Extrapolation In the field there is often a lack of data on existing pumps, while manufacturers cannot be reached or do not respond to requests for information. To be able to compare pump alternatives with existing pumps it is necessary to complete these data. Typically, there is a little metal plate, with the main characteristics stamped into it, fixed to every pump. It is possible to extrapolate pump characteristics from these data, using graphs depicted in Figures 10, 22, 23 and 24: (1) optimum efficiencies as a function of specific speed and flow rate; (2) approximate head and efficiency evolution as a function of pump delivery for various specific speeds (3) NPSH at 3% head drop for pumps with overhung impeller; and (4) approximate NPSH characteristics as a function of delivery flow for various specific speeds. An example of extrapolation is shown in the sheet “extrapolation” of PumpSelect (i.e. [PUMPSELECT]Extrapolation! for the case of Kirloskar NW1+).

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Fig. 22: Approximate head and efficiency evolution as a function of pump delivery for various specific speeds

Fig. 23: NPSH at 3% drop for pumps with overhung impeller

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Fig. 24: Approximate NPSH characteristics as a function of delivery flow for various specific speeds

The suffix “00” in Q00, H00 and NPSH00 means the value of Q, H and NPSH at the best efficiency point.

4.13 Net Positive Suction Head (NPSH) and cavitationIt is critical that the inlet of a pump allows a smooth flow of water to enter the pump at a sufficiently high pressure to avoid creating vapour bubbles in the fluid. As the pressure on a fluid decreases, the temperature at which vapour bubbles form (like boiling) also decreases. Therefore, it is essential that the suction pressure at the pump inlet be well above the pressure at which vaporization would occur for the operating temperature of the liquid. This is called a net positive suctions head, abbreviated NPSH.

If the suction pressure is allowed to decrease to the point where vaporization occurs, cavitation is created inside the pump. Instead of a steady flow of liquid, the pump will draw a mixture of liquid and vapour, causing the delivery to decrease. Furthermore, as the vapour bubbles proceed through the pump, they encounter higher pressures which cause the bubbles to implode. Excessive noise, vibration and greatly increased wear of pump parts would result.

Pump manufacturers supply data about the required net positive suction head (NPSHR) for satisfactory operation. The person selecting the pump must ensure that there is a sufficiently high NPSH available (NPSHA). That is: NPSHA > NPSHR. Standards have been set by ANSI/HI calling for a minimum of 10% margin. Traditionally, in centrifugal pumps in non-critical applications the minimum margin is 0,5 m.

The value of NPSHA is dependent on the nature of the fluid being pumped, the suction piping, suction head, and the ambient pressure. This can be expressed as: NPSHA = Hsp + Hs – Hf – Hvp. The vapour pressure and specific weight of water vary with temperature, and ambient pressure varies with altitude.

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Figure 25: Pump suction line details and definitions of terms for computing NPSH (Mott, 2000)

The data given in pump catalogues for NPSH are for water and apply only to the listed operating speed. If the pump is operated at a different speed, the NPSHR at the new speed can be calculated from:

(Nouwen, 2000)

4.14 Common pumps with lower than optimum efficiency

Self-priming pumpsConventional centrifugal pumps, mixed flow and axial flow pumps are not self-priming because when dry they are unable to create suction at the intake. This limitation can be overcome by operating such pumps on flood suction (normal in submerged pumps), or fitting the pump with a suitable self-priming device. Self-priming devices include priming nozzles, diffusers and ejectors.

Figure 26 : the principle of a nozzle-type of self-priming centrifugal pump (from Nouwen, 1976, and Varisco, 1997)

The most common type of self-priming pumps employs the principle of the priming nozzle (see figure 26). Air is drawn into the pump by the vacuum produced as the impeller rotates and is emulsioned with the water contained in the pump casing. The air/water mixture is driven into the priming chamber where the air is separated from the water and vents through the delivery line. The water is recirculated until all the air is evacuated from the suction line,

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the pump primes and starts operating like a normal centrifugal pump. Other designs exist to cause the so-called “nozzle” effect.

Alternatively, an ancillary priming device may be used to prime a pump lacking self-priming characteristics. This may be a hand-operated membrane pump or a traditional farm pump. In that case the pump be fitted with a shut-off valve on the discharge orifice. Or the pump can be primed with an outside water supply or by hand, i.e. by pouring water into the pump until the suction end and the pump are completely filled with water. A foot valve will be required to prevent the water from draining out of the suction line during priming. The efficiency of self-priming pumps is often 10 to 15% less than conventional centrifugal pumps. Therefore, self-priming pumps, especially those with an open impeller, should only be resorted to when conventional pumps cannot be used. Self-priming pumps are very often used in shallow wells, but this seems hardly justified.

Impeller typesThere are many types of impeller, including the hydrostal, turbine, and impulse impellers. These will not be discussed here. Very common types are the open, closed and semi-open impellers in centrifugal and mixed-flow pumps. They cannot be distinguished by their specific speed. The open types are used for water containing solids or debris. The more open the impeller, the lower the efficiency. Therefore, open impellers should be avoided where possible. If there is a risk that the pump gets clogged, it is generally better to make improvements at the intake, e.g. by providing a screen or by using a foot valve with strainer.

Figure 27 semi-open impeller and open impeller (Nouwen, 1976). For closed impellers see fig. 3, 5, 10 and 11.

4.15 Dismantling a volute-mounted pump with an overhung impellerPreferably a pump is dismantled and reassembled using good drawings and a manual. There are many differences in pump construction. Special care must be taken if the pump has a mechanical seal: the seal parts must not be damaged. Even then, the seal may leak after reassembly and may have to be renewed. Here follows the dismantling procedure of a volute-mounted pump with soft sealing (another common type, the pedestal-mounted pump, will not be discussed here):

remove bolts 1 from volute casing 2. Pull bearing housing 3, complete with impeller assembly, backwards. In larger pumps the bearing housing may have to be supported.

remove impeller bolt or nut 7. It is best to block the axle at the coupling end while unscrewing.

pull off the impeller 6 with a pulley and remove the impeller key 8. unscrew gland nuts 9 from seal gland 10. unscrew bolt 11 and remove seal cover 4. remove gland 10 from axle 5 and take out the gland packing 12 from the packing

housing. pull off the coupling part from the axle and remove key 13.

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remove bearing cover 14 and rings 15. pull axle 5 with bearings 16 carefully out of bearing housing 3 (use a pipe, place it

against the inner ring of the bearing, and hit the pipe carefully with a hammer). remove the bearings from the axle. clean all parts and check for damage. follow reverse order for reassembly. Bearings are best heated in oil of about 80C. The

pump axle and the cooled down bearings are put back in the preheated bearing housing.

damaged or hardened seal glands 12 and packings 17 should be renewed. when replacing suction side wear ring 18 care should be taken that it is not tilted. The

gap between the ring and the impeller is normally about 0,15 mm (so 0,3 mm on the diameter).

test the pump (make sure that it is lubricated and that the pump does not run dry).

Figure 28: horizontal volute-mounted centrifugal pump

The most common type of axle seal is the gland packing. With poor sealing liquid is pumped out or air sucked in. It is a common source of malfunction. Manufacturers often sell packing rings to precisely fit their pumps. The packing is gently squeezed into position with a gland to ensure that leakage is no more than a drops. This leakage provides lubrication and cooling and prevents wear on the axle.

A common type of packing is the so-called soft packing which has a soft nucleus with a metal (lead) exterior. They can be applied for pressures up to 15 bar. The procedure for renewing the packing is as follows:

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Remove all old packing material (e.g. using special corkscrew-type tools). Clean axle and packing housing thoroughly.

If rings of the manufacturer are not available, make your own rings from packing strands.

Open the rings as indicated in Figure 28 and put the rings into the housing using a flat tool, e.g. a half pipe (never use a screwdriver!!).

Make sure that the ring openings are rotated 90 or 180 in each successive ring. After gently tightening the gland, it should be slightly loosened. The new packing can now be run in for several hours. If necessary the pump should

be stopped, to prevent the packing from overheating. The packing is allowed to leak profusely during this phase.

After running in the gland should be squeezed to reduce leakage to a low drip.

Figure 29: Opening new packing rings correctly and tools for removing old packing

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Chapter 5

Engines

5.1 Introduction

Since the mid-1940s the diesel engine has become the predominant source of industrial power throughout the world for units up to roughly 5,000 horsepower, principally because it is capable of burning a low-grade fuel at a comparatively low rate of consumption per horsepower per hour. Relatively unrefined fuels can be burned by a diesel engine because of the nature of its fuel-injection system and combustion process. Low fuel consumption results primarily from the higher compression ratio used. A greater fuel saving is effected at partial load than at full load since it is not necessary to throttle the inlet air, as in the case with spark ignition, to maintain an inflammable fuel-air mixture. (Only about two-thirds as much fuel is required.) Diesel engines do have some disadvantages, however. They are, for example, handicapped somewhat by their higher initial cost and greater weight per horsepower, by their emission of high levels of air pollutants (e.g., nitric oxide and soot) and odour, and by their greater operating noise and vibration.

5.2 Parameters involved in engine selectionWhen selecting an engine for a particular application, the following factors must be considered:

1. The range of power and speeds required;2. Rotational direction of both the pump and the engine;3. Space, weight, minimum and maximum speedvi, and position limitations;4. Cost of engine purchase and installation;5. Availability of certain accessories;6. Availability of after-sales service, such as spare parts;7. The conditions of operation (temperature, altitude, humidity);8. Governing codes and standards.

Most of these factors are taken into account in PUMPSELECT. Engine catalogues and manufacturer’s representatives will supply the necessary information to assist in the selection and specification of pumps.

5.3 Characteristic curvesThe characteristic curves describe the behaviour of an engine under changing operating conditions. The power P is plotted against the speed n. Normally a distinction is made between maximum power at intermittent load and maximum power at constant load, which is about 10% lower. In some cases a further distinction is made between maximum power at constant load and a kind of further derated maximum power. How this affects the engine derating discussed in Section 5.5 is not clear. In engine selection for pump applications the second value is used.

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Figure 30: Power output of Kirloskar TAF-1 and TAF-2

5.4 Engine efficiencyMaximum engine efficiency is normally mentioned in terms of specific fuel consumption at maximum torque speed. A typical best value for diesel engines of more than 2 kW is 235 g/kWh. This works out at a best efficiency of about 35% if diesel has the following propertiesvii:

Fuel Energy content Specific gravityDiesel 39020 kJ/l 0.815-0.855*Petrol 35560 kJ/l 0.75*In this text and in PUMPSELECT 0.845 at 15.55 ºC will be used.

Text books normally quote 30-40%, but this is an optimistic estimate, considering that ageing of the engine, poor quality maintenance, excessive power consumed by cooling fans and injectors all bring down efficiency (FAO, 1992). In addition engine efficiency is directly affected by the load factor. If the engine operates at partial load, efficiency may decrease as followsviii:

Figure 31: specific fuel consumption and partial load of Kirloskar TV-1 (8 HP at 1800 rpm)

Normally, partial load should not be less than 40% of maximum load. Below 40% there is a increasing risk of soot depositing in the combustion chamber. Besides, engine efficiency at 20% of maximum load may well be around 15% instead of 30 to 35%.

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5.5 Engine deratingThe output power of a reciprocating engine is proportional to the density of the inlet air, if all other factors remain equal. Change in air density occurs due to change in elevation, weather, or temperature. Engine derating is the reduction of maximum continuous output as a result of these changes. The following derating is applied to maximum output of engines in PumpSelect:

Altitude: approx. 3.5% for every 300 m above 150 m above sea level; Air inlet temperature: 2% for every 5.5ºC above 30ºC;

In PumpSelect humidity is not taken into account. A maximum derating percentage of 6% is mentioned by certain manufacturers.

5.6 Rotational directions of the pump and the engineIn most small engines the direction of rotation is anti-clockwise looking at the fly wheel. In India, standard rotation is often clockwiseix. The rotation of the impeller in most small pumps is anti-clockwise when viewed from the suction end. Again, the exception is most small Indian pumps.

It is obvious that the pump and engine cannot be coupled directly if the directions of rotation are opposite. Certain Indian engines have two opposite flywheels and can be coupled directly from both ends. Certain other engines have reverse rotation as an option.

5.7 Minimum and maximum speedSpeed is an important limitation: each engine has a minimum and maximum speed, beyond which it cannot be operated.. Many engines are designed for a speed range of 1500 to 3000 rpm, but other speed ranges are quite common: 1800-3600 for small, modern engines; 1200 to 2000 rpm for older models, and even 600-1000 rpm for some very old models that are still produced in India. With some of the smaller engines it is possible to get the same power at half the speed using power offtake at the gear end. Many, but not all, engines are suited for v-belt arrangements. V-belts are the easiest way to match engines and pumps with wide differences in speed. However, they may entail transmission losses in the order of 10-15%.

As mentioned in section 4.9, the power required by a pump varies with the cube of the speed, whereas the power provided by an engine varies directly with speed. Therefore, care must taken not to exceed a certain maximum speed. This is defined by the speed where the power required by the pump does not exceed the maximum continuous power that can be delivered by the engine. A margin of 10 to 15% is often applied to allow for engine wear.

5.8 Space, weight and position considerationsIn mobile pump sets there are not normally any space considerations. Both the engine and the pump are often compact. It can be useful to know how many engines or pumps or pump sets fit in a standard 20 feet container, to determine transportation costs. Normally about 40 10 hp engines fit in one container.

Other considerations are:

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A major space problem is presented by the transportation of rigid pipes or suction hoses.

Weight considerations apply especially to mobile pump systems. This explains to some extent the popularity of 2-stroke petrol engines in the smallest pump sets.

The largest mobile pump systems may weight as much as 650 kg. In that case, they will have to be trailer-mounted using large car wheels. For weights of about 300 kg small wheels can be used. When mobile pumps are meant for use in combination with a tractor, their maximum weight will depend on the size of the tractor.

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Chapter 6

Pipes, Appendages and Configurations

6.1 IntroductionThe characteristics of pipes and appendages used in a pipe line system determine its energy or head losses. Friction loss is the energy lost as the water flows along straight lengths of pipe and tubing. Minor or form losses occur as the water flows through flow system components, such as valves, fittings and bends. Other common components are reducers, enlargements, foot valves with strainers, entrance losses and exit losses.

In low lift pumping systems the head loss in pipes and appendages can easily surpass the static head. The easiest ways to redress this situation is by increasing diameters and decreasing the lengths. There is a wide variety of other solutions, some of them expensive, others locally made (see section 6.4)

6.2 Pipe and tubingThe actual outside and inside diameters of standard commercially available pipe and tubing may be quite different from the nominal size given. The nominal sizes for commercial pipe still refer to an "inch" size in spite of an international trend to the SI system. Larger pipe comes in the following diameter classes: 3, 4, 6, 8, 10, 12, 14, 16, 18, 20 and 24 inches, but sometimes other sizes, such as 7, 9 and 11 inch pipes, are also available.

Pipes can be made of galvanized iron, steel, concrete, PVC, or cast iron, hoses can be made of canvass, rubber, polyethylene or other plastics. Some hoses have two or three layers: an impervious inner layer, a middle layer to prevent the hose from expanding under pressure, and a third to prevent it from being damaged by sharp or abrasive objects. Some hose materials were not originally designed as such. In the Philippines fertilizer bags are sewn by local tailors into low-pressure lay-flat hose and in Australia Bartlett (http://www.bartlett.net.au) makes hoses of up to 778 mm diameter using UV stabilized PVC coated woven polyester material similar to that used in the manufacture of truck tarpaulins. Generally, relatively low-cost, large-diameter hoses can be used under low-pressure conditions only. If bending equipment is available, pipes can be made locally using iron sheets. Another option is fibre-reinforced raisin.

Because of their durability and the relatively low cost of local casting, cast iron pipes continue to be used in India. Elsewhere the three-layer blue lay-flat hoses, galvanized iron pipes and PVC pipes are commonly applied. Galvanized iron quick couplings are available for locally produced PVC pipes, to save on transport costs.

Rigid tubing, such as galvanized iron pipes can be used both for suction and discharge. Another common solution is the black, flexible, heavy, rubber-like, reinforced suction hose. Cheaper flexible, reinforced suction hose made of P.V.C. or another plastic material is also available.

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http://www.bartlett.net.au/


Hazen-Williams coefficients Ch for different types of pipe are tabulated below (formula in Section 2.3.5):Type of pipe Average for new, clean pipe Design valueSteel, ductile iron, or cast iron with centrifugally applied cement or bituminous lining

150 140

Plastic, copper, brass, glass 140 130Steel, uncoated 130 120Concrete 120 100Unknown 100 100Corrugated steel 60 60

Sometimes it is possible to suppress the use of discharge pipe, wholly or partially. Several configurations are shown in Section 6.4. Other options include the use of gutter-like, open-air structures, provided the water can first be lifted to the required height or suppressing the use of a fixed, concrete or stone, main canal inlet structures. The latter are often located about 10 m further away from the pump than strictly necessary to allow for river bank erosion. A wooden inlet structure is often cheaper and can be moved in case of erosion. If the canal is suitably lined so that it cannot be eroded, there is no need for an inlet structure, especially if the speed of water can be reduced before it leaves the discharge pipe, e.g. by way of a bubbler.

The pipe size for the suction line should never be smaller than the inlet connection on the pump. It can be somewhat larger to reduce flow velocity and friction losses. Pipe alignment should eliminate the possibility of forming air bubbles or air pockets in the suction line, as this will cause the pump to lose capacity and possibly to lose prime. Long pipes should slope upward toward the pump. Elbows in a horizontal plane should be avoided. If a reducer is required it should be of the eccentric type.

Fig. 32: Life-cycle cost principle for pumped fluid distribution systems

In general pipes should be sized for lower velocities based on the ideal of minimizing the energy losses. Practical installation considerations may lead to the selection of smaller pipes. Some of these practical considerations include the cost of pipe, valves and fittings. Figure 35 shows the principle of considering the life cycle of a pumped fluid distribution system. The primary parameters are total cost on the vertical axis and pipe size on the horizontal axis. The system cost curve indicates that larger pipe sizes along with the larger valves and fittings are more expensive. These aspects will be dealt with in more detail in chapters 8 and 9.

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6.3 Hose maintenance and repairHigh quality hose is expensive. Lower quality hose tends to be more vulnerable. In both cases, it is wise to take care at all times to avoid repair and early replacement:

1. When storing a lay-flat hose for long periods ensure that it is properly wound without twisting and kinking.

2. Hose life can be extended by turning it quarter-turns clockwise at regular intervals, e.g. once a month.

3. When storing lay-flat hose at the end of the irrigation season, drain out any water left in the hose. Store properly.

4. Small cuts or holes in lay-flat hose can be repaired easily provided the correct adhesives and parts are available. These normally come in kit form from the manufacturer. Holes are first filled with a mushroom-shaped patch filled with adhesive. A protective sleeve is then slid along the hose and glued over the repair.

Fig. 33: Lay flat hose repair (in Kay, 1983): (a) hole is cleaned out; (b) mushroom shaped repair patch coated with adhesive is folded, pushed into a hole and opened out with “stem” projecting outwards; (c) repair is weighted during setting using a spacer with hole in centre positioned over roughly trimmed “stem”; and (d) stem is finally trimmed and protective sleeve glued over repair

6.4 AppendagesCast iron and galvanized iron are the most common materials for appendages. Appendages can be bought or locally made. In Bangladesh, most foot valves and strainers are locally made of iron sheets. There seems to be no standard set of designs for locally made appendages. As a result it can be difficult to predict friction losses in them. This is not much of an issue in reducers and enlargements. The advantages of locally made appendages are: (1) creation of local employment; (2) development of local expertise in pump installation; and (3) appendages can always be made to fitx.

Considerable friction losses can occur in foot valves. These losses are relatively more important with decreasing static lift. End-users are often unaware of the inefficiency of the pumping system sold to them, because they are not able to carry out an analysis. It is not uncommon that importers sell pipes and appendages with the same diameters as those of pump orifices. So, a 100 mm pump will have 100 mm hoses and a 100 mm foot valvexi. With a discharge of 25 l/sec head loss in a 100 mm foot valve is likely to be in the 1-2 m range,

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which is a lot if you want to lift water only 3 m. In practice small pipe diameters occur often in conjunction with small foot valves to create even more abominable situations.

Fig. 34: foot valve with strainer – poppet disc type (left, K=6) and foot valve with strainer (right, K=1.2)

The friction coefficient K varies considerably from one type of foot valve to another: there is a factor 5 difference between strainer-poppet disc type foot valves and hinged disk ones. There are limits to increasing the diameter of foot valves in view of decreasing head loss: (1) an economic limit as the higher efficiency of larger foot valves cannot always pay for its higher cost; and (2) a technical as the valve requires a minimum nominal water speed to open properly (often about 1 m/sec).

Pumps operating from open sumps, rivers or lakes should have strainers or screens on the intake side to prevent entrance of objects that might damage the impellers. Strainers or screens may become plugged and when this happens pump cavitation can occur. For the same reason the intake orifice of a pump is often larger than the discharge one. Curiously, in low-lift, mobile applications with long discharge hoses the reverse would often seem more desirable.

6.5 ConfigurationsThere are many pumping system configurations. It would exceed the scope of this manual to review or even just mention all of them. The following configurations are common in low-lift, diesel-powered pumps using surface sources, such as rivers and lakes with variable water levels:

1. On natural river bank: this is by far the most common. It can be both fixed and mobile (see figure 35)

2. on a well3. floating (see figure 36)

Many other systems exist. tractor-driven systems hydraulic motor-driven (see figure 37) mobile or stationary axial-flow pumps (see figures 38, 39 and 40) locally-made axial-flow, mixed-flow and centrifugal pumps (e.g. from Thailand,

Vietnam, Indonesia and the Philippines, see figure 6) Increasingly, submersible (electric) pumps are used, because they are easier to install in a

wide variety of configurations. They require a source of electricity, such as a diesel generator, but normally overall efficiency will decrease by about 20 to 30% as a result of 2 additional energy conversions.

35


Fig. 35 Small mobile 55 l/sec pump set with 2 cylinder 15 hp engine (Niger River)

Fig. 36 Floating pump set (75 l/sec), Senegal River

Fig. 37 Hydraulically driven water pump with diesel prime mover

36


Fig. 38: Diesel-driven low-cost installation method for vertical propeller pumps (Batescrew, Australia)

Fig. 39: Low-cost installation methods for low-lift electric drainage pumps (VoPo, Netherlands), probably applicable in diesel applications, too.

6.6 Pump set specificationAfter pump selection, the following items must be specified (Mott, 2000):

Type of pump and manufacturer; Size of pump; Size of suction connection and type (flanged, screwed, etc.); Size and type of discharge connection; Speed(s) of operation; Specifications for driver (for example: for an electric motor – power required, speed,

voltage, phase, frequency, frame size, enclosure type); Coupling type, manufacturer, and model number; Mounting details; Special materials and accessories required; Shaft seal design and seal materials.

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Chapter 7

Design and Technical Analysis

7.1 IntroductionThe principles involved in designing a pumping system were dealt with in Chapter 4 and subsequent chapters. The procedure in Section 7.2 can be used to design a system. Reference will be made to an Excel 5.0 application called PumpSelect [PUMPSELECT.xls]. Normal Excel cell references are used to identify cells: GMP!N19 means cell N10 on sheet GMP. The application can do the necessary calculations very quickly and is useful for exploring the constraints of your system and for comparing it with alternatives, both in efficiency and economic terms. It also contains a database of pumps and engines.

7.2 Design procedureThe design procedure will carry you through the design of a simple, mobile pumpset which is required to deliver 250 m3/h (GMP!N19; by using 69,5*3.6 instead of 250 you will be aware that the required discharge of 250 m3/h is equivalent to 69,5 l/sec) at 4 m of static head (PDC!I3; water temperature 25 ºC PDC!O16; air temperature 40 ºC GMP!N17; 260 m above sea level GMP!N18).

Step 1: Propose the general lay-out of the systemThe following are its primary features:The pump is placed near the surface source and located 1 m (PDC!O17) above the level of the water. Therefore the pump requires priming, while cavitation is unlikely to pose a problem;The suction line is a total of 4 m long (PDC!C18)A foot valve and strainer is connected to the suction pipeThe discharge line contains one standard elbow and a total of 30 m of lay-flat hose (PDC!C26)

Step 2: Specify the sizes for the pipes and foot valve, use the table in Section 4.3 as a guide.A 6 inch (150 mm, PDC!E18) suction line gives a head loss of 0,63 m (PDC!I18) if the suction line material is not known (“pas connu”, PDC!G18)A 6 inch (150 mm, PDC!E26) discharge line gives a loss of 2,91 m (PDC!I26) if the discharge line material is very smooth (“tres lisse”, PDC!G26)An 8 inch (200 mm, PDC!C14) foot valve with strainer (“type Berselli avec crépine”, PDC!E14) gives a loss of 0,50 mThere is a reducer between the foot valve and the suction line: diameter a is 200 mm (PDC!C20) and diameter b is 150 mm (PDC!E20). This gives a loss of 0,11 m.There is one elbow at the discharge orifice of the pump with a diameter of 150 mm (PDC!C24) and a radius of 1 (PDC!E24) giving a loss of 0,40 m.Make sure that the second enlargement/reducer is neutralized (150 mm in both PDC!C22 and PDC!E22).Also neutralize the first elbow (“coude”) by inputing 999 mm in PDC!C16. Simply set the radius at 1 (PDC!E16).You will notice that the exit losses are 0,79 m. This is the totally useless kinetic energy that is imparted to the discharge and that will erode your earthen channel if you take no precautions.

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The total energy loss is 5,33 m and the total head on the pump is 9,33 m (PDC!I6) when the flow is 250 m3/sec. This is the desired operating point for the pump.Step 2 could be considered a reiterative process. If the total energy loss would seem exorbitant compared to the static head it seems wise to make the necessary adjustments. On the other hand it may be worth checking if pipe and element sizes could be reduced without excessive energy loss. Ultimately, it is a mix of cost, efficiency and practical considerations that will determine your choice.

Step 3: Specify a suitable pump in GMP!B4:D4. In this case one could choose a Chinese BC150xii or a European Johnson (Storkxiii) CN 150-125 or the CN 150-160. Unfortunately, the Chinese option is (at the time of writing and to the best knowledge of the author who has never travelled to China) insufficiently documentedxiv, but for cost reasons it would seem the best choice: its FOB price is about 10 times lower (US$ 200 instead of US$ 2000).

Because the author’s technical documentation of the CN 150-160 and of the 150-125 is much better it is possible to illustrate some useful aspects concerning impeller diameters. As mentioned in 4.9 the main method for matching pump characteristics to service data when using electric motors is by reducing impeller diameter. However, even when using direct-coupled diesel engines a certain impeller diameter will have to be chosen. This cannot be done before specifying an engine. Meanwhile the CN150-160 with the smallestxv, 176 mm impeller will be selected temporarily to be able to proceed with the design process without iterations. At a speed of 1980 rpm (GMP!F3) the pump matches the desired operating point with a pump efficiency of 64% (GMP!N27).

Note: Standards have been set by ANSI/HI calling for a preferred operating region (POR) for centrifugal pumps to be between 70% and 120% of the best efficiency point (BEP). Probably the aim of such standards is to help avoid pumps operating at relatively low efficiencies and avoid cavitation (>120%). There is no reason to purposely deviate from this standard, but there is no reason why one couldn't, provided cost, efficiency and cavitation aspects are dealt with properly.

Step 4: Specify a suitable engine in GMP!B10:D10. If the Chinese pump would have been selected it would have been logical to select a Chinese engine, like the S-1100. Since a European pump was chosen it is probably better to select one of the worldwide represented European makes like Hatz, Deutz, Lombardini or Lister-Petter. It should be noted that Hatzxvi and Lombardinixvii are produced in licence and at low cost in a large number of countries. Because Lombardini is widely represented in Africa and Asia and because the author has a fair set of data sheets on the main models, Lombardini will be opted for. The largest in the data base is the Lombardini 9LD625-2 which delivers a maximum of 17,5 kW (continuous rating, 10% overload capacity) at 3000 rpm. At the same speed of the pump (GMP!F10 = + F3) the engine has a nominal safety margin of 15% (GMP!N30) not taking into account the engine derating of 4.9% (GMP!N29). This leaves a net safety margin of 15 - 4.9 = 10.1%. This is adequate: in the industry the applied safety margin is often 10-15%, with smaller petrol engines having a wider margin than larger diesel engines. The application of a safety margin is not obligatory. It is not intended to prevent overheating of the engine. The main reason is to be able to compensate the effect of wear and tear on pumping efficiency and available power of the pump and the engine, respectively. Another reason is to avoid the risk of engine surcharge as a result of incidental changes in the pumping conditions: this is particularly relevant for pumps with a negative P-Q curve, such as axial-flow pumps, in which an unexpected blockage, e.g. by floating debris, at the pump entry can cause a strong

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rise in the required power. In our case the selected pump is of the mixed-flow type and has a flat P-curvexviii.

Step 5: Check if NPSHR > 0.5 m. In our case NPSHR (GMP!N28) is 3.7 m, which is more than enough to prevent cavitation problems. Step 6: Check the system for other pumping conditions, if necessary.Normally, the upper extreme with the highest dynamic head is already taken care of in step 1 to 5, so only the lower extreme needs to be checked. In some cases this means not only adjusting the static head (PDC!I3) but also the length of the discharge line (PDC!C26), and possibly other parameters. In our case we will hypothesize a mathematic relationship between static head and discharge length, as follows: PDC!C26 = 7.5*PDC!I3. If the minimum static head is 1 m, total dynamic head becomes 4.15 m and the engine speed needs to be lowered to 1560 rpm for the same discharge of 250 m3/h. Engine speed is not too low: GMP!F3 >= GMP!N4. The power margin of the engine is 48%, which is acceptable (it should be less than 60% to maintain reasonable fuel efficiency and less than 75% or whatever else the manufacturer indicates to avoid soot depositing in the combustion chamber). NPSHR (GMP!N28) is enough at 3.2 m.

7.3 A Simple Energy Audit The above system is more or less typical of the pumping systems that are used for low-lift conditions along Sahelian rivers in West Africa. The questions is: what is the scope for improving its efficiency?

The end-users standard for pumping efficiency is fuel consumption. Fuel consumption is a direct expression of overall water-lifting efficiency and could be used for a simple energy audit of water lifting systems, including those using diesel generator sets for electric pumps or other forms of energy transmission and conversion.

Instead of calculating the efficiencies in (see Section 3.1), it is possible to calculate overall2 directly, by dividing energy output by energy input, where energy output is (Epot=m.g.h on an hourly basis so multiplied by 3600) and energy input is fuel consumption per hour multiplied by the energy content (Section 5.6), so: overall2 = 69,5*3600*9,81*4/(2,91*39020*1000) = 8.6% for a lift of 4 m and overall2 = 69,5*3600*9,81*1/(1,58*39020*1000) = 4% for a lift of 1 m. On average this is 6.3%, which is less than half of what is considered acceptable in Section 3.1. (13.1%), so there should be considerable scope for improvement.

7.4 Efficiency ImprovementsA useful procedure for examining efficiency improvements is as follows:

Step 1: Increase the dimensions of hoses and appendagesHow flexible your are in specifying alternative dimensions will largely depend on what is available on the national market and at what price. It is assumed here that Bartletxix lay-flat hoses and hinged-disk valves are available at reasonable cost. Increase discharge line diameter from 150 to 231 mm (9 inch) Increase suction hose diameter from 150 to 200 mm Add gradual reductions and enlargements

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Step 2: Change the friction characteristics of hoses and appendages Change the foot valve type from "Berselli" to hinged-disk. A similar effect could be

achieved by increasing the "Berselli" diameter, but this would require an additional reduction.

Step 3: Examine the 2 extreme pumping conditions At a static head of 4 m:

1. P margin is 39-4.9% = 34.1%. The wide margin suggests that there may be scope to change to a smaller engine.

2. NPSHR margin = 4.9 m3. pump speed = 1679 rpm, which is less than 1800 or 2400 rpm.4. engine speed = 1679 rpm, which is well in between 1500 and 2200 rpm (for

continuous use).

At a static head of 1 m it is not possible to maintain the discharge constant at 250 m3/h (not even at the lowest engine speed of 1500 rpm), so a higher discharge of 277 m3/h will have to be accepted. This suggests that there may be scope for a slightly smaller pump:1. P margin is 39-4.9% = 34.1%. The wide margin suggests that there may be scope to

change to a smaller engine.2. NPSHR margin = 4.9 m3. pump speed = 1679 rpm, which is less than 1800 or 2400 rpmxx.4. engine speed = 1679 rpm, which is well in between 1500 and 2200 rpm (for

continuous use).5. Note that pump efficiency is only 41%.

Step 4: Specify an alternative pump, if desirableIn Step 3 it was concluded that there may be scope for a smaller pump. The Johnson CN 150-125 (with the largest, 176 mm impeller) seems worth trying. At a static head of 1 m the discharge can be maintained at 250 m3/h at an engine speed of 1508 rpm. Pump efficiency is much better at 50%.

Step 5: Specify an alternative engineStill at a static head of 1 m, it can be shown that by changing from a P margin of 67% with the larger engine, fuel consumption can be reduced from 1.14 l/hour to 0.99 l/hour using the smaller Lombardini 4LD820, or a 15% improvement in overall pumping efficiency, not to mention the lower cost of the smaller engine.

Step 6: Examine the 2 extreme pumping conditions At a static head of 4 m: 1. P margin is 21-4.9% = 16.1%. 2. NPSHR margin = 5.0 m3. pump speed = 1783 rpm, which is less than 1800 or 2400 rpm.4. engine speed = 1783 rpm, which is well within 1500 and 2200 rpm (for continuous use).

At a static head of 1 m, pump speed must be 1508 rpm for a discharge of 250 m3/:1. P margin is 47-4.9% = 41.9%, which is well within 60%.2. NPSHR margin = 4.4 m3. pump speed = 1508 rpm, which is less than 1800 or 2400 rpmxxi.4. engine speed = 1508 rpm, which is well between 1500 and 2200 rpm (for continuous use).

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Step 7: Check overall efficiencyAbove alternative system is of a type that could much improve low-lift pumping efficiency along Sahelian rivers in West Africa. The question is: by how much?

overall2 = 69,5*3600*9,81*4/(1,64*39020*1000) = 15.3% for a lift of 4 m and overall2 = 69,5*3600*9,81*1/(0,99*39020*1000) = 6.4% for a lift of 1 m. On average this is 10.9%, which is reasonably close to what is considered acceptable in Section 3.1. (13.1%). There is still scope for further improvement, but it will become increasingly difficult and costly.

i The pumpsets are equipped as set out in the table of subsection 3.2. Fuel consumption is corrected for engine load. Specific cost includes: fuel cost, interest cost (10%), repair and maintenance cost (10% of initial system cost) and depreciation (lifespan in hrs/1000 hrs per year, one crop per year).ii Speed is an important limitation: each pump has a maximum speed, beyond which it cannot be operated.. Many pumps are designed for maximum speeds of 1500 or 1800 rpm. Smaller pumps (up to a discharge diameter of 100 or 125 mm) are made for maximum speeds of 3000 or 3600 rpm. Some manufacturers offer the possibility to modify pumps for higher speeds, e.g. by using heavier bearings. iii Standards have been set jointly by the American National Standards Institute (ANSI) and the Hydraulic Institute (HI) calling for a “preferred operating region” (POR) for centrifugal pumps to be between 70% and 120% of the best efficiency point (BEP). See ANSI/HI, 1997.iv With the exception of friction loss that occurs in the column pipe for a vertical turbine pump.v E.g. there is no well drawdown when pumping from a large surface source. vi Speed is an important limitation: each engine has a minimum and maximum speed, beyond which it cannot be operated.. Many engines are designed for a speed range of 1500 to 3000 rpm, but other speed ranges are quite common: 1800-3600 for small, modern engines; 1200 to 2000 rpm for older models. With some of the smaller engines it is possible to get the same power at half the speed using power offtake at the gear end. Many, but not all, engines are suited for v-belt arrangements. This may entail transmission losses in the order of 10-15%.vii Specific gravity is also a function of temperature : for every 13º C temperature rise, specific gravity will decrease by about 1%.viii This phenomenon has been incorporated in PumpSelectix In many cases reverse rotation (i.e. anti-clockwise rotation) can be provided if specially ordered.x Various flanges and couplings are used to join pumps, pipes and appendages. Problems can occur when flanges are normalized to different standards, when a mistake is made in the procurement process, or when certain parts are simply not available. xi Before the post-war energy glut pump manufacturers and buyers were more energy conscious and similar pumps used to have larger, trumpet-like orifices. Under the pressure of economic competition, all pump manufacturers gradually eliminated these enlargements. As a result, energy-conscious pump buyers are now obliged to buy separate enlargements and reducers. xii It is produced by a Chinese company called Hunan Tianyi Pump Holdings Co., Ltd. See the 6BX-15 on http://www.tianyipump.com/production/pump/depp-e.htm.xiii Stork Pumps has been sold to Johnson Pumps a few years ago. Apparently, margins were no longer attractive. In the early 1980s, licences have been issued to companies in India and Zimbabwe. At the same time the Indian licensee also delivers most of the castings to the original company. xiv There is little doubt that proper documentation is available somewhere, but probably it is not easily accessible.xv In reality there is no smallest diameter. As the diameter gets smaller, efficiency decreases. Most manufacturers do not normally offer diameters of less than 20-30% smaller than the maximum diameter.xvi Some Hatz engines and a wide range of Ruggerini engines are produced by Pancar in Turkey. It seems Hatz is also produced in countries like Spain and Poland.xvii Lombardini is also produced in Turkey (Antor) and India (Greaves).xviii Centrifugal pumps have a positive P-Q curve, i.e. the required power increases with lower dynamic head. In the case of constant-speed electric motors it will be necessary to increase installed power by a factor of 10 or 20% when lower dynamic head is expected. In the case of a variable-speed internal combustion engine, it is possible to simply reduce speed and thereby power requirements. As a result NPSHR could surpass the threshold value and has to be checked.

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http://www.tianyipump.com/production/pump/depp-e.htm


7.5 Archimedes versus Rotodynamic Pump EfficiencyIs it feasible to design a rotodynamic system for very low lifts that is as efficient as an Archimedean screw with the added advantages of mobility, reliability, flexibility and affordability?

It is generally believed that the Archimedean screw is hard to beat when it comes to low-lift efficiency. However, the Archimedean screw is a bulky and costly apparatus, which is relatively difficult to transport and install. A small screw with a discharge of 70 l/sec for a lift of 2 m, has a diameter of 70 cm, a length of 4 m, weighs about 2 t and costs about US$ 30 000 and is unlikely to have better overall efficiency than 65%, not counting engine efficiency. With the efficiencies of engine (30%) and transmission (95%) included, overall2 will not easily surpass 18.5%.

For various reasons screw pumps (i.e. axial-flow or propellor pumps) are discarded: (1) (as yet) unknown characteristics of locally made axial-flow pumps from the Philippines, Vietnam, Thailand and Indonesia; and (2) lack of mobility and relatively high cost of industrial axial-flow pumps. Bowl-type mixed-flow pumps are rarely used for the application under study, so only volute-type mixed-flow pumps will be examined here.

Looking at the table in Section 4.3 we note that a discharge of 70 l/sec or 252 m3/h is can be achieved with a 6 inch or an 8 inch pump. We have seen what can be achieved with 6 inch pumps, although its limits have not been fully explored, so the logical 8 inch alternative will be dealt with now. There are 3 solutions for 8 inch, 70 l/sec volute-type mixed-flow pumps that seem particularly interesting:

From 2 or 3 companiesxxii in Turkey: direct, crankshaft-end driven 8 inch pumps; From many companies in China: v-belt driven 8 inch pumps; and, From Kirloskar, India: gear-end driven 8 inch pumps.

Because the scope of this manual is limited and because it seems to be the most efficient of the three, only the Kirloskar will be examined in detail. It is assumed that it will be installed on a low-cost sump made from concrete pipe (see Section 6.5). The pipe system of the pump consists of: (1) a 250 mm hinged-disk foot valve; (2) 3 m 250 mm iron suction pipe; (3) a 250 mm elbow of r=1d diameter; (4) a 250-200 mm reducer; (5) a 200-250 mm enlargement; and (6) direct discharge in irrigation canal via 2 m 250 mm normal iron pipe. The pump is a Kirloskar MF-200 with 220 mm impeller, gear-end driven by a TAF-1 variable-speed, air-cooled engine. Because the gear-end drive is used, engine speed is twice that of the pump (i.e. GMP!F10 = 2*F3). For a discharge of 70 l/sec at a static head of 2 m, pump speed will be 790 rpm and fuel consumption will be 0.76 l/hour. Overall efficiency is:

overall2 = 70*3600*9,81*2/(0,76*39020*1000) = 16.7%.

This is very close to the 18.5% of the Archimedean screw, while it is about 10 times cheaper. Similarly equipped, but with 5 m suction pipe it can be used for static heads up to 4 m. With expected fuel consumption of 1.16 l/hour, overall efficiency is a staggering 21.8%:

xix Bartlett is an Australian company that manufactures affordable lay-flat hoses in non-standard diameters , such as 5, 7, 9 and 11 inches.xx The Johnson CN 150-160 can be delivered with bearings for speeds up to 1800 or 2400 rpm.xxi The Johnson CN 150-125 can be delivered with bearings for speeds up to 1800 or 2400 rpm.xxii Standart, Pancar and possibly Mas Pompa.

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overall2 = 70*3600*9,81*4/(1.16*39020*1000) = 21.8%.

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Chapter 8

Economic Analysis

8.1 IntroductionUltimately end-users are not interested in efficiencies, not even in fuel efficiency, but in cost. Heavy subsidies distort prices and can easily lure farmers into using inefficient systems, often at the expense of tax-payers or economic development: examples abound from the United States of America to Asia. On the other hand, the imports of agricultural equipment can be taxed or otherwise hampered by inefficient government customs agencies. As a result, large groups of farmers remain under-equipped and cannot attain higher levels of productivity.

It is impossible to carry out an economic analysis without prices. In many cases, these prices will have to be derived from ex-factory or FOB prices in the country of origin. The effect of market imperfections on the price formation is such that equipment prices in Africa are often 1.5 to 4 times as high as in the countries of origin. These higher prices cannot be analysed away. Because some types of equipment have never been exported on a regular basis to Africa, it is difficult to know their prices. In many cases, equipment has been imported by development organizations, that do not use procedures for assigning costs to the equipment transportation and handling. Finally, the known price of equipment sometimes only concerns the pump set in a narrow sense, without taking into account the need for accessories, such as hoses, pipes or valves, or the need for making local changes, such as adding wheels or modifying the chassis. Thus, a description of how local prices are derived is very useful.

8.2 Water pumping costThe cost of pumping includes both fixed and variable costs. Fixed costs include such items as initial investment costs and replacement costs. Variable or operating costs are dominated by energy costs, but also include the cost of lubricants, labour, purchase of repair parts, overhaul costs, and regular maintenance costs.

Although the total investment cost can be obtained by adding the costs of the individual components, it is more desirable to obtain the annualised investment cost. The annual operating cost can be added to the annual operating cost to obtain an annual pumping cost. Annualized investment costs can be computed by several methods. One approach is to take the cost of a particular item and divide it by its expected life to obtain an average annual cost. This method disregards interest on the investment. A second method to obtain annualised investment costs is to compute an amortized cost considering the expected life of the item and the interest rate.

Both a simple and a more elaborate method for calculating water pumping costs can be carried out in PumpSelect. The simple method can be found in [PUMPSELECT.xls]Comparaison and the more elaborate, better method can be seen in [PUMPSELECT.xls]Evaluation. The simple method is intended to be used for a quick comparison of a large number of system, the elaborate method can be used for detailed case studies of one or several pumping systems. There can be a 10 to 15% difference in pumping

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costs between the two methods. This is not much considering that there are often enormous uncertainties in the estimates of useful life of pump components and their cost.

8.3 A simple comparison of pumping costsA simple method for comparing pumping costs for a large number of pumping systems is shown in spreadsheet [PUMPSELECT.xls]Comparaison. It calculates the sum of: (1) maintenance cost; (2) capital cost; (3) depreciation; and (4) fuel cost. It involves the following steps:

Step 1: Specify a list of equipment in B7:B26

Step 2: Estimate the local price for each type of equipment in French francs, useful life in hours, average discharge in m3/hour, and the fuel consumption in litres/hour.

Step 3: Specify the five parameters that are common to all systems in H29:H34. The parameters are: (1) maintenance cost as a percentage of installation cost (normally 10%); (2) local interest rate (often in the range of 10-15%); (3) the price of fuel (in French francs per litre); (4) specific discharge (in litres/second/hectare; for small-scale basin irrigation often in the range of 3,5-5 l/sec/ha); and (5) the number of pumping hours per year.

The results of this calculation should be interpreted with caution. The difference in pumping costs found with the detailed evaluation method described in Section 8.4 can be more than 10 or 15%. To limit this kind of discrepancies it is important to estimate the average useful life of the equipment properly. This is particularly important when the detailed evaluation makes a distinction between the estimated useful life (years) of different pump components, such as the pump, the engine or the accessoriesxxiii.

Of course, the spreadsheet can be modified to suit specific needs. Also, there is no need to restrain the comparison to only 20 types of equipment, although experience indicates that it is not a small task to identify 20 different types of equipment.

Note that in [PUMPSELECT.xls]Evaluation!I37:M47 the simple method is used in supplement of the detailed method. In it the estimated useful life is calculated as a weighted average of the various components (Evaluation!M39). Below table shows estimated life periods for various pump components in the United States of America (ASAE, 1983)xxiii:

Estimated useful life (years) of various pump components

Annual hours of useComponent 500 1000 2000 3000Well 25 25 25 25Pump 15 15 15 10Gearhead 15 15 15 10Drive shaft 15 15 7 5Engine 15 15 10 7Gas line 25 25 25 25Engine foundation 25 25 25 25

xxiii In practice, the situation in West Africa is such that often little distinction is made between the life of the engine and the life of other components. This is perhaps surprising because engine life is often much shorter than under normal conditions in more temperate climates.

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Electric motors 25 25 25 25Electric controls and wiring 25 25 25 25

8.4 A detailed evaluation of pumping costsThe method used here is based on the one shown in “A comparison of decentralized minigrids and dispersed diesels for irrigation pumping in Sahelian Africa” (Perlack et al., 1988) in which an example representative of irrigated cropping systems found in the Senegal Valley is used to compare costs of water pumping through the use of commonly-used, small diesel-driven pumpsets versus a decentralized generator connected to a small electrified grid.

Several modifications have been added to the original method to allow the integration of: 1. Crop water requirements specified in Evaluation!C7:C18;2. Irrigation system characteristics in Evaluation!D7:D18 and F7:F18;3. Seasonal variation in water levels, specified in Evaluation!H7:H18;4. Pumping system characteristics, by using the information generated with

[PUMPSELECT.xls]GMP and PDC in Evaluation!I7:P18 of table 1: crop pumped water and energy requirements; and,

5. Economic data.

The reasons for considering 1, 2, and 3 are as follows. Crop water requirements are included because some crops require more water than others, which indirectly affects pumping costs per hectare. Similarly, some irrigation systems are more efficient than others, thus reducing pumping requirements and costs. The seasonal variation in water levels can normally be expected to affect the energy requirements of the pumping systemxxiv.

Since basin-irrigated rice is the main crop in farmer-managed irrigation schemes in the Sahel, the simple approach for assessing water requirements in Evaluation!C7:F18 seems justifiable. Evapotranspiration values for Timbuktu, Mali, have been used, multiplied by a crop coefficient Kc of 1,2 for ricexxv (no distinction is made for crop stage, but this would be a simple matter). The only form of irrigation water loss considered here is percolation loss at field level. In small-scale flood irrigation systems percolation losses can be expected to account for 95% of all lossesxxvi.

In Evaluation!E7:E18 the number of days of irrigation can be added. In this case, irrigation starts at the beginning of the last decade of August. It would be easy to change the table to a decade-wise one in stead of the month-by-month system used. The 2000 m3/ha extra water is the amount of water needed to prepare the soil for the rice transplants.

xxiv It is the lack of relationship between static head and fuel consumption found in pumping systems in Timbuktu, Mali, that triggered the author’s interest in the subject. The 12 pump sets used in the irrigation development programme were composed of a Lister-Petter HR3 and a Caprari 200 BHR-D with a discharge of about 135 l/sec for irrigating 34 ha village rice schemes. Static head varied from 1 to 4 meters, but diesel consumption remained constant at 4,2 litres/hour.xxv The FAO publication Crop evapotranspiration guidelines for computing crop water requirementscan be used for calculating reference and crop evapotranspiration from meteorological data and crop coefficients. The online document can be viewed at: http://www.fao.org/docrep/X0490E/X0490E00.htmxxvi In practice, percolation losses could be estimated by subtracting crop evapotranspiration from the depth of water supplied to the system. As a result, percolation losses would include the irrigation distribution losses. As said earlier: in flood irrigation this seems acceptable, especially because the management of small irrigation systems is such that certain losses that can be expected in large schemes do not normally occur.

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How to use [PUMPSELECT.xls]Evaluation? Generally, specify all the cells that are marked in bold. This means that one will have to take the following steps:

Step 1 derive and specify evapotranspiration values in Evaluation!C7:C18Step 2 estimate daily percolation values in Evaluation!D7:D18Step 3 indicate the number of days you plan to irrigate in each monthStep 4 in case additional irrigation is needed for crop establishment or drainage purposes,

specify in Evaluatio!E7:E18 (in m3/ha)Step 5 for each static head calculate (using [PUMPSELECT.xls]GMP and PDG]: (a)

discharge; (b) engine speed; (c) manometric head; (d) absorbed power (in kW); and (e) fuel consumptionxxvii.

Step 6 indicate the maximum power of the engine in kW in Evaluation!G23. This isn’t very meaningful and therefore not used in any further calculations

Step 7 specify the investment costs in Evaluation!G33:G36 of the engine, the pump, the accessories and the installation.

Step 8 specify the life of the engine, the pump and the accessories in Evaluation!G38:G40.

Step 9 specify the cost of fuel in US$/litre in G42Step 10 estimate the maintenance and labour costs as a percentage of the total investment

costs in F44 (often 7-10%)Step 11 specify the interest rate in N23Step 12 enter the evaluation period, ideally the smallest common multiple of the lives of

engine, pump and accessories, but less than 20, possibly 30 years;Step 13 the calculation of the present value of engine, pump and accessories replacement is

not automated, so it will have to be specified in N27:N29.

8.5 CaveatIt cannot be overemphasized that the results in this type of economic analyses are sensitive to relative assumptions regarding capital costs, operating costs, energy efficiencies and useful lives. Changing key assumptions in different options could alter the results significantly (Perlack et al., 1988) xxviii.

xxviii If the reader would ever contemplate carrying out case studies using PumpSelect he is advised to use this phrase when he starts writing his conclusions.

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Chapter 9

Procurement and Prices

9.1 Pump marketing in West AfricaIf marketing efficiency is a useful measure of the performance of the pump market in West Africa, it would seem that the sector is not doing very well. As mentioned in Section 8.1 equipment prices are often 1.5-4 times as high as in countries of origin. In addition, the terms of trade have been moving against agricultural mechanization in Africa over the past decades. Irrigation pumps are a good example of this. As a result of increasing globalization, agricultural equipment from low-wage, industrialized countries, such as China and India, is becoming more easily accessible. Provided this equipment can be imported efficiently, there is considerable scope for lowering equipment cost in Africa, thus enhancing its affordability.

Pump manufacturers or dealers are not necessarily interested in overall water-lifting efficiency. The reasons are: (1) pump units are often sold by diesel engine representatives, who are generally more preoccupied by selling the largest possible diesel engine; (2) the iterative process involved in attaining better efficiencies is cumbersome to those with limited experience or knowledge; and (3) the general tendency is to focus on manometric head and pump efficiency, rather than static head and overall pumping efficiency. Furthermore, the conditions for after-sales service are not always well defined. It is not always clear what guarantees mean at 1000 km away from the sales point.

The purchase of a pump unit typically involves the following steps: The customer chooses the representative of an engine manufacturer, whose products

he considers good value or reliable The customer asks the sales person if he has a suitable pump set for a given discharge Sales person shows a model in a catalogue or in the showroom. He makes sure that no

matter what, the pump will always be able to deliver the required discharge. In fact, the often limited selection of pumps available is such that they are suitable for a wide range of dynamic heads. Generally, this means that for low heads no efficient solution is available.

No attempt is made to analyse pumping costs and few options are available to improve efficiencies.

Shopping around is of little use. There is little choice on the local market. Usually, equipment with acceptable low-lift efficiency is often not available

More efficient hoses, pipes and appendages are often extremely expensive. As a result it the effect of their use on overall pumping costs will be negligiblexxix.

Finally, the client ends up buying a pump with too big an engine for pumping conditions that are not really his. As a result the pump set is more expensive, he will consume perhaps twice as much fuel, and he will pay more in maintenance. In many cases the client will not buy it, because it is too expensive. He will wait for the next rains, which is not a very good strategy in the northern Sahel if your village is next to a big perennial river with a tendency to almost flood your land. Maybe, one day, a

xxix In addition, “medium” lift pumps do not have favourable pump characteristics for better accessories: what can be gained in system efficiency is lost almost entirely in pump efficiency.

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development organization will come to give him a pump with not too many strings attached. Of course, the development organization will not buy from the local pump supplier because he is too expensive.

9.2 Alternative suppliersDiesel-powered mobile pump sets are manufactured by the millions in Asian countries, such as India, China and Turkey. Indian importers have been supplying East and southern Africa with Indian equipment for a long time. Turkey is exporting mainly to the Middle East and North Africa. West Africa has been dominated by suppliers of European equipment. Relatively little Chinese equipment seems to have entered Africaxxx. Engineering firms from China that carry out infrastructural works in the region sometimes import equipment.

Asian equipment is about 5 to 10 times as cheap in the countries of origin as European equipment. Yet, Asian equipment has difficulty entering Africa. The reasons for this are manifold: (1) part of the price advantage is lost in transport and custom procedures; (2) access to foreign exchange to pay Asian manufacturers may be difficult for new importers; (3) equipment markets are dominated by a few importers of European equipment. The established importers are happy with the situation as it is, emphasizing durability and reliability to justify their margins; (4) these importers rely on the expertise of marketing departments of European manufacturers, whereas Asian manufacturers offer only equipment and spares; (5) local equipment markets are very fragmented, e.g. development organizations buy abroad instead from local suppliers; (6) local purchasing power is very limited; (7) very little and isolated experimentation is carried out to get acquainted with Asian equipment; and (8) whoever successfully imports Asian equipment will not lack competitors, because the same equipment is made by many other manufacturers. The normal system of sole representation does not apply.xxxi

Apart from the lower prices, there are several reasons why Asian equipment should be imported more to Africa: (1) a wide range of mobile, low-lift pumping equipment is produced in Asia, thus increasing choice enormously; (3) engines are often quite simple, requiring little or no extra training of local mechanics; (4) in pump sets only the engine is a bit difficult to maintain in good order. With a bit of adaptive engineering another engine could easily be fitted on an otherwise very cheap and reliable pump set; and, finally, (5) there seems to be no reason why a concerted effort to properly introduce certain equipment could not be successful. Of course, this presupposes a determination to obtain the necessary expertise.

9.3 Improving the performance of the African pump marketGenerally speaking, the African pump market is not efficient. Therefore, competition will need to be enhanced. Very competitive sources of irrigation equipment exist in Asia. The best way to promote this equipment is by improving the availability of technical information on this equipment. During the sub-regional workshop on irrigation technology transfer in support of food security, which was held in Harare, Zimbabwe, in April 1997, it was recommended (Zhou Weiping, 1997) that:

“Irrigation experts, government officials, staff of enterprises and other institutions should be organized by UN organizations, regional organizations or through bilateral

xxx This is only based on impressions of the author. It would be nice to have some statistics on this subject.xxxi This is both an advantage and a disadvantage. The disadvantage is that nobody is likely to invest a lot in setting up an importing business. The advantage is that once certain equipment is imported the competition will keep prices low, thus improving market efficiency.

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arrangements to engage in technical exchanges on the technologies and applications of small irrigation equipment. In particular, study tours should be conducted to other countries so that there is co-operation on the content, objectives, methods, direction of technological development and product choices of the major types of equipment.

Demonstration areas of different types of equipment from other developing countries can be established in the region to provide local farmers with an opportunity to observe the effects of these small equipment and arouse interest to try out the equipment. Training bases can be situated nearby to provide farmers with training.

Under the support of the United Nations or other international organizations, training courses and workshops can be conducted in the developing countries with the relevant experiences. Experts can be invited to lecture to local participants, visit facilities and train local technical extension staff.

International financial institutions should provide assistance for this activity. Governments in the region should make requests to countries with which they share

experiences and develop relationships to promote technology transfer through inter-governmental arrangements; and,

The United Nations organizations, especially FAO, should continue to encourage the development of the whole process, formulate further actions and programmes, and guide more governments, enterprises and groups to participate in this programme.”

xxvii Both fuel consumption and absorbed power are needed because power doesn’t translate directly into fuel consumption but is affected by engine load and specific fuel consumption of the engine.

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References

Allen, R.G., Pereira, L.S., Raes, D. and Smith, M. (1998) see: FAO (1998)

ANSI/HI (1997) Standard for centrifugal and vertical pumps for allowable operating region

ANSI/HI (1998) Standard for centrifugal and vertical pumps for NPSH margin

ASAE (1983) Design and operation of farm irrigation systems

Bom, G.J. and Van Steenbergen, F. (1997) Fuel efficiency and inefficiency in private tubewell development

CIRAD (1999) La motorisation dans les cultures tropicales

Delacretaz, B (1982) Les pompes dans les ouvrages d’hydraulique agricole

Encyclopaedia Britannica (1999) Pumps

FAO (1986a) Irrigation in Africa south of the Sahara

FAO (1986b) Water lifting devices (by Fraenkel, P.L.)

FAO (1987) Irrigation and water resources potential for Africa

FAO (1997) Irrigation technology transfer in support of food security

FAO (1998) Crop evapotranspiration guidelines for computing crop water requirementsAvailable online: http://www.fao.org/docrep/X0490E/X0490E00.htm.

Hecq, J. and Dugauquier, F. (1990) Perimetres irrigues villageois en Afrique sahelienne

Kay, M. (1983) Sprinkler irrigation : equipment and practice

Milnars Pumps Ltd. (1994) Centrifugal pumps

Mott, R.L. (2000) Applied fluid mechanics

Nouwen. A. (1976, 1998) Pompen 1

Nouwen. A. (1977, 1996) Pompen 2

Nouwen. A. (2000) Pers. comm.

Perlack, R.D., Petrich, C.H. and Schweitzer S. (1988) A comparison of decentralized minigrids and dispersed diesels for irrigation pumping in Sahelian Africa

Purkey, D.R. and Vermillion, D. (1995) Lift irrigation in West Africa: challenges for sustainable local management

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http://www.fao.org/docrep/X0490E/X0490E00.htm


RNAM (1991) Regional catalogue of agricultural machinery: Bangladesh, India, Indonesia, Iran, Nepal, Pakistan, China, Philippines, South Korea, Sri Lanka, Thailand

Savatier, A. (1978) Les pompes et les petites stations de pompage

Sulzer Brothers Ltd. (1989) Sulzer centrifugal pump handbook

Van ’t Hof, S. (1992) Irrigation d’appoint à Bourem (Rapport de mission, 18-25 March 1992)

Van ’t Hof, S. (1994) Economical low-lift pumps for village irrigation along the Niger River

Van ’t Hof, S. (1998) The design of a low-lift irrigation pump pilot project: improving the availability of affordable pumpsets to African farmers

Violet, J.A., Soumaila B.D.I.A., Van Steekelenburg, P.N.G., and Waldstein, A. (1991) The development of irrigated farming in the Sahel: irrigation policy limitations and farmer strategies (OECD, Club de Sahel , CILLS, ILRI).

Zhou Weiping (1997) Review of the irrigation equipment manufacture and supply sector in China (in: FAO, 1997)

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affordable low-lift pumping forsmall-scale irrigation development : appropriate equipment selection

Documents

pumping costs

overall pumping efficiency

kinetic pumps

mixedflow pumps

pump selection

development organizations

depreciation costs

mobile pump sets