Best Irrigation Water Pumping Manual

7/28/2019 Best Irrigation Water Pumping Manual

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Small-scale pumped irrigation - energy and cost iii

PREFACE

This manual is about reducing the costs involved in small-scale pumped irrigation schemes.

Too often, schemes are designed and constructed with thought given only to the immediate cost

of constructing the scheme and of buying and installing equipment. Little or no attention is

given to operating costs, with the result that some schemes may well be cheap to install but very

costly to run. When water is pumped, every litre has a real cost because of the energy needed.

If more water is pumped than is needed or is pumped inefficiently, then operating costs can rise

significantly because of the additional energy which is wasted.

Ways of approaching scheme design and equipment selection are described so as to take

account of the operating costs. Simple examples are used to show how this can be done, andhow true comparisons can be made between different designs. Guidelines are given, based on

experience in many developing countries, so that sound practical choices can be made.

The manual is not just for those starting a new scheme. It is also for those who wish to

evaluate and improve existing schemes, and practical ways of reducing operating costs by

improving the efficiency of water use and pumping are described.

The readership envisioned is that group of people with some practical experience in

small-scale irrigation but who have little or no technical or engineering knowledge and wish to

be able to advise farmers on appropriate equipment selection and its proper and efficient use.

Although not numbered in the same series as the FAO/ILRIIrrigation Water Management

Training Manuals, this particular publication is seen as being complementary to that series,and, as a consequence, numerous cross-references are made in the text to the various volumes

of the Training Manuals series.

The text is substantially the work of Dr Melvyn Kay, of Silsoe College, UK, with additional

technical input from N. Hatcho of the Land and Water Development Division, FAO, Rome.

The text was edited and prepared by Thorgeir Lawrence for publication by FAO.

Any comments on the text as it stands and any suggestions for potential improvements

that could be included in subsequent editions are welcomed, and should be addressed to:

Water Resources, Development and Management Service , AGLW

FAO

Viale delle Terme di CaracallaI-00100 ROMA,

Italy


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Small-scale pumped irrigation - energy and cost v

CONTENTS

Page

1.INTRODUCTION 1

1.1 Small-scale irrigation 1

1.2 Problems 1

1.3 Solutions 2

1.4 Making choices 2

2.SOME BASIC CONCEPTS 5

2.1 Introduction 5

2.2 Pressure 5

2.3 Discharge 7

2.4 Energy 9

2.5 Power11

2.6 Efficiency 13

3.CHOOSING A NEW IRRIGATION SYSTEM 15

3.1 Introduction 15

3.2 Water sources 18

3.3 Pumps and power units 18

3.3.1 Pump types 18

3.3.2 Pump Characteristics 22

3.3.3 Pump selection 233.3.4 Power units 24

3.3.5 Efficiency 26

3.4 Distribution systems 29

3.4.1 Open channels 29

3.4.2 Pipelines 31

3.4.3 Distribution efficiency 35

3.5 Methods of irrigation 37

3.5.1 Surface irrigation 37

3.5.2 Sprinkler irrigation 39

3.5.3 Trickle irrigation 40

3.5.4 Selecting an irrigation method 403.6 System capacity 41

3.6.1 Crop water requirements 42

3.6.2 Peak scheme water demand 44

3.6.3 Seasonal scheme water demand 45

3.7 Peak power and energy demand 45

3.8 Costs 46

3.8.1 Capital cost 47

3.8.2 Operating cost 47

3.8.3 Overall cost 49

3.8.4 Effects of changes 52

3.8.5 Some general conclusions 533.8.6 Some practical considerations 54


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4.CASE STUDY-1 55Introduction 55

4.1 Options available 55

4.2 Scheme water demand 56

4.3 Peak power and energy demand 57

4.4 Overall costs 59

4.5 Conclusions 59

4.6 Guidelines 62

5.CASE STUDY-2 63

5.1 Options available 63

5.2 Scheme water demand 63

5.3 Overall power and energy demand 64

5.4 Overall costs 65

5.5 Conclusions 67

5.6 Guidelines 70

6.IMPROVING EXISTING SCHEMES 71

6.1 Introduction 71

6.2 Inefficient water use 72

6.3 Inefficient equipment 73

6.4 Effect of inefficiency 74

6.5 Evaluating a scheme 74

6.6 Obtaining data 766.6.1Observing and questioning 76

6.6.2Some basic data 76

ANNEX 79


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Small-scale pumped irrigation - energy and cost vii

TABLES

Page

1. Energy content of fuels and foods 10

2. A guide to selecting centrifugal pumps 21

3. Pump selection for small-scale schemes 23

4. Indicative values of distribution efficiency (%) 35

5. Typical field application efficiencies for irrigation methods 37

6. Typical sprinkler data 39

7. Factors affecting selection of irrigation method 41

8. Indicative values for crop water needs and growing periods 43

9. Useful life of irrigation system components 47

10. Indicative maintenance and repair costs 48

11. Capital recovery factors (CRF) 50

12. EAC values for pumps at various discount rates 51

13. EAC values for pumps for different life expectancies 51

14. Changing the distribution system and its effects on energy and cost 52

15. Calculating scheme water demand 56

16. Overall power and energy demands 57

17. Overall cost comparisons 58

18. Calculating scheme water demand 64

19. Overall power and energy demands 64

20. Overall cost comparisons 6521. Efficiency of surface irrigation methods 73


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FIGURES

Page

1. Making choices , the design process 4

2. Relationship between force and pressure 5

3. Measuring pressure in a pipe 6

4. Calculating discharge 7

5. Measuring discharge 8

6. Energy conversion - analagous systems in people and machines 10

7. Illustration of the problem considered in Example 3 11

8. Relationship between rate of energy use and power 12

9. Graph relating flow, static head and power 13

10. Choosing irrigation system components 15

11. Components of a typical irrigation scheme 16

12. Typical axial flow pump 19

13. The radial flow (centrifugal) pump 20

14. Typical mixed flow pump 21

15. Pump characteristics of the three pump types 22

16. Pump selection based on head and discharge parameters 24

17. Manufacturers data for a centrifugal pump 25

18. Efficiency of components of pumping plant 26

19. Suction lift limitations 28

20. Energy demand for open channel distribution 29

21. Channel design: dimensions and drop structures 3022. Pipe system and its energy demand 32

23. Hydraulic gradient 33

24. Nomograph relating pipe diameter, discharge, head loss and velocity 34

25. Basin, border and furrow irrigation 36

26. Sprinkler irrigation 38

27. Hose-pull sprinkler system 39

28. Trickle irrigation 40

29. Peak and seasonal scheme water demands 42

30. The concept of water requirements in mm 43

31. Relationship between pipe size and seasonal energy cost 53

32. Effect on EAC values of reducing pump efficiency 6033. Effect on EAC values of changing interest rate 60

34. Effect on EAC values of a greater depth to the groundwater table 61

35. Effect of reducing pumping efficiency on EAC values 66

36. Effect of greater depth to groundwater on EAC values 66

37. Effect of increasing scheme size on capital and operating costs 68

38. Evaluating irrigation scheme performance 71

39. System efficiency value ranges 74

40. The relationship between efficiency and seasonal operating costs 75


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Small-scale pumped irrigation - energy and cost 1

Chapter 1

Introduction

1.1 SMALL-SCALE IRRIGATION

Small-scale irrigation is an important aspect of irrigation development in many countries.

Approximately half of the irrigated area in Sub-Saharan Africa, for example, is irrigated in this

way. It involves individual or small groups of farms, organized and managed by farmers,

usually independent of government resources. This type of development has often proved

successful in places where the larger-scale, primarily government controlled, projects havenot. This is not to say that small-scale is therefore better than large-scale farming, or indeed

that small-scale is more simple to develop. It is a different approach to irrigated farming, with

its own challenges. Irigation development requires careful design, construction and management

to be successful. It is, perhaps, in the management element that the key difference lies. In a

small system there are no tiers of management, as in the large-scale schemes. Farmers alone

decide when to irrigate and how much water to apply; start and stop the pumps; and generally

run the entire scheme with the help of the family or local community.

Small-scale farming can be highly productive in terms of yield per hectare of land. The

energy input into large-scale schemes can be up to 15 times greater than that required for small-

scale farming for the same output of crops produced. This is in sharp contrast to large-scale

schemes where the ratio is normally less than 4. Thus, on a national or regional scale, whenconsidering the use of commercial fuel in agriculture, which in many countries is both scarce

and expensive, the small-scale approach can be an attractive one.

1.2 PROBLEMS

Despite their apparent attractiveness in terms of potential productivity, small-scale schemes

are, however, not always as efficiently run as they could be. Most schemes rely on pumping to

supply their water needs and are often designed on the basis of minimum investment cost, with

little or no thought given to the effect that this might have on operating costs over many years.

For example, a farmer may purchase a cheap pump which runs at a very low level of efficiency.

The energy cost may be considerable and it may require much servicing and spare parts. If thefarmer were to purchase a better and more appropriate pump then more money might be spent

initially but there should be much more money saved over the years through reduced fuel

(energy) costs and maintenance. Similar issues arise when selecting other components of an

irrigation system.

An equally important issue to consider is how well the scheme is managed once it is operating.

The most appropriate system design and selection will be of little use in the hands of an

inexperienced or unskilled irrigator. Good equipment is no substitute for good management

and, here too, considerable savings in energy and operating costs can be made by ensuring

good equipment and water management practices.


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Introduction2

1.3 SOLUTIONS

This manual describes ways of approaching scheme design and equipment selection which

take account of both investment and operating costs, and, in particular, emphasise the significance

of energy costs.

Some basic concepts need to be understood about water flow, energy and power, and, for

those who have little or no knowledge of these, they are described in Chapter 2.

In Chapter 3 the basic components of an irrigation scheme are described together with ways

of choosing between different pumps, distribution systems and methods of irrigation. There

may be many different ways of irrigating a farm and a basis for comparison and selection is

needed. Cost is often the dominant factor. Thus the idea ofcost effectiveness is introduced,

showing that both capital costs and operating costs must be considered when selecting equipment,

and that the one affects the other. This is demonstrated in Chapters 4 and 5, where two contrasting

case studies show how the principles and practices of Chapter 3 can be applied.

Many small-scale irrigation schemes are already in operation, and one question here might

be how to get the best results from what is already there. Chapter 6 examines ways of looking

at existing schemes to determine energy use and operating costs, and to find ways of reducing

them through improved efficiency of equipment and water use.

1.4 MAKING CHOICES

Much of this manual is about the process ofdesign the process of making logical choices

between systems of irrigation and equipment (Figure 1). It is important to realize at the outset

that there is unlikely to be just one ideal choice; there may be many alternatives, any one of

which might be quite appropriate. The job of the designer is to present the options available in

relation to good irrigation practice, water availability, equipment, its reliability and cost. The

farmer can then choose the system which he or she feels is most appropriate.

The design process

Apreliminary design is usually done first. This is often done quickly in order to establish the

options available. Once a choice has been made, work then proceeds to a detailed design

which details every nut and bolt to be purchased and every canal and structure to be constructed.

To undertake a preliminary design, basic information is needed about the land and crops to

be irrigated. However, accurate details about land areas and crops may not be necessary at this

stage. To understand this it is important to realize what preliminary design is about. It is to

establish the maximum capacity or size of the system to be constructed and the choices available

to the farmer. The system capacity must be enough to satisfy the maximum amount of water

needed by the crops, and there are simple ways of assessing this without detailed knowledge of

the cropping. Clearly the answer will not be exact but great accuracy is not needed at this stage.

Remember that when a scheme is operating it will run for most of the time at well below its

maximum capacity. It may only run at full capacity for a very short period when the crops are

maturing and need most water. It is very much like designing and using a car. It may be

designed to operate at a maximum speed of 150km/h, but most drivers would travel well below

this speed and only use the maximum speed occasionally. Thus whether the maximum speed is

150 or 160km/h is not really very critical to the overall use of the car if it otherwise meets allthe demands made upon it by the driver. If the actual maximum performance is less than


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150km/h, then the car will still get there it will just take a little longer. In the same way the

capacity of an irrigation system need not be determined with great accuracy as long as thecapacity will meet most, if not all, of the operating demands that the farmer will make. If the

capacity falls a little short of demand then the difference can be made up by running the system

for a longer period.

A further aspect of design is considering How will the final cost of the scheme be affected

by the decisions made during the design process?. If, for example, the crop water requirement

is changed by 10%, or a channel is increased in size by 20%, does this significantly affect the

overall cost of the scheme? If it does, then this figure needs to be chosen with considerable

care. If it does not, then such accuracy is not needed. A good designer will concentrate on the

important factors which will have significant effects on the outcome. The inexperienced designer

will need to experiment a little to determine which are the critical factors in the design process.

A final aspect of design, which the inexperienced designer may not realize at first, is that

there are no formulae which can help with the initial decision making. For example, there is no

formula which would show that a pipe should be used instead of an open channel. This is a

matter of choice, which may eventually be decided by cost or some other constraint. The

designer would thus consider both options, prepare a preliminary design for each one, and then

see which was best. Several designs may be done in this way before the best one can be chosen.

In other words, the designer will often choose what seems to be appropriate and then set about

proving that the choices made are indeed the best. This is where an experienced designer can

be invaluable. On the basis of past experience of similar situations the designer may well be

able to greatly simplify the design process because he or she may have a very good idea of what

will be the best solution. Unfortunately, the inexperienced designer must go through a morerigorous process to arrive at the best solution. This manual is to help the inexperienced designer,

and to try and pass on some of the experience of others in order to shorten and simplify the

design process.

Cost

Cost will be an important factor when making choices. In this manual typical costs are used to

demonstrate the selection process, but the reader must take great care when using conclusions

drawn from this because local costs may vary considerably from those shown. The reader is

thus encouraged to go through the design process using local costs and to make judgements

based on local solutions. Throughout the text the unit of currency used is the United States

dollar ($US).


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Introduction4

FIGURE 1Making choices - the design process


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Chapter 2

Some basic concepts

2.1 INTRODUCTION

This chapter provides a guide to some of the basic principles which affect energy needs in

small-scale irrigation. SI units the International Metric System are used throughout the

text. Reference is made to other units where appropriate, because it is an unfortunate fact of

life that many different systems are in use in irrigation, and sometimes it can be confusing and

lead to serious mistakes.

The fundamental units in the SI systems are:

Measurement Unit Symbol

Length metre m

Volume cubic metre m3

Mass kilogramme kg

Force newton N

2.2 PRESSURE

Pressure is a commonly used term, but it does have a special meaning in hydraulics. It describes

the force exerted by water on each square metre of some object submerged in water. It may be

the bottom of a tank, the side of a dam, or a pipeline.

Pressure is normally measured in kilonewtons per square metre (kN/m2). An alternative to

this in irrigation is the bar, where 1 bar is equal to 100kN/m2. Pressure is calculated by:

Thus pressure is force per unit area (Figure 2).

Pressure (kN/m2) =force (kN)

area (m2)

FIGURE 2

Relationship between force and pressure


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Some basic concepts6

Pressure measurement

Pressure in pipes can be measured using a

bourdon gauge (Figure 3). Inside the gauge isa curved tube of oval section, which tries to

straighten out when the system is under

pressure. The tube is linked to a pointer which

moves across a graduated scale and indicates

pressure. Irrigators normally measure pressure

in the field using these gauges as they are robust

and simple to use.

However, engineers often refer to pressure

as a head of waterin metres (m) rather than a

pressure in kN/m2. If the bourdon gauge was

replaced with a long vertical tube, the waterpressure in the pipe would cause water to rise

up the tube. The height of this water column

is a measure of the pressure in the pipe. For

example, a pressure of 3bar in the pipe would

result in water rising to a height of 30m in the

tube. Thus, engineers may refer to the pressure

as 3bar or 30m head of water.

EXAMPLE 1

Calculate the pressure when a force of 10 kN is applied to an area of 2 m2.

- We know that Pressure = force / area, so P= 10 / 2 = 5 kN/m2.

If the area is increased to 4 m2, what will be the nre pressure?

- P= 10 / 4 = 2.5 kN/m2.

Thus the force has remained the same but the pressure is reduced by spreading the force over

A typical operating pressure for a sprinkler system is 3bar pressure, or 300kN/m2. This

means that every square metre of the inside of the pipes and pump has a uniform force of

300kN acting on it. Other common units of pressure are kilogrammes-force per squarecentimetre (kgf/cm2) and pounds-force per square inch (lbf/in2).

For conversion from one unit to another:

1 bar = 14.7 lbf/in2 = 1 kgf/cm2 = 100 kN/m2

FIGURE 3

Measuring pressure in a pipe

In this manual both the termspressure and headare used to mean the same thing.

Head of water in metres (m) = 0.1 x pressure (kN/m2

) = 10 x pressure (bar)

It is simple to change from pressure to head of water:


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Atmospheric pressure is important to the understanding of suction when pumping water

(Section 3.3.3) and particularly its effects on the efficiency of pumping (Section 3.3.5).

2.3 DISCHARGE

The speed at which water flows in a pipe or channel is called the velocity and is measured inmetres per second (m/s). The discharge is the volume of water flowing along the pipe or

channel each second, and is measured in cubic metres per second (m3/s). To understand this,

consider the case of water flowing in a 100mm diameter pipe at a velocity of 1.5 m/s (Figure4).

In one second the quantity of water moving past some point in the pipe will be equal to the

shaded volume shown. This volume is numerically equal to the water velocity multiplied by

the cross-sectional area of the pipe, i.e., 1.5 0.008 = 0.012m3/s.

Importance of Pressure

Pressure is important to the successful operation of both sprinkler and trickle irrigation.

Sprinklers must be operated at the right pressure so that the water jet breaks up properly and a

uniform water application is achieved (Section 3.5.2.). The right pressure is also required in

trickle systems so that each emitter gives the same discharge throughout the scheme

(Section3.5.3).

Atmospheric pressur e

Atmospheric pressure is the pressure of the atmosphere around us, pressing down on our bodies

and the surface of the earth. Although air seems very light, when there is a large depth, as at the

earths surface, it creates a pressure of approximately 100kN/m2. This is equivalent to lbar or

10m head of water.

Atmospheric pressure = 100 kN/m2 = 1 bar = 10 m head of water

In general terms:

Discharge (m3/s) = cross-sectional area of pipe (m2) x velocity of water (m/s)

FIGURE 4

Calculating discharge


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For most small irrigation systems the unit of discharge (m3/s) is much too large and so litres

per second (l/s) is very often used. The conversion is made by multiplying by 1000.

FIGURE 5

Measuring discharge

Figure 5-A

Figure 5-C

Figure 5-B

Discharge (l/s) = discharge (m3/s) x 1000


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2.4 ENERGY

Energy is another word commonly used in everyday language, but in hydraulics and irrigation

it has a very specific meaning: Energy enables useful workto be done

People and animals require energy to do work. This is obtained by eating food and converting

it into useful energy for work through the muscles of the body.

In irrigation, energy is needed to lift or pump water. Water energy is supplied by a pumping

device driven by human or animal power, or a motor using solar, wind or fossil fuel energy.

Energy measurement

Energy is normally measured in units of watt-hours. One watt-hour is a very small amount of

energy and so engineers tend to use a larger unit, the kilowatt-hour (kWh) instead, where

1kilowatt-hour = 1000watt-hours.

Here are some examples of energy use which may be familiar to the reader and which will

provide some practical indication of energy use:

A farmer working in the field uses 0.2-0.3kWh every day.

An electric desk fan uses 0.3kWh every hour.

An air-conditioner uses 1kWh every hour.

Notice how a time period (e.g., every hour, every day) is always given when describing the

amount of energy needed. The farmer using 0.2 kWh every day, for example, indicates that thisenergy must be supplied from food each day otherwise he or she would not be able to work

properly. In irrigation, energy requirements may be determined on a daily, monthly or seasonal

basis.

Discharge measurement

Discharge in a pipeline can be measured using a flow meter (Figure 5-A). The meter indicates

the volume of water passing through the pipeline. By noting the time for a given volume of

water to pass the discharge can be determined using the formula:

Discharge (m3/s) = volume of water (m3) / time (s)

A simple way of measuring discharge from a pipe or sprinkler is to catch the flow in a

bucket of known volume, measuring how long it takes to fill (Figure 5-C). The discharge is

calculated using the above formula. See Example 2.

Discharge in open channels can be measured using a weir or flume measuring structure(Figure 5-B). If no measuring structure is available, a rough guide can be obtained by estimating

the velocity of flow using a float; measuring the cross-sectional area of the channel; and

multiplying the velocity and the area together. (See Training Manual 7: Canals)

EXAMPLE 2

A small plastic tube is connected to a sprinkler nozzle to collect water in a bucket. If the bucket

holds 5 litres and it takes 15 seconds to fill, calculate the sprinkler discharge.

Discharge = volume / time = 5 / 15 = 0.33 l/s


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Energy sour ces

Energy comes from food, in the case of animals and people, and from fossil fuel, wind and

sunshine in the case of engines and motors.

Foods have energy values which our bodies convert into useful energy so that we can do

useful work. In the same way fossil fuels, wind and sunshine have energy which can be converted

into useful energy to pump water.

Table 1 gives some indication of energy values for typical foods, fossil fuels and energy

sources.

TABLE 1

Energy content of fuels and foods

Changing energy

An important aspect of energy is that it can be changed from one form of energy to another.

People and animals can convert food into useful energy to drive their muscles (Figure 6). In a

typical pumping system powered by a diesel engine, the energy is changed several times beforeit is usefully used by the water. Chemical energy contained within the fuel (diesel oil in this

case) is burnt in a diesel engine to produce mechanical energy. This is passed to the pump via

Fuel or

food

Energy Indicative

efficiency (1)Comment

Maize

Wood

Diesel

Petrol

Wind

Solar

1 kWh/kg

4 kWh/kg

11 kWh/l

9 kWh/l

0.01-41 kWh/m2

1 kWh/m2

10%

10%

20%

10%

20%

5%

As animal and human consumption

Sometimes also expressed as fuel consumption

(0.09 l/kWh for diesel and 0.11 l/kWh for petrol)

For wind speeds from 2.5 to 40 m/s respectively

Maximum solar energy at sea level

Note: 1. Approximate efficiency when converted to mechanical power.

FIGURE 6

Energy conversion - analogous systems in people (top) and machines (bottom)


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a drive shaft, and finally to the water. Thus the discharge, pressure or both can be increased. A

pump can be thought of as a device for putting additional energy into a water system.

The system of energy transfer is not perfect and energy losses occur through friction between

the moving parts and are usually lost as heat energy (the human body temperature rises when

work hard; an engine heats as fuel is burnt to provide power). Energy losses can be significant

in pumping systems, and so can be costly in terms of fuel use. This concept is discussed further

in Section 2.6.

Calculating energy requi rement

The amount of energy needed to pump water depends on the volume of water to be pumped and

the head required and can be calculated using the formula:

Increasing either the volume of water or the head will directly increase the energy required

for pumping.

2.5 POWER

Poweris often confused with the term energy. They are related, but they have different meanings.

Energy is the capacity to do useful work whereas power is the rate at which the energy is used.

Water energy (kWh) =volume of water (m3) x head (m)

367

EXAMPLE 3

600 m3 of water is pumped each day to a tank 10 m above ground (Figure 7). Calculate the

amount of energy reguired to do this.

Water energy (kWh) = (600 x 10) / 367 = 16.3 kWh.

This is the energy required each day.

FIGURE 7

Illustration of the problem considered in Example 3


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Power is the rate of using energy and is commonly measured in kilowatts (kW). The

power needed to pump water is called water power.

Another commonly used measure of power is horse power (HP). As it is not part of the

metric units system it will not be used in this manual. However, if comparison is needed the

relationship is 1kW = 1.36HP.

An air conditioner may have a power rating of 3kW. This means that it uses 3kWh of

energy every hour. In 24 hours it will consume 72kWh (3kW 24h) of energy at the rate of

3kW every hour. Thus, power is describing the rate at which the energy is used. The greater

the energy use rate the greater is the power need (Figure 8).

Another way of calculating power and energy is to use the pump discharge rather than

the volume of water to be pumped.

In this case the water power required can be calculated by first using the formula:

Figure 9 is a graph of this formula and from which water power can be obtained.Energy can then be calculated from power. It is the amount of power used in a given

time period and so:

Power (kW) =energy (kWh)

time (h)

EXAMPLE 4

In Example 3 it was calculated that the water energy required each day to lift 600 m 3 of water

through 10 m was 16.3 kWh. Calculate the water power needed to do this.To calculate water power from water energy it is necessary to know the time over which pumping

takes place.

If pumping continues for 24 hours per day:Water power (kW) = energy used per day (kWh) / time (h) = 16.3 / 24 = 0.68 kW.

If the pump operates only 12 h/day:Water power = 16.3 / 12 = 1.35 kW.

If pumping is only 6 h/day:Water power = 16.3 / 6 = 2.7 kW.

Note that the water energy is the same in each case, but that the rate of using the energy - the

power - changes with the time period. More power is needed when less time is available for pumping

the same volume of water.

FIGURE 8

Relationship between rate of energy use and power

Water power (kW) = 9.81 x discharge (m3/s) x head (m)


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Example 5 demonstrates this approach and shows that the results are the same whichever

method is used to calculate power and energy.

2.6 EFFICIENCY

When pumping irrigation water it is not enough just to meet the water power and energy

requirements. Additional energy and power must be provided because losses occur in transferring

fuel energy to water energy via the power unit and pump. The losses in the system are caused

by friction and water turbulence and are usually expressed as efficiency. This can be expressed

both in terms of energy use and of power use.

FIGURE 9Graph relating flow, static head and power

Water energy (kWh) = water power (kW) x operating time (h)

EXAMPLE 5

Referring to Example 4, if 600 m3 of water is pumped 10 m each day, calculate the water power

and energy required, using the pump discharge approach if pumping is for only 6 h/day.

Discharge (m3/s) = volume (m3) / time (s) = 600 / (6 x 3600) = 0.028 m 3/s.

Using the above equations:

Water power (kW) = 9.81 x discharge (m3/s) x head (m) = 9.81 x 0.028 x 10 = 2.7 kW.

Water energy (kWh) = water power (kW) x operating time (h) = 2.7 x 6 = 16.3 kWh.

These answers are the same as those obtained in the previous example, thus demonstrating

that water power and energy can be calculated using either approach.


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Energy use eff iciency

This provides an overall indication of the way energy is used. It would usually be assessed on

a seasonal or annual basis.

Power use eff iciency

This provides an assessment of the efficiency with which power is converted from the fuel to

the water, but only at the moment of measurement. The efficiency may vary over time,

particularly if there is wear in the engine and pump.

A system with no friction would have an efficiency of 100% and all the energy and

power input would be transferred to the water. However, this is not the case in real life and

there are always friction losses in all the components of the power unit and pump. This is

discussed more fully in Section 3.3.5.

Sometimes efficiencies can be very low without pump users being aware of the problem.

This can result in excessive energy use and high pumping costs. This is an important aspect of

pumping and is discussed more fully in Chapter 5.

For the purposes of this manual, the efficiencies of energy and power use are assumed to

be the same. In practice this may not be the case. A seasonal assessment of energy use efficiency

may not always give the same value as power use efficiency measured only once or twice

during the season. Note that, in calculations using efficiencies, we always use the decimal

form [(efficiency in %)/100] of the value.

Pumping plant efficiency (%) = (water energy / actual energy) x 100

Pumping plant power efficiency (%) = (water power / power input) x 100


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Chapter 3

Choosing a new irrigation system

3.1 INTRODUCTION

Choosing a new irrigation system is about choosing the various components which make up the

system. In this chapter the main components are listed, and guidance is given in how to choose,

for preliminary design purposes, between the various options and component configurations

available.

Figure 10 illustrates the process of preliminary design and the decisions to be made.

Each part of the process is described in this chapter.

Small-scale pumped irrigation systems are made up of the following components (Figure11):

Water source;

Pump and power unit;

Distribution system; and Method of irrigation.

FIGURE 10

Choosing irrigation system components


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Choosing a new irrigation system16

FIGURE 11

Components of a typical irrigation scheme


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The water source, the distribution system and the method of irrigation determine

the energy demand.The pump and power unit provide the energy supply.

Water sour ce

The water source may be a river or lake (surface water) or a shallow well or borehole

(groundwater). In some cases, water can be abstracted from rivers by gravity, but in many

cases pumping will be needed. In the case of groundwater abstraction, pumping is essential.

(See also Training Manual 6: Scheme irrigation water needs and supply.)

The amount of water abstracted and the height through which it must be lifted from the

river or borehole add to the energy demand.

Pump and power uni t

The pump may be driven by a power unit such as a diesel or petrol engine, or an electric motor.

In some special cases solar or wind power, or even hand or animal power, may be used to

provide the power source for the pump, but they are not so common and are generally limited

to very small irrigated plots. In this manual the primary concern is with the use of pumps

driven by diesel or petrol engines, as these are usually the main sources ofenergy supply

available to most small-scale farmers.

Distr ibut ion system

The distribution system conveys water from the pump to the fields and may consist of pipes oropen channels. Some systems are a combination of both. The choice of distribution system has

a significant effect on the energy demand.

Method of ir ri gation

The method of irrigation may be surface, sprinkler or trickle irrigation. This may also affect

the choice of distribution system and is also significant in determining the energy demand.

Surface irrigation may be supplied by either pipe or open channel systems. Sprinkler and

trickle irrigation systems would normally use piped distribution systems. (See also Training

Manual 5:Irrigation Methods.)

Typi cal systems

The most common combinations of components for an irrigation system are:

Pump open channel surface irrigation.

Pump pipe supply surface irrigation.

Pump pipe supply sprinkler or trickle irrigation.

The first system is the most common for small-scale irrigation, although the advantages of

the second are now being more fully realized. Sprinkle, and especially trickle, irrigation are

growing in importance in some areas where soils are very sandy and water is scarce, or energy

costs are high, or both, but surface irrigation is the dominant method and is likely to remain so

in many countries for the foreseeable future.


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3.2 WATER SOURCES

Rivers and lakes

Many small irrigation schemes are located close to natural river channels and lakes and obtain

water by pumping from these sources. They provide a supply which can be seen by the farmer

and be judged whether sufficient or not for the seasonal needs of the farm. Usually, the pumping

pressures , and hence energy requirements, needed to use such sources are small because the

difference in elevation between the source water level and the level of the field are usually not

large.

Shallow groundwater

This is an ideal source of supply for farms located some distance from a river or lake. Usually

the groundwater table is fed by seepage from a river or lake and may be only a few meters

below ground level. This source may be less reliable than surface water because except through

pumping experience there is no easy way of assessing whether there is a sufficient reserve of

water to ensure adequate irrigation. However, the farmer can save the cost of an expensive

canal or pipe system to bring water from a more distant surface supply.

As with surface supplies, the energy costs involved in pumping are relatively low.

Deep gr oundwater

This may be water which has permeated through the ground from a surface source many

kilometres away or water which has been trapped in the ground by impermeable soils for many

thousands of years (fossil water).

Pumping deep groundwater which may be 20 - 100m or more below ground level can be

expensive in terms of energy use, as well as in the cost of drilling the borehole, and requires

special, deep borehole, pumping equipment, which may also be expensive to buy.

3.3 PUMPS AND POWER UNITS

A pump is a machine which changes fuel energy into useful water energy and needs a petrol or

diesel engine or an electric motor to drive it. In special circumstances it may also be possible to

use wind or solar energy. For surface irrigation the pump lifts water from a river or groundwater

into a channel or pipe system. For sprinkler and trickle irrigation the pump provides the energyfor the pressure and discharge needed to distribute water in the pipes to the sprinklers and

emitters, in addition to the energy needed to lift water from the source.

3.3.1 Pump types

Although there are many types of pumps and water lifting devices, many are unsuited to irrigation

use. The most commonly used types are the axial flow (or propeller) pump, the radial flow (or

centrifugal) pump, and the mixed flow pump. These are looked at in detail below.

Axial flow pump

An axial flow pump consists of a propeller hence its alternative name housed inside atube which is located below the water level (Figure 12). The tube acts as the discharge pipe,

and the power unit turns the propeller by means of a long shaft running down the middle of the


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pipe and this lifts the water up the pipe. This pump is very efficient for lifting large volumes of

water at low pressure and is ideally suited to lifting water from a river or lake to provide surface

irrigation water to a farm with open channel distribution. However, these pumps tend to be

very expensive because of the high cost of materials, particularly the drive shaft and bearings to

support the shafted propeller. For this reason there are no small axial flow pumps manufactured

of a size suitable for the small farm of 1 - 2ha. They tend only to be used on larger farms and

for communal schemes, where several small farms are irrigated from the same pump. They are

particularly suited to paddy rice schemes because of the large volumes of water usually needed

for this crop.

Radial fl ow (centri fugal) pump

Centrifugal pumps are the most common type of pump used on small schemes because they are

much cheaper than axial pumps to buy and maintain. Small pump sets are often readily available

in most developing countries (Figure 13). They are best suited to sprinkler and trickle irrigation,

where a higher pressure is needed than for surface irrigation.

To understand how a centrifugal pump works, consider first how centrifugal forces occur.

Most readers will at some time have spun a bucket of water around at arms length and observed

that no water falls from the bucket even when it is upside down (Figure 13). Water is held in

the bucket by the centrifugal forces created by spinning the bucket. A centrifugal pump makes

use of this idea and can be thought of as many buckets all spinning around together. Thebuckets are replaced by an impellerwith blades or vanes which spin at high speed inside the

pump casing (Figure 13). Water is drawn into the pump from the source of supply through a

FIGURE 12

Typical axial flow pump


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FIGURE 13

Radial flow (centrifugal) pump


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short length of inlet pipe called thesuction pipe. As the impeller spins, water is thrown outwards

and is collected by the pump casing and guided towards the outlet. This is called thedelivery.

Some pumps have very simple impellers with

straight vanes. These tend to be inefficient because

they create a lot of turbulence in the flow and hence

energy losses. However they are cheap to make and

are used in cases where efficiency is not important.

Most irrigation pumps have curved vanes so that the

water enters and leaves the impeller smoothly. This

means lower energy losses and higher energy use

efficiency. Some impellers have side plates and are

called closed impellers. When there is debris in the

wateropen impellers are used to reduce the risk ofblockage.

Centrifugal pumps can be classified into two

types: volute pumps, and turbine (diffuser) pumps. The main difference between them is that

the turbine type has diffuser vanes, which provide diverging passages to direct the water flow.

Centrifugal pumps are often described by the diameter of the delivery connection pipe,

e.g., a 50mm pump. Table 2 is a guide to selecting centrifugal pump sizes for different flow

ranges.

Mixed flow

This pump is a mixture of the axial flow and the centrifugal pump and has the advantage of

combining the best features of both pump types (Figure 14). Mixed flow pumps are more

efficient at pumping larger quantities of water than centrifugal pumps and are more efficient at

pumping to higher pressures than axial flow pumps.

They can also operate as submersible pumps, i.e., being completely below the source

water surface.

FIGURE 14

Typical mixed flow pump

Pump size (mm) Discharge (l/s)

255075100125

0 - 55 - 1515 - 2525 - 3535 - 50

TABLE 2

A guide to selecting centrifugal pumps


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3.3.2 Pump Characteristics

Axial flow, centrifugal and mixed flow pumpsare designed to run at a constant speed and

their performances are described by the fol-

lowing characteristics:

Head and discharge;

Power requirements; and

Efficiency of operation.

Typical characteristics for operating head

and discharge for the three pump types appear

as Figures 15-A, 15-B and 15-C. They show

how head, power and efficiency vary as the

discharge changes. For example, when the

head requirement is 120% of the design head

value, discharge is reduced to 60%, 80% and

90% of design discharge for centrifugal, mixed

flow and axial flow pumps respectively.

Head and discharge

Pumps can deliver a wide range of discharges depending on the pressure required and the speed

at which the pump is operated. However, there is a trade off between head and discharge. If

more discharge is needed the head drops, and if less discharge is needed, then the head rises. Adifferent set of curves would be obtained if the pump was running at a different speed. The

faster it runs the greater the head and the discharge.

FIGURE 15-A

Pump characteristics: discharge - head

FIGURE 15-B

Pump characteristics: discharge - power

FIGURE 15-C

Pump characteristics: discharge - efficiency


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Power

All pumps need power to rotate the impeller. The amount of power needed depends on thespeed of the pump and the discharge that is required. The faster the pump rotates, the more

power is needed.

For axial flow pumps there is a very large power demand as the pump is starting because

there is a lot of water and a heavy pump impeller to get moving. Once the pump is under way

the power demand drops to its normal running level.

Centrifugal pumps behave quite differently. The power demand is very low when starting,

but as the discharge increases the power also gradually increases.

Mixed flow pumps operate in between these two contrasting conditions and have a more

uniform power demand over the discharge range.

Efficiency

The concept of efficiency was first developed in Section 2.6. It measures how well the mech-

anical energy and power from the power unit is converted into water energy and power in the

pump. The pump power efficiency is calculated by:

The efficiency generally increases to some maximum value and then falls again over the

discharge range. The maximum efficiency is usually between 30 - 80% and there is only a

limited range of discharges and heads over which the pumps operate at maximum efficiency.Outside this range the pump will be less efficient and so more power and energy will be needed

to operate the system. Smaller pumps tend to operate at lower efficiencies than larger ones

because they have more friction to overcome relative to their size.

3.3.3 Pump selection

There are many pumps on the market and the designer must try to select a pump which will

provide the discharge and head needed for the scheme while the pump is operating within its

maximum efficiency range.

Table 3 indicates the range of good operating conditions for different pump types.

TABLE 3Pump selection for small-scale schemes

Note: 1. The ideal pump type, but not usually available for small-scale farming.

A large number of irrigation schemes use surface irrigation and open channel distribution

pumping from shallow water supplies. This situation is ideal for axial flow pumps but

unfortunately few, if any, pumps are available at a reasonable price for the small discharges

Pump power efficiency (%) = (water power output / actual power output) x 100

Irrigation system Pressure or Head(bar) (m)

Discharge(l/s)

Pump type

Surface irrigation- open channel distribution- pipe distribution- deep tube wellSprinkler systemTrickle system

0.5 51.0 10

>2.0 >202 - 6 2 - 601 - 2 10 - 20

any dischargeany dischargeany dischargeany dischargeany discharge

axial1 or mixedaxial1 or mixed

mixed or centrifugalcentrifugalcentrifugal


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required on many farms. The only alternative is to use centrifugal pumps instead and accept

that they will run at well below their peak efficiency (Figure 16).

For sprinkler and trickle irrigation much higher pressures are needed and so centrifugal

pumps are ideally suited to this use and will operate more efficiently.

A typical example of pump selection using the data supplied by a manufacturer would be

as follows:

3.3.4 Power units

There are two main types of power unit: internal combustion engines, and electric motors.

I nternal combustion engines

Many small irrigation schemes do not have access to electricity and so rely on petrol (spark

ignition) engines or diesel (compression ignition) engines to drive the pumps. These engines

have a good weight:power output ratio, and are compact in size and relatively cheap due tomass production techniques.

FIGURE 16

Pump selection based on head and discharge parameters

EXAMPLE 6

A centrifugal pump is required for a small sprinkler irrigation system. The discharge required is12 l/s, at a pressure of 2 bar. Using the information supplied by the manufacturer (see Figure 17),determine the pump efficiency.

If the same pump was to be used to pump water into an open channel and the pressure neededfor this was only 1 bar, show what effect this would have on the pump discharge and the efficiency.

From Figure 17, the efficiency of the pump at a discharge of 12 l/s and pressure of 2 bar (20 m ofhead) is 52%. This is within the high efficiency zone of the pump.

If the pressure required was only 1 bar (10 m of head) the discharge would increase to 18 l/s, butat the much reduced efficiency of only 12%.

Thus, using an inappropriate pump for the surface irrigation option has a significant effect on theefficiency of pumping.


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Diesel engines tend to be heavier and more robust than petrol engines and are more

expensive to buy. However, they are also more efficient to run and if operated and maintained

properly they have a longer working life and are more reliable than petrol. In some countries

petrol-driven pumps have needed replacing after only 3 years of operation. Diesel pumps

operating in similar conditions could be expected to last at least 6 years. However, it must not

be forgotten that engine life is not just measured in years, it is measured in hours of operation

and its useful life depends on how well it is operated and serviced. There are cases in developing

countries where diesel pumps have been in continual use for 30 years and more.

A diesel-engined pump can be up to four times as heavy as a petrol-engined pump of

equivalent power, and so if portability is important a petrol pump may be the answer.

Electri c motors

Electric motors are very efficient in energy use (75 - 85%) and can be used to drive all sizes and

types of pumps. The main drawback is the reliance on a power supply which is beyond the

control of the farmer, and which in many places is unreliable. Inevitably electrical power

supplies usually fail when they are most needed. Heavy demands occur when crops are needing

most water and so a power failure over several days can have disastrous consequences for a

crop. When using trickle irrigation on light sandy soils, serious crop losses may well occurafter only a few days without power.

FIGURE 17

Manufacturers data for a centrifugal pump


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3.3.5 Efficiency

The efficiency of power units and pumps is very variable, and few data are available on actualfield performance of small-scale irrigation pumping installations. The data that are available

indicate that efficiencies are very low, in the range 0.5 to 8%, and that such poor levels are quite

common.

Many of the common causes of low efficiency can be corrected at little cost once the

problem is identified, but unfortunately it is easy to run an inefficient pumping system without

even realizing it. Any shortfall in output is simply made up by running the system for longer

than would otherwise be necessary.

Pumping efficiencies are likely to be much higher for sprinkler and trickle systems as the

head needs of these systems are more favourable to the hydraulic characteristics of centrifugal

pumps.Figure 18 shows the main components of a small pumping system and the poor efficiencies

that can commonly occur. The main reasons for inefficiency are listed below. Note that improved

efficiency can be achieved by rectifying the common faults.

Fuel efficiency 90-100%. Fuel is often spilt or leaks from tanks, or from joints in the

fuel pipeline.

Power unit efficiencySmall petrol engines (1kW) 10%.

Small diesel engines (1.5 - 2kW) 15-35%.

Large diesel engines 30-40%. (Text books normally quote 30-40% for engines

but these are optimistic. Ageing of engine, poor quality maintenance, excessive

power consumed by cooling fans, injectors, etc., all bring down efficiency.)

Electric motors have much higher efficiencies 75-85% but a reliable electricity

supply may be difficult to obtain in many locations.

FIGURE 18

Efficiency of components of pumping plant


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Power unit to pump transmission If the engine and pump are direct coupled, then

transmission efficiency nears 100%.

Pump efficiency A pump running at optimum head and speed has an efficiency of between40% and 80%. Many pumps are not run at optimum head and speed, and so their efficiency

could be much lower. This is particularly true for small pumps where the frictional

losses are a higher proportion of the total power requirement.

The overall efficiency of the pumping system can be found by multiplying together the

efficiencies of each component:

Note that in any calculation of this type the decimal equivalent of the percentage is used,i.e., an efficiency of 10% becomes 0.1 in the calculation, 20% becomes 0.2, and so on.

Taking the worst and best possible combinations of all the above efficiencies provides

some indication of the most likely range of overall efficiencies:

This implies that the worst likely efficiency is around 3%. Even this seems good when

compared to the actual field measurements of 0.5% referred to earlier in this section!

Although an efficiency of 30% might be expected from a centrifugal pump operating a

sprinkler or trickle system, it is unlikely to reach this level of efficiency for surface irrigation.

The best that can be achieved would be around 10%.

Peak power demand

The water power and overall efficiency of the pumping plant are used to calculate the overall

power demand.

Developing the formula from Section 2.5:

Pumping plant efficiency (%)= fuel efficiency x power unit efficiency x transmission efficiency x pump efficiency x 100

EXAMPLE 7

Worst condition = 0.9 x 0.1 x 0.9 x 0.4 x 100 = 3%Best condition = 1.0 x 0.35 x 1.0 x 0.8 x 100 = 28%

Overall power demand = water power (kW) / pumping plant efficiency

Overall power demand (kW) =9.81 x discharge (m3/s) x head (m)

pumping plant efficiency

EXAMPLE 8

A small diesel-driven pump delivers a discharge of 2 l/s when lifting water 3 m from a river.

Calculate the peak power demand when the overall efficiency of pump and power unit is 10%.

Peak power demand = (9.81 x 0.002 x 3) / 0.1 = 0.59 kW

Note that the discharge of 2 l/s must be divided by 1000 to convert it into m3/s.


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Pump suction

An aspect of using centrifugal and mixed flow pumps which is not always fully understood,

and which can seriously impair efficiency, is the suction side of the pump.

In cases of shallow groundwater or surface water pumping, the pump is located above

the water surface and water has to be sucked up a short length of pipe into the pump, as shown

in Figure19. The difference in height between the water surface and the pump is called the

suction lift.

When a pump is operating it draws in water in much the same way as a person sucks

water up through a drinking straw. There is a limit to how high water can be lifted in this way

and it depends on atmospheric pressure (Section 2.2). At sea level this is approximately 10m

head of water. Sucking creates a low pressure in the drinking straw and the outside pressure of

the atmosphere pushes down on the water surface and forces water up the straw. As atmospheric

pressure is the driving force, this puts a practical limit on the height to which water can be lifted

in this way.

Ideally it should be possible to lift water 10m, but because of friction losses in the pipe

and pump a practical limit is 7m. Even at this level many pumps will have difficulty sucking

water. Considerable energy will be needed to suck the water and the pump operator may have

difficulty keeping the pump primed (i.e., keeping the pump and suction pipes full of waterwhen starting the pump). For this reason, pumps should be located so that the suction lift is less

than 3m if possible. If the depth to the water is greater than 3m, then a small shelf can be

excavated and the pump located nearer to the water surface (Figure 19).

Note that these rules only apply when operating in areas close to sea level. Here the

atmospheric pressure is approximately 10m head of water. For schemes operating at higher

altitudes in mountainous regions the atmospheric pressure may be much lower than 10m and

so the suction lift will need to be reduced well below 3m to ensure proper pump operation.

However, not all pumps suffer from suction lift limitations. Pumps designed to work

below the water surface submersible pumps have no such problems.

An example of the effects of variations in suction lift on pump discharge is given by thecase of a small centrifugal pump, which delivered 6.5l/s when operating at 3m suction. When

the suction lift was increased to 8m the discharge dropped to 1.2l/s a loss in flow of 5.3l/

FIGURE 19

Suction lift limitations


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s, or a loss of 85% of the original discharge! Thus, at the greater suction lift the pump would

have to be operated considerably longer to meet water demand, and at such a low flow rate thepump may be well away from its best operating efficiency. This example was cited by Wagner

and Lanoix1 (1969).

3.4 DISTRIBUTION SYSTEMS

The distribution system conveys water from the pump to the fields. This may be by open

channels or through pipes. The choice of distribution system affects both the power and energy

requirements.

3.4.1 Open channels

The most common method of distribution is through open channels, which may be lined or

unlined. Channel design affects the energy demand of the system in three ways:

by determining the energy requirement to lift water from its source into the channels;

by influencing energy losses resulting from friction between the water and the canal; and

by influencing the extent of any additional energy required to pump water which is lostthrough seepage, canal breaches and misuse.

Water will only flow downhill in open channels and so the layout of canals should ensure

that the highest point in the canal system is near to the pump and water source. In this way

water will then flow downhill under the force of gravity and out onto the fields. Sufficient

power must be provided in this case to lift water from its source into the channels (Figure 20).

The head required is determined by the difference in level between the water source and the

water level in the channel. The water level in the channel at the source must be high enough toensure an adequate flow of water to the field, and must include adequate head to allow effective

flow from the channel to the field.

Large water losses can easily occur in open channels. This may be due to seepage

through the bed and sides of a channel. However, open channels, particularly unlined ones, are

prone to breaching, whereupon considerable amounts of water can be lost. They are also easily

FIGURE 20

Energy demand for open channel distribution

1 Wagner, E.G. & Lanoix, J.N. 1969. Water Supply for Rural Areas and Small Communities. Geneva:

World Health Organization.


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misused. Channels may be left open, particularly when control gates are not working properly,

and water runs to waste. These features of open channels mean that considerable amounts ofwater may be pumped which are wasted, using additional energy and fuel for which there is no

benefit in terms of additional crops.

Of course channels can be lined to reduce seepage, but this requires additional capital

expenditure. A choice must then be made between the additional cost of lining and the cost of

pumping the water which would be lost through seepage. This involves a comparison between

capital expenditure and operating costs, which is discussed later in Section 3.8.3.

Lining canals can often seem an attractive way of reducing seepage losses. It can also

reduce maintenance costs and improve irrigation system distribution efficiency. However, if

linings are to be successful they must be constructed with great care. A concrete lining, for

example, needs to be well vibrated as it is poured so as to be impermeable, and must be placed

on channel beds and banks that have been well compacted. If settlement occurs after construction

and the lining cracks, then not only will seepage losses be high but the cost of the specialist

repairs will also be significant.

Water losses in channels for typical irrigation schemes expressed in terms of efficiency

are shown in Table 4.

Channel hydraul ics

Most irrigation channels excavated in the natural soil are trapezoidal in shape and slope downhillaway from the water source. Channels usually follow the natural ground slope but if the land is

steep, then drop (or fall) structures may be needed to avoid serious erosion problems (Figure

21). Channels with longitudinal bed slopes of less than 1:1000 will usually avoid serious

erosion problems, but a minimum slope of greater than 1:5000 is needed to discourage siltation

and plant growth problems.

Channels which are lined may be trapezoidal but can also be rectangular or semi-circular.

The main aspect of channel design is choosing the bed width and depth of flow. This can

present some difficulties because choosing a value for one affects the other. Thus channel

design is a little more complicated than pipe design because pipes are always circular and so

only one value is chosen the pipe diameter. The reader must look to other texts for the

detailed design of channels, but as guidelines:

FIGURE 21

Channel design: dimensions and drop structures


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Unlined channels are designed so that the velocity is low and the bed and sides are not

eroded by the water. For this reason, unlined channels tend to be wide and shallow, spreadingthe flow over a large area to reduce the erosive power of the water.

Lined channels are expensive to construct. For this reason they tend to be narrow and deepwhich ensures the minimum area of lining for a given channel carrying capacity. The velocity

also tends to be high, but this is not usually a problem as the channel is protected from

erosion by the lining.

3.4.2 Pipelines

Pipelines are often thought to be too expensive for many small irrigation schemes except when

sprinkler or trickle irrigation is used, as then the use of pipes is essential. However, expensive

is a relative word and does not convey a specific meaning. It may well be that when the fulloperating advantages of pipes are considered they may be a viable alternative to open channels.

For small-scale surface irrigation schemes, recent research has shown many advantages

for piped distribution systems:

Very low distribution losses even less than lined channels, as it is much easier to closeoff the flow in a pipe than in an open channel (See Table 4 for water losses in pipelines

expressed as an efficiency).

Less land area is taken up by buried pipes. Channels can take up 0.5-2% of the commandarea.

Pipes can often be installed at lower cost than lined canals.

Pipe systems can provide a more flexible and reliable system of supply.

Reduced contact with water has potential health benefits.

Pipelines for surface irrigation usually operate at low pressures, typically around 0.5bar

(5m of head).

Pipelines are essential for the use of sprinkler and trickle irrigation, and they need to

operate at much higher pressures (typically 2 - 6bar for sprinkler and 1 - 2bar for trickle

systems) and need to be strong enough to withstand up to twice the working pressure. The

reason for this is that pressure surges which are much greater than the normal working pressure

can occur in pipes, and can cause bursts. It is thus important to install a pipe with the correctpressure rating to avoid the expense of repair or even replacement of a complete system.

Energy is needed in pipe systems not only to pump water from the source to the pipe but

also to overcome the energy losses due to friction as water flows down the pipe (Figure22). If

surface irrigation is used, then water can flow freely from the pipe into the field. If sprinkler or

trickle irrigation is used, then additional energy is needed to ensure the water sprays or drips

properly.

Predicting head losses in pipes is not an exact science, and it easy to make mistakes

when calculating them. In addition, losses can increase as the pipe ages and becomes rougher

inside through continued use. For these reasons the losses in the distribution system should be

kept low at the design stage by choosing pipe diameters that are large enough for friction to notdominate the operation of the system at some later date. As a guideline, energy losses in the

pipes should be less than 30% of the total pumping head.


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Pipeli ne hydraul ics

Energy is lost when water flows along a pipe. This is due to friction between the flowing water

and the pipe wall. This energy loss means that the pressure near the pump will always be

greater than at the far end of the pipe. The change in pressure is called thehydraulic gradient

(Figure 23). Additional power and energy must be supplied by the pump to overcome that

friction so that sufficient water is still delivered to the scheme at the right pressures.

Energy loss in pipelines can be measured as a head loss in metres (m). It depends on the

following factors:

Discharge small changes in discharge can cause very large changes in head loss.

Pipe diameter small changes in pipe diameter can cause very large changes in head loss.

Pipe length changes in pipe length cause similar changes in head loss. Increasing a pipe

length from 100m to 200m will double the head loss.

Pipe layoutthe kinds and numbers of bends and junctions.

FIGURE 22Pipe system and its energy demand energy neededto pressurize sprinklers


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Pipe materialit determines frictional resistance by its smoothness or otherwise.

Energy loss in pipes can be determined from information supplied by pipe manufacturers.

A typical nomograph for PVC pipes is shown in Figure 24. The following examples will

demonstrate effects of discharge, pipe diameter and pipe length on the head loss.

A good guide to selecting the right pipe diameter is to keep the velocity below 1.6m/s. This

is good engineering practice. It ensures that head losses are low and it will help to avoid the

surge and water hammer (sudden oscillations in water pressure) problems which can cause

pipes to burst.

Practical considerations

Different pipe materials have different friction characteristics. The example used in thistext is PVC. If other pipes are used, then values for friction head losses must be obtained

from the supplier.

The smallest diameter pipe may be the cheapest, but it is not always the best choice. Pressurelosses can be very high and so can the cost of providing the extra energy to overcome the

losses. It may be cheaper in the long term to use a larger pipe size, which may have a higher

capital cost but requires less energy in use and so has a much lower operating cost. This

issue is discussed in detail in Section 3.8.5.

Think long term when selecting pipes. Will more water be needed in the future? Will thesystem be extended? If so, investment now in a larger pipe size may save high energy costs

later when trying to pump an increased discharge down a pipe which is too small. A common

problem across the world is that farmers install pipelines which are too small. Many regret

the decision later when they see the potential for irrigation and wish to expand their system.

It is not necessary to use a pipe size which is the same diameter as the pump delivery pipe.For example, a 50mm diameter pump does not mean the farmer must use a 50mm diameter

FIGURE 23

Hydraulic gradient


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pipe. The diameter is selected according to the above guidelines and if it is different from

the pump diameter then a special section of pipe with a varying diameter (a reducer) is

simply used to connect the pump to the pipeline.

It is important to see what pipe sizes and pumps are available in the local market and to

design around this equipment. This may not always give the most efficient system from anenergy use point of view but it will mean that local support for servicing, maintenance and

repair will be available. Such an advantage may far outweigh any fuel efficiency use issues.

FIGURE 24

Nomograph relating pipe diameter, discharge, head loss and velocity


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3.4.3 Distribution efficiency

Water is not always distributed efficiently, and losses may occur from channels through seepage,

evaporation and mismanagement of the system. In the case of open channels this may involve

gates being left open when no one is irrigating, and canal banks breaching through poormaintenance. For pipe systems, there may be leakage from the joints because of poor sealing

and, again, valves may not always be closed properly. However, it is likely that pipelines have

a potentially higher efficiency than open channels. For design purposes, Table 4 indicates

typical values of distribution efficiency.

EXAMPLE 9

An irrigation scheme uses a 100 mm diameter pipeline, 130 m long, to deliver a discharge of 8 l/s.Determine the head loss.From Figure 24:

When discharge is 8 l/s through a pipe of 100 mm, head loss is 10 m/km.Therefore, over 130 m [= 0.13 km] head loss will be 10 x 0.13 = 1.3 m.

What will be the increase in head loss is the discharge is increased to 16 l/s?From Figure 24:


The increase in head loss is 4.8 - 1.3 = 3.5 m.Increasing discharge causes a large increase in head loss.

Determine the change in head loss if a pipe of 80 mm is used to deliver the same discharge [8 l/s]over the same distance [130 m].FromFigure 24:


Difference is 3.8 - 1.3 = 2.5 m, i.e. anincrease in head loss.A decrease in pipe diameter causes an increase head loss.

Determine the change in head loss if the 100 mm pipe is used to deliver the same discharge [8 l/s] over twice the distance [260 m].FromFigure 24:


Difference is 2.6 - 1.3 = 1.3 m, anincrease in head loss.An increase in pipe length causes a corresponding increase in head loss.

TABLE 4

Indicative values of distribution efficiency (%)

Scheme size (ha)

Earth canals Lined canals Pipes

sand loam clay

Large: >2 000 haMedium: 200 - 2 000 haSmall:


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FIGURE 25

Basin, forder and furrow irrigation


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3.5 METHODS OF IRRIGATION

There are three methods of irrigation commonly used on small schemes (See also Training

Manual 5:Irrigation Methods):

Surface irrigation

Sprinkler irrigation

Trickle irrigation

The main objectives of these methods are to:

Apply an adequate amount of water to meet crop needs

Apply water uniformly across the field

Ensure there are no long-term problems (e.g., soil erosion, salinization).

3.5.1 Surface irrigation

This is the most common method used on small schemes and involves flooding water across

the soil surface so that it can infiltrate into the root zone and be used by the crop. Basin

irrigation, border irrigation and furrow irrigation are all surface methods (Figure 25). The

choice of surface method depends on the crop, cultivation practices, soils and topography, and

farmer preferences.

Surface irrigation is a labour-intensive method but generally requires less energy than

other methods because of the low head required for distribution.

Although surface irrigation is considered to be a simple method of irrigation this can bevery misleading. Surface irrigation design and construction is relatively simple and little or no

imported specialist materials are needed. However, the proper management of the method is

very complex. The efficient use of irrigation water all depends on the skill of the farmer, who

must decide when to irrigate and how much to apply, and then provide the right discharge into

the field so that water infiltrates adequately and uniformly into the root zone. This is not an

easy task, as the soil and topographic conditions can be very variable and the farmer may not

have the necessary degree of control over the discharge and timing of the application

Potentially, surface irrigation can be very efficient

if all the factors involved are under the careful control

of an experienced irrigator. More often however, the

water management skills are lacking and efficiency tendsto be low. As the designer will not know the level of

field application efficiency that the farmer will achieve

once the scheme is built, a typical value is used for design

purposes (Table 5). If the actual efficiency is less than

the typical value once the scheme is operating, then the

farmer will need to operate the system for longer each

day, or to reduce the cropped area to compensate. This

fall in efficiency will increase the energy demand

(Section 5.2).

For additional information on surface irrigation see Kay (1986)1.

TABLE 5

Typical field application efficiencies

for irrigation methods

Irrigationmethod

Efficiency (%)

SurfaceSprinklerTrickle

607590

1. Kay, M. 1986. Surface Irrigation: Systems and Practice. Cranfield, UK: Cranfield Press.


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3.5.2 Sprinkler irrigation

Sprinkler irrigation involves distributing water in pipes under pressure and spraying it into the

air so that it breaks up into small droplets and falls to the ground like natural rainfall. Sprinkler

systems are generally more efficient and use less labour than surface irrigation and can be

adapted more easily to sandy and erodible soils on undulating ground. There are many types of

sprinkler system available, but the most common is a system using portable pipes (aluminium

or plastic) supplying rotary impact sprinklers (Figure 26).

An individual rotary impact sprinkler produces a circular wetting pattern with poor

uniformity. To obtain good uniformity, several sprinklers are always operated close together

so that the patterns overlap.

Pressure is an important factor in successful sprinkler operation. Typical operating

pressures range from 2 to 6bar, and so energy requirements can be much greater than for

surface irrigation. If sprinklers are working at the pressure recommended by the manufacturer

then the distribution will be good. If the pressure is above or below this value then the distribution

will be adversely affected. The most common problem is when pressure is too low and this

happens when pump and pipes wear, increasing friction and so reducing pressure.

Typical data for rotary impact sprinklers are shown in Table 6.

It is usually assumed that sprinkler irrigation is more efficient than surface irrigation.

Potentially this is the case, but it largely depends on how well the system is operated and

maintained. If pipe seals leak or burst, and if sprinklers are left running for longer than necessary,

then wastage is inevitable. For design purposes, a field application efficiency of 75% is generally

used.

Traditional sprinkler irrigation is not so well suited to small farms. Typical spacings forsprinklers are 18m 18m, and so they are not so flexible and adaptable to the multitude of

small plots usually found on many farms. An alternative which may be more applicable to

FIGURE 26

Sprinkler irrigation


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small farms is the use of smaller sprinklers connected to the mainline by flexible hoses (Figure27). This is often called a hose-pull system. These sprinklers have great flexibility in operation

and are easily re-located around the farm.

For fuller details of the methods, their design and management the reader should refer to

standard text books and other publications3.

3.5.3 Trickle irrigation

Trickle irrigation involves dripping water onto the soil at very low flow rates (2-20l/h) from asystem of small diameter plastic pipes fitted with outlets calledemitters. Water is applied close

to the plants so that only the part of the soil volume in which the roots develop is wetted.

Applications are usually frequent (every 2-3 days) and this can provide a favourable high moisture

level condition in which the plants can flourish. Many other claims are made about the method,

including increased crop yields, greater efficiency of water use, possible use of saline water,

reduced labour requirements and its adaptability to poor soils. An important advantage is the

ease with which nutrients can be applied with the irrigation water. The relative importance of

each of these attributes will vary depending on the situation.

A typical trickle irrigation system is shown in Figure 28.

TABLE 6

Typical sprinkler data

FIGURE 27

Hose-pull sprinkler system

1. Two publications for further reading are: FAO/ILRI. [1988]. Irrigation methods. Irrigation Water

Management Training Manual5. Kay, M. 1983. Sprinkler Irrigation: Equipment and Practice.

London: Batsford.

Nozzlediameter

(mm)

Pressure(bar)

Diameter ofwetted circle

(m)

Flow(m3/h)

Application rate (mm/h) for spacings:

18 x 18 m 18 x 24 m 24 x 24 m

456810

3.03.03.04.04.5

2932354348

1.021.672.444.968.13

3.25.27.515.325.1

..3.85.711.418.9

..

..4.28.614.0


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Trickle irrigation is potentially a very efficient method of applying water to crops. Field

application efficiency can be as high as 90%, but like any other method it relies very much on

the skill of the irrigator to achieve this. Field measurements on trickle systems have shownapplication efficiencies as low as 25%. This was the result of poor system management rather

than design. The farmers had not fully understood the concept of partial wetting of the root

zone and so they wasted a lot of water trying to wet up the entire area.

Because of the potentially higher efficiency and the operating pressure of only 1-2bar

this method can use less energy than sprinkler irrigation and in some cases less than surface

irrigation.

Trickle irrigation is very adaptable to small-scale irrigation. It can be ideal for small

plots of trees and row crops requiring different amounts of water. Trickle laterals may also be

moved from one crop row to another to reduce the cost of the system.

Many claims are made about trickle irrigation, such as that it saves irrigation water,increases yield, etc., but care should be taken in accepting such claims. Crops need a certain

amount of water to grow (Section 3.6.1) and generally they are not aware of where the water is

coming from. If it comes from surface flooding, sprinkling or trickle, it makes little difference

to the plants they respond to water. The saving in water comes from the efficiency with

which the water can be applied and it is here that trickle has a distinct advantage. Some yield

increases have been shown with trickle and this may be due to the favourable soil water conditions

and the nutrients added to the water.

For further detailed information reference should be made to specialist publications1.

3.5.4 Selecting an irrigation method

The selection of an appropriate irrigation method depends on a wide range of t

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