CHAPTER FIVE

5 - i

CHAPTER FIVE

INDEX

CHAPTER FIVE - PUMPING PLANTS AND SYSTEMS .........................................................1

5.1 Introduction ............................................................................................................................15.1.1 General ................................................................................................................15.1.2 The Poor Track Record of Pumping Plant in Tanzania ...................................................1

5.2 Pump Classification and Characteristics ................................................................................35.2.1 Pump Classification .........................................................................................................3

5.2.1.1 Rotodynamic .......................................................................................................35.2.1.1.1 Centrifugal ..........................................................................................................45.2.1.1.2 Peripheral Pumps ..............................................................................................125.2.1.1.3 Special Pumps ...................................................................................................13

5.2.1.2 Positive displacement pumps ..................................................................................155.2.1.2.1 Reciprocating ....................................................................................................155.2.1.2.2 Rotary ................................................................................................................18

5.2.2.1 Construction of Centrifugal Pumps .........................................................................225.2.2.2 Specific Speed .....................................................................................................30

5.2.3 Pump Suction Requirements ..........................................................................................335.2.5 Operations of Centrifugal Pumps ...................................................................................345.2.6 Efficiencies and Fuel Consumption of Rotodynamic Pumps and their Motors .............35

5.3 Prime Movers .......................................................................................................................365.3.1 Electric Motors for Pump Drives ...................................................................................36

5.3.1.1 The AC Induction Motor .........................................................................................375.3.1.1.1 General ..............................................................................................................375.3.1.1.2 Electric Motor Failure Modes ...........................................................................395.3.1.1.3 Increasing the Power Factor..............................................................................425.3.1.1.4 Power Factor, Installed Capacity and Demand .................................................44

5.3.1.2 Different Starting Methods .....................................................................................465.3.1.3 Different Applications .............................................................................................495.3.1.4 Speed Control of Pumps with Frequency Converters ............................................53

5.3.2 Diesel Engines and Generators ......................................................................................59

5.4 Pumping Plant Categories ....................................................................................................625.4.1 Surface Water Pumping .................................................................................................62

5.4.1.1 Suction Lift Pumping ..............................................................................................625.4.1.2 Gravity inflow Source Pumping .............................................................................63

5.4.2 Pumping from an Inaccessible Spring ...........................................................................645.4.2.1 General Considerations ...........................................................................................645.4.2.2 Electrical power system design for remote control .................................................64

5.4.3 Pumping from Deep Wells .............................................................................................65

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5.4.4 Pumping from Shallow Wells ........................................................................................655.4.5 Booster Pumping ............................................................................................................665.4.6 Pumping Capacity ..........................................................................................................665.4.7 Pumping Period ..............................................................................................................665.4.8 Pumping Sequence .........................................................................................................665.4.9 Pump Arrangement ........................................................................................................67

5.5 Pumping Systems .................................................................................................................675.5.1 Single Stage Pumping ....................................................................................................675.5.2 Two Stage pumping .......................................................................................................675.5.3 Branched Pumping Systems. .........................................................................................675.5.4 In Line Boosting ............................................................................................................675.5.5 Maximum Suction Lift Calculation ...............................................................................68

5.6 Equipment Type Design and Selection ................................................................................695.6.1 Type Design ..............................................................................................................695.6.2 Selection ..............................................................................................................705.6.3 Metallic Materials of Pump Construction (and their Damage Mechanisms) ................71

5.6.3.1 Types of Corrosion ..................................................................................................725.6.3.2 Types of Wear .....................................................................................................775.6.3.3 Fatigue .....................................................................................................825.6.3.4 Materials of Construction ........................................................................................835.6.3.5 Selection of Materials of Construction ...................................................................86

5.7 Plant Pumping Systems and Equipment Protection .............................................................895.7.1 Dry-Running Protection .................................................................................................905.7.2 Water Hammer or Surge ................................................................................................90

5.7.2.1 Introduction .....................................................................................................905.7.2.2 Initial Water Hammer Analysis Calculation ...........................................................915.7.2.3 Subsequent Analysis of a Potentially Dangerous Water Hammer Situation ..........915.7.2.4 Flow Separation .....................................................................................................935.7.2.5 Pipeline Topography ...............................................................................................935.7.2.6 Water Hammer Protection Systems ........................................................................94

5.7.3 Protection Against Cavitation ......................................................................................1005.7.4 Over Voltage Protection ..............................................................................................1025.7.5 Cathodic Protection ......................................................................................................1055.7.6 Overload (O/L), Current and Single Phasing Protection .............................................107

5.8 Erection Operation and Maintenance Instructions .............................................................1085.8.1 Pumpset Lining up and Final Coupling .......................................................................1085.8.2 Parallel Alignment .......................................................................................................1095.8.3 Angular Alignment ......................................................................................................109

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5.8.4 Routine Maintenance Schedule Pumps .....................................................................1105.8.5 Pipework Installation ...................................................................................................1115.8.6 Operating Procedure ....................................................................................................112

5.8.6.1 General ...................................................................................................1125.8.6.2 Priming ...................................................................................................1125.8.6.3 Starting ...................................................................................................1125.8.6.4 Stopping ...................................................................................................113

5.8.7 Maintenance Chart .......................................................................................................113

5.9 Economics of Electrical Power Systems ............................................................................1155.9.1 Power Factor Correction ..............................................................................................1155.9.2 Requisite Reactive Power Calculations .......................................................................1165.9.3 Cable Sizing for the Capacitor Connection .................................................................117

5.10 Energy Source Considerations ...........................................................................................117

5.11 Instrumentation ...................................................................................................................118

5.12 Life Cycle Analysis, an Introduction .................................................................................118

5.13 Formula and Conversion Factors .......................................................................................120

TABLES AND FIGURES Figure 5.1: Radial Flow Impellers 4 Figure 5.2: Mixed Flow Impellers 5 Figure.5.3: Axial Flow Impellers 5 Figure 5.4: Single Suction Close Coupled Pump 6 Figure 5.5: Double Suction Axially Split Pump 6 Figure 5.6: Single Suction Axial Flow Pump 6 Figure 5.7: Section through Single Stage End Suction Centrifugal Pump 7 Figure 5.8: Radially Split Ring Section Multistage Pump 7 Figure 5.9: Radially Split Barrel Casing Type Multistage Pump 8 Figure 5.10: Axially Split Multistage Pump 8 Figure 5.11: Section through a Typical Nine Stage Ring-Section

Through-Bolt-Multistage Centrifugal Pump 9 Figure 5.12: Closed - Coupled Pump 9 Figure. 5.13: Long - Coupled Pump 9 Figure 5.14: Turbine Type Pump 10 Figure 5.15: Submersible Pumpset 10 Figure 5.16: Illustration of a Vertical Spindle Borehole Pump 11 Figure 5.17: Electro Submersible Pump 12 Figure 5.18: Peripheral Pumps 12 Figure 5.19: Jet Pump 13

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Figure 5.20: (a) Shallow Well (b) Deep Well 13 Figure 5.21: Gas Lift/Air Lift Pump 14 Figure 5.22: Horizontal Single-Acting Plunger Pump 16 Figure 5.23: Horizontal Double-Acting Piston Pump 16 Figure 5.24: (a) Diaphragm (b) Horizontal Plunger 17 Figure 5.25: Hydraulic Actuated Diaphragm 17 Figure 5.26: Generalized Form of Rotary Pump Performance 18 Figure 5.27: Sliding Vane Pump 19 Figure 5.28: External Vane Pump 19 Figure 5.29: Axial Piston Pump 19 Figure 5.30: (a) Flexible Vane Pump (b) Flexible Tube Pump 20 Figure 5.31: Single Screw Pump (progressing cavity) 20 Figure 5.32: External Gear Pump 20 Figure 5.33: Internal Gear Pump (with crescent) 21 Figure 5.34: (a) Single Lobe Pump (b) Three-Lobe Pump 21 Figure 5.35: Circumferential Piston Pump 21 Figure 5.36: Multiple (three) Screw Pump 21 Figure 5.37: Impellers 22 Figure 5.38: Characteristic Curve for Radial Flow Centrifugal Pump 23 Figure 5.39: Typical Curves for (a) Axuial and (b) Mixed Flow Pumps 23 Figure 5.40: Casing (a) Volute Type (b) Diffuser Type 25 Figure 5.41: (a) Single Volute Casing (b) Double Volute Casing 25 Figure 5.42: (a) Multi-Vane Radial Diffuser (b) Axial Diffuser 26 Figure 5.43: (a) Axial Suction Thtust and (b) Hydraulic Unbalanced Thrust 26 Figure 5.43: (c) Thrust Balance Method 27 Figure 5.44: Hydraulic Axial Balance with Double Inlet Impeller 27 Figure 5.45: Axial Balance Typical Balance Disc Arrangement 27 Figure 5.46: Axial Balance Typical Balance Drum Arrangement 28 Figure 5.47: Axial Balance Thrust 28 Figure 5.48 (a) 100% BEP Flow and (b) Reduced Flow 29 Figure 5.49 (a) Radial Thrust and (b) Double Volute 30 Figure 5.50: Units of Specific Speed 31 Figure 5.51: Relationship between Pump Efficiency and Specific Speed 32 Figure 5.52: Preferred Arrangement of Pumping Stations 33 Table 5.1: Pump Type, Characteristics and Applicability 34 Figure 5.53: Effect of Changing Speed of Pump 34 Table 5.2: Comparative Efficiencies for Different Pumps and Motors 35 Table 5.3: Overall Fuel Consumption 36 Table 5.4: Degrees of Displacement and Power factor 45 Table 5.5: Power Factor Surcharges 45 Figure 5.54: Direct-On-Line Start 46

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Figure 5.55: Star-delta Start 47 Figure 5.56: Frequency Converter 48 Figure 5.57: Soft starter 49 Figure 5.58: (a-d) Different Applications 50 Figure 5.59: Speed Control of Pumps with Frequency Converters 53 Figure 5.60: Altitude De-rating for a Naturally Aspirated Diesel Engine 61 Figure 5.61: Temperature and Humidity De-rating for a Diesel Engine 62 Table 5.6: Altitude and Temperature Values (Factor B) 69 Figure 5.62: Headloss through Strainer and Footvalve 69 Figure 5.63: Cavitation Erosion of an Impeller 76 Figure 5.64: The Fretting Damage of a Shaft beneath an Impeller that Experience

Small Amplitude Motion 78 Figure 5.65:The three-body abrasive wear of a laser-hardened shaft sleeve in an

abrasive service 80 Figure 5.66: (a-d) Examples of Erosion 81 Figure 5.67: Fatique fractures 83 Table 5.7: Calculated Wear Factor 86 Table 5.8: Material Selection Chart for Volute Casing Pumps 87 Table 5.9: Material Selection Chart for Wet-Pit Diffuser Pumps 88 Table 5.10: Material Selection Chart for Reciprocating Pumps 89 Table 5.11: Combination to be Avoided When Area of Metal Considered Is Small

Relative to Area of Coupled Metal 89 Table 5.12: Combination to be Avoided when Area of Metal Considered Equal to

Area of Coupled Metal 90 Figure 5.68: Superimposition of Surge on a Pipeline Profile 93 Figure 5.69: Pipeline Profile Illustrating Suitable Locations for Surge Protection Devices 95 Figure 5.70: Vortex Prevention Assessment 102 Figure 5.71: Sacrificial Anodes for Cathodic Protection 107 Table 5.13: Typical Maintenance Chart 114 Table 5.14: TANESCO Tariff (2008) 116 Figure 5.72: Typical Overall Pumpset Life Cycle Cost Breakdown 120

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CHAPTER FIVE - PUMPING PLANTS AND SYSTEMS

5.1 INTRODUCTION

Revisions to the 1997 edition of this Chapter are generally minor except for the addition of a new Section on Pump Classification and Characteristics providing a discussion on Prime Movers in general and an introductory descriptive and illustrative evaluation of different types of Pumps; and new Sections providing an overview of materials of pump construction; and an introduction to Life Cycle Assessment of Pumping Plant.

5.1.1 General

The main purpose of any pumping plant and pumping system is to lift water from a lower to a higher level.

The laws of physics dictate the minimum power requirements for lifting a given mass of water through a given distance in a given time. The challenge for the pump manufacturer, system designer, and subsequently the operator is to ensure that this is done as reliably, and efficiently as possible with the minimum of energy consumption.

The running and the economy of a water production line relies mainly on the success of following project planning sub-components:

Intake and Plant Design Pumping System Design Equipment Type Design Equipment Selection Plant, Pumping System and Equipment Protection Accuracy and comprehensiveness of Erection, Operation and Maintenance Instructions Economics of Electrical Power Systems or other power systems Energy Considerations Compliance with Instructions Observation of the Factory Ordinance

5.1.2 The Poor Track Record of Pumping Plant in Tanzania

The recent history of Pumping Plant in this country is far from satisfactory. It has shown that smooth and economic running of most of the existing water schemes including those designed by foreign consulting firms has been impaired by deficiencies in some if not most of the areas relating to pumping systems cited above.

This has in part been due to the fact that the previous edition of this Design Manual on the Electrical and Mechanical part of a water scheme design has either been ignored or inadequately followed. Hence most existing designs do not exhibit a sufficiently high degree of technical soundness in this respect.

The situation has been exacerbated by the failure to effectively ensure the quality of manufacture, the competence of installation contractors and the appropriate training and provision of competent and dedicated system operators.

The economic life time of water pumping equipment under normal operation and maintenance practices is at least 10 and can be 15 years. In this country however, it is not uncommon to need

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pumps replacing much earlier following premature and permanent breakdowns, even after 5 years of operation or less. Reasons for premature breakdown include cavitation, incorrect or inadequate design, selection and pump specification, incorrect erection, silt/chemical erosion, lack of effective protection, poor operation and maintenance, etc.

Furthermore water supplies are frequently interrupted due to lack of realistic principles of investment which results in cheap pumping equipment being selected, lack of adequate standby pumping capacities and systematic operating sequences.

An analysis of the enumerated failures has amongst other things shown that:-

1. Silt and chemical erosion due primarily to poor source selection and location and poor intake design as well as to the use of clear water pumps that are not designed for such conditions.

2. Cavitation has often been due to poor sump-pump layout which results in turbulent flow and allows air entrainment in the pump; further to this the use of complicated suction pipe lines which makes it virtually impossible to effect perfect pump priming processes (displacement of all the air in the suction line by running the pumped medium into it) and this results in poor pumping system as the pump is subjected to objectionably low net positive suction head (NPSH).

3. Lack of sound protection has forced the water authorities to spend, untimely, millions, of shillings on rewinding or replacement motors following burn-outs due to overloads, phase failures, under-voltage, lightning and switching surges. Also lack of hydraulic, mechanical and electrical protection has exposed plants to damage and operators to accidents.

4. Poor erection has contributed greatly towards severe and often irreparable damage to pumps and drives (prime movers), equipment shaft breakages, and burning of electric motors.

It should be noted in this connection that improper equipment alignment, wrong clearances at couplings, force fitting of foundation base plates, and pipes especially at pump branches can cause excessive stress in the respective items. In such cases breakages of couplings, shafts, bearing and foundation failures, etc. are bound to occur.

5. Poor construction/installation materials or inappropriate sizes of the same have been quite predominant problems; evidently, this is because the technical know-how in this regard has not been effectively propagated to those concerned. This has also caused a lot of damage to motor-windings, foundations and drives as well as reduced expected plant outputs and life.

6. Equipment and material handling and storage is yet another area that needs to be given due consideration at the project planning stage. Failure to do so can result into premature failures of plant and equipment and therefore affect the planned project economy.

7. The degree of protection being specified and provided presently leaves much to be desired. Electric motor installations in areas prone to lightning especially along the shores of Lake Victoria are not provided with lightning arrest6rs. Hence motor windings have been and are still being burnt-out due to lightning surges.

8. Failure to recognise that electricity supply, nominally of 400V, 50Hz or 11kV, 50Hz is often intermittent and below those values.

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9. Poor operation and maintenance has also caused a number of schemes to stop running due to frequent breakdowns including failure to maintain adequate stocks of spare parts. It is thus necessary that with a view to minimising these problems, designers should observe the following factors as being among the important design criteria apart from ensuring that the operation and maintenance of any scheme is planned for from the design stage.

i. Technical level of education of plant operator and maintenance staff; ii. Provision of clear and planned maintenance facilities; iii. Comprehensive lists of likely number and timing for spare parts; and iv. Due care and attention to the economics of project running.

10. It is rare to find factory ordinances being observed. Numerous pumping sets can presently be found without coupling guards, and some operators have been and are still likely to fall victims of even fatal accidents at pumping stations. It is important therefore to draw the attention of plant managers and operators to some clearly defined factory regulations. Even more important is the need to observe such regulations in design so as to avoid loss of human resources.

5.2 PUMP CLASSIFICATION AND CHARACTERISTICS

5.2.1 Pump Classification

Although there are a wide range of pumps available for numerous applications they generally fall into two major groups. These are:

Rotodynamic Pumps. Positive Displacement Pumps.

5.2.1.1 Rotodynamic

Rotodynamic pumps are essentially rotary machines in which energy is continuously imparted to the pumped liquid by a rotating impeller or rotor and consist of three types: Centrifugal, Peripheral and Special Pumps as indicated in the diagram below:

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5.2.1.1.1 Centrifugal This group of pumps consists of a shaft mounted impeller(s) rotating unidirectionally within a casing. The liquid enters the impeller eye and acquires energy in the form of velocity as it passes though the impeller passages. The velocity head is converted into pressure head by the volute or spiral shaped outer casing of the pump which directs the liquid from the outer perimeter of the impeller to the pump discharge. A less common method of developing pressure head is to surround the impeller with concentric diffusing passages.

A well designed centrifugal pump, suitable for purpose, should achieve around 80% efficiency with not more than 20% losses in energy conversion, bearing friction and hydraulic losses. However, actual efficiency depends on several factors including size and flow rate, with efficiency generally increasing with size.

It must be recognised that efficiency drops with time due to wear and tear and the all too frequent use of design flow rates to predict actual output needs to be actively discouraged as it is increasingly misleading with age. Actual operating efficiency determined from measured output and power consumed is a useful criterion in judging whether it is time for refurbishment or replacement.

A major problem with all centrifugal pumps is how best to accommodate end thrust on the shaft. The pressure of the water at the inlet side of the impellor is low relative to that on the back of the impellor beyond the outlet, which is almost equal in surface area, so that an axial thrust is imparted. Unless this is balanced in some way this would quickly cause wear on the pump and shorten its life. In small pumps this end thrust is dealt with by the use of purpose-designed thrust bearings. For larger pumps, a double entry, back-to-back impellor design may be used.

Centrifugal pumps can be divided into two main groups depending on the type of impeller.

(a) Radial Flow Impellers.

The liquid enters the impeller axially and discharges radially, in effect changing its direction by 90o. In this case the head developed is due to the centrifugal force exerted on the fluid by the impeller.

FIGURE 5.1: RADIAL FLOW IMPELLERS

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(b) Mixed Flow Impellers.

The liquid enters the impeller axially and discharges in both axial and radial directions. In this case the head developed is the result of a combination of the centrifugal force and the lift produced by the vanes on the liquid.

FIGURE 5.2: MIXED FLOW IMPELLERS

(c) Axial Flow Impellers.

The liquid enters leaves the impeller in an axial direction. In this case the head developed is entirely due to the lift produced by the vanes on the liquid.

FIGURE.5.3: AXIAL FLOW IMPELLERS

Radial and mixed flow impellers can be subdivided into:

(i). Single suction the liquid enters the impeller from one side only. (Fig. 5.4)

(ii). Double suction the liquid enters the impeller from both sides. (Fig. 5.5)

(iii). Axial flow impellers which are of the single suction type. (Fig. 5.6)

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FIGURE 5.4: SINGLE SUCTION CLOSE COUPLED PUMP

FIGURE 5.5: DOUBLE SUCTION AXIALLY SPLIT PUMP

FIGURE 5.6: SINGLE SUCTION AXIAL FLOW PUMP

Centrifugal pumps, irrespective of impeller type can be either singe stage, whereby the total head is the result of one impeller, or can be multistage.

Standard, above ground centrifugal pumpsets will either be horizontally or vertically mounted with the prime mover either at one end or immediately above the pump.

For general waterworks purposes, the maximum pressure normally developed by a single stage pump will be 80 -100 m so that for heads greater than this a multi-stage pump is usually required although increased head can also be achieved by increased speed and/or larger impellors. However, and as a general rule, the lower the speed, the longer the life of the pump.

With single stage horizontal pumps, an end entry suction with side or top outlet is offered by some manufacturers whilst for vertically mounted units and for multistage pumps either side or top entry and exit ports are necessary. An end entry, single stage pump is illustrated in Figure 5.7 and a multi-stage pump in Figure 5.11.

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FIGURE 5.7: SECTION THROUGH SINGLE STAGE END SUCTION CENTRIFUGAL PUMP

Centrifugal pumps can be manufactured with a split casing, especially in larger sizes. A great advantage with this type of pump is that the upper half of the casing is easy to remove giving access to the diffuser chamber and rotating element with the impellor and bearings visible for inspection and maintenance without the need to disconnect either pipework or the prime mover. In addition, new or refurbished rotating elements can be kept in stock enabling a speedy removal and replacement with such things as bearing replacement or impellor rehabilitation then being undertaken at a more leisurely and careful pace without the pressure of minimising downtime being present.

In multistage units the total head is the end result of a series of impellers within the one casing. Under these conditions the required head is achieved by the summation of the heads developed by each individual impeller.

Multistage unit casings can be either axially split (Fig. 5.10) or radially split. Radially split pumps can consist of a series of ring sections fastened together by external tie bolts (Fig. 5.8); alternatively for higher pressures or to cater for thermal shock, the ring sections can be encased in a barrel casing which also contains the suction and discharge branches (Fig. 5.9). Multistage pumps can be either horizontally or vertically disposed.

FIGURE 5.8: RADIALLY SPLIT RING SECTION MULTISTAGE PUMP

inletport

shaft forcouplingto primemover

outletport

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FIGURE 5.9: RADIALLY SPLIT BARREL CASING TYPE MULTISTAGE PUMP

FIGURE 5.10: AXIALLY SPLIT MULTISTAGE PUMP

An advantage of multistage pumps is that it is practicable to initially incorporate one or more dummy stages, a dummy stage simply being a diffuser without an impeller. Then as demand increases, impellers can be added to increase delivery pressure and hence flow. Pump efficiency is not much affected by the dummy stages, however the additional impellors must be considered when ensuring the prime mover will have enough power to drive the pump after they are added.

A section through a typical multi-stage pump is illustrated in Figure 5.11.

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FIGURE 5.11: SECTION THROUGH A TYPICAL NINE STAGE RING-SECTION

THROUGH-BOLT-MULTISTAGE CENTRIFUGAL PUMP

Centrifugal pumps can be either self priming or non self priming and can have open, semi open or closed impellers depending on the specific requirements of the particular pump.

Centrifugal pumps can also be identified by their basic mechanical configuration and characteristics:-

Overhung Impeller Type the impeller(s) is mounted on the end of the shaft which is cantilevered from its bearing supports. In addition, this type of pump can either be of the close coupled design, in which the pump casing is fixed directly to the driver frame and the impeller is mounted on the driver shaft (Fig 5.4 & Fig. 5.12) or long coupled, where the pump is mounted on a base plate and driven through a coupling. (Fig 5.13)

FIGURE 5.12: CLOSED - COUPLED PUMP

FIGURE. 5.13: LONG - COUPLED PUMP

Impeller Between Bearings Type the impeller(s) is mounted on a shaft between bearings situated at both ends.

inletport

outletport

couplingto primemover

endthrust

bearing

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inletport

risingmain

FIGURE 5.14: TURBINE TYPE PUMP

Turbine Type/Submerged Centrifugal Pumps this rather ambiguous term usually applies to vertical multistage deep well pumps that are constructed with diffuser casings screwed or bolted together. The bearings are lubricated, cooled and flushed by the pumped liquid (Fig 5.14). A special type of turbine pump is the electro submersible. In these deep well units, the motor is close coupled to the pump and submerged in the well. This type of pump is used for high head applications where long intermediate shafts are undesirable

There are two types of submerged centrifugal pumps, the fully submerged or submersible pumpset and the vertical spindle pump.

In the submersible pumpset, both the pump and its motor are submerged such that the term submersible pump is now synonymous with that of the submersible pumpset. When designed as long narrow units they are especially well suited as borehole pumps being considerably cheaper than vertical spindle pumps. The motor is located in a shroud subsequent to the pump with the water flowing around it so as to cool it. Power is supplied to the motor through waterproof cables. They also tend to be two-pole driven enabling them to run at the highest available speed.

There major disadvantage is that the motor is out of sight of any operator and easy to forget until it fails. Such failures are often the result of failure of the motor-cable water seals and the motor can burn-out under such circumstances. On the other hand they are reasonably quick and easy to install and remove as there is no need for a drive shaft or spindle. Nor do they have to be truly vertical in their positioning. Similarly and providing there is access at the wellhead for a mobile crane or space for shear legs, no surface structure is needed other than for the switchgear which can be mounted in a simple waterproofed housing.

FIGURE 5.15: SUBMERSIBLE PUMPSET

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They do however tend to be less efficient than the vertical spindle design because of the special design of the motor and because they usually require a larger number of stages due to the relatively small impellor diameter.

Specially designed submersible pumpsets can be mounted horizontally and are then often used as in-line booster pumps. The vertical spindle pump is one where the prime mover is at the surface, mounted above flood water level in the case of river intakes, and the pump is immersed in the water. The connecting shaft or spindle rotates within the riser pipe, often within a tube or sleeve held centrally to the rising main by spider bearings which allow the water to pass through the annular space between the shaft or its containing sleeve and the rising main. Such rising mains and their contents are often in 3 m lengths allowing for slow but progressive removal and replacement. Such removal and especially replacement is a highly skilled task as true alignment is critical to avoid excessive wear and tear or even failure. An example is shown below:

FIGURE 5.16: ILLUSTRATION OF A VERTICAL SPINDLE BOREHOLE PUMP

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In addition to the centrifugal pumps described previously, there are other types which have their own unique characteristics:

Single stage electro-submersible pumps, sump pumps, glandless and canned motor pumps, sewage pumps and abrasion resistant pumps.

FIGURE 5.17: ELECTRO SUBMERSIBLE PUMP

5.2.1.1.2 Peripheral Pumps

In a peripheral pump, energy is imparted to the fluid within the cells of a vane wheel impeller. Alternatively, the cells are arranged peripherally on the outer sides of a wheel disc. Peripheral pumps come in various configurations, such as overhung impeller, impeller(s), mounted between bearings, single stage and multistage.

Alternative names are side channel pump and regenerative turbine pump.

FIGURE 5.18: PERIPHERAL PUMPS

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5.2.1.1.3 Special Pumps

(a) Jet Pump

The term jet pump describes a pump in which a high velocity jet of fluid is utilized to create a low pressure area in a mixing chamber, causing the suction fluid to flow into this chamber. The venturi is positioned so that gradual velocity conversion occurs with minimum losses. (Fig. 5.19)

FIGURE 5.19: JET PUMP

When the jet/venturi are attached directly to the pump ahead of the suction impeller and activated by the liquid from the pump it is termed a shallow well jet pump. When the jet/venturi are set at the base of an extended suction pipe and activated by the liquid from the pump it is termed a deep well jet pump.

(a) (b) FIGURE 5.20: (a) SHALLOW WELL (b) DEEP WELL

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The jet pump or ejector pump, works on the principle of ejecting a small stream of high pressure water to entrain a larger volume of low pressure water and force it to a higher elevation. Its application is normally restricted to shallow wells in combination with a single stage centrifugal pump located at the wellhead. Here a proportion of the pump discharge is directed down to the bottom of the well and through an ejector to promote the flow up the suction pipe. Lifts of up to 75 m can be realized by this method, although, in general, the higher the lift, the lower the efficiency of the pump.

(b) Gas Lift/Air Lift pumps.

A gas pressure source is used to lift or pump the liquid handled by mixing it with gas under pressure, usually compressed air. An air-lift pump is commonly used to test boreholes. It is based on pressurising the water surface with compressed air to force it up the delivery pipe. Lift is directly proportional to the pressure developed on the surface, i.e. the head of equivalent less friction losses in the pipe. The system involves no moving parts in the well but requires that the well be sealed above the water level to maintain the surface pressure. Such types of pump usually have a low efficiency.

FIGURE 5.21: GAS LIFT/AIR LIFT PUMP

(c) Electro magnetic pump

An electro-magnetic pump is a glandless pump without rotating parts in which a magnetic field acts upon a susceptible medium (liquid metals in principle).

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5.2.1.2 Positive displacement pumps

Positive displacement pumps are essentially rotary or reciprocating machines in which energy is periodically added by application of force to movable boundaries of enclosed fluid containing volumes, resulting in a direct increase in pressure. The various types are illustrated on the diagram blow:

5.2.1.2.1 Reciprocating

Most reciprocating pumps are either suction or lift pumps, both functioning as free delivery or force pumps, and working as single-acting or double-acting pumps, and are used in wells and boreholes. They can be manually (hand or foot) driven.

In a suction pump, the plunger and its cylinder are located above the water level, usually within the pump stand itself. A suction pump relies on atmospheric pressure so that it is in practical terms limited to 5 to 8 metres primarily dependant upon elevation. (See section 5.2.5 on NPSH.)

In a lift pump, the term deep or shallow refers to the depth of water in the well and not to its total depth. The pump element comprising cylinder and plunger is located below the water level and can lift water from depths as great as 180 m.

They can either be driven by a lever action or for greater depths by a rotating hand wheel or pair of such mounted on either side of the headstock. For more information on a handwheel operated type of reciprocating pump, designers are advised to visit the web-site www.duba.com.

A variation on the reciprocating well-pump is the hydrostat or hydraulic ram pump used for lifting surface water whereby a large volume of water flowing in one pipe or waterway is used to drive a ram which is connected to a smaller pump which pumps part of the water to a higher elevation through a branch pipe, with the majority of the flow continuing down the main pipe or discharging from the pump.

Their use tends to be restricted to the supply of small volumes of water from steep, fast flowing streams or rivers in remote locations without an accessible power supply to hillside communities above.

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1. Piston, Plunger or Diaphragm

A reciprocating pump set generally consists of a piston / plunger (or diaphragm) inlet and outlet valves and a means by which the piston / plunger can be actuated. This is usually done by a reciprocating engine or an electric motor / steam turbine, crank and connection rod combination. These pumps can be either.

(a) Single acting in which the liquid is discharged during the forward motion of the piston.

(b) Double acting in which the liquid is discharged during both the forward and backward motions of the piston.

Reciprocating pumps can be further classified as:-

(i) Simplex pumps contain one single or double acting piston / plunger.

(ii) Duplex pumps contain two single or double acting pistons / plungers.

(iii) Triplex pumps contain three single or double acting pistons / plungers.

(iv) Multiplex pumps contain more than three single or double acting pistons / plungers.

FIGURE 5.22: HORIZONTAL SINGLE-ACTING FIGURE 5.23: HORIZONTAL

DOUBLE-ACTING PLUNGER PUMP PISTON PUMP

(a) Piston / Plunger Type

The piston / plunger pump offers a positive means of metering a wide variety of fluids by utilising a range of gland packing materials and long life plungers. With this type of pump the flow rate accuracy and discharge pressure greatly exceed that of the mechanical diaphragm type. The wet end consists of a piston / plunger reciprocating within a cylindrical housing.

(b) Mechanical Diaphragm Type

With glandless construction and simple wet end designs, the mechanical diaphragm pump offers the advantage of a positively sealed pumping chamber, in which the risks of corrosion and erosion are negligible.

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FIGURE 5.24: (a) DIAPHRAGM (b) HORIZONTAL PLUNGER

(c) Hydraulically Actuated Diaphragm Type

This type of pump combines the glandless construction of the mechanical diaphragm pump and the accuracy, repeatability and high pressure capability of the piston / plunger pump. The diaphragm is hydraulically coupled to the piston or plunger. This type of pump can incorporate diaphragm rupture detection devices and various hydraulic fluids which are compatible with the fluid being pumped.

CONTROLLED VOLUME PUMP PISTON DIAPHRAGM LIQUID END

FIGURE 5.25: HYDRAULIC ACTUATED DIAPHRAGM

2. Controlled Volume Pump

Another type of reciprocating positive displacement pump is the controlled volume pump which provides precision control of very low flow rates up to a maximum of about 3 l/s. Flow rate accuracy is typically within 1%. Other names for these pumps are proportioning pumps and metering pumps. Controlled volume pumps are generally available in three construction styles, piston or plunger; mechanical diaphragm and hydraulically actuated diaphragm type (piston diaphragm). Usually, the driver is an electric motor. Basically, the design criteria that applies to large motor driven reciprocating pumps also applies to controlled volume pumps. Flow rate variations are normally achieved by manual resetting of the stroke length. Automatic controls are available for stroke length resetting and motor speed. The typical efficiency of this type of pump is around 90o%.

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5.2.1.2.2 Rotary

There are a number of different varieties of rotary pumps but in all, the essential working element is a rotor which can take a number of different forms, such as an impellor, vane or screw or a combination of these.

Such pumps differ from reciprocating pumps in that the delivery is continuous and hence smoother. However, internal losses are normally somewhat higher through slip (internal leak-back). Slip increases with increasing pressure so rotary pumps are less suited to high pressure systems. Rotary pumps are physically much larger than centrifugal pumps for equivalent capacities and therefore occupy greater space.

The water output of a rotary pump is almost proportional to the rotating speed as indicated in Fig 5.26. One such pump is the helical rotary (Mono) pump which consists of a single thread helical rotor which rotates inside a double thread helical sleeve or stator, giving it the alternative names of the eccentric screw or progressive cavity pump.

Both diesel engines and electrical motors are suitable for driving this type of pump, whilst some smaller units are also capable of hand operation.

FIGURE 5.26: GENERALIZED FORM OF ROTARY PUMP PERFORMANCE

Being a positive displacement pump, the helical rotor pump has a head discharge characteristic that is almost vertical to the capacity and for each power input are parallel to each other.

For more specific information of the commonest make of this pump, designers should refer to the web site www.mono.com.

Rotary pumps generally consist of gears, screws, vanes or similar elements enclosed within a casing. They have no separate inlet or outlet valves and the liquid flows through the pump in a uniform stream as a result of the rotation of elements. These pumps are characterised by their close running tolerances. Rotary pumps can be divided into two general groups:

1. Single Rotor, and

2. Multiple Rotors.

Quantity

Hea

d

P1 P2 P3 P4 P5 P6

P = power input

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1. Single rotor types

(i) Vane

The Vane of various forms, including blades, buckets, rollers, or slippers are radially displaced inwards or outwards by a cam which has the effect of drawing liquid into and then out of the casing. There are two types of vane pumps:

FIGURE 5.27: SLIDING VANE PUMP FIGURE 5.28: EXTERNAL VANE PUMP

(ii) Piston

The rotor contains cylinders in which piston like elements reciprocate. As the rotor turns, so the movement of the pistons draws the fluid into the casing, the rotation of the cylinder of the piston and cylinder being relative to the parts.

There are two variations of this type of pump.

In one variation, the pistons reciprocate in an axial direction and in the other the reciprocating action of the piston is in a radial direction.

FIGURE 5.29: AXIAL PISTON PUMP

(iii) Flexible Member

The flexible member, being a vane, tube or liner is of sufficient elasticity to accomplish sealing and transfer the fluid within the casing.

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(a) (b)

FIGURE 5.30: (a) FLEXIBLE VANE PUMP (b) FLEXIBLE TUBE PUMP

(iv) Screw Progressive Cavity Pump

The progressive cavity or helical rotor pump consists of a resilient stator in the form of a double internal helix and a single helical rotor.

The rotor maintains a constant seal across the stator and this seal travels continuously through the pump giving uniform positive displacement. The single helical rotor rolls in the stator with an eccentric motion. FIGURE 5.31: SINGLE SCREW PUMP

(PROGRESSING CAVITY)

2 MULTIPLE ROTOR TYPES

(i) Gear Pump

The meshing of two or more gears provides the pumping action. This meshing of gears also forms part of the moving fluid seal between the inlet and outlet ports.

Gear pumps can be either:-

(a) External gear in which all gear teeth are cut externally and may be of spur, helical or herringbone tooth pattern.

FIGURE 5.32: EXTERNAL GEAR PUMP

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(b). Internal gear in which one of the rotors has teeth cut internally and the other has externally cut teeth.

FIGURE 5.33: INTERNAL GEAR PUMP (WITH

CRESCENT)

(ii) Lobe Pump

The rotors consist of one or more lobes, the interaction of which transports the fluid from the inlet to outlet. As in the gear pumps, the shape of the rotors and their operations provided part of the fluid seal.

(a) (b)

FIGURE 5.34: (a) SINGLE LOBE PUMP (b) THREE-LOBE PUMP)

(iii). Circumferential Piston

The liquid is transported in the spaces between the piston surface to the outlet. The operation is similar to that of a lobe pump; however, a fundamental difference is that the rotors do not form any type of fluid seal.

Circumferential piston pumps can either be external or internal. The internal version must have two or more piston elements and has no need for timing gears, whereas the external type may have one or more pistons and requires timing gears.

FIGURE 5.35: CIRCUMFERENTIAL PISTON PUMP

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(iv). Multiple Screw

Usually the screw type rotors in this type of pump cannot drive each other and timing gears are required. The principle of operations is similar to that of the single screw pump.

FIGURE 5.36: THREE SCREW PUMP

5.2.2 Characteristics of Centrifugal Pumps

5.2.2.1 Construction of Centrifugal Pumps

A centrifugal pump is a machine which moves liquid by accelerating it radially outward in a rotating impeller to a surrounding stationary housing or casing. It has two main parts; 1) a rotating element consisting of a impeller mounted on a shaft. 2) a stationary element consisting a casing, staffing box and bearings.

Impeller

The rotary motion of impeller imparts velocity energy to the liquid some of which is converted into pressure within the impeller passages. Impellers may be classified as radial, axial or mixed flow, depending on design.

FIGURE 5.37: IMPELLERS

(1) Radial Flow

The radial flow impeller discharges the fluid radially at 90o to the shaft axis. The characteristic curve for a typical, true, radial flow centrifugal pump is shown in Figure 5.38. The head-flow curve is relatively flat up to the design point and the power at zero flow is only about 40% of that required at design duty.

As a result it can be started against a closed delivery valve which can also be closed before the pump is shut down. No problems then occur provided the valve is opened before over-heating occurs. This reduced power requirement at start up is also beneficial as it reduces the starting current when an electric motor is used as the prime mover, whilst the delivery pressure rise can be carefully controlled by slowly opening the valve thus reducing pressure transients.

Unfortunately such transients are difficult to avoid when stoppage results from an unexpected power outage.

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Whilst the maximum head developed for a given speed is not greatly in excess of the design head, the shape of the head-output curve is important. In the illustration shown here an unsatisfactory situation occurs which should be avoided because at heads higher than the duty head there are two possible outputs.

The pump is therefore unstable when operating in this region and this can be particularly troublesome when two or more pumps are being operated in parallel.

FIGURE 5.38: CHARACTERISTIC CURVE

FOR RADIAL FLOW CENTRIFUGAL PUMP

Designers are advised to carefully check the head-output curve of any envisaged pump and/or to clearly specify that only units with a continuously falling head are acceptable. In addition, the maximum head at zero flow, the closed valve head should not be excessive and the efficiency curve should be reasonably flat about the design duty point so that there is no great reduction in efficiency if the actual head is slightly different from that expected.

(2) Axial Flow

The axial flow impeller discharges fluid along the shaft axis in which the rotation of the impeller forces the water forward axially. For this reason an axial flow pump is by definition not centrifugal in its pumping action.

(3) Mixed Flow Mixed flow pumps act partly by centrifugal action and partly by propeller action, the blades of the impeller being given a degree of twist. However, in practical terms, there are no precise dividing lines between radial flow (centrifugal), mixed flow, and axial flow pumps.

The mixed flow impeller discharges fluid in a conical direction using a combined radial and axial pumping action as suggested by the title.

FIGURE 39: TYPICAL CURVES FOR (a) AXIAL AND (b) MIXED FLOW PUMPS

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They act partly by centrifugal action and partly by propeller action, the blades of the impeller being given a degree of twist.

Characteristic curves are shown in Figure 39.However, in practical terms, there are no precise dividing lines between radial flow (centrifugal), mixed flow, and axial flow pumps.

In general, axial and mixed flow pumps are primarily suited for pumping large quantities of water against low heads, whilst centrifugal pumps are best suited for pumping moderate outputs of water against high heads. Axial flow pumps in particular have poor suction capability and must be submerged for starting. They are best suited for transferring large quantities of water from a river to some nearby ground-level storage.

The starting power required by a mixed flow pump is much the same as the duty power, but for axial flow pumps, the starting power is substantially in excess of the duty power. Axial flow pumps are therefore not started against a closed valve which would overload a motor correctly sized for the expected duty.

It should be noted that even the smallest of pumps may be mixed flow rather than truly centrifugal, due to specific requirements or space restrictions and then mixed flow multi-stage pumps are deployed.

Vertical Trunk Slung (VTS) or Wet Well Pumps are often such axial flow pumps that are supported from above with the drive provided by a rotating shaft within the riser part of the pumpset assembly. This rotating shaft is supported by bearings that may be contained in a separate sleeve so that they can be provided with clean water for lubrication, although some special material bearings are now manufactured which, it is claimed, need no special lubrication. Designers are however advised to seek good references for instances in which such special material bearings have been successfully used in situations similar to those proposed before opting for such.

All wet well vertical spindle pumps have a number of disadvantages however including the need for a relatively lengthy down-time to remove / maintain / re-install the impeller and the bearings and require very accurate and precise realignment upon re-assembly if either rapid wear or even bearing failure is to be avoided. They are also purpose built units so are rarely available off-the-shelf and are also expensive and in general, it can be recommended that less expensive dry well axial flow pumps or centrifugal pumps are used instead as indicated in Figure 5.39 above, notwithstanding that the civil engineering costs are usually higher than the difference in price between the two types of pumps. However and when taking into account the key role played by pumping in other than gravity water supplies, the additional cost that may be involved is considered to be well worth it.

(4) Inducers

These are fitted on the suction side of first stage impellers. They are basically axial flow impellers with extended vanes. This enables them to operate in a cavitating condition with only a small head drop since the vapour cavity occupies only a relatively small length of passage. It thus generates enough head whilst cavitating to enable the first impeller to operate without cavitation.

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Casing

The velocity head of water leaving the impeller is converted into pressure head in the casing either by means of a volute (Fig. 5.40a) or by a set of stationary diffuser vanes surrounding the impeller periphery (Fig. 5.40b)

(a) (b)

FIGURE 5.40: CASING (a) VOLUTE TYPE (b) DIFFUSER TYPE

(1) Volute Casing

The volute is the most common form of casing (Fig. 5.41a). The volute increases in cross sectional area from the cut water to the inner end of the discharge cone so as to give a near constant average water velocity.

Most of the conversion from velocity head to pressure head takes place in the discharge cone. However uneven pressure distributions around the impeller may give rise to undesirable radial loading on the shaft, particularly when operating at reduced flows. The double volute design (Fig. 5.41b) reduces this radial loading.

FIGURE 5.41: (a) SINGLE VOLUTE CASING (b) DOUBLE VOLUTE CASING

(2) Diffuser

The diffuser fits inside the pump casing and guides the flow smoothly into the discharge pipe (or the next impeller in the case of a multi stage pump).

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The diffuser incorporates a number of vanes which form radially diverging water passages around the periphery of the impeller and recover a significant percentage of the total head. Most diffusers have radial vanes. Axial diffusers are used when it is desirable to reduce outside diameter of the casing or to increase the clearance between the impeller and diffuser vanes in order to reduce vane tip erosion in very high speed pumps.

(a) (b)

FIGURE 5.42: (a) MULTI-VANE RADIAL DIFFUSER (b) AXIAL DIFFUSER

Shaft & Bearings

The primary function of the shaft is to transmit the driving torque to the impeller. With the support of the bearings, the shaft must also locate the impeller radially and axially within the casing. Single stage overhung impeller pumps normally operate in the stiff shaft mode i.e. below the first shaft critical speed.

Multistage pumps may often operate in the flexible shaft mode, running above the first critical speed. With these pumps the hydrodynamic support and damping afforded by the internal clearances normally guarantees satisfactory wet operation.

Axial Thrust

Axial thrust is generated by the internal pressures acting on the impeller and shaft end. One component to this axial thrust is that due to, and dependent only on, suction pressure and is shown on Fig. 5.43a.

The other major thrust component for a horizontal pump is that created by a hydraulically unbalanced impeller as shown in Fig. 5.43b. This effect is due to the presence of an unopposed pressure at the impeller back shroud. For small pumps with a suction diameter similar to the shaft diameter, this effect is minimal.

FIGURE 5.43: (a) AXIAL SUCTION THRUST AND (b)

HYDRAULIC UNBALANCED THRUST

(a) THRUST DUE TO SUCTION PRESSURE

(b) HYDRAULIC UNBALANCED THRUST

Absolute suction pressure

Atmospheric pressure

Atmospheric pressure

(balanced)

Pressure acting on impeller

shrouds

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However, for larger pumps and / or higher head pumps the generated thrust can be very large.

The most common solution to this problem is to use an hydraulically balanced type of impeller fitted with a back ring and balance holes, which reduce the pressure at the rear of the impeller hub by allowing leakage though to the suction side A similar reduction in pressure can be achieved through the use of impeller rear (pump out) vanes. Both of these methods are shown in Fig. 5.43c.

Multistage pumps may have opposed impellers for axial thrust balance, or a balancing device such as a balance disc or balance drum.

FIGURE 5.43: (c) THRUST BALANCE METHODS

(2) Single Stage Pump with Double inlet Impeller

Theoretically a pump of this typeshould be in complete axial balance;however, the presence of slightcasting differences will cause theflow pattern to each impeller entry(eye) to marginally differ thuscreating a residual axial thrust.

This is taken by a ball bearing of thecombined radial / thrust type.

The arrangement is illustrated inFig. 5.44.

FIGURE 5.44: HYDRAULIC AXIAL BALANCE WITH DOUBLE INLET IMPELLER

(3) Multistage Horizontal Pump with Balance Disc

The unbalanced axial thrust is approximately equal to the pump differential pressure acting on the annular area of the impeller back shroud; this being roughly equivalent to the area of the impeller eye minus the shaft area.

FIGURE 5.45: AXIAL BALANCE TYPICAL BALANCE DISC ARRANGEMENT

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The balance disc rotates with the shaft and automatically adjusts the gap at A so that the pressure in the inner balance chamber opposes the unbalanced axial thrust of the impellers. This balance device is entirely self compensating and no thrust bearing is required.

(4) Multistage Pump with Balance Drum

The area of this balance drum (see Fig. 5.46) is approximately the same as the impeller unbalanced area but slightly undersized to ensure a residual unbalanced load in order to keep the shaft in tension. This axial load is taken by a thrust bearing, in this case a tilting pad bearing of the Michell type as shown in Figure 5.47.

FIGURE 5.46: AXIAL BALANCE TYPICAL BALANCE DRUM

ARRANGEMENT

FIGURE 5.47: AXIAL BALANCE DEVICES

Radial Thrust

When a single volute pump is operated at the best efficiency flow rate, the velocities and hence the pressures acting on the impeller are uniform around the volute. This is shown in Fig. 5.48a.

At flow rates other than best efficiency point, the pressure distribution is no longer uniform. At reduced flow rates, the pressures increase spirally towards the cutwater (see Fig. 5.48b) resulting in a radial reaction F. A similar situation exists at flow rates beyond best efficiency flow rate, with an approximately opposite (in direction) reaction. Fig. 5.49a) shows typical variation of radial thrust with flow rate.

Note that maximum radial thrust occurs at zero flow with the minimum at the best efficiency flow rate. The magnitude of the radial thrust is a function of the total head, impeller diameter and impeller width. Thus high head pumps with large impeller diameters will experience very high radial thrusts. If pumps of this type are operated at

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consistently low flow rates, bearing life will be reduced and the bending stresses associated with large shaft deflections may eventually lead to shaft failure.

The manufacturer may design for this problem by supplying an oversize shaft with large bearings; however, this solution leads to a more expensive unit, especially in the case of large high head pumps.

An alternative is to reduce the value of radial thrust through the use of a double volute as shown in Fig. 5.49b. In such a casing the flow is divided into two almost equal streams by two cutwaters 180o apart. Although the volute pressure inequalities remain, there are now two opposing radial forces which almost cancel out. In practice the cancellation is not complete but nevertheless a major reduction in radial thrust at partial capacities is achieved.

Double volute casing pumps are most often found in the petro-chemical industry where consistent operation at partial capacities is commonplace. They are also occasionally found in high head per stage water pumps. Efficiency is slightly lower due to the additional wetted area.

Diffuser casings also virtually eliminate radial thrust in the same way as a double volute.

This is one of the reasons behind the reliable operation of diffuser type deep well vertical turbine pumps in which radial shaft support can only be supplied by water lubricated plain bearings.

FIGURE 5.48 (a) 100% BEP FLOW AND (b) REDUCED FLOW

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(a) (b)

FIGURE 5.49: (a) RADIAL THRUST AND (b) DOUBLE VOLUTE

5.2.2.2 Specific Speed

The characteristic differences between pumps can be described by a criterion referred to as specific speed, this being the speed of an ideal pump geometrically similar to the actual pump, which when running at this speed will raise a unit of volume in a unit of time of time through a unit of head.

The performance of a pump is expressed in terms of pump speed, total head and flow rate. Specific speed is calculated from the formula using data at the best efficiency point as follows:

Specific Speed Ns = N Q H

Where, in SI units [USS units]

N = speed of the pump in r.p.m

Q = flowrate in m3/s [US gpm] H = total dynamic head in m [ft]

Specific speed relates to the geometry of the pump rotor and is independent of the pump size. When applying the formula above, it should be noted that:

(a) With a double suction impeller, half the best efficiency flowrate should be used. (b) With a multistage pump, best efficiency conditions for the first stage impeller

only are considered in calculating specific speed.

The normal range of specific speed (SI units) is from 10 to 300 and the relationship between specific speed and basic impeller shape is illustrated in Fig. 5.50.

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Units of Specific Speed

(specific speed in US units)

FIGURE 5.50: UNITS OF SPECIFIC SPEED

Since specific speed is used only as an index or type number, certain liberties are permissible in selecting the units used as long as the relationship between the different units are clearly understood.

Consequently the numerical value of Ns will vary according to the units in which H and Q are expressed. The speed of the impeller is always given in r.p.m. The relationship of specific speed in various units is as follows:

Relationship of Specific Speed in Various Units

Metric Ns Q (m3/s) H (m)

1.00

British Ns Q (igpm) H(ft)

47.13

USA Ns Q (usgpm) H(ft)

51.64

Specific Speed and Pump Type

By categorising centrifugal pumps in terms of specific speed, the pump user can visualise the type of pump he is likely to need.

e.g suppose a pump is required to deliver 0.10 m3/s at a head of 89m at 1450 r.p.m. Using the equation:

Ns = 1450 0.10 89

= 15.82 (817 in US units)

This is indicative of a fairly low specific speed radial flow pump. Hence the user can expect a large pump with relatively low efficiency. However, if the speed is doubled to 2900 r.p.m the specific speed of the pump will be 31.64 (1635 in US units).

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Consequently the pump can be smaller with a higher efficiency which may allow a less powerful driver, and make the overall pumpset package much less expensive. Associated civil costs would also be correspondingly reduced.

Hence specific speed can be a valuable guide to the pump user in planning and designing a more cost effective pumping station.

Radial flow pumps have a specific speed between 10 and 90, mixed flow pumps between 40 and 160, and axial flow pumps between 150 and 420.

Specific Speed and Pump Efficiency

The maximum attainable efficiency of centrifugal pumps depends to a great extant on pump geometry as categorised by specific speed. In general, the efficiency increases as the value of Ns rises.

FIGURE 5.51: RELATIONSHIP BETWEEN PUMP EFFICIENCY AND SPECIFIC SPEED (US UNITS)

Fig. 5.51 illustrates the relationship between pump efficiency and specific speed. The trend of the curve indicates that for economical operation very low values of Ns are to be avoided; a condition that can be overcome by employing multistage pumps.

Specific Speed and Characteristic Curves

Specific speed not only relates to impeller geometry, but also the trend to the characteristic curves for total dynamic head, power consumption and efficiency, examples of which are shown in Fig. 5.50. The steepness of the H Q curve increases as specific speed increases, whilst the efficiency curve remains relatively unchanged, except to exhibit a narrow plateau when Ns is high.

At low specific speed, power consumption is a minimum at zero flowrate and rises with flowrate. This is an overloading characteristic and the motor must be sized to meet the extreme conditions that may occur in operation. At medium specific speed the power curve has a pronounced peak at approximately the design duty. This is a non overloading characteristic and therefore the pump can work safely over the entire flow range with a motor sized to meet the peak requirements.

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High specific speed pumps have a failing power characteristic with maximum power occurring at minimum flow. Such pumps should not, therefore, be used in zero flow conditions. Discharge throttle valves or by-pass systems should be avoided with this type of pump otherwise a motor of considerably greater power that necessary for normal duty will be required.

5.2.3 Pump Suction Requirements There are two basic arrangements of surface water pumps namely those including a suction lift and those with a positive or gravity head on the inlet side. It is preferable that any rotodynamic pump is sited so that negative pressure does not develop on the suction side. Negative pressures can result in reduced performance and may prevent the pump from being automatically primed. There is also the risk of vortexing at the suction inlet causing further problems. If negative pressures are unavoidable for a surface mounted pump then a self priming pump must be specified.

Unfortunately, it is not always possible to avoid suction lift with raw water pumping on account of the costs that would be involved to do so. It is however almost always possible to avoid negative suction heads with onward or treated water pumping.

It is recommended however that negative suction be avoided whenever possible, even in raw water pumping stations by using a more suitable arrangement of design as illustrated in Figure 5.52. Suction lift pumping and maximum suction lift calculations have been detailed in sections 5.4.1.1 and 5.5.5.

FIGURE 5.52: PREFERRED ARRANGEMENT OF PUMPING STATIONS

5.2.4 Common Pump Types used in water supply

Water pumps may be considered as belonging to one of six types, namely:

Centrifugal pumps; Axial and mixed flow pumps; Reciprocating pumps; Rotary pumps; Jet pumps; and Air lift pumps.

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Their principle characteristics and applicability are summarised in the following table:

TABLE 5.1: PUMP TYPE, CHARACTERISTICS AND APPLICABILITY

TYPE OF PUMP CHARACTERISTICS AND APPLICABILITY 1. Centrifugal (radial flow) a. Standard (dry) b. Submerged

Medium to high speed of operation (usually 880 3,500 rpm). Smooth, even discharge. Motor powered. Efficiency ranges from 50% - 85% largely depending upon operational speed and pumping head. Capacity range 25 10, 0000 l/min. Less suitable for variable heads. Requires skilled maintenance. Specific speed 10 -90.

TYPE OF PUMP CHARACTERISTICS AND APPLICABILITY 2. Axial and Mixed Flow High capacity and low lift pumping somewhat similar to centrifugal

pumps. Can pump water containing sand and silt. Requires skilled maintenance. Specific speeds 40-60 for mixed flow and 150 -240 for axial flow.

3. Reciprocating

a. Suction b. Lift (deep well)

Low speed of operation: Hand, animal, wind, solar or motor powered. Low efficiency (range 25% - 60%) Capacity range: 10 50 l/min Suitable for pumping against variable heads. Valves and cup seals require frequent maintenance or replacement

4. Rotary Helical rotor (Mono type)

Medium speed of operation. Hand, animal, solar, wind or motor powered (using gearing). Medium to good efficiency (range 60% - 70%). Best suited for low capacity and high lift (submerged) pumping.

5. Jet Used for increasing the suction depth of small centrifugal pumps down to 75 m, thus allowing the pump unit to be placed on the ground. Relatively low efficiency. Capacity ranges 10 -800 l/min, suitable for sandy waters as the sand can be removed before entering the pump.

6. Air lift Commonly used to test boreholes. Usually of low efficiency

5.2.5 Operations of Centrifugal Pumps When the rotational speed of a centrifugal pump is changed, there is little change in efficiency but the output, head developed and the power required alter according to the following relationships which are known as the affinity laws.

Where N is the rotational speed, Q the output, and P the power required:

Q1 / Q2 = N1 / N2; H1 / H2 = (N1)2 / (N2)2; and P1 / P2 = (N1)3 / (N2)3.

These relationships are illustrated in Figure 5.53.

FIGURE 5.53: EFFECT OF CHANGING SPEED OF PUMP

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Air in a pump will reduce its efficiency and may induce corrosion as a result of cavitation. The presence of air is usually apparent by a hard crackling noise as if the pump had some gravel inside and if such a sound does not disappear shortly after starting, the cause should be investigated.

Net positive suction head (NPSH) which is largely dependent on temperature and elevation determines the maximum suction head possible.

Specific speed enables the classification of geometrically similar pumps and is determined from the expression,

Nsp = (N Q1/2) / H3/4 5.1

Where,

N is the impeller speed (rpm), Q is the output at maximum efficiency (m3/s), and H is the equivalent head (m)

5.2.6 Efficiencies and Fuel Consumption of Rotodynamic Pumps and their Motors

Giving efficiencies and fuel consumption is difficult because they vary so widely with every individual case having something unique about it. However the figures given in the following tables may be taken as a guide to the efficiencies and fuel consumption that can be expected from a well designed installation, although wide variations occur according to the power rating and type of pump and motor.

Because electricity tariffs in Tanzania are high, power costs can account to 30 40 % of total running costs. It is therefore necessary to select as efficient pumping plant as practical. It is further wise to assume an annual efficiency drop of at least 1% per year.

TABLE 5.2: COMPARATIVE EFFICIENCIES FOR DIFFERENT PUMPS AND MOTORS

UNIT EFFICIENCY RANGE THAT CAN BE EXPECTED Horizontal Centrifugal Pump Medium sized 80 82%, up to 85% for large size and even

higher with special construction involving higher price Submersible Pump Small size 70%, larger sizes 75 81%. Generally about 3%

less than for an equivalent duty, vertical spindle pump due to its restricted diameter.

Vertical spindle shaft driven pump

Generally about 3% less than a similar duty horizontal pump.

Horizontally mounted electric motor

Fixed speed, alternating current (AC) induction motor, 93 95%

Vertically mounted electric motor

Fixed speed AC induction motor, 90 94%

Submersible pumpset motor Because of restricted diameter usually between 85 89%

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TABLE 5.3: OVERALL FUEL CONSUMPTION

UNIT FUEL CONSUMPTION Electrically driven pumps

About 1.0 kW for every 0.75 kW of water power output which implies an efficiency of about 75%. Can however be up to 1.3 kW per 0.75 kW water power output or higher for small pumps and for variable speed pumps.

Diesel Engines Average 0.28 kg of diesel fuel oil consumed per kWh of engine power exerted to 0.21 kg of diesel fuel oil consumed per kWh of engine power good. For lubricating oil add 5% to fuel oil cost.

5.3 PRIME MOVERS

Other than for the small pumps where wind, human or animal power is used, prime movers used to drive pumping machinery are essentially either:

Electric motors, or Diesel engines.

Diesel engines can either be coupled directly or through gearing or used to generate electricity which is then used to power an electric motor.

However, it must be noted that there are disadvantages with all prime movers.

For a water supply, the key is dependability and electric motors can only be as reliable as the supply of electricity to them. Unfortunately, much of Tanzania is yet to have a reliable virtually continuous power supply with both voltage fluctuations and unplanned power outages a not uncommon feature. In addition, there are many parts of the country and particularly rural areas that as yet have no such mains supply.

Diesel engines and diesel generators require the supply of fuel, usually in 45 gallon drums or by fuel tanker for the larger schemes. Fuel drums are often prone to pilferage unless stored securely and with use carefully monitored by scheme managers. Tanker delivered fuel requires permanent storage tanks into which the fuel can be transferred. The use of diesel generators to firstly generate electricity and then for that electricity to power electric motors loses efficiency at each stage of the process. Direct coupled diesel engines do not provide power for any ancillary equipment or site lighting.

Even where diesel generators are provided as a standby electrical source, they add significantly to the capital and operational costs and require periodic running as well as maintenance to keep them in an operational state to deal with mains power outages.

These prime movers and their characteristics are discussed below.

5.3.1 Electric Motors for Pump Drives

An electric motor converts electrical energy into mechanical energy. The most common type of electric motor used to run pumping machinery is the three-phase alternating current (AC) motor. They can be classified as follows:

AC induction motors of either the cage rotor or wound rotor type; synchronous motors; and commutator motors.

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AC induction fixed speed cage motors are the simplest, most robust, reliable and least cost of all electric motors. Synchronous motors are more expensive because of the need for more complex control equipment as well as a DC supply for the rotor. Commutator motors provide a variable speed source and in the days before electronic variable frequency drives used to be used where variable speeds were required.

Because by far the most commonly used electric motor for operating pumps are the AC induction motors, only these are described in more detail here.

5.3.1.1 The AC Induction Motor

5.3.1.1.1 General A typical AC motor consists of two parts:

1. An outside stationary stator having coils supplied with AC current to produce a rotating magnetic field, and;

2. An inside rotor attached to the output shaft that is given a torque by the rotating field.

In an AC induction motor, the phase differences between the three phases of the polyphase electrical supply create a rotating electromagnetic field in the motor.

Through electromagnetic induction, the rotating magnetic field induces a current in the conductors in the rotor, which in turn sets up a counterbalancing magnetic field that causes the rotor to turn in the direction the field is rotating. The rotor must always rotate slower than the rotating magnetic field produced by the polyphase electrical supply; otherwise, no counterbalancing field will be produced in the rotor.

Induction motors up to about 500 kW in output are produced in highly standardized frame sizes, making them nearly completely interchangeable between manufacturers (although European and North American standard dimensions are different).

There are two types of rotors used in induction motors.

Cage rotors: Most common AC motors use the cage rotor, which takes its name from its shape - a ring at either end of the rotor, with bars connecting the rings running the length of the rotor. It is typically cast aluminium or copper poured between the iron laminates of the rotor, and usually only the end rings will be visible. The vast majority of the rotor current will flow through the bars rather than the higher-resistance and usually varnished laminates. Very low voltages at very high currents are typical in the bars and end rings; high efficiency motors will often use cast copper in order to reduce the resistance in the rotor.

In operation, the cage motor may be viewed as a transformer with a rotating secondary - when the rotor is not rotating in sync. with the magnetic field, large rotor currents are induced; the large rotor currents magnetize the rotor and interact with the stator's magnetic fields to bring the rotor into synchronization with the stator's field. An unloaded cage motor at synchronous speed will consume electrical power only to maintain rotor speed against friction and resistance losses; and as the mechanical load increases, so will the electrical load - the electrical load being inherently related to the mechanical load. This is similar to a transformer, where the primary's electrical load is related to the secondary's electrical load.

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Furthermore, a stalled cage motor (overloaded or with a jammed shaft) will consume current limited only by circuit resistance as it attempts to start. Unless something else limits the current (or cuts it off completely) overheating and destruction of the winding insulation is the likely outcome.

Wound Rotor: An alternate design, called the wound rotor, is used when variable speed is required. In this case, the rotor has the same number of poles as the stator and the windings are made of wire, connected to slip rings on the shaft. Carbon brushes connect the slip rings to an external controller such as a variable resistor that allows changing the motor's slip rate. In certain high-power variable speed wound-rotor drives, the slip-frequency energy is captured, rectified and returned to the power supply through an inverter.

Compared to cage rotors, wound rotor motors are expensive and require maintenance of the slip rings and brushes, but they were the standard form for variable speed control before the advent of compact power electronic devices. Transistorised inverters with variable-frequency drive can now be used for speed control, and wound rotor motors are becoming less common. Transistorised inverter drives also allow the more-efficient three-phase motors to be used when only single-phase mains current is available.

Motor starting: Several methods of starting a polyphase motor are available. Where a large inrush current and high starting torque can be permitted, the motor can be started across the line, by applying full line voltage to the terminals (Direct-on-line, DOL). Where it is necessary to limit the starting inrush current (where the motor is large compared with the short-circuit capacity of the supply), reduced voltage starting using either series inductors, an autotransformer, thyristors, or electronic soft starters. A technique sometimes used is star-delta starting, where the motor coils are initially connected in wye arrangement for acceleration of the load, and then switched to delta arrangement when the load is up to speed. This technique is more common in motors manufactured in Europe than in those from North America. Transistorised drives can directly vary the applied voltage as required by the starting characteristics of the motor and load.

Soft starters are typically computerised devices that can fault find. They are however sophisticated and need suitably qualified technicians trained in their use. Should they go unserviceable they need returning to the manufacturer for attention.

Motor speed: The speed of an AC motor is determined primarily by the frequency of the AC supply and the number of poles in the stator winding, according to the relation:

Ns = (120 F) / p 5.2

Where,

Ns = Synchronous speed, in revolutions per minute (RPM) F = AC power frequency p = Number of poles per phase winding

Actual RPM for an induction motor will be less than this calculated synchronous speed by an amount known as slip, that increases with the torque produced. With no load, the speed will be very close to synchronous. When loaded, standard motors have between 2-3% slip, special motors may have up to 7% slip, and a class of motors known as torque motors are rated to operate at 100% slip (0 RPM/full stall).

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The slip of an AC motor is calculated by:

S = (Ns Nr) / Ns 5.3 (percentage slip = (Ns Nr) / Ns 100)

Where, Nr = Rotational speed, in revolutions per minute, and S = Normalised Slip, 0 to 1.

As an example, a typical four-pole motor running on 60 Hz might have a nameplate rating of 1725 RPM at full load, while its calculated speed is 1800 RPM.

Submersible pump motors: These are typically a two-phase AC servo motor with a squirrel-cage rotor and a field consisting of two windings: 1) a constant-voltage (AC) main winding, and 2) a control-voltage (AC) winding in quadrature with the main winding so as to produce a rotating magnetic field. The electrical resistance of the rotor is made high intentionally so that the speed-torque curve is fairly linear. Two-phase servo motors are inherently high-speed, low-torque devices..

5.3.1.1.2 Electric Motor Failure Modes

- New Methods for Low Resistance Protection

Mostly due to low insulation resistance, as many as 90% of motor and generator electrical problems occur at start-up. These are trickier than over-current or mechanical failures, but new products and techniques can reliably insure against low resistance failures automatically and safely.

When electric motors or generators fail during use, it invariably means extensive equipment repair or replacement the high cost of which is often minor compared to the impact of unscheduled system downtime. Understanding the more common failure modes is essential to minimizing their impact in an operating environment.

Types of Failure: Electric motors typically fail due to one of three causes:

low resistance (electric ground) mechanical (bearing failure, vibration, etc.) over-current (electrical overload)

Low resistance is one of the most common failure causes. Also known as Monday Morning or Start-up failure (these are the circumstances in which it usually strikes), low resistance is traditionally the most difficult electrical problem to protect against, since it can occur either through conventional, predictable wear or sudden, catastrophic fashion.

A combination of protective devices and/or predictive maintenance tests and procedures has long helped users guard against mechanical and over-current failures

However, devices to protect against low resistance failures, are recent developments and have not established uniformly good reputations for effectiveness, or more importantly for safety.

Distinctions between Failure Modes; How is Low Resistance Different?

Mechanical Failures: Mechanical failures happen for a wide variety of reasons, including inadequate lubrication, unbalance and vibration, and misalignment. What most mechanical failures have in common is that they happen gradually; they display characteristic warning signs which intensify over time. An increasing assortment of

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sophisticated analytical techniques and services can detect impending mechanical failure at ever-earlier stages, allowing corrective measures as part of an overall, scheduled preventive maintenance program.

Any good electrical apparatus shop can recommend suitable diagnostic tests and instrumentation. For all practical purposes, mechanical failures can be averted through routine preventive and predictive maintenance.

Over-current Failures: Over-current failures happen most often when operating conditions cause devices to draw substantially more current than their rated load capacity. They tend to happen suddenly, and are not conducive to preventive procedures or predictive mea