Pumps and Pumping System

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					                      Best Practices for Agriculture Pump Sets and Rural Demand Side Management (DSM)
                           Distribution Reform, Upgrades and Management (DRUM) Training Program




                   AGRICULTURAL PUMPING SYSTEMS




                                        PREPARED BY




     ENERGY ECONOMY & ENVIRONMENTAL CONSULTANTS
          #506, 15TH CROSS, INDIRA NAGAR II STAGE
                      BANGALORE 560 038
                      PHONE: 080 – 25213986 – 89, FAX: 080 – 25259172
                             EMAIL: mail@3ecindia.com

                                            JULY 2005



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                                                        Table of Contents

1             Introduction ................................................................................................................. 6
2             Definition of Important Terms .................................................................................. 8
2.1           Key parameters ..............................................................................................................8
      2.1.1        Capacity .................................................................................................................. 8
      2.1.2        Head ........................................................................................................................ 9
      2.1.3        NPSH .................................................................................................................... 12
      2.1.4        Power and Efficiency ............................................................................................ 15
      2.1.5        Specific Speed ....................................................................................................... 17
3             Understanding Performance Curves....................................................................... 22
3.1           Performance Curve ......................................................................................................22
3.2           Developing a System Curve.........................................................................................23
3.3           Developing a Pump Performance Curve......................................................................23
3.4           Normal Operating Range .............................................................................................23
4             Components of pumping System ............................................................................. 24
5             Basic Concepts of Operation, Maintenance of Centrifugal Pumps ...................... 26
5.1           General Components of Centrifugal Pumps ................................................................26
5.2           Working Mechanism of a Centrifugal Pump ...............................................................26
      5.2.1        Generation of Centrifugal Force ........................................................................... 27
      5.2.2        Conversion of Kinetic Energy to Pressure Energy ............................................... 27
5.3           Requirements for Consistent Operation .......................................................................28
6             NPSH: A Critical Consideration When Selecting Pumps ..................................... 30
6.1           Net Positive Suction Head (NPSH) and Cavitation .....................................................30
6.2           Vapor Pressure and Cavitation.....................................................................................32
6.3           Two values of NPSH ...................................................................................................33
      6.3.1        Calculating NPSHA .............................................................................................. 34
      6.3.2        Effect of low NPSH .............................................................................................. 35
6.4           NPSHr for single suction, mixed flow and axial flow pumps .....................................35
7             Types of Pumps ......................................................................................................... 38
7.1           Pump classes ................................................................................................................38
      7.1.1        Centrifugal Pumps ................................................................................................ 39


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      7.1.2        Positive Displacement Pumps ............................................................................... 39
      7.1.3        Between Centrifugal or Positive Displacement Pumps ........................................ 40
7.2           Centrifugal Pumps .......................................................................................................40
7.3           Deep Well Turbine Pumps ...........................................................................................41
      7.3.1        Operating Characteristics ...................................................................................... 44
      7.3.2        Using the Pump Curve .......................................................................................... 45
      7.3.3        Installation of Vertical Turbine Pumps ................................................................. 46
7.4           Submersible Pumps......................................................................................................47
7.5           Propeller Pumps ...........................................................................................................48
7.6           Pump Selection Criteria ...............................................................................................50
7.7           Factors to Consider in Selecting an Irrigation Pump ...................................................50
8             Pump Selection .......................................................................................................... 53
8.1           Introduction: .................................................................................................................53
8.2           Basic Pump Operating Characteristics: .......................................................................53
      8.2.1        Total Dynamic Head: ............................................................................................ 53
      8.2.2        Total Static Head: ................................................................................................. 54
      8.2.3        Pressure Head: ...................................................................................................... 54
      8.2.4        Friction Head: ....................................................................................................... 55
      8.2.5        Velocity Head: ...................................................................................................... 55
      8.2.6        Suction Head: ........................................................................................................ 55
      8.2.7        Pump Power Requirements: .................................................................................. 56
      8.2.8        Effect of Speed Change on Pump Performance: ................................................... 56
      8.2.9        Pump Efficiency: .................................................................................................. 57
      8.2.10       Reading a Pump Curve: ........................................................................................ 58
      8.2.11       Changing Pump Speed: ......................................................................................... 58
8.3           Submersible Pumps......................................................................................................58
8.4           Consideration of the Parameters of Head, Discharge and speed in the
              selection of the pump ...................................................................................................59
8.5           Summary View of Application Parameters and Suitability of Pump ..........................60
8.6           Quick Selection: ...........................................................................................................61
      8.6.1        Factors for selection of Pump set .......................................................................... 61
      8.6.2        Yield of the bore well: .......................................................................................... 61


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       8.6.3        Depth to low water level: ...................................................................................... 62
       8.6.4        Height and length of delivery point: ..................................................................... 62
8.7            Pump Installation .........................................................................................................63
8.8            SELECTING PUMPSETS wrt acre.............................................................................63
8.9            Precautions to be taken while erecting a pump ............................................................64
8.10           Trouble Shooting: ........................................................................................................65
       8.10.1       Basic complaint ..................................................................................................... 65
       8.10.2       Advanced troubleshooting .................................................................................... 68
9              Concepts Of Efficiency And Its Assessments ......................................................... 71
9.1            Power and Efficiency ...................................................................................................71
       9.1.1        Brake Horse Power (BHP) .................................................................................... 71
       9.1.2        Best Efficiency Point (BEP) ................................................................................. 72
       9.1.3        Significance of BEP .............................................................................................. 72
9.2            Pump set Efficiency – Field Evaluation.......................................................................72
9.3            Distribution of losses in a pumping system .................................................................74
9.4            Symptoms that Indicate Potential Opportunity ............................................................75
9.5            Check List for Energy Savings in Pumping Systems ..................................................75
10             Proposed options ....................................................................................................... 76
10.1           Defects in the system: ..................................................................................................76
10.2           Options for Improvement:............................................................................................76
10.3           Analysis of proposed options for pumps .....................................................................76
       10.3.1       Partial rectification ................................................................................................ 76
       10.3.2       Complete Replacement ......................................................................................... 77
10.4           Energy efficient pumping system ................................................................................77
10.5           Retrofitting of Pump set ...............................................................................................79
10.6           Flow control Strategies using Throttling .....................................................................81
10.7           Effect of Pipe Diameter on cost savings ......................................................................81
10.8           Capacitor usage PF improvement: ...............................................................................82
10.9           Cost Benefit Analysis for replacement of pump sets – Case Study.............................83
       10.9.1       Methodology ......................................................................................................... 83
       10.9.2       Efficiency Analysis ............................................................................................... 83
       10.9.3       Cost benefit analysis – F12 ................................................................................... 84

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    10.9.4     Cost benefit analysis for F13 ................................................................................ 86
10.10      Replacement of GI pipes with HDPE pipes – Case Study...........................................87
    10.10.2          Effects of Change in Pipe material: .................................................................. 89
10.11      Common Causes and Remedies ...................................................................................91
    10.11.1          Cost vs. Savings From Repair or Replacement ................................................ 92
10.12      Conservation tips for agriculture pumps ......................................................................92




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                                                CHAPTER I
                                 1           INTRODUCTION


An Overview

Pumping systems account for nearly 20% of the world‟s electrical energy demand and range
from 25-50% of the energy usage in certain industrial plant operations. Pumping systems are
widespread; they provide domestic services, commercial and agricultural services, municipal
water/wastewater services, and industrial services for food processing, chemical, petrochemical,
pharmaceutical, and mechanical industries.

Although pumps are typically purchased as individual components, they provide a service only
when operating as part of a system. The energy and materials used by a system depend on the
design of the pump, the design of the installation, and the way the system is operated. These
factors are interdependent. What‟s more, they must be carefully matched to each other, and
remain so throughout their working lives to ensure the lowest energy and maintenance costs,
equipment life, and other benefits. The initial purchase price is a small part of the life cycle cost
for high usage pumps. While operating requirements may sometimes override energy cost
considerations, an optimum solution is still possible.

A greater understanding of all the components that make up the total cost of ownership will
provide an opportunity to dramatically reduce energy, operational, and maintenance costs.
Reducing energy consumption and waste also has important environmental benefits.

Pumping System in India
Pumping systems account for an estimated 30% of the electricity used in the industrial sector in
India, or almost 15% of total national electricity consumption. In many process plants, pumping
systems are estimated to account for 40 – 60 % of the total electricity consumption and therefore
warrant particularly close attention. Essentially all electricity used in the agricultural sector is for
pumping.

The Indian power sector is facing severe capital and capacity shortages. Scheduled and
unscheduled power cuts are common in most parts of the county. The low power tariffs for
irrigation pump sets (IPS) are said to be the major reason for the poor financial health of the
power sector. The electricity subsidy for IPS was Rs. 101.13 billion (US$ 2.89 billion) in 1995.
The subsidised power tariff (based on the connected load) and poor efficiency of pumping
systems is a cause of concern for the power sector. In the last two decades, the growth rate of
electricity use by IPS has been about 12% per annum. This growth rate is twice as high as that of
other sectors. In 1995, IPS consumption, as claimed by the power sector, was 28% of total sales
(Planning Commission 1995).[1] And nearly 500,000 IPS continue to be added to the number in
service each year. The efficiency of IPS is dismally low. Field studies and



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pilot projects have demonstrated that IPS electricity consumption can be reduced by 30 to 50%
by simple measures, such as the use of higher efficiency pump sets and pipes of larger diameter.
The payback period for such investments is 1 to 2 years [NABARD, 1984; Patel and Pandey,
1993]. But past efforts have been mostly directed towards rectification of old IPS and a lot needs
to be done to ensure efficient installation of half a million new IPS each year.

The agricultural sector accounts for about 30% of electricity consumption in India. The largest
population of inefficient pumps and systems is also to be found in this sector. Two factors that
adversely impact electricity consumption are, efficiency of the pumping system, and inadequate
standards for motors and pump-sets.


Purpose of this manual
This manual entry addresses the basic concepts of pumping system, selection of pumpsets,
various factors affects the system efficiency and the available techniques to improve the same
with case studies.




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                                                 CHAPTER II
                        2   DEFINITION OF IMPORTANT TERMS


2.1 Key parameters

The key performance parameters of pumps are capacity, head, BHP (Brake horse power), BEP
(Best efficiency point) and specific speed. The pump curves provide the operating window
within which these parameters can be varied for satisfactory pump operation. The following
parameters or terms are discussed in detail in this section.

    Capacity
    Head
         o       Significance of using Head instead of Pressure
         o       Pressure to Head Conversion formula
         o       Static Suction Head, hS
         o       Static Discharge Head, hd
         o       Friction Head, hf
         o       Vapour pressure Head, hvp
         o       Pressure Head, hp
         o       Velocity Head, hv
         o       Total Suction Head HS
         o       Total Discharge Head Hd
         o       Total Differential Head HT
         o       Shut-off head
   NPSH
         o       Net Positive Suction Head Required NPSHr
         o       Net Positive Suction Head Available NPSHa
 Power (Brake Horse Power, B.H.P) and Efficiency (Best Efficiency Point, B.E.P)
 Specific Speed (Ns)

   Affinity Laws

2.1.1 Capacity

Capacity means the flow rate with which liquid is moved or pushed by the pump to the desired
point in the process. It is commonly measured in either gallons per minute (gpm) or cubic

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meters per hour (m3/hr). The capacity usually changes with the changes in operation of the
process. For example, a boiler feed pump is an application that needs a constant pressure with
varying capacities to meet a changing steam demand.

The capacity depends on a number of factors like:

           Process liquid characteristics i.e. density, viscosity
           Size of the pump and its inlet and outlet sections
           Impeller size
           Impeller rotational speed RPM
           Size and shape of cavities between the vanes
           Pump suction and discharge temperature and pressure conditions

For a pump with a particular impeller running at a certain speed in a liquid, the only items on the
list above that can change the amount flowing through the pump are the pressures at the pump
inlet and outlet. The effect on the flow through a pump by changing the outlet pressures is
graphed on a pump curve.

As liquids are essentially incompressible, the capacity is directly related with the velocity of flow
in the suction pipe. This relationship is as follows:




2.1.2 Head

Significance of using the “head” term instead of the “pressure” term
The pressure at any point in a liquid can be thought of as being caused by a vertical column of
the liquid due to its weight. The height of this column is called the static head and is expressed
in terms of feet of liquid.


A given pump with a given impeller diameter and speed will raise a liquid to a certain height
regardless of the weight of the liquid


The same head term is used to measure the kinetic energy created by the pump. In other words,
head is a measurement of the height of a liquid column that the pump could create from the



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kinetic energy imparted to the liquid. Imagine a pipe shooting a jet of water straight up into the
air, the height the water goes up would be the head.


The head is not equivalent to pressure. Head is a term that has units of a length or feet and
pressure has units of force per unit area or pound per square inch. The main reason for using
head instead of pressure to measure a centrifugal pump's energy is that the pressure from a
pump will change if the specific gravity (weight) of the liquid changes, but the head will not
change. Since any given centrifugal pump can move a lot of different fluids, with different
specific gravities, it is simpler to discuss the pump's head and forget about the pressure.
So a centrifugal pump‟s performance on any Newtonian fluid, whether it's heavy (sulphuric acid)
or light (gasoline) is described by using the term „head‟. The pump performance curves are
mostly described in terms of head.

Pressure to Head Conversion formula

The static head corresponding to any specific pressure is dependent upon the weight of the liquid
according to the following formula:




Newtonian liquids have specific gravities typically ranging from 0.5 (light, like light
hydrocarbons) to 1.8 (heavy, like concentrated sulfuric acid). Water is a benchmark, having a
specific gravity of 1.0.

This formula helps in converting pump gauge pressures to head for reading the pump curves.
The various head terms are discussed below.

Note: The Subscripts„s‟ refers to suction conditions and „d‟ refers to discharge conditions.

     o          Static Suction Head, hS
     o          Static Discharge Head, hd
     o          Friction Head, hf
     o          Vapor pressure Head, hvp
     o          Pressure Head, hp
     o          Velocity Head, hv
     o          Total Suction Head HS
     o          Total Discharge Head Hd

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     o          Total Differential Head HT
     o          Net Positive Suction Head Required NPSHr
     o          Net Positive Suction Head Available NPSHa

Static Suction Head (hS) : Head resulting from elevation of the liquid relative to the pump
center line. If the liquid level is above pump centerline, hS are positive. If the liquid level is
below pump centerline, hS are negative. Negative hS condition is commonly denoted as a
“suction lift” condition




Static Discharge Head (hd): It is the vertical distance in feet between the pump centerline and
the point of free discharge or the surface of the liquid in the discharge tank.

Friction Head (hf): The head required to overcome the resistance to flow in the pipe and
fittings. It is dependent upon the size, condition and type of pipe, number and type of pipe
fittings, flow rate, and nature of the liquid.

Vapor Pressure Head (hvp): Vapor pressure is the pressure at which a liquid and its vapor co-
exist in equilibrium at a given temperature. The vapor pressure of liquid can be obtained from
vapor pressure tables. When the vapor pressure is converted to head, it is referred to as vapor
pressure head, hvp. The value of hvp of a liquid increases with the rising temperature and in
effect, opposes the pressure on the liquid surface, the positive force that tends to cause liquid
flow into the pump suction i.e. it reduces the suction pressure head.

Pressure Head (hp): Pressure Head must be considered when a pumping system either begins or
terminates in a tank which is under some pressure other than atmospheric. The pressure in such a
tank must first be converted to feet of liquid. Denoted as hp, pressure head refers to absolute


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pressure on the surface of the liquid reservoir supplying the pump suction, converted to feet of
head. If the system is open, hp equals atmospheric pressure head.

Velocity Head (hv): Refers to the energy of a liquid as a result of its motion at some velocity
„v’. It is the equivalent head in feet through which the water would have to fall to acquire the
same velocity, or in other words, the head necessary to accelerate the water. The velocity head is
usually insignificant and can be ignored in most high head systems. However, it can be a large
factor and must be considered in low head systems.

Total Suction Head (HS): The suction reservoir pressure head (hpS) plus the static suction head
(hS) plus the velocity head at the pump suction flange (hVS) minus the friction head in the suction
line (hfS).

HS = hpS + hS + hvS – hfS

The total suction head is the reading of the gauge on the suction flange, converted to feet of
liquid.

Total Discharge Head (Hd): The discharge reservoir pressure head (hpd) plus static discharge
head (hd) plus the velocity head at the pump discharge flange (hvd) plus the total friction head in
the discharge line (hfd).

Hd = hpd + hd + hvd + hfd

The total discharge head is the reading of a gauge at the discharge flange, converted to feet of
liquid.

Total Differential Head (HT): It is the total discharge head minus the total suction head or

HT = Hd + HS (with a suction lift)
HT = Hd - HS (with a suction head)

Shut-off head:

A pump's vertical discharge "pressure-head" is the vertical lift in height - usually measured in
feet or m of water - at which a pump can no longer exert enough pressure to move water. At this
point, the pump may be said to have reached its "shut-off" head pressure. In the flow curve chart
for a pump the "shut-off head" is the point on the graph where the flow rate is zero.

2.1.3 NPSH

When discussing centrifugal pumps, the two most important head terms are NPSHr and NPSHa.
Net Positive Suction Head Required, NPSHr




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NPSH is one of the most widely used and least understood terms associated with pumps.
Understanding the significance of NPSH is very much essential during installation as well as
operation of the pumps.


Pumps can pump only liquids, not vapours
The satisfactory operation of a pump requires that vaporization of the liquid being pumped does
not occur at any condition of operation. This is so desired because when a liquid vaporizes its
volume increases very much. For example, 1 ft3 of water at room temperature becomes 1700 ft3
of vapour at the same temperature. This makes it clear that if we are to pump a fluid effectively,
it must be kept always in the liquid form.
Rise in temperature and fall in pressure induces vaporization
The vaporization begins when the vapour pressure of the liquid at the operating temperature
equals the external system pressure, which, in an open system is always equal to atmospheric
pressure. Any decrease in external pressure or rise in operating temperature can induce
vaporization and the pump stops pumping. Thus, the pump always needs to have a sufficient
amount of suction head present to prevent this vaporization at the lowest pressure point in the
pump.
NPSH as a measure to prevent liquid vaporization
The manufacturer usually tests the pump with water at different capacities, created by throttling
the suction side. When the first signs of vaporization induced cavitation occur, the suction
pressure is noted (the term cavitation is discussed in detail later). This pressure is converted into
the head. This head number is published on the pump curve and is referred as the "net positive
suction head required (NPSHr) or sometimes in short as the NPSH. Thus the Net Positive
Suction Head (NPSH) is the total head at the suction flange of the pump less the vapour
pressure converted to fluid column height of the liquid.

NPSHr is a function of pump design

NPSH required is a function of the pump design and is determined based on actual pump test by
the vendor. As the liquid passes from the pump suction to the eye of the impeller, the velocity
increases and the pressure decrease. There are also pressure losses due to shock and turbulence
as the liquid strikes the impeller. The centrifugal force of the impeller vanes further increases the
velocity and decreases the pressure of the liquid. The NPSH required is the positive head in feet
absolute required at the pump suction to overcome these pressure drops in the pump and
maintain the majority of the liquid above its vapor pressure.

The NPSH is always positive since it is expressed in terms of absolute fluid column height. The
term "Net" refers to the actual pressure head at the pump suction flange and not the static suction
head.

NPSHr increases as capacity increases



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The NPSH required varies with speed and capacity within any particular pump. The NPSH
required increase as the capacity is increasing because the velocity of the liquid is increasing, and
as anytime the velocity of a liquid goes up, the pressure or head comes down.                  Pump
manufacturer's curves normally provide this information. The NPSH is independent of the fluid
density as are all head terms. Note: It is to be noted that the net positive suction head required
(NPSHr) number shown on the pump curves is for fresh water at 20°C and not for the fluid or
combinations of fluids being pumped.

Net Positive Suction Head available, NPSHa

NPSHa is a function of system design

Net Positive Suction Head Available is a function of the system in which the pump operates. It
is the excess pressure of the liquid in feet absolute over its vapor pressure as it arrives at the
pump suction, to be sure that the pump selected does not cavitate. It is calculated based on
system or process conditions.

NPSHa calculation

The formula for calculating the NPSHa is stated below:




Note:

1.      It is important to correct for the specific gravity of the liquid and to convert all terms to
units of "feet absolute" in using the formula.
2.      Any discussion of NPSH or cavitation is only concerned about the suction side of the
pump. There is almost always plenty of pressure on the discharge side of the pump to prevent the
fluid from vaporizing.


NPSHa in a nutshell


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In a nutshell, NPSH available is defined as:

NPSHa = Pressure head + Static head - Vapor pressure head of your product – Friction head loss
in the piping, valves and fittings.

“All terms in feet absolute”

In an existing system, the NPSHa can also be approximated by a gauge on the pump suction
using the formula:

NPSHa = hpS - hvpS  hgS + hvS

     1.     hpS = Barometric pressure in feet absolute.
     2.     hvpS = Vapor pressure of the liquid at maximum pumping temperature, in feet
                    absolute.
     3.     hgS = Gauge reading at the pump suction expressed in feet (plus if above
                    atmospheric, minus if below atmospheric) corrected to the pump centerline.
     4.     hvS = Velocity head in the suction pipe at the gauge connection, expressed in feet.

Significance of NPSHr and NPSHa

The NPSH available must always be greater than the NPSH required for the pump to operate
properly. It is normal practice to have at least 2 to 3 feet of extra NPSH available at the suction
flange to avoid any problems at the duty point.

2.1.4 Power and Efficiency

Brake Horse Power (BHP)

The work performed by a pump is a function of the total head and the weight of the liquid
pumped in a given time period.

Pump input or brake horsepower (BHP) is the actual horsepower delivered to the pump shaft.

Pump output or hydraulic or water horsepower (WHP) is the liquid horsepower delivered by
the pump. These two terms are defined by the following formulas.




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The constant 3960 is obtained by dividing the number or foot-pounds for one horsepower
(33,000) by the weight of one gallon of water (8.33 pounds).

BHP can also be read from the pump curves at any flow rate. Pump curves are based on a
specific gravity of 1.0. Other liquids‟ specific gravity must be considered.

The brake horsepower or input to a pump is greater than the hydraulic horsepower or output due
to the mechanical and hydraulic losses incurred in the pump.

Therefore the pump efficiency is the ratio of these two values.




Best Efficiency Point (BEP)

The H, NPSHr, efficiency, and BHP all vary with flow rate, Q. Best Efficiency Point (BEP) is
the capacity at maximum impeller diameter at which the efficiency is highest. All points to the
right or left of BEP have a lower efficiency.

Significance of BEP

BEP as a measure of optimum energy conversion

When sizing and selecting centrifugal pumps for a given application the pump efficiency at
design should be taken into consideration. The efficiency of centrifugal pumps is stated as a
percentage and represents a unit of measure describing the change of centrifugal force (expressed
as the velocity of the fluid) into pressure energy. The B.E.P. (best efficiency point) is the area on
the curve where the change of velocity energy into pressure energy at a given gallon per minute
is optimum; in essence, the point where the pump is most efficient.

BEP as a measure of mechanically stable operation


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The impeller is subject to non-symmetrical forces when operating to the right or left of the BEP.
These forces manifest themselves in many mechanically unstable conditions like vibration,
excessive hydraulic thrust, temperature rise, and erosion and separation cavitation. Thus the
operation of a centrifugal pump should not be outside the furthest left or right efficiency curves
published by the manufacturer. Performance in these areas induces premature bearing and
mechanical seal failures due to shaft deflection, and an increase in temperature of the process
fluid in the pump casing causing seizure of close tolerance parts and cavitation.

BEP as an important parameter in calculations

BEP is an important parameter in that many parametric calculations such as specific speed,
suction specific speed, hydrodynamic size, viscosity correction, head rise to shut-off, etc. are
based on capacity at BEP. Many users prefer that pumps operate within 80% to 110% of BEP
for optimum performance.

2.1.5 Specific Speed

Specific speed as a measure of the geometric similarity of pumps

Specific speed (Ns) is a non-dimensional design index that identifies the geometric similarity of
pumps. It is used to classify pump impellers as to their type and proportions. Pumps of the same
Ns but of different size are considered to be geometrically similar, one pump being a size-factor
of the other.

Specific speed Calculation

The following formula is used to determine specific speed:




As per the above formula, it is defined as the speed in revolutions per minute at which a
geometrically similar impeller would operate if it were of such a size as to deliver one gallon per
minute flow against one-foot head.

The understanding of this definition is of design engineering significance only, however, and
specific speed should be thought of only as an index used to predict certain pump characteristics.



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Specific speed as a measure of the shape or class of the impellers

The specific speed determines the general shape or class of the impellers. As the specific speed
increases, the ratio of the impeller outlet diameter, D2, to the inlet or eye diameter, D1,
decreases. This ratio becomes 1.0 for a true axial flow impeller. Radial flow impellers develop
head principally through centrifugal force. Radial impellers are generally low flow high head
designs. Pumps of higher specific speeds develop head partly by centrifugal force and partly by
axial force. A higher specific speed indicates a pump design with head generation more by axial
forces and less by centrifugal forces. An axial flow or propeller pump with a specific speed of
10,000 or greater generates its head exclusively through axial forces. Axial flow impellers are
high flow low head designs.

Specific speed identifies the approximate acceptable ratio of the impeller eye diameter (D1) to
the impeller maximum diameter (D2) in designing a good impeller.

Ns: 500 to 5000; D1/D2 > 1.5 - radial flow pump
Ns: 5000 to 10000; D1/D2 < 1.5 - mixed flow pump
Ns: 10000 to 15000; D1/D2 = 1 - axial flow pump

Specific speed is also used in designing a new pump by size-factoring a smaller pump of the
same specific speed. The performance and construction of the smaller pump are used to predict
the performance and model the construction of the new pump.

Suction specific speed (Nss)

Suction specific speed (Nss) is a dimensionless number or index that defines the suction
characteristics of a pump. It is calculated from the same formula as Ns by substituting H by
NPSHr.

In multi-stage pump the NPSHr is based on the first stage impeller NPSHR.

Specific speed as a measure of the safe operating range

Nss is commonly used as a basis for estimating the safe operating range of capacity for a pump.
The higher the Nss is, the narrower is its safe operating range from its BEP. The numbers range
between 3,000 and 20,000. Most users prefer that their pumps have Nss in the range of 8000 to
11000 for optimum and trouble-free operation.

Pump affinity laws
The running speed of the electric induction motors is at slip from its synchronous speed. The
running speed of the motors is also influenced by variations in the supply frequency. Since the
pump characteristics furnished by the pump manufacturers is at certain nominal speed,
depending upon the actual speed while running, the actual pump performance would be different



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from declared characteristics. Estimates of the pump performance in actual running can be
worked out from the declared characteristics, by using the following affinity laws.
If n‟‟/ n‟ = k then Q‟‟/Q‟= k
H‟‟/H‟= k2, then P‟‟/P‟ = k3
In the above formulae, n denotes the speed of the pump, p denotes the power input to the pump,
the subscript ‟‟ denotes the values at the actual speed and the subscript‟ denotes the values at the
nominal speed.
Recalculating the pump – performance at actual speed would reveal the following.
     If the actual speed is less than the nominal speed, then the values of the discharge, head and
      power required to be input to pump would all be less than the values from the declared
      characteristics
     Similarly, if the actual speed is more than the nominal, it should be checked that the higher
      power input required would not be overload the motor.
     When the actual speed is more, because the discharge is also correspondingly more, the
      NPSHr would also be more, varying as per the following formula
           o     NPSHr‟‟/ NPSHr‟ = k2


Scope for Adjusting the Actual Characteristics
To avoid overloading or cavitation, marginal adjustment to the pump performance may be done
at the site, either by employing speed – change arrangements or by trimming down the impeller.
The modifications in the performance on trimming the impeller can be estimated using the
following relations:
If D‟‟/D‟ = k, then Q‟‟/Q‟ = k
H‟‟/ H‟ = k2
Then P‟‟/ P‟ = k3
Such modifications are recommended to be done with in 10 – 15 % of the largest diameter of the
impeller. The percentage depends upon the design, size and shape of the impeller. The pump
manufacturer should be concluded on this.




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                                      Figure 4.1: Affinity Laws

The Affinity Laws are mathematical expressions that define changes in pump capacity, head, and
BHP when a change is made to pump speed, impeller diameter, or both. According to Affinity
Laws:

        Capacity, Q changes in direct proportion to impeller diameter D ratio, or to speed N
         ratio:

         Q2 = Q1 x [D2/D1]
         Q2 = Q1 x [N2/N1]

        Head, H changes in direct proportion to the square of impeller diameter D ratio, or the
         square of speed N ratio:

         H2 = H1 x [D2/D1]2
         H2 = H1 x [N2/N1]2

        BHP changes in direct proportion to the cube of impeller diameter ratio, or the cube of
         speed ratio:

         BHP2 = BHP1 x [D2/D1]3
         BHP2 = BHP1 x [N2/N1]3



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Where the subscript: 1 refers to initial condition, 2 refer to new condition

If changes are made to both impellers diameter and pump speed the equations can be combined
to:

Q2 = Q1 x [(D2xN2)/ (D1xN1)]

H2 = H1 x [(D2xN2)/ (D1xN1)] 2

BHP2 = BHP1 x [(D2xN2)/ (D1xN1)] 3

This equation is used to hand-calculate the impeller trim diameter from a given pump
performance curve at a bigger diameter.

The Affinity Laws are valid only under conditions of constant efficiency.




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                                                CHAPTER III
                     3   UNDERSTANDING PERFORMANCE CURVES
3.1 Performance Curve

The capacity and pressure needs of any system can be defined with the help of a graph called a
system curve. Similarly the capacity vs. pressure variation graph for a particular pump defines
its characteristic pump performance curve.

The pump suppliers try to match the system curve supplied by the user with a pump curve that
satisfies these needs as closely as possible. A pumping system operates where the pump curve
and the system resistance curve intersect. The intersection of the two curves defines the
operating point of both pump and process. However, it is impossible for one operating point to
meet all desired operating conditions. For example, when the discharge valve is throttled, the
system resistance curve shift left and so does the operating point.




Figure 3.1: Typical system and pump performance curves




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3.2 Developing a System Curve

The system resistance or system head curve is the change in flow with respect to head of the
system. It must be developed by the user based upon the conditions of service. These
include physical layout, process conditions, and fluid characteristics. It represents the
relationship between flow and hydraulic losses in a system in a graphic form and, since friction
losses vary as a square of the flow rate, the system curve is parabolic in shape. Hydraulic losses
in piping systems are composed of pipe friction losses, valves, elbows and other fittings,
entrance and exit losses, and losses from changes in pipe size by enlargement or reduction in
diameter.



3.3 Developing a Pump Performance Curve

A pump's performance is shown in its characteristics performance curve where its capacity i.e.
flow rate is plotted against its developed head. The pump performance curve also shows its
efficiency (BEP), required input power (in BHP), NPSHr, speed (in RPM), and other information
such as pump size and type, impeller size, etc. This curve is plotted for a constant speed (rpm)
and a given impeller diameter (or series of diameters). It is generated by tests performed by
the pump manufacturer. Pump curves are based on a specific gravity of 1.0. Other specific
gravities must be considered by the user.

3.4 Normal Operating Range

A typical performance curve (Figure 3.1) is a plot of Total Head vs. Flow rate for a specific
impeller diameter. The plot starts at zero flow. The head at this point corresponds to the shut-off
head point of the pump. The curve then decreases to a point where the flow is maximum and the
head minimum. This point is sometimes called the run-out point. The pump curve is relatively
flat and the head decreases gradually as the flow increases. This pattern is common for radial
flow pumps. Beyond the run-out point, the pump cannot operate. The pump's range of operation
is from the shut-off head point to the run-out point. Trying to run a pump off the right end of the
curve will result in pump cavitations and eventually destroy the pump.

In a nutshell, by plotting the system head curve and pump curve together, you can determine:

1.        Where the pump will operate on its curve?

2. What changes will occur if the system head curve or the pump performance curves changes?




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                                                CHAPTER IV
                        4    COMPONENTS OF PUMPING SYSTEM
Pumping systems are comprised of several components
1.        Prime mover
2.        Pump
3.        Piping
4.        Valves and other fittings
5.     Controls and instrumentation (like, Starter, Energy meter, Protection against dry run,
open delta)




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                                                                Prime mover
                                                                    Pressure Gauge
                                                                                                 Pump
                                            BFV


     Pipes



                                                      NRV


                            Figure 1.1: A typical Pumping System



Note: BFV – Butterfly Valve, NRV – Non Return Valve

High pumping system efficiency is a function of not only individual component efficiencies but
also of system design. Indeed, poor system design can erode the gains achieved by selecting high
efficiency components.




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                                                CHAPTER V
5    BASIC CONCEPTS OF OPERATION, MAINTENANCE OF CENTRIFUGAL
                             PUMPS
5.1 General Components of Centrifugal Pumps

A centrifugal pump has two main components:

I. A rotating component comprised of an impeller and a shaft

II. A stationary component comprised of a casing, casing cover, and bearings.

The general components, both stationary and rotary, are depicted in Figure. The main components
are discussed in brief below. Figure2.1 shows these parts on a photograph of a pump in the field.




                       Figure 2.1: General components of Centrifugal Pump

5.2 Working Mechanism of a Centrifugal Pump

A centrifugal pump is one of the simplest pieces of equipment in any process plant. Its purpose
is to convert energy of a prime mover (an electric motor or turbine) first into velocity or kinetic
energy and then into pressure energy of a fluid that is being pumped. The energy changes occur
by virtue of two main parts of the pump, the impeller and the volute or diffuser. The impeller is the
rotating part that converts driver energy into the kinetic energy. The volute or diffuser is the
stationary part that converts the kinetic energy into pressure energy.

Note: All of the forms of energy involved in a liquid flow system are expressed in terms of feet
of liquid i.e. head.

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5.2.1 Generation of Centrifugal Force

The process liquid enters the suction nozzle and then into eye (center) of a revolving device
known as an impeller. When the impeller rotates, it spins the liquid sitting in the cavities
between the vanes outward and provides centrifugal acceleration. As liquid leaves the eye of the
impeller a low-pressure area is created causing more liquid to flow toward the inlet. Because the
impeller blades are curved, the fluid is pushed in a tangential and radial direction by the
centrifugal force. This force acting inside the pump is the same one that keeps water inside a
bucket that is rotating at the end of a string. Figure 2.2 below depicts a side cross-section of a
centrifugal pump indicating the movement of the liquid.




                            Figure 2.2: Liquid flow path inside a
                            centrifugal pump

5.2.2 Conversion of Kinetic Energy to Pressure Energy

The key idea is that the energy created by the centrifugal force is kinetic energy. The amount of
energy given to the liquid is proportional to the velocity at the edge or vane tip of the impeller.
The faster the impeller revolves or the bigger the impeller is, then the higher will be the velocity
of the liquid at the vane tip and the greater the energy imparted to the liquid.

This kinetic energy of a liquid coming out of an impeller is harnessed by creating a resistance to
the flow. The first resistance is created by the pump volute (casing) that catches the liquid and
slows it down. In the discharge nozzle, the liquid further decelerates and its velocity is converted
to pressure according to Bernoulli‟s principle.

Therefore, the head (pressure in terms of height of liquid) developed is approximately equal to
the velocity energy at the periphery of the impeller expressed by the following well-known
formula:




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A handy formula for peripheral velocity is:




This head can also be calculated from the readings on the pressure gauges attached to the suction
and discharge lines.

           One       fact      that     must       always      be      remembered:
           A pump does not create pressure, it only provides flow. Pressure is a just
           an indication of the amount of resistance to flow.

Pump curves relate flow rate and pressure (head) developed by the pump at different impeller
sizes and rotational speeds. The centrifugal pump operation should conform to the pump curves
supplied by the manufacturer. In order to read and understand the pump curves, it is very
important to develop a clear understanding of the terms used in the curves. This topic will be
covered later.

5.3 Requirements for Consistent Operation

Centrifugal pumps are the ultimate in simplicity. In general there are two basic requirements that
have to be met at all the times for a trouble free operation and longer service life of centrifugal
pumps.

The first requirement is that no cavitation of the pump occurs throughout the broad operating
range and the second requirement is that a certain minimum continuous flow is always
maintained during operation.

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A clear understanding of the concept of cavitations, its symptoms, its causes, and its
consequences is very much essential in effective analyses and troubleshooting of the cavitations
problem.

Just like there are many forms of cavitations, each demanding a unique solution, there are a
number of unfavorable conditions which may occur separately or simultaneously when the pump
is operated at reduced flows. Some include:

            Cases of heavy leakages from the casing, seal, and stuffing box
            Deflection and shearing of shafts
            Seizure of pump internals
            Close tolerances erosion
            Separation cavitations
            Product quality degradation
            Excessive hydraulic thrust
            Premature bearing failures

Each condition may dictate a different minimum flow low requirement. The final decision on
recommended minimum flow is taken after careful “techno-economical” analysis by both the
pump user and the manufacturer.

 The consequences of prolonged conditions of cavitations and low flow operation can be
disastrous for both the pump and the process. Such failures in hydrocarbon services have often
caused damaging fires resulting in loss of machine, production, and worst of all, human life.

Thus, such situations must be avoided at all cost whether involving modifications in the pump
and its piping or altering the operating conditions. Proper selection and sizing of pump and its
associated piping can not only eliminate the chances of cavitations and low flow operation but
also significantly decrease their harmful effects.




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                                               CHAPTER VI
    6    NPSH: A CRITICAL CONSIDERATION WHEN SELECTING PUMPS
6.1 Net Positive Suction Head (NPSH) and Cavitation

The Hydraulic Institute defines NPSH as the total suction head in feet absolute, determined at the
suction nozzle and corrected to datum, less the vapor pressure of the liquid in feet absolute.
Simply stated, it is an analysis of energy conditions on the suction side of a pump to determine if
the liquid will vaporize at the lowest pressure point in the pump.

The pressure which a liquid exerts on its surroundings is dependent upon its temperature. This
pressure, called vapor pressure, is a unique characteristic of every fluid and increases with
increasing temperature. When the vapor pressure within the fluid reaches the pressure of the
surrounding medium, the fluid begins to vaporize or boil. The temperature at which this
vaporization occurs will decrease as the pressure of the surrounding medium decreases.

A liquid increases greatly in volume when it vaporizes. One cubic foot of water at room
temperature becomes 1700 cu. ft. of vapor at the same temperature.

It is obvious from the above that if we are to pump a fluid effectively, we must keep it in liquid
form. NPSH is simply a measure of the amount of suction head present to prevent this
vaporization at the lowest pressure point in the pump.

NPSH required is a function of the pump design. As the liquid passes from the pump suction to
the eye of the impeller, the velocity increases and the pressure decreases. There are also pressure
losses due to shock and turbulence as the liquid strikes the impeller. The centrifugal force of the
impeller vanes further increases the velocity and decreases the pressure of the liquid. The NPSH
required is the positive head in feet absolute required at the pump suction to overcome these
pressure drops in the pump and maintain the liquid above its vapor pressure. The NPSH required
varies with speed and capacity within any particular pump. Pump manufacturer's curves
normally provide this information.

NPSH Available is a function of the system in which the pump operates. It is the excess pressure
of the liquid in feet absolute over its vapor pressure as it arrives at the pump suction. Fig.6.1
shows four typical suction systems with the NPSH Available formulas applicable to each. It is
important to correct for the specific gravity of the liquid and to convert all terms to units of "feet
absolute" in using the formulas.




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Figure 6.1 Calculation of system Net Positive Suction Head Available for typical suction
conditions.

In an existing system. The NPSH Available can be determined by a gauge on the pump suction.
The following formula applies:

NPSHA= PB= Vp+ Gr + hv

Where:
Gr = Gauge reading at the pump suction expressed in feet (plus if above atmospheric minus if
below atmospheric) corrected to the pump centerline.

hv = Velocity head in the suction pipe at the gauge connection, expressed in feet.

Cavitation is a term used to describe the phenomenon which occurs in a pump when there is
insufficient NPSH Available. The pressures of the liquid are reduced to a value equal to or below
its vapor pressure and small vapor bubbles or pockets begin to form. As these vapor bubbles
move along the impeller vanes to a higher pressure area they rapidly collapse.

The collapse or "implosion" is so rapid that it may be heard as a rumbling noise, as if you were
pumping gravel. The forces during the collapse are generally high enough to cause minute



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pockets of fatigue failure on the impeller vane surfaces. This action may be progressive, and
under severe conditions can cause serious pitting damage to the impeller.

The accompanying noise is the easiest way to recognize cavitation. Besides impeller damage,
cavitation normally results in reduced capacity due to the vapor present in the pump. Also, the
head may be reduced and unstable and the power consumption may be erratic. Vibration and
mechanical damage such as bearing failure can also occur as a result of operating in cavitation.

The only way to prevent the undesirable effects of cavitation is to insure that the NPSH
Available in the system is greater than the NPSH required by the pump.

6.2 Vapor Pressure and Cavitation

The energy of the of water at the pump suction, even after deducting the NPSHr should be more
than the vapor pressure Vp, corresponding to the pumping requirement. The vapor pressure in
meters of the water column (mWC), for water at different temperatures in degree Celsius is in
the table below


Vapor Pressure of Water


                     Degree Celsius (0C)                    mWC
                             0                              0.054
                             5                              0.092
                             10                             0.125
                             15                             0.177
                             20                             0.238
                             25                             0.329
                             30                             0.427
                             35                             0.579
                             40                             0.762
                             45                             1.006
                             50                             1.281
                       Table: 6.1 Vapour Pressure of Water with temperature


If the energy of the water at the pump suction would be less than the vapor pressures, the water
would tend to evaporate. Vapor bubbles so formed will travel entrained in the flow until they
collapse. This phenomenon is known as cavitation. In badly devised pumping systems, cavitation

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can cause extensive damage due to cavitation erosion or due to the vibration and noise associated
with the collapsing of the vapor bubbles.
The critical role net positive suction head (NPSH) plays in the selection of pumps that
operate at the higher inlet water temperatures found in steam condensate service.

Anyone who has experienced cavitation damage to a pump impeller, shaft, or seals knows that
problems relating to net positive suction head (NPSH) can also mean "not pumping so hot."1 To
prevent damaging cavitation, it is necessary that engineers clearly understand the critical
importance of NPSH when selecting pumps.

 As liquid enters the eye of the impeller in a centrifugal pump, its pressure is reduced. If the
absolute pressure at the impeller eye drops down to the vapor pressure of the fluid, vapor pockets
begin to form. As these vapor pockets travel in the fluid along the vanes of the impeller, pressure
increases and the pockets collapse. This collapse is called cavitation. Cavitation is not only noisy
but also damages the pump impeller, shaft and seal, and over time, may reduce pumping
capacity. NPSH refers to the minimum suction pressure, expressed in feet of water column, that
is required to prevent the forming and collapsing of these vapor pockets.

Figure 6.2 shows the change in system pressure (Ps) as the fluid travels through the impeller. To
prevent cavitation, Ps must remain above the vapor pressure.




Figure 6.2 Top curve shows system pressure (Ps) remaining above fluid vapour pressure as it
passes through the pumps; cavitation cannot occur. Bottom curve shows Ps falling below the
vapour pressure as it enters the impeller eye. This will cause cavitation. Cutaway view of a pump
volute on the right shows the passage of flow through the impeller.

6.3 Two values of NPSH

There are two values of NPSH: NPSHR and NPSHA. NPSHR (required) is the amount of
suction head required to prevent pump cavitation, is determined by the pump design, and is


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indicated on the pump curve. NPSHA (available) is the amount of suction head available or total
useful energy above the vapour pressure at the pump suction. This is determined by the system
conditions. NPSH typically is measured in ft of liquid.

6.3.1 Calculating NPSHA

NPSHA, measured in feet, can be calculated by using the equation noted below. The vapor
pressure Pv of the liquid is subtracted from the system pressure Pa it is then converted from psia
to feet by multiplying by 2.31 and dividing by the specific gravity of the liquid. The static head
He, measured in feet, is determined by the elevation of the water line above the pump suction.
This value can be negative if the application is a suction lift. Finally the friction losses Hf,
measured in feet, are calculated and subtracted from He. This quantity is then added to the first
term of the equation.




In summary, NPSH is a critical factor in selecting condensate pumps due to the high
temperatures experienced in steam condensate systems. The NPSHA consists of many factors.
These include condensate temperature, pressure in the condensate receiver, elevation of the
condensate receiver, and pipe friction losses. Changing any of these has a direct effect on the
NPSHA.

The NPSHR is dependent on the manufacturer of the pump and its design of it. The manufacturer
needs to take into consideration such factors as motor speed, impeller inlet flow angle, number of
vanes, and the use of an inducer.

When selecting a condensate pump, the consulting engineer should be aware of these various
factors, specifically the use of an inducer and the speed of the motor. Taking these factors into
account will ensure that the correct choice is made to best fit the specific application.




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6.3.2 Effect of low NPSH


                                           Low NPSH required



                                  Higher Ss (Specific speed) Value




                                     Large Impeller Eye Diameter



                             Higher Capacity at Suction Recirculation



                                         Higher Minimum Flow



                            Narrower Range of Trouble Free Operation



6.4 NPSHr for single suction, mixed flow and axial flow pumps

1.      For vertical pumps, mainly of the vertical turbine type and of the bore – well submersible
type, suction lift has to be totally avoided. Even for these pumps, when the discharge required is
high, they have to be installed providing the minimum submergence. The minimum submergence
required may at times demand submerging more than the first stage of the pump. It should be
checked whether the submergence would be adequate for the vortex – free operation.
2.      Jet centrifugal combinations can work for lifting from depths up to 70 m. however; the
efficiency of the pump is very low.
3.      Positive displacement pumps are normally self – priming. However this should not be
confused with the NPSHr. Even if the NPSHa is not adequate, the pump may prime itself and
run, but would cavitate.




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Figure 6.3 a – NPSHr for single suction pump with overhung impeller




                                                                                                                   Min
                                                                                                                   NPSHr

                                                                                                                   Max.
                                                                                                                   Suction lift




     Figure 6.3 b – NPSHr for single suction pump with shaft through eye of impeller




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                Figure 6.3 c – NPSHr for single suction, mixed flow and axial flow pumps




H in m




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                                               CHAPTER VII
                                       7     TYPES OF PUMPS


7.1 Pump classes


Pumps may be classified in two general types, dynamic and positive displacement. Positive
displacement pumps are those in which energy is imparted to the liquid in a fixed displacement
volume, such as a casing or a cylinder, by the rotary motion of gears, screws, or vanes, or by
reciprocating pistons or plungers. Centrifugal pumps are dynamic pumps. Energy is imparted to
the liquid by means of a disk with curved vanes rotating on a shaft called the impeller. The
impeller imparts kinetic energy to the fluid by means of its shape and high rotational velocity.
This energy is transformed to pressure energy when the fluid reaches the pump casing (see
Figure 7-1). The pressure head difference between the inlet and the outlet, or Total Head
produced by the pump, is proportional to the impeller speed and diameter. Therefore, to obtain a
higher head, the rotational speed or the impeller diameter
can be increased.

How a pump produces pressure, an interesting experiment
you can try at home will illustrate a similar process. A
small plastic bottle is required to which a string is attached.
Twist a rubber band around the bottle‟s neck a few times
and attach two 3-foot long strings, one on each side of the
glass. Tie the other ends of the string together, fill the glass
half full with water and hold it suspended from the strings.
Start spinning. As you may have guessed, the fluid inside
the glass will become pressurized. How do you know that
the fluid is pressurized? To prove it to yourself, make a
very small hole in the glass bottom. Make the hole just
large enough for water to dribble through. Now spin the
glass again. The water will spray out of the glass bottom no
matter what its position, up or down

                                                                        Figure 7-1 Using a spinning Bottle
                                                                        to demonstrate centrifugal force.



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7.1.1   Centrifugal Pumps

The centrifugal pump produce a head and a flow by increasing the velocity of the liquid through
the machine with the help of a rotating vane impeller.
The centrifugal pump can be classified as:
    End suction pump
    In-line pump
    Double suction pump
    Vertical multistage pump
    Horizontal multistage pump
    Submersible pumps
    Self-priming pumps
    Axial-flow pumps
    Regenerative pumps

Centrifugal Pumps are "constant head machines".

7.1.2   Positive Displacement Pumps

The positive displacement pump operates by alternating of filling a cavity and then displacing a
given volume of liquid. The positive displacement pump delivers a constant volume of liquid
against varying discharge pressure or head.
The positive displacement pump can be classified as:
    Reciprocating pumps
    Power pumps
    Steam pumps
    Rotary pumps

A Positive Displacement Pump, unlike a Centrifugal Pump, will produce the same flow at a
given speed, RPM, no matter the discharge pressure.
    Positive Displacement Pumps are "constant flow machines".

A Positive Displacement Pump must not be operated against a closed valve on the discharge side
of the pump because it has no shut-off head like Centrifugal Pumps. A Positive Displacement
Pump operating against a closed discharge valve, will continue to produce flow until the pressure
in the discharge line are increased until the line bursts or the pump is severely damaged - or both.

A relief or safety valve on the discharge side of the Positive Displacement Pump is therefore
absolute necessary. The relief valve can be internal or external. The pump manufacturer has
normally the option to supply internal relief or safety valves. The internal valve should in general
only be used as a safety precaution, an external relief valve installed in the discharge line with a
return line back to the suction line or supply tank is recommended.




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7.1.3 Between Centrifugal or Positive Displacement Pumps

Selecting between a Centrifugal Pump or a Positive Displacement Pump is not always straight
forward.

7.1.3.1   Flow Rate and Pressure Head
The two types of pumps behave very differently regarding pressure head and flow rate:

     The Centrifugal Pump has varying flow depending on the system pressure or head
     The Positive Displacement Pump has more or less a constant flow regardless of the system
      pressure or head. Positive Displacement pumps generally gives more pressure than
      Centrifugal Pump's.

7.1.3.2   Capacity and Viscosity
Another major difference between the pump types is the effect of viscosity on the capacity:

     In the Centrifugal Pump the flow is reduced when the viscosity is increased
     In the Positive Displacement Pump the flow is increased when viscosity is increased

Liquids with high viscosity fills the clearances of a Positive Displacement Pump causing a higher
volumetric efficiency and a Positive Displacement Pump is better suited for high viscosity
applications. A Centrifugal Pump becomes very inefficient at even modest viscosity.

7.1.3.3   Mechanical Efficiency
The pumps behaves different considering mechanical efficiency as well.

     Changing the system pressure or head has little or no effect on the flow rate in the Positive
      Displacement Pump
     Changing the system pressure or head has a dramatic effect on the flow rate in the
      Centrifugal Pump

7.1.3.4   Net Positive Suction Head - NPSH
Another consideration is the Net Positive Suction Head NPSH.

     In a Centrifugal Pump, NPSH varies as a function of flow determined by pressure
     In a Positive Displacement Pump, NPSH varies as a function of flow determined by speed.
      Reducing the speed of the Positive Displacement Pump pump, reduces the NPSH



7.2 Centrifugal Pumps

Centrifugal pumps are used to pump from reservoirs, lakes, streams and shallow wells. They are
also used as booster pumps in irrigation pipelines. All centrifugal pumps must be completely


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filled with water or "primed" before they can operate. The suction line as well as the pump have
to be filled with water and free of air. Air tight joints and connections are extremely important on
the suction pipe. Priming a pump can be done by hand operated vacuum pumps, internal
combustion engine vacuum, motor powered vacuum pumps or small water pumps that fill the
pump and suction pipe with water.

Centrifugal pumps are designed for either horizontal or vertical operation. The horizontal
centrifugal has a vertical impeller connected to a horizontal drive shaft as shown in Figure 7-2.
Horizontal centrifugal pumps are the most common in irrigation systems. They are generally less
costly, require less maintenance, easier to install and more accessible for inspection and
maintenance than a vertical centrifugal. There are self-priming horizontal centrifugal pumps, but
they are special purpose pumps and not normally used with irrigation systems.




                           Figure 7-2. A horizontal centrifugal pump.

Vertical centrifugal pumps may be mounted so the impeller is under water at all times. This
makes priming unnecessary, which makes the vertical centrifugal desirable for floating
applications. Also, a self priming feature is very desirable in areas where there are frequent
electrical power outages or off-peak electrical price reductions are available. Self priming also
lends itself to the new control panels for center pivots where automatic restart is a programmable
function. A note of caution: because the bearings are constantly under water, a higher level of
maintenance may be required.

7.3 Deep Well Turbine Pumps

Deep well turbine pumps are adapted for use in cased wells or where the water surface is below
the practical limits of a centrifugal pump. Turbine pumps are also used with surface water
systems. Since the intake for the turbine pump is continuously under water, priming is not a
concern. Turbine pump efficiencies are comparable to or greater than most centrifugal pumps.
They are usually more expensive than centrifugal pumps and more difficult to inspect and repair.
The turbine pump has three main parts: (1) the head assembly, (2) the shaft and column assembly
and (3) the pump bowl assembly as shown in Figure 7-3. The head is normally cast iron and
designed to be installed on a foundation. It supports the column, shaft and bowl assemblies and
provides a discharge for the water. It also will support either an electric motor, a right angle gear
drive or a belt drive.



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The shaft and column assembly provides a connection between the head and pump bowls. The
line shaft transfers the power from the motor to the impellers and the column carries the water to
the surface. The line shaft on a turbine pump may be either water lubricated or oil lubricated.

The oil-lubricated pump has an enclosed shaft into which oil drips, lubricating the bearings. The
water-lubricated pump has an open shaft. The bearings are lubricated by the pumped water. If
there is a possibility of fine sand being pumped, select the oil lubricated pump because it will
keep the sand out of the bearings. If the water is for domestic or livestock use, it must be free of
oil and a water-lubricated pump must be used. In some states, such as Minnesota, there is no
choice; water-lubricated pumps are required in all new irrigation wells.
Line shaft bearings are commonly placed on 10-foot centers for water-lubricated pumps
operating at speeds under 2,200 RPM and at 5-foot centers for pumps operating at higher speeds.
Oil-lubricated bearings are commonly placed on 5-foot centers.

A pump bowl encloses the impeller. Due to its limited diameter, each impeller develops a
relatively low head. In most deep well turbine installations several bowls are stacked in series
one above the other. This is called staging. A four-stage bowl assembly contains four impellers
all attached to a common shaft and will operate at four times the discharge head of a single-stage
pump.
Impellers used in turbine pumps may be either semi-open or enclosed as shown in Figure 7-4.
The vanes on semi-open impellers are open on the bottom and they rotate with a close tolerance
to the bottom of the pump bowl. The tolerance is critical and must be adjusted when the pump is
new. During the initial break-in period the line shaft couplings will tighten, therefore, after about
100 hours of operation, the impeller adjustments should be checked. After break-in, the tolerance
must be checked and adjusted every three to five years or more often if pumping sand.




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                           Figure 7-3. A deep well turbine pump.




Figure 7-4. Both enclosed and semi-open impellers are used in vertical turbine and
centrifugal pumps. Only enclosed impellers are used in submersible pumps.


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Both types of impellers may cause inefficient pump operation if they are not properly adjusted.
Mechanical damage will result if the semi-open impellers are set too low and the vanes rub
against the bottom of the bowls. The adjustment of enclosed impellers is not as critical; however,
they must still be checked and adjusted.

Impeller adjustments are made by tightening or loosening a nut on the top of the head assembly.
Impeller adjustments are normally made by lowering the impellers to the bottom of the bowls
and adjusting them upward. The amount of upward adjustment is determined by how much the
line shaft will stretch during pumping. The adjustment must be made based on the lowest
possible pumping level in the well. The proper adjustment procedure if often provided by the
pump manufacturer. The adjustment procedure for many of the common deep well turbine
brands is outlined in Nebraska Cooperative Extension Service bulletin EC 81-760, entitled "How
to adjust vertical turbine pumps for maximum efficiency."

7.3.1 Operating Characteristics

The operating characteristics of deep well turbine pumps are determined by test and depend
largely on the bowl design, impeller type and the speed of the impeller shaft. Flow rate, TDH,
BHP, efficiency, and RPM are similar to those given for centrifugal pumps. Vertical turbine
pumps are generally designed for a specific RPM setting.

A vertical turbine pump curve is shown in Figure 7-5. This pump curve is similar to the
centrifugal pump curve except instead of curves for various RPM's, the curves are for different
diameter impellers. Decreasing the diameter of impellers is called "trimming." Manufacturers
will trim impellers to the proper size to match the TDH and flow rate requirements of a specific
irrigation installation. Pump curves for turbine pumps are normally shown for a single stage so
the TDH obtained will be determined by multiplying the indicated head on the pump curve by
the number of stages. The brake horsepower requirements must also be multiplied by the number
of stages. Note that the flow rate will not change no matter how many stages are added.




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Figure 7-5. Deep well turbine pump curve. The brake horsepower and total head are for
one stage. If the pump had five stages, multiply the brake horsepower and the total head
values by five. The gallons per minute will stay the same no matter how many stages are
added.


7.3.2 Using the Pump Curve

As an example, suppose the pump curve in Figure 7-5 is for a 5-stage pump, with a 7.13 inch
impeller supplying 800 GPM. What would be the TDH and BHP values?

Solution: Follow the dashed vertical line from 800 GPM up to where it meets the 7.13-inch
impeller curve on the upper portion of the chart. Follow the dashed horizontal line left to where it
shows 26 feet of TDH. Multiplying 26 by 5 gives 130 feet of TDH. Next, follow the dashed
vertical line from 800 GPM up to the 7.13-inch impeller BHP curve on the lower portion of the
chart and then follow the horizontal dashed line left to where it shows 6.5 BHP. Multiplying 6.5
BHP by 5 stages produces a 32.5 BHP requirement for this pump. Also note that the pump is
operating at its peak efficiency of 80 percent. At this efficiency the calculated BHP (equations 1
and 2) is 32.8.




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7.3.3 Installation of Vertical Turbine Pumps

Deep well turbine pumps must have correct alignment between the pump and the power unit.
Correct alignment is made easy by using a head assembly that matches the motor and
column/pump assembly. It is very important that the well is straight and plumb. The pump
column assembly must be vertically aligned so that no part touches the well casing. Spacers are
usually attached to the pump column to prevent the pump assembly from touching the well
casing. If the pump column does touch the well casing, vibration will wear holes in the casing. A
pump column out of vertical alignment may also cause excessive bearing wear.

The head assembly must be mounted on a good foundation at least 12 inches above the ground
surface. A foundation of concrete (Figure 7-6) provides a permanent and trouble-free installation.
The foundation must be large enough to allow the head assembly to be securely fastened. The
foundation should have at least 12 inches of bearing surface on all sides of the well. In the case
of a gravel-packed well, the 12-inch clearance is measured from the outside edge of the
gravel packing.




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Figure 7.6. Recommended concrete base with access pipe for water level measurement and
chlorination.


A well access pipe at least 1.5 inches in diameter must extend through the foundation into the
well casing. The access pipe serves two purposes. The first is to measure both static and pumping
water levels in the well and the second is to allow chlorination of the well. A 3/4-inch diameter
polyethylene tubing with the bottom end closed inserted into the access pipe and extending to the
pump level will make measuring water levels much easier. Small holes must be drilled into the
tubing to allow water to move in and out of the tubing easily. More information on well
maintenance can be found in NDSU circular AE-97, "Operation and Maintenance of Irrigation
Wells."

7.4 Submersible Pumps

A submersible pump is a turbine pump close-coupled to a submersible electric motor as shown in
Figure 7.7. Both pump and motor are suspended in the water, thereby eliminating the long drive
shaft and bearing retainers required for a deep well turbine pump. Because the pump is located
above the motor, water enters the pump through a screen located between the pump and motor.




Figure 7.7. A submersible pump installed in a well.


The submersible pump uses enclosed impellers because the shaft from the electric motor expands
when it becomes hot and pushes up on the impellers. If semi-open impellers were used, the pump

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would lose efficiency. The pump curve for a submersible pump is very similar to a deep well
turbine pump.

Submersible motors are smaller in diameter and much longer than ordinary motors. Because of
their smaller diameter, they are lower efficiency motors than those used for centrifugal or deep
well turbine pumps. Submersible motors are generally referred to as dry or wet motors. Dry
motors are hermetically sealed with a high dielectric oil to exclude water from the motor. Wet
motors are open to the well water with the rotor and bearings actually operating in the water.

If there is restricted or inadequate circulation of water past the motor, it may overheat and
burn out. Therefore, the length of riser pipe must be sufficient to keep the bowl assembly
and motor completely submerged at all times. In addition, the well casing must be large
enough to allow water to easily flow past the motor.

Small submersible pumps (under 5 horsepower) use single-phase power. However, most
submersible pumps used for irrigation need three-phase electrical power. Electrical wiring from
the pump to the surface must be watertight and all connections sealed. The electrical line should
be attached to the column pipe every 20 feet to prevent it from wrapping around the column pipe.
Voltage at the motor leads must be within plus or minus 10 percent of the motor nameplate
voltage. If there is a 5 percent voltage drop in the submersible pump cable, voltage at the surface
must not be less than 95 percent of rated voltage. Because the pump is located in the well,
lightning protection should be wired into the control box. Lightning hits on wells with
submersible pumps is a leading cause of pump failures.

Submersible pumps can be selected to provide a wide range of flow rate and TDH combinations.
Submersible pumps more than 10 inches in diameter generally cost more than comparably sized
deep well turbines because the motors are more expensive.

Many manufacturers make submersible booster pumps. These pumps are usually mounted
horizontally in a pipeline. An advantage to using a submersible as a booster pump instead of a
centrifugal is noise reduction. This is a desirable attribute in residential settings and near golf
courses. Submersibles have also been used as booster pumps in the suction lines of centrifugal
pumps. This application is used in situations where the water level will fluctuate a considerable
amount over the season. Having a submersible in the suction line will change the head at the inlet
of the centrifugal pump from a suction head to a positive head.

7.5 Propeller Pumps

Propeller pumps are used for low lift, high flow rate conditions. They come in two types, axial
flow and mixed flow. The difference between the two is the type of impeller. The axial flow
pump uses an impeller that looks like a common boat motor screw and is essentially a very low
head pump. A single-stage propeller pump typically will lift water no more than 20 feet. By
adding another stage, heads from 30 to 40 feet are obtainable. The mixed-flow pump uses either
semi-open or closed impellers similar to turbine pumps.



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In permanent installations, propeller pumps are mounted vertically as shown in Figure 7-8. For
portable pumping platforms, they are mounted on trailers or they are mounted on pontoons for
use as floating intakes. Portable propeller pumps are commonly mounted in almost horizontal
positions (low angles) to allow them to pump into pipelines easily as well as to be backed into a
water source. Portable propeller pumps are commonly powered by the power-take-off (PTO) on
tractors. On many farms, propeller pumps are used to pump out waste storage lagoons.




                                    Figure 7-8. A propeller pump.


Power requirements of the propeller pump increase directly with the TDH so adequate power
must be provided to drive the pump at maximum lift. Propeller pumps are not suitable under
conditions where it is necessary to throttle the discharge to reduce the flow rate. It is important to
accurately determine the maximum TDH against which this type of pump will operate.

Propeller pumps are not suitable for suction lift. The impeller must be submerged and the pump
operated at the proper submergence depth. The depth of submergence will vary according to
various manufacturers recommendations, but generally, the greater the diameter of pump, the

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deeper the submergence. Following recommended submergence depths will ensure that the flow
rate is not reduced due to vortices. Also, failure to observe required submergence depth may
cause severe mechanical vibrations and rapid deterioration of the propeller blades.

7.6 Pump Selection Criteria

The selection of an irrigation water pump is based almost entirely on the relationship between
pump efficiency and the TDH the pump will provide at a specific flow rate. As shown before,
these parameters are also the basis of the pump characteristic curve. Table 2 can be used to
narrow the selection of a pump type for a broad range of flow rates and total dynamic heads. One
item not included in the TDH values in Table 2 is the suction lift. If your application needs to lift
the water to the pump then a centrifugal pump will have to be used.

Table 2. A chart showing the most desirable pump types to use for a given range of flow rates and total
dynamic heads.
---------------------------------------------------------------------------------
                                               TDH (feet)
Gallons per -----------------------------------------------------------------
Minute            50 or less             50 to 500                   500 or more
---------------------------------------------------------------------------------
0 to 300       Propeller                 Centrifugal                Centrifugal
                Centrifugal              Vertical Turbine           Vertical Turbine
                                         Submersible                 Submersible
--------------------------------------------------------------------------------
300 to 5000 Propeller                    Centrifugal                Centrifugal
                                         Vertical Turbine           Vertical Turbine
                                         Submersible                Submersible
---------------------------------------------------------------------------------
5000 or more Propeller                   Centrifugal                Centrifugal
                                         Vertical Turbine           Vertical Turbine
                                         Propeller
                                         Submersible
---------------------------------------------------------------------------------

7.7 Factors to Consider in Selecting an Irrigation Pump

----------------------------------------------------------------------------------------------------
Pump Type         Advantages                                        Disadvantages
----------------------------------------------------------------------------------------------------



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Centrifugal 1. High efficiency over                                 1. Suction lift is limited.
                    a range of operating                             It needs to be within
                           conditions.                                 20 vertical feet of the
                 2. Easy to install.                                water surface.
                 3. Simple, economical and 2. Priming required.
                    adaptable to many                               3. Loss of prime can damage
                           situations.                               pump.
                 4. Electric, internal                              4. If the TDH is much lower
                    combustion engines or                            than design value, the
                           tractor power can be                        motor may overload.
                           used.
                 5. Does not overload with
                           increased TDH.
                 6. Vertical centrifugal
                           may be submerged and
                           not need priming.
-------------------------------------------------------------------------------------------------------
Vertical      1. Adapted for use in                                 1. Difficult to install,
Turbine                    wells.                                      inspect, and repair.
                 2. Provides high TDH and                           2. Higher initial cost
                           flow rates with                          high than a centrifugal pump.
                           efficiency.                              3. To maintain high
                 3. Electric or internal                             efficiency, the impellers
                           combustion power can                        must be adjusted
                           be used.                                  periodically.
                 4. Priming not needed.                             4. Repair and maintenance
                 5. Can be used where                               is more expensive
                           water surface                               than centrifugals.
                           fluctuates.
-------------------------------------------------------------------------------------------------------
Submersible 1. Can be used in deep                                  1. More expensive in larger
                           wells.                                      sizes than deep well
                 2. Priming not needed                              vertical turbines.
                 3. Can be used in crooked                          2. Only electric power
                           wells.                                      can be used.
                 4. Easy to install.                                3. More susceptible to


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                 5. Smaller diameters are                           lightning.
                           less expensive than                      4. Water movement past
                           comparable sized                          motor is required.
                           vertical turbines.
-------------------------------------------------------------------------------------------------------
Propeller      1. Simple construction.                              1. Not suitable for suction
                 2. Can pump some sand.                              lift.
                 3. Priming not needed.                             2. Cannot be valved back
                 4. Efficient at pumping                            to reduce flow rate.
                           very large flow rates                    3. Intake submergence depth
                           at low TDH.                                 is very critical.
                 5. Electric, internal                              4. Limited to low (less
                           combustion engine and                       than 75 feet) TDH.
                           tractor power can be
                           used.
                 6. Suitable for portable
                           operation.
------------------------------------------------------------------------------------------------------




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                                                CHAPTER VIII
                                        8     PUMP SELECTION


8.1 Introduction:

The heart of most irrigation systems is a pump. To make an irrigation system as efficient as
possible, the pump must be selected to match the requirements of the water source, the water
piping system and the irrigation equipment.
Pumps used for irrigation include centrifugal, deep well turbine, submersible and propeller
pumps. Actually, turbine, submersible and propeller pumps are special forms of a centrifugal
pump. However, their names are common in the industry. In this circular the term centrifugal
pump will refer to any pump located above the water surface and using a suction pipe.
Before selecting an irrigation pump, a careful and complete inventory of the conditions under
which the pump will operate must take place. The inventory must include:
1.        The source of water (well, river, pond, etc.)
2.        The required pumping flow rate
3.        The total suction head
4.        The total dynamic head
There usually is no choice when it comes to the source of the water; it is either surface water or
well water and availability will be determined by the local geology and hydrologic conditions.
However, the flow rate and total dynamic head will be determined by the type of irrigation
system, the distance from the water source and the size of the piping system.


8.2 Basic Pump Operating Characteristics:
"Head" is a term commonly used with pumps. Head refers to the height of a vertical column of
water. Pressure and head are interchangeable concepts in irrigation, because a column of water
2.31 feet high is equivalent to 1 pound per square inch (PSI) of pressure (1 PSI = 0.07 Kg/cm2).
The total head of a pump is composed of several types of head that help define the pump's
operating characteristics.


8.2.1 Total Dynamic Head:
The total dynamic head of a pump is the sum of the total static head, the pressure head, the
friction head, and the velocity head. An explanation of these terms is given below and
graphically shown in Figure 8.1.




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Figure 8.1:The Total Dynamic Head (TDH) is the sum of the total static head, the total
friction head and the pressure head. The components of the total static head for a surface
water and well water pumping system are shown.


8.2.2 Total Static Head:


The total static head is the total vertical distance the pump must lift the water. When pumping
from a well, it would be the distance from the pumping water level in the well to the ground
surface plus the vertical distance the water is lifted from the ground surface to the discharge
point. When pumping from an open water surface it would be the total vertical distance from the
water surface to the discharge point.


8.2.3 Pressure Head:


Sprinkler and drip irrigation systems require pressure to operate. Center pivot systems require a
certain pressure at the pivot point to distribute the water properly. The pressure head at any point
where a pressure gage is located can be converted from pounds per square inch (PSI) to feet of
head by multiplying by 2.31. For example, 20 PSI is equal to 20 times 2.31 or 46.2 feet of head.
Most city water systems operate at 50 to 60 PSI, which, as illustrated in Table 1, explains why
the centers of most city water towers are about 130 feet above the ground.

                      Table 1. Pounds per square inch (PSI) and


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                      equivalent head in feet of water.

                           PSI                               Head (feet)

                            0                                      0
                            5                                     11.5
                           10                                     23.1
                           15                                     34.6
                           20                                     46.2
                           25                                     57.7



8.2.4 Friction Head:

Friction head is the energy loss or pressure decrease due to friction when water flows through
pipe networks. The velocity of the water has a significant effect on friction loss. Loss of head
due to friction occurs when water flows through straight pipe sections, fittings, valves, around
corners, and where pipes increase or decrease in size. Values for these losses can be calculated or
obtained from friction loss tables. The friction head for a piping system is the sum of all the
friction losses.


8.2.5 Velocity Head:

Velocity head is the energy of the water due to its velocity. This is a very small amount of energy
and is usually negligible when computing losses in an irrigation system.


8.2.6 Suction Head:

A pump operating above a water surface is working with a suction head. The suction head
includes not only the vertical suction lift, but also the friction losses through the pipe, elbows,
foot valves and other fittings on the suction side of the pump. There is an allowable limit to the
suction head on a pump and the net positive suction head (NPSH) of a pump sets that limit.
The theoretical maximum height that water can be lifted using suction is 33 feet. Through
controlled laboratory tests, manufacturers determine the NPSH curve for their pumps. The NPSH
curve will increase with increasing flow rate through the pump. At a certain flow rate, the NPSH
is subtracted from 33 feet to determine the maximum suction head at which that pump will
operate. For example, if a pump requires a minimum NPSH of 20 feet the pump would have a
maximum suction head of 13 feet. Due to suction pipeline friction losses, a pump rated for a
maximum suction head of 13 feet may effectively lift water only 10 feet. To minimize the
suction pipeline friction losses, the suction pipe should have a larger diameter than the discharge
pipe.




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Operating a pump with suction lift greater than it was designed for, or under conditions with
excessive vacuum at some point in the impeller, may cause cavitation. Cavitation is the
implosion of bubbles of air and water vapor and makes a very distinct noise like gravel in the
pump. The implosion of numerous bubbles will eat away at an impeller and it eventually will be
filled with holes.

8.2.7 Pump Power Requirements:

The power added to water as it moves through a pump can be calculated with the following
formula:


                Q x TDH
      WHP = -----------      (1)
                 3960
where:
 WHP = Water Horse Power
 Q = Flow rate in gallons per minute (GPM)
 TDH = Total Dynamic Head (feet)

However, the actual power required to run a pump will be higher than this because pumps and
drives are not 100 percent efficient. The horsepower required at the pump shaft to pump a
specified flow rate against a specified TDH is the Brake Horsepower (BHP) which is calculated
with the following formula:
                         WHP
BHP =           ----------------------                 (2)
                Pump Eff. x Drive Eff.
BHP -- Brake Horsepower (continuous horsepower rating of the power unit).
Pump Eff. -- Efficiency of the pump usually read from a pump curve and having a value
between 0 and 1.
Drive Eff. -- Efficiency of the drive unit between the power source and the pump. For direct
connection this value is 1, for right angle drives the value is 0.95 and for belt drives it can vary
from 0.7 to 0.85.


8.2.8 Effect of Speed Change on Pump Performance:




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The performance of a pump varies with the speed at which the impeller rotates. Theoretically,
varying the pump speed will result in changes in flow rate, TDH and BHP according to the
following formulas:

 RPM2
( ----- ) x GPM1 = GPM2        (3)
 RPM1

  RPM2
( ----- )2 X TDH1 = TDH2        (4)
  RPM1

  RPM2
( ----- )3 x BPH1 = BPH2       (5)
  RPM1

where:
 RPM1 = Initial revolutions per minute setting
 RPM2 = New revolutions per minute setting
 GPM = Gallons per Minute
         (subscripts same as for RPM)
 TDH = Total Dynamic Head
         (subscripts same as for RPM)
 BHP = Brake Horsepower
         (subscripts same as for RPM)

As an example, if the RPM are increased by 50 percent, the flow rate will increase by 50 percent,
the TDH will increase (1.5 ÷ 1)2 or 2.25 times, and the required BHP will increase (1.5 ÷ 1)3 or
3.38 times that required at the lower speed. It is easy to see that with a speed increase the BHP
requirements of a pump will increase at a faster rate than the head and flow rate changes.

8.2.9 Pump Efficiency:

Manufacturers determine by tests the operating characteristics of their pumps and publish the
results in pump performance charts commonly called "pump curves."
All pump curves are plotted with the flow rate on the horizontal axis and the TDH on the vertical
axis. The curves in Figure 2 are for a centrifugal pump tested at RPM of 2800.
Each curve indicates the LPM versus Head relationship at the tested RPM. In addition, pump
efficiency lines have been added and wherever the efficiency line crosses the pump curve lines
that number is what the efficiency is at that point. Brake horsepower (BHP) curves have also


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been added; they slant down from left to right. The BHP curves are calculated using the values
from the efficiency lines. At the top of the chart is an NPSH curve with its scale on the right side
of the chart.

8.2.10 Reading a Pump Curve:

When the desired flow rate and TDH are known, these curves are used to select a pump. The
pump curve shows that a pump will operate over a wide range of conditions. However, it will
operate at peak efficiency only in a narrow range of flow rate and TDH.

8.2.11 Changing Pump Speed:

In addition, suppose this pump is connected to a diesel engine. By varying the RPM of the
engine we can vary the flow rate, the TDH and the BHP requirements of this pump. As an
example, let's change the speed of the engine from 1600 RPM to 1700 RPM. What effect does
this have on the GPM, TDH and BHP of the pump?
Solution: We will use equations 3, 4 and 5 to calculate the change. Using equation 3, the change
in GPM would be (1700/1600) x 900, which equals 956 GPM. Using equation 4, the change in
TDH would be (1700/1600)2 x 120, which equals 135.5 feet of TDH. Using equation 5, the
change in BHP would be (1700/1600)3 x 37.9, which equals 45.5 BHP. This point is plotted on
Figure 2 as the circle with the dot in the middle. Note that the new operating point is up and to
the right of the old point and that the efficiency of the pump has remained the same.
When a pump has been selected for an irrigation installation, a copy of the pump curve should be
provided by the installer. In addition, if the impeller(s) was trimmed, this information should also
be provided. This information will be valuable in the future, especially if repairs have to be
made.

8.3 Submersible Pumps

A submersible pump is a turbine pump close-coupled to a submersible electric motor as shown in
Figure 3. Both pump and motor are suspended in the water, thereby eliminating the long drive
shaft and bearing retainers required for a deep well turbine pump. Because the pump is located
above the motor, water enters the pump through a screen located between the pump and motor.

The submersible pump uses enclosed impellers because the shaft from the electric motor expands
when it becomes hot and pushes up on the impellers. If semi-open impellers were used, the pump
would lose efficiency. The pump curve for a submersible pump is very similar to a deep well
turbine pump.

Submersible motors are smaller in diameter and much longer than ordinary motors. Because of
their smaller diameter, they are lower efficiency motors than those used for centrifugal or deep
well turbine pumps. Submersible motors are generally referred to as dry or wet motors. Dry
motors are hermetically sealed with a high dielectric oil to exclude water from the motor. Wet
motors are open to the well water with the rotor and bearings actually operating in the water.



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If there is restricted or inadequate circulation of water past the motor, it may overheat and burn
out. Therefore, the length of riser pipe must be sufficient to keep the bowl assembly and motor
completely submerged at all times. In addition, the well casing must be large enough to allow
water to easily flow past the motor.

Small submersible pumps (under 5 horsepower) use single phase power. However, most
submersible pumps used for irrigation need three phase electrical power. Electrical wiring from
the pump to the surface must be watertight and all connections sealed. The electrical line should
be attached to the column pipe every 20 feet to prevent it from wrapping around the column pipe.
Voltage at the motor leads must be within plus or minus 10 percent of the motor nameplate
voltage. If there is a 3 percent voltage drop in the submersible pump cable, voltage at the surface
must not be less than 95 percent of rated voltage.

Submersible pumps can be selected to provide a wide range of flow rate and TDH combinations.
Submersible pumps more than 10 inches in diameter generally cost more than comparably sized
deep well turbines because the motors are more expensive.
1.   Pump Material of construction,
2.   NPSH, and
3.   Working temperatures and cooling or flushing media plans size, and
4.   various other features as governed by liquid to be pumped and pumping conditions;


8.4    Consideration of the Parameters of Head, Discharge and speed in the selection of the
       pump

The parameters are combined together in term specific speed of a pump, which calculated by the
following formula

nq = (3.65NQ0.5)/ H0.75

Where, nq - Specific speed

N – The operating speed of the pump in rpm

Q – The rate of flow in cubic meters per second

H – The rated head per stage of the pump in meters

Most aspects of the performance characteristics of the different types of pumps can be compared
based on their specific speed. Some useful observations are summarized below.

1.     Centrifugal pumps are made with specific speeds above 36.
2.     For high discharges, by which specific speed becomes high the corresponding Net Positive
       Suction Head required (NPSHr) also becomes high, it can be arranged that the discharge be
       shared by two impellers or by two sides of an impeller as in a double suction pump. While


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       estimating the attainable efficiency for such pumps, only half of the total Q should be
       considered.
3.     Similarly for high heads, by which the specific speed becomes low, and hence the attainable
       efficiency becomes low, it can be arranged that the head be distributed amongst a number of
       impellers as in multi stage pumps, thus improving the specific speed of each stage and
       consequently the attainable efficiency.

8.5 Summary View of Application Parameters and Suitability of Pump

Based on the considerations of all the above stated points a summary view as compiled of the
application parameters and suitability of pumps of various types and presented in the below
table. However these are general guidelines. Specific designs may either not satisfy the limits or
certain designs may exceed the limits.

Table 6.1: Application of pumps

     Pump Type          Suction Capacity to life                 Head Range                      Discharge Range
                        Low Medium High                 Low       Medium High               Low Medium         High
                        3.5      6m       8.5 m         upto      10 – 40   above           upto upto 500 Above
                         m                              10 m        m       40 m             30        l/s    500 l/s
                                                                                             l/s
Centrifugal             Ok    Ok             Ok         Ok        Ok             Ok         Ok      Ok        No
Horizontal end –
suction
Centrifugal             Ok    No             No         Ok        Ok             Ok         Ok        Ok                 Ok
Horizontal
Axial        split
casing
Centrifugal             Ok    Ok             No         No        Ok             Ok         Ok        Ok                 No
Horizontal
multistage
Jet centrifugal         When limitations of Ok                    Ok             No         Ok        No                 No
combinations            suction lift are to be
                        overcome
Centrifugal             When suction lift is to be Ok             Ok             Ok         Ok        Ok                 Ok
Vertical turbine        avoided
Centrifugal             When suction lift is to be Ok             Ok             Ok         Ok        Ok                 Ok
Vertical                avoided
submersible
Positive                Normally self priming               Limited only by the             Ok       Ok        No
displacement                                             pressure which casing can          Easy adaptation for dosing
pumps                                                            withstand                  or metering




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8.6 Quick Selection:

8.6.1 Factors for selection of Pump set

1.      Yield of the bore well
2.      Depth to low water level of bore
3.      Height and length to which water is to be pumped
4.      Water requirements

8.6.2 Yield of the bore well:

The continuous unrestrained flow through a bore is called yield of the bore well. The yield of the
bore well depends upon
          Nature of source
          No of veins (source) tapped
          Subsoil water level
A well drilled in summer normally shows a low yield which is likely to improve in the monsoon.
Similarly a well drilled in October may even become dry in summer.
It is a good practice to select a pump such that it does not exceed the maximum yield of the well.
Thus ensures that
          The pump does not run dry in the bore thereby enhancing the pump‟s life
          Water of the bore does not become saline.
Normally the driller of the well is supposed to provide the accurate information regarding,
          No. of veins and distance from ground level at which these veins have been trapped
          The maximum yield of the well


The yield of the well is expressed by many drillers in terms of height over a V notch. The below
table give discharge in LPM to corresponding heights over V notch.


Distance over V Notch


                Height over V Notch                                   Yield in LPM
                         In Inches                   Over 45deg V                    Over 90deg V



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                     1.0                                3.9                             9.4
                     1.25                               6.7                             16.2
                     1.5                               10.6                             25.6
                     1.75                              15.5                             37.4
                     2.0                               21.5                             51.9
                     2.5                               37.5                             90.5
                     3.0                               58.5                            142.0
                     4.0                              120.0                            290.0
                     5.0                              209.0                            505.0
                     6.0                              328.0                            794.0
                     7.0                              480.0                           1159.0
                     8.0                              670.0                           1620.0
                     9.0                              897.0                           2166.0
                     10.0                             1165.0                          2813.0


8.6.3 Depth to low water level:

If the pumped capacity of the well is matched to the yield of the well then obviously the
maximum depth to low water level has to adjust to the height at which the last vein is tapped in
the bore.

8.6.4 Height and length of delivery point:

The height to which the water is to be pumped has to be precisely estimated. This is most
important especially on long upward inclined terrains. The length of the pipeline and the height
to which the water is to be pumped together with the depth of low water level decides the total
head f the pump set. Friction in long pipeline is to be calculated. The higher diameter pipes
gives lower frictional heads. This helps to reduce load on pump and thus increase it life.




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8.7 Pump Installation




1.     Minimum immersion depth below lowered to a minimum Ht (operation water level) in
       relation to the upper edge of the pump 0.5m and more,
That means---------------------He-Ht>0.5m
2.     It must be ensured that the unit is freely suspended from the riser pipe and does not sit on
       the base of the well and also does not touch the side of the well or the basin.

8.8 SELECTING PUMPSETS wrt acre

1 H.P Pump set gives a discharge of 2,200 gallons/hr. normally. Viz. 22,000 gallons in 10 hrs (1
inch of water in one acre - 1 acre inch)
Discharge according to size of pump set.


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1 H.P., 1" x 1", 1 1/2" x 1 1/2" pumpsets - 2,000 - 3,000 gph.

1 1/2 H.P., 1 1/2" x 1", 2" x 1 1/2" pumpsets - 3,000 - 4,000 gph.

2 H.P., 2" x 2", 2 1/2" 2", 3" x 2 1/2" pumpsets - 5,000 - 6,000 gph.

3 H.P., 2 1/2" x 2", 3" x 2 1/2" 2" x 2" pumpsets - 6,000 - 7,000 gph.

5 H.P., 4" x 3", 3" x 2 1/2", 2 1/2" x 2" pumpsets - 10,000 - 12,000 gph.

Working out area and depth of irrigation.

Example: 5 H.P. Pumpset. If this pump is worked for 10 hrs, it gives a discharge of 1,10,000
gallons of water approximately. If the area of the farm is five acres, the depth of water will be
1,10,000/22,000 which is five inches (22,000 gallon = 1 inch in one Acre).

8.9 Precautions to be taken while erecting a pump

Maximum suction head, shall not exceed 7 mts. (20 ft). Suction head is measured from water
surface of source to pump level vertically.

Suction pipe shall be errected vertically. It shall never be allowed in a slanting position.

There shall not be any leak on the suction side.

Foot valve must rest at least 1 mt. above the water bed. If foot valve touches ground, it will such
dirt and sand, and pumping system will fail.

Polythene pipes are preferable to avoid frictional loss.

Usually centrifugal pumps are used where suction is less than 7 mts. A single stage pump is used
where total head (suction head + delivery head) is not more than 40 ft. (13 mts). Stage pump at
the rate of 1 stage for every 40 ft. may be used. Thus for 120 ft. total head, a 3 - stage pump shall
be used.

Where suction head is more than 20 ft. Ejecto Pumps (Jet pumps) shall be used. A jet pump will
pump water from wells upto 100 ft. deep.

Single phase pumpsets are available upto 1.5 H.P. Above 2 H.P., only 3 phase pumpsets are
available. So small farmers having an area upto 3 acres may go for single phase pumpsets rather
than 3 phase electric connections which may not be readily available.

Area-wise requirements of pumpsets:

(1) H.P. upto 2 Acres (1 Ha.) (2) H.P. upto 4 1/2 Acres (1.5 Ha.) (3) H.P. upto 5 Acres (2 Ha.)

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Source : Agricultural Engineering Wing, Directorate of Agriculture, Thiruvananthapuram

8.10 Trouble Shooting:

8.10.1 Basic complaint

1. No water

 A. Motor runs - you can hear it or feel the pipe vibrate or amp check if you have an
amprobe.
     a) Hole in drop pipe or coupling, bleeder valve blown out.
     b) Massive leak in your system. Pump is delivering water just not where you want it to go.
     c) Jammed or backward check valve. It happens.
     d) Pump is out of the water
     e) Pump inlet screen plugged. Very rare.
     f) Pump worn out. Impellers worn. If it has pumped sand or is very old this is possible.
     g) Pump shaft broken or coupling stripped. Very rare these days.
     h) Pump air locked.
     j) Water level has dropped so far pump can't lift to surface.

B. Motor doesn't run
    a) No power to pump - this is the most common thing.
    b) Motor failed
    c) Wires down well broken or bad splice.
    d) Control box problem, bad capacitor or relay or cover is not on.
    e) Pressure switch problem - easy to fix but usually wishful thinking.

1) Look at the contacts. If they aren't closed figure out why.
           The switch thinks the pressure is at shutoff level. Did it freeze last night?
           Possibly bad pressure switch or plugged inlet.
2) Burned contacts don't mean much. Bugs in the contacts are a common problem. Clean them
off with the eraser end of a wooden pencil. These contacts are always
electrically hot.
     f) Overload tripped. Look for a red button on or under control box.
     g) Pump locked up.
     h) Both wires to motor or control box are connected to the same leg in the panel.

2. Not enough water, or pressure - motor runs, perhaps runs all the time
   A.. Leaks - surprisingly small leaks can lose a lot of water. Common problem.
     a) Leaks in your house system.
        Shut off line between tank and house and see if pump builds up pressure normally.
     b) Down the well: Holes in drop pipe or bleeder valve.
   B. Pump problems
     a) Pump too small for demand
     b) Pump impellers worn by sand


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    c) Water level has dropped below what pump is designed for
    d) Check valve jammed either down well or on surface. The nut can also come off the
plunger and improper pipe fittings can prevent plunger travel.
    e) Plugged inlet screen. Very rare.
    f) No water in well or pump not set deep enough.
    g) Motor coupling stripped or shaft broken. Sometimes can still pump.

C. Tank problems
   a) Waterlogged tank will cause pump to go on and off continually.
      This also results in apparent low pressure. This is very common.
   b) Surface check valve stuck open allowing water to run back down the well or stuck closed
preventing water from
      getting up.
D. Electrical problems
   a) Improper connections at control box. If color codes were not kept the pump will attempt to
start on the
      run winding and will not be able to continue running
   b) Low voltage. 230 volt pumps will run on 115 volts but not very well and will cut out and
reset.
      This happens when one pole of a two pole circuit breaker has tripped. Pull both poles all the
way to off,
      then back to on.
   c) Motor has internal short which is not bad enough to make it stop totally but results in
intermittent operation
      or less than full speed operation. This is a frequent motor death mode.

3. Bad water
A. Milky -air or gas in water.
   1) Natural entrained air or gas - not much you can do about it.
   2) Tank air problem
      a) Bad air volume control
      b) Pumping water level too low allowing air to be sucked into pump
      c) Excessive draw from tank allows air into house lines
B. Sandy - well problem, made worse by frequent starts, well driller problem
C. Tastes bad - try an activated carbon filter
D. Looks bad - particulates in water, try a cartridge filter
E. Stains sink -Iron and/or manganese in water, water treatment problem
F. Stinks - hydrogen sulfide gas or methane
G. Slime in strainers - iron bacteria, chlorinate well

4. Fuses blow, breakers trip, overloads trip
A. Happens immediately when power applied to motor
   1) Short to ground in motor, cables or supply wires to pressure switch.
      Remove control box cover or disconnect leads to motor to see where the problem is.
      Shorts make things trip very fast.


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   2) Worn out breaker, wrong size breaker, non-time delay fuses can't take starting current.
   3) Control box problem causing start winding in motor not to operate. Usually times several
seconds to trip.
   4) Low voltage
   5) Pump locked up
B. Happens when motor has been running
   1) Low voltage
   2) Short cycling, too many starts
   3) Control box too hot due to sun or other heat source.
   4) Control box problem - bad capacitor, relay, or wrong size
   5) Fuses or overloads too small.
   6) Circuit breakers worn out - they will only trip so many times.
   7) Frequent low head starting causing up thrust
   8) Worn pump - usually causes low amps but can also cause high amps.
   9) Pumping a lot of sand.
   10) Wires too small or contacts somewhere very bad causing high voltage drop.
   11) Well is so crooked the pump and moor have been forced into a bind. You have to work
at it to create this one.

5. Pumps starts and stops too often. This is very hard on submersible pumps and motors.
A. Water logged tank.
   1) Galvanized tank
      a) No air charging system - drain tank and open a fitting to break vacuum.
         This can always be used as a temporary fix on any tank.
      b) Air leak in tank above water level
      c) Surface check valve is leaking and preventing snifter valve from taking in air.
      d) Snifter valve (usually screwed into check valve) is not working. It should suck in air
every time
         the pump stops. Frequent problem area.
      e) Bleeder in well is not letting water leak out of the pipe so air can be sucked in by the
snifter.
      f) Pump runs constantly and so never cycles to put air in tank.
      g) Air volume control letting too much air out.

2) Bladder tank

a) Bladder is ruptured. Tank will feel heavy and water will come out of tire core valve
on top of tank.
b) Tank has too little pre-charge air in it or, too much. It needs to be just right which is 2
pounds less than the start pressure of the pump, measured with the tank drained and the pump
off.

B. Air logged tank - air volume control bad or too much air being pumped in.
C. Defective pressure switch or set wrong



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D. Tank too small for pump size and demand.
E. Check valve on surface may be jammed or partially open


8.10.2 Advanced troubleshooting

This is for people who are familiar with electricity and have a voltmeter, ammeter and ohmmeter
and enough common sense not to fry themselves.


There are two basic symptoms:

1) Motor does not run

2) Something trips out


Motor does not run

A) Makes no sounds - this most likely means no power to motor. First make sure you have put he
cover back on the control box if it is 1 HP or less.

Start at the pressure switch with the switch wedged open with a non-conductor and measure
voltage leg to leg-AND to ground.

If you do not have 230 volts (unless it is a rare 115 volt motor) trace back to
the circuit breaker or fuse box. If you have 115 volts to ground on both legs
at the pressure switch, you have both legs on the same hot leg and thus zero potential difference
between them. Put one leg on the other hot leg.

If you have 115 volts to ground on one leg and zero on the other, one wire is broken or one half
of the 230 volt breaker is defective or tripped.

If everything is zero at the pressure switch the wires are broken or the breaker is bad, or tripped,
or the main power is out.


If everything checks out then there is an open in the motor or in the control box or the wiring to
the motor. Start by disconnecting the power at the breaker then disconnecting the wires that go
down the well from the control box. Use an ohm meter to check for continuity between all three
wires (or two if it is a two wire pump). Also check each leg to ground. All should be infinity or
at least 10 megohms to ground. The resistances leg to leg are small. The yellow is common and
the yellow red (start) should be more than the black (run) to yellow. An open indicates a broken
wire, bad splice or bad motor. A low resistance to ground indicates a bad motor or sub cables
that are damaged.


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B) Motor hums, buzzes . This is either low voltage, a bad control box, mixed wire color code ,
shorted motor.

Do all the checks listed in ( A) above. If it is not covered in (A):

1) If the pump is new

a) Ohm check the wires from the motor. The highest amp reading will be Red to Black. The
next highest Yellow to Red and the lowest Yellow to      Black. If your readings don‟t agree,
the color code is mixed down below.

2) Wrong voltage control box. Only possible on ½ HP pumps where 230 volt or 115 volt motors
are made. If 115 volt box is used on a 230 system, the control box relay will be expecting much
higher amps and so will not drop out the start winding.

3) Control box problems. Sometimes they are bad out of the box.                               11/2 HP and above
sometimes have incorrect connections. Rare but it happens.

There are four possible components in a control box:

Start capacitors- black cylinders- most likely to fail. Look for burned off connectors, black gunk
oozing out. If it looks OK, you need an analog ohm meter. Short across the capacitor to
discharge it, then put the ohm meter on it. It should show a low reading which increases to
infinity over several seconds as the capacitor charges. These are cheap and readily available at
any electric motor shop.

Run capacitors - usually metal cylinders - almost never fail- almost. Overload relays - “Klixons”
the red button. They fail. If they trip out, check the amp draw. If it is normal, the overload is bad.
By-pass it with a jumper until you can get one. ( or forget about it)

Start relay- black or Blue Square. Most difficult to diagnose. It depends on whether they are solid
state (blue, or on some original, a small semi-conductor looking thing) or electro-mechanical, a
2" square with MARS written on it somewhere. If you get to this point, just replace the control
box.

Control box problems are often caused by short-cycling of the pump.

8.10.2.1 Something trips out.

This means the pump overload or a circuit breaker or fuse. This does not mean the pressure
switch. First check for proper voltage starting at the circuit breaker, then the pressure switch,
then the leads going down the well. This can be difficult with control boxes that have covers that
pull the guts out with them. These are for your safety and the manufacturers safety from lawyers,
but they are a pain to troubleshoot. People in the industry make jumpers from two old control


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boxes. Your best bet is to put a short jumper on the three pump leads and wire nut them where
you can get a probe on them. This also lets you make amp readings and ohm readings.

 A. Circuit breaker trip. If there are no voltage abnormalities, this is either a dead short
somewhere or a bad breaker. If it takes some time to trip, look for bad breaker, too small a
breaker or hot breaker box. It may also be a small ground fault resulting in high amps but
usually the pump overload will trip first.

If you are looking for a short or ground fault, open the circuit breaker so you don‟t blow up your
ohm meter, then start at the pump, disconnect the leads going down the well and check each leg
to ground. You should get near infinity. Next check the yellow to red and yellow to black. These
reading should be very low, 2 to 12 Ohms. The yellow to red should be higher than the yellow to
black. If you don‟t find anything down the well, start working your way back to the pressure
switch, then to the breaker, until something shows up. Fix it. This will probably require pulling
the pump or digging. The good news is that you will get your exercise without paying health club
dues.

B. Overload trip. This means high amps or bad overload. Again, assuming nothing showed up
on the voltage check, take amp readings on all three wires. These motors are actually designed
for the service factor, i.e. a 2 HP motor is actually a 2.3 HP motor, so it doesn‟t hurt them to run
at SFA. If the amps are uniformly high by 10 to 15% it probably means the motor and/or pump
end are shot. If one leg is high it indicates a ground fault. The red leg is the start winding, the
black is the run winding and the yellow is common. Any electrons that go down the red and
black have to come up the yellow or go to ground. A single high leg is probably a ground fault.
If you put your amprobe around all three legs at once and have any current show, it is a ground
fault. It can be motor or sub cable.

When the motor starts you should see a momentary blip on the red lead amps which may fall off
to zero on small pumps, or fall to a low level on capacitor start/capacitor run control boxes. If
you don‟t see this, look for control box problems or an open in the start circuit. This usually is
accompanied by high amps on the black-yellow leads as the pump tries to start. It is possible for
the pump to start sometimes without the start circuit.




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                                               CHAPTER IX
            9        CONCEPTS OF EFFICIENCY AND ITS ASSESSMENTS


9.1 Power and Efficiency

When we talk about automobiles and discuss efficiency, we mean how many miles per gallon, or
liters per 100 kilometers. When we discuss centrifugal pumps we are comparing the amount of
work or power we get out of the pump to the amount of power we are putting into the pump.

9.1.1 Brake Horse Power (BHP)

The work performed by a pump is a function of the total head and the weight of the liquid
pumped in a given time period.

Pump input or brake horsepower (BHP) is the actual horsepower delivered to the pump shaft.

Pump output or hydraulic or water horsepower (WHP) is the liquid horsepower delivered by
the pump. These two terms are defined by the following formulas.

     BHP = (Q * H * s.g) / η * 3960                          WHP = (Q * HT * s.g) / 3960

     Where                                                   Where

     BHP - Pump horsepower input required                    WHP - Pump horsepower input required

     Q – Flow in Gallons per minute (GPM)                    Q – Flow in Gallons per minute (GPM)

     H – Head (in ft)                                        HT – Total Differential Head (in ft)

     s.g – specific gravity of the fluid                     s.g – specific gravity of the fluid

     η – Pump Efficiency at the operating point

The constant 3960 is obtained by dividing the number or foot-pounds for one horsepower
(33,000) by the weight of one gallon of water (8.33 pounds).

BHP can also be read from the pump curves at any flow rate. Pump curves are based on a
specific gravity of 1.0. Other liquids‟ specific gravity must be considered.

The brake horsepower or input to a pump is greater than the hydraulic horsepower or output due
to the mechanical and hydraulic losses incurred in the pump.



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Therefore the pump efficiency is the ratio of these two values.



Pump Efficiency        =        WHP
                                BHP

9.1.2 Best Efficiency Point (BEP)

The H, NPSHr, efficiency, and BHP all vary with flow rate, Q. Best Efficiency Point (BEP) is
the capacity at maximum impeller diameter at which the efficiency is highest. All points to the
right or left of BEP have a lower efficiency.

9.1.3 Significance of BEP

BEP as a measure of optimum energy conversion

When sizing and selecting centrifugal pumps for a given application the pump efficiency at
design should be taken into consideration. The efficiency of centrifugal pumps is stated as a
percentage and represents a unit of measure describing the change of centrifugal force (expressed
as the velocity of the fluid) into pressure energy. The B.E.P. (best efficiency point) is the area on
the curve where the change of velocity energy into pressure energy at a given gallon per minute
is optimum; in essence, the point where the pump is most efficient.

BEP as a measure of mechanically stable operation

The impeller is subject to non-symmetrical forces when operating to the right or left of the BEP.
These forces manifest themselves in many mechanically unstable conditions like vibration,
excessive hydraulic thrust, temperature rise, and erosion and separation cavitation. Thus the
operation of a centrifugal pump should not be outside the furthest left or right efficiency curves
published by the manufacturer. Performance in these areas induces premature bearing and
mechanical seal failures due to shaft deflection, and an increase in temperature of the process
fluid in the pump casing causing seizure of close tolerance parts and cavitation.

BEP as an important parameter in calculations

BEP is an important parameter in that many parametric calculations such as specific speed,
suction specific speed, hydrodynamic size, viscosity correction, head rise to shut-off, etc. are
based on capacity at BEP. Many users prefer that pumps operate within 80% to 110% of BEP
for optimum performance.

9.2 Pump set Efficiency – Field Evaluation

The following steps for determination of efficiency
   Make available necessary instruments for power & flow measurements

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   Install the flow meter on the pump deliver pipe
   Start the pump set and allow to stabilize
   Measure input power
   Measure discharge for a specific period and calculate output in Litres per second
 Taking into consideration the suction & delivery heads convert the water discharge into
“Output HP”
   Determine efficiency as ration of output calculated to measured input
For testing of pumpsets the following instruments can be utilized for measuring various
parameters.
        Parameter                                           Instrument
 Electrical power         Power analyzer instrument of accuracy Class 1 or 0.5 with
                          capability to measure Voltage, Current, Power factor,
                          Frequency, kW, kVA, kVAr simultaneously
 Water Discharge          Flow meter of suitable size

 Working head             Dip Level Indicator / Measuring tape / Pump installation
                          depth


A sample of data inputs and methodology for determination of efficiency is explained below

Sanctioned power: 5 HP

Electrical data collected:

                Voltage      Current         P.F. (cos phi)         Power input (kW)
 R phase         418.1        6.93               0.858                     4.32
 Y phase         411.1        7.26               0.898                     4.44
 B phase         417.1        7.04               0.818                     4.14
 Average         415.4        7.08               0.858                4.30 (5.76 HP)

Pipe/Head data:

Suction head (Hs):    11.59 m
Discharge length (Hd):       13.7 m
Diameter of delivery pipe: 0.051 m
Number of bends (B): 2

Flow meter readings

Volume Discharges: 1000 litres

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Time duration: 318 seconds


Flow calculation

Flow = Q = Volume discharged / time duration = 1000/318 = 3.15 LPS
Conversion from LPS to m³/sec = LPS/1000
Q = 3.15/1000 = 0.00315 m³/sec

Velocity calculation

Velocity = v = Q/Area of pipe = 0.00315/(3.14159*0.051*0.051/4) = 1.58 m/sec

Head Calculations:

Total Head = Static head + Frictional head + gravitational loss + Head due to bends
= Hs + Hf + (velocity²/2g) + (B/2)
Hf = (2*0.001*(Hs + Hd)*Velocity²)/(9.81*Dia of pipe) = 0.25
Total head (H) = 12.97 m = (11.59 + 0.25 + ((1.58)²/(2 x g)) + 1

Efficiency calculations:

        Total output = 9.81 x H x Q (kW) = 9.81 * 12.97 * 0.00315 = 0.41 kW
Efficiency of the pump set = (output power/input power) x 100
Efficiency = (0.41/4.3) x 100 =       9.5%


9.3 Distribution of losses in a pumping system

Table1.1: Typical distribution of losses in a pumping system. (Piping includes friction
losses and static head)
(Source: BEE manual for Energy Efficiency in electrical utilities)


                                   Total Energy Input – 100
 Component                       Energy Received                          Energy Lost/
                                                                          dissipated
 Motor                           100                                      12
 Coupling                        88                                       2
 Pump                            86                                       24
 Valves                          62                                       9


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 Piping                           53                                       11
 Work done on fluid               42                                       -
 Energy Balance for a typical pumping system


9.4 Symptoms that Indicate Potential Opportunity

Common symptoms that indicate opportunities for energy efficiency in pumps are as follows:


Table 9.1: Symptoms that Indicate Potential Opportunity


              Symptom                             Likely Reason                             Best Solutions
Throttle value-controlled systems         Oversized pump                        Trim impeller, smaller impeller,
Bypass line (partially or                 Oversized pump                        Trim impeller, smaller impeller,
completely) open
Constant pump operation in a              Wrong system design                   Match pump capacity with
continuous process                                                              system requirement
High maintenance cost (seals,             Pump operated far away                Match pump capacity with
bearings)                                 from BEP                              requirement.


9.5 Check List for Energy Savings in Pumping Systems
     Operate pumps near best efficiency point.
     Modify pumping system and pumps losses to minimize throttling.
     Use booster pumps for small loads requiring higher pressures.
     Repair seals and packing to minimize water loss by dripping.
     Balance the system to minimize flows and reduce pump power requirements.
     Conduct water balance to minimize water consumption
     Provide booster pump for few areas of higher head
     Replace old pumps by energy efficiency pumps.
     In the case of over designed pump, provide downsize / replace impeller or replace with
      correct sized pump for efficient operation.
     Rationalise number of stages in multi-stage pump in case of head margins.
     Optimise system resistance by pressure drop assessment and pipe size optimisation.


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                                                   CHAPTER X
                                       10 PROPOSED OPTIONS

10.1 Defects in the system:

It has been observed that the overall efficiency of pump-sets are low. The study has identified
the following reasons:


1. Inefficient motors.
2. Inefficient pumps.
3. Inefficient foot valves and piping system.
4. Low voltage.


10.2 Options for Improvement:

There are two options for improving the overall pump-set efficiency. The options are:
1. Partial Rectification
2. Complete Replacement


10.3 Analysis of proposed options for pumps

10.3.1 Partial rectification

In the partial rectification programme, except for motor and pump, the following are covered:


i.          Replacement of inefficient foot valves.
ii.         Removal of unnecessary bends.
iii.        Removal of unnecessary lengths.
iv.         Reduction in height of pipe above the ground.
v.          Replacement of GI pipes with HDPE/PVC pipes.


This requires much lesser investment than complete replacement. Here, the farmers benefit in
terms of more water output from the pumping system. It does not reduce the energy requirement
of pumping. In other words, no energy savings in grid loading is realised by using these options.



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10.3.2 Complete Replacement

In this option, motors and pumps are also replaced along with the partial rectification programme
as stated above. The complete replacement will involve the following:
i.          Installation of energy efficient motors.
ii.         Installation of energy efficient pumps.
iii.        Replacement of inefficient foot valves.
iv.         Removal of unnecessary bends.
v.          Removal of unnecessary lengths.
vi.         Reduction in height of pipe above the ground.
vii.        Replacement of GI pipes with HDPE/PVC pipes.


The selection of this option would help in reduction of grid loading but would require much
higher investment (Rs. 30,000 or more). In addition, it would help in saving electrical energy. It
is suggested that the new energy efficient motors must be able to run quite efficiently even down
to a voltage of 340 V. Thus, this programme‟s technical success is dependent on whether power
can be supplied to each pump-sets above 340 V. A line improvement programme should precede
the complete rectification programme for pump-sets.

10.4 Energy efficient pumping system

The following steps were adopted to work out the ratings of new energy efficient pump-sets1 to
replace the existing inefficient pump-sets.

The kW required for the new energy efficient pumps is calculated using:

kW required for the new energy efficient pump

                           Existing efficiency x (existing pump demand in kW)

                    =      -----------------------------------------------------------
                                    New pump efficiency




11
   Head will be reduced in Energy Efficient Pumping system, but in view of continuous dropping in water level we
have assumed head to be same as measured value.
Implementing agencies will determine the exact HP of the pump to be installed later. Motor rating now have been
calculated on measured value. Hence savings have been put more conservatively.



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It is assumed that the efficiency of new energy efficient pump-sets is 50%. The average
efficiency of the existing pump-sets is 24%. Therefore, on an average a 10 HP pump-set needs
to be replaced by a 5 HP pump. However, there are very low efficiency pump-sets where a 10
HP can be replaced by 3 HP and on the other hand the 10 HP pumps replaced by 7.5 HP.
Another issue as been the higher consumption of the pump than the official ratings. We have
taken the actual consumption and replaced it with new energy efficient pumps.
e.g. : A pump of official rating of 7.5 HP from Annexure – B & C, Tr. #16 and pump # 4,
actually consumes 10 HP.


We have determined the efficiency (28.03%) and replaced it as follows
                (10 x 0.28) / 0.5 = 5.6 HP
We have provided the farmer a new energy efficient pump of 7.5 HP. Therefore, apparently we
are replacing a 10 HP pump by 7.5 HP efficient pump.


The following table gives an example of complete replacement involved for the pumps
connected to one of the transformers. The details of Transformer No. 16 (100kVA) is provided
below:
Table 10.1 : Existing Scenario and modified values of motors.
                                                                                After Modification
  Name of the         Existing Scenario of the Pumping System
   Farmer
                  Official Rated      Actual     Efficiency %    Efficiency %      Required          Provided HP
                        HP             HP                                            HP

1. Vaidpal                25            25          40.88            50             20.44               20

2. Punni Devi            7.5           12.5         29.61            50               7.4               7.5

3. Rulla Ram             7.5           12.5         28.03            50                7                7.5

4. Suraj Ban             7.5            10          28.07            50               5.6               7.5

5. Jile Singh            7.5            10          22.89            50               4.6                5

6. Arjun Dev             7.5            10          29.73            50               5.9               7.5

7. Ram Singh             7.5            10          30.26            50                6                7.5

Thus, the HP of the new energy efficient motor is selected to be nearest available HP of the
motor. The results of the analysis are presented below.




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Table 10.2 : The HP ratings and the number of pumps required
Rating in HP                 3         5         7.5        10         12.5      15        20        Total

No.of pumps required        19       131        112         14         10         2        4         292
The replacement of existing pump-sets would lead to a saving of 46%, which was arrived by the
following calculation,


                         Existing kW of all pumps – kW of all new pumps
% saving         =       -----------------------------------------------------------            x 100
                         kW of all existing pumps


The existing pump demand is 2646.58 kW consisting of 2256.98 kW for 246 pumps (for which
measurements are available) and 389.60 kW for 46 pumps (where no measurements could be
carried out but estimated).


The projected pump demand after complete replacement has been calculated to be 1431.2 kW
leading to a saving of 1215.38 kW. This would lead to an annual saving of 1.82 million units for
1500 hrs of operation.

10.5 Retrofitting of Pump set

The efficiency of the pump sets can be less due to the following factors:
      Impeller worn out
      Rotor imbalance
      Poor quality of material used for rewinding of motors
      Also poor quality of material used for the construction of the pump/while repairing the
       pump
There are pump sets of 5 to 10 years old in the field. Due to poor quality of power supply and
operating the pump sets during two-phase supply, the motor get burnt out frequently. From one
of the field survey, it was observed while having discussion with farmers that for most of the
pumpsets, once/twice in a year, the rewinding is being done. Due to the use of poor quality of
material for rewinding the efficiency of the pump sets come down. Also, the pump sets of more
than 5 years old are having different design from the current scenario. The material of
construction and impeller are entirely different. Also the olden design had maximum of 30-35%
efficiency.
The low efficiency of the pump sets due to the above factors, can be improved slightly by
changing worn out impeller with new one, balancing the rotor, winding the motor with quality


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material and doing entire overhauling of the pump. An effort has been made to identify the cost
benefit analysis of the retrofitting of the pump set.


Cost Benefit Analysis for Retrofitting of Pump sets:


Efficiency of the pump set before retrofitting             20%
Investment required for the retrofitting                12,000 Rs.
Efficiency improvement by retrofitting                       5%
Before Retrofitting
Working Head                                                  75 m
Discharge                                                   0.96 LPS
Input Power                                                 3.53 kW
Efficiency                                                 20%
After Retrofitting
Working Head
Working Head                                                  75 m
Discharge                                                   0.96 LPS
Efficiency                                                 25%
Input Power                                                 2.83 kW
Savings in energy                                           0.71 kW
Running hours per year                                    2,000 hrs
Energy savings per year                                   1,413 kWh
Cost of electricity considered                               2.5 Rs./kWh
Monetary savings per year                                 3,532 Rs./year
Simple Payback period                                       3.40 Years


The above analysis has been made on the basis that by improving the efficiency of the pump set
the energy consumption will get reduce and the discharge will remain same. But normally it
depends on the performance curve.
From the above analysis it can be seen that even by doing the retrofitting of the pump sets, the
efficiency can be marginally improved up to only 5%. The investment required will be around
12,000 Rs. The following points are for your discussion:


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     The each unit/pump set has to be look after by the same manufacturer
     If the pump set is very old/undergone more modifications during repairing, then a new
      impeller and other parts has to be put in place, which will increase the cost of retrofitting.
     For the local made pump sets, the retrofitting will be very difficult, and all the components
      has to be replaced with a new one of reputed brand, if the casing design matches with them.
      Otherwise, whole body with other components has to be replaced which will be equivalent
      to replacing with a new unit.
     During the period of retrofitting the farmer has to be supplied with a spare pump set.

10.6 Flow control Strategies using Throttling
When the flow rate varies with time, the control strategy to be employed has a major impact on
the energy costs of the operating the system. Several control strategies can be considered.
To achieve lower flow rates, a bypass valve could be used, whereby the pump continues to
operate at full speed, with liquid re circulated back to the pump inlet. This provides zero power
savings at reduced loads.
Alternatively, full pump speed can be maintained and the pump output throttled with a valve to
achieve a desired lower flow. This corresponds to increasing the system pressure drop; which
changes the locus of the system head – flow curve moves the operating point along the pump
curve. The additional pressure that must be provided by the pump as a result of throttling will
generally be lower for a pump with a “flatter” head – flow curve (i.e. where head increases
relatively little with reduced flow). For any pump, however, operating away from the original
design point can mean reduction in pump efficiency, and at very low flows there is a potential for
problems due to hydraulic imbalances.
The power required with throttling will be some what less than when using by pass control, nut
nevertheless is significantly higher than would be required by reducing the speed of the pump
instead. Speed reduction can mean large electricity savings, since input power requirements
changes with cube of the volume flow.
Varying pump speed is generally the most efficient option to meet varying demands. This
corresponds to moving down the system curve. In this case different flow rates are achieved with
corresponding changes in head with relatively little change in pump efficiency.

10.7 Effect of Pipe Diameter on cost savings

Pipe diameter is a key factor in determining energy costs. The energy requirement of a pumping
system operating over a period of time t can be expressed as

Ep = t * Q * ∆P/ η

Where Q is the volume flow rate, ∆P is the total pressure drop, η is the motor pump set efficiency.



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Assuming that the only losses are due to friction, ∆P is proportional to the square of volume flow
(∆P ~ Q2). Therefore the power required to overcome friction increases as the cube of the volume
flow. This relationship can be used to quantify the effect of diameter on energy costs.

For a given volume flow rate, the above expression for (∆P)f indicates that pumping energy is
inversely proportional to piping diameter raised to the 5Th power and directly proportional to friction
factor. Since the friction factor also has a slight dependence on diameter, the pumping energy
required to overcome friction in piping different diameter is

Ep1 / Ep2 = (D2/D1)4.84
Friction loss in pipe: 4*f*l*V2/ 2*g* D
Where
f – Friction Factor
l – Length of the pipe in m
V – Velocity of the fluid m/s
D – Diameter of the pipe in mm
Pumping power requirement – refer efficiency formulae in earlier sections.

10.8 Capacitor usage PF improvement:

Usage of capacitors will improve the Power Factor which intern gives the following benefits:
     Reduces the line losses
     Improves the Terminal voltage
     Reduction in Line current
     Marginal improvement in motor efficiency
     Overall improvement in system efficiency
The following tables show the recommended capacitor rating for direct connection to induction
motors to improve power factor to 0.95 or better at all loads


                                   CAPACITOR RATING IN KVAR
                   MOTOR H.P. 3000 RPM 1500 RPM 1000 RPM 750 RPM
                                        KVAR              KVAR             KVAR            KVAR
                        3                   1                 1               1.5              2
                        5                   2                 2               2.5             3.5
                       7.5                 2.5                3               3.5             4.5
                       10                   3                 4               4.5             5.5


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                            15                  4                 5                6              7.5
                            20                  5                 6                7               9
                            25                  6                 7                9             10.5


10.9 Cost Benefit Analysis for replacement of pump sets – Case Study

10.9.1 Methodology

A detailed cost benefit analysis was carried out for the option of replacement of pump sets with
higher efficiency pump sets of low rating
     Estimate the average energy consumption of existing pump sets of low efficient pumpsets
      and the energy requirement for the new pump sets of higher efficiency
     Estimate investments required for carrying out the above
     Estimate benefits achievable from implementation of the project viz. pump set energy
      savings, feeder demand savings losses

10.9.2 Efficiency Analysis

The data obtained by testing a sample of 109 pumpsets was used to determine the efficiencies of
the units. The efficiencies determined range from 3% to 38%. A graphical representation of
range is given in the following chart.
Chart 1: Pump efficiency summary – Feeder F12

             Feeder 12 - Pump set Efficiency Range




                       6%        7%
                                                            Less than 10
                                                            10% - 20%
                                                            20% - 30%
       44%                                    43%
                                                            above 30%




Chart 2: Pump efficiency summary – Feeder F13




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             Feeder 13 - Pump set Efficiency Range




             20%               13%
                                                       Less than 10
                                                       10% - 20%
                                                       20% - 30%
                                         29%
                                                       above 30%
           38%




The number of samples were 54 & 55 for Hegedehalli and Melekote feeders respectively. As
can be seen from the chart most of the pumpsets have efficiency levels in the range of 11 to 30%
(i.e. 84 out of 109 pumpsets i.e. 77%). The average efficiency of the pumpsets is in the range
comes to about 21%.

10.9.3 Cost benefit analysis – F12

All pumpsets having efficiencies less than or equal to 20% are recommended to be replaced by
pumpsets of higher efficiency, Lower HP rating to assure same amount of water discharge. The
numbers of pumpsets coming under this category represent 27 out of the total 54 samples under
the efficiency determination test. This means that about 50% of the pumpsets operate under
efficiency less than 20%. Considering this as representative sample, the total inefficient
pumpsets could be about 180 out of 361 pump sets, which are running (here running and
temporarily not running pump sets were considered as running which can be replaced) in F12.
The efficiency can be raised to 40% by replacing with new pump set and using HDPE pipes
instead of MS/GI pipes. The new high efficiency pump set will be designed in such that it
delivers the same amount of water but consuming less power. Undertaking pump set replacement
in coordination /consent with the consumer would result in substantial savings in demand and
consumption.


Considering Pumps of efficiency below 20 % is replaced
Average rating of pumpsets                           7.25 kW
Modified rating                                       3.2 kW
Number of pumpsets for replacement                   27.0 Nos out of 54
kW Saved                                              4.1 Per IP
kW Saved                                           110.2 For 27 IP
Annual hrs                                         2,000 Hrs
Energy savings                                 220,342 kWh


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Rate                                                2.5 Rs./kWh
Monetary savings                             550,855 Rs
Investment                                     35,000 Per IP
Investment                                   945,000 For 27 IP
Simple Payback                                      1.7 Years


Total Pumpsets tested                                              54
Pump set to be replaced                                            27
% Of pumps to be replaced                                          50%
Total no of IP sets in the feeder (Running)                        361 Nos
Total HP                                                           2957.5 HP
Avg. HP                                                            8.19 HP
Based on motor efficiency of 75%, the
7.9 HP motor will consume                                          8.19 kW
As a result of the efficiency measurement the pumps to be replaced is 50%
No of pump set to be replaced                                      180.50 ~ 180 Nos
Average rating of pumpsets                           8.19      KW
Modified rating                                      3.58      KW
Number of pumpsets for replacement                 180.0       Nos. out of 361 IP
kW Saved                                              4.6      Per IP
kW Saved                                           829.4       For 180 IP
Annual hrs                                         2,000       Hrs
Energy savings                                     1.659       Million units
Rate                                                  2.5      Rs./kWh
Monetary savings                                   4.147       Million Rs
Investment                                       35,000        Rs Per IP
Investment                                         6.300       Million Rs For 180 IP
Simple Payback                                       1.52      Years


Taking an average actual kW rating of 8.19 being replaced with a 3.5kW it is estimated that the
demand savings will be about 846 kW resulting in energy savings of 1.69 million units annually.

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The investments required would be of the order of 6.3 Million Rs and the investments will be
repaid in 1.52 years2.

10.9.4 Cost benefit analysis for F13

All pumpsets having efficiencies less than or equal to 20% are recommended to be replaced by
pumpsets of higher efficiency, Lower HP rating to assure same amount of water discharge. The
numbers of pumpsets coming under this category represent 25 out of the total 55 samples under
the efficiency determination test. This means that about 45% of the pumpsets operate under
efficiency less than 20%. Considering this as representative sample, the total inefficient
pumpsets could be about 137 out of 302 pump sets, which are running (here running and
temporarily not running pump sets were considered as running which can be replaced) in F13
feeder.
The efficiency can be raised to 40% by replacing with new pump set and using HDPE pipes
instead of MS/GI pipes. The new high efficiency pump set will be designed in such that it
delivers the same amount of water but consuming less power. Undertaking pump set replacement
in coordination /consent with the consumer would result in substantial savings in demand and
consumption.
Taking an average actual kW rating of 7.97 being replaced with a 2.63 kW it is estimated that the
demand savings will be about 731.7 kW resulting in energy savings of 1.463 Million units
annually. The investments required would be of the order of 4.795 Million Rs. and the
investments will be repaid in 1.31 years3.
Considering Pumps of efficiency below 20 % is replaced
Average rating of pumpsets                              7.1 KW
Modified rating                                         2.3 KW
Number of pumpsets for replacement                    25.0 Nos out of 55
kW Saved                                                4.7 Per IP
kW Saved                                             118.6 For 25 IP
Annual hrs                                           2,000 Hrs
Energy savings                                   237,215 KWH
Rate                                                    2.5 Rs./kWh
Monetary savings                                 593,038 Rs
Investment                                         35,000 Per IP
Investment                                       875,000 For 25 IP
Simple Payback                                          1.5 Years

2
    Tariff for saved energy – 2.5 Rs./kWh & Investment/pump set – Rs. 35,000
3
    Tariff for saved energy – 2.5 Rs./kWh & Investment/pumpset – Rs. 35,000


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Total Pumpsets tested                                            55
Pump set replaced                                                25
% Of pumps to be replaced                                        45%
Total no of IP sets in the feeder (Running)                      302 Nos
Total HP                                                         2407 HP
Avg HP                                                           7.97 HP
Based on motor efficiency of 75% the
motor HP of 7.9 will consume                                     7.97 kW


As a result of the efficiency measurement the pumps to be replaced is 50%
No of pump set to be replaced                                    137.27~ 137.00 Nos


Average rating of pumpsets                            7.97       KW
Modified rating                                       2.63       KW
Number of pumpsets for replacement                137.00         Nos. out of 302 IP
kW Saved                                                5.3      Per IP
kW Saved                                            731.7        For 137 IP
Annual hrs                                          2,000        Hrs
Energy savings                                      1.463        Million units
Rate                                                    2.5      Rs./kWh
Monetary savings                                    3.658        Million Rs
Investment                                        35,000         Rs Per IP
Investment                                          4.795        Million Rs For 137 IP
Simple Payback                                        1.31       Years


10.10 Replacement of GI pipes with HDPE pipes – Case Study

The entire bore wells in the two feeders F12 & F13 generally have the GI pipe for pumping the
water from the bore well up to ground level. And from there to the storage tank is normally of
PVC/HDPE pipes.
The frictional loss in the GI pipe is more when compared to HDPE/PVC pipes. There is an
option, that the frictional loss of the total pumping system can be reduced by replacing the GI

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pipes with HDPE pipes. The following sample calculation explains, how the change in material
of pipe from GI to HDPE affects the total head of the system.
System Components:
Discharge                       : 2.0 LPS
Length of discharge pipe        : 122m
(from pump to ground level)
Diameter of this pipe           : 2 inch
Material of this pipe           : GI (to be replaced with HDPE)
Pipe fittings
Elbow / Bend                    : 3 Nos.
Couplings                       : 20 Nos.

10.10.1.1       System with GI Pipe
Absolute roughness of GI pipe              : 0.15mm
Relative Roughness                         : Absolute roughness/diameter of pipe
                                           : 0.002953
Reynold‟s Number                           : Density x Velocity x Diameter of pipe/ Viscosity
                                           : 50,121
From Moody‟s Diagram (using Reynold‟s Number and Relative Roughness),
the friction factor                        : 0.007
Equivalent length for Pipe fittings        : Sum of empirical resistance coefficients of
individual fittings x Diameter / (4 x Friction factor)
Considering 0.75 & 0.04 as Resistance coefficients for Bends and Couplings respectively, the
equivalent length   : 5.44m
Frictional Loss                            : 4 x Friction factor x Total length of Pipe x
                                  Velocity 2 / (2g x Diameter of pipe)
                Hf      : 3.55m

10.10.1.2       System with HDPE Pipe
Absolute roughness of HDPE pipe            : 0.0015mm
Relative Roughness                         : 0.00003
Reynold‟s Number                           : 50,121
From Moody‟s Diagram (using Reynold‟s Number and Relative Roughness),
the friction factor                        : 0.005

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Equivalent length for Pipe fittings      : 7.4m
Frictional Loss                Hf        : 2.65m
(Calculated on the same discharge basis)


Reduction in Friction Loss               : 0.9m


It can be seen that the reduction in frictional loss is around 1.0m. So the total working head of
the pump set can be reduced by 1.0m by replacing the GI pipe with HDPE pipe.

The friction loss reduction will again depend on the discharge of the system. If the discharge is
more than there will be a significant amount of reduction can be gained by changing the pipe
material.

10.10.2Effects of Change in Pipe material:

1. For a new bore well or for a bore well where the existing pump is going to be replaced with a
new pump, the actual operating head of the system has to be calculated. Along with the yield of
the bore well, the operating head is used to select the suitable, correct sized pump set. So that the
selected pump set will operate in its best efficiency range.
In this case, the reduction in frictional loss by changing the GI pipe with HDPE pipe will have
significant effect on the selection of new pump set which will replace the existing pump set of
low efficiency. Here the actual operating head of the new pump will be less than the old pump,
because of reduction in frictional loss. Along with the higher efficiency, the less working head
by the pipe replacement will further reduce the energy requirement of the pump set.
Figure 1: Performance Curve of Pump




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2. For an existing pump set, only by replacing the GI pipe with HDPE pipe will have the
following effects.
     The change in pipe material will give counter productive effect. Means, the discharge of
      the pump set will be increased due to the reduction in operating head.
     The energy consumption of the pump set may remain constant or may increase little further
      based on its performance curve. That means there is no savings in energy also.
     The efficiency of the total system may come down further due to shifting the operating
      point towards right side of the curve.
     If the discharge increase intern increase the power consumption then the current flow in the
      windings will increase. This makes easy burn out of windings.
     Handling problems will arise due to the inadequate strength of HDPE pipe material while
      using for higher capacity pump sets.

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10.11 Common Causes and Remedies
There are many reasons for poor pumping plant efficiency. Some of the more common causes of
unsatisfactory performance and their remedies are as follows:
1.    Impellers that are out of adjustment is the easiest and least expensive problem to correct.
      Both pumping rates and efficiency are reduced because energy is used to pump water that
      is recirculated around the impellers instead of being pumped into the irrigation system.
      Impeller adjustment is especially critical with semi-open impeller pumps. Impellers may be
      out of adjustment because of improper initial adjustment or because of wear. To avoid
      pump damage, only experienced pump people should attempt to make impeller
      adjustments.

2.     Pump bowls designed for a higher pumping rate than the well can supply is one of the
       most common reasons for poor pumping plant efficiency. Overestimating well yield often
       results from poor testing of the well after drilling. If well testing was inadequate, the yield
       of the well may have been less than anticipated. In other cases, the pump supplier
       recommended oversize pump bowls in order to require fewer stages, thereby reducing
       initial cost. Furthermore, declining water tables in some areas have reduced well yields. In
       this situation, a pump is forced to operate at a lower flow rate and higher lift than that for
       which it was designed. If for any of these reasons the pump capacity does not fit the well
       characteristics, a high pumping plant efficiency can be achieved only by replacing the
       bowls with new (not rebuilt) bowls that meet the well requirements.

3.     Damaged impellers also will result in poor performance. Three common causes of
       impeller damage are cavitation (low temperature boiling of pumped water), sand pumping
       and improper impeller adjustment. Sometimes only the impellers need to be changed, but
       more often the permanent solution is to replace the entire bowl assembly. If this is done, it
       is likely that a different model of pump bowls should be used to fit present well conditions.

4.     Incorrect power unit selection is another major cause of low efficiency. This is much
       more important for engines than for electric motors. While the efficiency of electric motors
       does not vary greatly with loading, it should be noted that over-loaded motors have shorter
       lives, are less dependable and are more expensive to maintain. On the other hand, because
       of graduated energy costs, underloaded motors often increase the cost per kilowatt of
       power used. Incorrect engine selection is a major cause of low efficiencies among the
       natural gas pumping plants. Many are overloaded. Automotive-type V-8 engines often are
       used for applications where heavy-duty industrial engines should be used. Operating speeds
       of the smaller engines are increased so that they will produce adequate power. As a result,
       they wear out rapidly and require much more fuel.

5.     Failure to perform required maintenance, including tune-ups, is often a cause of low
       efficiency in engine-driven pumping plants. Electric motors, on the other hand, usually
       operate efficiently if they operate at all. Finally, a change in operating conditions from
       those for which a pumping plant was designed will result in a drop in efficiency. Three
       common situations that result in increased pumping lifts or pressures are a drop in water

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      table elevation, converting from open discharge to a pipeline, and changing from surface
      irrigation to sprinkler irrigation. On the other hand, a reduction in operating pressure results
      when center pivot sprinklers are converted from high pressure to low pressure in an attempt
      to save energy. Usually the pump will operate less efficiently under the new lower pressure
      conditions than it did under high pressure. As a result, anticipated savings in energy costs
      may not be realized.

10.11.1Cost vs. Savings From Repair or Replacement

Once it has been found that a pump is not performing up to par, the next step is to consult a
reputable pump supplier to determine the cost of repair or replacement. If it is necessary to pull
the pump, these costs will be substantial.

How does one decide whether pump repair or replacement will pay off? There are certain
conditions under which pump bowls will almost certainly need to be replaced.

     The potential well yield is adequate, but the pump will not supply the required flow rate at
      the required pressure.
     The water table has declined dramatically; this was not anticipated in the original pump
      selection.
     A major change in the irrigation system has occurred, either from surface irrigation to
      sprinkler irrigation or vice versa, or from high pressure to low pressure sprinklers.

For other reasons, it will be better to perform a cost benefit analysis before going for repair or
replacement option.

10.12 Conservation tips for agriculture pumps

1. Which is the foot-valve, that saves energy?

                                          The  foot-valve shown in Fig-2, because it has a wider
                                          mouth and a larger area of opening.
                                          The larger valve helps to reduce the loss by friction.
                                           Frictionless foot valves are now in use to improve
                                          the system efficiency
                                          An efficient low friction “ISI” mark foot-valve, though
                                          costlier, pays back fast the extra cost by saving energy.

                                          Fig-2


2. Which type of pipeline helps save energy ?
The rigid PVC pipeline, with a larger diameter, shown in Fig-4. More energy required to
pump water through small diameter pipes because it offers higher friction. If the pipe is
larger than the pump flange size, a reducer must be used.




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How does a 20 mm decrease in diameter increases the friction three times: ,if, in place of
100 mm (four inch) pipe, an 80 mm (three inche) pipe is used, the loss due to friction for
drawing the same quantity of water will be three times more, which will cause higher energy
consumption. Also pipes made of rigid PVC cause lower frictional loss as compared to
pipes made of conventional galvanized iron. Such pipes thus help save energy.




      Fig-4                                                                     Fig-5
3. Which type of pipeline arrangement helps save energy?
The one shown in Fig-6.
The pipeline arrangement shown in the figure 7 has many bends and unnecessary fittings,
which causes higher energy consumption. Each bend in an 80 mm (three inch) diameter
pipeline leads to as much friction loss as an additional pipe length of three meters.
Therefore, the fewer the number of bends and fittings in a pipe, the more the energy saving.




               Fig-6                                                                     Fig-7

4 Which type of bend should be used in a pipeline?
The type of bend shown in Fig-8. Sharp bends and L-joints in the pipe lead to 70
per cent more frictional loss than standard bends.




               Fig-8                                                                     Fig-9




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5. Which type of installation is better?
The one shown in Fig-10.
The pump works most efficiently when it is not more than 10 feet above the water level. If
the well is deep, the pump should be installed on a platform at the right height.




                 Fig-10                                                           Fig-11

6. Which length of pipe helps save diesel?
The pipe of short length shown in Fig-12. The pipe shown in Fig-13 is unnecessarily high
and would require more fuel for pumping water. A farmer can save 15 litres of diesel every
month simply by reducing the pipe height by 2 metres (if he is using diesel engine).




                 Fig-12                                                           Fig-13

7. Which transmission is better?
The transmission shown in Fig-14. The belt in Fig-15 is old and worn out. It could slip or
snap anytime, causing loss in the transmission of power and hence increase fuel
consumption.




                                                                                                                

Fig-14                                                                                     Fig-15

Check points for efficient transmission.
 Reduce the number of joints in the belt.
 Check and adjust belt tension frequently.
 Check alignment of the pump with the engine.


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