Selection of Centrifugal Pumping
D. Z. Haman, F. S. Zazueta and F. T. Izuno
Florida Cooperative Extension Service
Institute of Food and Agricultural Sciences
University of Florida
John T. Woeste, Dean
Dorota Z. Haman and Fedro Zazueta are Associate Professors of Agricultural Engineering in the Institute of Food and Agricultural
Sciences, University of Florida, Gainesville, FL 32611. Forrest T. Izuno is an Associate Professor of Water Management at the
Everglades Research and Education Center, PO Box 8003, Belle Glade, FL 33430-1101.
Table of Contents
Selection of Centrifugal Pumping Equipment .................1
Introduction .......................................... 1
Pump performance parameters ............ .............. .1
Capacity .......................................... 1
Power requirements ..................
Net positive suction head ..............
Specific speed .....................
Determination of operating conditions ........
System Capacity ........................
Total dynamic head required by the system ...
Static head .........................
Static lift ........................
Static discharge ..................
W ell drawdown ......................
Operating head ......................
...... ......... 3
. .............. 3
............ .. .4
Friction loss ............ ..... .....................4
Velocity head ....................................... .5
System-head variations ................................5
Characteristic curves for centrifugal pumps .................. .5
Head versus pump capacity ..............................5
Efficiency versus pump capacity .............................6
Brake power versus pump capacity ..................... ...6
Net positive suction head required versus pump capacity ....... .6
Pump operating point .................... ..................7
Pump selection ........................................... .8
Pump types .......................................... .8
Pumps operating in series .............................. 10
Pumps operating in parallel ............................. 10
Efficiency considerations ..................... .........10
References ........................................... 11
U2NIVrIF.ITY C FLUJDA Li)a"AKlES
The pump is an essential component of an
irrigation system. Proper selection of pumping
equipment that will provide satisfactory
performance requires good understanding of
existing conditions. Design restrictions, operating
conditions of the irrigation system, and required
flexibility in system operation must be understood
before an efficient pump can be selected for a given
Pump performance parameters
Capacity, head, power, efficiency, required net
positive suction head, and specific speed are
parameters that describe a pump's performance.
The capacity of a pump is the amount of water
pumped per unit time. Capacity is also frequently
called discharge or flow rate (Q). In English units
it is usually expressed in gallons per minute (gpm).
In metric units it is expressed as liters per minute
(Vmin) or cubic meters per second (m3/sec).
Head is the net work done on a unit weight of
water by the pump impeller. It is the amount of
energy added to the water between the suction and
discharge sides of the pump. Pump head is
measured as pressure difference between the
discharge and suction sides of the pump.
Pressure in a liquid can be thought of as being
caused by a column of the liquid that, due to its
weight, exerts a certain pressure on a surface. This
column of water is called the head and is usually
expressed in feet (ft) or meters (m) of liquid.
Pressure and head are two different ways of
expressing the same value. Usually, the term
"pressure" refers to units in psi (pounds per in2) in
the English system or kilopascals (kPa) in metric
units. "Head" refers to ft in English units or meters
(m) in meteric units. A column of water that is 2.31
ft high exerts a pressure of 1 psi.
The power imparted to the water by the pump is
called water power. To calculate water power, the
flow rate and the pump head must be known. In
English units water power can be calculated using
the following equation:
WP = (Q* H)3960 (1)
WP = water power in horsepower units
Q = flow rate (pump capacity) in gpm
H = pump head in ft
In metric units, water power is expressed in kilo-
watts, pump capacity in liters per minute, head in
meters, and the constant is 6116 instead of 3960.
There are always losses that must be accounted
for in any physical process. As a result, t4 provide
a certain amount of power to the water a larger
amount of power must be provided to the pump
shaft. This power is called brake power (brake
horsepower in English units). The efficiency of the
pump (discussed below) determines how much more
power is required at the shaft.
BP = WP/E (2)
where E is the efficiency of the pump expressed as
a fraction, BP and WP are brake power and water
Pump efficiency is the percent of power input to
the pump shaft (the brake power) that is trans-
ferred to the water. Since there are losses in the
pump, the efficiency of the pump is less than 100%
and the amount of energy required to run the pump
is greater than the actual energy transferred to the
water. The efficiency of the pump can be calculated
from the water power (WP) and brake power (BP):
E% = (WP/BP)*100(3)
The efficiency of a pump is determined by con-
ducting tests. Efficiency varies with pump size,
type and design. Generally, larger pumps have
higher efficiencies. Materials used for pump con-
struction also influence efficiency. For example,
smoother impeller finishes will increase pump effi-
Net positive suction head
The required net positive suction head (NPSH,)
is the amount of energy required to prevent the for-
mation of vapor-filled cavities of fluid within the
eye of impeller. The formation and subsequent col-
lapse of these vapor-filled cavities is called cavita-
tion and is destructive to the impeller.
The NPSHr to prevent cavitation is a function of
pump design and is usually determined experimen-
tally for each pump. The head within the eye of the
impeller should exceed the NPSH, to avoid cavita-
Specific speed is an index number correlating
pump flow, head and speed at the optimum
efficiency point. It classifies pump impellers with
respect to their geometric similarity. Two
impellers are geometrically similar when the ratios
of corresponding dimensions are the same for both.
This index is important when selecting impellers
for different conditions of head, capacity and speed
(Figure 1). Usually, high-head impellers have low
specific speeds and low- head impellers have high
There is often an advantage in using pumps with
high specific speeds. For a given set of conditions,
operating speed is higher. As a result, the selected
pump can generally be smaller and less expensive.
However, there is also a trade-off since pumps op-
erating at higher speeds wear out faster.
. 60o -w 100
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Determination of operating
Before selecting a pump, it is necessary to deter-
mine the head (H) and discharge (Q) required by
the irrigation system. The system head versus dis-
charge relationship should be evaluated for the en-
tire range of operating conditions. When the sys-
tem head and/or discharge vary significantly, spe-
cial attention must be given to selecting a pump (or
set of pumps) that can satisfy all conditions. Be-
cause most pumps are not very efficient over wide
ranges in operating heads, the most prevalent con-
ditions should be determined and a pump that op-
erates efficiently over this set of conditions, and can
operate under all other possible conditions, should
Over 10,000 gpm
'i 1000 3000 gpm
S500 1000 gpm
.200 500 gpm
100 200 gpm
II 1, Iia I9i ^- Axis of
Centrifugal Mixed Flow Propeller
Figure 1. Theoretical pump efficiencies as functions of specific speed, Impeller design, and pump capacity.
The flow rate required by the irrigation system
depends on the size and type of the irrigation sys-
tem, crop water requirement, time of operation,
and efficiency of the system.
Frequently, the discharge is constant for a given
irrigation system. However, it may vary, espe-
cially for large systems with several zones. A typi-
cal example of a variable discharge system is one
where the same pump is used for several irrigation
zones consisting of solid-set sprinklers and when
the zones are not the same size. If possible, the
system should be designed to minimize these types
Total dynamic head required by the
For a given irrigation system a pump must pro-
vide the required flow rate at the required head (or
pressure). The total dynamic head (TDH) curve of
the system (Figure 2) illustrates that more head is
required to increase the total flow through the sys-
The pressure required for operating a given
sprinkler or emitter represents only a portion of
the total dynamic system head. Additional pressure
must be produced by the pump to lift water from
the well or other water source, to overcome friction
losses in the pipe and other components of the sys-
tem, and to provide velocity for the water to flow
through the pipe. As a result, the total dynamic
head for the system is the sum of static head (dis-
tance the water must be lifted), well drawdown, op-
erating pressure (pressure required at the sprin-
kler or emitter), friction head (energy losses) and
velocity head (energy required for water to flow).
These components are illustrated in Figure 2. The
SYSTEM HEAD CURVE
Static Discharge Static Head
Figure 2. Total system curve and its components.
system total dynamic head can be expressed as:
H, = H. + H + Ho + H + H (4)
Ht = total dynamic head of the system (TDH)
H = static head (static lift + static discharge)
H = drawdown
Ho = operating head
H, = friction loss head
H, = velocity head
Static head is the vertical distance from the wa-
ter level at the source to the highest point where
the water must be delivered. It is the sum of static
lift and static discharge. Static head is indepen-
dent of the system discharge (gpm) and is constant
for all discharge values. However, it is possible
that the static head may vary with time due to the
changes in the system.
The static lift is the vertical distance between
the center line of the pump and the elevation of the
water source when the pump is not operating. If the
water elevation of the source is below the pump el-
evation, the static lift is positive. If the pump is lo-
cated at the elevation below the water surface el-
evation, the static lift is negative.
The static discharge head is a measure of the el-
evation difference between the center line of the
pump or top of the discharge pipe and the final
point of use. When pumps discharge directly into
canals a short distance from the pump at the same
elevation, the static discharge head is zero. If, how-
ever, a pump supplies water to some distant point
at another elevation, then it is necessary to com-
pute the static discharge head. To obtain this value,
subtract the elevation of the pump or discharge
pipe from the elevation of the final point of deliv-
As a well is pumped the water level in the well
declines. This phenomena is commonly called the
well drawdown. The amount of the drawdown is a
function of the pumping rate, the aquifer proper-
ties, well size, method of construction (well screen,
etc.) and the time the pump is operated. The best
way to determine the well drawdown is to test
pump a well at various rates and observe the draw-
down. Testing of wells is described in detail in IFAS
Extension Circular 803 "Water Wells for Florida
When the water is to be pumped from the well it
is important to know the drawdown to account for
the additional lift. For a surface water source such
as lake or river this water level may drop during a
dry season. Any changes in static lift must be ac-
counted for in the static head portion of the total
Some irrigation systems require pressure to
operate. The range of this pressure varies among
systems. High pressure systems, such as traveling
guns and high pressure center pivots or sprinkler
systems, may require large operating pressures (up
to 100 psi). Micro irrigation systems can operate at
much lower pressures (8-30 psi). For gravity
irrigation systems (furrow, flood or open ditch
subirrigation) the operating pressure can be close
For most irrigation systems, the operating
pressure is constant. However, some systems may
have a variable operating pressure. A good
example is a center pivot system with an end gun
for corner irrigation. Operating the gun requires
additional pressure head for a relatively short time.
When water flows through a pipe there is a loss
of head due to friction. This loss can be calculated
using hydraulic formulas or can be evaluated using
friction-loss tables, nomographs, or curves provided
by pipe manufacturers. The pump must add
energy to the water to overcome the friction losses.
As the discharge of the system increases the
velocity also increases. The friction loss increases
as the square of the flow velocity. Due to the high
cost of energy, it is often recommended that a
larger pipe size be used to decrease the velocity for
the same discharge. This is usually economically
feasible if the water velocity is more than 5 ft/sec.
For a system having very long pipelines or
undersized pipe for a given flow rate, the friction
loss can be very significant.
Friction losses must be considered on both the
intake and discharge sides of the pump. It is
especially necessary to compute or evaluate the
friction loss on the suction-side of centrifugal
pumps to assure enough net positive suction head
available (discussed below) to prevent pump cavita-
Pump cavitation is caused by the reduction in
pressure behind the impellers to the point that the
water vaporizes. It can be very damaging to the
pump. Cavitation is described in IFAS Extension
Circular 832 "Pumps for Florida Irrigation and
In every irrigation system there are some addi-
tional friction or minor losses due to various fit-
tings and other components such as flow meters
and intake strainers. Minor losses can be esti-
mated using tables that relate each type of fitting
to the certain equivalent length of the same diam-
Velocity head is the amount of energy required
to provide kinetic energy to the water. For systems
with a high total head this component is very small
compared with other components of the total sys-
tem head. Velocity head is calculated using the
H = V2/2g (5)
H = velocity head ft (m)
V = water velocity in the system ft/sec (m/sec)
g = acceleration of gravity 32.2 ft/sec2 (9.81 in
In most installations velocity head is less than
one foot (.3m). An increase of water velocity in the
system will not usually result in large increases in
velocity head. However, velocities that are too
high will increase friction losses as discussed above
and also may result in water hammer which
should be avoided. Water hammer is a sudden
shock wave propagated through the system. To
avoid it, the velocity is generally maintained below
The total system head can vary with time due to
variations in well drawdown, friction, operating
conditions, and static water level changes through-
out the seasons. The friction losses increase with
system age due to corrosion or deposits in the pipe
and other components. The static lift component of
the total dynamic head may vary due to fluctuating
water levels throughout the season, or from year to
In some systems there is a periodic change in
the operating head. It may not be possible to select
a pump that is efficient under a wide range of
system heads. In some cases an additional
(booster) pump, in series with a main pump, may
provide the additional head when necessary (see
the section on pumps in series). This arrangement
is frequently used in center pivot systems, where a
small booster pump provides the additional operat-
ing head required for end gun operation at the cor-
ners of the field.
Characteristic curves for
A set of four curves known as the pump's charac-
teristic curves is used to describe the operating
properties of a centrifugal pump. These four curves
relate head, efficiency, power, and net positive suc-
tion head required to pump capacity (Figure 3).
Pump manufacturers normally publish a set of
characteristic curves for each pump model they
make. Data for these curves are developed by test-
ing several pumps of a specific model. The operat-
ing properties of a pump depend on the geometry
and dimensions of the pump's impeller and casing.
( DISCHARGE )
Figure 3. Characteristic curves for a single-stage centrifugal
Head versus pump capacity
Figure 4 shows a typical head, H (ft), versus ca-
pacity (discharge), Q (gpm), curve for a single-stage
pump. This curve relates head produced by a pump
to the volume of water pumped per unit time.
Generally, the head produced decreases as the
amount of water pumped increases. The shape of
the curve varies with the pump's specific speed and
impeller design. Usually, the highest head is pro-
duced at zero discharge and is called the shut-off
shut off head
CAPACITY (gpm, I/min)
Figure 4. A typical head versus capacity characteristic curve
of a centrifugal pump.
Efficiency versus pump capacity
Figure 5 shows the curve relating efficiency (E)
to discharge (Q). The E-Q relationship can also be
drawn as a series of envelope curves upon the H-Q
curve (Figure 6). The efficiency of a pump steadily
increases to a peak, and then declines as Q in-
creases further. Efficiencies vary between types of
pumps, manufacturers and models.
Efficiency is defined as the output work divided
by the input work. The input work is usually ex-
pressed as the size of the engine required to drive
the pump. It is commonly expressed in English
units as the brake horsepower.
Brake power versus pump capacity
The shape of the brake power versus discharge
curve is a function of the head versus discharge
and efficiency versus discharge curves. The most
common form of the BP-Q curve for centrifugal
pumps is presented in Figure 7. In some cases the
highest power demand is at the lowest discharge
rate and continues to decline as the discharge in-
creases. It is important to notice that even at zero
discharge, when the pump is operating against the
shut-offhead, an energy input is needed.
CAPACITY (gpm, I /min)
Figure 5. A typical efficiency versus capacity characteristic
curve of a centrifugal pump.
CAPACITY (gpm, l/min)
Figure 6. Efficiency expressed as a series of envelope curves
upon the head versus discharge characteristic curve
of a centrifugal pump.
It is recommended that the power requirement
(brake power) be calculated using equation (1) be-
cause the vertical scale for most BP-Q curves can-
not be read accurately.
Net positive suction head required
versus pump capacity
One of the curves typically published by manu-
facturers is the net positive suction head required,
CAPACITY (9pm, I/min)
Figure 7. A typical brake power versus capacity characteristic
curve of a centrifugal pump.
NPSHr, versus capacity, Q. For a typical centrifu-
gal pump the NPSH, steadily increases as Q in-
creases. To assure that the required energy is
available, an analysis must be made to determine
the net positive suction head available NPSH.,
which is a function of the pumping-system design.
Net positive suction head available in the system is
calculated using following formula:
NPSH. = BP SH FL VP (6)
BP= barometric pressure
SH= suction head or lift
FL = friction losses in the intake pipe
VP= water vapor pressure at a given tempera-
(all terms should be expressed in feet of water).
After these calculations are performed the
NPSH, versus Q curve can be used. The NPSH.
must be greater than NPSH, at a given Q to avoid
pump cavitation. A typical curve representing
NPSH, versus capacity Q is shown in Figure 8.
Pump operating point
A centrifugal pump can operate at a combination
of head and discharge points given by its H-Q
curve, called the operating point. Once this point is
determined, brake power, efficiency, and net posi-
tive suction head required for the pump can be ob-
tained from the set of pump curves.
CAPACITY (gpm, I/min)
Figure 8. A typical characteristic curve representing net
positive suction head required (NPSHr) versus
capacity for a centrifugal pump.
The operating point is determined by the head
and discharge requirement of the irrigation system.
A system curve, which describes the head and dis-
charge requirements of the irrigation system, and a
head-discharge characteristic curve of the pump
are used to determine the pump operating point
(Figure 9). At the operating point the head-dis-
charge requirements of the system are equal to the
head-discharge produced by the pump.
A system curve is produced by calculating the
total dynamic head Ht (see equation 4) required by
the irrigation system to deliver a certain volume of
CAPACITY ( gpm, I /min)
Figure 9. Determination of the operating point for a given
centrifugal pump and water system.
water per unit time. By determining the system
head curve for a range of discharges above and be-
low the design discharge, sufficient information will
be available to evaluate pump performance for all
expected operating conditions.
It is important to realize that the system curve
does not always remain constant over time. Any
change in the system curve results in a shift of the
pump operating point. The total range of possible
conditions of head versus discharge for a given sys-
tem must be evaluated before an efficient pump (or
pumps) can be selected. Two examples of possible
changes with time are in Figures 10 and 11. In
Figure 10 the change in the total system head is
due to increased friction losses, while in Figure 11
the change is due to a change in static head.
The pump selection should consider these
changes. If the range of discharge is not very large
it is possible to select a pump that can operate un-
der all these conditions with reasonable efficiency.
Pump selection is the choice of the most suitable
pump for a particular irrigation system. The per-
formance requirements of the water system must
be specified and the pump type must be selected.
Alternate pumps that meet the requirements of the
system also should be specified. Normally, the
most suitable pump is chosen from these pumps,
considering economic factors.
CAPACITY ( gpm, I/min)
CAPACITY (gpm, I/min)
Figure 11. A change in the total dynamic system head due to
the drop of the water table.
Various types of centrifugal pumps are most
commonly used in irrigation and drainage systems.
Centrifugal pumps can be classified by type of im-
1. Radial-flow pumps
2. Axial-flow pumps
3. Mixed-flow pumps
In addition, a centrifugal pump can be classified in
one of four major groups depending on its design
1. Volute pumps
2. Diffuser pumps
3. Turbine pumps
4. Propeller pumps
Volute pumps are used where water is obtained
from depths generally less than 20 ft (7m). The ex-
act value of possible lift is determined by the net
positive suction head required by the pump and in-
take-side conditions. Whenever possible, it is rec-
ommended to use a horizontal volute pump, be-
cause it is less expensive and easy to install. Deep
wells and some surface water sources, however, will
require a submersible pump to provide their NPSH,
needs. If the suction-side lift is more than 20 ft at
any time during pumping, a horizontal volute cen-
trifugal pump may have cavitation problems. A
turbine pump should be used in this system.
Figure 10. A change In the total dynamic system head due to
the Increased friction losses In the old pipe.
Pumps using axial-flow types of impellers are
designed for conditions where the capacity is high
and the head requirements are low. These impel-
lers are used in propeller pumps. Most axial-flow
pumps operate on installations where suction lift is
not required. They are installed in such a way that
the impeller is submerged in the water. In Florida,
they are most frequently found in the south, where
the water is pumped from canals. Characteristic
curves of these pumps vary from those for centrifu-
gal pumps discussed in this publication. They will
not be discussed here; however, for more informa-
tion, see James (1988).
Mixed-flow impellers with diffusers are often
used in deep-well turbine and submersible turbine
pumps. These pumps are used for pumping water
from depths larger than 20 ft.
More details on various pump types, classifica-
tions, operation, and applications of various pumps
are provided in IFAS Extension Circular 832
"Pumps for Florida Irrigation and Drainage Sys-
Manufacturers' catalogs are consulted to identify
pumps of the proper type capable of supplying the
-- -- --,- r--- -,-
discharge and head requirements of the water sys-
tem. Characteristic curves for these pumps are ex-
amined to determine which of these pumps are
suitable for the irrigation system. Proper selection
of a pump requires knowledge, understanding, and
use of the pump's characteristic curves.
For example, in the case of the variation in sys-
tem discharge, select a pump with flat Q-H charac-
teristic curve where the head doesn't change sig-
nificantly with a change in discharge (Figure 12).
This pump can deliver 800 gpm at approximately
107 ft of pressure at 70% efficiency. If the flow
rate increases to 1000 gpm the head will drop only
about 15 ft (6.5 psi) and efficiency will remain the
same. If the system can operate with a 15 ft lower
head, the same pump can be used for both flow
It is not always possible to select one pump for
all anticipated operating conditions. If the dis-
charge varies significantly, an additional pump in
parallel with a main pump may be used in time of
larger flow demands (see section on parallel
Figure 12. A typical set of characteristic curves for a deep well submersible turbine pump with a relatively flat H-Q characteristic
Pumps operating in series
To connect two pumps in series means that the
discharge from the first pump is piped into the inlet
side of the second pump (Figure 13). In this type of
arrangement all the flow successively passes from
Pumps operating in parallel
Figure 15 presents a parallel configuration of
two pumps. A typical example of this arrangement
is a situation where two or more pumps draw water
from a single source and all the flows are dis-
charged into a single pipe. Another example is a
situation where several small wells are providing
the required discharge. Parallel arrangements are
Figure 13. Two pumps connected in a series.
one pump to the next with each pump adding more
energy to the water. This is a typical arrangement
in multi-stage turbine or submersible pump where
the same discharge passes through all stages and
each builds additional head. Often, series configu-
rations are used when head requirements of the
system exceed that which can be supplied by indi-
vidual pumps. They are also used in systems with
variable head requirements. A typical example is a
small centrifugal pump used as a booster pump for
corner irrigation on a center pivot system or, for
that matter, any booster pump, in any water sys-
tem, which works in addition to the main water
pump. Figure 14 shows head-discharge curves for
two pumps operating in series.
Pumps A and B
CAPACITY (gpm, I/min)
Figure 14. Head versus discharge characteristic curves for two
pumps operating in series.
Figure 15. Two pumps connected in parallel.
also common methods of meeting variable dis-
charge requirements of the system. Figure 16
shows a head-discharge characteristic curve for two
pumps operating in parallel.
During the pump selection process, only pumps
having high efficiencies (above 70%) for the design
discharge should be considered for a system. It is
common practice to select a pump capable of pro-
Combined Curve for Pumps
E A and B Operating In Parallel
I \> Pump 8
CAPACITY (gpm, l/min)
Figure 16. Head versus discharge characteristic curves for two
pumps operating In parallel.
during higher head and larger flow rate (approxi-
mately 10%) than the design parameters. This will
assure that as the pump wears, its performance will
remain adequate. The impact of low efficiency on
power consumption is very significant. Total pump
operating costs may justify the purchase of a more
expensive pump that can operate with higher effi-
ciency under needed conditions. Economics is often
the primary criterion for pump selection. It is im-
portant to estimate the cost of pump operation and
consider this cost together with the initial cost of
the pumping equipment. The economic analysis is
beyond the scope of this publication. For additional
information, see Jensen (1980) or James (1988).
Haman, D.Z., F. T. Izuno, A.G. Smajstrla. 1989.
Pumps for Florida Irrigation and Drainage Sys-
tems. Extension Circular 832. IFAS. University of
Florida, Gainesville ,FL, 32611.
Haman, D.Z., A.G. Smajstrla, G.A. Clark. Water
Wells for Florida Irrigation Systems. Extension
Circular 803. IFAS. University of Florida, Gaines-
ville, FL, 32611.
James, L.G. 1988. Principles of Farm Irrigation
System Design. John Wiley and Sons. New York.
Jensen, M.E. 1980. Design and Operation of
Farm Irrigation Systems. American Society of Agri-
cultural Engineers, St. Joseph, MI 49085.
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