Title: Selection of pumps and power units for irrigation systems in Florida
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Permanent Link: http://ufdc.ufl.edu/UF00049917/00001
 Material Information
Title: Selection of pumps and power units for irrigation systems in Florida
Translated Title: Circular / Agricultural Extension Service ; no. 330 ( English )
Physical Description: Book
Language: English
Creator: Harrison, D. S.
Choate, Rush E.
Publisher: Agricultural Extension Service, IFAS, Dept. of Agricultural Engineering, University of Florida
Publication Date: 1968
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Bibliographic ID: UF00049917
Volume ID: VID00001
Source Institution: University of Florida
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Full Text

Circular 330


SELECTION OF PUMPS AND

POWER UNITS FOR IRRIGATION

SYSTEMS IN FLORIDA


Dalton S. Harrison and Rush E. Choate


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Agricultural Extension Service
Institute of Food and Agricultural Sciences
Department of Agricultural Engineering
UNIVERSITY OF FLORIDA


1968








INTRODUCTION
Dalton S. Harrison and Rush E. Choatel


It is estimated that there are
some 15,000 irrigation systems
now being operated in Florida, rep-
resenting a total of 1,500,000 acres
subject to irrigation. This is twice
the combined total irrigated acre-
age of nine southeastern states. Of
this total, approximately 500,000
acres are sprinkler irrigated.
Crops subject to and which need
intensive irrigation to insure ade-
quate yields and quality include
citrus, peaches, vegetables, orna-
mentals, tobacco, sugarcane, sub-
tropical fruits, and grass-clover
pastures, representing a combined
area of some 1,450,000 acres.
The predominant type of irriga-
tion systems used in Florida are:
seepage or subsurface, permanent
(overtree) spring k ler, portable
sprinkler or volume guns, radial
(moved mechanically), perforated
pipe and travelling guns.
In many pump installations,
there is the question of "what type


pump should be used?" This de-
serves careful study because usu-
ally the type pump is dictated by
the installation and there is no
choice between a horizontal vs. a
vertical pump. When the source of
water is a deep well, there is no
choice but a vertical or "deep well
turbine," and then it becomes a
question of choosing the most effi-
cient and economical unit.
An engineering analysis should
be made of all pumping situations,
considering such key points as:
space requirements, NPSH (net
positive suction head), priming,
flexibility, corrosion, useful life,
efficiency, maintenance, and eco-
nomics involved.
The purpose of this bulletin is to
give some insight to installers,
production managers, and dealers
in irrigation equipment as to the
characteristics and operation of
pumps and power units used in ir-
rigation in Florida.


TYPES OF PUMPS


There are two basic classes of
pumps; namely, positive displace-
ment and kinetic. Each class has
various subclassifications. The cen-
trifugal pump, a subclassification
of kinetic pumps, is most com-
monly used in irrigation. It will be
discussed in detail.

Centrifugal pumps are generally
of two basic types vertical and


horizontal, and each of these has
many variations. As the name im-
plies, centrifugal pumps employ
centrifugal force to move water
against certain forces from one
point to another. This type of pump
in its simplest form consists of an
impeller, fixed on a shaft, which ro-
tates in a volute type casing. Water
reaching the center of the impeller
is picked up by the impeller vanes


L Associate Agricultural Engineer, Agricultural Extension Service, and Agricultural Engineer, Agri-
cultural Experiment Stations.








and accelerated to a high velocity
by the impeller rotation. It is then
discharged by centrifugal force


into the casing where the high ve-
locity head is converted to pressure
head.


PRINCIPLES OF OPERATION


SUCTION OR LIFT:
A centrifugal pump will operate
either with the impeller and its
casing submerged in the source of
water (that is, below the surface),
or with the impeller some limited
distance above the surface but
with the inlet side of the pump con-
nected to an air tight pipe or hose
which extends below the surface
of the water source. In the latter
instance, it is said to be operating
under a suction lift condition. Ac-
tually the pump does not "lift" the
water, because water does not pos-
sess tensile strength. It must be
pushed up to the pump by a higher
pressure on the surface of the
water source than exists at the
center of the impeller.
All centrifugal pumps must be
completely filled with water before
they can operate. After the pump
is primed, it is started and the ro-
tation of the impeller throws the
water that is contained in it to-
ward the outside by centrifugal
force. This creates a vacuum, or an
area of lower pressure, at the cen-
ter of the impeller. Since atmos-
pheric pressure is pushing down
ward on the surface of the source
of supply, the water is thus forced
through the suction pipe or hose to
the lower pressure area at the cen-
ter of the impeller to replace the
water being thrown outward. Thus,


there is a continuous flow of water
through the pump.
NET POSITIVE SUCTION
HEAD (NPSH):
When a pump is being operated
with its impeller above the surface
of the supply, it is operating under
a suction lift. The permissible ver-
tical distance between the center
of the impeller and the surface of
the supply is not equal for all type
centrifugal pumps. The allowable
distance varies with the design of
the pump and other operating con-
ditions. The greater the suction lift
under which a pump will operate
satisfactorily does not necessarily
mean that it is a better pump. It
merely means that the design
characteristics are different. Usu-
ally, to design a pump that will
operate under maximum lift means
that some other desirable charac-
teristics must be sacrificed.
Theoretically, if a pump could be
designed to produce a perfect vacu-
um at its center and it were being
operated at sea level, the atmos-
pheric pressure of about 14.7
pounds per square inch acting
downward on the surface of the
source would force water up the
suction line to the pump a distance
of 34 feet. In practice, of course,
this is impossible; first, because a
perfect vacuum cannot be created
at the center of the impeller, and







second, because there are losses
due to the friction created by the
flow through the suction line and
losses due to turbulence at the en-
trance to the suction line and at
the entrance to the impeller. Usu-
ally a vertical suction lift of about
15 feet should be considered the
maximum for reasonably efficient
operation.
Most manufacturers test their
pumps to determine the suction
lift under which they will operate,
and plot this data on the pump
performance curve as "NPSH"
(Net Positive Suction Head) re-
quired by the pump. It is expressed
in absolute terms. For example, if
at a given condition of tempera-
ture, speed, capacity, and head,
with all data referred to sea level
and reduced to absolute terms, a
pump on test is found to require
a minimum of 24 feet Net Positive
Suction Head, the pump would
operate at a maximum suction lift
of 10 feet. This in effect is the
equivalent of 14.7 psi atmospheric
pressure, or 34 feet, minus the 24
feet of NSPH required. The suction
lift includes vertical elevation
above the water surface while
pumping, and friction losses in the
suction piping system. When allow-
ance is made for these it may de-
velop that an actual lift of only
8 feet or even less is a safe limit
without involving damage to the
pump.
Operation of a pump under a lift
greater than that for which it was
designed, or under other conditions
which result in excessive or rapidly


fluctuating vacuum at some point
in the impeller, may cause cavita-
tion which will severely damage
the pump.
HEAD:
A pump is a device employed to
move water, or any liquid, from one
point to another. The difference in
elevation between the lower and the
higher level is called the "static
head" at which the pump operates.
It usually is expressed in terms of
feet but, bearing in mind that 2.31
feet of elevation is equivalent to 1
psi of pressure, the performance
of a pump is often expressed in
terms of pounds of discharge
pressure.
The total head that a centrifugal
pump will develop is a function of
the speed at which the impeller is
turned and the diameter of the im-
peller. If a pump impeller is being
turned at its rated speed and a
valve on the discharge side of the
pump is closed, it will develop a
certain maximum head. Under
these conditions, this head must
be read on a pressure gauge. The
guage reading translated into feet
will register the height to which
the pump is capable of elevating
the water. This is known as the
"shut-off-head." If the valve is
slowly opened it will be found that
as the flow increases the pressure
gauge reading will fall and this will
continue until some point of maxi-
mum flow and minimum head is
reached. If the total head being de-
veloped at any given rate of flow is
plotted against the quantity of
water being delivered in terms of








gallons per minute the result will
be a performance curve for this
particular pump at this particular
speed. If the power required to
turn the pump is observed during
this process it will be noted that
for a typical centrifugal pump the
power is at a minimum when there
is no water being discharged from
the pump and that it will gradually
increase as the rate of flow increas-
es and the head decreases. The
maximum efficiency will be some-
where about midway between zero
flow and maximum flow. A typical
pump performance curve is shown
in Figure 1.
PERFORMANCE CHARACTER-
ISTICS AND SPEED OF OPERA-
TION:
The performance of a pump va-
ries with the speed at which it is
turned. The capacity or rate of flow
varies directly as the change in
speed; that is, if the speed is in-
creased by 50 percent the capacity
is increased by 50 percent. The
head that the pump will develop,
however, varies as the square of
the ratio of the new speed to the
old speed. Thus, an increase of 50
percent in the speed means that
the developed head will increase
(15 )2or 2.25 times. Further, the

required horsepower increases as
the cube of the ratio of change.
this means that by increasing the
speed by 50 percent the required
horsepower will increase (.)3 or
3.38 times that required at the
lower speed.


EFFECT OF SPEED CHANGE
ON PUMP PERFORMANCE:
Theoretically, varying the pump
speed will result in changes in ca-
pacity, head and brake horsepower
according to the following for-
mulae:
rpm2 ) x gpm1 = gpm2
rpm/
rpm22 x Hi = H2 (in feet)
rpmi
(rpm2 x bhpi = bhp2
rpmi/
If the pump is driven by a con-
stant speed electric motor connect-
ed directly to its shaft, the pump
must turn at the same speed as the
motor. The pump speed can be
changed by connecting it to the
motor through a belt drive with
pulleys of different diameter or
through a speed changing gear.
This arrangement, however, pro-
vides only a different fixed speed
and not a variable speed.
When the pump is driven by an
internal combustion engine, it is
possible to vary its speed of opera-
tion within the speed range of
which the engine is capable; how-
ever, this will often change the effi-
ciency. Also, the horsepower ca-
pacity of an engine varies when
the rpm of the engine is changed.

EFFICIENCY:
The efficiency of a pump is a
measure of the degree of its hy-
draulic and mechanical perfection.
It is defined as the ratio of the
energy output to the energy input
and is expressed in percent.










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IL 5118 PUMP EFFICIENCY (%)
z < 60
30

25 8o
78
W 20 75
70
4, 15 -6

I10 4 4-
z
o 3 "
-J

-2


0 100 200 300 400 500 600 700
PUMP OUTPUT (IN GMP)

FIG. I. A TYPICAL PERFORMANCE CURVE OF AN 8-INCH TURBINE PUMP 1760 RPM








Since the unit horsepower is de-
fined as the power required to raise
a weight of 33,000 lbs. a vertical
distance of 1 foot in 1 minute, the
work performed by a pump (en-
ergy output in terms of horse-
power) is determined by the
weight of the water it delivers per
minute multiplied by the total
equivalent vertical distance in feet
through which it is moved. For ex-
ample, a pump delivering approxi-
mately 396 gallons per minute at a
total head of 10 feet, is performing
work at the rate of 1 horsepower.
If it were possible to achieve 100
percent efficiency it would only be
necessary in this instance to apply
1 horsepower to the pump shaft.
Actually, of course, the energy in-
put must be greater than 1 horse-
power.
Some of the energy losses which
result in a lower efficiency are the
friction in the bearings which sup-
port the pump shaft, friction be-
tween the shaft and the packing
in the stuffing box, unavoidable
leakage between areas of high
pressure and adjacent areas of low


pressure inside the pump case, and
the friction caused by the water
moving across the metallic sur-
faces in the pump. There are also
other losses of a more complex
nature.
The efficiency of a pump is de-
termined by actual test. Referring
to the example above, if it should
be found that 1.25 horsepower
must be applied to the input shaft
when the pump is doing work
equivalent to 1 horsepower, the
pump efficiency would be 80 per-
cent (1 divided by 1.25).
It is often necessary that a pump
designer sacrifice some degree of
efficiency in order to achieve some
other characteristic which is desir-
able in providing a unit with maxi-
mum usefulness. The efficiency
range to be expected varies with
the pump size, type, etc., but usu-
ally it should be between 65 per-
cent and 80 percent. Insofar as is
feasible, a pump should be selected
to operate near the point on its
curve where maximum efficiency
is obtained. This is referred to as
the "design point."


SELECTION OF TYPE PUMP


Before a pump may be selected
to perform a given job, three
things must be known: (1) the
total head against which it must
operate, (2) the desired flow in
gallons per minute at this point,
and (3) the suction lift. Once this
information has been established,
selection of a proper pump can
begin.
CENTRIFUGAL PUMPS (For sur-


face supply and shallow wells):
If the source of water is a sur-
face supply, such as a lake, stream,
pond or other body of surface
water, the pump most commonly
used is the horizontal centrifugal
type, commonly referred to as a
centrifugal pump. This is one in
which the shaft is normally in the
horizontal position. This type is
usually subdivided into two groups,







single suction (end suction), and
double suction (often called split
case). Either of these may be single
or multistage; that is, they may
have only one impeller or they may
have two or more impellers so con-
structed that the water in passing
through the pump is conducted
from the discharge of one impeller
to the suction of the second and
thus the total head is that devel-
oped by a single impeller multiplied
by the number of impellers in the
pump.
The most common pump and the
lowest in cost is the end suction,
single stage. Available sizes vary
considerably with different manu-
facturers. In general, if the desired
performance exceeds about 1000
gallons per minute of capacity and
150 feet of head, consideration
should be given to the split-case
type which is more rugged in con-
struction and is capable of a much
greater range of performance.
Vertical types of pumps (deep
well turbine) may also be used
with a surface supply and in many
applications they offer a distinct
advantage because they are inher-
ently more flexible in performance
and construction.
If the source of water is a well
and its pumping level is greater
than the suction lift allowable for
a centrifugal or shallow well jet
(15 to 22 ft.), a possible alternative
is a pump which can be partially
or completely installed inside the
well.
JET PUMPS:
For very low capacity require-


ments (5 to 20 gallons per minute),
such as a home water system, one
of the most common types is a jet
pump. This consists of a small cen-
trifugal pump located at ground
level connected to a jet installed
below the water level in the well.
By circulation of part of the water
from the pump, back through the
jet, water is forced up to the im-
peller in the pump and a continu-
ous flow at reasonable pressure is
provided.
Shallow well jet pumps operate
on the same recirculation principle
but the jet is installed above
ground and the allowable lift is
limited to about 22 feet; whereas,
deep-well jet pumps have a maxi-
mum lift of about 85 feet.
Jet pumps were designed for
home water systems and their ca-
pacities are seldom adequate for
irrigation purposes. Also, the jet
pump requires almost twice the
horsepower that a submersible
would require to deliver the same
amount of water from the same
depth.
SUBMERSIBLE PUMPS:
The submersible pump consists
of a multistage vertical pump con-
nected directly to an electric motor
(3450 rpm) which is designed to
operate under water. Both the
pump and motor are suspended in
the well below the water level by
means of a pipe which conducts the
water to the surface. This type is
available in a wide range of capaci-
ties for 4-inch wells and larger, but
in the larger sizes it may be rela-
tively higher in cost because the








waterproof motor is quite expens-
ive.
Some advantages of a submer-
sible are: maximum efficiency of
performance, can be used in wells
of any depth, no priming of pump
is necessary, well can be located
any distance from point of water
use, no possibility of air leaks,
positive air charging system for
pressure storage tank, and no
water-logging.
DEEP WELL TURBINE PUMPS:
For a deep well and for capaci-
ties above about 100 gallons per
minute, the most widely used irri-
gation pump is a vertical type cen-
trifugal, commonly referred to as a
"deep well turbine." Basically, this
is a centrifugal type of pump de-
signed for installation in a well. It
consists of four major components:
(1) Bowl assembly This con-
tains one or more impellers, each
in its own housing; (2) Column
and shaft assembly This con-
sists of the pipe to suspend the
bowl assembly in the well and con-
vey the water to the surface. Inside
this pipe (or column) is the shaft
which connects the impeller shaft
to the driver located at ground
level. The shaft may be either
water lubricated or oil lubricated;
(3) Discharge assembly This
often is called the "head" or
"base." It is normally cast iron, de-
signed for installation on a founda-
tion. It supports the column and
shaft assembly and the bowl as-
sembly in the well and provides a
discharge for the water being de-
livered. It also accommodates the


driver for the pump; and (4)
Driver This may be either an
electric motor or a right angle gear
for connection to a power unit.
When an electric motor is used the
usual type is a vertical hollow
shaft design which permits the
pump shaft to come up through its
center for securing at the top. The
right angle gear is also normally
of the hollow shaft type for the
same reason. It also has a hori-
zontal shaft for connection to the
engine drive or power take-off. The
internal gears are available in var-
ious ratios in order to accommo-
date an engine with an operating
speed which is different from the
desired pump operating speed.
Because of the limited diameter
of its impellers, each one develops a
relatively low head and it is neces-
sary for the average application to
stack several impellers in series
one above the other with each in
its own bowl or diffuser housing.
This is called staging. Thus, a four-
stage bowl assembly contains four
impellers all attached to a common
shaft through the separate hous-
ing or bowls. The bowl shaft is at-
tached to the line shaft through
the center of the pump column pipe
and must have a length as neces-
sary to locate the bowl assembly
below the level of the water in the
well.
Total Dynamic Head: For any
given capacity and speed, each im-
peller develops a certain amount
of head. For an example, let us as-
sume a well with 10-inch casing
and a static water level 100 feet








below the surface, and that it is
desired to select a pump to deliver
500 gallons per minute at a pres-
sure of 40 psi measured at the
point of discharge from the pump.
In this example the static water
level is 100 feet below the surface
but when 500 gallons per minute is
being pumped from the well this
level will fall. The distance the level
falls when pumping is called draw-
down and this varies with localities
and the formation into which the
well is drilled. Assuming the draw-
down, in this case, at a pumping
rate of 500 gallons per minute is 10
feet, then the total distance to the
surface becomes 110 feet. In this
example, 40 psi pressure is desired
at the surface. This is equivalent
to about 92 feet, so the total head
the pump must develop is 110 feet
plus 92 feet or 202 feet. To this
must be added an allowance for
friction loss within the pump col-
umn and the discharge assembly
which would normally be about 5
feet, assuming proper column size.
Thus, the total dynamic head or
the total operating head, will be
207 feet. If, for instance, there
were available an impeller that de-
livered 500 gallons per minute at
head of 22 feet, a 10-stage pump
would be selected, but the diameter
of the impellers could be reduced
("trimmed") to the point where
the average head developed by each
would be 20.7 feet.
Because of its inherent flexibil-
ity the vertical turbine pump is
sometimes used in sumps, lakes, or
other surface sources with a mini-


mum length of column between the
bowl assembly and the discharge
assembly.
OPTIONAL PUMP FEATURES:
Most pumps can be obtained
with various features of construc-
tion to suit individual preferences
and the various types of applica-
tions. These are discussed in the
following sections:
(a) Shaft Lubrication-
Water lubricated pump column
assembly is provided with fluted
rubber bearings to permit lubrica-
tion of the shaft by the water
being pumped. If the pump is.to be
operated at less than about 2200
rpm these bearings, which are
fixed in the column pipe couplings,
are usually placed at 10-foot inter-
vals. For higher speeds the bear-
ings are on 5-foot spacings.
Oil lubricated pump column as-
sembly includes a tube which en-
closes the shaft. Inside the tube are
bronze shaft bearings which are
threaded on the outside to serve as
couplings for connecting the pieces
of tube. The bearings spacing is
usually on 5-foot centers. Lubricat-
ing oil is fed into the top of the
tubing and passes by gravity over
the surfaces of the bearings. At
the bottom of the column is an
opening which permits the oil to
flow out. It then floats on top of the
water in the well.
Water lubricated turbine pumps
are simpler, cheaper, and more
commonly employed. If more than
four or five of the rubber shaft
bearings are above the water level
and thus become dry when the








pump is not operating, some means
of prelubrication is required, such
as a small pre-lub tank from which
water can be spilled over the bear-
ings prior to starting the pump.
With smaller size pumps, a foot-
valve can be installed below the
bowl assembly to keep the column
pipe full of water.
When the water level is very
deep, oil lubrication is normally
used. While there is no definite
point at which it becomes neces-
sary, it is usually recommended for
depths in excess of 150 to 200 feet.
(b) Enclosed vs. Semi-Open Impel-
lers-
An enclosed impeller is one in
which the vanes are covered on
both the top and bottom edges. A
semi-open impeller is one in which
only the top edge of the vanes is
covered. The bottom edge of the
vanes runs at a close clearance
with the pump bowl. By raising the
setting of the semi-open impellers
the clearance between the vanes
and the bowl seat is increased and
water is allowed to circulate
through this area. This makes pos-
sible a variation in the perform-
ance of the pump at any given
speed. Thus, by adjustment of the
impeller clearance, a specified per-
formance can be maintained even
though there is a change in the
water level. With enclosed impel-
lers the performance and the ef-
ficiency are not affected by small
differences in their position and
constant performance can be main-
tained over a longer period of time.
(c) Non-Reverse Ratchet-


At a small additional cost, either
an electric motor or a right angle
gear drive can be provided with a
ratchet to prevent pump rotation
in the reverse direction. An electric
motor will operate in either direc-
tion and if it should accidentally be
operated in the wrong direction
by reason of phase reversal in the
power supply, damage to the pump
may result. While an engine cannot
be operated in the wrong direction,
except in case of backfire, it is
preferable the pump not be permit-
ted to spin in the reverse direction,
which will occur after stopping,
because the water flowing back
down the pump column through
the impellers turns them like a
water wheel. As the water in the
column recedes, the shaft is being
turned in its bearings without lub-
rication, which causes undue wear.
HIGH VOLUME AXIAL FLOW
PROPELLER PUMPS-
Axial flow propeller pumps are
an efficient means devised for
either irrigation or drainage pump-
ing at low heads of up to 50 feet
and above 500 gallons per minute.
Their efficiency is high, especially
when the total head is in the range
of 8 to 20 feet.
There are some 5 types of axial
flow pumps on the market (1)
angle, (2) vertical, (3) two-way
(4) inner-well, and (5) horizontal.
The pumping element of an axial
flow propeller pump consists pri-
marily of a revolving propeller in
a stationary bowl which contains
vanes above and below the pro-
peller. Water enters the pump







through the intake bell. It is dis-
charged into the distributor section
and then out the discharge elbow.
Flow is essentially a straight line
along the pump axis keeping fric-


tion and turbulence to a minimum.
Capacities of axial flow pumps
vary from 500 gpm with a 6-inch
diameter propeller up to 100,000
gpm using a 60-inch propeller.


WELL SIZE AND CAPACITY LIMITS


Bearing in mind that the vertical
turbine pump is installed inside the
well, there are definite capacity
limitations for a given diameter of
well casing. Since the capacity of a
centrifugal pump varies directly
with its speed of operation, it is
necessary to increase the pump
speed to obtain maximum capacity
from a given well size. The maxi-
mum permissible speed depends
upon a number of factors but for
4, 6, and even 8-inch pumps, a
speed of 3600 rpm is not uncom-
mon. For larger sizes this speed is
not advisable. Since nominal elec-
tric motor speeds used in irriga-
tion pumping are either 1760 rpm
or 3450 rpm, intermediate speeds
may be obtained only with right
angle gears of suitable ratio or
with belt and pulley drives.
The following are the approxi-


mate or normal maximum capaci-
ties that should be expected from
deep wells:
Size: Up to:
4-inch 90 gpm
6-inch 400 gpm
8-inch 600 gpm
10-inch 1000 gpm
12-inch 2000 gpm
The above figures are not limiting.
Much depends on the water level,
yield characteristics of water bear-
ing formation and the pressure to
be developed, but they serve as a
general guide line. In any case, the
overall installation must be care-
fully evaluated. For instance, al-
though 1000 gpm may be obtained
from a 10-inch pump with reason-
ably good efficiency, the cost of a
12-inch pump installation may be
less, even including the higher cost
of the larger well.


POWER UNITS FOR IRRIGATION


ELECTRIC MOTORS:
Electricity is a very satisfactory
power source. The dependability
and long life of electric motors
make them a desirable power
source. The most common type of
motor for pumping plants is the 60-
cycle, 220/440 volt, 3-phase, squir-
rel cage induction motor. The speed
of these motors under full load is
nearly constant.


Single phase motors are often
used for loads up to and including
five horsepower. However, three-
phase motors are more efficient.
Above 5 to 71/2 horsepower, single
phase motors are not generally
adapted to irrigation pumps. Elec-
tric motors above 5 horsepower
will generally have an efficiency of
between 88 and 90 percent. Most
squirrel cage induction motors are








designed to operate satisfactorily
under a continuous overload of 10
to 15 percent; however, it is not
wise to plan on an overload.
Electric motors should always be
provided with protection against
excessive heating due to overload-
ing or undervoltage. In addition,
larger motors will require a starter
or starting compensator.
If any adequate electric power
supply is available, the electric
motor may be the cheaper power
source in many cases. Including
the cost of the control, the initial
investment will normally be less
than that of an internal combus-
tion engine drive. The cost of elec-


tric power for operation and stand-
by charges may be higher than the
fuel cost for an internal combus-
tion engine, but the maintenance
cost must also be considered. For
an electric motor, maintenance is a
minimum but it may be consider-
ably more for an internal combus-
tion engine.
The following formulae may be
useful in computing pump oper-
ating costs in arriving at a decision
whether to use electric motors or
internal combustion engines. Both
fixed and operating costs should
be weighed before a final decision
is made. Tables 1-4 will be helpful
in computing these total costs.


FUEL OR ENERGY COSTS:
INTERNAL COMBUSTION ENGINES-
QhFcd
Hourly Pumping Costs == F (in cents)
3960E
Where Q = Discharge in gpm
h = Pumping head in feet (TDH)
Fc = Fuel consumption in gal. per hp hour
d = Cost of fuel in cents per gallon
e = Pump efficiency
ELECTRIC MOTORS:
Qhe
Hourly Pumping Costs = 531E (in cents)
5310Ee
Where Q = Discharge in gpm
h = Pumping head in feet (TDH)
c = Cost of elec. in per kwh
E = Pump efficiency
e = Efficiency of electric motor


INTERNAL COMBUSTION
ENGINES:
Selection of an engine for a pow-
er source for irrigation pumping
should be based on the continuous
service rating, rather than the


maximum BHP rating, with ade-
quate allowance for high tempera-
tures as well as power loss in drive
components. Taking 80 percent of
the manufacturer's maximum BHP
rating for continuous duty would








not be unreasonable, when the con-
tinuous horsepower rating is not
indicated.
Gasoline, diesel, LP-gas, and
electric power units are all used to
drive irrigation pumps in Florida.
Each has its advantages and dis-
advantages.
Gasoline engines have two prin-
cipal advantages over diesel and
LP-gas engines. These are (a) low-
er initial cost, and (b) more read-
ily available maintenance and re-
pair service. On the other hand,
diesels have a longer life. LP-gas
engines require less maintenance
than gasoline and the fuel may cost
less. Another advantage of LP-gas
is that fuel is less likely to be tak-


en by "night raiders."
The life expectancy of internal
combustion engines is less than
that of an electric motor (Table 1).
An electric motor provides only
constant speed of operation but an
internal combustion engine pro-
vides flexibility of pump perform-
ance through easy speed variation,
which may be desirable. It is also
necessary in selecting an engine
power unit to make certain that
adequate power is available at the
desired speed at which the pump
will be operated. A reserve horse-
power allowance must also be made
for wear, drives, and the other fac-
tors which reduce the power an
engine will provide as it gets older.


INSTALLATION AND OPERATION


Any type of pump is a valuable
piece of precision equipment, and
in order to operate satisfactorily
over its normal life expectancy it
should be properly and carefully


installed by competent personnel.
Further, like any machine, it
should be maintained and operated
in accordance with the recommen-
dation of its manufacturer.


ACKNOWLEDGMENTS


The authors wish to acknowledge
the contributions of the education-
al committee of the Florida Irriga-


tion Society, George
D. S. Bentley, H. E.
J. M. McNamara.


L. Black, Jr.,
Buckley, and


EXPLANATION OF TERMS


horsepower
hour
inches
kilovolt-amperes
kilowatts
kilowatt-hours
pound
pounds per square inch
revolutions per minute


tdh total dynamic head
bhp brake horsepower
C degree centigrade
cfm cubic feet per second
dia diameter
eff efficiency
F degrees Fahrenheit
ft feet
fps feet per second


hp
hr
in
kva
kw
kwh
lb
psi
rpm








gal gallon (U.S.)
gpm gallons per minute
H head
Head (H) A term related to
pressure but usually expressed in
"feet" rather than "psi." It is de-
rived from the fact that a column
of fluid exerts pressure at a given
point in relation to the height of
the column above that point, and
the density (weight per unit vol-
ume )of the fluid. For example, a
column of water one foot high ex-
erts a pressure of approximately
0.433 psi on its base. Thus, 1 ft.
head (water) = 0.433 psi. or 2.31
ft. head = 1 psi.
Static Head The difference in
elevation (in feet) between the
source of supply and the point of
free discharge, when there is no
flow (sometimes called elevation
head).
Dynamic Head The head condi-
tion that exists when water is flow-
ing through a system of pipes, etc.
Friction Head Pressure loss (in
feet) due to frictional resistance
when water flows through pipe, fit-
tings, orifices, etc.
Discharge Head (Dynamic) The
elevation (in feet) between the
center line of the pump, if a hori-
zontal type, or the center line of
discharge of a vertical turbine
type, and the point of free dis-
charge, plus the friction head be-
tween these two points, plus the
residual pressure existing at the
point of discharge (expressed in
feet), plus the velocity head.
Suction Head The elevation (in


feet) between the source of supply
and the center line of the pump,
(assuming that the source is un-
der atmospheric pressure), plus
the friction head in the suction
line. If the source of supply is be-
low the center line of the pump it
is commonly referred to as suction
lift.
Drawdown The difference, in
feet, between the pumping level
and the static level of the source.
Velocity Head The energy con-
tained in a stream of water by rea-
son of its velocity. It represents
the force necessary to accelerate
the water in the pipeline and is
equivalent to the distance in feet
through which it would have to
fall in a vacuum to attain that ve-
locity. This is a relatively small
factor and need not be considered
in the design of the average irri-
gation system.
Total Dynamic Head (tdh) The
total system head when water is
flowing through it. It is the sum
of the static head, plus the friction
head, plus the residual pressure at
the point of discharge (expressed
in feet), plus the drawdown. (It
also includes the velocity head and
the entrance and exit losses, but
these are relatively small and usu-
ally are ignored.)
Irrigation Application of water
to soil for the purpose of supplying
moisture essential to plant growth.
Irrigation Frequency The fre-
quency between irrigations (in
days) when the crop is using water
at the design use rate. It is de-







termined by dividing the available of water actually reaching the root
moisture by the design water use zone of the plant divided by the
rate. gross amount applied multiplied by
Irrigation Efficiency The amount 100.
Determination of Brake Horsepower at the pump shaft:
gpm x tdh
bhp =3960 x eff brake horsepower required by the pump.
(To this must be added the power consumed
by mechanical friction in the bearings, gears,
etc. in order to arrive at the continuous bhp
required at the input shaft.)
where tdh = total dynamic head, gpm = gallons per minute
eff = hydraulic efficiency of the pump, expressed as a decimal
Pumping Rate Determination for Irrigation Systems:
453 x I x A
gpm= HxD
where I = inches of water (gross) to be applied each
irrigation
A = total acres to be irrigated
H = number of hours of operation per day
D = number of days to complete one irrigation.
(This should not exceed 75% of the irrigation
frequency.)
Pumping Rate of Permanent Systems:
Applied Rate Desired (gross) gpm/acre
0.10 inches/hr 45
0.15 inches/hr 67.5
0.20 inches/hr 90
0.25 inches/hr 112.5
Application or Precipitation Rate for Sprinkler Systems:
gpm/sprinkler x 96.3
PR= SxL
where S = spacing of sprinklers along the lateral (in feet)
L = spacing between laterals (in feet)
(This applies to square, rectangular, or triangular spacing.)
Table of Equivalents:
Length
1 millimeter = 0.03937 inch
1 centimeter = 0.3937 inch
1 meter = 39.37 inches
3.2808 feet
1 kilometer = 3,280.8 feet







1 mile


rod
chain
link


1 acre
Area of a circle
Area of a triangle

1 acre-inch
1 gallon of water
1 gallon of water
1 gallon of water
1 cubic foot of water
1 cubic foot of water
1 litre
1 cubic meter
Volume of a cylinder

1 cfm
1 acre-inch/hr
1 second foot (or 1 cfs)

1 gallon of water
1 cubic foot of water

Degrees C
Degrees F

1 atmosphere

1 foot of water
1 psi
1 foot of water
1 inch of Mercury(Hg)


1 horsepower

1 kilowatt


5,280 feet
= 1.60935 kilometers
= 16.5 feet
= 100 links or 66 feet
= 7.92 inches
Area
= 43,560 square feet
= 3.1416 x radius2
=1/2 altitude x base
Volume
= 27,154 gallons
= 231.0 cubic inches
= 0.1337 cubic foot
= 3.7853 litres
= 7.48 gallons
= 28.316 litres
= 0.2642 gallon
= 264.0 gallons
= 3.1416 x radius2 x height
Rate of flow
= 7.48 gpm
= 452.57 gpm
=448.83 gpm
Weights
= 8.326 pounds
= 62.428 pounds
Temperature
=5/9 (F 32)
=9/5 (C + 32)
Pressure
= 14.7 psi
=33.947 feet of water
= 0.433 psi
= 2.31 feet of water
= 0.883 inches of Mercury (Hg)
=1.133 feet of water
Mechanical and Electrical
= 745.7 watts
=33,000 foot pounds per minute
= 1,000 watts
= 1.341 horsepower







ELECTRIC MOTOR DATA
(Irrigation Pumping)

DIRECT ALTERNATING CURRENT
REQUIRED CURRENT SINGLE-PHASE 3-PHASE
Amps when 746 (hp) 746 (hp) 746 (hp)
hp is known (E) (Eff) E (Eff) (pf) 1.73 (E) (Eff) (pf)
Amps when 1000 (KW) 1000 (KW) 1000
KW are known E E (pf) 1.73 (E) (pt)
Amps when 1000 (KVA) 1000 (KVA)
KVA is known E 1.73 (E)
W (I) (E) (E) (I) (p) 1.73 (I) (E) (pf)
K 1000 1000 1000
KVA(I) (E) 1.73 (I) (E)
KVA 1000 1000
(I) (E) (Eff.) (I) (E) (pf) (Eff.) 1.73 (I) (E) (p9) (Eff.)
HP Output 746 746 746

I = amperes pf = power factor
E = volts KW = kilowatts
Eff efficiency (as a decimal) KVA = kilovolt amperes
hp = horsepower

Table 1. Suggested Depreciation Period for Components of an Irriga-
tion System*

UNIT DEPRECIATION (YRS.)
Well, Cased 25
Pump 15
Power Unit:
Diesel 15
LP-Gas 12
Electric 25
Gasoline 9
Pipe:
Aluminum, Sprinkler 15
Asbestos Cement** 25
PVC plastic, C1 160** 25
Steel coated, underground 20
Sprinklers 8
Data from "Sprinkler Irrigation", 2nd. Ed. 1959.
** Estimate by authors.








Table 2. Capital Recovery Factors for Different Compound Interest
Rates and Estimated Life.
Estimated Compound Interest Rate (%)
Life (years) 3 4 5 6
5 .2184 .2246 .2310 .2374
8 .1425 .1485 .1547 .1610
9 .1284 .1345 .1407 .1470
10 .1172 .1233 .1295 .1359
12 .1005 .1066 .1128 .1193
15 .0838 .0899 .0963 .1030
20 .0672 .0736 .0802 .0872
25 .0574 .0640 .0710 .0782

Table 3. Field Performance of Irrigation Power Units.

Power Fuel Consumption (Gal./Hp.Hr.) Annual**
Maint./ Useful*
nit Maximum* Probable** Avg.* 100 hrs. Life (yrs.)
Diesel 0.066 0.078 0.089 $15 15
Gasoline 0.089 0.112 0.145 $11 9
Propane 0.115 0.138 0.175 $10 12
Electric*** 0.830 0.866 1.08 $ 1 25
* Sprinkler Irrigation, 1959 2nd Ed., Sprinkler Irrigation Association,
Washington, D. C.
** Suggested by authors, based on Florida conditions.
***KWH per HP Hr.
To compute fuel consumption per day:
Cont. HP x Hrs. Operation/Day x Gal./HP Hr. = Gal./Day
To compute electric energy per day:
Cont. HP x Hrs. Operation/Day x KWH/HP Hr. = KWH/Day
Table 4. Example for Computing Fixed and Operating Costs of an Irri-
gation System, using Electric Power.
(A) FIXED COSTS
Est. Life Initial CRF* Fixed Annual
Unit (years) Cost x @ 6% Costs
Well 25 $1,500 .0782 $117
Pump 15 $1,850 .1030 $191
Elec. Motor 25 $1,275 .0782 $100
A-C Mainline 25 $1,115 .0782 $ 87
Al. Laterals 15 $1,750 .1030 $180
Sprinklers 8 $ 460 .1610 $ 74
*Capital Recovery Factor, from Table 2. Total $749
This example does not include annual tax, housing, and insurance costs.
19








Annual Operating
(B) OPERATING COSTS Costs
(1) Labor: (0.5 man hrs./acre-inch)
480 ac-in x 0.5 man-hrs./ac-in x $1.25/man-hr. = $ 300

(2) Elec. Power = 30 BHP x 0.866 KWH/BHP = 26KW
26 kw x 497 hrs. = 12,922 KWH = $ 323
12,922 kwh x $0.025/kwh

(3) Maint. = $1.00/100 hrs. x 496 hrs. = $ 5

Total $ 628

TOTAL ANNUAL COSTS
Fixed Costs $ 749
Operating Costs $ 628

Total $1,377

Costs per acre-inch = $ 2.87

Situation: 24 Acre Nursery System, 2340 ft. portable 3-inch aluminum
pipe laterals, and 580 ft. A-C mainline. Water source is well
(8-inch) 375 ft. deep and 100 ft. casing. Irrigation require-
ments are: 1 inch water every 41/2 days, irrigating 8 hrs. per
day, at an application rate of 1/3-inch per hour; irrigating
2.9 acres per set. Pump and well requirements: 450 gpm
@ 190 ft. TDH, from an 8-inch well, using 30 hp electric
motor; 20 applications annually. Cost of electricity $0.025
per kwh. Labor at $1.25 per hour.












COOPERATIVE EXTENSION WORK IN AGRICULTURE AND HOME ECONOMICS
M. O. Watkins, Director
and United States Department of Agriculture, Cooperating
(Acts of May 8 and June 30, 1914)
Agricultural Extension Service, University of Florida,




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