Group Title: Circular
Title: Energy efficient main pipelines for drip-irrigated tomato production
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Title: Energy efficient main pipelines for drip-irrigated tomato production
Series Title: Circular
Physical Description: 4 p. : ; 28 cm.
Language: English
Creator: Smajstrla, A. G ( Allen George )
Clark, Gary A
Zazueta, F. S ( Fedro S )
Florida Cooperative Extension Service
Publisher: Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida
Place of Publication: Gainesville
Publication Date: 1994
Subject: Microirrigation -- Equipment and supplies   ( lcsh )
Pipelines -- Energy conservation   ( lcsh )
Microirrigation -- Energy conservation   ( lcsh )
Tomatoes -- Irrigation -- Energy conservation   ( lcsh )
Genre: government publication (state, provincial, terriorial, dependent)   ( marcgt )
bibliography   ( marcgt )
non-fiction   ( marcgt )
Bibliography: Includes bibliographical references (p. 4).
Statement of Responsibility: A.G. Smajstrla, G.A. Clark and F.S. Zazueta.
General Note: Title from caption.
General Note: "April 1994."
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Bibliographic ID: UF00008564
Volume ID: VID00001
Source Institution: University of Florida
Holding Location: University of Florida
Rights Management: All rights reserved by the source institution and holding location.
Resource Identifier: ltqf - AAA6828
ltuf - AKA2282
oclc - 30691016
alephbibnum - 001926321


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Circular 1132
April 1994



Florida Cooperative Extension Service

Energy Efficient Main Pipelines for Drip-Irrigated Tomato

A.G. Smajstria, G.A. Clark and F.S. Zazueta2


The main pipeline in an irrigation system is the
pipeline that carries water from the irrigation pump
to the distribution pipelines in the field. The purpose
of the main pipeline is to efficiently deliver the flow
rate and pressure required to irrigate the subunits or
zones. Energy losses can be minimized by properly
selecting and installing components so that friction
losses will be low and the pipelines will work properly
throughout the life of the irrigation system.

For drip-irrigated tomato production, a field is
normally divided into several zones, and one zone is
irrigated at a time. This allows the mainline (and
pumping system) to be as small as possible because it
needs only enough capacity for the largest zone.
Sometimes, however, more than one zone or the
entire field is irrigated at once. In those cases, the
main pipeline must have sufficient capacity for the
largest area irrigated at one time.


Main pipelines are selected on the basis of both
economic and hydraulic considerations. Selection
based on economic considerations means that the
pipeline will have the lowest annual cost for the
entire life of the irrigation system compared with both
the next larger and the next smaller available pipe

diameters. Selection based on hydraulic consider-
ations means that the velocity of flow will be low
enough to minimize surge pressures (water hammer)
and that the pipe will have a pressure rating adequate
to withstand both expected static and surge pressures.
When these economic and hydraulic considerations
produce different results, the larger (more
conservative) pipe size must be chosen.

Economic Considerations

When main pipelines are selected on the basis of
economic considerations, a detailed economic analysis
is required. In this analysis, the initial pipe and
installation cost for various commercially available
pipe sizes (amortized over the expected life of the
system at current interest rates) is compared with the
annual pumping cost for each of the commercially
available pipe sizes. When the amortized initial cost
is added to the annual operating cost, the pipeline
with the lowest total cost is shown to be the most
economical for the buyer. This is the pipe size that
should be selected.

For small pipe sizes, the initial cost is low but the
pumping cost is high because friction losses are higher
in smaller pipe sizes. Conversely, the initial cost is
high for large pipe sizes, but the pumping cost is low
because friction losses are lower in larger pipes. The
actual pipe size selected depends on the pipeline

1. This document is Circular 1132, a series of the Agricultural Engineering Department, Florida Cooperative Extension Service, Institute of Food
and Agricultural Sciences, University of Florida. Publication date: April 1994.
2. A.G. Smajstrla, Professor, Agricultural Engineering Department; G.A Clark, Associate Professor, Gulf Coast Research and Education Center,
Bradenton, FL; F.S. Zazueta, Professor, Agricultural Engineering Department, Cooperative Extension Service, Institute of Food and Agricultural
Sciences, University of Florida, Gainesville FL 32611.
The Institute of Food and Agricultural Sciences is an equal opportunity/affirmative action employer authorized to provide research,
educational information and other services only to individuals and institutions that function without regard to race, color, sex, age, handicap,
or national origin. For information on obtaining other extension publications, contact your county Cooperative Extension Service office.
Florida Cooperative Extension Service / Institute of Food and Agricultural Sciences / University of Florida / John T. Woeste, Dean

Energy Efficient Main Pipelines for Drip-Irrigated Tomato Production

length, the intended flow rate, the initial cost of the
pipe and installation and the estimated hours of
operation and cost of pumping throughout the life of
the system.

When drip systems are used to irrigate annual
crops with a relatively short growing season, such as
vegetable crops in Florida, the annual hours of
operation are comparatively low. This is because
drip-irrigated tomatoes rarely require more than 2 to
3 hours of irrigation per day, even under peak water
use conditions, and growing seasons are relatively
short (normally about 100 days per season). Under
these conditions, a detailed economic analysis favors
the selection of smaller pipe sizes than does a
hydraulic analysis. Thus, the hydraulic analysis almost
always controls the pipe size selected.

The reader is advised to make an initial selection
of the mainline pipe size using the hydraulic
considerations given in the following sections of this
publication. The annual operating cost for the
mainline pipe size selected can then be calculated.
Only if the annual pumping cost is high with respect
to the amortized initial cost will it be necessary to
compare the total annual cost with that of the next
larger pipe size to determine which of these pipe sizes
should be selected. For more information on how to
conduct a detailed economic analysis of a main
pipeline, see the IFAS Agricultural Engineering
Extension Report entitled Selection of Main Irrigation
Pipelines: Economic Analysis.

Hydraulic Considerations

One hydraulics question must be answered: Is the
water velocity low enough to avoid hydraulic shocks
(water hammer) that could damage the pipeline?
This is actually a two-part question. To answer it,
both the pipe size and the pressure rating must be

Pipe Size .,

In general, the diameter of the mainline pipe
should be large enough to limit the velocity of water
flow to 5 feet per second (fps). This velocity will
minimize friction losses and will prevent water
hammer problems if the pipe has an adequate
pressure rating and is properly installed.

Table 1 gives the maximum flow rates that can be
carried in Class 160 PVC irrigation pipe and in
aluminum irrigation pipe so that the 5 fps velocity is

not exceeded. To use Table 1, determine the flow
rate required for the largest irrigation zone (or zones,
if more than one zone will be operated at once), then
select the pipe diameter that can carry this flow rate.
For example, if the irrigation system requires 180
gallons per minute (gpm), a 4-inch PVC or aluminum
pipeline will be adequate because the 4-inch PVC
pipe can carry up to 211 gpm and the 4-inch alumi-
num pipe up to 187 gpm before the 5 fps velocity is
exceeded. As another example, if the irrigation
system requires 700 gpm, 8-inch PVC or aluminum
pipelines can be used.

If more than one zone will be operated at the
same time, their flow rates must be added to select
the required mainline pipe size. For example, if two
zones, each requiring 160 gpm, will be operated at the
same time, the mainline size must be based on the
total flow rate of 320 gpm. In this case, 6-inch PVC
or aluminum pipelines would be required.

Notice that the capacities of the PVC and alumi-
num pipes in Table 1 are slightly different for pipes
of the same nominal diameter. This is because the
capacity is based on the inside pipe diameter (I.D.)
and the inside diameters for PVC pipes are slightly
different than for aluminum pipes. The blank lines in
Table 1 indicate that 5- and 7-inch PVC and 2.5-inch
aluminum irrigation pipe are not commercially

Pressure Rating

Class 160 PVC pipe refers to plastic irrigation
pipe with a pressure rating of 160 pounds per square
inch (psi). Aluminum irrigation pipe has a pressure
rating of 145 to 150 psi. These pressure ratings will
normally be adequate for mainlines in drip irrigation
systems. In some cases, pipe with higher pressure
ratings may be required or lower pressure ratings may
be adequate, as discussed in the following paragraphs.

The pressure rating of a main pipeline must be
considerably greater than the normal system operating
(static) pressure so that the mainline will be safe and
functional for the life of the irrigation system. The
mainline must be able to withstand the pump dis-
charge pressure during normal operating conditions
and the potentially destructive pressures generated by
water hammer. In addition, PVC pipe pressure
ratings must be de-rated (decreased) for water
temperatures above 73.4'F. This de-rating is
necessary because the strength of plastic pipe is
reduced at high temperatures. The pressure rating

Page 2

Energy Efficient Main Pipelines for Drip-Irrigated Tomato Production

Table 1. Flow rates that limit velocities to 5 fps for Class 160 PVC an(
aluminum irrigation pipe.

Class 160 PVC Aluminum Irr. Pipe
Diameter Inside Flow Inside Flow
(Inches) Diameter Rate Diameter Rate
(inches) (gpm) (Inches) (gpm)
2 2.193 59 1.900 44
2.5 2.655 86 N/A1 N/A
3 3.230 128 2.914 104
4 4.154 211 3.906 187
5 N/A N/A 4.896 294
6 6.115 458 5.884 424
7 N/A N/A 6.872 579
8 7.961 777 7.856 756
10 9.924 1207 9.818 1181
12 11.770 1698 11.872 1727
N/A: Not available in this size.

stamped on the pipe is determined
temperature of 73.4'F.

at the standard

Water Hammer

Water hammer pressures are the pressure surges
that occur because of sudden stoppage or reduction in
flow, or because of a change in direction of flow.
These pressure surges occur when valves close
quickly, when air is suddenly released from a pipeline
or when water changes direction at a tee, elbow or
other pipe fitting. Although water hammer pressure
surges last for only a brief moment, they can be large
enough to burst pipelines. Damage from water
hammer typically occurs at fittings, particularly if the
fittings are not properly installed.

In irrigation pipes, the actual water hammer
pressure depends on pipe material, wall thickness,
velocity of flow and other factors, such as how quickly
a valve closes or how abruptly the water changes
direction. A detailed surge pressure analysis must be
made to estimate the amount of surge pressure that
will be developed in a specific pipeline. Procedures
for conducting a detailed surge analysis are given in
IFAS Extension Circular 828, Water Hammer in
Irrigation Systems.

ASAE (1991a) and FIS (1991) irrigation design
standards state that water hammer in irrigation
pipelines can be minimized by limiting the velocity of

d flow to 5 fps. The small surge pressures that
can occur at this velocity will not cause
damage if the working pressure in the
pipeline is limited to 72 percent of the
pressure rating stamped on the pipe. This
limits the working pressure of Class 160 PVC
pipe to 115 psi (0.72 x 160 psi = 115 psi).

Temperature Effects

Pressure ratings of PVC pipe are deter-
mined at the standard temperature of 73.4'F
(23*C). A temperature increase above 73.4'F
reduces the strength of PVC pipes, a factor
that must be considered when PVC pipes are
selected. For Class 160 PVC pipe, the
amount of reduction is approximately 20 psi
for each 10'F elevation in temperature. The
actual reductions are shown in Table 2.

When water is
underground aquifer,
normally in the range

pumped from an
its temperature is
of 70*to 75*F; such

temperatures will have little effect on the pressure
rating of the pipe. Water pumped from ponds is
considerably warmer during summer months.
Summertime pond water temperatures may reach
90'F or more. When this is the case, the pipe
pressure rating will be reduced, as shown in Table 2.
For example, the pressure rating of Class 160 PVC
pipe will be 141 psi when the water temperature is
80*F. The rating will be only 120 psi when the water
temperature is 90*F.

Table 2. Temperature effects on PVC pipe pressure ratings.

Temperature PVC Pipe De- Class 160 PVC
Degrees F Rating Factor Pipe Pressure
(multiplier) Rating (psi)
73.4 1.00 160
80 0.88 141
90 0.75 120
100 0.62 99
110 0.50 80
120 040 64


A mainline pipe diameter is selected on the basis
of both the maximum allowable velocity of 5 fps and
a pressure rating adequate for the normal operating
pressure plus surge pressures, after the pressure

Page 3

Energy Efficient Main Pipelines for Drip-Irrigated Tomato Production

rating has been adjusted for temperature. The
following example illustrates the selection procedure.

Assume that a mainline pipe is to be selected to
carry 400 gpm, and that PVC pipe will be used. As
shown in Table 1, a 6-inch Class 160 PVC pipe will
keep the velocity of flow below 5 fps. (Note that the
6-inch pipe can carry 458 gpm at a 5 fps velocity.)
Assume that the water source is a pond and that the
maximum expected water temperature is 90*F. Then,
as indicated in Table 2, the Class 160 pipe pressure
rating must be decreased to 120 psi. Finally, to avoid
damage from water hammer, the operating pressure
should not be allowed to exceed 72 percent times the
available pressure rating of 120 psi, or 86 psi. Thus,
a 6-inch, Class 160 PVC pipeline will be adequate if
the normal operating pressure in the pipeline does
not exceed 86 psi.

The pressure rating for aluminum pipe is not
sensitive to temperature. Thus, the 145 to 150 psi
pressure rating of aluminum irrigation pipes would be
adequate for most drip irrigation systems. Using the
same example, a 6-inch aluminum irrigation pipeline
would be adequate to carry 400 gpm at a velocity of
less than 5 fps.


ASAE. 1991a. "Design, Installation, and
Performance of Underground, Thermoplastic
Irrigation Pipelines." ANSI/ASAE Standard
S376.1. In: Standards 1991. 38th Edition.
American Society of Agricultural Engineers, St.
Joseph, MI. pp. 606-616.

ASAE. 1991b. "Minimum Standards for Aluminum
Sprinkler Irrigation Tubing." ASAE Standard
S263.3. In: Standards 1991. 38th Edition.
American Society of Agricultural Engineers, St.
Joseph, MI. pp. 570-571.


An irrigation pipeline must be properly installed
to function effectively. This publication has
addressed only pipe sizing and selection. The buyer
must be certain that the pipeline is properly installed
and that air and pressure relief valves are installed as
needed. See ASAE (1991a,b) or FIS (1991) irrigation
standards for more information on proper pipeline


Main pipelines are selected on the basis of both
economic and hydraulic considerations. When these
two criteria produce different results, the larger
(more conservative) pipe size should be chosen. For
annual crops requiring relatively few hours of pump
operation per year, hydraulic considerations almost
always control the pipe size selected by limiting the
velocity of flow to 5 fps. Pipe pressure ratings must
be adequate to withstand the normal operating
pressure plus surge pressures. PVC pipe pressure
ratings must be decreased for water temperatures
above 73.4'F. Normally, the pressure ratings of Class
160 PVC or aluminum irrigation pipe are adequate
for main pipelines in drip irrigation systems.

FIS. 1991. Standards and Specifications for
Agricultural Solid-Set Sprinkler and Microirrigation
Systems. Florida Irrigation Society, Inc. Winter
Park, FL. 65 pp.

James, L.G. 1988. Principles of Farm Irrigation
System Design. John Wiley and Sons. New York.
543 pp.

Pair, C.H. (Ed.-in-Chief). 1983. Irigation. The
Irrigation Association. Silver Spring, MD. 686

Clark, G.A, A.G. Smajstrla and D.Z. Haman. 1989.
Water Hammer in Irrigation Systems. IFAS Ext.
Cir. 828. Fla. Coop. Ext. Svc., Univ. Fla.
Gainesville, FL. 6 pp.

Director, in cooperation with the United States Department of Agriculture, publishes this information to further the purpose of the May 8 and June
30, 1914 Acts of Congress; and is authorized to provide research, educational information and other services only to individuals and institutions that
function without regard to race, color, sex, age, handicap or national origin. Single copies of extension publications(excluding 4-H and youth
publications) are available free to Florida residents from county extension offices. Information on bulk rates or copies for out-of-state purchasers is
available from C.M. Hinton, Publications Distribution Center, IFAS Building 664, University of Florida, Gainesville, Florida 32611. Before publicizing *
this publication, editors should contact this address to determine availability. Pnnted 4/94.

Page 4

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