Florida Cooperative Extension Service
Measuring Pump Capacity for Irrigation System Design1
A.G. Smajstria, D.Z. Haman and F.S. Zazueta2
Proper design of an irrigation system requires that
the pumping system be precisely matched to the
irrigation distribution system. Then the pressure and
flow rate required can be efficiently provided by the
When an irrigation system is designed or modified
to use an existing pumping system, it is necessary to
measure the capacity of the existing pump. The
irrigation system can be properly designed only if the
flow rate and pressure of the pump are accurately
It is not adequate to visually estimate pump
capacity or to use the manufacturer's specifications to
determine current pump capacity. Visual estimates
are normally not accurate, and manufacturer's
specifications do not include the effects of site-specific
factors such as well characteristics or suction and
discharge pipe sizes. Manufacturer's specifications
also do not include the effects of age and wear on
pumping system performance.
MEASURING PUMP CAPACITY
The capacity of a pump has two components, the
pump discharge rate and the discharge pressure. The
discharge rate is normally measured in gallons per
minute (gpm) in English units or liters per second
(lps) in metric units. Pressure is normally measured
in pounds per square inch (psi) in English units or
kiloPascals (kPa) in metric units. It is necessary to
measure both discharge rate and pressure under
normal operating conditions in order to determine
how the pumping system will operate as a part of an
The discharge rate can be measured using one of
the following three methods.
* A flow rate meter is the most direct method
because it measures the flow rate directly in gpm
or lps. The cost of flow rate meters varies widely.
Costs range from about $50 for small pitot meters
which measure only flow rate, to several hundred
dollars for impeller meters which measure flow
rate and totalize the flow through the meter.
Pitot meters are inserted through a hole drilled
into-an .irrigation pipe. -They divert a small
stream of water through the meter and float a
ball or disk flow indicator. The flow rate is read
from a graduated scale at the height of the flow
indicator. One type of commercially-available
pitot flow rate meter is shown in Figure 1.
An impeller flow rate meter measures both the
flow rate and the total flow through the meter.
The flow rate is read directly from a needle
similar to a car speedometer, and the total flow is
1. This document is Circular 1133, Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida.
Publication date: February 1994.
2. A.G. Smajstria, Professor; DZ. Haman, Associate Professor, and 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 Scienoes 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
W!VERSITY OF FLORIDA LIBRARIES
Measuring Pump Capacity for Irrigation System Design
I *MINIMUM ACCEPTABLE DIMENSIONS
Figure 1. Pitot tube flow rate meter.
registered on a meter similar to the odometer
(mileage gauge) on a car. An impeller meter is
used as a component of the pump capacity
measuring apparatus shown in Figure 2.
A totalizing flow meter measures the total volume
of water that has passed through the meter in
gallons (gal) or liters (1). Totalizing water flow
meters are relatively inexpensive because they are
commonly used for many applications ranging
from metering homeowner's water use to
metering agricultural irrigation systems. Their
costs range from about $50 for 1/2-inch meters
which have capacities up to 10 gpm to $2,000-
3,000 for large meters with capacities of several
thousands of gpm.
To determine pump discharge rates with a
totalizing flow meter, a stop watch or other watch
with a second hand is needed. The volume of
water metered must be divided by the time during
which the volume was measured to determine the
flow rate. For example, the meter register reads
200.0 gal at the beginning and 395.0 gal at the
end of a 10 minute measurement period. Then
the flow rate is (395-200) gal /10 min = 19.5 gpm
or 1.23 lps. / C I
SFlow rates can be measured using a container of
known volume and a stop watch. For example,
flow is directed into a 50-gal graduated tank for
10 minutes. The volume of water collected is 47
gal. Then the flow rate is 47 gal / 10 min = 4.7
gpm or 0.30 Ips.
The discharge pressure can be measured easily
and inexpensively using a standard pressure gauge.
Standard pressure gauges are available in 15, 30, 60
and 100 psi (100, 200, 400 or 700 kPa) ranges for
about $10. Select a pressure gauge that is accurate
within the range of pressures that the pump can
produce. For example, if your pump can produce a
maximum pressure of 50 psi (345 kPa) select a 60-psi
(400-kPa) gauge as the next-largest commercially-
MEASURING HEAD-DISCHARGE CURVES
For centrifugal and turbine irrigation pumps, the
discharge rate depends on the pressure that the pump
operates against. If the pressure is high, the
discharge rate will be low, and conversely, if the
pressure is low, the discharge rate will be high. The
relationship between pressure and discharge rate is
known as the head-discharge curve for the pump.
The head-discharge curve may be different for each
pump because of the pump characteristics and many
An apparatus which can be used to measure a
pump head-discharge curve is shown in Figure 2. The
apparatus consists of a pressure gauge, flow meter,
and regulating valve installed on a section of straight
pipe. The pipe must be long enough so that the flow
meter works properly. Many flow meters require
certain minimum lengths of straight pipe upstream
and downstream of the meter for accuracy. See the
flow meter manufacturer's specifications. Common
straight pipe lengths required are 10 pipe diameters
upstream and 6 pipe diameters downstream of the
meter, although these lengths may be reduced if the
flow meter is equipped with straightening vanes.
Straightening vanes are installed inside the meter tube
and help improve meter performance by reducing
extreme turbulence-as-water is directed through the
Measuring Pump Capacity for Irrigation System Design
Figure 2. Pump capacity measuring apparatus, including a
pressure gauge, flow meter, and regulating valve.
A pipe fitting or connecting hose is needed so
that the head-discharge measurement apparatus can
be connected to the pump discharge. All components
must be sized and pressure-rated to permit the
measurement of the complete range of pressures and
discharge rates that the pump produces.
Measure the head-discharge curve in the following
1. Connect the head-discharge apparatus to the
pump discharge with sufficient lengths of straight
pipe to obtain an accurate flow rate
2. Operate the pump to remove air from the pump
and pipelines and to reach normal operating
3. Slowly close the regulating valve and measure the
shut-off head. (Warning: Be certain that all
components are pressure-rated to withstand the
maximum pressure that the pump can deliver).
The shut-off head is the maximum pressure that
the pump delivers when there is no flow. Record
Do not leave the pump operating under no-flow
conditions for long periods of time, as this may
overheat and damage some pumps.
4. Open the regulating valve a small amount and
measure the pressure and flow rate at this valve
5. Repeat Step 4 at other valve openings until the
valve is completely open. Valve adjustments
should be made to-produce at least 6 to 8
pressure-discharge data points over the flow range
from completely closed to wide open.
It is convenient to use the pressure gauge to
uniformly distribute the measured data points
over the range that the pump can produce. For
example, assume that the pump shut-off head was
45 psi. Then, for convenience, adjust the
regulating valve to set 5 psi changes in pressure
as the valve is opened. First, open the valve until
the pressure drops to 40 psi and measure the
discharge rate at that pressure. Then, open the
valve until the pressure drops to 35 psi, etc., until
the valve is completely open and the final
pressure is 0 or nearly 0 psi. The final pressure
may not reach 0 depending on the specific design
of your head-discharge measurement apparatus.
Example Head-Discharge Data
An example set of head-discharge data is shown
in Table 1. These data were developed following the
previous example in which the shutoff head was
assumed to be 45 psi (310 kPa), and flow rates were
measured at 5 psi (34 kPa) increments until the
regulating valve was completely open. This
measurement strategy produced a total of 10 data
Table 1. Example pump head-discharge data
Figure 3 is a graph of the data shown in Table 1.
Notice that the relationship between pressure and
discharge-rate is-a curve rather than a straight line.
This is the reason that at least 6 to 8 data points are
necessary to accurately describe this relationship.
From the data shown in Table 1 and Figure 3, the
designer can read the information needed to properly
Measuring Pump Capacity for Irrigation System Design
Pump Capacity Curve
Pressure versus Discharge
I Press e (psi) I
Figure 3. Example pump capacity curve using the data from
design an irrigation system using this pump. For
example, if a sprinkler irrigation system requires 35
psi (214 kPa) at the pump, then 13.0 gpm (0.82 Ips)
is the maximum flow rate that is available from this
pump. If the system requires more than 13.0 gpm,
then it will need to be designed and operated in zones
so that no zone requires more than 13.0 gpm.
As another example, if a drip irrigation system
requires 20 psi (138 kPa) from the pump, then this
pump can deliver 25.2 gpm (1.59 Ips) at this pressure.
Factors Affecting Head-Discharge Curves
Many factors affect the pump head-discharge
curves measured, including the design of the pump
and the size of the pump and power unit. Other
factors include worn impellers or other components,
speed at which the pump is operated, size of well,
aquifer characteristics, depth to water table or surface
water level, and size of suction and discharge pipes.
The size, design, and maintenance of intake screens
and check valves also affect pump capacity.
The measurement procedures described in this
publication allow the effects of all of these factors to
be included, and the results are valid only when the
same conditions exist during the operation of the
irrigation system. If operating conditions change
greatly from the test conditions, then the pump may
produce much different outputs when connected to an
irrigation system. In the following paragraphs, several
examples are given to illustrate changes in pumping
conditions that could cause changes in pump capacity.
If water is being pumped from a pond, and the
pond water level drops greatly during the irrigation
season, the pump output will be reduced as the water
level drops. Likewise, if the water level in a well
drops, then the pump output will also drop. Thus,
pump capacity tests should be scheduled when water
levels are low so that the minimum pump output can
be measured, or tests should be made at both high
and low water levels so that the effects of water levels
can be measured.
If the pump is driven by an internal combustion
engine, its output will be greatly affected by the
engine speed (RPM). Therefore, pump capacity tests
must be run at the same RPMs that will be used
If the intake strainer on the suction pipeline
becomes partially clogged by debris in a pond, the
pump capacity will be reduced. Thus, pump capacity
tests should only be conducted after strainers have
been cleaned and other maintenance has been
performed. Likewise, routine maintenance is
required so that the pump capacity is not reduced for
this reason during irrigation.
When an existing pumping system will be used as
a component of an irrigation system, it is necessary to
measure the capacity of the pump so that the
irrigation system can be designed to operate
efficiently using the available flow rate and pressure.
This requires measuring pump discharge rates and
pressures at several points over the available range.
Flow meters or volumetric methods can be used to
measure discharge rates, while pressures are easily
measured with pressure gauges. Procedures for
measuring pump capacity were presented, and factors
affecting pump capacities under field conditions were
OCLC 30323953 Held by FUG 8 other holdings
Rec stat n Entered 19940425 Replaced 19950613
Type a ELvl I Srce d Audn Ctrl Lang eng
BLvl m Form Conf 0 Biog MRec Ctry flu
Cont GPub s LitF 0 Indx 0
Desc a Ills a Fest 0 DtSt s Dates 1994 ,
040 FBA *c FBA
092 631.587 *2 20
100 1 Smajstrla, A. G. *q (Allen George)
245 1 0 Measuring pump capacity for irrigation system design / *c A.G. Smajstrla, D.Z. Haman and F.S. Zazueta.
260 [Gainesville]: *b Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida,
300 4 p.: *b ill.; +c 28 cm.
490 1 Circular; *v 1133
500 Title from caption.
500 "February 1994."
650 0 Irrigation pumps.
650 0 Irrigation engineering.
700 1 Haman, D. Z. *q (Dorota Z.)
700 1 Zazueta, F. S. *q (Fedro S.)
710 2 Florida Cooperative Extension Service.
830 0 Circular (Florida Cooperative Extension Service); 4v 1133.
Delete Holdings- Export- Label- Produce- Submit- Replace- Report Error- Update Holdings- Validate-
,b / AEco 7
3 1262 05331 9405
Page 1 of 1