# Improving pumping plant efficiency does not always save energy

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```							                                                                                     charge of the pump and the pumping
water level in the well). The higher the
The efficiency of a pumping plant is calculated as follows:                          efficiency, the more power that is pro-
QxH                                             duced per unit of input power.
EO =                                                         Electricity users pay for energy
3,960 x IHP
based on the number of kilowatt-hours
where Eo is the overall pumping efficiency, Q is the pump flow rate or capacity      (kwh) consumed (plus fixed charges).
(gallons per minute [gpm]), H is total head or lift (feet), IHP is the input         The kilowatt is the power demand of
horsepower and 3,960 is a conversion factor, which converts the product of           the electric motor; 1 kilowatt equals
Q x H into horsepower. The total head is the sum of the pumping lift (elevation      1.34 horsepower. The hours are the op-
difference between the pump discharge pipe and the pumping water level in            erating time of the motor. To reduce
the well) and the discharge pressure head (discharge pressure in pounds per          electrical energy use, the kilowatt-
square inch [psi] multiplied by 2.31). The discharge pressure must be converted      hours must decrease because of fewer
to feet of head to make it compatible with pumping lift, which is also in feet.      kilowatts or less operating time, or
(Note: a column of water 2.31 feet high creates 1psi of pressure at its base.)       both. Regardless of the claims about a
The input horsepower depends on the energy source. For electric motors,           proposed energy-saving measure, if
IHP can be calculated using                                                          the number of kilowatts or the operat-
48.2 x Kk                                    ing time is not reduced, no energy sav-
IHP=
ings will occur.
where Kh is the meter constant (stamped on the power meter faceplate) and t is          The 2001 energy crisis resulted in a
the time in seconds for 10 revolutions of the meter disc. Newer power meters         state-funded program for testing
display the kilowatt demand of the electric motor. Kilowatts are multiplied by       pumps and improving pumping plant
1.34to obtain horsepower.                                                            efficiency. The goal, of course, was to
The input horsepower of a diesel engine can be calculated by                      reduce energy use in California. Yet
IHP = 55 x q                                      the grower response at the Salinas
meeting suggests that improving effi-
where 9 is the fuel consumption of the engine in gallons per hour. The fuel          ciency may not actually translate into
consumption can be measured by disconnecting the fuel line from the fuel tank,       cost savings. This article discusses the
placing it into a container filled with a known volume of fuel and measuring         reasons behind the growers’ response.
the time it takes to fill the container with fuel. The discharge end of any bypass
fuel line should also be inserted into the container. The input horsepower of a      Improving efficiency
diesel engine may be three to four times that of an electric motor because of           Options for improving pumping
differences in engine efficiency. However, both are rated based on the brake or      plant efficiency include adjusting im-
shaft horsepower: An electric motor rated at 100 horsepower produces the same        pellers, repairing or replacing worn
power as a 100-hprsepowerdiesel engine.                                              pumps, replacing mismatched
An inefficient ingine may cause low overall efficiency even when the pump        pumps, and converting to energy-
itself is efficient.Separating engine efficiency from pump efficiency requires       efficient electric motors. The effect of
specialized equipment.The efficiency of an electric motor tends to be relatively     these options on energy use was
-
constant as long as the motor will run unless the motor becomes severely             evaluated using data collected over
underloaded, making separation easier.                                   -B.R.H.     the past 20 years from numerous
pumping plant tests conducted by
the author, utility companies and

124   CALIFORNIA AGRICULTURE, VOLUME 56, NUMBER 4
Semi-open impeller                      Enclosed impeller

companies that install and maintain        Fig. 2. To improve pump performance, semi-open impellers (A) can be adjusted,
but enclosed impellers (B) cannot.
pumps.
the appropriate clearance between the      crease in pump capacity was insuffi-        operating requirements of the particu-
bottom of the vanes of a semi-open im- cient to offset the increase in input            lar irrigation system; as a result, it may
peller and the bowl housing is critical    horsepower because part of the in-          not be possible to reduce operating
for efficient pump performance. Wear       creased pump output also contributed time in many cases. The practice of not
caused by sand in the well water can       to increased total head.                    reducing operating times is what led
increase the clearance between the im-        Repairing worn pumps. Repairing         to the growers’ response in Salinas; it
pellers and housing, and reduce             a worn pump can increase capacity,         is possible that most irrigators were
pumping plant efficiency. Efficiency        total head and overall efficiency, as      unaware of the need to reduce operat-
can be partially restored by adjusting     shown by the pump test data in table        ing times and as a result were using
the impellers. This involves slightly      2. However, for this data the input         more energy after their pumps were
lowering the pump shaft and, in turn,      horsepower increased from 83 to 89.         repaired.
the impellers, by rotating the nut at      This behavior may be typical of many            Replacing mismatched pumps. A
the top of the shaft (fig. 1).This adjust- repaired pumps. A summary of 63             performance characteristic of deep-
ment will not work for enclosed impel-     data sets of pump performance before        well turbine and centrifugal (booster)
lers (fig. 2).                             and after repair shows increases of         pumps is that as pump capacity in-
When we adjusted the impellers of      39%, 0.5% and 33% in pump capacity,         creases, pump efficiency increases to a
four pumps, both pump capacity and         total head and overall efficiency, re-      maximum and then decreases. New
overall efficiency increased consider-     spectively (Hanson 1988).The small          pumps should be selected to provide
ably (table 1).Total head increased        increase in total head occurred be-         the desired flow rate and total head
slightly, because pumping lift only        cause pumping lift was the main con-        near the point of maximum efficiency,
contributed to total head (see box,        tributor. However, the repair               which minimizes the horsepower de-
page 124). However, for all four           increased the input horsepower for          mand of the pump. Initially efficient
pumps, impeller adjustments in-            58% of the pumping plants, with an          pumps can become inefficient because
creased input horsepower. Therefore,       average increase of 17%. For these          of changes in operating conditions,
if the pumps are operated for the same pumping plants, using the same oper-            such as different groundwater levels
amount of time after the adjustment (a ating time before and after the pump            or alterations in discharge to pressur-
common practice), energy use will in-      repair will increase energy use by          ized irrigation systems, even though
crease. Energy use will decrease only      17%. However, pumping the same              the pump is operating properly (no
if the operating time is decreased by      volume will decrease the average en-        wear).
pumping the same volume of water af- ergy use by 22%. For many pumping                     A pump operating properly but not
ter the adjustment as before. For these    plants, reducing the operating time         near the point of maximum efficiency
data (table l),the increase in energy      may be necessary to realize any en-         is said to be mismatched to the operat-
costs ranged from 8.3% to 18.2%for         ergy savings from pump repairs.             ing conditions. Used pumps are also
the same operating time before and af-         Operators of irrigation pumping         candidates for being mismatched. To
ter adjustment. For tests 1,3 and 4, the plants commonly run repaired or ad-           restore the pumping plant’s efficiency,
decrease in energy costs ranged from       justed pumps for the same amount of         the mismatched pump must be re-
12% to 22.4% to pump the same vol-         time after repairs or adjustments as        placed with one providing the desired
ume of water. For test 2, energy costs     they did before. Operating times are        total head and capacity near maxi-
increased by 6%. In this case, the in-     often based on the management and           mum efficiency. This change reduces

CALIFORNIA AGRICULTURE, JULY-AUGUST 2002   125
Worn impeller and a centrifugal pump.

the kilowatt demand of the pump and          pumping plant efficiency is calculated        that are irrigated. An efficiency of
results in energy savings even if the        from these data, and can then be              about 60% or more for one of the tests
operating time is unchanged.                 compared with the standards for               indicates a mismatched pump under
A review of pump test data from a         correcting electric pumping plants            the normal operating conditions.
pump (table 3) revealed an overall effi-     (table 5)(Hanson 2000).                          A second approach is to compare
ciency of 48%. However, an efficiency           Pump wear or a mismatched                  pump test data under normal operat-
of 57% was found by testing the pump         pump can cause poor efficiency.               ing conditions with the manufact-
under several different conditions.          Likewise, repairing a mismatched              urer’s performance data. Manu-
This relatively high efficiency sug-         pump may not improve efficiency.              facturers provide information for
gested a mismatched pump. An analy-         So how can one determine if a pump            each pump on the relationships be-
sis showed that replacing the ineffi-        is mismatched or worn?                        tween capacity and total head, effi-
cient pump with one producing the               One approach is to conduct mul-            ciency and brake horsepower. By
same output at an efficiency of 60%         tiple pump tests, each under different        comparing the total head of the
would reduce the input horsepower           operating conditions. These conditions        pump test with the manufacturer’s
from 112 to 90, for a 19.6% reduction        can be imposed on a pump simply by            total head at the measured capacity,
in energy use.                               partially closing a valve in the dis-         one can evaluate if a low efficiency is
Using energy-efficient electric          charge pipe or changing the number            due to wear or mismatched operat-
motors. Energy-efficient electric mo-       of sprinkler pipelines or drip lines          ing conditions.
tors need less input horsepower than
standard motors (table 4). Buying an
energy-efficient motor for a new irri-
gation pumping plant is more eco-
nomical than retrofitting an existing
pumping plant. For example, an
energy-efficient, 100-horsepower
motor can cost \$6,000, compared with
\$5,000 for a standard motor. The input
horsepower of the energy-efficient
motor will be 104 compared with 109
for the standard motor. At a typical
cost of \$0.1 per kilowatt-hour operat-
ing the pump for 2,000 hours per year
will save \$746, with a simple payback
period of 1.3 years. The payback
period for retrofitting, on the other
hand, is 8 years.                                                  0 Mismatched pump
2
-0
0 Wearing pump
Evaluating plant performance
Pumping plants should be eval-
uated every several years to determine              0
the status of the pump and possible                     2,000        2,400             2,800                 3,200             3,600
reasons for poor efficiency. Evaluating
a pumping plant requires a pump test,                                         Pump capacity (gpm)
during which capacity (flow rate), lift,
Fig. 3. Comparison of pump test data with manufacturer‘s total head-capacity curve of a
discharge pressure and input horse-          mismatched pump and a wearing pump. Numbers in parenthesesare pumping plant
power are measured. The overall              efficiency.

126   CALIFORNIA AGRICULTURE, VOLUME 56, NUMBER 4
For example, a pump test con-              tion of the pumping plant. Cascading          ciency does not guarantee energy sav-
ducted in 1984 showed an efficiency of        water in a well may prevent good              ings. In fact, adjusting or repairing
54% (fig. 3, mismatched pump). How-           measurements of pumping lift. Poor            worn pumps may increase energy use
ever, the deviation of the test point         test conditions may also prevent good         unless the operating time of the pump-
from the manufacturer’s curve was             flow-rate measurements. The test sec-         ing plant is reduced. Sometimes, ad-
about the same as that of the 1983 test       tion should have eight to 10 pipe di-        justing the operating time still will not
point. The following year (1985),             ameters of straight pipe immediately          save energy if part of the increase in
pump efficiency rebounded. This be-           upstream of a flow meter and two             pump output contributes to a signifi-
havior suggests that the 1984 effi-           pipe diameters downstream to prevent         cant total head increase in addition to
ciency was due to a mismatched                errors due to excessive turbulence in        a capacity increase. With a higher flow
condition. The 1985 point, which              the water. For a 10-inch-diameterpipe,       rate, pumping plant operators can re-
shows efficiency similar to the 1983          a straight section 80 to 100 inches long     duce operating time, with either less
test, verifies this. However, a second        is recommended. However, research has        irrigation time per set or greater acre-
data set showed that the deviation be-        shown that propeller flow meters are         age irrigated per set. Opportunities for
tween pump test data and the                  less susceptible to large errors from tur-   reducing the operating time will de-
manufacturer’s performance curve in-          bulence caused by bends or elbows and        pend on site-specific conditions, such
creased with time, indicating possible        checks valves upstream from the meter        the irrigation method and its design
increasing pump wear (fig. 3, wearing         (Hanson and Schwankl1998).                   and management characteristics.If re-
pump).                                                                                     ducing the operating time is not pos-
Some caution is necessary in taking         Energy savings                              sible, the improved efficiency may
this approach. Poor-quality pump test            This series of studies shows that         result in more crop yield and revenue
data may prevent an accurate evalua-          simply improving pumping plant effi-         due to more water applied to a field.
Both growers and others apparently
did not recognize this fact at the Sali-
nas meeting.
Multiple pump tests (at least three)
of a pumping plant are recommended,
to help evaluate possible reasons for
low efficiency. Pumping plant opera-
tors should also obtain the manu-
facturer’s performance curves to use in
the evaluation process.

B.R. Hanson is Extension lrrigation and
Drainage Specialist, Department of Land,
Air and Water Resources, UC Davis.
References
Hanson BR. 1988. Benefits and costs of
improving pumping efficiency. Cal Ag
42(4):2 1-2.
Hanson BR. 2000. Irrigation Pumping
Adjustments t o impellers, made by turning the nut at the top o f the pump shaft, can      Plants. UC DANR Pub 3377. 126 p.
improve pump efficiency. But that may not necessarily translate into energy savings,          Hanson BR, Schwankl LJ. 1998. Water
unless operating time is reduced.                                                          turbulence disrupts accuracy of some
flow meters. Cal Ag 52(1):25-30.
CALIFORNIA AGRICULTURE,JULY-AUGUST2002   127

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