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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.
Adjusting impellers. Maintaining
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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