Trim or Replace Impellers on Oversized Pumps Page: 2 of 2
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In practice, impeller trimming is typically used to avoid throttling losses associated with
control valves, and the system flow rate will not be affected.
Note that, in contrast to centrifugal pumps, the operating regions of mixed-flow and axial-
flow pumps are limited because of flow rate instabilities. Therefore, consult with the pump
manufacturer before changing the impeller diameter. Removing stages is often advisable
for a multistage centrifugal pump that is oversized for current operating conditions.
When a pump serves a critically important process, it might not be possible to wait for the
impeller to be trimmed. In that case, consider ordering another impeller and continuing
operation until the new impeller can be installed.
Example
A double-suction centrifugal pump equipped with an impeller 14 inches in diameter is
throttled to provide a process cooling water flow rate of 3,000 gpm. The pumping system
operates for 8,000 hours per year with a head of 165 feet (ft) and pump efficiency (fl)
of 80%. The pump requires 156 bhp. Pump and system curves indicate that a trimmed
impeller can supply the 3,000-gpm required flow rate at a head of 125 ft. From the affinity
laws, the diameter of the trimmed impeller is approximately as follows:(H2Q2)/ (HQ,) = (D2 /D1)3
Holding Q constant,
D2 = D x (H2 / H,)1/3
= 14 x (125 / 165) 13
= 12.76 inchesAssuming that the pump efficiency remains unchanged, installing a 123/4-inch trimmed
impeller reduces input power requirement to the following:
bhp2 = (H2 X Q2) / (3,960 x r-)
= (125 x 3,000) / (3,960 x 0.8)
= 118.4 bhp
Estimated energy savings, assuming a 94% motor efficiency, are as follows:
(bhp, - bhp2) x 0.746 kW/hp x 8,000 hours/year / 0.94 = 238,720 kWh/year
At an electricity cost of 5 cents per kWh, total cost savings are estimated to be $11,936 per
year.
Reference
Match Pumps to System Requirements, U.S. Department of Energy Pumping Systems Tip
Sheet #6, 2005.BestPractices is part of the Industrial
Technologies Program Industries of the
Future strategy, which helps the country's
most energy-intensive industries improve
their competitiveness. BestPractices brings
together emerging technologies and best
energy-management practices to help
companies begin improving energy efficiency,
environmental performance, and productivity
right now.
BestPractices emphasizes plant systems,
where significant efficiency improvements
and savings can be achieved. Industry gains
easy access to near-term and long-term
solutions for improving the performance of
motor, steam, compressed air, and process
heating systems. In addition, the Industrial
Assessment Centers provide comprehensive
industrial energy evaluations to small- and
medium-size manufacturers.
FOR ADDITIONAL INFORMATION,
PLEASE CONTACT:
EERE Information Center
1-877-EERE-INF
(1-877-337-3463)
www.eere.energy.gov
Industrial Technologies Program
Energy Efficiency
and Renewable Energy
U.S. Department of Energy
Washington, DC 20585-0121
www.eere.energy.gov/industryA STRONG ENERGY PORTFOLIO
FOR A STRONG AMERICA
Energy efficiency and clean, renewable
energy will mean a stronger economy, a
cleaner environment, and greater energy
independence forAmerica. Working
with a wide array of state, community,
industry, and university partners, the U.S.
Department of Energy's Office of Energy
Efficiency and Renewable Energy invests in
a diverse portfolio of energy technologies.
DOE/G0-102006-2226
September 2006
Pumping Systems Tip Sheet #7
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Trim or Replace Impellers on Oversized Pumps, report, September 1, 2006; Golden, Colorado. (https://digital.library.unt.edu/ark:/67531/metadc874990/m1/2/: accessed April 24, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.