A pump’s efficiency can degrade as much as 10% to 25% before it is replaced, according to a study of industrial facilities commissioned by the U.S. Department of Energy (DOE), and efficiencies of 50% to 60% or lower are quite common. However, because these inefficiencies are not readily apparent, opportunities to save energy by repairing or replacing components and optimizing systems are often overlooked.
Define Pumping System Efficiency
System efficiency incorporates the efficiencies of the pump, motor, and other system components, as shown in the area of the illustration outlined by the dashed line.
Pumping system efficiency (ηsys) is defined as follows:
Only the required head and flow rates are considered in calculating system efficiency. Unnecessary head losses are deducted from the pump head, and unnecessary bypass or recirculation flow is deducted from the pump flow rate.
Conduct Efficiency Tests
Efficiency tests help facilities staff identify inefficient systems, determine energy efficiency improvement measures, and estimate potential energy savings. These tests are usually conducted on larger pumps and on those that operate for long periods of time. For details, see Hydraulic Institute standards ANSI/HI 1.6-2000, Centrifugal Pump Tests, and ANSI/HI 2.6-2000, Vertical Pump Tests.
Flow rates can be obtained with reliable instruments installed in the system or preferably with stand-alone tools such as a sonic (Doppler-type) or “transit time” flow meter or a Pitot tube and manometer. Turbulence can be avoided by measuring the flow rate on a pipe section without fittings at a point where there is still a straight run of pipe ahead.
Improve System Efficiency
Internal leaks caused by excessive impeller clearances or by worn or misadjusted parts can reduce the efficiency of pumps. Corrective actions include restoring internal clearances and replacing or refurbishing worn or damaged throat bushings, wear rings, impellers, or pump bowls. Changes in process requirements and control strategies, deteriorating piping, and valve losses all affect pumping system efficiency.
Potential energy savings can be determined by using the difference between actual system operating efficiency (ηa) and the design (or optimal) operating efficiency (ηo), or by consulting published pump curves, as available, for design efficiency ratings.
Software tools like DOE’s Pumping System Assessment Tool (PSAT) also provide estimates of optimal efficiency. When the required head and flow rate, as well as actual electrical data, are input into the software, PSAT will account for artificial head and flow losses.
The equation for calculating potential energy savings is as follows:
Savings = kWin x t x ( 1 – ηa/ηo ) ,
savings = energy savings, in kilowatt-hours (kWh) per year kWin = input electrical energy, in kilowatts (kW)
t = annual operating hours
ηa = actual system efficiency, calculated from field measurements
ηo = optimal system efficiency.
Efficiency testing and analysis indicate that a 300-horsepower centrifugal pump has an operating efficiency of 55%. However, the manufacturer’s pump curve indicates that it should operate at 78% efficiency. The pump draws 235 kW and operates 6,000 hours per year. Assuming that the pump can be restored to its original or design performance conditions, estimated energy savings are as follows:
Savings = 235 kW x 6,000 hours/year x [ 1 – (0.55/0.78) ] = 415,769 kWh/year.
At an energy cost of 5 cents per kWh, the estimated savings would be $20,786 per year.
- Survey the priority pumps in your plant and conduct efficiency tests on them.
- Identify misapplied, oversized, or throttled pumps, or those that have bypass lines.
- Identify pumps with operating points below the manufacturer’s pump curve (if available); estimate energy savings of restoring the system to its original efficiency.
- Identify pumps with flow rates of 30% or more from the BEP flow rates, or with system imbalances greater than 20%.
- Determine the cost effectiveness of each improvement.
About Jonathon Bell
Jonathon Bell is an entrepreneur, focused on building his family's legacy in the industrial pump market. Currently, he is focused in Latin America, building Dynapro Pumps Mexico from the ground up while contributing in Canada & the United States with Sales & Marketing efforts.
His commitment is developing teams through individual and partnered coaching, to bring out the best in each team member and giving them the tools to help them reach their goals. Guiding and teaching the core values of passion, evolving, and team communication, his teams and members become top performers in their respective fields.
He is honest, generous, and passionate about others success for them individually, their families, and their communities
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Centrifugal Tests (ANSI/HI 1.6-2000), Hydraulic Institute, 2000.
Conduct an In-Plant Pumping System Survey, DOE Pumping Systems Tip Sheet, 2005. Match Pumps to System Requirements, DOE Pumping Systems Tip Sheet, 2005.
Trim or Replace Impellers on Oversized Pumps, DOE Pumping Systems Tip Sheet, 2005.
DOE/GO-102005-2158 September 2005 Pumping Systems Tip Sheet #4
U.S. Department of Energy Washington, DC 20585-0121 www.eere.energy.gov/industry
“Control Valve Replacement Savings,” U.S. Department of Energy Performance Optimization Tip, Energy Matters, July 1998; available online at: http://www.nrel.gov/docs/legosti/fy98/23382.pdf