In pumping applications with variable ﬂow rate requirements, adjustable speed drives (ASDs) are an efﬁcient control alternative to throttling or bypass methods. ASDs save energy by varying the pump’s rotational speed. In centrifugal pumping applications with no static lift, power requirements vary, as the cube of the pump speed and small decreases in speed or ﬂow rate can signiﬁcantly reduce energy use. For example, reducing the speed (ﬂow rate) by 20% can lower input power requirements by approximately 50%. Due to drive inefﬁciencies, however, ASDs do not save energy in applications that operate close to fully loaded most of the time. For example, ASDs are seldom cost-effective in ﬂuid transfer pumping systems with on/off control when static lift is a signiﬁcant portion of the total head. In moving a ﬁxed volume of ﬂuid, increases in operating hours can offset the power savings resulting from reducing ﬂow rates. Developing a system curve is the ﬁrst step in understanding a given pump system’s characteristics at various ﬂow rates. Then, process requirements can be displayed in histogram, ﬂow rate duration curve, or load-duty cycle format. The load-duty cycle is a frequency distribution indicating the percentage of time that a pump operates at each system operating point; it can be useful in calculating potential energy savings. You can obtain the load-duty cycle by using historical measurements of ﬂuid ﬂow rates or using a recording watt-meter to monitor the electrical power input to the pump motor (see Table 1).
After establishing values for ﬂow rate and head, you can extract the pump efﬁciency and shaft horsepower required from the pump curve. Using weighted averages for power at each operating point, factor in the motor’s efﬁciency to calculate weighted input power (see Table 2).
*Base on a 50-hp, 1,800-rpm, totally enclosed, fan cooled standard efficiency motor from MotorMAster+ 4.0 data.
Perform similar calculations to obtain the average input power for the same pump when using an ASD to control ﬂow rate. Afﬁnity law equations used in conjunction with the system curve can help you calculate pump shaft horsepower requirements at each ﬂow-rate point. (Afﬁnity laws are valid for circulating water pumping applications or ﬂuid transfer applications with little static head.) Factor in motor and drive efﬁciency at each operating point to calculate weighted input power (see Table 3).
- Program drive controllers to avoid operating pumps at speeds which may result in equipment or systems resonances
- Install a manual bypass to keep the motor operating at a ﬁxed speed if the ASD should fail
- Install a single ASD to control multiple pump motors
- Use caution when reducing the ﬂow velocities of slurries.
- Flow control can be achieved by using ASDs, trimming impellers, installing multiple pumps, or adding a multi-speed motor.
- Consider ASDs as an option when pumps operate at least 2,000 hours per year and process flow rate requirements vary by 30% or more over time.
About Jonathon Bell
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References: DOE/GO-102007-2230 May 2007 Pumping Systems Tip Sheet #12
Industrial Technologies Program Energy Efficiency and Renewable Energy
U.S. Department of Energy Washington, DC 20585-0121 www.eere.energy.gov/industry
Improving Pumping System Performance: A Sourcebook for Industry, U.S. Department of Energy, 2006
Adjustable Speed Pumping Applications, DOE Pumping Systems Tip Sheet, 2007
Variable Speed Pumping: A Guide to Successful Applications, Hydraulic Institute, 2004