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Control Strategies for Centrifugal Pumps with Variable Flow Rate Requirements for Reduced Energy Consumption?

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).

 

Table1-Load-Duty-Cycle-for-an-existing-centrifugal-pump-with-throttle-valve-control

*brake horsepower

 

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).

 

 

 

Installation Considerations

  • 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.

 

Suggested Actions

  • 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

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

 
About Dynapro:

A professional, trustworthy company, committed to create and maintain lasting relationships with our customers and our community.  Our focus is on constantly evolving our business practices and dedicated service to always be aligned with our clients and the environment.  

Our strong sense of responsibility to the environment and the communities we live and work in help encourage our clients and other companies to join forces with us to make a difference.  

We manufacture our own pump models and interchangeable high quality products, improve products, and materials.  We deliver them for less and faster to help achieve our goal of reduced consumption; energy & materials, and reduce maintenance.

For more information, please visit https://dynaproco.com/about-us-page

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