HI pump faqs
How Motor Speed Affects Energy Consumption & Are There Sustainability Standards in Pump Systems?
HYDRAULIC INSTITUTE
| IMAGE 1: Rotodynamic (left) and positive displacement (right) variable speed pump curves with system curves demonstrate operating point (flow and head) reduction as a function of speed, which depends on the pump and system curve shape. (Images courtesy of the Hydraulic Institute)
QUESTION
How does reducing motor speed affect energy consumption in pump systems?
For pumps driven by electric motors, reducing motor speed is commonly used to manage pump output to match the system requirements and can lead to energy savings. Motor speed is typically reduced using a variable frequency drive (VFD), which controls the voltage and frequency supplied to the motor to adjust its speed.
When motor speed is reduced, the pump performance curve (head or pressure as a function of flow rate) changes. Rotodynamic pump curves demonstrate reduced head and flow as a function of reducing speed, and positive displacement pumps demonstrate reduced flow as a function of speed. The key distinction is that the maximum head developed by a rotodynamic pump will decrease as a function of speed, but positive displacement pumps can develop maximum differential pressure at reduced speed.
Power consumption is a function of fluid density, flow rate, developed head or pressure and the pump and motor efficiency. The operating point is defined by the pump performance curve and system curve intersection (Image 1). An important consideration is that the pump efficiency for rotodynamic pumps will change and decrease as the operating point transitions to the left of the pump’s best efficiency point. Positive displacement pumps will maintain more constant efficiency as the operating point changes.
With the understanding that power consumption is dependent on the operating point, reduced motor speed typically results in savings due to a reduction in flow rate or differential pressure. There are also rotodynamic pump applications where the system has high proportions of static head, and this results in low flow and low efficiency operation where savings are minimal or the flow rate is below the pump’s minimum allowable. This often causes premature wear and failure.
The actual reduction in power consumption depends on the interaction between the pump and system curves, as well as the efficiency of the pump, motor and VFD at the load point. Assuming the fluid properties do not change, the reduction in power will be proportional to the reduction in flow rate, developed head or pressure and change in efficiency. Taking the electrical input power in kilowatts (kW) and the operating time in hours (h) provides energy in kilowatt hours (kWh). The energy savings will then be influenced by factors such as:
- The shape of the system curve (i.e., the proportion of static head and friction head)
- The pump type and performance curve shape
- The proportion of time the system operates at part load (i.e., reduced flow, head or both) and how pump efficiency changes as speed is reduced
- The efficiency characteristics of the motor and drive at the part load operating points
For more information on variable speed operation and evaluating energy use at the system level, refer to pump system training courses at training.pumps.org.
QUESTION
Is there a standard way of looking at sustainability for pump systems?
In the context of pump systems, sustainability does not have a single definition. An obvious consideration for sustainability involves the efforts manufacturers have made to further improve the efficiency of pumps and the integration of sensors and controls so the next generation of pumps match the system demand, which greatly reduces energy consumption.
However, sustainability extends beyond reducing energy consumption. It includes the energy required to manufacture the equipment, designing the equipment for a long service life and repairability, and provisions for recycling the raw materials at the equipment end of life. Additionally, responsibility for sustainability falls on the pump system owner to ensure the system is operated as designed and properly maintained to ensure a long service life, reliable operation and efficient use of resources over time. This should be thought about with a circular economy view, where everyone is doing their part to ensure the efficient use and responsible recycling or disposal of materials.
From a pump manufacturer perspective, leading companies have stated sustainability key performance indicators (KPIs) that they measure and grade themselves against. For example, as a manufacturer expands its operations, it will likely consume additional power, but energy offset provisions may be implemented that reduce its energy or carbon per facility size or unit output. This could include things like sourcing recycled materials, sourcing energy from low-carbon generating sources, implementing energy efficiency improvements for facilities or generating their own electricity.
Similarly, from a pump system ownership perspective, the manufacturer can focus on total life cycle cost in its procurement instead of prioritizing the lowest initial cost. This requires a system level examination and recognizing that improvements to one component alone may not deliver meaningful long term benefits if the overall system is not well matched to its application. Energy consumption is typically 40% of an industrial pump’s life cycle cost (Image 2) and can be much more for systems with high operating hours. Therefore, having an active assessment program that measures and analyzes existing system performance to identify optimization investments that will lower the system’s life cycle cost can have a great impact on improving sustainability.
| IMAGE 2: Typical industrial pump life cycle cost
For additional guidance on sustainable pump system design and operation, refer to HI & Europump Pump Life Cycle Cost Guidebook at pumps.org.
HI Pump FAQs® is produced by the Hydraulic Institute as a service to pump users, contractors, distributors, reps and OEMs.
For more information, visit pumps.org.
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