Optimising pump speed to save energy

Pumps are huge energy users; therefore, small percentages saved in this area can have a significant impact on a company's bottom line profits. As costs of greenhouse gas emissions increase so does the cost of energy and as a result the incentive to lower these emissions. One method which is of increasing interest to the pumping industry is energy efficiency technology that results in using less energy for the same tasks.

A study by the Lappeenranta University of technology and a Finnish paper mill reveals that the consumption of specific energy using throttling control may require up to three times the energy compared to a solution using variable speed drives combined with optimised pump control.

According to the study, pump control based on variable speed drives can deliver energy saving of up to approximately 70% in parallel pumping installations. The biggest savings can be achieved in situations where there are significant fluctuations in the flow. The project leading to this conclusion involved both computer simulations and practical work on laboratory-scale equipment.

The project was undertaken at the LUT's Department of Energy and Environmental Technology. It set out to quantify the differences in energy consumption in four applications with three different flow control methods. The simulations were performed with Matlab v 6.1 and Simulink software, and the results were verified with actual measurements. The control methods compared were throttle control, standard pump control and optimised pump control.

Throttle control: One pump is throttled and the others are on/off controlled.

Standard pump control: One pump is controlled by a variable speed drive (VSD) and the others are on/off controlled.

In optimised pump control, each pump has its own VSD and the required flow is divided evenly between all the pumps. As a result, their rotational speed is the same. This case differs from the standard model in that the pumps are switched on and off in an optimised way. (Optimised pump control technology is subject to a patent application by ABB.)

The first simulated industrial example is typical of real-life industrial pumping situations where new control technology can be applied. The example was taken from a Finnish paper mill, where Ahlstrom APP22-65 centrifugal pumps are used to pump chemically treated water to a desalination unit.

The simulations, which relied on simplified system and duration curves, showed that in this case, throttle control uses considerably more energy than other control methods. Optimised pump control is by far the more energy-efficient method. The difference between standard and optimised pump control is over 45%. The consumption of specific energy with throttling is almost threefold of that used with optimised pump control.