Running far from the best efficiency point can be damaging.
08/26/2019
The best efficiency point (BEP) of a pump is the operating point, flow rate (capacity) and head where the pump offers the peak of efficiency. However, the importance of BEP is more than efficiency and best performance. This is the operating point where the possibility of energy waste and unwanted force, vibration, noises, etc., are at the lowest. Therefore, BEP is closely related to the highest theoretical reliability and lowest possible risk or maintenance.
Working Far From BEP
A centrifugal pump might be operated far from its BEP for many reasons. The most common causes have been wrong selection of the pump, an error in operation or a misjudgment of operators. Unfortunately, there are still operators who think operation far from BEP is just a matter of lower efficiency and higher energy waste. However, it could heavily affect reliability. Many pumps cannot be operated safely and reliably far from BEP for long durations of time.
Image 1. An example of a large, critical pump during installation. This variable speed pump is designed to operate most of the time around its BEP. (Image courtesy of the author)
The complex liquid pattern inside a centrifugal pump operates in 3D with many complicated effects such as recirculation flows at suction and discharge sections, flow separation, etc. The curvature of the blades, the rotation of impeller assembly and separation of flow at different places have great influence on this complicated flow field inside a pump.
Even when a centrifugal pump is operated at BEP, there is some energy waste and loss, and there could be issues and challenges. When a pump is operated far from its BEP, these problems would be intensified and new issues would arise.
A large portion of wasted energy when a pump is operated far from BEP is converted to unwanted heat, vibration or noises. Another effect of such an operation is high forces or loads on bearings and seals, which can reduce their life. The rise of liquid temperature and internal circulation (within the pump) are other damaging effects of such a poor operation. These effects can be minimized with the use of a variable speed drive or other suitable methods to keep the operating point close to BEP.
Higher forces and loads on bearings experienced for operation far from BEP can be a major issue, particularly for constant-speed pumps using rolling-element bearings. This can be partially rectified by specifying certain types of heavy-duty and long-life rolling-element bearings. For small- and medium-size pumps, only rolling-element bearings would be used and other more reliable bearings such as sleeve bearings cannot be used.