Is it meaningful to continue replacing the small impeller or cutting the impeller after implementing variable frequency control on a centrifugal pump?
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Dear experts, several centrifugal pumps currently have a low frequency of operation, around 20Hz; as a result, their flow rate and head are well below the design values. The model numbers of these pumps are 100AY120 and 150AY150. I’m wondering whether it’s necessary to replace the impellers with smaller ones or to cut them down in size. Is it more energy-efficient to use low-frequency control, or is it better to reduce the size of the impellers? The power of the pump motors is between 55–75 kW. I seek your advice.
If the output frequency of the centrifugal pump is low, and the current flow rate and head are much lower than the design values, replacing the impeller with a smaller one or cutting it can effectively reduce the pump’s operating point, enabling it to meet lower flow rate and head requirements and thereby improving the pump’s efficiency and energy-saving performance. By adjusting the impeller size, it is possible to better match the actual operating conditions and reduce energy consumption. Variable frequency control can also achieve energy savings, but it does so primarily by adjusting the operating speed to meet the requirements of different working conditions, offering a wider range of applications and greater flexibility in adjustment. However, when operating at extremely low frequencies, the efficiency of the pump may decrease. It is recommended to first conduct a detailed analysis of the system’s actual operating parameters and requirements, and after performing a technical-economic comparison, select the most suitable solution. If it is only necessary to reduce flow rate and head temporarily or on a small scale, variable frequency control can be considered as a preferred option. If the operating conditions remain below the design parameters over a long period, replacing or adjusting the impeller size may be a more economical and effective solution. At the same time, ensure that the selected impeller is compatible with the existing pump body, and that the pump as a whole remains within its efficient operating range. .
For centrifugal pumps, the frequency ratio equals the speed ratio, which in turn is equal to the square of the head ratio; therefore, the frequency ratio results in a significant reduction in head. In other words, at 20 Hz, the head is approximately 0.16 times that at 50 Hz. Impeller modifications can only reduce the head by a small amount and will also significantly decrease the pump’s efficiency. If the actual operating range is limited, it is advisable to use a pump that is better suited to these parameters.
It is best to choose a new pump directly. When making this selection, it is important to thoroughly understand the performance curve provided by the manufacturer, identify the flow rate and power requirements of the pump for one’s specific operating conditions, and then select an appropriate motor