A question about centrifugal pumps

The most convenient and reliable way to obtain the H-Q curve of a centrifugal pump is to conduct direct measurements through experimental methods. By understanding the H-Q characteristic curve and other characteristic curves of centrifugal pumps, we can select the appropriate pump and determine the power required for the motor, ensuring that the pump operates under the most favorable conditions.

Characteristic curve of a centrifugal pump: The head H, shaft power N, and efficiency η of a centrifugal pump all vary with the flow rate Q, and their relationships can be represented by the pump’s characteristic curve or the performance curve of the centrifugal pump. Before the centrifugal pump leaves the factory, the manufacturer determines curves such as H-Q, N-Q, and η-Q, which are included in the product catalog or manual for reference by the users when selecting and operating the pump. Various models of centrifugal pumps have their own unique characteristic curves, which are not affected by the characteristics of the piping. But they all share some common patterns: (1) The head of a centrifugal pump generally decreases as the flow rate increases (with possible exceptions at very low flow rates), which is in line with the basic equation for centrifugal pumps. (2) The shaft power of a centrifugal pump is minimum when the flow rate is zero, and increases as the flow rate rises. Therefore, when starting a centrifugal pump, the pump outlet valve should be closed to reduce the starting current and protect the motor. Closing the outlet valve first when stopping the pump is mainly to prevent high-pressure liquid from flowing back and damaging the impeller. (3) The pump achieves its highest efficiency at the rated flow rate. This point of maximum efficiency is known as the pump’s design point, and the corresponding values are referred to as the optimal operating parameters. The performance parameters indicated on the centrifugal pump’s nameplate are those corresponding to the point of maximum efficiency. Centrifugal pumps are generally unlikely to operate exactly at their design point, but they should work as much as possible within the high-efficiency range, that is, within 92% of their maximum efficiency.

The characteristic curves of all pumps are obtained by the manufacturers in their testing facilities using water at normal pressure. These curves can serve as a reference when applied in actual production; some pumps have a correction factor relative to water, and by referring to the data in the manual, it is possible to determine the pump’s characteristic parameters under actual operating conditions.