The maximum operating efficiency of a centrifugal pump is different when it is parallel operated with different pumps. To study the reduction in the maximum operating efficiency of pumps in parallel operation compared with their rated efficiency, 28 pairs of pumps with the same rated head and different rated flows were tested orthogonally in parallel operation. A determination of the pumps operating flow and head was done to calculate the maximum operating efficiency in different combinations. The results showed that the maximum operating efficiency of a pump in parallel operation was higher than 85% of rated efficiency when the flow ratio of two pumps was within 1.6. When the flow ratio was increased to 2.0, the maximum operating efficiency of the pump will sharply decline to 70% of rated efficiency or less. Therefore, the operating efficiency a pump can keep is 70% or more under the condition that the flow ratio between the pumps is less than 2.

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