Reasonable methods for determining the optimum number of blades in a low specific speed centrifugal pump with closed-type impeller with splitters, i.e. compound impeller have been rather rare in the literature so far. In the article, a new analytical method was put forward to determine such an optimum number of blades by including the effect of turbulent boundary layer over impeller blades. Three conventional impellers with different numbers of full-length blades and two compound impellers with different numbers of splitters were designed and manufactured. The corresponding performance tests were then conducted. Results showed that the optimum numbers of blades exist for two kinds of impeller in terms of head at design point, pump efficiencies at design point and best efficiency point, and slope of head-flow rate curve at shut-off point. The estimated optimum numbers of blades are in good agreement with the numbers based on the experiments. The conventional impellers with full-length blades are more prone to the hump phenomenon than the compound impellers at the optimum numbers of blades. For the compound impellers, however, the hump effect is negligible at the optimum number of blades, and their head and efficiency are higher than those for the impellers with full-length blades. The method is applicable to compound impeller design in low specific speed centrifugal pumps.
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