You can reduce the outside diameter of an impeller if you want to reduce either the head, or flow rate of your centrifugal pump.

When you do this reduction, the affinity laws predict that the new rate of flow is equal to the original rate of flow times the ratio of the new diameter to the old-as in the equation
Q = Q1 x D2/D1

If the model of a larger pump is made to a smaller size, the rate of flow is predicted as the cube of the diameter ratio or Q2= Q1 x (D2/D1)3

Why did the smaller size change by the cube of the ratio?  

  • In the first case, only one dimension of the impeller is affected, the Impeller OD. The clearance between the tip of the impeller and the casing increases. That is why most pump manufacturers limit the reduction to about 10% of the impeller diameter to prevent slippage.
  • In the second case, all dimensions of the pump impeller are changed as well as the diameter. The pump casing design is also changed with the same ratio.
  • Less slippage between the impeller outside diameter and the casing means more head and more capacity.

Take a look at the following illustration: