In comparing similar centrifugal pumps it is often useful to use a parameter called specific speed. For similar designs major pump dimensions are proportional to specific speed. Thus, if the performance curve is known for one pump, it can be estimated for another pump of similar design but different impeller diameter.

The specific speed is given by the following formula:

A pump’s specific speed is always calculated at its point of maximum efficiency. It is not a dimensionless number, so that it is critical that the units used in calculating the specific speed be known.

Specific speed is used by pump designers to help determine the required impeller geometry. The lower the specific speed, the more the impeller shape will approach true radial flow. The higher the specific

speed, the more closely the impeller needs to approach true axial flow.

Specific speed is also useful in estimating maximum attainable pump efficiency. This is done from Figure 11-4, which is published by the Hydraulic Institute. This figure is useful to help estimate brake horsepower and to help validate vendor’s quoted efficiencies.