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This program is designed to help you calculate the diameter of the impeller required to achieve a permanent change in your system's flow rate. The calculations are based on the affinity laws which in turn are derived from a dimensionless analysis of three important parameters that describe pump performance (ref: The Pump Handbook by McGraw-Hill, chapter 2). The analysis is based on the reduced impeller being geometrically similar and operated at dynamically similar conditions or equal specific speed.

The affinity laws or equations are:

where subscripts 1 and 2 denote the value before and after the change. P is the power, n the speed, D the impeller diameter , H the total head.

If the speed is fixed they become:

If the diameter is fixed they become:

The process of arriving at the affinity laws assumes that the two operating points that are being compared are at the same efficiency. The relationship between two operating points, say 1 and 2, depends on the shape of the system curve (see next Figure). The points that lie on system curve A will all be approximately at the same efficiency. Whereas the points that lie on system curve B are not. The affinity laws do not apply to points that belong to system curve B. System curve B describes a system with a relatively high static head vs. system curve A which has a low static head.

Diameter reduction To reduce costs pump casings are designed to accommodate several different impellers. Also, a variety of operating requirements can be met by changing the outside diameter of a given radial impeller. Euler's equation shows that the head should be proportional to (nD)2 provided that the exit velocity triangles remain the same before and after cutting. This is the usual assumption and leads to:

Which apply only to a given impeller with altered D and constant efficiency but not a geometrically similar series of impellers.If that is the case then the affinity laws can be used to predict the performance of the pump at different diameters for the same speed or different speed for the same diameter. Since in practice impellers of different diameters are not geometrically identical, the author's of the section called Performance Parameters in the Pump Handbook recommend to limit the use of this technique to a change of impeller diameter no greater than 10 to 20%. In order to avoid over cutting the impeller, it is recommended that the trimming be done in steps with careful measurement of the results. At each step compare your predicted performance with the measured one and adjust as necessary.

The applet is designed to be used in English or French.