Q.  Should I use the pump affinity rules to correct my pump curve data when trimming the impeller?

A. While the affinity rules can give a good approximation of pump performance for a given impeller trim, in some cases the error can be as high as 15 to 20%.

Generally speaking, lower specific speed pumps follow the affinity rules better than higher specific speed pumps. The inaccuracies of the affinity rules for impeller trims can be attributed to changes in geometry caused by trimming the impeller. An impeller trim can change the vane angle, the width of the cutwater, as well as other aspects of the design.

Often pump manufacturers will provide performance curve data at various trims and more accurate adjustments can be attained by interpolating between the curves for two impeller trims of a particular pump.

The book Pump Characteristics and Applications by Michael Volk, P.E. was referenced for this article. To read more about the affects of trimming impellers has on a pump's curve check out this extensive resource.