Pump Affinity Laws: Use with Caution
Plumbing Systems and Design 2007; Vol. 6: No. 8
by Ahlgren, Roy C. E.
The pump affinity laws provide a useful way to estimate the effect of changes in either the revolutions per minute (rpm) or the diameter of the impeller on a pump’s performance. In this article, Roy C. E. Ahlgren, discusses why these laws can be better used to predict performance in some cases as opposed to others. Using an example, he demonstrates how the pump affinity laws predict actual performance with better accuracy at variable rpms than after trimming the impeller. In addition to better predictions when efficiency changes are smaller, the affinity laws assume geometric similarity and noncavitating pump operation with cold water. Finally, the affinity laws cannot take into account system curve, which also affects pump efficiency. The article includes figures showing several curves and tables of curve estimates.
Available at : ASPE Publications