This article is the second of three on centrifugal pump radial thrust. The first article (Pumps & Systems, July 2012) related the author’s experience with the use of the traditional equation to calculate radial thrust, subsequent measurements of radial thrust and comparison of the two. This article shows a plot of measured radial thrusts superimposed on a performance curve and discusses the pattern revealed. The third will discuss variations in casing and impeller designs that reduced the radial thrust.
The actual measured radial thrusts for the 3 x 2 x 11 vertical, in-line pump are plotted on the pump performance curve (see Figure 1). The load varied from 240 pounds, at shut-off, with the maximum-diameter impeller, to zero, at capacities that were about 70 percent of the best efficiency point (BEP) for all impeller diameters. Why wasn’t the zero thrust closer to the BEP? The 70-percent, zero-load point is indicative of a casing that contains a blend of a volute and a concentric circular collector. It also indicates that a true volute collector, with this impeller, could produce a radial thrust, at shut-off, even higher than the 240 pounds that we measured.
|Figure 1. Measured radial thrusts superimposed on the performance curve
The direction of the low-capacity thrust was approximately 90 degrees downstream from the cutwater. At capacities above zero thrust, the thrust increased in the opposite direction—approximately 270 degrees downstream from the cutwater. These angles compared favorably with those reported by Stepanoff.1
Calculations using the traditional equation and K values shown in the first article in this series indicated that the maximum radial load would be experienced at an impeller diameter less than the maximum. Tests proved otherwise. The load dropped significantly as the impeller diameter was reduced.
1. Stepanoff, A.J., Ingersoll-Rand, Centrifugal and Axial Flow Pumps, John Wiley & Sons, New York, 1948.