Shaft Failure of Large Axial Flow Pumps

Failure of shafts driving large axial flow pumps was found to be caused by intake flow pattern. Extensive tests on prototype pump and laboratory model were used to identify and correct the problem. Strains measured on the shaft during pump operation showed the existence of a cyclic bending moment sufficiently large to cause a fatigue failure. The bending moment was not a critical speed phenomenon and was assumed to be caused by hydraulic forces acting on the impeller. Laboratory model tests identified a vortex in the pump intake. The vortex was not centered below the pump impeller and generated an unbalanced force on impeller blades as these passed through the irregular water flow. Changing the shape of the intake eliminated the vortex and large bending moment in the shaft.