Practical Solutions to Machinery and Maintenance Vibration Problems
Chapter 9, Fluid Flow
Section 2, Recirculation
When the output of a centrifugal pump is reduced by throttling the discharge vale or other discharge restrictions, the fluid flow through the pump is disturbed. The velocity of the fluid exiting the tips of the impeller vanes is reduced, and therefore, the fluid no longer passes smoothly into the volute and discharge piping. Instead, it tends to impinge upon the "cutwater" and induce a vibration at a frequency equivalent to number of vanes x rpm. Its amplitude often exceeds alert values, especially if resonance is involved. Often associated with the vanepass frequency is a random low amplitude wide frequency vibration similar to cavitation and turbulence, but occurring in the lower frequency ranges. This often results in misdiagnosis. However, it is important to note that recirculation occurs on the discharge side of the pump and cavitation occurs on the suction side. The cure for recirculation is to operate the pump, at or close to its rated capacity for the operating conditions and avoid excessive throttling. If permanent reduction in capacity is required, then the OD of the pump impeller can be reduced slightly to increase the gap between the impeller tips and the cutwater. Scalloping the impeller tips is another means to reduce vanepass vibration without major reductions in pump capacity.
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