Unbalanced Hydraulic Forces Caused by Geometrical Manufacturing Deviations of Centrifugal Impellers

Abstract

When an impeller has geometrical manufacturing deviations, synchronous vibration occurs from an unbalanced hydraulic force rotating with the impeller. This phenomenon is termed “Hydraulic unbalance.” In this study, experiments and analyses were carried out to study the hydraulic unbalance. We made several model impellers with intentional “manufacturing deviations”: in vane angle, vane pitch, and with eccentricity. Hydraulic forces were estimated from the rotor vibrations. The results of the experiments showed that the magnitude of the hydraulic force increased as the increase of the extent of manufacturing deviations. It was also shown that the amplitude and the phase of the vibration due to the unbalanced hydraulic force depend on the flow rate. This character of the unbalanced hydraulic force is quite different from that of the mass unbalance force, which is independent of the flow rate. A two-dimensional inviscid flow analysis was carried out to clarify the characteristic of the unbalanced hydraulic force. It was found that the hydraulic force can be divided into three components which are constant, linear and parabolic function of the flow rate. Applications of the present results show that the “hydraulic unbalance” of a high-speed pump impeller manufactured within a geometrical tolerance recommended by a typical standard can be larger than the ordinary “mechanical unbalance.” It was thus shown that the hydraulic unbalance is one of the important factors in the balancing of high-speed pump rotors.