A series of centrifugal pumps were to be purchased and installed in a petrochemical plant in Taiwan. Prior to purchase, the user contracted MSI staff to perform a complete mechanical design evaluation, including rotor dynamic, torsional critical speed, and rotor forced response Analysis. In addition, the pressure-carrying capability of all casings at full and part load was to be assessed. Some of the pumps were driven by motors through variable frequency drives (VFDs, which have unique and strong torsional excitation harmonics), and some of them were driven by steam turbines.

Our analysis showed that all aspects of the design were acceptable with sufficient factor of safety, except for the second torsional natural frequency, which was excited by twice line frequency of the motor (this is usually the strongest torsional excitation frequency in electrical motors), and by the partial admission steam turbines second harmonic of running speed, which could be easily excited by misalignment. A low-speed-shaft coupling modification was determined through "what-if" analysis that allowed the second torsional mode to be shifted up and out of the range of excitation by these harmonics, without creating any new problems. The pumps were installed, and have operated for several years without any mechanical difficulties.