Abstract
This paper shows the results of the analysis of the dynamic behaviour of a power unit, whose shaft-train alignment was significantly influenced by the machine thermal state, that was affected in operating condition by high subsynchronous vibrations caused by oil-whip instability phenomena. The dynamic stiffness coefficients of the oil-film journal bearings of the generator were evaluated considering the critical average journal positions that caused the instability onsets. By including these bearing coefficients in a mathematical model of the fully assembled machine, the real part of the eigenvalue associated with the first balance resonance of the generator rotor became positive. This paper shows the successful results obtained by combining diagnostic techniques based on mathematical models of journal bearings and shaft train with detailed analyses of monitoring data aimed to investigate the effects of the hot alignment of rotating machines on the occurrence of oil-whip instability onsets.
Highlights
The oil-whip instability phenomenon is one of the most important malfunctions that cause subharmonic components in the frequency spectrum of rotating machine vibrations [1,2,3]
This paper shows the results of the analysis of the dynamic behaviour of a power unit, whose shaft-train alignment was significantly influenced by the machine thermal state, that was affected in operating condition by high subsynchronous vibrations caused by oil-whip instability phenomena
This paper shows the successful results obtained by combining diagnostic techniques based on mathematical models of journal bearings and shaft train with detailed analyses of monitoring data aimed to investigate the effects of the hot alignment of rotating machines on the occurrence of oil-whip instability onsets
Summary
The oil-whip instability phenomenon is one of the most important malfunctions that cause subharmonic components in the frequency spectrum of rotating machine vibrations [1,2,3]. These coefficients were included into a further mathematical model of the fully assembled rotating machine in order to perform a parametric analysis The results of this investigation showed that the bearing coefficients, evaluated considering the experimental average journal positions that induced unstable subsynchronous vibrations, caused a positive real part of the eigenvalue of the machine-train model associated with the first flexural critical speed of the generator rotor. The first critical speed of the generator evaluated at the operating rotational speed, that is considering the proper estimates of the bearing coefficients that corresponded to the critical hot alignment of the shaft-train, was very close to the rotational speed associated with the frequency order of the subharmonic unstable vibrations experienced in operating conditions This experimental evidence confirmed the reliability of the results obtained by the authors’ study. This paper shows the successful results obtained by combining diagnostic techniques, based on models of journal bearings and of shaft-train, with detailed analyses of raw monitoring data, in order to investigate the effects of thermally induced changes in the shaft-train alignment on the generation of oil-whip instability onsets in rotating machines
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