Diagnostic Damage Control of Shaft Trains in Turbine Units According to Torsional Vibration Parameters
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Vol. 54, No. 4, November, 2020
DIAGNOSTIC DAMAGE CONTROL OF SHAFT TRAINS IN TURBINE UNITS ACCORDING TO TORSIONAL VIBRATION PARAMETERS D. M. Popov,1 A. Z. Zile,1 D. V. Taradai,2 and S. B. Tomashevskii1 Translated from Élektricheskie Stantsii, No. 6, June 2020, pp. 35 – 40.
An analysis of existing methods for determining the natural frequencies of torsional vibrations of the shaft trains in power turbine units is carried out in order to assess the potential for using changes in natural frequencies to diagnose shaft damage. A computational modeling of torsional vibrations of a shaft train having a circumferential crack allowed informative natural frequencies to be determined for the purposes of detecting damages and the locations of natural frequency measurements to be established to a satisfactory degree of accuracy. An assessment of changes in natural frequencies from depth and characteristic locations of a circumferential crack is carried out. An alternative method for detecting violations of the shaft train integrity, based on monitoring changes in the angles of static twisting, is also considered. Keywords: rotor; shaft train; turbine unit; torsional vibrations; natural frequencies; circumferential crack; modeling; static twisting; vibration monitoring; vibration; diagnostics.
crease in the stiffness of the shaft during the development of a defect. For assessing diagnostic capabilities, let us turn to a design model representing a circumferential crack in the form of a M(ã) concentrated torsional stiffness [4]. Expressions for the relevant calculation, obtained using fracture mechanics, take the form:
One of the known approaches for determining the operational reliability of power turbine units uses the natural frequency detuning of shaft train torsional vibrations (TV) according to the exciting torque frequencies [1 – 3]. Due to the low damping, the level of TV and the resulting stresses depends significantly on proximity to resonance. This determines the importance of accurately determining the natural frequencies of the TV and ensuring their resonance detuning at the fundamental excitation frequencies of 50 and 100 Hz during the design stage. However, the accuracy of the calculated determination of natural frequencies by the higher natural modes is insufficient for guaranteed resonance detuning. In this connection, the experimental determination of natural TV frequencies for the shaft trains of head turbine units is also relevant. An accurate experimental determination of natural TV frequencies is necessary for diagnosing the development of transverse — and, above all, circumferential — cracks in shafts. The use of transverse vibration parameters for the identification of such faults appears to be ineffective due to the very weak influence of axisymmetric shaft damages on the level and characteristics of bending vibrations. In order to detect circumferential-type defects, it is necessary to record small changes in the natural TV frequencies caused by a de1 2
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