PSI - Issue 22

E. Georgievskaia / Procedia Structural Integrity 22 (2019) 229–234 E. Georgievskaia/ Structural Integrity Procedia 00 (2019) 000 – 000

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Fig. 1. Examples of fatigue cracks in Francis turbine runner: (a) crack length 315 mm by Frunzăverdel D. et al. (2010); (b) long through crack by Nennemann B. et al. (2016).

Fig. 2. Comparison: (a) vibration amplitude of turbine guide bearing; (b) vibration amplitude of generator guide bearing; (c) amplitude of trust bearing; (d) dynamic stress intensity.

From the presented in Fig. 2 data, it is clear that the changes in the vibration parameters and dynamic stresses do not correspondent each other either qualitatively or quantitatively. Therefore, vibration diagnostic system, in the current form, does not allow predicting the appearance and development of cracks and assessing the unit lifetime. 4. Description of the design model The design features and the operating modes determine the intensity and frequency spectrum of dynamic stresses in the blades, for example, Georgievskaia E. (2018). The distribution of dynamic stresses in runner predominantly regulates the hydraulic unit lifetime. However, defining the real dynamic stresses under the actual operating conditions of the hydraulic unit is a very difficult task even now. In order to expand the capabilities of vibration diagnostics systems, it is promising to study the regularities of the effect of cracks on the modal parameters of the hydraulic unit as a whole and its individual components. To research the effect of cracks on the hydraulic unit lifetime the dynamic system "runner – shaft – support bearings" of the vertical Francis hydraulic unit was studied. Multivariate calculations were performed for the 16-blade runner. To study the effect of the crack length on the vibration parameters, a series of calculations with simulated through cracks in the runner using the blade model taking into account the cyclic symmetry and for the runner as a whole was carried out. Model cracks were located along the interface line of the blade with the wheel hub from the output edge, which is typical for long operation turbines, as noted Frunzăverdel D. et al. (2010) or Nennemann B. et al. (2016) or Georgievskaia E. (2018) and many others. During the numerical experiment, the possibility of developing both a single crack and a mass cracking affecting all the blades of the runner was considered.