Effect of cracks on dynamic parameters and lifetime of hydraulic units

Abstract

Traditionally, an indicator of the reliability and quality of the hydraulic unit operation is the vibration state. However, numerous experiments show that there is often no correlation between the parameters measured during vibration tests and the level of dynamic stresses in runner – the one of the main unit elements. At the same time, it is important to remember that the dynamic stresses predominantly determine the hydraulic unit lifetime. Moreover, as practice shows, most modern diagnostic systems of hydraulic units cannot identify cracks not only in the early stage, but in also having a sufficiently long size.The dynamic system "runner – shaft – support bearings" of the vertical Francis hydraulic unit was studied in this work. The aim of research was learning the effect of cracks on the vibration parameters measured during regular vibration tests.The calculations have shown that due to the high rigidity of the structure, the appearance of cracks almost no affected on changes in the frequency spectrum or vibration amplitudes. It was also found the presence of eigenvalues modes, the excitation of which is not accompanied by a vibration response on the support nodes, but causes dynamic stresses in the runner blades that eventually lead to cracking in the zone of joint of the blades with the hub or rim. This means that the existing diagnostic systems do not allow identifying cracks and, accordingly, not allow to correctly assessing the lifetime of the hydraulic unit or the time of safe operation up to needed repair.The conducted studies show the shortcomings of the existing diagnostic systems of hydraulic units from the standpoint of lifetime assessment and make it possible to determine the priority directions of their development for the near future.