PSI - Issue 22

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

230

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

Keywords: crack; hydraulic turbine; lifetime; runner; vibration state.

1. Introduction Traditionally, an indicator of the reliability and quality of the hydraulic unit operation is the vibration state. The vibration state monitoring of the hydraulic unit can be carried out periodically using a portable system, or online mode. Modern diagnostic or monitoring systems combine up to 200 sensors that record the state of the unit main parameters and the vibrations at different points. As practice shows, even the most advanced diagnostic systems of hydraulic units cannot identify cracks not only in the early stage but also when they have sufficiently long sizes. σ dynamic stress intensity in the blade output edge at the interface point with the runner hub N unit power at the set operation mode f r revolving frequency f v vortex rope frequency f g guide vane frequency Z g numbers of guide vanes L crack length 2. Examples of fatigue cracks in Francis turbine runner Fig. 1 shows examples of large fatigue cracks in the area of welding the blade to the hub of the Francis hydraulic turbine runner. The diagnostic system did not detect such cracks at operation. They were found only under the unit inspection during plan repair. Presented on Fig. 1 cracks are typical examples of fatigue damage for the Francis turbine runner after long operation. They appear in the welding zone of blades to the hub or rim and, obviously, limit the hydraulic turbine lifetime, as noted many res earchers, for example, Frunzăverdel D. et al. (2010) or Nennemann B. et al. (2016). The mentioned above welding zone in Francis turbines is the most loaded both statically and dynamically. The growth of cracks occurs due to the impact of variable hydrodynamic loads accompanying unit work at all operation modes. Fatigue cracks, as a rule, begin from an output (thinner) edge and develop along the welding line. The most modern diagnostic systems do not allow detecting such macro-cracks not only at an early stage of their A large number of experiments show that there is often no correlation between the measured vibration and the level of dynamic stresses in blades. Fig. 2 shows the appropriate example – the processing of site tests performed by specialists of the hydropower department JSC “NPO CKTI”. Positions (a), (b) and (c) demonstrate amplitudes of vibration displacements (2A) at trust and guide bearings measured by vibration diagnostics system. Position (d) shows the summary dynamic stress value and main frequency components:  the vortex rope frequency (f v  about 0.2-0.5f r );  revolving frequency f r ;  guide vane frequency (f g = f r ·Z g ). For the presented example, f v  0.5 Hz, f r  2 Hz, f g  40 Hz. development but even at large length and disclosure values. 3. Ability to identify cracks using vibration diagnostics Nomenclature 2A amplitude of vibration displacement