61. Turbine Instability Explained by Multidimensional Cavitation Diagnostics Bajic Branko (Proceedings, Hydro 2003, Vol. I, Croatia, pp. 101–109). Abstract Since commissioning in 1989, the two 40 MW bulb units of the Dubrava HPP on the Drava River in Croatia have suffered from once-per-revolution power fluctuations, rather strong at unit A and somewhat weaker at unit B. These fluctuations cause increased dynamic loading on the machinery elements and thus shorten the units’ life. From 1990 to 2002 numerous assumptions about the cause of the fluctuations were investigated. Finally, in 2002 the explanation presented here was formulated and proven. Vibro-acoustic diagnosis of cavitation in unit A turbine which was carried out by Korto Cavitation Services’ multidimensional method, has shown that a connection between cavitation and the power fluctuations might exist. Following that, by using experimental data on the power fluctuations collected in one-month monitoring and data on cavitation in turbine A – collected in full-scale and model-scale tests – this possibility has been analysed in detail. One of the goals of the analysis was to find out to which extent the power fluctuations are the result of the fact that, since the planned downstream power plant has not been built, the Dubrava turbines operate at circa 1.5 m lower tail water than designed. This is a report on the results of the analysis. First, the diagnosis is presented, then the arguments the diagnosis is based on, and, at the end, the practical implications of the findings, including the influence of the tail water. The power fluctuations at the Dubrava HPP bulb units are caused by cavitation developed on the runner blades which, due to the shaft horizontal position and a large runner diameter, pulsate once per revolution. The fluctuations become strong when strong sheet cavitation is developed and especially strong when trash is caught by the upper guide vanes; this disturbs the inflow to the runner and supports cavitation development. It is interesting to note that cavitation causing these fluctuations is not very erosive. Not sufficiently accurate manufacturing of the runner with respect to cavitation, even when keeping the runner shape within tolerances prescribed by a standard, is responsible for the power fluctuations. The tail water lower than designed and the runner design that is risky with respect to cavitation amplify the strong effects of imprecise manufacturing. However, the fluctuations would not be eliminated even under the design operating conditions, with the tail water higher by 1.5 m. The disadvantages of the design: it permits excessively strong cavitation under normal operation and results in an unacceptably high sensitivity to inflow variations. The collected experimental data leads to the conclusion that the intense shaft vibrations at high power values have the same origin as the power fluctuations. As the trash cannot be eliminated entirely, and as the runner shaped more accurately than that required by the standard is hardly feasible, strong fluctuations can be eliminated only by designing the new runner for a given tail water, such that would have a sufficient cavitation resistance and a low sensitivity to inflow variations. The owner has decided to follow this course of action. Top Keywords Fluctuations, Dynamic loading, Vibroacoustic diagnosis, Cavitation, Model-scale tests. Top |