With the need for flexible operation, Francis runners are exposed to various operating conditions outside the traditional operating range. To grant sufficient fatigue life, a deep knowledge of both loads and material behavior is mandatory. With the change in operation diverse stress amplitudes and number of cycles have to be considered for the comparison with fatigue curves. Hydro power turbines are exposed to possible corrosion attack due to the surrounding water environment, which is a relevant impact on the fatigue curves. Therefore an overview of parameters that influence the corrosivity will be given. From an evaluation of a database that contains water quality data of worldwide hydro power projects, different water qualities are defined to perform fatigue tests with the influence of a corrosive medium. For selected water qualities, preliminary material test results will be presented.
The durability of plasma sprayed ceramic thermal barrier coatings subjected to cyclic thermal environments has been improved substantially by improving the strain tolerance of the ceramic structure and also by controlling the substrate temperature during the application of the coating. Improved strain tolerance was achieved by using ceramic structures with increased porosity, microcracking or segmentation. Plasma spraying on a controlled-temperature substrate also has been shown to improve durability by reducing harmful residual stresses. The most promising of the strain tolerant ceramic coatings have survived up to 6000 cycles of engine endurance testing with no coating or vane platform damage. In side-by-side engine tests, thermal barrier coatings have shown that they greatly reduce platform distress compared to conventionally coated vanes in addition to permitting reductions in cooling air and attendant increases in engine efficiency.
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