International competition in the field of power generation is increasing and customers are demanding economic and efficient power plants. In the long term, continuous power plant availability can only be guaranteed through an effective mode of operation in conjunction with a systematic maintenance and inspection concept. Apart from boiler, steam piping and valves, the rotating components of the turbine/ generator (turbine and generator rotor) also belong to the most highly stressed components in a power plant. Loads result for example from operating parameters, the mode of operation of the machinery, start-up processes, thermal stresses, prestressing, residual stresses from the manufacturing process, as well as loading from the centrifugal forces acting on the rotating components. During scheduled outages, highly-stressed components are subjected to non-destructive testing designed to reliably detect any possible service-induced damage (e.g. cracking) before this can lead to failure of a component and severe consequential damage. For example, damage to a blade in the low-pressure turbine of a South African power plant (600 MW) in January 2003 resulted in the entire turbine generator unit being destroyed. Quite apart from the risk to personal health, such damage can lead to unscheduled outages and plant downtime, as well as unplanned costs for expensive repair and maintenance work on the turbine/generator. In comparison to these risks, the cost of inspecting such highly-stressed components is easily justified, as is the need for reliable and qualified techniques in the field of non-destructive testing.
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