Power stations are required to be designed and maintained to operate for long periods reliably and safely, ideally with the minimum need to close the plant down for servicing, inspection or other reasons. Typically the plant is licensed to operate for a period of 4 years until a major refurbishment and inspection outage is needed. However, essential is up-to-date recording of the remaining life of at least the high-risk areas of the plant. Monitoring these when the plant is in operation is not always possible or reliable and alternative methods are often required. One important need is to be able to assess the remaining life of components, particularly those identified as high risk. The network and orientation of the steam pipelines can make access for inspection and assessment purposes extremely difficult. The main steam pipes operate, for example, at pressures of 180 bar and temperatures of 568℃ and require substantial lagging. The dimensions of the main steam pipes are typically of bore 240 mm and outside diameter 360 mm. The main threat to the integrity of the main steam pipe parent material is due to creep life exhaustion. Fortunately from design studies and operational experience of pipe degradation rates and failures, good information is available as to parts of the pipe system that need to be monitored to obtain reliable data on the remaining life of the pipes. Also known is that a good and feasible monitoring method to reveal the onset of failure processes is by the measurement of the increases in micro-strain generated in the outer skin of the pipe material, and this measurement can be made when the plant has been shut down during an outage. There are several methods to measure the change in micro-strain at designated points. Preferred, are more robust sensors that are little affected by the build up of scale on pipe surfaces and enable direct on-site physical measurements using optical methods.
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