Using conventional technology, only the outer stages (the first stage and the last one or two stages) of industrial steam turbines are generally accessible to visual inspection. Signs of problems such as erosion, or native or foreign object damage in the intermediate stages can only be detected by other diagnostic means, such as vibration monitoring. This deficiency can make turbine diagnosis difficult in some circumstances. A remote visual inspection technology that can penetrate to the intermediate stages through access from the first or last stages would provide more complete visual coverage. In a recent inspection of a large cross-compound steam turbine at Detroit Edison’s St. Clair Power Plant, new minimally invasive inspection tools were used to locate the cause of rotor imbalance in the low pressure turbine. Attempts to find the problem using conventional borescopes had failed — they could not be inserted far enough into the intermediate stages to locate the problem. Using the modified inspection tools, a missing shroud band segment was discovered on the L-3 stage of the low pressure turbine. Conventional tools had been unable to reach further than the L-1 stage. The new tools, featuring stiffness in the axial direction and flexibility in the radial direction, were maneuvered through ten stages of turbomachinery and were used to locate the problem and evaluate collateral damage to other turbine stages.

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