Inspection Management, a team within the RWE Group have been inspecting last-stage low pressure steam turbine blade root geometries with phased array ultrasound for over ten years on a range of blade designs, but have only encountered defective blades on rare occasions. This changed in 2013 and whilst the discoveries have helped to validate the capability/limitation of phased array ultrasound inspection method, they have also challenged pre-conceptions on the capability of other inspection methods. In all cases the blades were of a ‘fir-tree’ design, curved in the axial direction and known to have a risk of cracking. Defects were discovered within recommended inspection intervals for two-shifting plant but had developed quickly. Blades from more than one OEM, often containing multiple cracks have been identified during in-situ inspection and removed from service. Following removal from the rotor the blades have been surface inspected with magnetic particle inspection and also liquid penetrant inspection, prior to metallurgical assessment. The results presented in this paper show variation in the relative ability and merit of each NDT method to detect defects with application detail, crack location and blade type.
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