[April 2008] Generator rotor end rings are highly stressed, safety critical plant items found in power stations throughout the world. Inspection of these items has traditionally been difficult and has required removal of the rotor from its stator.
There has been a global history of end ring failures due to stress corrosion cracking, with crack initiation from internal ring bores. These bores are not accessible to high integrity surface inspection without removal of generator rotor from stator and subsequent removal of end ring.
To develop a high integrity end ring inspection that can be carried out with generator rotors both ‘in-situ’ and out of stator housings. This eliminates the high expense and risk associated with removing end rings from rotors, while overcoming the ultrasonically attenuative nature of the end ring material.
We have developed a bespoke computerised ultrasonic inspection process linking powerful data acquisition and analysis software with a purpose designed scanner for fast, accurate, end ring inspection results either in situ or ex-situ of stator housings. The equipment includes a low profile scanner designed to fits between the end ring and the stator, the purpose written software records data displayed multiple formats enabling ‘focussed’ analysis on critical shrink fit areas. We also use multiple probe arrays enabling a number of simultaneous pulse echoes and time of flight diffraction scans. Permanent electronic records provide an auditable fingerprints and the system has been verified on sample end rings with known defects. There is a significantly reduced risk of plant damage (five generator rotors were ‘dropped’ worldwide in 2001) and we have benefited from significant time/cost savings in subsequent monitoring programs.