3angles Non Destructive Testing
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Automated Inspection of generator Retaining Rings (AIRR)
Following a number of retaining ring failures in the 70’s and 80’s the industry has made significant strides in addressing maintenance and design issues concerning retaining rings. However, the mechanisms that were at fault thirty plus years ago are still in play. Although slowed by better material chemistry and a heightened awareness of maintenance issues rings can and have continued to fail. A combination of stress corrosion, high hoop stresses, and high cycle fatigue make up the failure scenario, with stress corrosion cracking (SCC) being the initiate.
For the inspection of generator retaining rings 3angles utilizes the Automated Inspection of Retaining Rings (AIRR) system. This system is equipped with ultrasonic (UT) and eddy current (ET) transducers. Based on the following scenarios an inspection plan will be developed to perform the most adequate inspection:
Mounted rings present unique challenges when it comes to their inspection. First is the character of incipient level SCC. In the initial phases of growth the individual cracks are below the threshold of detection in normal scanning planes and with standard ultrasonic transducers. Special techniques and/or transducers are required to optimize the test for a higher probability of detection. Second is materials in contact with the inside surface of the ring and the mounting contact with the field. These modify the acoustic impedance of the ID surface of the ring which in turn modifies the return test signal. Third is internal geometry, grooves and tapers that require scanning with different techniques to normalize the ultrasonic beam to the interrogation surface or plane. And fourth is the structure of some older rings that have large non homogeneous grain structures that can scatter and /or redirect the ultrasonic beam.