Implementation of ultrasonic immersion technique for babbitt metal debonding in turbine bearing

This study was aimed at the implementation of an ultrasonic method to assess the reliability of turbine bearings. Modified ultrasonic immersion technique was used in carrying out both laboratory experiment and field application. From studies conducted in the laboratory, we confirmed by C-Scan that the condition of the interface layer between the babbitt and base metal can be monitored. The C-scan image in the ultrasonic immersion test can be used successfully to observe the condition of interface layer. The testing, using a focused transducer, provided a promising approach for estimating the extent of the damaged region and observing the interface layer effectively. The difference of the ultrasonic reflection ratio between the bonding and debonding area at the interface layer is one of the key parameters for assessing the extent of the damaged area; additionally, the reflection amplitude exhibits a favorable correlation with the overall damage level. In this study, a developed technique was applied to inspect the turbine bearings at several power plants in Korea whereby the applicability in the field can be ascertained. The ultrasonic testing was analyzed to obtain key parameters such as amplitude and the position of transducer at the damaged interface layer. The amplitude and positional data were integrated and controlled by a PC-based ultrasonic equipment. The debonding size and area in C-scan image was calculated automatically by labeling and clustering algorithm, of which results showed a good correlation to the condition of actual defects. C-scan images provided clear results to end users, which then can be used to judge the reliability of the bearing.