Advanced ultrasonic inspection methodologies for fitness-for-service (F.F.S) assessment of hydraulic turbines

This paper presents an ultrasonic inspection methodology to improve the fitness-for-service (F.F.S) assessment of hydraulic turbine runners after manufacturing or during the in-service inspection. The improvement proposed here is to apply ultrasonic array inspection techniques with an emphasis on the total focusing method (TFM) to produce data compatible with fitness-for-service methodologies. Conventional ultrasonic inspection methods based on good workmanship are mandatory for manufacturing, and in-service inspections are generally limited to surface methods such as penetrant or magnetic testing. Our previous work found serious limitations with conventional ultrasonic testing (UT) applied to the high-stress area located in the welded joint between the blade and the band. Undetected flaws will likely remain in a weld after fabrication, which can reduce the component’s service life. Our work is centered on a real turbine runner using various ultrasonic array configurations to characterize detected flaws left after fabrication. According to an evaluation of a Francis turbine runner, our results suggest that a dedicated TFM transducer with passive axis focusing and encoded inspection results in a higher detection rate, more accurate flaw definition, and more accurate sizing for the fitness-for-service (F.F.S) assessment of hydraulic turbine runners.