On the performance of nondestructive testing methods in the hydroelectric turbine industry

Abstract

Welded joints of turbine runners are one of the most critical parts of Francis turbines due to the presence of welding discontinuity and high stress. Because of thermal cycles, solidification, cooling distortion and residual stresses, welded joints always include discontinuities of different types and sizes. Some specific parameters will limit welding flaw dimensions in some or all direction based on the joint geometry, material and welding procedure. If discontinuities of critical size remain undetected, fatigue cracks might initiate and propagate in these zones because of dynamic in-service stresses leading to high repair costs and long down times. Therefore, reliable NDT methods and good knowledge of the probability of occurrence of welding flaws is important for fatigue life estimations. Every NDT method has its weaknesses; therefore, even after meticulous inspections it is likely for some discontinuities of critical sizes to remain in the welded joint. Our objective is to clarify the probability of detection and occurrence of different types of welding flaws in hydroelectric turbine runners. Furthermore, an overview of current nondestructive inspection methods and their capability in characterizing flaw dimensions will be discussed. Finally, advanced NDT techniques, for the characterization of welded joints integrity, will be proposed.