Case study on the integrity and nondestructive inspection of flux-cored arc welded joints of Francis turbine runners

The welded joints of hydraulic turbine runners have a higher failure probability than other runner regions due to the inevitable presence of welding flaws and dynamic stress concentrations. Welding processes are always associated with flaws caused by technical or metallurgical factors. In order to lower the failure probability, critical welding flaws should be detected and removed before the runner commissioning; otherwise, they will propagate under dynamic stresses and endanger the structure before it reaches its expected life. In such a case, the electricity production process should be halted and a mandatory unplanned maintenance should be applied. The production halt and the maintenance process (or runner replacement) impose high costs to the utility owner. A remedy to this is to have prior knowledge of potential welding flaws so as to select proper nondestructive testing (NDT) methods to detect and characterize them. In addition, better characterization of the potential flaws and their dimensions leads to more precise life estimations. In this study, different NDT methods are used on a series of T-joints designed to be representative in scale of real turbine runner joints. The outcomes of these methods are compared and used to characterize some of the detected indications. In order to confirm the NDT results, part of the joint containing a targeted indication has been extracted from the T-joints to undergo detailed evaluations. The objective is to provide detailed information on some common welding flaws in hydraulic turbine runners and to discuss the capability of different NDT methods used to characterize such flaws.