Design against fatigue of Francis runners

The major threat to the structural integrity of runners for Francis type hydro power turbines is development of fatigue cracks initiated at defects in conjunction with welds connecting the runner vanes to the crown and band. The fatigue loading is typically caused by the pressure pulsation appearing in the wakes behind the guide vanes, thus a typical high cycle fatigue loading. A typical Francis turbine with 26 guide vanes and a rotational speed of 167 rpm is exposed to 260,520 cycles per hour. With the known crack growth rates of the steels normally used in Francis runners, stable crack growth under such loading circumstances would cause fatal failure after only a very short time of operation. The fatigue resistance of Francis runners is therefore determined by the level of the threshold for when fatigue cracks start. The important design criterion is thus the dynamic stress level that can be sustained given a certain weld defect, and fracture mechanics criteria should be used. This paper describes the structural design process of Francis runners at Kvaerner that features optimized choice of runner geometry and production methods.