Stress measurement of runner blade for a Francis prototype turbine

Fatigue crack on the runner blade is one of the severe threats to the operation safety of prototype turbine. Several factor, such as limited operation range, may lead to fatigue cracking commonly observed on Francis turbine. Several cases show that fatigue crack may reduce the working life of runner. The high stress during operation is one of main reasons to produce cracks. Hydraulic design, manufacture quality, deviation of operation condition and resonance may cause high stress on the runner blade. Because static and dynamic stress cannot be predicted with absolute certainty for all operating conditions, the most effective way to obtain the actual stress level of the blade is site measurement. In this paper, stress measurement is carried out on a Francis prototype turbine. Measurement results show that high static stress appears at a load range near full load, and high dynamic stress appears at lower loads. Finally, some practicable suggestions are proposed to reduce the stress of runner blade.