Experimental study on stress characteristics of Francis turbines under multiple operating conditions

Abstract

[Objective] The study aims to investigate the dynamic stress characteristics and fatigue damage mechanisms of turbine runner blades under complex operating conditions. [Methods] Based on field test data, Fourier transform(FT) technology was used to perform time-domain and frequency-domain analysis of stress signals at key parts(such as blade root and outlet edge) of the Francis turbine runner. The stress response characteristics of these key parts of blades were systematically investigated during variable load, no-load, and start-stop transient processes. [Results] The result showed that under variable load conditions, the maximum static stresses at measurement points 7FSG12 and 13FSG1 reached 27.56 MPa and 25.99 MPa, respectively, while the maximum dynamic stresses at measurement points 13FSG2 and 13FSG14 were 13.19 MPa and 13.04 MPa, respectively. Furthermore, the dynamic stress distribution at the junction of the blade and the lower ring showed a decreasing trend from the inlet edge to the outlet edge, specifically manifested as 7FSG10 > 7FSG8 > 7FSG6. Under the 320 MW condition, the dynamic and static stresses at most measurement points were lower than those under the 290 MW and 300 MW conditions. [Conclusion] The result show that significant stress concentration is observed in the connection zone between the runner blade and the upper crown(especially near the outlet edge), which is highly consistent with the actual crack initiation zone. It is recommended to add a stress-reduction triangular block to improve stress distribution. The intensity of the rotor-stator interaction effect decreases with increasing distance from the blade-free zone. The unit is capable of short-term safe power overgeneration under the condition of 320 MW over-rated output. However, it should be noted that the alternating stress amplitude on the blade under start-stop and no-load conditions is significantly higher than that under variable load conditions, and the fatigue damage risk increases sharply with the frequency of start-stop cycles. Therefore, to improve the structural reliability and operational safety of hydraulic units, it is recommended to optimize the operational strategies to minimize unnecessary start-stop operations.

Keywords

francis turbine / runner blade stress / fatigue crack / time-frequency domain analysis / field test / rotor-stator interaction / runner blade / influencing factor