Failure investigation of a large pump-turbine runner

A failure investigation was conducted on a high-pressure machine with a large pump-turbine runner that was 2.9 m in diameter and had a maximum discharge rate of 32 m3/s.A part of the runner broke off during operation. This released a piece of the crown, which went through the machine, causing further damage. An analysis of the broken runner revealed a fatigue problem, so the dynamic/vibratory behavior of the runner during machine operation was investigated to determine the cause.First, the excitation forces acting on the runner during operation were studied. The main excitation in pump-turbines is generated by the interference between the rotating blades and the stationary vanes, known as a rotor-stator interaction (RSI), which produces large pressure pulsations. The amplitude of the pressure pulsations was measured in the prototype using pressure transducers.Second, the modal response of the runner structure was analyzed. A finite element (FEM) model of the runner was developed and the main natural frequencies and associated mode shapes were identified.A dynamic analysis was then performed to determine the runner response. A harmonic excitation simulating the pressure pulsation of the RSI was applied to the numerical model of the runner. The results showed a large stress concentration in the T-joint between the blade and crown where the crack originated. Finally, the possible causes of the damage are discussed.

► We analyze an unusual fatigue failure in a large pump-turbine by the dynamic behavior.
► We simulate the main forces and structural dynamics using a FEM model.
► We determine deformations and locations with maximum stress during machine operation.
► The excitation frequency was close to a natural frequency and both had the same mode-shape.
► The radius of the T-joint where initial cracks were detected was modified.