Case study: Design Evaluation of a L-0 Blade

Stage: L-0 stage in a combined cycle LP steam turbine

Issue: Blade failure with the crack initiated at the trailing edge near hub 

Objectives: Design evaluation and root cause analysis using FEA

Diagnosis: Water droplet erosion under partial load operation

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Background

  • A L-0 blade failure occurred during an over-speed and trip operation.  Numerous L-0 blades experienced similar cracking on the airfoil trailing edge.

  • Finite element analysis (FEA) was used to examine the structure characteristics, such as frequencies, steady and dynamic stresses.

  • FEA was also applied to perform a fracture mechanics analysis to predict the critical crack size under different operating speeds.

  • Computational fluid dynamics (CFD) was employed to study the effect of load conditions on the flow patterns and its association with erosion.   

Summary of Analyses

  • The analytical results and metallurgical evidence both support a scenario where sustained and significant erosion first weakened the material and introduced stress risers along the trailing edge.

  • The initial cracks were then propagated by HCF (high cycle fatigue) most likely induced by a resonant or near resonant condition of a lower fundamental mode vibration.

  • CFD simulation identified that flow re-circulation occurring approximate to the trailing edge was the potential source for TE erosion.

  • Recommendations include: 1) Limiting number of partial load operations to prevent droplet erosion on trailing edge; 2) Blending damaged trailing edge.

  • The predicted critical crack size appears to be consistent with the crack dimension observed from the metallurgical examination.