Case study: Detuning and Re-design of Steam Turbine Blades

Stage: Stage 15 and 16 in a steam turbine with an earlier design

Issue: Crack and rupture found at the first hook of straddle mounted root

Objectives: Root cause analysis and blade re-design

Diagnosis: High cycle fatigue (HCF) due to resonance of synchronous vibration was identified as the driver

Solutions: Modification of airfoil profile guided by analysis to de-tune the frequency 

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Background

  • Cracks and rupture were found at the first hook of straddle mounted root after a relative short period of time in service.

  • TTI was solicited to perform modal analyses and identify potential resonant modes of vibration.

  • The main objective of the study was to evaluate various de-tuning strategies and optimize configurations to mitigate the resonance issue for both stages.

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Summary of Analyses

  • At-speed frequencies and mode shapes of the bladed-disk structure were calculated by FEA analysis. Interference diagrams were established to identify potential resonant modes.

  • Several options of de-tuning strategies including re-profiling airfoil,  re-grouping and use of different shroud material were examined.

  • Parametric studies were performed to achieve an acceptable solution in regards to margins to the resonant frequencies and level of steady stresses.

  • Increase of fillet radii was also studied to prolong fatigue life.

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