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
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.
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.