Case study: Modal Analysis of a L-0 Blade

Stage: Large L-0 steam turbine

Issue: cracks occurred in the root of multiple blades after a relative short period of time in service

Objectives: Root cause analysis of cracking, interpretation of BVM data and evaluation of new blade design

Diagnosis: Cracking was attributed to high cycle fatigue (HCF) under broad band excitation

Solutions: Adopt OEM redesigned blade featured with additional damping elements and comply with the guidelines developed for BVM system

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Background

  • Cracks were found in the root of multiple blades after a relative short period of time in service.

  • TTI was requested to undertake a design evaluation on both original freestanding and new damped L-0 blade proposed by OEM.  

  • TTI was also tasked to verify the prescribed guideline by deriving the relationship between BVM tip displacements and dynamic stress at the crack sites.

Summary of Analyses

  • At-speed frequencies and mode shapes of the bladed-disk structure were calculated by FEA analysis. Results were used to correlate against BVM data and provide the basis to estimate actual dynamic stresses under non-synchronous vibration.

  • Critical locations with predicted peak dynamic stresses are consistent with field observation.  Critical vibrating modes due to broad band excitation were identified.

  • A series of CFD simulations were conducted to assess the performance for both original and new blades under different load conditions.

  • Design of the new blade with introduction of the damping elements was evaluated and verified by analysis.