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
Background
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Cracks were found in the root of multiple blades after a relative short period of time in service.
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TTI was requested to undertake a design evaluation on both original freestanding and new damped L-0 blade proposed by OEM.
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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
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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.
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Critical locations with predicted peak dynamic stresses are consistent with field observation. Critical vibrating modes due to broad band excitation were identified.
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A series of CFD simulations were conducted to assess the performance for both original and new blades under different load conditions.
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Design of the new blade with introduction of the damping elements was evaluated and verified by analysis.
