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
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.