Due to the increased practice of load following, non-synchronous vibration has become a more common source of blade fatigue. TTI has used its blade modeling expertise to review, interpret and verify criteria set by operators for early warning of potential blade fatigue-failures.
Our BVM data analysis has ranged from turbine and compressor freestanding designs to blade groups with cover-bands.
Stalled flutter, unstalled flutter and broad band excitation are all non-resonant conditions that are difficult to predict, but are readily detectable. While monitoring blade tip displacements is the most effective way to detect and prevent high cycle fatigue failures where a non-synchronous source of vibration is suspected, not all detected responses are necessarily life threatening.
The maximum allowed vibration used to set alarms or trips is not a function of tip displacement alone. A higher than normal tip displacement is often not severe enough to produce a fatigue failure, even if the duration of the event is prolonged, or the event is experienced multiple times. Conversely, where the vibration is severe, even limited exposure can compromise the fatigue life of the blades within a relatively short period of time.
Using a dynamic model of the monitored row, recorded tip displacements can be converted to a vibratory stress at any location on the blade. The true measure of blade life (accumulated fatigue) is then projected based on the frequency of vibration, mode shape, magnitude of tip displacement and duration of the transient event.