Risk Identification
Known crashes on Fr 5, 7FA, 9FA plus casing damages on Fr 5, 7EA & 6B turbines.
AVOID CATESTROPHIC DAMAGE
Risk Factors
The Basic Cause is an apparent aerodynamic alternating force that can create relative movement ( e.g. fretting) of the vane bases with each other and within the casing. The risk is seemingly dependent on a variety of factors including:
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Position of the vanes relative to the horizontal joint….those with likely the least amount of inter-base friction are most commonly damaged
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Flow disturbances…proximity to extractions, FA rotor vane clocking, off-design compressor flows such has cold temperature, power augmentation and on the low flow side; part load operation with high IGV's angles used in DLN systems
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Casing cracks in vane base retention area starting at horizontal joint indicate potentially higher than expected stresses…potentially combined with thermal cycling (Starts effect)
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Shim protrusions and base tilt magnify flow disturbance exposure
Always check for vane looseness using vane tilt check when casings are removed!
Tolerances between vane bases and casing fits as well as vane base compression amongst stages may explain variations between units. Shim loosening accelerates risks and likelihood exposing the rotor vanes. Frictional reduction over time seems to suggest potential for Operating Hours and Starts effect. Starts also force the vanes through a resonance more frequently.
Shim Impact?
