Aerodynamic Instability Effects on Compressor Blade Failure: A Root Cause Failure Analysis

Diagnosing the root cause of compressor blade failures by high cycle fatigue (HCF) is elusive for gas turbines with a long history of successful operation; long operating times preclude design, fabrication, and material defect issues that are usually associated with short term failures. Long exposure time might implicate erosion or corrosion issues for compressor failures. An investigation of 3^rd stage stator vane and 4^th stage rotor blade failure in a Frame 7 gas turbine revealed that aerodynamic excitations associated with mild compressor instability (undetected by installed sensors) was the most probable cause. A Blade Vibration Audit (BVA) approach exploits information collected and compared from independent sources: 1) Fracture mode details and expected failure stress levels estimated in metallurgical examination; 2) Aerodynamic excitations due to internal airfoil wakes, rotating stall, and flutter applied to vibratory stress response data obtained by modal testing to estimate relative operating stress levels; 3) Design margins deduced from successful operating experience, which establish a base line for comparison with the excitation sources considered likely. Diagnosis of HCF failure causes must produce results that match observations of the specific failures in three ways: 1) The airfoils must be in resonance for sufficient time for fatigue to occur, 2) The vibratory stress must be greater at the failure location than elsewhere, and 3) The causal effect must significantly increase the stress at the failure location to be more than elsewhere. The analysis showed that the 3^rd stator vane failed at 2/3 span, which led to 4^th blade failure due to subsequent adverse aerodynamic excitation and impact damage. The most likely cause of the 3^rd stator vane failure was a combination of resonant excitation from excessive wakes of the downstream rotor blades and flutter associated with mild intermittent surge.