Who Won the Race to Fracture – Torque Tube or Air Separator?

Abstract A gas turbine failed catastrophically at a process plant during operation. The failure resulted in the torque tube and air separator breaking into multiple pieces. The bolts connecting the torque tube to the air separator, the bolts connecting the torque tube to the turbine discs and the bolts connecting the torque tube to the compressor section also failed. A detailed metallurgical root cause failure analysis was performed to understand what happened. The analysis included reconstruction of the torque tube and air separator from the broken pieces, and evaluation of all fracture surfaces. Based on the patterns seen on the fracture surfaces, the fracture mechanisms and crack propagation paths were determined for the torque tube and the air separator. Mechanical properties were tested using specimens extracted from the broken pieces. Microstructural evaluation was also performed. Similar analyses were performed on the broken bolts. Based on the fracture surface evaluation of the bolts, the bolt fractures were believed to be collateral damage. Multiple preexisting cracks and a material defect were found in the torque tube. Fracture had propagated from one of these preexisting cracks in the torque tube. The question was if the torque tube fractured first, or the air separator fractured first.