Analysis of multiple gas turbine failures due to high order (6e) resonant vibrations

Abstract

The results and conclusions are presented of a study concerning the durability problems experienced with gas turbine engines at a processing plant. The investigation encompassed the design and failure history of hot gas path components in the engines, focusing upon the power turbine (PT) blade failures responsible for repeated wrecks. Two of the wrecks followed shortly after replacement of bushings in the power turbine variable vanes. The root causes of these PT blade failures were resonant vibration, PT variable vane alignment and, in at least two instances, fabrication defects. Resonant blade vibration occurred in a sixth-order harmonic (6e) when operating at 4650 rpm, the mode shape being a six-nodal diameter coupled blade-disk bending of the blades predominantly in the axial direction. The dominant forcing function was due to misalignment of the 24 variable inlet vanes immediately upstream from the PT blades because of deviations in adjustable angle from the desired average setting. High amplitude vib...