“Introduction:

As depicted in Fig, 1, a dual shaft gas turbine drives a propane compressor through a speed reduction gearbox. The gas generator consists of a 16 stage air compressor driven by a single stage turbine wheel at 5,100 rpm. The power turbine output is rated at 38,000 HP at a speed of 4,500 rpm. A diaphragm load coupling is used between the power turbine and the pinion of a single helical reduction gear set with a ratio of 1.5957:1. The bull gear output is coupled to a six stage horizontally split propane refrigeration compressor operating at 2,820 rpm with another non-lubricated diaphragm coupling. It is significant to note that this machinery train had over ten years of successful operating history.

As part of a plant expansion, this train was retrofitted with various upgrades, and routine maintenance ac¬tivities were performed during a scheduled overhaul. Gas turbine output was increased, reduced ratio gears were installed, and all of the gearbox bearings were replaced. In addition, the compressor seals and bear¬ings were replaced which required the removal and reinstallation of the coupling hub. Approximately one hour after a smooth startup, an event occurred that produced a short duration highly audible noise, and briefly drove the gearbox vibration monitors upscale. The machinery train continued in operation for an¬other five hours, and was then shutdown in an orderly manner for a scheduled gear tooth inspection.

The teeth on both pinion and bull gear were visibly surface scored, and the unit was blocked in for further inspections. It was subsequently determined that the diaphragm on the power turbine load coupling was distorted (buckled or wrinkled), and both hubs had spun on the low speed coupling between the bull gear and compressor. Clearly, a significant and undefined event had occurred. The following paper discusses the measurements and instrumentation applied during the analysis of this event, and offers recommendations for minimizing the potential for this type of failure event on other units.

Measurements and Instrumentation: The machinery train was instrumented with a full suite of shaft sensing proximity probes as shown in Fig. 1. The gas turbine and propane compressor radial probes are installed at +45° from vertical top dead center (TDC). As viewed from the turbine inlet, the probes at 450 to the left of TDC are designated as the Y-Axis sensors. The transducers mounted at 45° to the right of TDC are identified as the X-Axis probes.”