Diagnosis of reduction gear failures of a power generation gas turbine due to excessive pipe strain

Abstract

This paper presents a case study in diagnosing a frequent fatigue failure of a reduction gear used in power generation in oil and gas industry. The reduction gear was connected to a main oil pump (MOP) via a coupling which in turn draw oil from oil reservoir via a series of connecting pipes. An initial investigation conducted by the plant personnel found that the root cause of the frequent gear failure was caused by the recurring misalignments between the reduction gear and the MOP which had resulted in many unscheduled downtimes. Misalignment of the machines recurred times and again in just a short period of time right after the completion of machines alignment works. In view of the complexity of the problem, a vibration Operational Deflection Shape (ODS) analysis of the entire machine train was deployed to obtain a more comprehensive insight into the problem. Results of the ODS analysis revealed that the actual root cause of them is alignment could be traced to pipe strain or misalignment of the pipes that connected to the MOP. Pipe strain was evidenced from the excessive out of phase between the two-connecting pipe flange that routed to MOP. Pipe strain exerts excessive pulling force that caused misalignment to MOP and the entire machines train. The condition of misalignment aggravated as the X2forcing frequency of misalignment coincided with the natural frequency of the MOP itself. MOP resonance had thus triggered a feedback loop that enabled the exchange of vibration energy between resonance and pipe strain. In short, pipe strain had initiated misalignment in MOP and subsequently triggered MOP resonance that fed more vibration energy into the connecting pipe creating a much larger pipe strain. The vicious cycle continues until failures eventually occurred in the reduction gear that connected to the MOP. The remedial works undertaken was thus to eliminate the pipe strain by replacing a section of a rigid pipe with a flexible braided pipe. This enables the system to be more tolerable to pipe strain and also decouples pipe strain from triggering misalignment and resonance in MOP. The remedy measure had successfully resolved the frequent gear failures and also reduced the overall vibration level of the entire machine train by almost half.