Failure analysis of first stage nozzle in a heavy-duty gas turbine

First stage nozzles are considered as one of the most critical components in the heavy-duty gas turbines because of their operational in-service condition in the hot gas path. In the present research, the failure analysis of a first stage nozzle in a 159 MW gas turbine is investigated in order to specify the modes and mechanisms of the failure and their root causes. Various examinations including macroscopic and visual inspection of the nozzle outer surface, the micro examination of the fracture surface, chemical analysis and metallographic analysis were carried out to complete the investigation. The micro and macro examinations revealed cracks, erosion and oxidation damages in the nozzle investigated. These failures were caused by overheating and improper filtration of the turbine inputs which activated the thermal fatigue failure mode. Finally, in order to estimate the nozzle life and predict the failure occurrence, a modified model is developed using Robinsons rule and its extension for total damages of first stage nozzle based on statistical data gathered in a periodic inspection conducted during operation.