Abstract
Cracking of 1st stage nozzles occurs routinely in industrial gas turbines. Such cracking, which sometimes is observed after only a few operating cycles, leads to short intervals between overhauls and expensive repairs or replacements of the nozzle sections. Despite the enormous economic costs associated with nozzle cracking; mechanisms, root causes, and optimum mitigation of such cracking has not been convincingly determined. In fact, some original equipment manufacturers (OEM) consider nozzle cracking as a “fact of life” and have only issued recommendations for their monitoring and replacements. The objective of this work was to identify the mechanisms and the root causes for the 1st stage nozzle cracking of a Frame-3 gas turbine. In this project, we measured, monitored, modeled, and analyzed the temperature and stress response of the nozzles during actual operating practices. This work concluded that the primary mechanism of such cracking is low cycle fatigue caused by high cyclic stresses. The major source of these high stresses was determined to be related to thermal and pressure shocks caused by transonic events which occur during the unit startups. This work offers recommendations to mitigate such cracking and a testing program to verify the root cause.
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