Failure assessment methodology for boiler tubes with localized external erosion defects

Boiler tubes used in power plants and manufacturing industries are prone to failure due to the harsh environment they operate in, usually involving high temperature, pressure, and some erosive-corrosive mechanisms. Among the wide range of failures associated with the tubes, localized external erosion continues to be a leading cause of tube leakages and unscheduled boiler outages in power plants and other utilities. This paper lays the foundation to develop a rapid decision-making tool to help prioritize the maintenance, repair, or replacement of these tubes. A failure assessment methodology is proposed, based on the analysis of failed localized thinned tubes from a power plant. Minimal geometric measurements are available for each failed tube (width of flaw, length and flaw and minimal remaining thickness). Finite element models of idealized flaws are created that match the measured data for all the failed tubes. Finite element models with higher remaining wall thickness than the measured values are also constructed. Using two material models, comprehensive nonlinear finite element analyses are conducted on the 160 modeled flawed tubes. The flawed tubes are assessed using the API-ASME fitness-for-service assessment protocol to demonstrate how these flawed tubes would be ranked from most severe to least severe. Allowable wall loss for some of the considered flawed tubes ranges from 44% to 81% of the original wall thickness, indicating that fitness-for-service assessments can establish safe operation even for substantially eroded boiler tubes.