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• Geometric functions to enable the accurate modelling of boiler tubes with localized erosion flaws were developed. • Effect of the flaw geometries on the stress concentrations and failure pressures of the tubes were investigated. • Failure of the tubes is insensitive to the choice of flaw shape, as long as the minimum remaining tube thickness is matched. • For various failure criteria thresholds, failure assessment of flawed tubes is fairly insensitive to their flaw shapes. Localized erosion is one of the most common failure mechanisms associated with boiler tubes, driven by impaction of the tube surface by fly ash, soot blowing steam, falling slag or other abrasive substances from the boiler's combustion chamber. The tubes may experience significant localized reduction in their wall thickness, becoming susceptible to plastic collapse and bursting. The replacement of failed tubes is one of the leading causes of unplanned and forced boiler outages in process and power plants. In this study, geometric functions to enable the accurate modelling of boiler tubes with localized erosion flaws were developed from conceptualized models and finite element analyses were conducted on the modelled flawed tubes. The effect of geometry on the stress concentration in the tubes and on the failure pressure associated with the tubes was investigated. Linear elastic stress analysis is sensitive to the flaw geometry – for a representative problem stresses vary 38% as the flaw shape is varied. However, nonlinear elastic-plastic analysis shows significantly reduced sensitivity to flaw shape. The collapse pressure of the same representative problem now varies only 2.45% as the flaw shape is varied. This finding was demonstrated for various failure thresholds sourced from the literature, based either on plastic strain or Von Mises equivalent stress, which demonstrates that the failure of the tubes is insensitive to the specific choice of shape parameterization, as long as the minimum remaining wall thickness is matched. The outcome of these investigations gives further insight into the behaviour of tubes with localized external flaws while in service. [ABSTRACT FROM AUTHOR]

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. • Proposed a rapid decision-making tool to prioritize the repair or replacement of boiler tubes under localized erosion. • Failure assessment methodology for localized thinned tubes when minimal geometric measurements are available is demonstrated. • Failure criteria for boiler tubes under localized erosion are proposed based on analysis of failed localized thinned tubes. • API-ASME fitness-for-service standard is used to confirm how flawed tubes could be ranked from most severe to least severe. • Fitness-for-service assessment can establish safe operation even for substantially eroded boiler tubes. [ABSTRACT FROM AUTHOR]