Failure assessment of boiler tubes under localized external erosion to support maintenance decisions

Abstract

Boiler tubes used in power plants and manufacturing industries are susceptible to numerous failures due to the harsh environment in which they operate, usually involving high temperature, pressure and erosive-corrosive environment. Among the wide range of failures associated with the tubes, localized external erosion is prevalent. In spite of efforts made over the years to solve this problem, localized erosion of boiler tubes continues to be a leading cause of tube leakages and unscheduled boiler outages in power plants and other utilities. There is, therefore, a need to approach this problem systematically and engage in rigorous studies that will allow improved management of this persistent problem. In this thesis, comprehensive studies were first carried out on modelled variants of localized external eroded boiler tubes with conceptualized flaw geometries, such as could be seen in real situations. The outcome of these investigations provided insights into the factors that influence the failure of these tubes while in use. The stress concentration, plasticity and flaw geometry all play critical roles in influencing the failure of tubes. Also, the failure pressures of the modelled tubes were analyzed in relation with several other failure criteria, to determine which failure criteria will be most suitable for the failure assessment of the localized tubes. Based on the result of the analysis, plastic strain in the range 5%-7% is recommended as a compromise between the extreme benchmark failure criterion of 20%, and the overly conservative 2%. The insights gained from the studies carried out on conceptualized variants of localized thinned tubes were extended to real localized external eroded tubes obtained from the industry and used to develop an improved and efficient failure assessment methodology framework for heat resistant seamless tubes while in service. This was done by treating the tubes as an inverse problem and using an optimization technique to obtain the flaw geometric properties of the tubes so as to effectively replicate them on the conceptualized geometries. Using two Material Properties Council (MPC) models generated based on the properties of the tubes as a function of their operating temperatures, comprehensive nonlinear finite element analyses (NLFEA) were conducted on the 160 finite element models. These tubes were assessed based on the maximum equivalent plastic strain and Von Mises stress produced at the deepest point of the flaw area within each of the tubes when subjected to their respective operating pressures at which they failed. The failure assessment outcome revealed that most of the heat resistant tubes while in service will remain intact and not fail if their remaining tube thicknesses were within (0.7 𝑡𝑚𝑖𝑛 to 𝑡𝑚𝑖𝑛), where 𝑡𝑚𝑖𝑛 is the minimum remaining thickness of the tube based on allowable stress. In addition, a 5% plastic strain ( 𝑃5%) and equivalent Von Mises stress criteria of 0.8 𝜎𝑢𝑡𝑠 were deduced as failure criteria to guard against the failure of these tubes while in service, and also avoid their early replacement. The developed methodology framework was checked and compared with the API-ASME FFS standard and found to be in good agreement with it, also more efficient and with reduced conservatism. Finally, sensitive studies were conducted based on the developed methodology to examine how the combination of the flaw geometry and material factors could possibly influence the failure of the tubes while in use. The study outcome shows that there were no appreciable changes in the normalized Von-Mises stress ratios and the plastic strain response for the normalized remaining thickness of the tubes. The proposed 𝑃5% and 0.8 𝜎𝑢𝑡𝑠 limits accurately predicted the failure for all the tubes and were reasonably safe limit for the tubes. Insights gained from the strain hardenability of the tubes studied will also provide guidance with taking proactive measures for the maintenance of the tubes. In summary, all the insights gained from this research and the developed failure assessment methodology framework will be helpful in categorizing the severity of localized external erosion on tubes while in use, and also support maintenance decisions on these critical assets. Keywords: Boiler tubes, localized external erosion, plastic deformation, stress concentration, flaw geometry, failure criteria, plastic strain, conceptualized finite element models, nonlinear finite-element analysis, equivalent Von Mises stress, API-ASME FFS Standard.