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Failure of carbon steel boiler tubes from waterside has been reported in the utility boilers and industrial boilers for a long time. In industrial boilers, most waterside tube cracks are found near heavy attachment welds on the outer surface and are typically blunt, with multiple bulbous features indicating a discontinuous growth. These types of tube failures are typically referred to as stress assisted corrosion (SAC). For recovery boilers in the pulp and paper industry, these failures are particularly important as any water leak inside the furnace can potentially lead to smelt-water explosion. Previous research shows that metal properties, environmental variables, and stress conditions are the major factors influencing SAC crack initiation and propagation in carbon steel boiler tubes. A significant volume of work has also been published to show that the use of carbon steel in high temperature water applications strongly depends upon the formation and stability of a protective magnetite oxide film on the waterside of boiler tubes. This present study is aimed at evaluating above stated variables individually and interactively to identify SAC crack initiation and crack propagation behavior in carbon steel boiler tubes. Other goal of this research is to understand the mechanism of bulbous SAC crack formation under industrial boiler operating conditions, thus to figure out a practical way to predict and prevent SAC type failures in the industrial boilers.

Made available in DSpace on 2016-06-02T19:51:48Z (GMT). No. of bitstreams: 1 3491.pdf: 1251165 bytes, checksum: 650dd47d0fec8bae05ba378f445f91dc (MD5) Previous issue date: 2011-02-11 / Universidade Federal de Sao Carlos / The aim of this work was to develop and apply a mathematical optimization model to support some key decisions in the steam production operation system with multiple industrial boilers. In the food industry, key decisions into this system are, normally, taken based on people experience, not through decision support tools based on operations research. The proposed model approaches the fuel replenishment decisions, fuel transportation and fuel inventory management. The model also determines the start-up and shutdown moments for each boiler, as well as, which of them will produce steam. An algebraic modeling language was used to implement computationally the model, which was solved by an optimization software. The adjustments and model validation were performed through a case study carried out in a large scale food industry localized in Goiania (GO). In computational experiments conducted with company s data, the results obtained by the model showed better economic performance than those from the company. In face of the good outcomes obtained by applying the model and the lack of protocols or decision tools for the studied system, this model presents itself as a feasible alternative to be considered to support some key-decisions in the steam production system with industrial boilers. / O objetivo deste trabalho foi desenvolver e aplicar um modelo matemático de otimização para apoiar algumas das principais decisões na operação de produção de vapor em múltiplas caldeiras industriais. Em indústrias de alimentos, essas decisões, em geral, são tomadas com base na experiência dos envolvidos e não com ferramentas de auxílio à tomada de decisão apoiadas pela pesquisa operacional. O modelo proposto contempla as decisões de aquisições de combustíveis dos fornecedores, o transporte para fábrica e o gerenciamento dos estoques dos combustíveis adquiridos no sistema. O modelo também determina os momentos de acionamento e desligamento das caldeiras, bem como em quais delas haverá produção de vapor. Uma linguagem de modelagem algébrica foi utilizada para implementar computacionalmente o modelo, que foi resolvido por meio de um software de otimização. Os ajustes e a validação do modelo ocorreram por meio de um estudo de caso realizado em uma indústria de alimentos de grande porte localizada em Goiânia (GO). Nos experimentos realizados com os dados fornecidos pela empresa, os resultados obtidos com o modelo apresentaram melhores desempenhos econômicos do que aqueles praticados pela operação real. Diante dos bons resultados alcançados na aplicação do modelo em situações reais e da ausência de protocolos ou ferramentas para apoiar as decisões no sistema estudado, o modelo proposto apresenta-se como uma alternativa viável para apoiar algumas das principais decisões na operação de produção de vapor em caldeiras industriais.