The thermal history and stress state of a fresh steam-pipeline influencing its remaining service life
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The service life of thick-walled power plant components exposed to creep, as is the case with pipelines of fresh- and re-heated steam, depend on the exhaustion rate of the material. Plant operation at elevated temperatures and at temperatures below designed temperatures all relates to the material exhaustion rate, thus complicating remaining life assessment, whereas the operating temperature variation is a most common cause in the mismatching of real service- and design life. Apart from temperature, the tube wall stress is a significant variable for remaining life assessment, whose calculation depends on the selected procedure, due to the complex pipeline configuration. In this paper, a remaining life assessment is performed according to the Larson-Miller parametric relation for a ?324?36 pipe bend element of a fresh steam-pipeline, made of steel class 1Cr0.3Mo0.25V, after 160 000 hours of operation. The temperature history of the pipeline, altogether with the pipe bend, is determined based on continuous temperature monitoring records. Compared results of remaining life assessment are displayed for monitored temperature records and for designed operating temperature in the same time period. The stress calculation in the pipe bend wall is performed by three methods that are usually applied so to emphasize the differences in the obtained results of remaining life assessment.Keywords:
Service life
Operating temperature
The creep behavior of metallic materials is frequently studied using creep tests carried out under constant load and constant temperature conditions. This technical note presents a general equation that precisely describes the creep-time relationship in such conditions in a succinct format. The equation allows all three classical stages of creep, primary, secondary, and tertiary, to be represented. It expresses creep strain explicitly in terms of real time in a single smooth, continuous curve. It uses six parameters; each independently describes a physical feature of the creep curves. The equation is useful for characterizing experimental results as well as for developing a creep model that can be incorporated into analysis models. Comparison with experimental results shows that the equation closely fits the creep curves of steel at various temperatures and stress levels.
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Creep life prediction is very important to maintain safe operation of thermal power plants for a long-term period. In this study, an alternative creep life prediction method was proposed based on a relationship between creep exponent n and creep coefficient k. A high temperature indentation creep test was conducted to identify the creep exponent and creep coefficient for each specimen with different creep damage ratio. Subsequently, the relationship between the creep coefficient and creep exponent for each creep damage ratio was focused on. As a result, it was confirmed that a new parameter B introduced based on both creep coefficient and creep exponent linearly increase as the creep damage ratio increases. Furthermore, the B is uniquely determined by the creep damage ratio regardless of the temperature and stress conditions. These results indicate that the creep life can be predicted with a high accuracy using the B.
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