Abstract
In order to understand the failures in some waterwall tubes of supercritical steam boilers, an analysis is required to estimate the temperature of the waterwall tubes and the oxide growth in these tubes. A review on the thermophysical properties of oxide grown in waterwall tubes was conducted. Specimens of waterwall tubes associated with thermal fatigue cracking were obtained and microstructural analyses, using metallographic and electron-optical techniques such as SEM and electron-probe microanalysis, were performed. A computer model is being developed to estimate the maximum temperature in waterwall tubes by considering several phenomena that take place in supercritical units. The comprehensive model includes the following boiler operation features and phenomena: oxide growth, distribution of the heat flux on the waterwall tube as a function of height distance, variation of steam flow rate due to load variation, variation of heat flux due to load variation, and variation of the heat transfer coefficient with steam conditions and furnace heat flux. The model will handle the transition subcritical-to-supercritical and will account for the heat transfer deterioration phenomena in tubes. The tube regions with deteriorated heat transfer regimes will be identified. The temperatures of steam, metal, and oxide will be obtained as a function of the height distance in the boiler. The mixing effects of the steam from different waterwall tubes and subsequent fluid dynamics effects on the heat transfer will be considered. The new model will enable the prediction of the maximum metal temperature for realistic boiler operation schedule, which include transitions from full-to-low loads. Based on the estimated data for the steam temperature, metal temperature, and oxide thickness, regions of the waterwall tubes that are exposed to the most severe conditions will be identified.
