Abstract The purpose of this dissertation is the experimental and mathematical analysis of the phenomenon of failure of water wall tubes of a sub-critical power plant boiler and to investigate the main factors affecting it. For this purpose, after statistical eva‎luation of defects in the boiler and field visits of the combustion chamber, various researches were performed in two parts. In the first part, chemical control of boiler water, microstructural changes of tube metal and phase and elemental analyzes of deposit layers with the aim of finding the source of depositions and its effect on the failure mechanism, and studying the effect of height, from the combustion chamber floor, on the microstructural changes and sedimentation . In the second part, the functional temperature of the inner, middle and outer surfaces of the pipe and the joint surface of the pipe / oxide layer were calculated and compared by two methods of analysis and numerical simulation. For this purpose, combustion calculations to find heat flux, calculation of working fluid rotation coefficient, calculation of two-phase fluid heat transfer coefficient in boiler water pipes, analytical solution of heat transfer in pipe, simulation of two-phase water-steam flow inside boiler pipe using computational fluid dynamics Critical state calculations were performed on the pipes, which could lead to overheating and burning of the pipes. The interaction between increasing the pipe temperature and the growth kinetics of sediment in the state without corrosion and in the state of corrosion on the inner surface of the pipe was investigated. Finally, stresses were simulated in hot water pipes in hot condition and in corrosion and non-corrosion conditions. Since the boiler was a gas burner, the corrosion caused by the combustion products on the outer surface of the pipes was very small and negligible. This study showed that increasing the thickness of oxide / sediment layer and boiler rotation coefficient has a significant effect on sediment growth kinetics and increasing pipe temperature, so that increasing the rotation coefficient by 8.5% reduces pipe temperature and sediment thickness by 30 and 70%, respectively. It turns. In the case of only sedimentation, the behavior of Hopi, axial, von Mises and strain stresses was linear with time, but in the case of corrosion in addition to sedimentation, the behavior of these parameters became nonlinear. On the other hand, mechanical properties tests showed that the relative reduction of these properties at higher altitudes than the furnace floor could be due to microstructural changes due to the interaction of increasing the metal temperature of the pipe and deposition at higher altitudes of the combustion chamber. Adaptation of the stress rupture test results to the stress-Larson-Miller parameter diagram also showed a decrease in the creep properties of the pipe metal. This was consistent with microstructural observations. The results of this study showed that the failure of water wall pipes in the boiler under investigation was due to accelerated creep due to corrosion and the two-way effect of increasing temperature and high deposition.

احمد كرمانپور، عبدالمجيد اسلامي

عباس بهرامي، ابوذر طاهري زاده، احمد كيواني