Abstract
Tube overheating may cause tube failure resulting in an unscheduled boiler shutdown that may interrupt plant operation. The impact of this problem is not only due to the cost of replacing defective parts but also due to the frequent need of system shutdown and the possible imminent safety hazards. This paper provides an investigation of the influence of rapid rise in steam flow rate (swing rate) on the thermal and flow characteristics of the riser tubes in natural circulation water tube boilers. A thermal model for the prediction of possible tube overheating was developed. The developed model incorporates a nonlinear state space dynamic model which captures the important physical interactions of the main variables of steam generation in drum boilers. The system under consideration includes the drum, the riser, and downcomer as its major components. A numerical scheme for the solution of the governing differential equations was established. The dynamic response of the system's state variables due to rapid rises in steam flow rate was investigated. The results show that the rapid rise in the steam flow rate results in decrease in the pressure and an initial increase in the steam quality which is followed by a decrease in the steam quality. The riser temperature increases partly due to the increase in the steam temperature and partly due to the dynamic influence resulting from a decrease in the heat transfer coefficient. The present calculations of the water level in the drum provide good comparison with those in the literature.