Dynamic overheating modeling of a super heater tube of waste heat boiler thermally coupled in ammonia oxidation reactor

Repeated failures have been observed in the outlet region of a few super heater tubes made of 1Cr–0.5Mo steel integrated in waste heat boiler thermally coupled in an ammonia oxidation reactor. The tube hardness measurements were carried out on the selected super heater tubes including those of ruptured, unruptured and headers. The relatively high value of hardness, 260 HV, of ruptured part in comparison to other parts indicates that some phase transformation probably occurred for such a low-alloy 1Cr–0.5Mo steel. A simple lumped dynamic mathematical model considering free convection and radiation was presented to show the temperature changes in case of shutdown of unit. It is found that the main cause of repeated failure of the super heater tubes were widespread sudden overheating and then rapid cooling of the super heater tubes due to sudden stoppage of operation (shutdown) and decreasing the thermal resistance of tubes. Shutdown of unit rooted in the blockage of steam flow inside connecting tubes which ended to tube overheating.