Failure analysis and modeling of super heater tubes of a waste heat boiler thermally coupled in ammonia oxidation reactor

Abstract

Repeated failures have been observed in the outlet region of a few super heater tubes made of 1Cr–0.5Mo steel integrated in a waste heat boiler thermally coupled in an ammonia oxidation reactor. A case study of a failed tube running at around 37,440h was presented. The bulging of some parts of super heater tubes were observed while the tubes were under normal operational temperature of 480°C and internal pressure of 44bar. 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, 260HV, of ruptured part compared to other parts indicates that some phase transformation probably occurred for such a low-alloy 1Cr–0.5Mo steel which led to lower thermal resistance. The dynamic modeling including conservation of energy has been developed in order to show the tube overheating. The main root cause of repeated failures of the super heater tubes was short-term overheating following rapid cooling of the tubes due to sudden stoppage of steam flow inside connecting tubes in the event of unit shut down which reduced the thermal resistance of the tubes over time.