Failure assessment of ASTM A213-T12 superheater boiler tubes in a natural gas liquid plant

This paper presents the results of an investigation into the failure of steam boiler tubes in a liquefied gas treatment plant. The failure appeared in the form of a window rupture in the boiler tubes of 1.25Cr-0.5 Mo grade steel. Study revealed that a thick magnetite iron oxide layer was formed in the inner surface of these tubes, disturbing the heat flow and rising the temperature of the tubes. Consequently, the internal oxide layer led to a series of metallurgical degradation and the loss in mechanical strength of the boiler tubes. Failure analysis of the steam tubes was carried out by optical and scanning electron microscopy examinations, chemical analysis of the tube material and the results showed that the scales were formed in the inner surface of the tubes. Furthermore, the rise in operational temperature and its distribution were simulated using fluent software and the results were compared against the designed temperature of the tubes. The simulation revealed that the operating temperature of the tubes reached beyond the design temperature, confirming the microstructural degradations i.e., oxidation, decarburization and reduction in the effective tube wall thickness. The extensive scale formation on the steam side and the resultant tube wall thinning was identified as the main cause of failure in the superheater tubes.