Critical Failure Analysis of Superheater Tubes of Coal-Based Boiler

Abstract

This paper highlights a methodology for failure investigation of superheater tubes made of the material T-22 of a coal-based boiler. The process includes visual observation, the identification of sampling locations, the determination of the bulk chemical composition of the base alloy, microstructural investigation using optical microscopy, the exploration of finer structural details using a scanning electron microscope (SEM), the evaluation of hardness over samples obtained from different locations, the fractographic analysis of different failed locations, the X-ray diffraction (XRD) study of corrosion products adhered to inner surfaces, and the determination of the nature of the failure. Within a span of four months, three successive failures of superheater tubes were reported. The tubes were observed to have undergone significant wall thinning. Microscopic examinations using SEM on the failed region and a region some distance away on the as-received tubes were conducted in order to determine the failure mechanism. Layer-wise oxidation corrosion (exfoliation) in the inner surface was observed. Apart from major cracking, a number of nearly straight line crackings were observed in the longitudinal direction of both tubes. Close to cracking/bulging, void formation/de-cohesion of grain boundary indicated creep deformation under service exploitation. The failure mechanism was identified to be a result of excessive oxidation corrosion along the inside wall to reduce thickness, the spheroidization of alloy carbides and the coarsening of precipitate as well as creep void formation along grain boundary leading to inter-granular cracking with material flow near regions covered with thick scales. Moreover, there was a drastic reduction in bulk hardness of alloy and finally 'thin lip fish mouth' fractures.

Preliminary Study of Failure Analysis on Tube Material Boiler Based on Pressure Aspect

“…In order for the process of phase change to take place properly, the tubes used must be in accordance with the criteria when the tube is operated. The choice of material from mechanical properties can be seen from the material's ability to accept loading without causing damage when the tube is being operated, thus extending the useful life of the material [1,3]. Analysis to find the best properties of materials has been done by performing various solutions to cases of tube failure.…”

“…In steam power plants, boilers have an important role. A boiler is a place for changing water from a liquid phase to a vapor phase [1,2]. Inside the boiler, there are tubes as a fluid place.…”

“…Inside the boiler, there are tubes as a fluid place. In the event of failure of the tubes in the boiler, it will cause the process of transfer of electrical energy to the consumer can be stopped [1]. If the transfer…”

Failure Analysis of Superheater Tubes Made from Material T91 in the Boiler of a 600 MW Power Plant

“…Some of the causes of failure in superheaters are localized prolonged heating, creep damage, thermal fatigue, excessive thermal stresses, water and fireside corrosion, erosion. General modes of failures observed in superheaters are wall thinning, reduced thickness, fish-mouth opening type bursting, creep cracks on tube surfaces and puncturing [3]. Approximately 35% of common failures in utility boilers are caused by long term overheating (creep) in superheaters, reheaters and wall tubes [4].…”

“…Methods for analyzing tube failures are performed in order of a. visual inspection, b. microscopic observation, c. estimation of creep mechanism, and d. corrosion products analysis. Gupta and Chattopadhyaya [3] studied a generalized methodology for the investigation of failed superheater tubes by considering feasible influential factors with suggestions for remedial actions. The tube made of the material T-22 failures that were observed ie.…”