Failure investigation of a water-wall tube of fossil-fuel fired boiler

Abstract

Failure investigation of water tube failure in boilers, especially in thermal power stations, is of paramount importance because it plays a crucial role in supplying energy to the dependent processing units in refineries. The failure investigation of the water-wall tube at the roof position in a boiler is presented in this work. In order to identify the root cause of the rupture, various destructive and non-destructive analyses were performed at multiple locations on the failed tubes. Visual inspection revealed a fish mouth-type rupture opening with excessive wall thinning about 60% before its failure and a diametrical growth of up to 150 % from the nominal diameter at the bulge location.From the chemical composition analysis, mechanical testing, and hardness measurements, the material was found to meet the requirement of SA 210 Grade A standards. This is a typical steel grade for high-pressure and temperature systems widely used in boilers. The microscopic examination revealed that the structure of the sound part of the tubes consisted of ferrite and pearlite structures. Whereas the microstructure of the bulged area exhibited a partial decomposition of the pearlite colonies, as well as formations of bainitic structures indicates localized overheating beyond the lower critical temperature (Ac1) of 723 °C for phase transformation. The absence of significant internal deposits and multiple bulges, ductile mode of failure, spheroidization in the ferrite matrix, and violent rupture are indications of short-term overheating. The failure mechanism of localized short-term overheating is identified as a result of the flame impingement due to the burner misalignment. A rupture of a tube in a high-pressure steam boiler can be catastrophic. A proper root cause analysis of the failure provides insight into the operating window for process variables, the interval of tracking feed water quality, and a planned shutdown inspection to avert any unpredictable shutdown in high-temperature applications. Understanding boiler-tube failures may improve stream-generating equipment performance, safety, availability, and reliability.