Failure study of secondary superheater tube out header damage in a 600-MW coal power plant

Herein, a failure analysis of the out header of a secondary superheater tube in a 600-MW coal-fired power plant that has been operating for the past 26 years was performed. This type of failure generally results in the plant being shut down. Therefore, it is crucial to identify the cause of the failure to determine the necessary action to be taken and to avoid similar failures in future. The investigation involved visual inspection, chemical composition analysis, microstructure analysis, hardness test, and deposit analysis. It was found that the tube failed with a bending and burst opening and experienced an increase in hardness in the fractured area. There was no sig-nificant difference between the microstructures of the fractured area and other areas located away from the fractured area. The X-ray diffraction and scanning electron microscopy and energy-dispersive X-ray spectroscopy results confirmed the presence of a magnetite layer (scale) on the inner wall tube, and the layer thickness ranged 293-345 mu m, which exceeded the peeling critical thickness value of 160 mu m. The failure resulted from the exfoliation of the magnetite layer on the inner wall tube, which blocked the flow of steam in the U-bend. The tube blockage caused local overheating, which reduced the maximum allowable stress until the bursting.