Failure Investigation of Secondary Superheater (SSH) Tube Boiler in the Coal Switching Case

Failure in the boiler tube is one of the major causes of the force outages in a coal-fired power plant. This paper presents a failure investigation on the high-strength alloy ASME SA213 T91 superheater tube of a 600 MW class coal-fired power plant due to coal switching. The research investigated the fuel quality and the materials using the material characterization method and mechanical testing. The potential slagging and fusibility were found based on coal properties. A wide-open rupture with thick and blunt edges is exposed in the tube failure. Several steps for failure investigation were performed. Microstructures of the failed tube show numerous creep voids estimated < 2.5 microns and decomposed into spheroids as evidenced by a decrease in the hardness number 165 HV nearest location of failure. Exposure to high temperatures in the long-term causes the formation of an oxide scale layer on the fireside tube. The thickness of the oxide scale on the inner side tube > 600 microns also accelerates tube failure and shortened creep life is shown by a time rupture value from 16.29 to 0.26 hours with higher operating hoop stress near to failure location. Changes in fuel characteristics are indicated by the value of fusibility of 1190°C and slagging index of 0.473 which can worsen the condition of overheating from the increase in metal temperature. Higher flue gas temperatures due to the coal switching characteristic and higher metal temperatures of tubes above 570°C were identified as the cause of overheating. The finding confirmed that the superheater tube failed by long-term overheating.