Short term rupture of boiler water wall tube

This article presents short‑term overheating of water wall tube failure especially on the repaired boiler tube. The aim of this brief article is to mitigate the high‑temperature failures not only by good engineering practices but also by proper repair, construction, maintenance, inspection and operation of circulating fluidised bed combustion coal fired CFBC boiler. Detailed failure analysis has been conducted on boiler tube including visual inspection, chemical analysis, microstructural analysis and hardness measurement to determine failure mechanism. Visual inspection demonstrated tube bulging with circumferential wide crack opened up to 39 mm along 122 mm longitudinal tube axis. The tube chemistry was as per original design BS3059/620 Pt2 S2 Category 2; however, slight reduction in wall thickness of 4.28 mm has been evidenced against 4.50 mm. The results revealed that short‑term overheating was responsible due to sudden rise in tube temperature beyond ~716 °C up to 750 °C, A1/A3 iron-carbon phase transformation temperature during start-up operation. Consequently, tube allowable stress has been decreased at increased temperature, resulting in rupture. Moreover, rapid cooling due to rupture produced a bainite structure in the failed boiler tube. The results were consistent and supported by increase in measured hardness of 330 BHN at failure zone. Several mitigation measures including removal of condensation steam prior to start-up operation were provided to avoid similar short‑term tube rupture failures.