Operational aspects, failures and design of radiant tube heater systems in a continuous strip annealing furnace

The focus of this Engineering Doctorate Thesis is the investigation into how the radiant tubes installed in the Continuous Annealing Process Line (CAPL) at Port Talbot steel works were failing and what measures could be taken to improve tube life. Radiant tube replacement and associated maintenance costs were one of CAPL’s biggest annual expenditures, with on average 33 tubes changed every year. Tube longevity was as low as 4 years in the hotter furnace zones, while in comparison, the cooler regions of the furnace had all original tubes still in operation after 12 years of service. A benchmarking process identified that most annealing furnaces were replacing on average 10% of total furnace tubes per year, while tube designs varied according to furnace manufacturer and tube supplier. Segal Galvanising line in Belgium, replaced the least amount at approximately 8.5%, through increasing material grades and subtle design changes Temperature analysis of the furnace at CAPL, highlighted that tube temperatures reached above 1000ºC with differentials of up to 75ºC across tube length in normal operation. Analysis of failed material identified that the tubes had been subjected to excessively high temperatures, which affected the microstructure and properties of the material, resulting in cracking failures at the end of the firing leg. Stress analysis showed that tube life was in the region of 4 years with current designs and maximum temperatures of 1000ºC. Installation of expansion bellows and increasing material grade resulted in longevity of the tube to increase by over double. Improvements have been made to the tube design and material specification throughout the project, with further changes employed in trial tubes, currently in use at CAPL furnace, with the aim of confirming theory discussed in this thesis and improving tube longevity