Radiant tubes lifetime prediction in steel processing lines using fluid-structure interaction modelling

One of the limiting factors for processing steel lines performances is the radiant tubes damage. This damage results from a combination of many parameters such as operating conditions (combustion performances, depending on burner design and control, furnace control, fuel quality and stability…) and radiant tube design (material selection, geometry, and manufacturing). Current operating conditions lead to upper temperature limit for radiant tubes, which are then exposed to several mechanisms, such as hot corrosion, creep, and thermal fatigue that ultimately may result to their failure. To improve equipment lifetime subject to severe thermal stresses it is essential to know precisely the radiant tube temperature distribution. The common approach to evaluate radiant tube lifetime is usually limited to thermal stresses calculations on the hypothesis of linear elastic behavior, based on calculated or measured temperature distribution. However, at high temperature, creep behavior implies large deformations that could change the fluid flow inside the radiant tube. A coupled analysis of fluid dynamics and thermomechanical behavior could lead to a more precise evaluation of thermal operating conditions and lifetime for radiant tubes. Thus, a development has been carried out by Fives to increase lifetime of W-shape NiCr cast radiant tube operating at temperatures up to 1000 °C. Combustion and heat transfer inside the tube are simulated using Computational Fluid Dynamics with RANS approach, for fuels including Natural Gas, Coke Oven Gas and residuals Mixed Gases from the steel production. Creep behavior is simulated using a Nonlinear Structural model with Norton's creep law model, using parameters from the literature. The deformation resulting from creep combined with material selection, tube design and tube supports contributes to evaluate radiant tube lifetime. Our results are satisfactory compared with radiant tubes surface temperatures and observed deformation after several years of operation, obtained in Fives test center and during industrial operation.