Thermal-Structural Analysis of the Super-Heater Tubes of a Unit of 158 MW

The thermal-structural analysis of the super-heater tubes for a 158 MW unit, applying FLUENT® (CFD) and ANSYS® (FEA), is presented. The analysis includes the spacers (union piece between tubes), welding and tubes. The failures of these elements are related with the operation of the unit and the selection of the weld and materials involved in the affected zone. The distribution of temperature in each metal depends of the thermal conductivity and coefficient of thermal expansion, provoking stress concentration in the rigid zones. The CFD study considers a three-dimensional model where the conjugate heat transfer, including internal flow (steam) and external flow (gases), was analyzed to full load of the unit in steady state. In the FEA study, the thermal-structural stresses were analyzed considering the temperature distribution obtained from the CFD study. The results obtained show that the spacer is of greater temperature than the tubes, provoking gradients of temperature through tube walls, spacers and welds. The highest stress located on the interior tube wall (on the direction of plane where the spacer is welded) is attributed to the different thermal dilatation and pressure expansion of the tube, spacer and weld. The study includes the analysis of some geometries of the union piece (spacer) to release the thermal-structural stresses.