Corrosion fatigue damage has resulted in the catastrophic failure of several riser and supply tubes in fossil boilers operating under sub-critical conditions. Most of the damage has been found on the neutral axis of tube bends and the damage mechanism was identified as corrosion fatigue. Significant stress concentrations will always be found associated with high tube ovality and significant thickness variations.
The purpose of this study is to demonstrate the effect tube bend geometry on the stress distributions developed due to the internal pressure. The work was performed to consider how the stresses present in tube bends vary with ovality and thickness variations (thinning/thickening). Ovality and thickness variations in tube bends, which are generally introduced during the bending process, were modeled into the three-dimensional finite element models (FEM). The finite element models considered five different degrees of ovalization of the cross-section at the center of the bend. For each model the maximum principal stress values and distribution of stresses within tube bends were evaluated. The finite element analysis (FEA) predictions were compared with the actual locations of corrosion damage on the tube bend and reasonably predicted where cracks should be expected to initiate and propagate, and where cracks should not be anticipated.