(22c) Radiant Coil Return Bend Stress Analysis and Failure Assessment

This paper presents a study, utilizing process analysis, pipe stress analysis, and metallurgical analysis, of repeated failures in the return bend of a radiant coil. These failures were initially characterized as thermal shock events, but the repeated nature of the failures led to a deeper investigation of the issue. Process analysis did not identify any irregularities that would be expected to lead to thermal shock of the components, while pipe stress analysis revealed that the return bends that were failing were not the most stressed components in the system, and the predicted stresses were well below the allowable stresses for all the load cases considered. However, metallurgical analysis determined that the initial characterization of the failures as being related to thermal shock was inaccurate, as the oldest fractures present were characteristic of fatigue failure. There were multiple fracture initiation sites at the joint between the return bend tubing and the hanger, which eventually grew and coalesced into a large fracture. This large fracture then caused a final overload failure that was initially characterized as thermal shock.

The fatigue cracking of the return bends was driven by thermal stresses that develop in the return bend and hanger during normal operation due to the geometry and insulation of the bend. Contributing to the fatigue fracture was severely decreased material ductility and toughness, as determined by tensile testing and Charpy impact testing of the failed return bends. While some reduction in material ductility is expected during service due to aging, the extent of ductility loss, particularly in the first failed return bend, raises questions regarding the casting process. Overall, this case study highlights the importance of detailed fractographic analysis of failed components, as preliminary or field evaluations sometimes fail to note significant features of the fracture surface that can lead to mischaracterization of the failure, and thus cause corrective actions to be ineffective.