Effect of Radial Offset Between Welded Back to Back Elbows on Their C₂ Stress Index

Pipe bends and elbows in process and power plant piping systems are sometimes added to provide additional flexibility along with their primary function of changing the direction of fluid flow. In special cases, piping systems may have double bends or elbows welded back to back with or without straight pipe in between. Due to manufacturing inaccuracies, back-to-back welded elbows may have a radial offset at the welded joint. Assessment of Class 1 piping following NB-3600 requires consideration of the C₂ stress index of the girth butt weld, which is to be multiplied with the elbow C₂ index. As an alternative, the C₂ stress index may be calculated by finite element analysis using the maximum linearized membrane plus bending stress intensity. The effect of offset between the elbows at the welded joint on the elbow C₂ stress indices is studied in this paper. Double elbows with elbow angles ranging from 30° to 90° and a radial offset of 1/32^″ and 3/32^″ at the weld are considered. Double elbows model with no weld in between and finite element model with single elbow are analyzed for comparison. The effect of angle between the bend planes of the two back-to-back elbows is studied. Results from the analyses are compared with C₂ stress indices calculated as per NB-3680.