STRESS INTENSIFICATION FACTORS FOR NON-CONCENTRIC ELBOW BRANCHES IN OLD CARBON-STEEL PIPELINES

摘要

Unreinforced branch connections manufactured using elbow ducts can be occasionally encountered during the Fitness-For-Service evaluation of very old, low-service, carbon-steel pipelines. For these peculiar components, no Stress Intensification Factors (SIFs) are usually available in the international codes and standards used by practicioners for stress analysis purposes. In a previous work from the author, the SIFs for various elbow branch connections were determined via FEA and a formula was derived through a parametric analysis in order to calculate the SIF for the elbow tees using the codified SIF of the equivalent standard joints. The investigation was restricted to concentric elbow branches, i.e., tees where the elbow duct had the same principal axis of the ran pipe: In this paper the case of non-concentric elbow branches was taken into account, considering the offset between elbow and ran pipes and the rotation of the curved duct in the two main meridional planes, thus bringing into focus lateral and non-planar elbow tees. The SIFs were calculated using FEA for different configurations of reducing elbow branches with diameters ranging from 2" to 10" and thicknesses from Piping Schedule 10, 40 and 80. Lateral and non-planar joints were analyzed for one reference geometry and at various offsets and rotation angles. A parametric analysis was then carried out to evaluate the offset effect on the stress intensification both on ran and branch side, for in-plane and out-of-plane bending. The results showed a significant impact only on the branch side where an increase in piping offset resulted in a reduction of the dominant out-of-plane bend- ing SIF. The original formula devised for the concentric elbow branch connections was hence modified to include this offset effect. Finally, the combined role of offset and rotation angle was parametrically analyzed for the special geometry investigated. The results demonstrated a significative increment of the SIFs for non-planar tees, both on ran and branch pipes, whilst in lateral joints the major SIF increased to a maximum only on the branch side.