Unreinforced branch connections are a typical component of steel pipelines, designed and manufactured according to the piping code standards of reference. These joints are constructed welding together two straight pipes at different angles respect to their axis and the Stress Intensification Factors (SIFs) of the welded area are readily available from the standards for stress analysis purposes. Nevertheless, branches manufactured using elbows are occasionally encountered during Fitness-For-Service evaluations of the older carbon-steel pipelines conveying fluids at low pressure and room temperature. In the present paper the SIFs for these peculiar joints were determined using FEA and compared with those obtained by FEA and by code for the classic 90° unreinforced branch connections manufactured with straight pipes. 42 tees were considered in the analysis with diameters ranging from 3″ to 10″ on the run side and from 2″ to 10″ on the branch side; thicknesses from piping Schedule 10, 40 and 80 were adopted in the investigation and long radius elbows were only considered. The SIFs for the elbow branch tees were found to be 1.1 to 1.35 times those of the standard joints for unitary diameter ratios and 0.8 to 1.1 in the other cases. The maximum SIF (out-of-plane branch) was found to be linearly related to the elbow flexibility. A simple formula was then proposed to conservatively calculate the SIFs of the elbow branch connections using the values provided by the codes for a standard stress analysis where the joint is modeled with straight pipes. Finally, the elbow tees were evaluated against fatigue via solid FEA obtaining a lower estimate of their safe life respect to the standard joints.
Stress Intensification Factors for Unreinforced Elbow Branch Connections in Old Carbon-Steel Pipelines
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Scano, L. "Stress Intensification Factors for Unreinforced Elbow Branch Connections in Old Carbon-Steel Pipelines." Proceedings of the ASME 2014 Pressure Vessels and Piping Conference. Volume 3: Design and Analysis. Anaheim, California, USA. July 20–24, 2014. V003T03A073. ASME. https://doi.org/10.1115/PVP2014-28528
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