A Method to Approximate the Steady-State Creep Response of Three-Dimensional Pipe Bend Finite Element Models Under Internal Pressure Loading Using Two-Dimensional Axisymmetric Models

Abstract

Pipe bends are regions of geometric discontinuities in the pipe systems used in power plants and most industry recorded failures have been located around similar regions. Understanding these potential locations of weakness is therefore of great interest for the safe and economic operation of piping components. Increased predictive accuracy would assist in component design, condition monitoring, and retirement strategy decisions. Modeling of piping components for finite element analysis (FEA) is complicated by the variation of the cross section dimensions (changes in wall thicknesses or cross section ovality) around the pipe bend due to the manufacturing procedure implemented. Quantities such as peak rupture stress and creep rupture life can be greatly affected by these geometric variation (Rouse, J. P., Leom, M. Z., Sun, W., Hyde, T. H., Morris, A., “Steady-state Creep Peak Rupture Stresses in 90 Pipe Bends with Manufacture Induced Cross Section Dimension Variations”International Journal of Pressure Vessels and Piping, Volumes 105–106, May–June 2013, pp. 1–11). Three dimensional (3D) models can be used to approximate to the realistic level of detail found in pipe bends. These simulations may however be computationally expensive and could take a considerable amount of time to complete. Two dimensional (2D) axisymmetric models are relatively straight forward to produce and quick to run, but of course cannot represent the full geometric complexity around the pipe bend. A method is proposed that utilises multiple 2D axisymmetric pipe bend models to approximate the result of a 3D analysis through interpolation, thus exploiting the greatly reduced computing time observed for the 2D models. The prediction of peak rupture stress (both magnitude and location) is assessed using a simple power law material model. Comments are made on the applicability of the proposed procedure to a range of bends angles (90 deg, 60 deg, and 30 deg), as well as the effect of the stress exponent (n) and tri-axial (α) material constants. Provided that peak stresses do not occur at the bend/straight interface, the magnitude and location of the peak rupture stress can be predicted by the 2D axisymmetric interpolation method with a typical percentage difference of less than 1%.