Stress Predictions at Elbow Ends Under Internal Pressure and System Moments

In order to assess postulated defects in the butt weld joining a 90 degree elbow to a seamless straight pipe, both axial and hoop stress components at this position are required. ASME III NB-3685 provides a method of calculating elbow stresses. However, this gives the maximum stress values in the elbow and applies to the central section of the bend. If these values are directly used in the defect assessments of welds at the ends of the elbow, the assessment results will be overly conservative. In order to obtain appropriate defect assessment results, more accurate axial and hoop stress distributions at the elbow ends are desirable. In this paper, the axial and hoop stress distributions at the elbow ends are predicted by deriving generalized stress relationships between the elbow end and the central section of the elbow, based on detailed finite element analyses and ASME III NB-3685 calculations. In order to do so, a series of small displacement elastic 3D finite element analyses have been performed. The finite element results were then compared with the ASME III NB-3685 stress predictions. Finally, the axial and hoop stress relationships between the elbow end and the central section of the elbow for internal pressure, in-plane moment and out-of-plane moment were derived. A comparison of the calculated stress values using the derived equations, the finite element analysis results and the ASME III NB-3685 stress calculations confirms that the derived stress relationships are appropriate to predict the axial and hoop stresses at the elbow ends. The objective of this paper is to show: 1) the ASME III NB-3685 stress calculations agree well with the 3D finite element analysis results at the central section of the elbow and 2) the derived stress relationships are appropriate to predict the axial and hoop stresses at the elbow ends.