A Margin-Consistent Procedure for Calculating the B2 Stress Index and a Proposed New Design Equation

In Section III of the ASME Boiler and Pressure Vessel Code, the design equation for primary stresses in piping contains stress equations for straight pipes that are modified by stress indices so that the equations can be applied to other piping components. In this paper we review the history of this equation in the context of elbows and then suggest a new procedure for calculating the B2 stress index that applies to stresses for bending. The resulting stress index equation, which is the ratio of the collapse moment of a straight pipe to the collapse moment for any component, gives a B2 value of 1.00 when applied to a straight pipe and a safety margin for the component that is always the same as for the straight pipe. This procedure is based on the Code-defined collapse moment, which we determine using nonlinear finite element analysis (FEA), supported by experimental data. We present B2 values for elbows with a wide range of pipe bend parameters. The values of B2 obtained using this equation are 14 to 48% lower than the values obtained using the current Code procedure when applied to stainless steel 90° butt-welding elbows.