A Review of Stress Intensification Factors for Reducers

Abstract Determining an accurate stress intensification factor (SIF) is important for piping stress analysis. This paper reviews the history of SIF guidelines for reducers and the significance of reducer geometry on the calculated B31J SIF value. ASME B31.3 Appendix D, which specifies a SIF of 1 for reducers, was removed in the 2020 version of B31.3 which now references ASME B31J. This means all the process piping stress qualified with SIF = 1 for reducers, may now have stress non-compliance with the new SIF of a value up to 2, resulting in layout or/and material changes in piping systems. This paper reviews the history of the development of SIF for reducers and the effect of this code update on default SIF values for reducers. Next, the significance of different reducer geometry parameters is investigated. In the initial stages of design, details such as exact reducer geometry are often not available. Initial analysis will therefore use default SIF values. These default SIF values should be conservative. If an area is a problem for stress, more detailed information can be used to refine the analysis. SIF values were calculated using default B31J values for reducer alpha, r2, and L2 parameters for a range of ASME B16.9 reducers at two different wall thicknesses. The SIF values are significantly higher than 1 and in many cases reach a value of 2. Most of these geometric parameters such as the L2 distance are not controlled by the ASME B16.9 reducer standard, yet they can have a significant effect on the SIF. It is suggested that L2, the distance between the transition radius and the small end of the reducer, should be specified as L2 >= (D2T2)^0.5 in B16.9 to keep reducer SIF values more predictable.