Study on the Stress Intensification Factors of Trunnions

The trunnion, which is the pipe welded to the outer surface of piping, is one of the most common pipe-supporting elements. According to para. 321.3.2 of ASME B31.3-2022, the design of such pipe-supporting elements and the local effects on the piping component are the responsibility of the designer, and no additional guidance is provided so far. Currently, conventional methods treating thin shells, such as beams on elastic foundations, the Bijlaard method, WRC 107/297/497, and BS 5500 Annex G, are often used, but a simpler approach consistent with the design of main piping components is demanded. Although ASME B31J-2023 does not provide Stress Intensification Factors (SIFs or i-factors) and Flexibility Factors (k-factors) for trunnion connections (without cutting a hole in the run pipe), it does provide them for similar connections, such as fabricated tees (with a hole in the run pipe). If the SIFs and k-factors for the trunnion connections are established, stresses for the trunnions can be easily evaluated with the same equations as for the branch connections in ASME B31.3 within standard piping system analysis software. This paper investigates SIFs and k-factors for trunnion connections based on Finite Element Analysis (FEA). First, SIFs and k-factors for unreinforced fabricated tees are reproduced using the analysis results and compared with those calculated using the equations provided by B31J to verify the validity of the FEA models. Second, SIFs for the trunnions are evaluated for various parameters. Finally, the results for trunnions are organized, and equations for them using dimensional parameters are proposed.