Case Study on Assessment of Fittings As Per ASME B31.3

Abstract ASME B31.3 is widely used in industries for design assessment of piping systems. ASME B31.3 has guidelines on design for various components for the piping systems. These components for which explicit design guidelines are provided are known as “listed” components. One key component for which ASME B31.3 is not very clear is requirements for fittings. There are certain fitting components which are listed in Table 326.1, which can be designed to other published standards. In our experience we have come across fitting types which do not have clear directions on design and qualification. In this paper, we will talk about some examples of fittings we have assessed and made compliant to ASME B31.3 for both normal fluid service and Chapter IX for High Pressure Fluid Service. In a general sense, any fitting type which is not covered in Table 326.1, can be considered as an “unlisted” component. We start with a calculation of shear stresses on the threads from the applied pressure and torque pre-load. We use ASME B1.13M non-mandatory Appendix B for calculation of the shear stress. To compute the value of allowable stress, we take the lesser of the value obtained from BPVC II D, Appendix 2-100 and ASME BPVC, Section VIII, Division 1, UG-23. This approach gives a conservative assessment on the thread holding capacity. For more complicated or detailed assessment, we have also used finite element approaches from ASME BPVC, Section VIII, Division 2 & 3. All these assessments do not provide a good indication on the actual leak tightness capacity of the seals, and thus for them we look at an experimental approach by testing as per UG-101. For B31.3 normal fluid service that would lead back to performance testing as per UG-101, and for Chapter IX, additional guidance is also provided in Section K314.3.2. For the lifetime assessment of the seals, we refer to Section K304.8, which refers back to doing a lifetime assessment with either ASME Section VIII, Division 3, Article KD-3 or Article KD-4. For some fitting configurations, we have also explored assessing their lifetime using an experimental basis.