The ASME B31.3 Code provides four basic methods for the design of components for internal pressure, as described in para. 302.2:
(a) Components in accordance with listed standards for which pressure ratings are provided in the standard, such as ASME B16.5 for flanges, are considered suitable by ASME B31.3 for the pressure rating specified in the standard. Note that the other methods of pressure design provided in ASME B31.3 can be used to rerate such listed components and∕or extend their temperature range.
(b) Some listed standards, such as ASME B16.9 for pipe fittings, state that the fitting has the same pressure rating as matching seamless pipe. ASME B31.3 modifies this slightly by stating that the fittings are accepted to have the same rating as the matching seamless pipe, considering only 87.5% of the wall thickness (removing the 87.5% allowance is presently under consideration). This takes into consideration the typical mill tolerance for pipe. Note that design calculations are not usually performed for these components; design calculations are performed for the straight pipe, and matching fittings are simply selected.
