This article is related to the ASME B16.9 fittings specified in an ASME B31.3 pipe class.

The suitability of the fittings and especially the Tee fitting of the ASME B16.9 in a ASME B31.3 pipe class is also depending various requirements of the ASME B31.3. The fittings ordered according the ASME B16.9 have the same pressure/temperature rating as per the connecting pipe of the same base material, refer to ASME B16.9 para. 2. The ASME B16.9 also stipulates: "In order to meet design or manufacturing requirements, it is expected that some portion of formed fittings may have to be thicker than the pipe wall with which the fitting is intended to be used. The mathematical analyses, if used, may take into account such thicker sections".

Furthermore  the ASME B31.3 gives criteria for the fittings in case a lower "stress intensification factor" may be used in the flexibility calculation. (The line routing can be simplified at higher temperatures). The crotch thickness, for example, should be minimal 1.5 times the nominal wall thickness of the pipe and the crotch radius should be as a maxim 1/8 times the diameter, refer to ASME B31.3 appendix D. Note: due to the tolerance (12.5%) the actual measured crotch wall should not be less than 1.3x the nominal wall of the connecting pipe.

It is recommended to list the above requirements in the pipe class to make sure the correct fittings are supplied. This means that the requirements of the ASME B31.3 should explicitly mentioned in the pipe class and in the purchase documentation. These details are usually not mentioned in a pipe class.

(Free to the email received of Hans Twigt, June 24, 2002)