Flanges are discussed in previous post (ASME B31.3 Limitations on Flanges). Additional requirements for flanged joints are provided in para. 312 of ASME B31.3. These discuss conditions where flanges with different strengths are bolted together. This can involve two flanges of the same or different materials or with different ratings, or a nonmctallic and metallic flange bolted together. In both cases, precautions are required to avoid overloading the weaker flange. In addition, the pressure rating of the joint is limited by the flange with the lower rating.
In either case, precautions are generally required to avoid over tightening the bolts, which could damage the weaker flange. In addition, for metallic-to-nonmetallic flanges, use of full face gaskets is preferred, again, to prevent excessive distortion of the nonmetallic flange due to bolt loads. If full face gaskets are not used with metallic/nonmetallic flange joints, the bolt torque is required to be limited, to avoid overloading the nonmetallic flange. #Little_PEng.
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