I think the concept is not to add any local stresses that may be created and transmitted to pipe due to "stress concentrations".

As Dave said:

Originally Posted By: Dave Diehl
Is it because B31.8 pipe, with simplified design rules to minimumize pipe wall for pressure, has little residual strength remaining for any additional localized stresses at attachments?


Any external steel members or supports should not be directly welded to the pipe, instead, you can put a steel pad fully encircled to the pipe and then be welded to the pipe by a full encirclement weld, and the support or steel member can now be attached to the steel pad (not directly to the pipe).

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Flexy