And my answer would be:
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?
Oh, yes
AAD, your understanding is correct!
In fact you can analyze how the Code considers the hoop stress by looking in 841.11 Steel Pipe Design Formula. You might note which thickness the Code considers and the fact all the Code (and the Code background and history) is focused to maximize MAOP/ minimize pipe wall. As you say, Code includes just simplified rules for hoop stress and having practically no margin here, the Code has to improve something where is necessary.
PS AAD, buy the book!
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