Ballbearing,
Fair point, mandatory requirement in a referenced spec becomes mandatory in the referencing code. I fully agree.

I don't think we can find the history of B16.5 and answer the question why the authors deemed it necessary to put this requirement. It does not affect strenght of the flange, does it? On the other hand, it may affect strength of the bolt (leak+corrosion), and B16.5 has requirements pertaing to bolts as well.

One more thought: it is more likely that bending moments due to excess loads will be about the horizontal axis (because gravity acts in vertical direction). It's better to have two equally overloaded bolts on top of the flange than just one on the centerline. How's that?

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Konrad Anikiel

Hi Konrad,
Back when I was on the tools in New Zealand I was led to believe that the Americans had their bolt holes on the centreline and the rest of the world straddled the centreline. This was maybe due to pipespools arriving from the US with flanges orientated wrong and having to be cut off and rewelded.
While researching this subject on the AWS forum it appears there are still American companies that use this practice.
I agree with your suggestion about the product dripping onto a single bolt located at the very bottom and maybe corroding it.
A couple more are - having 2 bolts level on the bottom makes it a lot easier to drop in a spiral wound gasket before final boltup and also with the bolts on the centreline you have 3 bolts to remove instead of 2 when removing a gasket,
Regards,
BB