Well, there is a couple of reasons for that
convention.
1. The flanges are not in isolation and as wished by
the piping designer. That is, as for a joint in the
field, withinn a spool, any orientation would work; no
matter, whether it is in the vertical or horizontal
axes or off that. The problem comes when a
prefabricated spool has to be coupled to a vendor item
like a machine flange. Some could be a cast component
as well. That is where the universal convention and
"GOOD ENGINEERING PRACTICE" comes into the picture.
The castings, universally are made with the philosophy
of "STRADDLE centre line". If the same policy is
followed in the spool fabrication also, there is
perfect harmony.
The second reason is just a fitter's option. If the
bolt holes follow the "ON THE AXIS" philosophy, one
bolt, the last one is likely to fall at a far end in
constricted spaces, and cause problem for
tooling-wrench space. If they are off the axes, the is
a little advantage in the wrench placement and its
rotation range.
I hope these two points adequately endorses the
general practice.
Regards/.
C. V. Gangadharan
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