orifice plate

Can anyone plese tell me why the tapping in orifice flanges are at 45 deg. and not at 90 deg.

Do you mean 45 to each other or to horizontal Typically they side by side at 90 degrees to vertical for a horizontal line. The fluid you are measuring will also effect the orientation. For liquid it could be facing down (45) or horizontal. For gas facing up or horizontal, steam nearly always horizontal.

If the line is vertical it doesn't matter where the ports are.

Hope this helps.

Regards
Roy