HGT 20592-2009: Issues regarding the inner diameter of socket weld flanges in steel pipe flanges (PN series)
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In HGT 20592-2009 for steel pipe flanges (PN series), the butt-welded flanges of DN250 have an outer diameter of 273 for both Series A and B; why is the inner diameter of the flanges different? The same is true for DN65 and DN80; the outer diameter of the steel pipes differs by only 0.1. Why can’t the inner diameter of the flanges be made the same? Why were the inner diameters of the A and B flanges for DN300 made the same?
Pipes of series B are normally larger than those of series A; for example, for DN40, the outer diameter of series B is 48 while that of series A is 45； DN50, outer diameter 60 for Series B, 57 for Series A. Some specifications are the same or only slightly different.
Actually, the smaller-diameter pipes are mostly of Series A; those with larger diameters belong to Series B, starting from 300 and above. This post is about the issue of the flange inner diameter.
Series A is equipped with British-style pipes, while Series B uses metric pipes; naturally, the choice depends on what is needed.