Issues with pipe flanges and steel pipes connection?

This is my personal understanding; please point out any inaccuracies! First question: The outer diameter of steel pipes is divided into two series, A and B. Series A is the internationally standard series (commonly known as imperial pipes), while series B is the series still in use in China (commonly known as metric pipes) ； If you understand this part, then you can realize that terms like SCH40/80 refer to the pipe wall thickness standards specified by the ASME code, and these naturally only apply to Series A pipes. Second question: First of all, this should be a flange made in accordance with the HG/T 20592-2009 standard (other standards also do not distinguish between Type A and Type B pipes). First question: Theoretically, it is supposed to be connected to Type B DN80 pipes, but for the DN80 size, there is not much difference among the pipes (the outer diameter of the 80mm pipe is 88.9 for Type A pipes and 89 for Type B pipes) ； The inner diameter of the flange is 90.5 for series A and 91 for series B (see HG/T 20592-2009 for details); it can still be used with some difficulty ； Question 2: Refer to the figures – Figure 1 shows a flange with a neck for butt welding: https://bbs.hcbbs.com/forum.php?mod=image&aid=1908695&size=300x300&key=fe88e3e545f102c8&nocache=yes&type=fixnone. Plate-type butt welding flanges are slightly thinner; using size to describe flanges of the same specification is not appropriate here, as flanges of the same specification but different types differ only in height. Figure 2 shows a flange for socket welding: https://bbs.hcbbs.com/forum.php?mod=image&aid=1908696&size=300x300&key=6dc61538ad775860&nocache=yes&type=fixnone. In both cases, the pipe needs to be inserted into the flange and then welded. (Additionally, there are no socket welding flanges of size 80 in standard 592.) Question 3: The inner diameter of a butt welding flange should be slightly larger than the outer diameter of the pipe, while the inner diameter of a socket welding flange should be close to the inner diameter of the pipe. The inner diameter of A-series flanges is slightly larger than the pipe’s inner diameter, whereas that of B-series flanges is slightly smaller ；

Socket weld flanges, up to DN50 in accordance with HG20592 standard. Which standard are you based on?

When selecting pipe fittings, some people use the SCH series according to American standards, while others use the metric series; sometimes the same project involves both American and metric standards. Why isn’t there a standardization? For example, tees and elbows use SCH standards, but valves and flanges use metric standards

It must be unified; otherwise, the outer diameter, inner diameter, and bolt holes will all be different. A proper design institute will not encounter such problems. When the locations where interfaces are arranged differ between public works and process teams, design institutes may face issues related to metric and imperial units

The standards need to be unified, and in fact they are. First, there’s the SCH series; the national standards for it are based on those in Europe, but they’re a poor imitation. In the end, I turned to the design institute’s criteria for selecting materials for the SCH series. Secondly, when I purchase materials, the national standards currently serve only as a reference. Many of the material specifications include those that I and my company usually buy (of course, these still need to meet the national standards). Finally, I’d like to talk about acceptance testing – it’s a task that requires careful attention, and I’ve suffered greatly because of it.