Technical summary: Are there 4 holes or 8 holes for the flange bolt holes on DN65 valves?

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In actual production, DN65 flanges often have 4 or 8 bolt holes – what is the reason for this? Here is a summary post on this situation. The chemical industry flange standard HG/T 20592-2009 specifies that flanges with DN65 size and pressures of PN10, PN16, and PN25 should have 8 bolt holes. However, the flanges actually purchased come in both versions with 4 bolt holes and those with 8 bolt holes, and the markings on these flanges also conform to HG/T 20592-2009. Was it the manufacturer who made a mistake? 1. Origin of the issue: The reason for this problem is that the predecessor of standard HG/T 20592-2009 was HG/T 20592-1997; under that 1997 standard, DN65 flanges indeed had 4 bolt holes. Some manufacturers were unable to make the necessary changes immediately, which led to both 4-hole and 8-hole flanges coexisting over several decades. 2. Cost: There is another situation in which, in order to save costs or out of a desire to be clever, some manufacturers deliberately combine 4-hole and 8-hole flanges into one type, in order to reduce material and management costs. 3. So, what problems might arise from making DN65 flanges with 4 holes or 8 holes from a technical perspective of flange connections? Based on my experience in work, using DN65 flanges with 4-hole or 8-hole connections is no problem, but two points need to be noted: (1) When 4 holes and 8 holes are used together, 4 of the connection holes lack studs, which makes the appearance unsightly. Ordinary customers will not accept it ； (2) In the chemical industry, flanges require that the bolt holes be welded at their midpoints; when such flanges are connected to pipeline fittings such as ball valves or pneumatic valves, if the orientation of these valves is not vertical, customers will not accept it. (3) According to current standards, the flange manufacturers are definitely at fault, but in actual procurement both types are available – what can you do about it? In our work, when purchasing DN65 flanges, we usually need to specify whether they have 4 holes or 8 holes. Especially when purchasing pipe fittings, it is necessary to specify this; otherwise, if the installed pipe fittings become tilted in appearance, it will be difficult to deal with the issue. (4) Some suppliers provide pipe fittings with 4 holes; knowing that this does not meet the standards, they will offer matching flanges as a remedy. As the designers, we generally would not agree to this, because when valve and other piping fittings are replaced, the same issue of tilted appearance in the installation of piping valves as mentioned in point (3) can occur. Therefore, it is best to ask the pipe fitting supplier to replace them with standard-compliant fittings. 4. In this regard, suppliers do a much better job by providing flanges that meet the standard HG/T 20615-2009 (American CLASS series).