ASME B16.5 has clear regulations on the number of bolt holes and the diameter of bolt holes in flanges. The number and diameter of flange bolt holes are different due to different flange pressure levels, but all types of flanges of the same pressure level are the same. The flange bolt holes are evenly distributed on the bolt hole diameter, and the number of holes is always an even number (4, 8, 12, 16, etc.).
In the manufacture of prefabricated parts with flanges, the positioning of flange bolt holes is particularly important. Perhaps in a drawing you will see the following sentence: All flange bolt holes are centered
This means that for the vertical flange sealing surface (the flange surface is vertical and the pipeline runs horizontally), the bolt holes should span both sides of the vertical line and the horizontal center line.
The above is the correct vertical flange positioning.
The above is the wrong vertical flange positioning, this positioning is forbidden.
For the horizontal flange surface (the flange surface is horizontal, the pipeline is vertically upward or downward), the flange bolt holes should span both sides of the factory north line.
The above is the correct horizontal flange positioning.
The above is the wrong horizontal flange positioning, this kind of positioning is prohibited.
In short, deviation from the standard bolt hole positioning is not allowed. Unless there is a clear requirement, for example, the customer clearly requires a different positioning method. In most cases, when we see a bolt hole positioning method that is different from the above regulations, we can think that the positioning method is wrong.