Do you know the positioning principle of flange bolt holes?
ASME B16.5 specifies the number of bolt holes and bolt hole diameters for flanges. The number and diameter of flange bolt holes vary depending on the flange pressure rating, but all types of flanges of the same pressure rating are the same. The flange bolt holes are evenly distributed on the bolt hole diameter, and the number of holes is always even (4, 8, 12, 16, etc.).
The positioning of flange bolt holes is particularly important in the fabrication of prefabricated parts with flanges. Maybe you see the following sentence in a drawing:
All Flange Bolt Holes Straddle the Centerlines.
For vertical flange sealing surfaces (the flange surface is vertical and the pipeline runs horizontally), the bolt holes should span both sides of the vertical line and the horizontal centerline.
For a horizontal flange surface (the flange surface is horizontal and the pipeline is vertically up or down), the flange bolt holes should span both sides of the factory’s northbound line.
This is important, deviations from standard bolt hole positioning are not allowed. Unless there are clear requirements, for example, the customer explicitly requires different positioning methods. In 99% of the cases, when we see a bolt hole positioning method different from the above, we can consider that the positioning method is wrong. This centerline rule is well known and followed by responsible equipment manufacturers and piping system builders.
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