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I'm was looking into the ASME B16.5Pipe flanges and pipe fittings and it has 2 dimension tables metric and inches.

What I found was rather strange for me. I was looking at a 2 inch welding neck flange, and the hub diameter must match the pipe size.

For the metric table this is 60.3mm and it is the same in all pipe sizes charts that I have available. When I look at this dimension in the Inch table, it is 2.38in when I convert this to mm this varies from the metric table.. 2.38in * 25,4 = 60,452mm

So there is a ,152 difference between these values. What is correct? These differences are to be found on more flanges then just the 2" flange size.

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    $\begingroup$ I'm not familiar with ASME, but I'm going to guess that the correct size for a 2in pipe is 2.38in, and the correct size for a 50.8mm pipe is 60.3mm. i.e. Pick whether you're working in metric or imperial units and stick with it. $\endgroup$ – AndyT Oct 21 '16 at 8:43
  • $\begingroup$ Well, i'm working with a double annotation metric units are my base units but our client is based in America and wants to have imperial dimensions. So to avoid calculation mistakes all dimensions are in double metric and imperial. $\endgroup$ – Mech_Engineer Oct 21 '16 at 9:54
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I would guess that the discrepancy is down to rounding fractional inch sizes to decimal inches ie 2.38 is not exactly 2 3/8"

Note that 2.375 (two and three quarter eighth inches) is 60.325mm

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  • $\begingroup$ I would like to upvote your answer but a reputation of 15 is required. Will need to research something before I can accept this as an answer. $\endgroup$ – Mech_Engineer Oct 21 '16 at 12:55
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    $\begingroup$ Last sentence: 2.375 is two and three eighths (not quarter) $\endgroup$ – Byron Wall Oct 26 '16 at 21:29
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As of the 2013 edition of ASME B16.5, the scope section explains:

This Standard states values in both SI (Metric) and U.S. Customary units. As an exception, diameter of bolts and flange bolt holes are expressed in inch units only. These systems of units are to be regarded separately as standard. Within the text, the U.S. Customary units are shown in parentheses or in separate tables that appear in Mandatory Appendix II. The values stated in each system are not exact equivalents; therefore, it is required that each system of units be used independently of the other. Except for diameter of bolts and flange bolt holes, combining values from the two systems constitutes nonconformance with the Standard.

This is actually a pretty common practice where metric and inch systems are regarded separately. While the numbers may not match exactly, the intention is that they work as a system within each unit of measure. Of course, as far as I'm aware, you would not buy a metric flange for an NPS or DN size pipe separately, they will be the same product manufactured to some company's internal drawings, so in essence the standard is accepting that any rounding errors are within tolerances intended by the standard. One reason they don't allow intermixing when interpreting the standard is that they don't want you to be able to cherry-pick the most permissive set of dimensions and tolerances for each feature.

The forward also notes:

The 2003 Edition included metric units as the primary reference units while maintaining U.S. Customary units in either parenthetical or separate forms.

So it would be reasonable to consider the metric information in the standard to be slightly more authoritative than the inch information.

The forward to ASME B36.10M-2004, which covers the dimensions of the actual pipe, explains

The standard was revised in 1978 to include SI metric dimensions. The outside diameter and wall thicknesses were converted to millimeters by multiplying the inch dimensions by 25.4. Outside diameters larger than 16 in. were rounded to the nearest millimeter, and outside diameters 16 in. and smaller were rounded to the nearest 0.1 mm. Wall thicknesses were rounded to the nearest 0.01 mm. These converted and rounded SI metric dimensions were added to Table 2. A formula to calculate the SI metric plain end mass, in kilograms per meter, using SI metric diameters and thicknesses was added to section 5. The SI metric plain end mass was calculated and was added to Table 2. These changes in the standard were approved and it was designated an American National Standard on July 18, 1979.

So they do acknowledge some use of rounding. For 2" NPS pipe they list the OD as 2.375 inches and 60.3mm (DN 50 size.) There there is only an error of .025mm which is consistent with their rounding philosophy. While your observed error of .152mm is definitely larger than I can directly justify, it is unlikely to cause a serviceability issue in a welded joint - .006" in below the manufacturing tolerances of most metal parts that are to be welded. Tolerances that small are usually achieved by precision machining. I would not be concerned about the actual function of the joint.

To address your specific question in the title, the pipe should be sized according to its own spec. That standard may have its own dimension tables, or it may reference ASME B36.10M and then specify manufacturing tolerances on top of it. In the US, if you'r ejust looking at plain black pipe, the relevant standard would be ASTM A36, but a multitude of standards exist for other materials, tighter tolerances, and application-specific pipes.

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