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Why is valve 3 "80 caliber instead of 75?

The nominal size of the piping assembly is a number without units.

Its function is that only valves, fittings and flanges with the same nominal size and nominal pressure can be connected to each other.

The English system is represented by NPS and the metric system is represented by DN. See article 1.5.2 of ASME B 16.34-2009 for their relationship.

For those less than NPS4, the corresponding DN is not calculated.