USED FOR:
Pipework components – Definition and selection of DN (nominal size)
Defining what “DN” actually means and how it’s selected across all pipework-related ISO standards.
RELATED STANDARDS:
- ISO 7268 “Pipe components – Definition and selection of PN (nominal pressure)”
- ASME B36.10M / ASME B36.19M “Welded and Seamless Wrought Steel Pipe / Stainless Steel Pipe (NPS system counterpart)”
- ISO 4200 “Plain end steel tubes, welded and seamless – General tables of dimensions and masses per unit length”
- EN 1092-1 “Flanges and their joints – Circular flanges for pipes, valves, fittings and accessories, PN designated – Part 1: Steel flanges”
DIMENSIONAL RANGE:
This is really the exception among the pages in this library: ISO 6708 doesn’t reference an external dimensional range, it establishes one. Its actual technical content is a list of preferred DN values, dimensionless whole numbers such as DN 6, DN 15, DN 25, DN 50 and so on up through the very large sizes used in major pipeline and duct work, with no fixed upper ceiling since individual product standards can extend the series where needed. What the standard explicitly does not do is tie any DN value to one specific bore or OD measurement in millimetres, that mapping is left to each product standard (flanges, valves, fittings) referencing DN, which is exactly why we never use ISO 6708 itself to look up an actual pipe diameter.
DESCRIPTION:
A short but foundamental document. DN is a dimensionless designation loosely related to bore, not an exact measurement, and this standard is what formally establishes that convention across every other ISO piping component standard. We mention it because engineers who’ve only ever worked in NPS/inches sometimes assume DN is a literal millimetre bore, which it isn’t.
IMPORTANT NOTES:
Always cross-reference actual OD/ID tables (ISO 4200 or the relevant product standard) rather than treating DN as a precise dimension for design calculations.
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