Overview
ISO 6708:1995 specifies the definition and selection of DN (nominal size) for components of a pipework system. This second edition cancels and replaces the 1980 edition and includes an expanded scope and the addition of clause 3. The standard was prepared by ISO/TC 5 (Ferrous metal pipes and metallic fittings), Subcommittee SC 10 (Metallic flanges and their joints).
DN is an alphanumeric designation used for reference purposes only. It comprises the letters DN followed by a whole number that is indirectly related to the physical size - typically the bore or the outside diameter of end connections - but the DN number itself is not a direct measurable dimension and should not be used for calculations except where explicitly permitted by a relevant standard.
Key Topics
- Definition of DN: DN is a nominal designation for pipework components; it is dimensionless and intended for reference and specification.
- Relationship to dimensions: When standards employ the DN system they should specify any explicit relationship between DN and component dimensions (for example DN/OD or DN/ID) so that designers and manufacturers can determine actual sizes.
- Preferred DN values: ISO 6708 provides a list of preferred DN values used across industry standards to promote harmonization and interchangeability of components.
- Scope and revision: The 1995 edition replaces the 1980 edition and expands coverage to include additional guidance (clause 3).
Applications
ISO 6708 is essential for professionals involved in the design, specification, procurement and manufacture of pipework systems. Typical practical benefits include:
- Standardized selection: Use of preferred DN values simplifies specification and reduces errors when selecting valves, flanges, fittings and other components.
- Compatibility checks: Clarifies how DN relates to bore or outside diameter so teams can verify component compatibility across suppliers and standards.
- Procurement and inventory: Harmonized DN designations support clearer purchasing descriptions and help manage spare parts and stock.
- Documentation consistency: Ensures piping documents, drawings and bills of materials use a consistent size designation system.
Related Standards
ISO 6708 is intended to be used alongside standards that define detailed dimensions and connections (e.g. flange standards, valve and fitting specifications) where the DN designation is applied. Where other size designation systems are used (NPS, OD, ID, thread types, welded ends), those methods remain valid but are outside the definition provided by ISO 6708.
Keywords: DN, nominal size, pipework, selection, preferred DN values, components, bore, outside diameter, compatibility, flanges, fittings.
