Overview
EN ISO 6708:1995 - “Pipework components - Definition and selection of DN (nominal size)” is an international standard (adopted by CEN) that defines the DN (nominal size) designation used for pipework system components. The second edition (1995) cancels and replaces the first edition (1980) and expands the standard by adding clarification (clause 3). It provides the formal definition of DN, guidance for its use in component standards, and a list of preferred DN values for consistent specification.
Key Topics
- Definition of DN: DN is an alphanumeric designation composed of the letters “DN” followed by a dimensionless whole number. This number is indirectly related to the physical size (bore or outside diameter) of end connections and is used for reference purposes.
- Dimensionless nature: The DN number does not represent a direct measurable value and should not be used for engineering calculations unless the relevant product standard explicitly specifies a relationship (e.g., DN/OD or DN/ID).
- Preferred DN series: The standard lists preferred DN values to standardize sizing across components. Examples include DN 10, 15, 20, 25, 32, 40, 50, 65, 80, 100 up to large sizes such as DN 1000, DN 2000 and beyond (full list contained in the standard).
- Usage guidance: Where a relationship between DN and actual dimensions is required (for example DN/OD or DN/ID), that relationship should be stated in the product-specific standard that uses the DN designation.
Applications
EN ISO 6708:1995 is widely used to ensure consistent size designation across pipework components such as:
- Flanges, fittings and metallic joints
- Valves, meters and pipeline accessories
- Component datasheets, procurement specifications and technical drawings
Who uses it:
- Mechanical and piping engineers
- Standards writers and product designers
- Manufacturers of pipes, fittings and valves
- Specifiers, purchasers and construction contractors
- Regulatory bodies and conformity assessors
Related Standards
- ISO 6708:1995 is the International Standard adopted by CEN as EN ISO 6708:1995.
- The standard notes other sizing methods (for example NPS, OD, ID or thread-based designations) exist and may be used in relevant standards; users should refer to product-specific standards for any DN-to-dimension relationships.
Keywords: EN ISO 6708:1995, DN nominal size, nominal diameter, pipework components, DN selection, preferred DN values, DN/OD, DN/ID.