The nominal diameter (DN - "Diamètre Nominal") is a standardized parameter for describing the pipe diameter in piping systems. It plays a central role in the planning, design and installation of systems in industrial water and wastewater technology. From the dimensioning of pipes to the selection of valves and components such as valves, pumps and fittings, the nominal diameter is an indispensable parameter that ensures that all elements of a system can be connected together smoothly.

This article provides a comprehensive explanation of the technical principles of the nominal diameter, its standards and classifications, as well as its significance and application in practice.

Definition of the nominal width

The nominal diameter (DN) is a dimensionless numerical value that specifies the internal diameter of a pipe or the connection dimensions of fittings and other components. It is based on a standardized system and allows the simple assignment and compatibility of different components.

Important features:

• Dimensionless: The nominal diameter itself is not an exact physical diameter, but a reference value.
• Standardization: Specified in international standards such as DIN EN ISO 6708 to ensure the interchangeability and compatibility of components.
• Unit of measurement: As a rule, the nominal diameter is specified in millimeters (e.g. DN 50).

Technical principles and standards

The nominal diameter is specified in various standards that regulate the dimensions and the assignment to actual pipe diameters:

1. ISO 6708 and DIN EN 1333:

• These standards define the nominal diameter as a reference dimension, regardless of the material or wall thickness of the pipe.
• They ensure compatibility between components from different manufacturers.

2. connection with the pressure stage (PN):

• The nominal diameter is often specified in conjunction with the pressure rating (PN - "Pressure Nominal").
• Example: DN 50 PN 16 refers to a pipe with a nominal diameter of 50 mm that is designed for an operating pressure of up to 16 bar.

3. dependence on material and wall thickness:

• While the nominal diameter describes the inside diameter, the actual outside diameter can vary depending on the wall thickness and material.
• Example: A DN 50 pipe made of steel typically has a different wall thickness than a DN 50 pipe made of plastic, which affects the outside diameter.

Importance of the nominal diameter in water and wastewater technology

The correct selection of the nominal size is crucial for the function and efficiency of a system. It influences:

1. hydraulic power:

• The diameter of a pipe has a direct influence on the flow velocity and the pressure loss in the system.
• A pipe diameter that is too small leads to high pressure losses, while an oversized pipe causes unnecessary costs.

2. compatibility:

• A standardized nominal width ensures that pipes, fittings and other components from different manufacturers can be connected to each other without any problems.

3. operational safety:

• Selecting the right nominal size minimizes the risk of flow problems such as cavitation or turbulence, which can cause damage to systems.

4. cost efficiency:

• Correct dimensioning reduces material and operating costs by minimizing pressure losses and maximizing pump efficiency.

Typical applications of the nominal size

The nominal diameter is used in almost all areas of water and wastewater technology:

1. pipe dimensioning:

• The hydraulic calculation determines the optimum nominal size based on the required flow rate and the permissible pressure difference.

2. selection of pumps:

• Pump connections are often standardized according to DN dimensions to enable simple integration into piping systems.

3. valves and fittings:

• Shut-off valves, non-return valves, filters and other components are available in standardized nominal sizes to facilitate installation in existing systems.

Challenges when selecting the nominal size

1. non-standardized systems:

• In older systems or international projects, nominal widths may differ, which makes integration more difficult.
• Solution: Use of reducers or adapter connections.

2. deviating wall thicknesses:

• Differences in wall thickness lead to different outer diameters, which can make installation more difficult.
• Solution: Use of transition pieces or flexible sleeves.

3. hydraulic inaccuracies:

• Incorrect dimensioning of the nominal diameter can lead to high pressure losses or insufficient flow.
• Solution: Precise hydraulic calculation taking into account pressure losses, flow velocities and flow rates.