How to Convert Pipe Size from Nominal Bore (NB) to Millimeters (mm)

When working with pipes, it's often necessary to convert the nominal bore (NB) size to millimeters (mm) for specific applications or standards. This guide will help you understand the Nominal Bore (NB) system, provide a straightforward conversion method, and discuss specific pipe standards that may require different considerations.

Understanding Nominal Bore (NB)

Nominal Bore (NB) is a standardized designation for the nominal diameter of a pipe. While the NB size does not directly correspond to the physical measurement, it is often used as an approximation for the internal diameter (ID) of the pipe in millimeters. This system is widely used for water, gas, and other liquid or gaseous systems.

Conversion from NB to mm

Converting NB to millimeters can be a simple process, especially for common pipe sizes. Here are some guidelines to help you convert:

For pipes with common NB sizes: The NB number is often equal to the approximate internal diameter (ID) in millimeters. For example: NB 15 15 mm NB 20 20 mm NB 25 25 mm NB 32 32 mm NB 40 40 mm NB 50 50 mm And so on...

Conversion Table

Below is a table showing the approximate conversion from NB to mm:

NB Size Approximate ID (mm) NB 15 15 NB 20 20 NB 25 25 NB 32 32 NB 40 40 NB 50 50 NB 65 65 NB 80 80 NB 100 100

Specific Pipe Standards

For specific pipe standards like Schedule 40 or Schedule 80, the actual internal diameter (ID) may vary, requiring you to reference a pipe size chart for the specific standard you are working with.

For example:

If you are dealing with ASME B 36.10 and B 36.19 standard pipes with sizes up to 12 inches (300 NB), no calculation is necessary. For 14 inches (350 NB) and above, multiply the NB size by 25.4 to get the outside diameter (OD) in millimeters. For instance, 14 x 25.4 355.6 mm.

Conclusion

To ensure accuracy in your pipe size conversions, it is always best to refer to the specific pipe specifications or standards you are working with. The dimensions can vary based on the material and manufacturing standards.

Understanding the conversion from Nominal Bore (NB) to millimeters (mm) is crucial for various applications, from plumbing to industrial systems. By using the guidelines provided here, you can make more informed decisions and ensure compatibility in your projects.