Nominal Diameter (DN) or Nominal Pipe Size (NPS)

The nominal diameter is the primary concept guiding the definition of a pipe. While this guide doesn't aim to explain the standards and regulations in detail, a brief explanation can be useful.

The Nominal Pipe Size (NPS) is a standardized system used to designate the size of pipes in the field of piping and hydraulics. It is a nominal designation rather than an actual measurement of the pipe’s external diameter. NPS is mainly used in North America, whereas other regions might use different systems such as DN (Nominal Diameter) in Europe.

Here are the key points about Nominal Pipe Size:

  • Nominal Size: NPS does not represent the actual size of the pipe. It is rather a nominal size used for identification purposes. For example, a pipe labeled as NPS 4 does not necessarily have an external diameter of exactly 4 inches.
  • Standardization: NPS is standardized by organizations such as the American National Standards Institute (ANSI) and the American Society for Testing and Materials (ASTM). These standards ensure uniformity and compatibility in the manufacturing and use of pipes.
  • Relationship with Actual Diameter: The actual external diameter of a pipe marked with a given NPS size may vary depending on the wall thickness and material. For example, a pipe labeled NPS 4 can have a slightly different external diameter compared to another pipe also labeled NPS 4, but with a different schedule (wall thickness).
  • Schedule: In addition to NPS, pipes are often classified by their schedule, which indicates the wall thickness. Common schedules include Schedule 10, Schedule 40, and Schedule 80. The schedule affects the internal diameter, pressure rating, and the pipe's weight.

Understanding the concept of NPS is crucial for selecting the correct pipes and fittings for a piping system. Correctly matching pipe sizes ensures compatibility and efficient fluid flow within the system.

In summary, Nominal Pipe Size (NPS) is a standardized system used to designate pipe sizes, providing a common language for communication within the piping industry. It helps ensure consistency and compatibility in the design, production, and installation of piping systems.