What is the difference between NPS and DN in relation to ASME B16.5 Flanges?

In the world of industrial piping, flanges play a crucial role in connecting pipes, valves, pumps, and other equipment to form a complete piping system. ASME B16.5 is one of the most widely recognized standards for pipe flanges and flanged fittings in North America. When dealing with ASME B16.5 flanges, two terms that often come up are NPS (Nominal Pipe Size) and DN (Diamètre Nominal). As a supplier of ASME B16.5 Flange, I've encountered numerous inquiries regarding the differences between these two concepts. In this blog post, I'll delve into the details of NPS and DN, their significance in relation to ASME B16.5 flanges, and how they impact the selection and use of these essential components.

Understanding NPS (Nominal Pipe Size)

NPS is a North American standard used to designate the size of pipes and piping components. It is a dimensionless number that represents the approximate inside diameter (ID) of the pipe in inches for pipes with a wall thickness up to a certain limit. However, it's important to note that the actual inside diameter of a pipe may vary depending on its schedule (wall thickness). For example, a pipe with an NPS of 2 inches does not necessarily have an inside diameter of exactly 2 inches. The actual ID will depend on the schedule of the pipe, which can range from Schedule 5 to Schedule 160 or higher.

In the context of ASME B16.5 flanges, NPS is used to specify the size of the flange. Flanges are available in a wide range of NPS sizes, typically from 1/2 inch to 24 inches, although larger sizes may also be available for special applications. The NPS size of a flange determines its dimensions, such as the outside diameter (OD), bolt circle diameter (BCD), number and size of bolts, and the thickness of the flange. These dimensions are standardized according to ASME B16.5 to ensure compatibility and interchangeability between different manufacturers' flanges.

Understanding DN (Diamètre Nominal)

DN is a European standard used to designate the size of pipes and piping components. It is a metric unit that represents the approximate inside diameter of the pipe in millimeters. Similar to NPS, DN is a dimensionless number, and the actual inside diameter of a pipe may vary depending on its wall thickness. DN sizes are commonly used in countries that follow the metric system, such as most European countries, as well as in many other parts of the world.

In the context of ASME B16.5 flanges, DN sizes are sometimes used in international trade or in applications where metric measurements are preferred. While ASME B16.5 primarily uses NPS sizes, it also provides conversion tables to allow for the use of DN sizes. The conversion between NPS and DN is based on a simple ratio, where 1 inch is approximately equal to 25.4 millimeters. For example, an NPS 2-inch pipe is equivalent to a DN 50 pipe.

Key Differences between NPS and DN

1. Measurement System: The most obvious difference between NPS and DN is the measurement system they belong to. NPS is based on the imperial system (inches), while DN is based on the metric system (millimeters). This difference can lead to confusion when working with international suppliers or in projects that involve a mix of imperial and metric components.
2. Size Designation: NPS is a dimensionless number that represents the approximate inside diameter of the pipe in inches, while DN is a metric unit that represents the approximate inside diameter of the pipe in millimeters. The relationship between NPS and DN is not always a direct one, as the actual inside diameter of a pipe may vary depending on its wall thickness.
3. Standardization: NPS is a North American standard, while DN is a European standard. ASME B16.5 primarily uses NPS sizes, but it also provides conversion tables to allow for the use of DN sizes. However, it's important to note that there may be some differences in the dimensions and tolerances between NPS and DN flanges, even when they are supposed to be equivalent in size.
4. Usage: NPS is widely used in the United States and other countries that follow the imperial system, while DN is commonly used in Europe and other countries that follow the metric system. In international trade, both NPS and DN sizes may be used, depending on the requirements of the customer and the specifications of the project.

Impact on ASME B16.5 Flange Selection

When selecting an ASME B16.5 flange, it's essential to consider both the NPS and DN sizes to ensure compatibility with the piping system. Here are some key factors to keep in mind:

1. Pipe Size: The NPS or DN size of the flange should match the size of the pipe it will be connected to. Using a flange with the wrong size can result in leaks, improper alignment, or other issues that can compromise the integrity of the piping system.
2. Pressure Rating: The pressure rating of a flange is determined by its class, which is based on the maximum allowable working pressure (MAWP) at a given temperature. ASME B16.5 flanges are available in various classes, such as Class 150, Class 300, Class 600, etc. The class of the flange should be selected based on the operating pressure and temperature of the piping system.
3. Flange Type: ASME B16.5 flanges come in different types, such as weld neck, slip-on, blind, socket weld, and threaded flanges. The type of flange should be selected based on the application, the type of pipe connection, and the installation requirements.
4. Material: Flanges are available in a wide range of materials, including carbon steel, stainless steel, alloy steel, and High Nickel Alloy ASME B16.5 Flanges. The material of the flange should be selected based on the corrosiveness of the fluid or gas being transported, the operating temperature and pressure, and the environmental conditions.

Examples of ASME B16.5 Flange Sizes

To illustrate the relationship between NPS and DN sizes, here are some examples of ASME B16.5 flange sizes:

These dimensions are based on a Class 150 Flange ASME B16.5 Class 150 and are for illustrative purposes only. The actual dimensions may vary depending on the specific requirements of the application and the manufacturer's specifications.

Conclusion

In conclusion, NPS and DN are two different standards used to designate the size of pipes and piping components. NPS is a North American standard based on the imperial system, while DN is a European standard based on the metric system. In the context of ASME B16.5 flanges, NPS is the primary size designation, but DN sizes are also used in international trade and applications where metric measurements are preferred. Understanding the differences between NPS and DN is essential for selecting the right flange for your piping system and ensuring compatibility and interchangeability.

As a supplier of ASME B16.5 flanges, I'm committed to providing high-quality products and expert advice to our customers. If you have any questions or need assistance with selecting the right flange for your application, please don't hesitate to contact us. We'll be happy to help you find the best solution for your needs.

References

• ASME B16.5 - Pipe Flanges and Flanged Fittings: NPS 1/2 through NPS 24 Metric/Inch Standard
• ISO 6708 - Pipework - Nominal sizes - Definition and selection