In the pipeline industry, the understanding of pipe size is often confusing. In particular, the difference between outside diameter (OD) and nominal diameter (NB) can lead to misunderstandings in design and application. This article will explore the relationship between outside diameter and nominal diameter in detail to help readers clearly understand these key concepts.
1. Basic concepts of pipe size
Outside diameter (OD)
The outside diameter (OD) refers to the external measurement of the pipe and is a fixed physical dimension. It reflects the overall width of the pipe and has nothing to do with the wall thickness of the pipe. The outer diameter of the pipe is usually represented by the letter "D" followed by the specific outer diameter size. For example, a
seamless steel pipe with an outer diameter of 108 mm can be expressed as D108.
Nominal diameter (NB)
The nominal diameter (NB) is a name introduced to simplify pipe specifications. It does not directly represent the actual inner or outer diameter of the pipe, but is a rough standard used to facilitate design and manufacturing. NB is usually an approximation and is expressed in millimeters. For example, a pipe with a nominal diameter of 100 mm may actually have different inside and outside diameters due to different wall thicknesses.
The main purpose of the nominal diameter is to uniformly represent the specification name of the pipe in the design drawings to avoid confusion in different situations.
Nominal Pipe Size (NPS)
Nominal Pipe Size (NPS) is a standard size for high-pressure, low-pressure and high-temperature pipes in North America. The definition of NPS is based on the outside diameter (OD) of the pipe, but its actual size has no direct relationship to the inside diameter. For example, a pipe with a 2 NPS has an outside diameter of 2.375 inches (or 60.3 mm) regardless of the wall thickness of the pipe.
2. The difference between outside diameter and nominal diameter
Measurement standards
Outside diameter (OD): is the actual outside diameter of the pipe and is usually an exact measurement.
Nominal diameter (NB): is an approximate value that provides a standard specification for the pipe and may not actually be equal to the actual inside or outside diameter of the pipe.
Influence of wall thickness
The wall thickness of the pipe directly affects its inner diameter. Therefore, although the nominal diameter (NB) can help us understand the standard size of the pipe, it does not reflect the actual inner diameter of the pipe. For example, a seamless steel pipe with a nominal diameter of 100 mm may have an outer diameter of 108 mm and a wall thickness of 5 mm. The inner diameter is calculated as: inner diameter = outer diameter − 2 × wall thickness = 108 mm − 2 × 5 mm = 98 mm
Application scenarios
Outer diameter: In actual applications, engineers and technicians usually use the outer diameter directly for installation and matching.
Nominal diameter: Nominal diameter is usually used in design drawings and standardization to facilitate the connection size of components such as pipes,
pipe fittings, valves and
flanges.
3.International standards for nominal diameter
Internationally, nominal diameter has different names and standards. Taking Europe as an example, the nominal diameter is usually called DN (Diamètre Nominal), which is equivalent to NB. In North America, the nominal pipe size (NPS) and NB are often used interchangeably, but in specific applications, attention should be paid to their applicable standards.
For example, for a pipe with a nominal diameter (DN) of 100 mm, although the inner diameter may vary due to different wall thicknesses, 100 mm is usually used as the standard.
4. Relationship between pipe specifications and wall thickness
The wall thickness of a pipe is an important factor affecting the strength of the pipe. Different wall thickness standards usually include:
Standard (STD): Common wall thickness, suitable for general pressure applications.
Extra Strong (XS): Suitable for occasions that need to withstand higher pressures.
Extra Strong Double (XXS): Used for applications that require higher pressures.
Increasing the wall thickness of a pipe can increase its mechanical strength, allowing the pipe to withstand higher design pressures. When designing, the specific application requirements of the pipe must be taken into account.
5. Summary
Outer diameter (OD) and nominal diameter (NB) play an important role in the pipeline industry. Understanding these two concepts and the difference between them is essential for the design, manufacture and application of pipelines. Although the nominal diameter is a convenient standard, in actual applications, accurate measurement of the outer diameter and wall thickness is still indispensable. I hope that through the explanation of this article, it can help readers better understand and apply pipe size standards.