Flange Dimensions: What Do ID and OD Mean?

When we look at flange dimensions, we often see the unit identifiers ID and OD. What do they mean? This article will tell you.

oD and iD meaning

In flange nomenclature, OD denotes outer diameter and ID denotes inner diameter. You can understand the relationship between OD and ID from the picture below.

OD (Outside Diameter)

The OD (outer diameter) of a flange means the measured diameter of its outermost edge. This is a significant parameter on flange design and engineering drawings since it dictates the flange’s size and the pipe or equipment it can accommodate.

ID (Inside Diameter)

ID (Inner Diameter) means the diameter of the flange’s central hole. This dimension is crucial to ensuring that the flange can accommodate a specified pipe size because the pipe must pass through the flange’s center hole.

Note: Whether it is OD or ID, these two are dimensions with actual measurement meaning. Following them is a number and a unit, typically in millimeters (mm). For example, OD 60 mm, ID 40 mm means that the outer diameter of the tube is 60 millimeters and the inner diameter is 40 millimeters.

These two dimensions are crucial for ensuring that the flange can be properly placed and is compatible with the pipe system or equipment. Flanges are commonly used to connect pipes, valves, pumps, and other components. They use bolts and screws to connect two flanges, and gaskets to seal the connection.

The outer diameter is used to determine the size of fittings (such as flanges or connectors) fitted on the pipe’s outside. It also helps define the pipe’s strength and resistance to external forces.

For example, in construction, the outside diameter is critical for determining the proper pipe size to withstand the structure’s weight and pressure.

Conversely, the inner diameter (ID) is critical for defining the pipe’s flow capacity. The carrying capacity of a pipe is exactly proportional to its inner diameter. For example, in pipeline applications, knowing the ID ensures that water or other fluids flow effectively through the pipe.

The practical application of outer and inner diameters extends across multiple sectors. In the oil and gas business, determining the outside diameter is critical for selecting pipes that can endure harsh environmental conditions and high pressure. On the other side, inner diameter measurements are critical in industries such as heating, ventilation, and air conditioning (HVAC), where exact flow capacity is required for proper ventilation or cooling.

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You may also see these dimension labels: DN and NPS

DN (Nominal Diameter)

DN stands for nominal diameter, which is a general term for the sizes of pipes and fittings in a piping system. The nominal diameter is a convenient round number for reference and does not have a strict relationship with the actual manufacturing dimensions. Normally the numbers following the letters DN do not represent a measured value and cannot be used for calculation purposes.

NPS (Normal Pipe Size)

NPS stands for Normal Pipe Size. It is a piping standard, issued by the American National Standards Institute. The ratio of the pipe’s inside diameter or outside diameter to its wall thickness serves as a measure of the outside diameter in the NPS notation, where the unit of measurement is inches (inch).

For example, NPS 1/2 indicates that the standard diameter of the pipe is 1/2 inch.

In short, DN and NPS are both standard size designation methods used to describe the dimensions of pipes or pipe fittings, but they are applicable to European and North American piping standards, respectively.

The main difference between them lies in the units used: DN uses millimeters as the unit, while NPS uses inches as the unit. Both standards aim to ensure good interchangeability of pipes and fittings produced by different manufacturers, facilitating design and engineering applications.