What Nominal Actually Means
The term “nominal” in engineering does not refer to a precise, measured dimension but rather a designated size used for reference and standardization. It is a value “in name only,” serving as a convenient label to identify a component. A nominal value is the base figure for a part, irrespective of the slight manufacturing variations or tolerances that occur during production. For example, a component labeled with a 50-millimeter nominal diameter may not measure exactly 50 millimeters, but it is accepted as the standard reference size for that part.
This concept of nominal sizing became necessary to ensure universal interchangeability of parts made by different manufacturers across the industry. Before standardization, custom measurements for every component made system design and repair a logistical nightmare. By establishing a fixed, non-dimensional nominal designation, engineers created a universal language that allows a pipe from one company to reliably connect with a fitting or valve from another.
Nominal Diameter Versus Actual Pipe Dimensions
The confusion surrounding nominal diameter stems from its relationship to the pipe’s physical measurements: the Outer Diameter (OD) and the Inner Diameter (ID). For any given nominal diameter, the Outer Diameter (the measurement across the outside edges) is designed to remain constant. This fixed OD is the fundamental measurement that ensures compatibility, as it is the dimension that connects to fittings, flanges, and other system components.
The Inner Diameter, which dictates the flow capacity of the pipe, is a variable measurement. The ID changes based on the pipe’s “schedule,” which is a non-dimensional number defining the wall thickness. If two pipes share the same nominal diameter, but one is a Schedule 40 and the other is a thicker Schedule 80, the pipe with the thicker wall (Schedule 80) will have a smaller Inner Diameter. This occurs because the wall thickness increases, but the fixed Outer Diameter must be maintained to ensure the pipe fits the standard fittings.
For instance, a nominal 2-inch pipe does not actually have a 2-inch OD or ID; its OD is fixed at 2.375 inches regardless of the schedule. The 2-inch nominal size is simply the name that tells you the fixed OD is 2.375 inches and that it will mate with all other 2-inch nominal components. This discrepancy between the nominal name and the actual measurements occurs for sizes up to 12 inches, with the nominal number being loosely based on an older, approximate ID standard. For sizes 14 inches and larger, the nominal diameter and the actual Outer Diameter are often equal.
Understanding NPS and DN Standards
The standardization of pipe sizing is primarily governed by two major systems used worldwide: Nominal Pipe Size (NPS) and Diameter Nominal (DN). The NPS system is the North American standard and uses imperial units, with the nominal size designated in inches (e.g., NPS 6). This system is used to designate the approximate size of the pipe, with the actual dimensions defined in standards like ASME B36.10.
The Diameter Nominal (DN) system is the metric equivalent, widely used in Europe and other parts of the world that adhere to metric standards. DN represents the approximate internal diameter of the pipe in millimeters, and it is usually referenced by the letters DN followed by a number (e.g., DN 150). While NPS is based on inches and DN is based on millimeters, both systems serve the same function of providing a standardized, non-dimensional reference for sizing. The two systems can be roughly related by multiplying the NPS number by 25 to get the corresponding DN value for sizes NPS 4 and above.