steel pipe sizes and dimensions

Steel pipes are categorized according to various criteria: by cross-sectional shape into circular, square, rectangular, and special-shaped types; by material into carbon structural steel, low-alloy structural steel, alloy steel, and composite pipes; and by application into those used for conveyance pipelines, engineering structures, thermal equipment, petrochemical industries, mechanical manufacturing, geological drilling, and high-pressure apparatus. Based on manufacturing methods, they are classified into seamless steel pipes (which include hot-rolled and cold-rolled/drawn varieties) and welded steel pipes (further divided into straight-seam and spiral-seam welded types).

Pipe dimensional parameters can be expressed in several ways. The following outlines commonly used designations: NPS, DN, OD and Schedule.

(1) NPS (Nominal Pipe Size)
NPS is a North American standard applied to pipes under various pressure and temperature conditions. It is a dimensionless numerical designation used to specify pipe dimensions. The value following "NPS" corresponds to a standardized pipe size.
This system evolved from the earlier IPS (Iron Pipe Size) standard, which was developed to distinguish between different pipe sizes based on approximate inner diameter measured in inches. For instance, an IPS 6″ pipe has an inner diameter near 6 inches. Consequently, pipes became commonly referred to by such nominal sizes (e.g., 2-inch, 4-inch, 6-inch).

(2) Nominal Diameter DN (Diameter Nominal)
DN serves as an alternative designation for nominal bore. It is used in piping systems as a alphanumeric identifier, comprising the letters "DN" followed by a dimensionless integer. It is important to note that the DN value represents a rounded reference size and does not correspond exactly to manufactured dimensions. In Chinese standards, pipe diameters are commonly indicated as DNXX (e.g., DN50), where the number is conventionally expressed in millimeters (mm).
Pipe diameter terminology includes outer diameter (OD), inner diameter (ID), and nominal diameter (DN/NPS). The nominal diameter (DN/NPS) does not equate to the actual OD or ID; rather, applicable standards specify the corresponding outer diameter and wall thickness required for manufacturing and installation, from which the inner diameter is derived.

(3) Outer Diameter (OD)
Denoted by the symbol Φ or the abbreviation OD, outer diameter is a critical dimension. Internationally, steel pipes for fluid transport are commonly grouped into two OD series: Series A (imperial, larger diameters) and Series B (metric, smaller diameters).
Multiple international standards define outer diameter series, including those set by ISO (International Organization for Standardization), JIS (Japanese Industrial Standards), DIN (German Institute for Standardization), and BS (British Standards).

(4) Pipe Wall Thickness Schedule
In March 1927, the American Standards Committee introduced a system specifying intermediate wall thickness values between two primary grades following an industrial survey. This system employs the term "Schedule" (abbreviated as SCH) to indicate the nominal wall thickness of pipes.

EHONG STEEL--steel pipe dimensions