Steel pipe dimensions

Steel pipes are classified by cross-sectional shape into circular, square, rectangular, and special-shaped pipes; by material into carbon structural steel pipes, low-alloy structural steel pipes, alloy steel pipes, and composite pipes; and by application into pipes for conveying pipelines, engineering structures, thermal equipment, petrochemical industries, machinery manufacturing, geological drilling, and high-pressure equipment. By production process, they are divided into seamless steel pipes and welded steel pipes. Seamless steel pipes are further categorized into hot-rolled and cold-rolled (drawn) types, while welded steel pipes are subdivided into straight seam welded pipes and spiral seam welded pipes.

 

There are multiple methods for representing pipe dimensional parameters. Below are explanations for commonly used pipe dimensions: NPS, DN, OD and Schedule.

(1) NPS (Nominal Pipe Size)

NPS is the North American standard for high/low-pressure and high/low-temperature pipes. It is a dimensionless number used to denote pipe size. A number following NPS indicates a standard pipe size.

NPS is based on the earlier IPS (Iron Pipe Size) system. The IPS system was established to differentiate pipe sizes, with dimensions expressed in inches representing the approximate inner diameter. For example, an IPS 6" pipe indicates an inner diameter close to 6 inches. Users began referring to pipes as 2-inch, 4-inch, or 6-inch pipes.

 

(2) Nominal Diameter DN (Diameter Nominal)

Nominal Diameter DN: An alternative representation for nominal diameter (bore). Used in piping systems as a letter-number combination identifier, consisting of the letters DN followed by a dimensionless integer. It should be noted that the DN nominal bore is a convenient rounded integer for reference purposes, bearing only a loose relationship to actual manufacturing dimensions. The number following DN is typically dimensioned in millimeters (mm). In Chinese standards, pipe diameters are often denoted as DNXX, such as DN50.

Pipe diameters encompass outer diameter (OD), inner diameter (ID), and nominal diameter (DN/NPS). The nominal diameter (DN/NPS) does not correspond to the actual outer or inner diameter of the pipe. During manufacturing and installation, the corresponding outer diameter and wall thickness must be determined according to standard specifications to calculate the pipe's inner diameter.

 

(3) Outer Diameter (OD)

Outer Diameter (OD): The symbol for outer diameter is Φ, and it can be denoted as OD. Globally, steel pipes used for fluid conveyance are often categorized into two outer diameter series: Series A (larger outer diameters, imperial) and Series B (smaller outer diameters, metric).

Numerous steel pipe outer diameter series exist globally, such as ISO (International Organization for Standardization), JIS (Japan), DIN (Germany), and BS (UK).

 

(4) Pipe Wall Thickness Schedule

In March 1927, the American Standards Committee conducted an industrial survey and introduced smaller increments between two primary pipe wall thickness grades. This system uses SCH to denote the nominal thickness of pipes.

 

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