2 Pipe Size
The standard uses two numbers to size pipe: nominal pipe size and schedule numbers. Nominal pipe size (NPS) is a North American set of standard sizes for pipe that refers to pipe diameter. Schedule numbers refer to wall thickness of the pipe.
Nominal Pipe Size
Pipe sizes are documented by a number of standards, such as ANSI/ASME B36.10M, API 5L, and ASTM A53/A53M in North America as well as ISO 65 internationally. The European equivalent of NPS is diameter nominal (DN).
For pipe sizes 1/8” though 12”, the NPS is approximately equal to the inside diameter (ID) in inches. For pipe sizes 14” and above, the NPS is equal to the outside diameter.
Schedule Numbers
Thin-wall pipe has a wall thickness of up to 5/16”. Pipe thicker than 5/16” is considered thick-wall pipe. For example, 6” schedule 40 pipe is considered to be thin-wall pipe because it has a wall thickness of 0.280”, while 6” schedule 80 pipe is considered to be thick-wall pipe because it has a wall thickness of 0.432”.
In the 1920s, the American National Standards Association (now ANSI) standardized the dimensions for wrought steel and wrought iron pipe based on the iron pipe size (IPS) system in use at that time. The association defined three designations for wall thickness: standard weight (STD), extra-strong (XS), and double extra-strong (XXS). Only three designations were required to represent wall thickness because there was only a small selection of wall thickness available.
In later years, a series of additional designations, known as schedule numbers, were added to the standard to better reflect the range of wall thicknesses used in industry. The original three designations are still used today, however extra-strong and double extra-strong are commonly referred to as extra-heavy (XH) and double extra-heavy (XXH). Extra-strong may also be represented as double E.H. or XX.