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Nominal Pipe Size
Carbon steel pipes come in nominal pipe sizes which are measured in inches. For example, you can purchase a pipe from one-quarter inch to 24 inches. For each nominal pipe size there are different schedule numbers or weights. For example, a three-inch nominal pipe comes in four different numbers, each with its own set of dimensions. When ordering by nominal pipe size and schedule number you acquire pipe with other set dimensions for inside and outside diameter and wall thickness.
Inside Diameter
The inside diameter of a pipe, measured from a point in the center of the pipe to the inner wall, is important because it defines the amount of space available. For each nominal pipe size and schedule number, there is a corresponding dimension for inside diameter. A three-inch nominal size pipe 40ST has an inside diameter of 3.068 inches, while a three inch 80XS has an inside diameter of 2.900 inches. The three-inch 160 is 2.624 inches and the three inch XXS is 2.300 inches.
Outside Diameter
Outside diameter defines the distance from a point in the center of the pipe to the outside wall. It provides information as to how space the pipe occupies. All four numbers of the three-inch nominal pipe size have the same outside diameter, 3.500 inches. Thus the difference in the schedule numbers of each nominal pipe size is in the interior of the pipe, not the exterior size. A four-inch nominal pipe size has an outside diameter of 4.500 inches while a five-inch increases to 5.563 inches.
Wall Thickness
Wall thickness is a measurement of the wall of the pipe. Since each nominal size pipe has the same outside diameter, the wall thickness is inversely related to the inside diameter. The smaller the inside diameter, the larger the wall thickness. For a three-inch nominal pipe size the wall thickness is 0.216 inches for the 40ST and 3.000 inches for the 80XS. For the 160 it is 0.438 inches and increases to 6.000 inches for the three-inch XXS.