Steel pipes are available in a number of different sizes, varying from 1/8 inch to 24 inches. The external diameter for a 1/8-inch steel pipe is normally 0.41 inch, while the internal diameter is 0.27 inch.
A 24-inch steel pipe, on the other hand, usually has an external diameter of 24 inches and an internal diameter of 22.63 inches. The sizes of steel pipes are identified by two designations: NPS and Sch. The term NPS stands for "normal pipe size" and Sch. is the short form for schedule, or the thickness of the wall. The alternative term used in Europe for NPS is DN or Diameter Nominal. In this system, the sizes of the steel pipes are measured in millimeters instead of inches.
Steel pipes are normally intended for pressure and mechanical applications. They can be used in water, air, gas and steam pipelines. Also, they are quite suitable for welding, bending and coiling. Stainless steel pipes are considered the strongest and most durable. They are used mainly in the oil and gas industry. Carbon steel pipes, on the other hand, are used in machine parts of aircrafts and heavy vehicles, mainly because they are moderately resistant to wear and tear.