The word “steel pipe” refers to round hollow sections used for transmission and distribution pipelines and piping systems that convey fluids and gases – such oil & gas, propane, steam, acids, and water.
The most important dimension for a steel pipe is the inside diameter (“pipe ID”), which indicates the rough (not the exact) fluid conveyance capacity of the tubular. The ID is expressed in NPS” or “DN” (nominal pipe size, or bore size).
The pipe outside diameter (OD) does not match the nominal size for pipes below NPS 14 inches (a 2 inches pipe, for instance, has an internal flow capacity of approximately 2 inches, but has an outside diameter of 2.375 inches). For pipes of a given NPS, the pipe outside diameter is fixed, whereas the pipe inside diameter decreases by increasing schedule values (pipe wall thickness).
The most important mechanical parameters for pipes are the pressure rating, the yield strength, and the ductility.
The standard combinations of pipe nominal diameter and wall thickness (schedule) are covered by the ASME B36.10 and ASME B36.19 specifications (respectively, carbon and alloy pipes, and stainless steel pipes).