Basic Knowledge of Steel Pipes 01

Commercial steel pipe is fabricated by piercing or extruding a hot billet(seamless pipe) or by bending then welding steel plates or skelp(longitudinal or spiral welded pipe). In either case, the fabricator produces a pipe with dimensions(diameter and thickness) that comply with a standard, such as ASME B 36.10 for carbon steel pipe, ASME B 36.19 for stainless steel pipe, API 5L for line pipe. Pipe mills also produce custom sizes, typically in the very large diameters. A standard schedule pipe up to 12" has an inner diameter close to its normal pipe size(NPS). Pipe 14" and larger has an outer diameter equal to its NPS.

Pipes are specified by their nominal size and schedule. Unlike pipes, tubes(or tubing) can have round or square cross section. Cylindrical tubing generally has an outer diameter equal to its nominal size, but not in all cases. Pipe schedules were introduced in the 1930's in an effort to standardize and replace the designations of Standard(STD), Extra Strong(XS), and Double Extra Strong(XXS), in use since the late 1800's. The schedule number of stainless steel pipe(ASEM B 36.19) is followed by the letter S, and includes lower schedules with thinner walls than carbon steel pipe(such as 5S and 10S) for low pressure corrosive service. What is the origin of these schedule numbers? According to the 1955's edition of ASME B 36.10, the relationship between pipe size and schedule was originally based on the following formula, a formula that unfortunately works well for certain sizes and schedules but not for others. The question of the origin of the schedule numbers(20, 40, 80, etc.) remains unanswered for now. The 1955 formula is 

t = pipe wall thickness, inch

sch = pipe schedule(ASME B 36.10 or B 36.19)

D = pipe outside diameter, inch