The American National Standards Institute (ANSI) classifies pipe thickness by the so called "Schedule Number" (Sch nº). Originally, many years ago, when the Sch. nºs were established, there was a relationship between the diameter, the allowable working pressure and the necessary pipe wall thickness to withstand it. As years passed, piping materials became better and able to withstand higher pressures, and that relationship is no longer valid. Schedule numbers, however, were (and still are) so deeply incrusted in the mentality of piping designers, erectors and users (including me), that they were left in the standards and are still widely used. Pipe dimensions are identyfied by the NOMINAL diameter and Schedule number.
Now, this is valid regarding piping made to ANSI. There are other piping standards such as API 5L, ASTM A-139 and others, which identify pipe dimensions by the OUTSIDE diameter and the ACTUAL wall thickness in inches. As the pipe made to these standards is welded, i.e, made by rounding up a plate and welding, the criteria in establishing the pipe wall thickness was the availability of plate. Regarding API 5L, several wall thicknessess coincide with Schedule numbers for pipe of the same diameter.
Giovanni S. Crisi
Sao Paulo - Brazil
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