Pipe Schedule is the term used to describe the thickness of a pipe. The outside diameter of a pipe is the same for all Schedules in a particular nominal pipe diameter. Standard standard pipe schedules or pipes sizes as given by ANSI / ASME B36.10M and API 5L. There are eleven schedule numbers commonly used: 5, 10, 20, 30, 40, 60, 80, 100, 120, 140, & 160.
The most popular schedule, by far, is 40. For pipe 8-inches and under, Schedule 40 and Standard Weight pipe wall thicknesses are the same. Schedules 5, 60, 100, 120, & 140 are rarely used. This should be considered when piping is specified for a particular project.
The schedule number is defined as the approximate value of the expression:
Schedule Number = (1,000)(P/S)
P = the internal working pressure, psig
S = the allowable stress (psi) for the material of construction at the conditions of use.
For example, the schedule number of ordinary steel pipe having an allowable stress of 10,000 psi for use at a working pressure of 350 psig would be:
Schedule Number = (1,000)(350/10,000) = 35 (approx. 40)
Note that the pipe is not always the "weakest link" in a piping system but this should be examined each and every time pipe is sized.
Method for Determing Schedule
Measure the inside diameter and divide it by the wall thickness. (inches) R= ID/Thickness
- API Standards
- API Spec 5L - Specification for Line Pipe
- ASME Standards
- ASME B36.10M - Welded and Seamless Wrought Steel Pipe