History of Pipe Schedules

I thought I would share this article.

Although this is common knowledge that is taken for granted among a lot of us old engineers, I have found that young engineers are lacking this information because both academic professors and we experienced engineers are both guilty of not passing on the information which used to be common and available when piping and fitting catalogs like Vogt, Tube Turns, Walworth, etc. used to be freely available to us. Now, these valuable free catalogs have become a thing of the past...

Because I regard this subject as very basic and important for all engineers to dominate, some years back I prepared the following explanation for young engineers working under me and with me in plant projects. I would like to share it with you and with any one else who hasn't had the opportunity to find out this logical explanation of how pipe is classified.

Industrial pipe thicknesses follow a set formula, expressed as the "schedule number" as established by the American Standards Association (ASA) now re-organized as ANSI - the American National Standards Institute. Eleven schedule numbers are available for use: 5, 10, 20, 30, 40, 60, 80, 100, 120, 140, & 160. The most popular schedule, by far, is 40. Sch 5, 60, 100, 120, & 140 have rarely, if ever been employed by myself in over 40 years as a practicing engineer. The schedule number is defined as the approximate value of the expression:

Schedule # = (1,000)(P/S)

Where,
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 = (1000)(350/10,000) = 35 (approx. 40)

This would be the proper schedule for welded joints and steel fittings but not for threaded connections and cast-iron or malleable-iron fittings. In practice, schedule 40 would be used for welded construction and Sch 80 (about 2x the computed value) for iron fittings. The higher schedule is required because of weaknesses the iron fittings and the metal lost in cutting the threads.

For all pipe sizes below 10", Sch 40 pipe is identical with what was once called "standard" pipe, and Sch 80 is identical with the former "extra-strong" pipe. There is no equivalent schedule number for "double-extra-strong" pipe, and Sch 160 is the only other weight in which pipe smaller than 4" is available.

Temperature has no direct bearing on the schedule, except as it either weakens (or strengthens) the material's allowable stress. Stainless steels (304ELC & 316ELC), for example yield a stronger allowable stress at the low temperatures near the cryogenic zone (-50 to -150 oF). Copper and Brass also exhibit the same behavior. I've used the rule of thumb that the softer the metal, the stronger it is at the lower temperatures.

I hope this has helped you in explaining how pipe is classified.