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Origin Of \"Schedule\" For Steel Pipe Wall Thickness?

Tony Conner
Tony Conner Member Posts: 549
Carrying on from the string on street elbows - where did THIS one come from?


  • Al Letellier
    Al Letellier Member Posts: 781
    pipe \"schedule

    Back in th olde days of pipe lore, steel pipe was available as standard weight, heavy weight and extra heavy. Then, as steel production increased, organization like ANSI were formed to give some standardization to manufacturing processes. Pipe size tables, or "schedules" were assigned to various thicknesses of pipe walls, which is known as pipe schedule today, such as Sch 40-80-120 etc.. As the schedule size goes up, so does the wall thickness. The outside diameter stays the same, otherwise, we'd need more threading gear than we could ever afford to buy.
    All my old reference and industry books indicate the term Schedule came from these tables of dimensions. However, I can't seem to find how Sch 40 became the standard pipe size, or where 40 came from.
    Hope someone else can find it. I used to know it, but I've been breathing too much PVC cement lately and my brain is stuck........

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  • Boilerpro_3
    Boilerpro_3 Member Posts: 1,231
    Al, you really should stop using PVC for steam headers

    Yes, I know its easier than threading steel, but the fumes are really a killer. Hee, hee, hee!

    Have a nice weekend!


  • Al Letellier
    Al Letellier Member Posts: 781

    Yeah, but when you're french and talented, too, you do the whole job and the rough plumbing usually comes first, if you know what I mean......good one!!!! Taken in the manner it was intended. Ain't this business a hoot???!!!???

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  • Tony Conner
    Tony Conner Member Posts: 549

    That's pretty close, Al! I'm impressed. I had to dig through some old books to find the official word for that question on another board. It was in the 1935 ASHVE Guide. The "standard weight", "extra-stong" (XS) "double-extra strong" (XXS) were done the same way as the "new" schedule designation - the walls got thicker by making the ID smaller. The change in designation, and increase in the number of standard wall thicknesses was forced by the increased use of welding for joining pipe. People figured (and correctly), that since you weren't cutting a thread into the wall of a pipe, and making it thinner, you could just start off with thinner wall material. Since there were no standards, there was this "wild west" effect with all kinds of pipe walls & fittings showing up that were thinner than the standard designations. There was a committee formed under the ASA (now ANSI) to standardize the piping situation - the "Sectional Committee on Standardization of Wrought Iron and Wrought Steel Pipe and Tubing".
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