BUYING GUIDE
Pipe schedules
A pipe schedule is a standardized wall thickness for a given pipe size. The outside diameter of each nominal pipe size is fixed — a 6-inch pipe is always 168.3 mm OD — and the schedule number (Sch 40, Sch 80, Sch 160) sets how thick the wall is, and therefore how much pressure the pipe can carry and how much it weighs per metre.
NPS and OD — the part everyone trips on
Nominal pipe size is a label, not a measurement. A '2-inch' pipe is not 2 inches anywhere: its OD is 60.3 mm, fixed by standard so that fittings, flanges and threads always match. From NPS 14 upward the label finally means something — NPS 14 is a true 14-inch (355.6 mm) OD.
Because OD is fixed per size, a thicker wall grows inward. A 6-inch Sch 40 and a 6-inch Sch 160 have identical outside diameters; the Sch 160 simply has a much smaller bore. This is why pipe is specified as size + schedule, never by bore alone.
What the schedule number buys you
Higher schedule = thicker wall = higher pressure rating and more kilograms per metre. Sch 40 is the general-service default; Sch 80 is the step up for higher pressure or where threading eats wall; Sch 160 and XXS live in high-pressure process work. STD and XS are the older weight-class names that mostly coincide with Sch 40 and Sch 80 in the common sizes.
Weight follows directly: each size-and-schedule pair has a published kg/m, which is how pipe is priced and how 2ndMetal calculates every pipe listing's theoretical weight. A 6-inch Sch 40 runs about 28.3 kg/m; the same size in Sch 80 about 42.5 kg/m — the schedule is half the cost of the metre.
Seamless vs ERW — a manufacture route, not a marketing word
Seamless pipe is pierced and rolled from a solid billet — no weld anywhere. ERW (electric resistance welded) pipe is rolled from strip and welded along its length. Both are legitimate; the specification decides. A106 is a seamless-only specification for high-temperature service; A53 comes in both routes — Type S seamless and Type E welded — and API 5L likewise allows both.
For structural and low-pressure work, ERW is usually the economical right answer. For high-temperature and high-pressure process lines, specifications often demand seamless. The route is stated on the mill certificate, and 2ndMetal listings carry it where declared — if your spec says seamless, filter for it rather than assuming.
Reading a surplus pipe listing
A well-described pipe piece gives you size (or OD), schedule or wall thickness in mm, grade, manufacture route where known, and length. Any two of OD, wall and schedule pin down the third — if a listing states OD and wall, the schedule tables tell you what it is.
Surplus pipe is often random-length — cut ends from spool fabrication rather than mill double-randoms. That is exactly what makes it cheap, and for supports, sleeves, casings and short spools, random lengths are all the job needs.
Common questions
Is Sch 40 the same wall thickness for every size?
No — the schedule keeps a similar pressure capability across sizes, so the actual wall grows with the pipe. Sch 40 wall is 3.91 mm at 2-inch but 7.11 mm at 6-inch. Always read the mm wall for the specific size, never assume a schedule is one number.
What do STD, XS and XXS mean?
The older weight-class system: standard, extra strong and double extra strong. STD matches Sch 40 and XS matches Sch 80 up to a point (from NPS 10–12 they diverge). XXS is thicker than Sch 160 in the small sizes. Standards list both systems side by side.
Is seamless pipe better than welded?
It is different, not better. Modern ERW welds are reliable and the pipe is cheaper; seamless is required where specifications demand it — typically high-temperature and high-pressure service, per A106 and similar. Buy the route your spec names, not the more expensive word.
Can I use pipe as a structural member?
Commonly done — pipe makes good columns, handrails and bracing. If the work is engineered, the engineer must accept a pipe grade in place of a structural hollow-section grade; for unspecified fabrication, surplus pipe is one of the cheapest circular sections you can buy.