Schedule 40 vs 40S: pipe schedules for stainless, nickel alloy and carbon steel
Sch 40S belongs to ASME B36.19, the schedule standard for stainless and nickel-alloy pipe; Sch 40 belongs to ASME B36.10 for carbon and alloy steel. Outside diameters are identical, and the walls of Sch 40S and Sch 40 are the same up to NPS 10. From NPS 12 upward they differ, so check before mixing them on one line.
Why there are two schedule standards
Pipe outside diameter is fixed by the nominal pipe size (NPS); the schedule sets the wall. Carbon steel pipe follows ASME B36.10, with schedules 10 to 160 plus STD, XS and XXS. Stainless and nickel-alloy pipe follows ASME B36.19, which uses the "S" schedules — 5S, 10S, 40S and 80S. Light walls such as 5S and 10S exist because corrosion-resistant alloys rarely need a corrosion allowance and cost far more per kilogram.
Where 40 and 40S match — and where they don't
Up to NPS 10, Sch 40S and Sch 40 have the same wall thickness, and so do 80S and 80 up to NPS 8. Above those sizes the S schedules stay thinner: for NPS 12, Sch 40S equals the STD wall, while Sch 40 is heavier. The pipe schedule chart lists every size.
Because outside diameters match, fittings and flanges fit either schedule — but bores and wall thickness for buttweld fittings and weld neck flanges must match the pipe wall, so state the schedule on every line.
How the wall is chosen
Under ASME B31.3 the pressure design thickness is t = P·D / 2(S·E·W + P·Y): P is design pressure, D the outside diameter, S the allowable stress of the material at temperature (from ASME Section II-D), E the joint factor (1.0 for seamless), W the weld strength reduction factor and Y a temperature coefficient. Add corrosion and erosion allowances, allow for the mill under-tolerance (typically −12.5% on seamless pipe), then choose the next schedule up.
Because S differs between materials, the same service can need a different schedule in 316L, duplex 2205 or A106 Gr B — duplex, being stronger, often allows a thinner wall.
Schedules by material
| Material | Usual schedules | Pipe standard |
|---|---|---|
| Carbon steel | Sch 40 / STD, 80 / XS, 160 | ASTM A106 |
| Low-temperature carbon steel | Sch 40, 80, 160 | ASTM A333 |
| Chrome-moly | Sch 40, 80, 160 | ASTM A335 |
| Austenitic stainless | 10S, 40S, 80S | ASTM A312 |
| Duplex & super duplex | 10S, 40S, 80S | ASTM A790 |
| Nickel alloys (825, 625, C276) | 10S, 40S, 80S | B423 · B444 · B622 |
Ordering pipe
- Material standard and grade, e.g. A312 TP316L or B423 N08825
- Seamless or welded
- NPS and schedule — write 40S, not 40, for stainless
- Length: single random, double random or cut lengths
- Ends: plain, bevelled (ASME B16.25) or threaded
- Certificate and tests: EN 10204 3.1/3.2, hydrostatic or electric test, IGC where required