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Steel pipe is ordered by two names: a nominal pipe size (NPS) and a schedule. Neither is a real dimension, which is why a "2 inch Schedule 40" pipe has an outside diameter of 60.3 mm and a bore of 52.5 mm. This article explains how the two names work, and how to get from them to weight and pressure.
Nominal size is not the outside diameter
Every NPS has a fixed outside diameter, whatever the schedule. From NPS ⅛ to 12 the outside diameter is larger than the nominal size: NPS 2 is 60.3 mm (2.375 in) and NPS 6 is 168.3 mm (6.625 in). From NPS 14 upwards the outside diameter equals the nominal size, so NPS 14 is 355.6 mm (14 in). The metric name DN is a label for the same pipe: DN 50 is NPS 2.
Because the outside diameter stays fixed, a heavier schedule makes the bore smaller, not the pipe bigger. That is why the same flanges, fittings and pipe supports fit every schedule of one size.
A schedule is a wall thickness for one size
The schedule number picks the wall thickness, but the thickness it gives depends on the size:
Scroll the table sideways to see all columns on a small screen.
| Size | Sch 40 wall | Sch 80 wall |
|---|---|---|
| NPS ½ (DN 15) | 2.77 mm | 3.73 mm |
| NPS 2 (DN 50) | 3.91 mm | 5.54 mm |
| NPS 6 (DN 150) | 7.11 mm | 10.97 mm |
| NPS 12 (DN 300) | 10.31 mm | 17.48 mm |
The wall grows with the pipe, but not in proportion, so a larger pipe in the same schedule usually takes less pressure than a smaller one.
STD, XS and XXS
The older names Standard (STD), Extra Strong (XS) and Double Extra Strong (XXS) are still common on orders. STD is the same as Sch 40 up to NPS 10, and XS is the same as Sch 80 up to NPS 8. Above those sizes they part company: NPS 12 STD has a 9.53 mm wall, but NPS 12 Sch 40 has 10.31 mm. Always check which one a drawing or a supplier means.
Stainless pipe: 5S and 10S
Stainless pipe is often thinner, because it rarely needs a corrosion allowance and costs more per kilogram. ASME B36.19M adds the light schedules 5S and 10S. Its 40S and 80S walls match STD and XS up to NPS 12.
Working out the weight
For carbon steel, the mass per metre is:
kg/m = 0.02466 × (D − t) × t, with the outside diameter D and the wall t in millimetres.
For NPS 2 Sch 40 that gives 0.02466 × (60.3 − 3.91) × 3.91 = 5.44 kg/m. Full of water, the same pipe weighs 7.60 kg/m, which matters when you size pipe supports and plan lifts.
Estimating the allowable pressure
Process piping codes such as ASME B31.3 set the minimum wall for straight pipe under internal pressure. Turned around, the same equation gives the pressure a wall can take:
P = 2 S E t / (D − 2 Y t)
- S is the allowable stress of the material at the design temperature. For A106 Grade B up to 38 °C it is 138 MPa, and it falls as the temperature rises.
- E is the weld joint factor: 1.0 for seamless pipe and 0.85 for electric resistance welded pipe.
- t is the wall you can count on: the nominal wall less the mill under-tolerance, usually 12.5%, and less any corrosion allowance.
- Y is 0.4 for steels below about 480 °C.
For seamless NPS 2 Sch 40 A106 B with a 1.5 mm corrosion allowance, the wall you can count on is 3.91 × 0.875 − 1.5 = 1.92 mm, and the allowable pressure is about 90 bar. Without the corrosion allowance it would be about 164 bar, which shows how much the allowance matters on small pipe.
This is a straight-pipe check only. Bends, branches, flanges and valves have their own ratings, and a real piping design is checked to the code that applies to it.
Flow velocity
The bore also sets the flow velocity. For water, designers often aim for about 1 to 1.5 m/s in pump suction lines and 1.5 to 3 m/s in discharge lines, to keep friction losses and noise down and to give the pump enough suction head. At 20 m³/h, NPS 2 Sch 40 runs at about 2.6 m/s: fine for a discharge line, but fast for a suction line.
