Pipe schedule dimensions: DN, OD, wall and internal diameter

And the wall thicknesses those bores come from:

These are the ASME B36.10M dimensions for welded and seamless wrought steel pipe. Fluid Network Studio tabulates the range above rather than the whole standard, which extends further in both directions and carries more schedules than the ones shown.

Note the change in behaviour at DN350 and above. Through DN300 the selectable schedules are 10, 40, 80 and 160. From DN350 up, the standard wall and extra-strong wall settle at fixed thicknesses of 9.53 mm and 12.70 mm regardless of size, and they no longer track schedules 40 and 80, so those are the designations offered at large bore. Full schedule 40, 80 and 160 tables for DN350 and above are deliberately not carried here rather than being extrapolated.

Stainless steel pipe, ASME B36.19M

Stainless pipe uses the same outside diameters as carbon steel, so it fits the same flanges, but the S-series schedules are thinner walls, reflecting the corrosion resistance of austenitic stainless and the smaller corrosion allowance it needs.

Where these came from, plainly. The outside diameters are the same B36.10M values used above and are firm. The S-series walls are transcribed from ASME B36.19M for the common sizes, and they have not been re-checked line by line against a controlled copy of the standard. Treat them as good working dimensions for hydraulic calculation and confirm against the standard itself before you use them for procurement or a stress calculation. We would rather tell you that than let you assume a level of checking that has not happened.

PE100 polyethylene pipe, AS/NZS 4130

Polyethylene pipe is not specified by schedule at all. It is specified by outside diameter and SDR, the standard dimension ratio, which is the outside diameter divided by the wall thickness. That makes the bore exact by definition rather than tabulated:

ID = OD * (1 - 2 / SDR)

SDR maps directly onto a pressure class for a given material, and the PN rating shown under each SDR heading is the PE100 mapping. A lower SDR is a thicker wall, a higher pressure rating and a smaller bore, which is the trade you are making every time you move up a class.

Two cautions on these numbers. The SDR wall is a minimum, so the mean bore of real pipe runs slightly smaller than the value above, and the difference is not large but it is one-sided. And polyethylene is a viscoelastic material whose bore changes measurably with temperature and internal pressure, so for a demanding calculation use the manufacturer's stated mean bore for the specific pipe rather than the SDR arithmetic.

Which diameter goes into a pressure-drop calculation

The internal diameter, always. Not the nominal size, and not the outside diameter.

This sounds obvious and it is one of the most common errors in hand calculations, because the nominal size is the number written on every drawing and every purchase order. For DN50 schedule 40 the nominal size is 50 mm and the bore is around 52.5 mm, an error of only a few per cent in the diameter, which is close to 30 per cent in the head loss. For DN15 schedule 80 the bore is under 14 mm against a nominal 15, and the error is worse again.

Relative roughness needs the internal diameter as well, since it is the absolute roughness of the material divided by the bore. The pipe roughness reference covers the numerator.

What these dimensions are not

They are nominal dimensions from the standards, which is what a hydraulic calculation should use, but they are not what will arrive on site to the last decimal.

• Manufacturing tolerance. The standards permit a tolerance on wall thickness, and mill practice usually means the wall is at or above nominal, so the delivered bore tends to be at or below the tabulated value.
• Corrosion and scale. An unlined carbon steel main loses bore over its life. If you are checking an existing system that has been in service for decades, the bore in the ground is not the bore in the table, and neither is the roughness.
• Lining. A cement-mortar lining sits inside the steel bore and takes a real bite out of it. Where a lining is specified, take the finished bore from the lining supplier rather than from the pipe standard.
• Fabrication. Bore reducers, weld penetration at butt welds and internal weld beads all narrow the flow path locally. These are minor-loss items rather than diameter items, and belong in the fitting K factors.

Frequently asked questions

What is the internal diameter of DN100 schedule 40 pipe?

102.26 mm. The outside diameter is 114.3 mm and the schedule 40 wall is 6.02 mm, so the bore is 114.3 less twice 6.02. The same pipe in imperial designation is NPS 4 schedule 40, and the same nominal size at schedule 80 has a bore of 97.18 mm.

What is the difference between DN and NPS?

They are two names for the same pipe. NPS is the imperial nominal pipe size in inches and DN is the metric designator, and the tables above carry both. Neither is a measurement of anything on the pipe: NPS 2 and DN50 pipe has an outside diameter of 60.3 mm.

Does a higher schedule number mean a bigger pipe?

No, it means a thicker wall and therefore a smaller bore. The outside diameter stays the same for every schedule at a given nominal size, which is what allows one flange, one support and one insulation size to serve all of them.

What is SDR, and how does it compare with a schedule?

SDR is the outside diameter divided by the wall thickness, so it is a ratio rather than a lookup, and the wall grows in proportion as the pipe gets bigger. A schedule number is a lookup in a table with no such proportionality. SDR 11 pipe has a wall one eleventh of its outside diameter at every size, and a bore of nine elevenths of the outside diameter at every size.

Are these dimensions valid for imperial pipe sizes?

Yes. ASME B36.10M is the metric presentation of the same dimensional standard used for NPS pipe, and the outside diameters above are the metric statements of the same imperial outside diameters, which is why they land on values such as 114.3 mm and 168.3 mm rather than round numbers.

Why is there no wall thickness table for PE100?

Because SDR makes it unnecessary. The wall is the outside diameter divided by the SDR, so a 110 mm SDR 11 pipe has a 10 mm wall by definition. Publishing that as a separate table would only add a place for a rounding error to enter.

References

• ASME B36.10M, Welded and Seamless Wrought Steel Pipe, for the carbon steel outside diameters and wall thicknesses.
• ASME B36.19M, Stainless Steel Pipe, for the 5S, 10S, 40S and 80S walls.
• AS/NZS 4130, Polyethylene (PE) pipes for pressure applications, for the PE100 outside diameter series and SDR classes.

Related

• Absolute pipe roughness values by material, the other input a friction calculation needs.
• Pipe fitting K factors for the minor losses on the same run.
• Pipe velocity calculator and the pipe flow and pressure drop calculator, both of which fill the diameter from this same size table.
• Open the Studio to pick DN and schedule on a real network instead of looking bores up by hand.