Absolute Roughness of Commercial Steel Pipe

Absolute roughness is the equivalent sand-grain height that reproduces the observed friction factor, not a profilometer measurement — Nikuradse glued graded sand into pipes to calibrate the scale, and every ε in the Moody chart traces back to that. For new commercial steel, 0.046 mm is the universally quoted figure, and it feeds the relative roughness ε/D that goes into Colebrook or Swamee-Jain.

The word doing the work is "new". Corrosion, tuberculation and scale in an old steel water main can raise effective roughness by a factor of ten or more, and the internal diameter shrinks at the same time — a double penalty, since head loss scales with roughness weakly but with diameter to the fifth power. Designers commonly use a corroded value or an aging allowance for water mains, while keeping the clean figure for compressed air and closed hydronic loops that never see fresh oxygen. Fortunately friction factor depends on ε/D only logarithmically, so a factor-of-two error in roughness moves head loss by only a few percent in turbulent flow.