Formula & Engineering Notes
Pipe bore from a velocity limit:
Density ρ = 1/vg is the saturated-vapour density at the line pressure. The recommended NPS is the smallest standard bore whose velocity stays at or below the limit.
All steam properties are interpolated from the shared IAPWS-IF97 steam-table datasets — see the full steam tables and the Steam Property Calculator.
Steam pipe sizing is a velocity game, not a pressure-drop game — at least for the first pass. Pick too high a velocity and you get noise, erosion at bends, and carry-over of any condensate; too low and you've bought oversized pipe and lagging for nothing. For saturated steam, 25–40 m/s is the usual window; superheated lines tolerate more.
The density does the heavy lifting here. Steam at 10 bar is roughly five times denser than at 1 bar, so the same mass flow needs a much smaller pipe at high pressure. That's the whole reason distribution headers run high and let down locally.