H-100Pipe Sizing and Head Loss

How it works

1. Size on velocity — Pick the bore from the velocity you want, not from the pump you happen to have. Below about 0.9 m/s solids drop out and settle in the invert; above 2.5 m/s you are paying for friction and inviting erosion and water hammer. Everything else follows from that choice.
2. Flow regime — Reynolds number decides which physics applies. Below 2300 the flow is laminar and the profile is a clean parabola. Above 4000 it is turbulent, the profile goes blunt, and mixing keeps solids suspended. The band between is unstable — never design into it deliberately.
3. Friction along the run — Darcy–Weisbach gives the distributed loss: h = f (L/D) v²/2g. The friction factor comes from Reynolds and relative roughness through Colebrook–White, or the Swamee–Jain fit. Note the v² term — double the velocity and the friction quadruples.
4. Minor losses — Every bend, valve and reducer costs a multiple of the velocity head. On a long transfer main these are genuinely minor. On a short, fitting-dense skid they routinely exceed the straight-run friction — and on a pump suction they are what pushes you into cavitation. Wind Σ K up and watch the split.
5. What governs it — Velocity sets the diameter; diameter sets the friction; friction sets the pump. Go one size down and you save on pipe and pay for it every hour the pump runs, because the head loss goes with v² and 1/D⁵. That trade is the whole of pipe sizing.
6. Ductile iron — Strong enough to bury under a road, which is why it dominates large potable mains. The number that catches people is roughness: cement lining starts at about 0.03 mm and climbs as it ages. Design a pump on the as-new figure and you will be short of head within a few years.

Design parameters

The panel opposite runs the same correlations as the H-100 design sheet, so the animation and the calculator cannot disagree. Every parameter below is live — drag one and the picture responds.

Open the full H-100 design sheet for the governing equations, the accepted design envelopes and the worked calculation.