Pipe Sizing: Velocity Limits, Head Loss, and Economic Diameter
Diameter is a trade between velocity (erosion, noise, NPSH) and head loss (pumping cost). Typical water velocities, continuity, and how to iterate D.
Key takeaways
- Q = V A fixes D once you pick a target velocity.
- Suction pipes run slower than discharge pipes.
- Head loss ∝ 1/D⁵ in turbulent Darcy flow — diameter is cheap insurance.
- Convert L/s and gpm to m³/s before you size.
Pipe sizing starts with continuity, not with a friction formula. Pick a velocity that the service can live with, compute D, then check head loss, NPSH, and surge. If any check fails, increase D and repeat.
Continuity
Example: 50 L/s = 0.05 m³/s at V = 2.0 m/s → D = 0.178 m. The next standard size is DN200 (0.2 m ID class), which drops velocity to about 1.6 m/s and cuts head loss.
Open solver: Pipe sizing calculatorVelocity bands
| Service | Typical V (m/s) | Why |
|---|---|---|
| Pump suction (water) | 0.8–1.5 | NPSH, air release |
| Pump discharge (water) | 1.5–3.0 | Balance of steel vs kWh |
| Building cold water | 1.0–2.5 | Noise and erosion |
| Gravity sewer (fullness check) | 0.6–3.0 | Self-clean vs scour |
| Compressed air (header) | 6–12 | Different fluid, still a ΔP budget |
Then check friction
Turbulent Darcy loss scales roughly as Q² L / D⁵. Doubling D (and keeping Q) cuts friction by about 32. That is why “one size up” is the usual answer to a high-head-loss complaint, not a smoother lining.
Open solver: Head-loss checkOpen solver: Suction-line NPSH checkOpen solver: L/s, gpm, m³/h converterFrequently asked questions
Suction: often 0.8–1.5 m/s. Discharge: 1.5–3 m/s. Building services sometimes allow 2–4 m/s on short runs. Fire mains and slurry have their own rules.