What the flow velocity depends on, how it is calculated from flow rate and diameter, and why too high a velocity causes noise and erosion.
Introduction
When a given flow has to pass through a pipe, the pipe's cross-section determines how fast the water has to move. A narrow pipe forces the water to flow faster at the same flow rate.
The formula
Watch the units: enter flow in l/s and diameter in mm, and convert to SI (÷1000) before inserting. Always use the internal diameter — not the nominal DN or outer dimension.
Why it matters
- 01Too high a velocity causes flow noise and can erode pipes and fittings over time.
- 02Too low a velocity can cause sedimentation and poor purging of air.
- 03In drinking water pipes, a maximum water velocity of approximately 1 m/s is recommended (DS 439/Viega — reduces biofilm and flow noise).
Permitted speed depends on the material
There is no universal speed limit. Copper is the most sensitive material: if water flows too fast — especially hot and with air bubbles or particles — the flow wears away the protective oxide layer on the inside, and erosion corrosion occurs (indentations and eventually perforation, typically in bends and after fittings). Stainless steel and plastic materials (PEX, PP) tolerate markedly higher velocities, because they are not dependent on a thin oxide layer in the same way. Therefore the limit is set lowest for circulating hot copper water.
| Material / application | Indicative maximum speed |
|---|---|
| Copper, cold water | ca. 2,0–2,4 m/s |
| Kobber, varmt vand ≤ 60 °C | ca. 1,5 m/s |
| Copper, hot water > 60 °C | ca. 0,5–0,6 m/s |
| Copper, constantly circulating hot water | under approx. 0.5 m/s |
| Stainless / pressure pipe | tolerates higher than copper (less erosion-sensitive) |
| PEX / PP (plastic) | is often limited by noise and pressure drop, not erosion |
| Service water generally (Danish practice) | ca. 1 m/s |
Tallene er vejledende og varierer mellem kilder og vandkvaliteter. Kobbertallene stammer fra Copper Development Association / branchelitteratur (koldt ≈ 2,4 m/s, varmt ≤ 60 °C ≈ 1,5 m/s, varmt > 60 °C ≈ 0,6 m/s); den danske anbefaling om at holde konstant cirkulerende varmt vand under ca. 0,5 m/s går igen i dansk korrosionsvejledning. De ~1 m/s for forbrugsvand er den branchepraksis DS 439/Viega arbejder efter. (Kilde: Copper Development Association / engineeringtoolbox; Miljøstyrelsen/danskbyggeskik korrosionsvejledning; DS 439:2024/Viega — vejledende.)
From velocity to pressure drop — Darcy-Weisbach
Velocity determines not only noise and erosion, but also how much pressure is lost along the way. Pressure loss in a straight pipe is described by the Darcy-Weisbach equation — and it grows with the square of velocity, so doubling velocity quadruples pressure loss.
Friktionsfaktoren λ afhænger af Reynolds-tallet Re = v · d / ν (ν = kinematisk viskositet, for vand ≈ 1,3 · 10⁻⁶ m²/s ved 10 °C). Ved Re under ca. 2300 er strømningen laminar og λ = 64 / Re; i praksis er vandinstallationer næsten altid turbulente, og λ aflæses da i et Moody-diagram eller beregnes iterativt med Colebrook-White-ligningen ud fra Re og den relative ruhed ε/d. Glatte materialer (kobber, plast) har lav ruhed og dermed lavere λ end gammelt, tæret galvaniseret stål.
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