Line velocity, Reynolds number, and flow regime from flow rate and pipe inside diameter.
v = Q / (πD²/4); Re = ρ v D / μ
How it works
Continuity converts volumetric flow and pipe inside diameter into velocity; the Reynolds number then classifies the flow as laminar or turbulent. It's the first check in any line-sizing exercise, before pressure drop is even computed.
Worked example
50 m³/h of water in a 100 mm line. ≈ 1.77 m/s and Re ≈ 176,000 — comfortably turbulent, and inside the 1–3 m/s guidance for pump discharge lines.
Inputs
Flow rate (m³/h)
Inside diameter (mm)
Density ρ (kg/m³)
Viscosity μ (Pa·s)
Frequently asked questions
What is a good velocity for liquid lines?
Common practice: 1–3 m/s for pump discharge, 0.3–1 m/s for pump suction (to protect NPSH), and below ~4 m/s to avoid erosion, noise, and water hammer. Gas lines are governed by pressure drop and noise rather than a simple velocity band.
Use pipe nominal size or inside diameter?
Always the actual inside diameter. A 4-inch schedule 40 pipe has an ID of 102.3 mm, not 100 mm — and velocity scales with 1/D², so small errors compound.
What does the Reynolds number tell me?
The ratio of inertial to viscous forces. Below ~2300 the flow is laminar (friction ∝ velocity); above ~4000 it is turbulent (friction ∝ velocity²). Most industrial water and hydrocarbon lines run fully turbulent.
Assumptions
Full-bore single-phase flow in a circular pipe.
Typical liquid-line guidance: 1–3 m/s pump discharge, 0.3–1 m/s suction; gas lines are sized by ΔP and noise instead.
References
Continuity and Reynolds number — any fluid-mechanics text (e.g. White, Fluid Mechanics).
Line-sizing velocity guidance per API RP 14E and common practice.
Pipe Pressure Drop — Frictional ΔP and head loss for single-phase flow in a circular pipe (Darcy–Weisbach, Swamee–Jain friction factor).
Fittings Pressure Drop (K-factors) — Minor losses through elbows, tees, and valves by the ΣK excess-head method — the companion to straight-pipe friction.
Friction Factor (Moody) — Darcy (and Fanning) friction factor from Reynolds number and relative roughness — Colebrook–White solved exactly, with the Swamee–Jain explicit fit for comparison.
Pipe Line Sizing (velocity) — Required pipe bore for a target line velocity, with the resulting velocity and typical service velocity guidance.