How pipe pressure drop is calculated
The Darcy–Weisbach equation gives friction loss from the friction factor, pipe length, diameter and the flow's velocity head. The friction factor depends on the Reynolds number and the pipe's relative roughness, found by solving the Colebrook equation.
- Reynolds number:
Re = ρ·V·D/μ - Colebrook:
1/√f = −2·log₁₀(ε/(3.7D) + 2.51/(Re·√f)); laminarf = 64/Re - Friction loss:
Δp = f·(L/D)·½ρV² - Elevation:
Δp = ρ·g·Δz
Worked example: well pump to a tank
Pumping 60 L/min of 12 °C water through 45 m of 32 mm plastic pipe flows at 1.24 m/s (Re ≈ 32,000, turbulent). Pipe friction costs about 25 kPa and lifting the water 18 m about 1.76 bar, so with fittings the pump must supply about 2.06 bar.
Good to know
- Keep liquid velocities between about 0.6 and 3 m/s (2–10 ft/s) to avoid settling at the low end and noise and erosion at the high end.
- Pro adds fittings and valves, pump shaft power and the system curve to compare with pump curves.