The energy lost to friction along the pipe, as pressure: what a pump must overcome. ΔP = f · (L/D) · (ρv²/2).
Re = vD/ν. Below about 2,300 flow is laminar; above 4,000 it is turbulent. Almost every pumped liquid system is turbulent.
Read off the Moody diagram from Re and relative roughness. Laminar: the clean f = 64/Re; turbulent: an empirical curve (Swamee–Jain here).
ε/D: a 0.05 mm pit barely matters in a 200 mm pipe but dominates in a 6 mm one, so small pipes are far more sensitive to material.
Pressure drop scales with the fifth power of diameter, so a small increase in bore dramatically cuts the loss: often the cheapest fix for an under-performing line is simply a larger pipe.
A lot. Below Re ≈ 2,300 flow is laminar and friction follows f = 64/Re. Above about 4,000 it is turbulent and roughness starts to matter; between the two lies an unpredictable transition zone: avoid designing in it. Turbulent eddies mix momentum across the pipe, giving a blunter profile with almost all the speed change in a thin layer at the wall, unlike laminar flow’s smooth parabola.
For water, a common comfortable band: slower and sediment settles, faster and you get erosion, noise and water hammer. Pump suction lines sit at the low end. The tool flags when you are outside typical ranges.
Elbows, valves, tees and entrance and exit effects add “minor losses” that can be significant in a real system and must be added separately.
A lot. Below a Reynolds number of ~2,300 flow is laminar and friction follows the clean f = 64/Re law. Above ~4,000 it's turbulent and roughness starts to matter. Most pumped liquid systems are turbulent.
Turbulent eddies constantly mix momentum across the pipe, dragging the slow fluid near the wall and the fast fluid at the centre toward each other — the result is a blunter, more uniform core with almost all the speed change crammed into a thin layer near the wall, unlike laminar flow's smooth parabola.
Because pressure drop scales with 1/D⁵. A small increase in bore dramatically cuts the loss — often the cheapest fix for an under-performing line is simply a larger pipe.
No — this is straight-pipe (major) loss only. Elbows, valves, tees, and entrance/exit effects add "minor losses" that can be significant in a real system and need to be added separately.
For water, very roughly 1–2.5 m/s is a common comfortable band — slow enough to limit erosion and noise, fast enough to avoid settling. The tool flags when you're outside typical ranges.
The Darcy–Weisbach equation: ΔP = f × (L / D) × (ρv² / 2), where f is the friction factor, L the pipe length, D the internal diameter, ρ the fluid density and v the mean velocity. Divide by ρg to express the same loss as a head of fluid. This page applies it for you from the flow rate, pipe size, length and fluid.
In laminar flow, f = 64 / Re exactly. In turbulent flow it depends on the Reynolds number and the pipe’s relative roughness, and is read from the Moody diagram or solved from the Colebrook equation. This page uses the explicit Swamee–Jain formula for turbulent flow and puts a marker on a Moody diagram that you can drag along the curve.
In fully turbulent flow the friction factor changes little, so the pressure drop grows roughly with the square of the velocity: about four times as much for double the flow through the same pipe. It is the reason an oversized pump, or a flow that has crept up, shows as a surprisingly large loss.
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