Pushing fluid through a pipe costs energy, lost to friction along the walls. The Darcy–Weisbach equation ties that loss to four things: how fast the fluid moves, how long and wide the pipe is, and how rough its walls are. The friction factor comes off the Moody diagram above — the iconic log–log chart relating friction to the Reynolds number and relative roughness. Drag the operating point to see how your pipe sits on it.
Water at 20 °C through 100 mm commercial steel pipe at 20 L/s gives a velocity of about 2.55 m/s and a Reynolds number near 250,000 — firmly turbulent.
The friction factor lands around 0.018, and over 50 m of pipe that's roughly 30 kPa of pressure drop. Step up one pipe size and that figure falls sharply — pressure drop scales with diameter to the fifth power.
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.