Hydraulics & Pressure
Friction Loss
FL
Friction loss is the pressure water gives up rubbing against the inside of the hose as it flows. It depends far more on the flow than on the hose, and it grows with the square of the flow — double the flow and the loss roughly quadruples.
Every length of hose subtracts pressure between the pump and the nozzle. The amount depends on the hose's friction coefficient, the length of the lay, and — most of all — the flow, which enters the formula squared. That squared term is why a small jump in flow produces a large jump in loss.
Larger-diameter hose loses far less pressure at the same flow, so a big line moving a lot of water can need a lower pump pressure than a small line moving less. Trusting the math instead of the apparent size of the line is the habit friction loss teaches.
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