Concept reference

Darcy-Weisbach

C.1Plain-language definition

Darcy-Weisbach says straight-pipe loss is a number of velocity heads. Longer pipe spends more head; larger diameter lowers both the length-to-diameter count and the velocity at a fixed flow. The friction factor carries the effects of flow regime and wall roughness.

C.2Governing relation or decision logic

hf = f(L/D)V²/(2g), where head loss h_f, length L and internal diameter D use consistent length units, V is m/s (ft/s), and Darcy f is dimensionless. At fixed flow in a circular pipe, the explicit velocity term makes loss approximately proportional to fLQ²/D⁵.

C.3The failure to name

The Darcy factor is four times the Fanning factor; confusing the two multiplies loss by four. Nominal diameter, as-new roughness and a friction factor borrowed from a different velocity are other quiet ways to understate long-term duty.

C.4Field review checklist

• Confirm the source uses the Darcy—not Fanning—friction factor.
• Use actual internal diameter and design-condition flow.
• Calculate Reynolds number and relative roughness for the chosen f.
• Run new and aged/credible roughness cases where pump selection depends on future loss.

C.5Where it fits in the course

See §3.2 for the equation and chart, and §3.5 for aging.