🎓 Prepared by students from Boğaziçi University

What is Pipe Flow Friction Loss?

Friction between flowing fluid and pipe walls dissipates energy, reducing flow velocity and pressure. This friction loss depends on pipe roughness, length, diameter, and flow rate. Engineers use the Darcy–Weisbach equation to predict and minimize losses in water supply, oil pipelines, and hydraulic networks.

Short answer

Pipe friction loss (head loss) is h_f = f(L/D)(v²/2g), where f is friction factor, L is pipe length, D is diameter, and v is flow velocity.

Head loss vs. flow velocity (f=0.025)

x: Velocity (m/s) · y: Head loss (m)

Try it: interactive calculator

Friction head loss h_f

2.87m

= 0.025*(100/0.1)*(1.5*1.5)/(2*9.81)

Friction factor f0.025 Pipe length L100 mPipe diameter D0.1 mFlow velocity v1.5 m/s

Step-by-step worked examples

Water through 100 m steel pipe (D = 0.1 m, v = 2 m/s, f = 0.025). Head loss?

h_f = f(L/D)(v²/2g)
h_f = 0.025 × (100/0.1) × (4/19.62)
h_f = 0.025 × 1000 × 0.204 = 5.1 m

50 m pipeline (D = 0.05 m, v = 1 m/s, f = 0.03). Head loss?

h_f = 0.03 × (50/0.05) × (1/19.62)
h_f = 0.03 × 1000 × 0.051 = 1.53 m

Reducing velocity by half (2 m/s → 1 m/s), head loss becomes?

h_f ∝ v², loss reduces by 1/(2²) = 1/4
New h_f = 5.1/4 = 1.28 m

Flashcards

Quick quiz

Q1.Darcy–Weisbach shows head loss is…

Correct answer: B. h_f ∝ v² — doubling velocity increases loss fourfold.

Q2.Longer pipes cause…

Correct answer: B. h_f ∝ L — longer length increases loss linearly.

Q3.Larger diameter reduces loss because…

Correct answer: C. Large D reduces velocity, lowering loss (h_f ∝ 1/D and ∝ v²).

Q4.Friction factor highest for…

Correct answer: C. Rough pipes at high Reynolds number show higher friction factor.

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Common mistakes

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Head loss is proportional to velocity. — Correct: Head loss is proportional to v².

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Longer pipes cause slightly more loss. — Correct: Head loss increases linearly with length.

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Friction factor is always the same. — Correct: Friction factor depends on Reynolds number and roughness.

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Pipe diameter doesn't matter much. — Correct: Larger diameter significantly reduces head loss.

FAQ

What is the Darcy–Weisbach equation?

h_f = f(L/D)(v²/2g) — predicts friction head loss in pipes.

Why do water utilities want large pipes?

Larger diameter reduces friction loss and energy cost.

How to reduce pipe friction loss?

Decrease velocity, use larger pipe, smooth pipes, shorter length.

What is friction factor?

Dimensionless number (0.01–0.1) accounting for pipe roughness and flow regime.