Question 1

What is the Darcy-Weisbach equation?

Accepted Answer

The Darcy-Weisbach equation calculates pressure loss due to friction in a pipe: ΔP = f·(L/D)·(ρV²/2), where f is the friction factor, L is length, D is diameter, ρ is density, and V is velocity.

Question 2

How do I determine the Reynolds number?

Accepted Answer

Reynolds number Re = ρVD/μ, where ρ is density, V is velocity, D is diameter, and μ is dynamic viscosity. Re < 2300 is laminar, Re > 4000 is turbulent, and between is transitional.

Question 3

What roughness value should I use?

Accepted Answer

Commercial steel: 0.045 mm. PVC and copper: 0.0015 mm. Concrete: 0.3-3 mm depending on finish. Cast iron: 0.26 mm. New pipes are smoother; old pipes may have significant buildup.

Question 4

How do I account for fittings and valves?

Accepted Answer

Use the equivalent length method: add the equivalent pipe length of each fitting to the straight pipe length. Alternatively, use K-factors and calculate minor losses separately.

Question 5

Why is flow in transition region problematic?

Accepted Answer

In the transition region (Re 2300-4000), flow alternates unpredictably between laminar and turbulent. Friction factor correlations are less accurate, so results have higher uncertainty.

Question 6

Can I use this for gases?

Accepted Answer

Yes, for low-pressure-drop applications where density change is small (<10% pressure drop). For significant pressure drops in gas systems, use compressible flow methods.