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How It Works
The Hazen-Williams equation is an empirical formula for calculating water velocity in pipes. It uses a roughness coefficient (C) that depends on pipe material and age, the hydraulic radius, and the slope of the energy grade line. The flow equation form uses pipe diameter (in inches) and outputs discharge in gallons per minute. It is simpler than Darcy-Weisbach but only valid for water near room temperature in turbulent flow.
Example Problem
A new PVC pipe (C = 150) has a hydraulic radius of 0.05 m and an energy slope of 0.004. What is the flow velocity?
- v = 0.849 × 150 × 0.050.63 × 0.0040.54
- v = 127.35 × 0.1312 × 0.0738
- v ≈ 1.23 m/s
Frequently Asked Questions
What is the Hazen-Williams C coefficient?
C reflects pipe interior smoothness. New PVC is about 150, new cast iron 130, and 20-year-old cast iron drops to around 100. Higher C means less friction and higher velocity.
When should I use Hazen-Williams vs. Darcy-Weisbach?
Use Hazen-Williams for quick water pipe sizing at normal temperatures. Use Darcy-Weisbach when working with other fluids, extreme temperatures, or when you need universal accuracy across flow regimes.
Does pipe age affect the Hazen-Williams coefficient?
Yes, significantly. Corrosion, scale, and biofilm reduce C over time. A new cast iron pipe at C = 130 may drop to C = 80–100 after 20–40 years without maintenance.
Related Calculators
- Darcy-Weisbach Calculator — the more general pipe friction formula for any fluid.
- Colebrook Equation Calculator — find the friction factor for Darcy-Weisbach.
- Pipe Flow Calculator — compute Reynolds number and flow rate for circular pipes.
- Manning Equation Calculator — open-channel flow counterpart to Hazen-Williams.
- Reynolds Number Calculator — determine if flow is laminar or turbulent in the pipe.
- Pressure Unit Converter — convert head loss between psi, Pa, and feet of water.