Water hydraulics
Darcy friction factor
- where
- Darcy friction factor
- Reynolds number
- absolute internal roughness (mm)
- internal pipe diameter (mm)
Explain this equation
Works out the Darcy friction factor for flow in a round pipe. Feed it into the straight-pipe friction loss calculation.
Re is the Reynolds number. ε is the absolute roughness of the inside of the pipe. d is the internal bore. ε / d is the relative roughness, which matters more as flow becomes more turbulent.
For laminar flow, below Re 2300, friction depends only on Re: f = 64 / Re. For turbulent flow the Colebrook equation combines a roughness term and a Reynolds term. Because f appears on both sides, the calculator solves it numerically. The 3.7 and 2.51 are the equation's fitted constants.
Sense check: smooth 20 mm pipe at Re 20,000 gives about 0.026. Laminar at Re 2000 gives 0.032.
Watch out: no figure is given in the transitional range of Re 2300 to 3999. Not for gas installation sizing.
Enter your values to see the result.
Good to know
Fully developed circular-pipe flow. Use actual roughness and fluid properties. No transitional-flow estimate; this factor is not a gas installation sizing approval.
Sources
Results are arithmetic only. Worked examples are not recommended design settings. This library does not provide automatic gas pipe selection, live gas tightness tests, purging procedures, combustion acceptance limits, installation approval or commissioning certification. Manufacturer data and the applicable current method remain essential.