A friction-factor number needs its convention. Otherwise a correct-looking pressure-loss calculation can carry a fourfold error.
Identify the Physical Basis
The Darcy and Fanning factors are different definitions: f_D = 4f_F. AFT’s verification example explicitly converts a Fanning factor to the Darcy-Weisbach convention by multiplying by four. For incompressible pipe-loss bookkeeping, Δp = f_D(L/D)(ρv²/2) uses the Darcy factor; an equation written with 4f_F is equivalent when its other assumptions match.
Work Through an Example
Suppose a hypothetical calculation has L/D = 100 and ρv²/2 = 500 Pa. A Darcy factor of 0.020 gives Δp = 1,000 Pa. The corresponding Fanning factor is 0.005. Substituting 0.005 directly into the Darcy expression gives only 250 Pa. The fluid and geometry did not improve; only the definition was mismatched.
Use the Result in the Review
When copying a coefficient from a handbook, software report or supplier calculation, copy the defining equation too. The symbol f alone is insufficient. Add a convention column to the worksheet and keep its source page or software setting. A comparison should normalize the definitions before debating the final pressure number. The same discipline applies when an equivalent-length method contains a friction factor.
For an interface discussion around a processing tank, submit the route calculation with its stated convention. Do not apply this arithmetic as a complete gas-flow method: compressibility and changing velocity can require additional treatment. Conversion of a definition fixes consistency; it does not validate the rest of the hydraulic model.
Which friction-factor convention is compatible with the selected pressure-loss equation?
This blank worksheet is for your own project. It contains no H M machine trial result.
| Review field | Reference or result | Responsible person |
|---|---|---|
| Factor source and defining equation | To complete | To complete |
| Darcy or Fanning convention | To complete | To complete |
| Geometry and velocity-pressure term | To complete | To complete |
| Normalized result and model assumptions | To complete | To complete |
Customer Questions
Can the symbol λ identify Darcy automatically?
Check the source definition; a symbol alone is weak evidence.
Should I multiply every factor by four?
Only when converting a Fanning value to the Darcy definition.
Will convention correction resolve all loss disagreements?
No. Diameter, properties, roughness and model scope may also differ.
Related Equipment References
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Primary References
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