Which factor has the greatest impact on friction loss per unit length in a pipe?

Which factor has the greatest impact on friction loss per unit length in a pipe?

• A. Pipe diameter
• B. Fluid velocity
• C. Pipe roughness
• D. Fluid density

Answer: (B)

Friction loss per unit length is driven primarily by how fast the fluid is moving. In the standard head-loss relation, the loss scales with the velocity squared, so doubling the flow speed more than doubles the friction loss. The friction factor, which accounts for pipe roughness and flow regime, does modify the loss, but the direct v^2 dependence makes velocity the dominant influence. Pipe roughness and diameter still affect losses—roughness changes the friction factor and smaller diameters increase losses for a given velocity—but their impact is secondary to the strong, squared relationship with velocity. Density doesn’t directly drive head loss for incompressible flow, so it’s not the main factor here.

Friction loss per unit length is driven primarily by how fast the fluid is moving. In the standard head-loss relation, the loss scales with the velocity squared, so doubling the flow speed more than doubles the friction loss. The friction factor, which accounts for pipe roughness and flow regime, does modify the loss, but the direct v^2 dependence makes velocity the dominant influence. Pipe roughness and diameter still affect losses—roughness changes the friction factor and smaller diameters increase losses for a given velocity—but their impact is secondary to the strong, squared relationship with velocity. Density doesn’t directly drive head loss for incompressible flow, so it’s not the main factor here.