Slack flow is a type of flow that can occur when the gravitational driving force exceeds full pipe friction loss. In what kind of routing does this typically occur?

• A. In horizontal pipes with no elevation change
• B. In piping over hills or uphill sections
• C. In low-viscosity pure gas systems
• D. In highly turbulent, straight runs

Answer: (B)

Slack flow happens when the energy available from gravity along the pipe is greater than the energy lost to friction as the fluid moves. Gravity contributes a head equal to the elevation change times the fluid’s weight, so if the route includes elevation changes, especially sections that climb uphill, gravity can provide a driving force that helps push the fluid through the line despite friction losses. When that gravitational head exceeds the full pipe friction loss, flow can be sustained by gravity alone, which is what we call slack flow.

This is why piping over hills or uphill sections is the typical scenario. In horizontal piping there’s no elevation change, so gravity doesn’t add a driving head. In low-viscosity pure gas systems, or in highly turbulent straight runs, gravity’s contribution is usually small compared to pressure and friction effects, so slack flow is not the usual outcome.