Friction loss is caused by resistance to flowing water caused by the walls of pipes. As such, these losses are inversely proportional to the diameter of the pipe. In practice very large pipe diameters for commercial scale applications experience nominal head loss, even over long distances. Conversely, very small pipe diameters for micro-hydro systems experience significant head loss over even modest distances. Irrespective, friction losses must be abstracted from the overall potential head to determine the total dynamic head available to drive the turbine. For small pipelines, head losses can be estimated from tables or graphical means. For final engineering the Hazen-Williams equation is arguably the best mathematical model for predicting head loss. The model is beyond the scope of this web site, but click here for a working Internet model.
Fluids flow in both laminar and turbulent manner, depending on the velocity and viscosity of the fluid. In the case of water at earth surface temperature and pressure regimes the boundary between laminar and turbulent flow occurs at a water flow velocity of 5 fps. Optimal hydroelectric production occurs in the upper range of laminar flow. Inside the threshold of the turbulent flow regime, the energy dynamics of pipe flow change and increase head loss. Regulation of a system in this optimal range of flow is a combination of the sizing of pipes, turbine, and nozzles.