HOMER Pro 3.9

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The frictional loss in the hydro pipeline, expressed as a fraction of the available head.

Water (like any viscous fluid) flowing through a pipe experiences a loss in pressure due to friction. We can express this pressure loss in terms of a loss of head, where head is the vertical drop through which the fluid flows. In HOMER, you specify the pipe head loss as a percentage of the available head.

Small high-head, low-flow hydro systems typically experience pipe head losses of between 10% and 20%. With low-head systems, pipe head losses are typically only a few percent.

The head loss percentage is defined in terms of the absolute head loss hl and the total available head h:

The Darcy?-Weisbach equation can be used to predict frictional losses in a circular pipe:

•hl = Absolute head loss due to friction, given in units of length

•fD = Darcy friction factor

•L = Pipe length

•D = Pipe diameter

•V = Flow velocity (where Q-dot is volumetric flow rate):

•g = Gravitational acceleration (i.e. 9.81 m/s2)

The Darcy friction factor fD can be calculated several different ways, including the well-known Moody diagram (below) or one of many on-line calculators. For laminar flows (Reynolds number, Re, less than 2300), a simple relationship can be used:

fD = 64/Re

The friction factor can vary for transition flows (2300 < Re < 4000), and a number of correlations have been proposed. The Moody diagram can provide a good estimate in this regime. For turbulent flows, the Moody diagram is a good reference, or fD can be computed by numerical solution of the Colebrook-White equation:

•? = Roughness height

•Dh = Hydraulic diameter (inside diameter for circular tubes)