Water Friction Objectives:
• You will understand the pressure loss called friction loss.
• You will appreciate the cause of friction loss and its importance.
• You will learn how friction loss influences the selection of pipe sizes.
• You will learn the relationship of pipe cross-sectional area and friction.
Water can drag on the
sidewalls of the pipe (friction) and use up energy intended to push water
through the pipe. If energy is used by friction along the pipeline, the water energy
or pressure drops. The pressure at points further along the pipeline will be
less than the initial pressure. Thus, the operating pressure of sprinkler nozzles
will vary from beginning to end in a system.
For example, imagine this,
it is (sort of) like riding down a dirty sliding board in a playground after
a dust storm. The rider slows down because of the drag or friction caused by
the dust. (A good cleaning and a piece of wax paper improve the ride!) The
cleaner and smoother the sliding board, the less the friction. Even the smallest
roughness interferes with movement.
Friction loss is an energy
loss or, in our case, a pressure loss. Pressure is energy.
The amount of energy
loss is related to the roughness of the inside surface of the pipe; the velocity
of water passing over that roughness; the number of elbows, valves and other
fittings; and the diameter of the pipe.
The faster that water moves over the roughness
of a pipe wall, the greater the turbulence of the water flow and the greater
the energy (pressure) loss.
On the other hand, slow flowing (low velocity) water
gently moves over the roughness without disturbing the flow very much and little
energy is lost.
Larger diameter pipes have less of the water volume passing
near the walls where the turbulence occurs so the wall roughness effect is less.
Knowledge about friction
loss is important for sizing pipes in an irrigation system. Friction loss is
commonly expressed in terms of "psi per 100 feet of pipe". Friction loss tables
are available for different types of pipes. An irrigation system designer relies
on the table to size pipe correctly. We will look at an example of such a table in the following section.