2. Water Friction

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.