1.3 Water Velocity

Remember that flow rate(Q) is velocity (V) times pipe cross-sectional area (A). Therefore, the flow (Q), is directly proportional to the velocity (V), if the pipe area (A) is constant. Within a given pipe, the flow rate increases as the velocity increases.

Water velocity in a pipe is very important to the proper operation of an irrigation system.

There is a danger to increasing the velocity. A mass of fast-moving water can create a large force within the enclosure if its path is suddenly blocked.

Closing a valve quickly can cause a surge pressure or water hammer in pipes filled quickly with high-velocity water. It can occur when flow reverses direction and goes back against a stopped pump.

To prevent water hammer that might damage the pipes, limit water velocities to a maximum of 5 to 7 ft/sec (360 to 420 ft/min).

Fortunately, a table of values is available to help the pipe selection process. We'll look at an example in section 4.2.1.

A second danger to increasing velocity comes in the form of a loss of flow energy or pressure as velocity increases.

At low velocities water flows over slight imperfections on the pipe walls smoothly. However, at high velocities the imperfections cause turbulence in the fast-moving flow. The turbulence slows the water flow and dissipates flow energy.