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.