Agriculture Reference
In-Depth Information
timer, pressure switch, irrigation controller, flow switch, and any combination of
the above.
12.10 Hydraulics in Pumping System
If you randomly select one pump, it will not automatically fit the needs of your
system; unless you have calculated the amount of flow your system needs and
determined how much pressure it will take to achieve the desired flow.
Velocity is used to increase pressure. Velocity of water being discharged is deter-
mined by the size of the casing and impeller as well as the speed of the impeller
spinning inside the case. Since water is essentially a noncompressible liquid, it
exhibits the unique trait of transferring pressure horizontally when in a confined
space. What this means is that water in a pipe exhibits the same pressure as it would
if the pipe were perfectly vertical, even if the pipe is not.
12.10.1 Pressure Vs Flow Rate
It is to remember here that there is always an inverse relationship between pressure
and flow (for a fixed conduit or pipe). Higher pressures mean lower flows. Lower
pressures result in higher flows.
12.10.2 Pressure and Head
The pumping head is normally expressed in feet of water; it is simply height of
elevation. Pressure is normally expressed as “pounds per square inch” (PSI). It is
the
weight
of the water in pounds per square inch. Pressure and head are related by
the equation:
P
=
ω
h
where
is the density of the fluid, and
h
is the height of the fluid column.
In most cases we measure water pressure in the static state when designing irri-
gation systems (or any other water piping system for that matter). Then we use
calculations to figure out the friction loss that will occur in the pipes and subtract it
from the static pressure to arrive at the dynamic pressure.
The following is oriented toward wells.
ω
12.10.2.1 Dynamic Water Depth
When the pump is running, the water level in the well drops below the water table.
It may drop a few inches or more than several meters depending on the type of soil
(or rock) the well is drilled into and the discharge rate. Dynamic water depth is the
depth of the water below the top of the well, when the pump is running. When a
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