In an irrigation system,
the major goal is to deliver a quantity of water from the source to the sprinkler
head or trickle emitter in an efficient manner. Flow of water in a closed pipe
can be described as a flow rate, i.e. volume per unit time. Imagine cutting
through a pipeline and catching the water coming out in one minute. If one catches
10 gallons in one minute, then the flow rate is 10 gallons per minute. The cut
exposes the cross-sectional area of the pipeline. The water passing through
that cross-sectional area in one minute was 10 gallons.
Flow is therefore a function of
water velocity and the pipe cross-sectional area.
The flow equation helps
to illustrate the relationship of velocity and area to the flow:
Q = 7.48 x V x A
Where:
Q = quantity of water flow
given in gallons per minute (gpm)
V = velocity of water given
in feet per minute (fpm). Charts may use feet per second (fps).
A = cross-sectional area
of the pipe given in square feet (sq ft)
Note: 7.48 = conversion factor
to change cubic feet to gallons. There are 7.48 gallons of water in a cubic
foot.
In our example above, the
water velocity through the pipe can be found by dividing the flow rate (10 gallons
per minute) by the cross-sectional area of the pipe in square feet. Water velocities
are shown in a commonly used Friction Loss chart that will be described soon.
So, the area and velocity determine
the volume per unit time (flow rate) of water that passes through a pipe.
In
the equation above, one can increase the velocity and decrease the area, or
vice versa, without changing the flow.
In the practical world, it says we might
use a smaller diameter pipe and save money by requiring the water to move faster.
However, later discussion of friction will illustrate additional power (energy)
is needed to put water through a pipe at a higher velocity while maintaining
a given end point usable water pressure. This subject confuses people and needs
careful reading and understanding.