SABI Magazine SABI Magazine June July 2017 | Page 7

Drip irrigation
The non-PC dripper in Figure 5 has flow
path dimensions of 0,60 mm x 0,59 mm, 44
teeth and a flow rate of 1,0 litre per hour at a
pressure of 10 m. Its turbulence coefficient is 7.
The crucial aspect of the labyrinth is the
creation of turbulence: the more, the better.
The more the turbulence, the less likely will
sediments be deposited on the walls of the
flow path and the more likely will they remain
in suspension until the water has left the
irrigation system.
The same flow rate in a costlier PC dripper with
a wider passage of 0,83 mm x 0,74 mm, 46
teeth and a flow rate of 1,0 litre per hour at a
constant pressure differential of 5 m across the
labyrinth has a turbulence coefficient of 10.
This turbulence is created by the design of a
labyrinth with teeth of varying shapes and
configurations. See Figure 6.
Figure 8. Labyrinth with meshed teeth.
Turbulence coefficien t
The ability of a dripper flow path to
create turbulence is measured by the
turbulence coefficient.
Figure 6. Turbulent flow through a labyrinth
The de signs of labyrinths are many and varied.
The most effective are those where the ends
of the teeth are sharp instead of rounded and
where there is a slight gap between the pitch
of the ends of the teeth. See Figure 7 and
compare it with Figure 8, whose teeth are
meshed. The latter’s design moves away from
turbulent flow and tends towards laminar flow,
just changing directions.
k=
254 ∆P (w d) 2
Q 2 n
k = Turbulence coefficient
differential through
∆P = Pressure
the labyrinth (m)
w = flow path width (mm)
d = flow path depth (mm)
Q = labyrinth flow rate (litres/h)
n = number of teeth
In a non-PC dripper, this pressure differential
is generally the inlet pressure of the dripper
at its nominal flow rate (usually 10m). In a PC
dripper, this is the pressure differential across
the labyrinth only, which is constant and is not
necessarily the difference between the pressures
at the inlet and the outlet of the dripper.
A dripper with high turbulence would have a
turbulence coefficient of about 8 to 10 or more.
Medium turbulence coefficients are about 5
to 6 and lower coefficients tend to be in the
region of 3 or lower.
The qualities of a dripper to
keep itself clean
Generally speaking, the following criteria
contribute to a clean dripper.
1. The area of the dripper’s inlet filter. The
greater the better.
2. The cross-sectional area of the labyrinth:
width x depth. The greater, the better.
3. The length of the labyrinth. The
shorter, the better.
4. The
turbulence
coefficient.
The
higher, the better.
Smaller drippers would tend to detract from
these criteria but that does not mean that they
should necessarily be avoided. Smaller drippers
have less material and are less expensive.
It makes sense to use a thin wall dripperline for
a seasonal crop, where the dripperline itself is
not expected to last more than a season or so,
with a small dripper that too only needs to last
a season or so.
Likewise, it makes sense to use a larger dripper
for a long-term crop.
Many PC drippers possess additional qualities
to keep a dripper clean such as self-flushing,
anti-siphon, and root intrusion prevention.
These will be discussed in Part 2.
Figure 7. Labyrinth with a gap between the
pitch of the ends of the teeth
SABI | JUNE/JULY 2017
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