SABI Magazine SABI Magazine June July 2017 | Page 6

Drip irrigation
I nside a c l e an
d ri ppe r
(Part 1)
by Jerry Austen, Netafim.
A
dripper has the difficult
task of delivering and
maintaining a low flow rate
of anything between 0,5 to 5 litres
per hour through a relatively large
flow path with its subsequent low
velocities and at the same time
keeping clean, so that the flow
rate does not decrease over time
from sediment build-up or even a
complete blockage.
A modern dripper is more sophisticated with
a flow path whose cross-section is no longer
circular but rectangular and whose length
is designed into special shapes: specifically,
teeth. The shape of the whole dripper body may
be cylindrical like the old drippers or maybe a
‘boat shape’ and welded onto the inside wall of
the dripperline.
Figure 5. The labyrinth of a non-PC dripper with
the exit ‘bath’ to the left, where a hole would be
made through the dripperline wall for the droplet
to exit the irrigation system
The internal features of a dripper
Fifty years ago, in the formative years of drip
irrigation, a dripper’s flow path was simply
a long tube with a circular cross section that
created little more than simple friction loss.
The shape of the whole dripper body was a
cylinder whose diameter approximated that of
the dripperline into which it was inserted.
Figure 1. An old-fashioned dripper
The flow was laminar. With laminar flow,
friction loss creates a slower velocity near
the wall of the tube and a higher velocity in
midstream. See Figure 2. Sediments suspended
in the water are deposited on the wall where the
velocity is lowest.
Figure 3. A modern ‘boat-shaped’ non-PC dripper,
welded onto the inside of a dripperline
In a non-pressure compensating dripper (non-
PC), the main features are usually an inlet filter,
an inlet orifice, a flow path whose shape is a
labyrinth with teeth, an exit ‘bath’ and finally
an orifice that is made through the wall of the
dripperline from which the droplet leaves the
irrigation system
Pressure
compensating
drippers
(PC)
have additional features such as a flexible
diaphragm that serve to keep the flow rate
constant regardless of the pressure at the inlet
to the dripper.
Inlet filter
The filter at the inlet is there to prevent solid
particles from entering the dripper and
potentially blocking its flow path. This means
that the openings or gaps of the inlet filter need
to be smaller than the passage of the labyrinth
that follows. Thus, if a particle manages to get
through the filter, it should be sufficiently small
not to block the labyrinth once it gets in there.
The gaps of the inlet filter tend to be about
25 to 30% less than the width and depth
of the labyrinth.
Of equal important is the actual size of the inlet
filter. This needs to be as big as possible. If
particles do get trapped on filter, there needs to
be sufficient space on the filter for the full flow
rate to get through.
Figure 2. Laminar flow
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SABI | JUNE/JULY 2017
Figure 4. The filter at the inlet to a modern
boat-shaped non-PC dripper.
(This is the underside of the dripper in Figure 5).
Labyrinth
The dripper’s inlet filter by itself is not sufficient
to keep the labyrinth clean.