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EuroWire – September 2009
79
technical article
Finally, a thin layer of water-swellable
•
gel is appropriately spread on the
surface of the aluminium or steel tape,
as shown in
Figure 1
The gel can be sprayed accurately onto the
metal tape, which means that the length,
width and thickness of the gel should be
selected according to the width of the tape
so that enough gel will be present at the
overlap without overflow. The spray of the
gel can be carried out either continuously
or intermittently (as shown in
Figure 1
).
Compared with traditional water-blocking
methods used in cables with metal tape
armour, the new method has the following
features:
The interstices in the cable core are
•
completely filled with ordinary gel,
just as in traditional methods. Since
the ordinary gel is cheaper than the
water-swellable gel, this behaviour
can effectively reduce the production
cost, which is a best choice for gel-
filled cables
A rubber eraser, with the inner dia-
•
meter almost the same as the outer
diameter of the cable core, is employed
to wipe away the superfluous gel; thus
the diameter of the cable core after
gel filling can be precisely controlled,
which makes subsequent structural
design and process control much easier
Since it is difficult to ensure the
•
water-blocking effect at the overlap of
the tape as well as the space between
the cable core and the metal tape, a
thinner layer of water-swellable gel is
sprayed onto the tape continuously or
intermittently
When water penetrates into the
•
cable, the gel will swell to fill up the
interstices between the cable core and
the metal tape. Meanwhile, water can
be also blocked at the overlap of the
tape due to the swelling of the gel
Thus, good water blocking perfor-
mance can be obtained over the whole
cross section of the cable
The gel can be sprayed onto the
•
metal tape at intervals, as long as
the water-blocking performance can
be ensured. In this way, the filling
compound material can be saved and
the production cost can be reduced
3 Implementation
of method
This new method can be easily
implemented
on
various
sheathing
lines by the adoption of the following
components:
Two gel pumps: one is for the
•
continuous ordinary gel filling in
the cable core, while the other is for
spraying the water-blocking gel onto
the metal tape
A set of pressure filling equipment for
•
ordinary gel used in the interstices of
the cable core
A set of injection equipment for inter-
•
mittent spraying of water-blocking
gel onto the metal tape with specified
length, width and thickness
A set of controls based on the line
•
speed: the original value can be set
according to different requirements and
the quantity of the gel sprayed onto the
tape can be automatically adjusted with
the variation of the line speed
Figure 2
illustrates the implementation of
the method. We can see that this method
is easy for operation without complicated
equipment alteration.
4 Water
penetration test
As mentioned above, the length, width
and thickness of the gel sprayed onto the
tape can be adjusted in order to achieve
the best result. Many experiments were
carried out to optimise these parameters.
Using the 24mm width steel tape armour,
for example, researchers selected 200mm
as the length of the gel and chose different
values for the width and the thickness of
the gel. The gel was injected onto the tape
at intervals of 1m. Water penetration test
was performed on the cable samples, with
different gel parameters, to verify whether
they could pass the test or not.
Gel thickness was calculated after the
value of gel weight, gel length and gel
width was known. The penetration test was
performed on 2m long cable samples (1m
waterhead, 24 hours), which is stricter than
that described in international standards.
Table 1
shows the water penetration test
results for above-mentioned trial cables
with different gel parameters. From the
table, it can be seen that optimum gel
parameters for the cable with 24mm wide
steel tape armour are 200×20×0.3mm
(if the gel is sprayed at a certain interval
of 1m).
Figure 2
▲
▲
: Implementation of the newmethod
Gel width
(mm)
Gel thickness (mm)
0.2
0.3
0.5
18
Failed
Failed, but water
leakage stops
after 1 min
Failed, but
water leakage stops
after 0.5 min
19
Failed
Failed, but water
leakage stops
after 0.5min
Passed, but gel is
likely to overflow
20
Failed, but water
leakage stops
after 1 min
Passed
Passed, but gel is
likely to overflow
22
Failed, but water
leakage stops
after 0.5min
Passed, but gel is
likely to overflow
Passed, but gel is
likely to overflow
Table 1
▼
▼
:
Water penetration test results
Cable core filled with ordinary gel
Water-blocking gel sprayed onto
steel type intermittently