Technical article
November 2014
49
Joints where the cables are welded
together are always critical, in particular
when talking about power cables for
offshore applications.
That is why cable manufacturers aim at
delivering large cable lengths with only
a minimum of joints as they contain a
potential risk for breakdowns.
This can be achieved by using highly pure
raw material. As the integration of screens
reduces the productivity of the line, an
approach is to omit the screens.
This requires, however, reliable alternative
methods to detect and sort out
contamination in the XLPE pellets at an
early stage.
Scorches can be avoided by tempera-
ture supervision and control of the melt
flow by an ultrasonic-based temperature
measurement.
Green technology
solution for clean
material
The new technology from Sikora inspects
the material for purity to 100 per cent.
The system combines X-ray technology
with an optical system. This allows
the detection of metallic and organic
impurities of 50
μ
m in the pellet itself and
on its surface.
The combination of these two tech-
nologies is unrivalled on the market. The
specially developed X-ray technology
is able to inspect transparent and
coloured (eg black) pellets as well as
semi-conductive
XLPE
material
for
impurities.
Contamination that is detected by
the X-ray or optical technologies is
identified by image processing software,
characterised as contamination and
automatically separated.
The basic principle of the X-ray technology
is the different attenuation of the material.
XLPE mainly comprises two carbon and
four hydrogen atoms. The carbon atom
has six protons in its core while hydrogen
has only one.
A typical contamination would be steel
particles from the extruder or granulator,
which is mainly iron (FE).
Iron has 26 protons in its core. Because
these 26 protons have a much higher
X-ray attenuation than the six protons
from the carbon it is possible to generate
a contrast between the two materials in
the X-ray image.
Regarding the optical inspection, the
illumination plays an essential role.
By using a special diffuse light construction
technique, the smallest contamination is
detected, among others, foreign objects,
foreign pellets or organic impurities.
In order to allow an industrial through-
put up to 2,000kg/h, modern camera
technologies are used.
They allow precise images of the pellets
to detect contamination down to a size of
50
μ
m.
In order to avoid contamination from
the outside, the feeding of the pellets is
carried out via a hermetically vibrating
ramp.
This at first avoids contamination that
might occur by conveyor belts, and
secondly an over-pressure avoids con-
tamination from dust of the ambient air.
The system is installed after the XLPE
transport box (ie octabin or bag), and
a hopper, before the hopper of the
extruder, whereas the compound is fed
by gravity.
▼
▼
Foreign objects, foreign pellets or other organic
contamination (Optical)
▲
▲
System for pellet inspection and sorting inside
▲
▲
Installation of the system for pellet inspection and sorting
Temperature measurement
1 Transport system
2 X-ray inspection
3 Optical inspection
4 Sorting unit
5 Bad stream
6 Good stream
CCV line
VCV line
System for inspecting
and sorting
