EuroWire November 2014

Technical article

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. 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. Green technology solution for clean material

1 Transport system 2 X-ray inspection 3 Optical inspection

4 Sorting unit 5 Bad stream 6 Good stream

▲ ▲ System for pellet inspection and sorting inside

System for inspecting and sorting

CCV line

Temperature measurement

VCV line

▲ ▲ Installation of the system for pellet inspection and sorting

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.

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.

▼ ▼ Foreign objects, foreign pellets or other organic contamination (Optical)

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November 2014