Technological cooperation between SIKORA and MAILLEFER for a 100 % quality control

Customer demands significantly determine which new technologies are pushed by SIKORA with high priority. Already 2 years ago, the company MAILLEFER raised urgent questions about the possibilities for the development of a system for the detection and sorting of contaminations in plastic pellets. The reason for the need of such a system was – and is – the continuously increasing demand for renewable energies in the international energy market. “The worldwide installed capa¬city has already increased from 1,250 GW (2010) to 1,700 GW (2013)”, says the International Energy Association (IEA). A part of this is supplied by wind turbines in offshore wind farms. For the power transmission to the mainland, submarine cables are used, which have to fulfill sophisticated requirements due to the specific environmental conditions. Therefore, the cable structure and its production process has to follow national and international standards. In these regards, the purity of the XLPE compound plays a critical role, especially with regard to the insulation: The purer the compound, the lower the risk of a breakdown. It is for that reason that the Chinese Standard for high voltage cables, for example, demands the exclusion of contaminations from 75 μm in the processed materials.
A very important aspect is always the cable joint, specifically when we talk about high voltage cables for off-shore applications. The joints where the cables are welded together are always critical. Therefore, energy suppliers want cable manufacturers to deliver large cable lengths with a minimum amount of joints, as each joint contains a potential risk for breakdowns. Consequently, the aim is to produce energy cables with long lengths with only a few joints while using highly pure raw material. In order to catch possible impurities in the XLPE compound before they get into the cable, cable manufacturers use screens which are positioned directly in the melt flow after the extruder, before the crosshead and, thus, prevent the contaminations from entering the product. But these screens can get clogged by degraded XLPE, i.e. scorches, or excessive amount of contaminants after certain run time and then the melt pressure in the extruder may increase significantly. Finally, the production has to be stopped in order to change the screens, which in turn means that later a joint is required at that position. An approach to increasing productivity is to omit the screens. This requires, however, appropriate methods to detect and sort out contaminations in the XLPE pellets at an early stage.
A solution to this problem is a system which inspects the pellets for purity before the XLPE pellets get into the extrusion process. Currently installed devices use optical measuring principles that randomly inspect pellets for contaminations. Since this is a sample check only, not 100 % of the material is controlled. Furthermore, with optics, only the outside of the pellet can be inspected, while impurities inside the pellets remain undetected. Moreover, the inspection of the raw material with existing devices is not done under clean room conditions, so that new impurities can emerge from the ambient air or as a result of the conveyor belt used.
“We have had a close cooperation with SIKORA for more than 20 years now, which brought us technologies such as the SICCON, a system for the non-contact measurement of the cable position in catenary lines, the CSS 2 for the monitoring of the purity of the XLPE melt at the production of high and extra-high voltage cables, or the ULTRATEMP 6000, a system for the continuous measurement of the melt temperature. It was a matter of course that we addressed SIKORA concerning the development of an inspection and sorting system”, explains Timo Mäkelä, Process Specialist for MV and HV products at MAILLEFER. “For MAILLEFER, it was important to develop a system, which inspects 100 % of the XLPE pellets and which sorts out reject material reliably”, Mäkelä goes on. The list of requirements mainly included 8 points:
1. Material throughput of 500 kg/h
2. Detection of metallic and organic contaminations
3. 100 % detection of small contaminations ≥ 50 μm
4. Detection of contaminations on and inside the pellet
5. Transport of the pellets without conveyor belt, as a conveyor belt may cause additional contaminations
6. Flow of material inside the devices under clean room conditions
7. Reliable sorting of contaminated pellets
8. Assuring purity of raw material for all potential suppliers
In initial experiments, various sensors were tested in order to ensure a reliable detection of contaminations with a size of 50 μm. Very soon it became clear that SIKORA had the know-how to develop the appropriate technology to meet MAILLEFER’s requirements. With the newly developed PURITY SCANNER, metallic and organic contaminations not only on the surface, but also inside the pellet can be detected. The system applies for XLPE compounds as they are used at the production of medium, high and extra-high voltage cables, as well as underground and submarine cables, including semi-conductive XLPE compound. With the combination of X-ray and optical technologies, a system was developed which inspects 100 % of the pellets for purity before they enter the production process. Rejected XLPE pellets are sorted out reliably. The transport of the pellets is carried out via a vibrating ramp. This transport system is hermetically sealed with the result that no external impurities get into the material flow.
“The PURITY SCANNER is the result of hard work and a trustful cooperation between MAILLEFER and SIKORA. With this system, the quality and consequently the reliability of the end products in the high voltage cable industry can significantly be increased. This also improves the safety and usability for the end user”, Mäkelä sums up.