SOPHISTICATED DESIGN REQUIREMENTS IN PLANT ENGINEERING FOR AN XLPE EXTRUDER FEEDING PLANT AS AN EXAMPLE

Transcription

1 SOPHISTICATED DESIGN REQUIREMENTS IN PLANT ENGINEERING FOR AN XLPE EXTRUDER FEEDING PLANT AS AN EXAMPLE Dipl.-Ing. Reza Izadpanah, Senior Manager Sales, Zeppelin Systems GmbH

2 CONTENTS 1. Material handling for the production of cable coatings 2. Material receipt 3. Gentle material transport 4. Air separator 5. Feeding tower 6. Sample configuration of an XLPE conveying plant 7. Requirements for material handling rooms 2

3 MATERIAL HANDLING FOR THE PRODUCTION OF CABLE COATINGS Material handling for the production of cable coatings from XLPE pellets is one of the most challenging tasks in bulk material handling. Requirements for purity of products used are especially high in these plants. Experience, know-how and accurately coordinated overall systems are decisive for the quality of the final product. Even the tiniest contaminations in the insulation of medium- and highvoltage cables can cause large damages. For this reason, handling of raw materials under clean-room conditions is a prerequisite for process safety. A successful process is only possible if peroxide nests and contaminations are prevented. For this reason, only components designed by Zeppelin are used for the plants. Special designs without clearances or dead spaces have been developed for this particular application. Gapless pipe connections, special fittings and gentle dense-phase conveying every detail matters for quality. Fig. 1: Cross section of a highvoltage cable. The light-colored insulation layer is surrounded by two black semi-conductive layers. 3

4 MATERIAL RECEIPT Fig. 2: Octabin emptying station Dustfree introduction of the material into the conveying system. XLPE (also called PE-X) pellets for high-voltage cable applications are usually supplied in so-called octabins. The drawing on the left (fig. 2) shows two octabin emptying stations. The end of the octabin lining is pulled over the conical seal of the station. When samples have been taken, the hose can be opened. Then, the material flows into the receiving hopper by gravity and is extracted from this hopper by means of suction conveying. The major advantage of this station is that it is a closed system. Dust or other foreign matter from outside cannot enter the system. A protective filter is installed upstream the air intake of the pneumatic conveying system. The hoppers are made of stainless steel , i. e. SS304 AISI, and product-contacted surfaces are electrolytically polished. In addition, weld seams of hoppers are manually precision-ground. 4

5 GENTLE MATERIAL TRANSPORT The material is transported to the extruder by means of pneumatic conveying. Due to dense-phase conveying, the system is operated at conveying speeds of 4 to 6 m/s. This avoids abrasion of the XLPE pellets to the highest extent. Conveying at high speeds would cause the formation of angel s hair and the material would no longer be suited for processing in the extruder. In addition, only stainless steel is used as piping material. A specially developed gapless pipe connection ensures that no pellet residues can accumulate. All the valves in the conveying system are ball valves which have no dead spaces to make sure that no dust or peroxide nests can be formed here. 5

6 GENTLE MATERIAL TRANSPORT Fig. 3: Flow diagram of a feed station with downstream filter The diagram on the right shows a typical PE-X conveying system. The separator (left) itself has no internal components or corners in which dust etc. can deposit. The transition from the body to the cone only has a curvature instead of an angular transition. As the internal surfaces are electrolytically polished, no dust can accumulate anyhow. The level switch of the conveying hopper has a sheathing made of PE so that PE-X only contacts PE-like materials, apart from stainless steel. The right side of the drawing shows the filter with an upstream vacuum pump. A protective filter, a so-called police filter, is installed between the vacuum pump and the main filter, in order to provide additional safety against contaminations from the conveying air. The fine particles which are produced during pneumatic conveying of XLPE pellets are separated by means of the filter and safely removed from the system. 6

7 AIR SEPARATOR ON TOP OF THE MAIN EXTRUDER FOR THE INSULATION LAYER The next safety step in the extruder feeding system downstream the feed station (fig. 3) and the receiving hopper is an air separator unit. Originally, air separators were developed for separating chaff from wheat. The cascade air separator used by Zeppelin in XLPE conveying plants considerably adds to quality improvements for cable compounds. Fig. 4: Cascade air classifier with metal separator and ionizer 7

8 AIR SEPARATOR ON TOP OF THE MAIN EXTRUDER FOR THE INSULATION LAYER The cascade air separator unit consists of three consecutive separating stages. Fig. 5 shows that the pellets are dosed into the first separating stage by means of a vibratory conveying chute. At the inlet of the air separator unit, there is a magnetic separator which holds back any ferromagnetic particles which might be present in the pellets. The second separator stage includes an ionizer for neutralizing the PE- X pellets electrostatic charge and, at the same time, for releasing the dust particles adhering to the pellets. The third stage of the air separator unit is the cascade air separator itself. The pellets trickle through the ring-shaped separator chamber in free fall, finely distributed in the air. Due to repetitive collisions on the cones, dust particles sticking to the pellets are mechanically released. 8

9 AIR SEPARATOR ON TOP OF THE MAIN EXTRUDER FOR THE INSULATION LAYER The ascending air flow, which can be easily controlled, carries the dust to the top to a dedusting filter (also refer to the overview diagram, fig. 6.) Please note that the air separator is also designed in such a way that it has no dead spaces, and its inner surfaces are electrolytically polished so that no dust can deposit on them. In addition, the air separator is completely maintenance-free and has no moving elements. Due to its short overall height, it can also be integrated in existing plants. When the pellets leave the air separator at the bottom outlet, they are free from dust and are fed into the extruder charge hopper. Samples can also be taken here, before extrusion of the XLPE pellets. In addition to designing complete XLPE conveying plants Zeppelin also offers upgrading or extension of existing plants. 9

10 FEEDING TOWER Fig 5: Feeding tower on top of the main extruder. The drawing on the left shows the typical structure of a feeding tower on top of the extruder. The material falls down from one unit into the other, without any assistance. The total height of such a feeding tower amounts to approx. 5,000 6,000 mm, depending on the extruder capacity. The plant can easily be installed on a steel frame. Additional platforms for maintenance purposes can be inserted on different levels, as requested by the customer. Fig. 5 shows an additional funnel on the left side of the extruder charge hopper. This funnel is used for feeding cleaning material pellets required to clean the extruder after extrusion of cable compound. The cleaning material is dumped manually out of bags into this funnel. 10

11 SAMPLE CONFIGURATION OF AN XLPE CONVEYING PLANT The overview shown below is an example for a possible configuration for an XLPE conveying plant. The drying system shown for the semi-conductive layer is mainly designed for surface drying of the pellets. The pellets are heated to approx. 80 C, also for increased extrudability. 11

12 REQUIREMENTS FOR MATERIAL HANDLING ROOM Most producers of high-voltage cables take special care that utmost cleanliness is ensured for material receipt (clean room conditions). The following is recommended for material receipt for the insulation layer: temperature: +25 ±3 C relative humidity: % lock / two-door system for entering the clean room filtered air in the clean room surfaces in the room made of coated aluminum surfaces of the octabins to be cleaned before they are brought into the clean room 12

13 REQUIREMENTS FOR MATERIAL HANDLING ROOM In addition, the following has to be ensured in the extruder room: air conditioning in the area of the downstream hopper, vibratory conveying chute, cascade air separator and extruder charge hopper The control panel room also needs to have air conditioning. The processor should make sure that the pellets are not stored for more than 6 months. The use of old material could possibly lead to quality deficiencies in cables. XLPE producers provide data sheets for this. Temperatures in the storage rooms should be from +10 to 25 C. Zeppelin Systems GmbH will be glad to provide further information. 13

14 THANK YOU FOR YOUR ATTENTION!