NO763069L - PROCEDURE AND CABLE MANUFACTURER - Google Patents

Description

Continuous tyérrbindlfngs-j

process for polymer cable- <

insulation. in

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The present invention relates to a method and

apparatus for making electrical cables which:comprises a conductor covered with a cross-linked polymer insulating material).

Several methods for producing such cables are previously known. The production of high-current cables requires a continuous cross-linking process where the cable's conductor is after it has been provided with one or more insulation layers and possibly with an inner and an outer semi-conductive layer<1>) in one or more ex-■ ■ :.. ^ ■■ i ... r truders, passes through a pressure pipe in which the insulated cable conductor first passes through a heated section of sufficient length to achieve full cross-linking of the polymeric material, and then passes through a cooling zone.

The continuously operating process equipment is often arranged

in the form of catenary lines and, in the case of large cables, as vertical lines. It is an absolute requirement that the cable insulation, while it is still hot, must not be touched mechanically.

The most common method that has been used to date uses saturated steam at high temperatures and high pressures to achieve the desired cross-linking of the polymeric material,

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followed by water cooling, also under high pressure, until the cable's insulation has reached a temperature at which it can be handled at atmospheric pressure without being destroyed. It is well known that when polymeric material such as polyethylene is crosslinked or cured by steam at the desired temperature of about 200°C and above, the steam introduces a significant number of small voids into the insulation. Such cavities are believed to have a negative effect on the electrical characteristics of the insulation.

For & avoid the formation of such cavities due to steam

curing, it has previously been proposed in US Patent No. 3,645,656

to use a non-condensing gas, such as e.g. nitrogen under pressure, as curing medium'. It has also been proposed to use gases such as helium, hydrogen, carbon dioxide and SF as a curing medium instead of saturated steam, and these proposals have resulted in better products.

There have also been several proposals for equipment to achieve this

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the desired temperature in the curing zone. In the above-mentioned patent, a method is described for circulating hot air or gas through a jacket surrounding the curing tube. In Swedish patent application no. 7411010-7

it is proposed to heat the curing tube by allowing an organic liquid to flow through the surrounding jacket. Furthermore, it is in US Patent No. 3,588,954

proposed to heat the inert gas by heat radiating elements which are placed inside the curing tube itself. Such heat-radiating elements

can be energized individually, but their temperature is so high that they will have a very destructive effect on the cable insulation if mechanical contact occurs.

In addition to external heating of the curing tube section, it has also been proposed to preheat the cross-linking gas medium before it is admitted into the tube. The use of circulating as well as stationary gas systems has been proposed and the direction of the gas flow has also been changed, but the most common flow direction for the gas is, however, in the direction of the cable's movement.

As an alternative to using gas in the curing section, it has been proposed in US Patent No. 3,773,872' to shorten the ordinary saturated steam section by exposing the cable's insulation to ultrasound. A hardening is thereby achieved both from the inside and the outside of the insulation.

Another option which is discussed in Swedish pat. application no. 72016409 to replace the curing tube with a very long extrusion nozzle in which the polymeric material is easily completely cross-linked.

After the curing process, the cross-linked polymeric cable insulation is usually cooled by water under pressure. In connection with the above-mentioned curing process, other cooling media such as gas (especially in a pre-cooling section), glycerol and organic liquids are also mentioned. The refrigerant is circulated inside a cooling pipe and usually enters the pipe at its lower end and leaves the pipe through an opening

at the end of the curing section. In some cases, the cooling pipe is divided into several individual cooling sections to provide gradual cooling down to an acceptably low cable temperature as described in, US Patent

However, it has been found that the use of water in the cooling process can have a destructive effect on the cross-linked polymeric cable insulation even if the curing zone is free of both steam and water. It has therefore been proposed in US Patent No.

3,909,177, to use silicone oil as a heat transfer medium in the curing section and also as a cooling medium in the cooling zone. While the silicone oil must be regarded as an advantageous cooling medium, and probably can also be useful as a heat transfer medium at temperatures below 200° C, the oil will soon lose its low viscosity at higher temperatures. The temperature required to cross-link the insulation of large cable cross-sections will lie in the range from to 300° C, and at such temperatures the oil tail will tend to, become polymerized and can stop at the valves in the circulation system in the hardening zone. Besides, will! that!, when using silicone oil in the entire curing/cooling pipe it will be impossible to separate the two circulation loops, so that clogging will also occur in the cooling loop. A mechanical seal between the two sections of the pipe should be avoided to prevent mechanical destruction of the insulation.

The proposed cold buffer zone to avoid the risk of destroying

laying the surface of the passing cable will lead to a continuous unwanted temperature shock for the cable insulation and this can also have destructive effects.

The purpose of the present invention is to provide a method and an apparatus for continuous crosslinking of polymer cable insulation; and thereby avoid the disadvantages that; can be found by the previously known methods and devices, and thus produce an electric high-current cable that is completely free of voids in the extruded insulation and in any semiconducting layers present.

The apparatus for carrying out the method according to the present invention makes use of either one; catenary-shaped or a vertical tube which is divided into a cross-linking and a cooling section. The two sections overlap each other so that a middle section can be used either for heating or for cooling, depending on the type of cable to be manufactured.

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Variations in conductor size and insulation thickness will

making it necessary to vary the ratio between the hardening zone and the cooling zone in order to obtain a maximum manufacturing speed over the entire production area. A high-current cable with low voltage has a large conductor cross-section and a small insulation thickness and will therefore require a short cross-linking section, but a long cooling section, while a high-current cable for high voltages and which therefore has a small; conductor diameter and large insulation

thickness will require a long cross-linking section, and a short cooling section.

In the hardening section, it is present as a cross-linking medium

an inert gas such as nitrogen, and in the cooling section there is silicone oil which is used as a cooling medium. The nitrogen gas is preheated before being admitted at the top of the curing tube shortly after the point where the cable leaves the extruder. It must be ensured that the extruder is not overheated and thus causes a cross-link already inside the extruder. A gas discharge is arranged at the desired location on the hardening section, preferably directly above the top of the required cooling section. The hardening section is equipped with out-

reversible heating elements that divide the curing tube into individually heated curing sections, so that the temperature of the cable can be maintained at a desirable high temperature level until the outer part of the insulation has been completely cross-linked. Then the temperature of the cable is lowered grad^yis<f>ra1 the original high curing temperature to a somewhat lower temperature because, in view of the desire to reduce the mechanical stresses that will be introduced into the material when it is cooled from the outside, it is necessary that the temperature of the insulation is not too high when it enters the cooling section. The gas can be circulated so that it enters the curing tube again

after it has passed a filter and a heating device, but can also alternatively be released in the manner in which it is discharged.

In the cooling section, the silicone oil is circulated from one intake at the lower end of the section near the cable outlet through the pipe to an outlet at the desired end of the curing (and cooling) section. The oil is cooled and filtered before it enters the pipe. No partitions are required between the gas-filled curing section and the oil-filled cooling section. The desired oil level is maintained by means of Irnnuonc-innel t- n A \ r& afvrl nnontnf-vr n*»fc lean henvttea more than one

refrigerant loop.

The main advantage of the present invention in relation to

the most related of the previously known devices, as described in US Patent No. 3,909,177, is that by using inert gas as a cross-linking medium, it will be possible to use higher temperatures in the curing section. This in turn opens up the possibility of increasing the production rate for a given length of curing pipe. A further advantage is that the transition from hardening section to cooling section can be varied and that it can be defined within small tolerances.

The advantages of the present invention in relation to it

mentioned Swedish patent application no. 7411010-7, is mainly that water is avoided in the cooling section. This application also mentions glycerol as a cooling medium, but glycerol, like water, is a polar liquid and it is also hygroscopic so that it should be avoided as a cooling medium. A further advantage compared to the aforementioned Swedish application is that the heating takes place gradually.

After the completion of the cross-linking and cooling process, the cable with the hardened insulation is pulled through a stuffing box at the termination of the pressure pipe after which the insulated cable core can be laid up temporarily on a cable drum or can proceed directly for the application of outer protective sheaths and reinforcement in equipment located in tandem with the apparatus, according to the present invention.

In order to provide a clearer understanding of the present invention, reference is made to the following detailed description and an embodiment example and to the accompanying drawings where: Figs. 1 and 2 schematically show a vertical and a chain-shaped, continuous cross-linking system that makes use of the features according to to the present invention, o^

Fig. 3 schematically shows the plant according to figures 1

and 2 in more detail,. When the production in fig. 3 is horizontal,

this has been done for the sake of simplifying the drawing, and the device 1 according to the invention should only be used for vertical or catenary installations.

In fig. 1, a vertical extrusion and cross-binding plant is shown. A cable core 1 passes through an extruder 2 in which the core is coated with polymeric insulation layers and possibly semi-conductive layers, after which it enters a continuous cross-linking and cooling arrangement 3 and is finally pulled out

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through a packing box 4. The other part of the arrangement 3 consists of a pipe 5 which is heated by external heating elements 6.

the elements are divided into several sections so that the temperature of the tube 5 can be varied gradually or in steps so that it follows a predetermined pattern. Inert gas is supplied through the top of the pipe 5 from a gas container and a preheater 7 and leaves the pressure pipe through an outlet 8 just above the cooling section. The used gas can be released into the open air, or can be recycled to the tank 7

via a filter device 9.

The cooling section is shown as a vertical pipe 10 which is followed by a turning wheel 11 and a horizontal pipe 12. After the cable insulation has been cooled to an acceptably low temperature, the cable with the hardened insulation is pulled out through the packing box 4. The cooling section is filled with silicone oil which is circulated through a cooling and filtering arrangement 13. The intake for cooling oil is located near the stuffing box 4

so that the cooling oil flows in the opposite direction to the movement of the cable and an outlet 14 is placed at the desired level.

It is important that the hot cable insulation is cooled sufficiently

in the vertical pipe 10 before it comes into contact with the turning wheel 11. If the height of the vertical pipe is sufficient to provide a complete crosslinking and cooling of the cable, the turning wheel 11 can

and the horizontal part of the cooling pipe 12 is omitted.

In addition to the outer heating elements 6 that heat the tube 5

and thereby ensures a desired gas temperature, the insulation can also be heated from the inside by introducing an inductive heating

(not shown) of the cable's conductor before or immediately after '!'•'■

the extruder.

However, care must be taken at the top of the pipe 5 that

the extruder is not overheated by the hot gas, because if this occurs then the cross-linking of the polymeric material may start inside the extruder and the outer surface of the cable insulation may then become uneven.

To initiate the cross-linking, it is considered advantageous to

bringing the polymeric material up from the extrusion temperature to the desired cross-linking temperature as quickly as possible. Temperatures between 200° C to 300° C are believed to be beneficial,

but temperatures above 300° C can be harmful to the material.

The gas should preferably be preheated to a temperature that corresponds to

the one found in the heated upper part of the tube section 5.

While the temperature is initially kept as high as possible without damaging the cable insulation, in the lower parts of the section it must be brought down to a lower temperature, e.g.

150° C to 200° C before the cable enters the section of the pipe where cooling is carried out using silicone oil. This gradual reduction of the temperature can be carried out during the continuous cross-linking process as the inner part of the insulation due to the heat wave from the outer layers moves inward into the cable even though the temperature in the outer layer is reduced.

As can be seen from the details in fig. 3, the transition level between the cross-linking section and the cooling section f the listener can be matched with the particular type of cable to be produced. As mentioned, some cabals require long cross-link sections and short chill-six ions while others require relatively short cross-link sections and long chill sections. Of course, it occurs at high voltage

in high current cables which require long cross-linking sections as well as cooling sections, and although the length of these sections can be determined directly depending on the achievable production rate, all plants must be built to meet the specifications.

In fig. 2 shows a cross-linking/cooling line of chain line shape and comprising the features described in connection with fig. 1. The differences between a chain line and a vertical line are well known and will not be discussed here, except that longer lines are easier to achieve in practice using the chain line principle. In addition to the silicone oil providing lubrication of the cable provides it also buoys the cable so that it becomes easier to center the cable in the pipe and thereby avoid eccentricity for the insulation due to sagging in the soft insulation material. In Figure 2, the same reference figures as already used in Figure 1 are used as far as possible.

Figure 3 shows schematically details for a vertical/chain line-shaped crosslinking/cooling system according to the present invention. The transition between the gas-filled cross-linking section and the section filled with silicone oil is only faintly implied, it being understood that while the oil level is determined by level control equipment, the transition line will be defined by the slope that the system has at the relevant point in the line.

A cable core 21 shown on the left in the drawing is pulled in

in an extrusion head 23 for extruding a semi-conductive layer on the core, the semi-conductive layer being of polymeric material and provided from an extruder 22. The cable core is then drawn into a double crosshead 25 for applying polymeric insulating material from an extruder 24 and outer semi-conductive polymeric material from an extruder! 26. The insulated cable is then fed into the cross-linking tube 27 and out through a packing box 28 as a finished cross-linked cable 29.

The pipe 27 preferably starts with a cooling zone 30 which is

arranged to prevent premature cross-linking inside the extruder 25. While the cooling zone 30 is filled with Jgas, the cooling effect can be achieved as shown, by circulating a cooling medium by means of a pump

30 A from a reservoir 30 B through a cooling jacket which is arranged around the upper part of the pipe 27. The directions of flow are shown by arrows in the figure. The cable is then passed through several cross-linking sections 31 A - 31 G. Each such cross-linking section is controlled by a thermostat 32 A - 32 G which is set in advance to a desired temperature and is controlled by a sensor 33 A - 33 G. These sections can preferably be heated using electric heating elements, such as electric heating cables wrapped around the pipe and insulated to the outside to prevent heat loss to the surroundings. The power of these electric heating elements is taken from the network 34 via a power distribution device 35.

As can be seen from the drawings showing an embodiment of the present invention, one or more of the compartments 31 E - 31 G can alternatively be used for cooling purposes. This will be explained below in detail. In a practical embodiment, the section 31 A 31 B can be kept at a high cross-linking temperature, the sections 31 C 31 E can be used to provide a gradual lowering of the temperature and the sections 31 F - 31 G can be used for cooling, i.e. that the heating elements in sections 31 F - 31 G may be out of order. cross-linking

the tube is provided with a gas inlet 36. Inert gas such as e.g. nitrogen is released from a gas storage tank 37 via a preheating arrangement 38 and a valve 39. A shunt 40 can optionally be used instead of the control valve.

The gas outlets are shown at points 41 A - 41 C of which only one is used at a time. These outlets are affected by the on-off valves 42 A - 42 C and 42 A - 42 C. Used gas is discharged through a valve

44 in open air at 45.

As an alternative to letting the gas pass out into the open air,

it can pass through a filter device 46 and be circulated back

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to the gas tank 37. To obtain a controlled flow of gas, a valve 39 can be used together with a shunt 47, while the valve 44

can be used together with the shunt 40. However, it is important that the gas is maintained at a predetermined pressure and a predetermined temperature at all times. The shunt 47 can be used in emergencies.

After it has left the cross-linking part of the pipe 27, the insulated cable conductor is fed into the cooling section 50 and the cooled cable 29 leaves the pipe through a packing box 28.

The cooling section 50 is filled with silicone oil under pressure

and this oil enters the pipe through an inlet 48 at the end of the pipe and possibly also through an inlet 49. The silicone oil is supplied from an oil reservoir 51 in which it is cooled to a temperature which is preferably not below -30° C because the viscosity at lower temperatures, we will prefer a lower temperature to reach the desired circulation. When the additional intake 49 is used, the silicone oil is also supplied from the additional reservoir 52.

As can be seen from the drawing, the cooling can take place gradually

by using more than one loop for the coolant.,The coolant will flow in all loops in a direction opposite to the cable movement and is therefore gradually heated by the contact with the cable. The additional loops can, as shown in the figure and as described below, be shunted by the primary loop.

The flow path for the silicone oil will be briefly described.

From the cooled reservoir 51, the oil is pumped into the cooling section 50 at the intake from 48 with the help of a high pressure pump 53 and also through a level control valve 54. A shunt 55 can be used in emergencies. The silicone oil flows in a direction opposite to the movement of the cable, and is forced out of the pipe at 5' 6 by opening a shut-off valve 57, and the flow rate is controlled by the control valve 58. Here, the shunt 59 can be used in emergencies. The silicone oil in the primary cooling loop is pumped back to the reservoir 51 which is equipped with cooling fins 60 to maintain a desired temperature in the reservoir. Before the oil enters the reservoir 51, it may be passed through a filter (not shown) to remove contaminants.

The silicone oil is pumped into the additional coolant loop

from a cooled reservoir 52 into the cooling section 50 through the entrance

a 49 by means of a high-pressure pump 61 and a level-controlling valve 62. The reservoir 52 will be cooled to a temperature which is somewhat higher than the temperature in the reservoir 5l. In emergencies, it is possible to use the shunt 63.

After the silicone oil in the last coolant loop has flowed through the coolant pipe in one direction opposite to the cable's movements, it is discharged through one of the outlets 41 B - 41 C. The desired outlet is selected by operating the shut-off valves 42 D, 42 C, 4i;A. The level of the o|jen is controlled by one valve and in an emergency the shunt 66 can be used. The oil is then pumped through a filter device (not shown) back to the reservoir

1 ■■ • • ■ i i 52 in which the cooling fins 67 are implied/supposed øt a shunt valve 68 is closed.

As shown, the additional loop for the refrigerants can be bypassed by closing the valves 56, 62, 63, 65, 66 and by opening the valve 68.

The cooling section 50 ends in a packing box 28 which can be

of any conventional type.i Any leakage in stuffing box 28 is collected by vessel 69 and pumped back to the main reservoir.

It should be noted that the cross-linking cooling pipe 27

may be provided with one or more conventional level detectors (not shown), which are arranged so that they can control the pressure/flow rate of the gas and/or oil, in such a way that the transition between these two pipe sections is maintained at a desired level. Conventional detectors can also be used for sensing the location of the cable in the pipes, but these must be of such a type that they do not come into contact with the cable surface. An incorrect positioning of the cable inside the pipe must be immediately corrected by adjusting the tension in the cable.

An example of how the equipment can work is that gas

under pressure flows from the tank 37 through the pipe 27 and out through the outlet 41 A, while the refrigerant is pumped, to maintain a corresponding pressure from the reservoir 51, through the pipe 27 and back through the outlet 41 B. The oil level is in this case chosen to lie between outlets 41 A and 41 B.

The valves 42 A, 42 B, 43 B and 43 C are in this case open, while the valves 42 c, 42 D and 43 A are closed. The heating elements in sections 31 31 F and 31 G will in this case probably be out of order. The number of outlets 41 A, 41 B, 41 C and 64 can therefore be varied and this also applies to the number of heating sections.

Claims (4)

1. Continuous cross-linking process for polymeric cable insulation where the cable after leaving the extruder passes through a pressure tube in which, the insulation is first heated to effect a complete cross-linking of the polymeric material and then cooled whereupon the cable is withdrawn through a sealing arrangement at the end of the tubular pressure chambers, and where silicone oil is used as a cooling medium which is kept under pressure and which flows in contact with the cable, but in the opposite direction to the direction of the cable's movement, characterized by a t in the insulated cable, when it enters the cross-linking pressure pipe to achieve fully cross-linked, is heated by direct contact with an inert gas which is preheated to a desired temperature and maintained at this temperature by external heating of the pressure pipe until the outer part of the insulation is completely cross-linked, after which the cable passes through part of the pressure pipe with a predetermined step-down temperature, before it goes into the cooling section. 2. Apparatus for carrying out the method according to Claim 1, characterized in that the transition zone between the cross-linking and cooling sections can be moved so that the length of the cross-linking part is increased while the length of the cooling section is reduced and vice versa. 3. Apparatus for carrying out a process according to claim H, characterized in that the predetermined step-down of the temperature in the pressure pipe is obtained by means of electric heating elements placed or wound in individually controlled sections on the outside of the pipe, which heating elements are isolated from the surroundings. 4. Apparatus according to claim 2, characterized in that the transition between the two sections is maintained at a predetermined level of the refrigerant by controlling, by means of one or more level detectors, the pressure and/or the flow rate of in the gas and/or the oil.