GB2093262A - Mineral-insulated cable - Google Patents

Abstract

Mineral-insulated cable is manufactured by introducing mineral insulant into a first tube (1) having two conductor rods (5) therein, drawing down tube (1) along part of its length, welding the undrawn end portion of tube (1) to a second tube (17), introducing mineral insulant into the unfilled portion of tube (1) and into the second tube having a continuation of the rods (5) and drawing down in one continuous operation the remaining end portion of tube (1) and part of tube (17). The steps are then repeated with further tubes. <IMAGE>

Description

SPECIFICATION Mineral-insulated cable manufacture The present invention relates to a method of, and apparatus for, making mineral-insulated cables, that is to say, cables of the kind comprising at least one conductor insulated from a surrounding metal sheath, and where there is more than one conductor from the other conductor or conductors, by compacted mineral insulating powder.

Mineral-insulated cables are generally made by placing one or more conductor rods in a preformed metal tube, filling the tube with a mineralinsulating powder which is densely packed in the tube and around the conductors by ramming and/or vibration, and then pulling the filled tube through one or more dies to draw it down.

According to the present invention, we provide an improved method of manufacturing mineralinsulated cable comprising the steps of: (i) introducing mineral insulant into a first tube, having at least one conductor rod positioned therein, so that a part of the length of the first tube is filled with mineral insulant; (ii) drawing down the first tube along part of its filled length; (iii) welding the undrawn end portion of the first tube to a second tube which surrounds a continuation of the at least one conductor rod of the first tube; (iv) introducing mineral insulant into the unfilled portion of the first tube and into the second tube so that at least a part of the length of the second tube is filled with mineral insulant; and (v) drawing down in one continuous operation the remaining end portion of the first tube and the second tube along at least part of its filled length.

Preferably the undrawn length of the at least one conductor rod is greater than the combined undrawn length of the first and second tubes and, prior to welding of the tubes, the second tube is passed over and around the free end of the rod or rods to enclose it or them and then is advanced towards the location at which the tubes are welded.

In another embodiment of the invention, the at least one conductor rod of the first tube is buttwelded to an additional rod or rods at a location remote from that at which the tubes are welded thereby forming the continuation of the at least one conductor rod, thereafter the second tube is passed over and around the free end of the rod or rods to enclose it or them and then is advanced towards the location at which the tubes are welded.

In yet another form of the invention, the at least one conductor rod is butt-welded to an additional rod or rods adjacent the location at which the tubes are welded thereby forming the continuation of the at least one conductor rod; preferably the additional rod or rods are supported within the second tube as it is advanced towards the location at which the tubes are welded.

Preferably, the method further comprises:- (a) welding the undrawn end of the second tube to a third tube which surrounds a continuation of the at least one conductor rod; (b) introducing mineral insulant into the unfilled portion of the second tube and into the third tube; and (c) drawing down in one continuous operation the remaining portion of the second tube and the third tube along at least part of its said filled length.

The steps (a), (b) and (c) may be repeated with a further tube or further tubes.

Preferably the conductor rod or rods are extended, and/or the third or subsequent tube is placed around it or them, in similar manner to that for the second tube.

Preferably, the insulant is introduced into the tubes as free flowing mineral-insulating powder which is subsequently compacted by ramming and/or by vibrating the tube. Alternatively, preformed blocks of solid mineral insulant with longitudinal holes are successively inserted in the tube and the conductor rods are passed through the aligned longitudinal hoies in the blocks.

Thus the present invention provides a simple method of producing any desirable length of mineral-insulated cable as a single unit with no joints.

Also, even when manufacturing a number of cables of a length which could be drawn conventionally from a single tube, cutting the number of desired lengths from a continuous single length of cable made by the method of the present invention reduces material wastage; this is because the amount of material discarded from the ends of the single length of cable of this invention (from which a number of the desired length can be obtained) is the same as that amount discarded in manufacturing each conventionally.

Preformed tubes used in the method embodying the present invention may be formed by extrusion, or they may be seam welded advantageously having been formed by suitably shaping a tape as described and claimed in our British Patent Specifications Nos. 1 554859 and 1 554860.

According to the present invention, we further provide improved apparatus for the manufacture of mineral-insulated cable, which apparatus comprises:- (i) a station at which free flowing mineralinsulating powder can be introduced into a tube in which at least one conductor rod is positioned; and (ii) means for drawing down an at least partially filled tube downstream of the station; and (iii) means for welding one partly drawn tube to a second tube, said welding means being positioned intermediate the station and the drawing means.

Preferably the welding means is an orbital T.I.G.

welder.

Thus conventional apparatus for manufacturing mineral-insulated cables can readily be modified to embody the present invention; moreover apparatus embodying the present invention occupies substantially the same space as conventional apparatus. The cost of modifying existing apparatus in accordance with the invention or of installing new apparatus is low.

In order that the invention may be more readily understood, a description is now given, by way of example only, reference being made to the accompanying drawings, in which: Figure 1 is a diagrammatic elevation of an installation in an initial stage of the manufacturing process; Figure 2 is a similar view to that of Figure 1 with the installation at a subsequent stage.

A stainless steel start tube 1, swaged at its front end, is fed from a supply 2 to installation 3, where it is initially fitted over a guide tube 4 which has two internal conductor shroud tubes (not shown) each containing a conductor rod 5; start tube 1 is supported on its outside by an automatically operable clamp 6. Front ends of conductor rods 5 emerging from the guide tube 4 are attached to a plug 7 which is then force-fitted into the swaged end of tube 1. This swaged end of start tube 1 is passed through a metal die case 8, which contains a guide die 9 mounted on a spacing ring 10 supported on a closing die 1 and then is gripped by a chuck 12.

A feed block 13 for selectively supplying either inert gas or mineral-insulating powder is fitted near the top of guide tube 4, so that free flowing fused or caustic magnesia can be fed into the guide tube 4 while the starter tube 1 is vibrated by two pneumatic hammers 14; the vibration encourages the powder to flow down the guide tube and densifies the powder within the starter tube before drawing. Figure 1 illustrates the installation at this stage.

The vibration is maintained as the tube is drawn through the closing die and wound onto a driven haul-off reel 15, a constant level of lubricating oii in the die case 8 being maintained throughout the drawing operation. When all but the upper most section (of about 200 mm) of tube 1 has been drawn through the die case 8 and the last portion of tube 1 has moved off guide tube 4, the drawing operation is interrupted by stopping the rotation of reel 15 and the vibration by hammers 14. Any excess powder flowing from the guide tube is removed by suction and approximately 12 mm depth of loose powder is also removed from the start tube 1.

The feed block 13 is removed and guide tube 4 is now held by clamp 16 so that clamp 6 can be released enabling a second tube 1 7 from supply 2 to be fed onto guide tube 4. Each length of start tube from supply 2 is less than the distance between the clamps 6 and 1 6, so that when a new start tube is lowered over the guide tube 4 the former is below the clamp 6 which can then support the guide tube; thus clamp 16 can be released to allow the new start tube to drop and fit on top of tube 1.

Then feed block 13 is again fitted to the top of guide tube 4 and an inert gas at low pressure is fed down it to purge any oxidising gasses from the space around the edges of the tubes. An orbital welding unit 1 8 is fitted around the edges of the tubes which are then autogenously T.l.G. welded together (see Figure 2).

Each weld is examined visually to determine whether it is flat with respect to the adjoining tube surfaces and whether there is a regular overlapping of weld in which case it can be drawn down successfully. If however the weld is convex (caused by excessive internal argon pressure) or concave, then subsequent drawing could form an irregularity insulation at the weld or even a break at the weld; thus any suspect welds are cut out and the new edges are welded.

After the weld has cooled, welding unit 1 8 is removed and the purging gas turned off. Tube 17 is filled with powder from feed block 13 in guide tube 4, and the remaining portion of tube 1 and part of tube 1 7 are drawn down through die case 8.

The process can be repeated with the uppermost undrawn part of tube 1 7 and a new tube from supply 2, the process being repeated as many times as required.

Continuous lengths of conductor rods 5 are produced by resistance butt welding individual rods; each tube passes over and around the free ends of conductor rods 5 from supply 2 towards the guide tube 4.

Claims (10)

1. A method of manufacturing mineralinsulated cable comprising the steps of: (i) introducing mineral insuiant into a first tube having at least one conductor rod positioned therein so that a part of the length of the first tube is filled with mineral insulant; (ii) drawing down the first tube along at least part of its filled length; (iii) welding the undrawn end portion of the first tube to a second tube which surrounds a continuation of the at least one conductor rod of the first tube; (iv) introducing mineral insulant into the unfilled portion of the first tube and into the second tube so that at least a part of the length of the second tube is filled with mineral insulant; and (v) drawing down in one continuous operation the remaining end portion of the first tube and the second tube along at least part of its filled length.

2. A method according to Claim 1. comprising, prior to introducing mineral insulant, inserting at least one conductor rod into the first tube, the or each undrawn rod being greater in length than the undrawn combined length of the first and second tubes.

3. A method according to Claim 1, comprising butt-welding the at least one conductor rod of the first tube to an additional rod or rods at a location remote from that at which the tubes are welded thereby forming the continuation of the at least one conductor rod.

4. A method according to any one of the preceding claims comprising passing the second tube over and around the free end of the rod or rods to enclose it or them and then advancing it towards the location at which the tubes are welded.

5. A method according to Claim 3, comprising supporting the additional rod or rods within the second tube as it is advanced towards the location at which the tubes are welded.

6. A method according to any one of the preceding claims comprising: (a) welding the undrawn end of the second tube to a third tube which surrounds a continuation of the at least one conductor rod; (b) introducing mineral insulant into the unfilled portion of the second tube and into the third tube; and (c) drawing down in one continuous operation the remaining portion of the second tube and the third tube along part of its said filled length.

7. A method according to Claim 6, comprising repeating the steps (a), (b) and (c) with a further tube or further tubes.

8. Apparatus for the manufacture of mineralinsulated cable, which apparatus comprises:- (i) a station at which mineral insulant can be introduced into a tube in which at least one conductor rod is positioned; (ii) means for drawing down an at least partially filled tube downstream of the station; and (iii) means for welding one partly drawn tube to a second tube, said welding means being positioned intermediate the station and the drawing means.

9. Apparatus according to Claim 8, wherein the welding means is an orbital T.l.G. welder.

10. A method or manufacturing mineralinsulated cable substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.

1 Apparatus for manufacturing mineralinsulated cable substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.