GB2195163A

GB2195163A - Heat-insulated conduit

Info

Publication number: GB2195163A
Application number: GB08719675A
Authority: GB
Inventors: Jurgen Friessner; Siegfried Fuhrmann; Bodo Knitter
Original assignee: Kabelmetal Electro GmbH
Current assignee: Kabelmetal Electro GmbH
Priority date: 1986-08-21
Filing date: 1987-08-20
Publication date: 1988-03-30
Anticipated expiration: 2007-08-20
Also published as: SE8703228L; GB2195163B; FI873607A; DK432787D0; IT8748312A0; CH673694A5; DK432787A; FR2606857A1; IT1211728B; FI873607A0; SE8703228D0; GB8719675D0

Abstract

To produce a more flexible heat- insulated conduit, consisting of two concentric pipes 1, 3, of which at least the outer pipe is a corrugated metal pipe, a heat-insulation layer 2 of foamed plastics based on polyurethane is situated between the pipes, the polyurethane foam having closed pores and an elongation at break of at least 25%. Further, the wall thickness S of the heat- insulating layer 2 is less than 30% of the average diameter D of the outer pipe, the corrugated depth t of the outer pipe 3 is greater than 0.05 D and the distance a between two corrugation crests of the outer pipe 3 is smaller than three times the value of the corrugated depth t. The wall thickness (s, Fig. 2 not shown) of the outer pipe 3 is smaller than 0.2 times the value of the corrugated depth t. A plastics jacket 4, helical spacer 6 and a bituminous layer 5 are also provided. Figs. 5 and 6 (not shown) illustrate terminal and through connections which involve the use of a helical spring (9) to achieve a connection for the outer pipe 3 and brazing at a connection 10 or 15 to join the inner pipes. <IMAGE>

Description

SPECIFICATION Heat-insulated conduit This invention relates to a heat-insulated conduit, in particular for the conveyance of district heat, comprising two coaxial pipes, of which at least the outer pipe is a corrugated metal pipe, a heat-insulating layer of foamed plastics material based on polyurethane disposed between these two pipes, and a plastics jacket surrounding the outer pipe.

Swiss Patent Specification No. 451,621 discloses a heat-insulated conduit intended for laying in the ground (or even above ground) and for conducting gases or liquids, which has substantially co-axially running metal pipes with a helical or an annular or bellows-shaped corrugation, of which the inner pipe serves as the conduit proper. Between the pipes there is a thermally insulating layer. The outer pipe has an external protective layer which acts (mechanically) as a corrosion-inhibiting layer. The special advantages of this conduit can be appreciated from the fact that it is continuously produced in great lengths, and, like an electric cable, can be despatched in known lengths on drums. The exact lengths required are cut off on the construction site.

This conduit has proved to be of particular advantage where expensive excavation work for the purpose of laying a conduit is to be avoided. Thermally induced changes in the lengths of the components of the conduit are of no concern, either during the assembly of the conduit layer by layer, or during laying, because the corrugation of the pipes accommodates these changes.

The main lines of district-heating conduits are generally laid by energy-supply companies.

However, the main lines are connected to individual consumers, e.g. individual houses, by subcontractors, e.g. heating-system contractors. Since the connecting conduits between the main line and the final consumer are as a rule less than 50 metres in length, sending out such short lengths on a cable drum is not rational.

It is an object of the present invention, therefore, to provide a heat-insulated conduit as first mentioned herein which, while retaining the above-mentioned advantages, will be substantially more flexible, so that, without the use of cable drums, it can be wound into coils whose diameter does not exceed 2.35 metres, so that the coils can be transported on ordinary commercial vehicles.

According to the present invention, there is provided a heat-insulated conduit comprising two coaxial pipes, of which at least the outer pipe is a corrugated metal pipe, a heat-insulating layer of foamed plastics material based on polyurethane, disposed between these two pipes, and a plastics jacket surrounding the outer pipe, wherein: (a) the foamed plastics material is a closed-cell foam having an elongation at break of at least 25%; (b) the wall thickness S of the heat-insulating layer, measured as the difference between the average diameter of the inner pipe and the average diameter of the outer pipe, is less than 30% of the average diameter D of the outer pipe; (c) the corrugation depth t of the outer pipe is greater than 0.05 D; (d) the distance a between two corrugation crests of the outer pipe is less than three times the corrugation depth t; and (e) the wall thickness s of the outer pipe is less than 0.2 times the value of the corrugation depth t.

As a result of the features adopted in accordance with the invention, it has been possible to produce a heat-insulated conduit, in the nominal sizes normal for house connecting conduits, such that it can be wound into coils less than 2.35 metres in diameter. This small size is achieved in a stress-free state with the invention, the wound diameter to which the conduit can be wound after completion actually being even less, approximately 12 D, D being the outside diameter of the conduit. The use of a closed cell polyurethane foam having an elongation at break of at least 25% ensures that the heat-insulation layer does not tear at the relevant small radii, when bending.The foam itself is also substantially more flexible, so that the heat-insulation layer, which in known conduits consisted of a rigid polyurethane foam, offers substantially less resistance to bending. Since the distances between the main line and the consumer are usually less than 50 metres, the heat losses over these distances are not unduly serious. By the reduction in the wall thickness of the heat-insulation layer, the diameter of the outer pipe can be substantially reduced if the nominal size, i.e. the diameter of the inner pipe, is maintained, so that the bending capacity or flexibility of the conduit is also increased by this feature. A further contributory feature for achieving the said object consists in changing the corrugation of the known pipe. By increasing the corrugated depth, in fact, the flexibility can be increased.

However, this measure alone is not sufficient. Since the distance between two corrugation crests of a corrugated pipe is also important, a reduction in the distance at a predetermined corrugated depth is also specified, to enable the flexibility of the corrugated pipes to be improved. It has also become evident that the wall thickness in corrugated pipes has a decisive effect on flexibility. Since the pressure resistance of corrugated pipes, at the same wall thickness, is substantially increased by the deeper corrugation, a reduction in the wall thickness can be accepted without the pressure load dropping too much.

The corrugation of the outer pipe may have a sinusoidal profile. It is also possible, however, to make both the corrugation peaks and the corrugation valleys (as viewed in cross-section) have a circular-arc shape.

Tests have shown that the plastics jacket in known heat-insulated conduits has a decisive effect on the bending capacity. Thus folds form in the compressive zone when bending at small radii. In the tensile zone, the plastics jacket is overloaded so that the over-extension, when laying in a straight direction, can no longer be reversed. Also, cracks can occur in the tensile stress region as a result of over-extension. For the reasons stated, it has proved to be advantageous if the plastics jacket follows the profile of the corrugation, and is bonded nonporously to the outer pipe. The non-porous bonding is intended to ensure that steam condensation on the corrugated pipe, which can lead to corrosion, is avoided.Moreover, to avoid corrosion, it has proved of value for a layer consisting of a copolymer material and/or a bituminous composition to be disposed between the outer pipe and the plastics jacket. The copolymer layer desirably tightly surrounds the corrugated metal pipe and is intimately bonded to this pipe Several alternatives are available for the design of the plastics jacket. Thus it is possible to extrude the jacket from a polyethylene or polyvinyl chloride material. The jacket can be brought into contact with the corrugation by a vacuum being produced between the outer corrugated pipe and the extruded pipe, or again by the plastics jacket, while still plastically deformable, being moulded into the corrugation valleys under pressure.

If a polyethylene material which is crosslinked is used as the plastics jacket, the wall thickness of the plastics jacket can be reduced, compared with a polyethylene which is not crosslinked, since crosslinked polyethylenes have substantially higher strength values than polyethylenes which are not crosslinked.

However, the plastics jacket can also consist of a sprayed-on casting resin, preferably based on polyurethane. Coatings of this type are relatively easy to apply to corrugated pipes. Here, too, the question of the casting resin being crosslinked is worth consideration. However, the plastics jacket can also consist of a tape wound on helically with overlapping edges, the pitch of which corresponds in size and direction to the corrugation pitch of the outer pipe. If prestretched plastics tapes are used, these are moulded into the corrugation valleys during subsequent heating. With particular advantage, however, the winding may consist of heat-shrinkable plastics tapes, the shrinkage effect of which is based on a cross-linking process.Shrinkage tapes of this type on their surface facing the corrugated pipe, can be provided with an adhesive layer, which performs the function of the previously mentioned copolymer layer.

For simpler laying of a conduit according to the invention, it has proved to be advantageous to design the corrugation of the inner pipe so that it corresponds to a metric thread. This has the advantage that tubular connection pieces produced on an ordinary lathe can be screwed on to or into the inner pipe, and can be brazed or welded to the inner pipe at the periphery.

The invention is described in greater detail with reference to the accompanying diagrammatic drawings, in which: Figure 1 is a fragmentary view in axial section of a conduit according to the invention, Figure 2 is a fragmentary side view largely in section showing a smaller portion of the conduit of Figure 1 on an enlarged scale, Figure 3 is a fragmentary sectional view on a still larger scale showing an outer pipe generally as used in the conduit of Figures 1 and 2 but with a different form of jacket, Figure 4 is a view similar to Figure 3 showing an outer pipe with another alternative form of jacket, and Figures 5 and 6 are sectioned side views showing respectively terminal and through connections for conduits according to the invention.

The heat-insulated conduit shown in Figures 1 and 2 comprises a helically corrugated inner pipe 1, e.g. of copper or high-grade steel, a thermally insulating polyurethane layer 2, a helically corrugated outer pipe 3#, of steel or an aluminium alloy, and an extruded plastics jacket 4 based on polyethylene. The valleys of the corrugation of the outer pipe 3 are filled with a bituminous composition 5, which inhibits corrosion. The outer pipe 3, particularly when it is of ordinary steel, may have a coating of an appropriate copolymer. A spacer running helically between the pipes 1 and 3 is shown at 6. The helical corrugation of the inner pipe 1 has a pitch which corresponds to the pitch of a metric thread; thus, for the end connection of the inner pipe 1, tubular connecting pieces can be used which have at one end a corrugation which corresponds to the corrugation of the inner pipe 1. Such tubular connecting pieces, for a corrugation with a metric thread, can be produced on ordinary lathes. The heat-insulation layer 2 consists of a polyurethane foam with an elongation at break of at least 25%. The wall thickness S of the heat-insulation layer 2 is less than 30% of the outside diameter D of the outer pipe 3. The plastics jacket 4, in the conduit shown in Figure 1, is an extruded jacket which may for example be of a silane-grafted polyethylene, which can be crosslinked by the action of moisture. This crosslinking will take place automatically during storage by virtue of the moisture content of the surrounding air. However, it can be accelerated by the application of heat, e.g. by an additional steam treatment.

Dimensions and materials for an exemplary conduit are given below.

Inner pipe; Inside diameter 30 mm Outside diameter 34 mm Pitch 5.08 mm Corrugation depth 1.5 mm Wall thickness 0.5 mm Material Copper Outer pipe; inside diameter 74 mm Outside diameter 85 mum Pitch 12.6 mm Corrugation depth 4.9 mm Wall thickness 0.6 mm Material Steel Anti-corrosion layer: Outside diameter 86.6 mm Material Bituminous compos ition Plastics jacket: Outside diameter 91.4 mm Material Polyethylene compos ition This heat-insulated conduit can be wound without difficulty into coils with an outside diameter of less than 2 m.

From Figure 2, it can be seen that the corrugation of the outer pipe 3 is substantially deeper than the corrugation of the inner pipe 1. The anti-corrosion composition 5 not only fills the corrugation valleys of the outer pipe 3, but in practice also covers the corrugation crests of this pipe 3.

In the case of Figure 3, the corrugation of the outer pipe 3 is of a configuration wherein both the corrugation peaks and the corrugation valleys have a circular arc shape. The plastics outer jacket 4 here exactly follows the profile of the corrugation of the outer pipe 3. In the case of Figure 3, in fact, the plastics outer jacket 4 may appropriately be produced by spraying on a casting resin.

In the case of Figure 4, a plastics jacket 5 in the form of a tape wound on helically is applied to the outer corrugated pipe 3 with the tape edges 6 overlapping one another. Such a jacket can be produced advantageously with the aid of heat-shrinkable plastics tapes which are stretched in the longitudinal direction and which, after the plastics jacket 5 has been heated, are moulded into the corrugation valleys of the outer corrugated pipe 3.

Figure 5 shows an advantageous terminal connection for a conduit according to the invention.

To produce this terminal connection, first of all the jacket 4, the outer pipe 3 and the heat insulation layer 2 are cut back with respect to the inner pipe 1, on the construction site, and additionally the heat-insulation layer 2 is removed over a certain length from the annular gap between the inner pipe 1 and the outer pipe 3. To the conduit is fitted a connector 20 which is prefabricated and which consists of a smooth pipe piece 21, an annular disc 7 brazed or welded on to the smooth pipe piece 21, three braces 8 which (distributed uniformly around the periphery) are welded to the annular disc 7, and a helical spring 9 welded to the braces 8. This connector 20 is screwed by means of the helical spring 9 into the helically corrugated outer pipe 3, the smooth pipe 21 being guided into the inner pipe 1. The smooth pipe 21 is then brazed to the end of the inner pipe 1 at 10.The connection thus produced can then be protected by insulating material 11 against radiation losses. The length of the part of the smooth pipe 21 which projects into the inner pipe 1 should preferably at least correspond to the diameter of the inner pipe. Moreover, it is advantageous if the smooth pipe 21, at its end remote from the inner pipe end 1, has a widened portion 21a which permits it to be used with standardised steel pipes. The terminal connection shown is a house or shaft connection, i.e. it is laid inside a building or shaft, through the masonry 12 of which the end of the conduit is guided. A seal inside the masonry is shown at 13.

Figure 6 shows a generally analogous through connection for conduits according to the invention. In this through connection, the inner pipes 1 are connected to one another via a sleeve 14 consisting of a corrugated metal pipe which is screwed on to the ends of the two inner pipes 1 and is brazed or welded to the latter at 15. At 16, welds are made between ends of respective braces 8, these being ends remote from respective helical springs 9. The welds 16, and also the brazed or welded joints 15, are made on the construction site. The through connection produced can then be covered by means 17 which may comprise a socket pipe, e.g.

a socket pipe made of a plastics tape, and a plastics-foam-forming mixture which will fully fill the annular gap can be introduced into the annular gap between the inner pipe 1 and the means 17. The opening in the means 17 required for introducing this mixture can finally be sealed by a plug 18. The ends of the means 17 are sealed to the outer jackets 4 of the conduits by means of heat-shrinkable sleeves 19.

It will be understood that the invention has been described above purely way of example, and that various modifications of detail can be made within the ambit of the invention.

Claims

1. a heat-insulated conduit comprising two coaxial pipes, of which at least the outer pipe is a corrugated metal pipe, a heat-insulating layer of foamed plastics material based on polyurethane, disposed between these two pipes, and a plastics jacket surrounding the outer pipe, wherein: (a) the foamed plastics material is a closed-cell foam having an elongation at break of at least 25%; (b) the wall thickness S of the heat-insulating layer, measured as the difference between the average diameter of the inner pipe and the average diameter of the outer pipe, is less than 30% of the average diameter D of the outer pipe; (c) the corrugation depth t of the outer pipe is greater than 0.05 D; (d) the distance a between two corrugation crests of the outer pipe is less than three times the corrugation depth t; and (e) the wall thickness s of the outer pipe is less than 0.2 times the value of the corrugation depth t.

2. A conduit according to claim 1, wherein the corrugation of the outer pipe has a sinusoidal profile.

3. A conduit according to claim 1, wherein the corrugation of the outer pipe is of a configuration wherein both the corrugation peaks and the corrugation valleys (as viewed in cross-section) have a circular-arc shape.

4. A conduit according to any of claims 1 to 3, wherein the plastics jacket follows the profile of the corrugation of the outer pipe and is bonded non-porously to the outer pipe.

5. A conduit accqrding to any of claims 1 to 4, wherein a layer comprising a copolymer material and/or a bituminous composition is disposed between the outer pipe and the plastics jacket.

6. A conduit according to any of claims 1 to 5, wherein the plastics jacket is an extruded jacket of a polyethylene or polyvinyl chloride material.

7. A conduit according to claim 6, wherein the jacket is of a polyethylene material which is crosslinked.

8. A conduit according to any of claims 1 to 5, wherein the plastics jacket comprises a sprayed-on polyurethane casting resin or other sprayed-on casting resin.

9. A conduit according to claim 8, wherein the casting resin is crosslinked.

10. A conduit according to any of claims 1 to 5, wherein the plastics jacket comprises a tape wound on helically and with overlapping tape edges, the pitch of the tape winding corresponding in size and direction to the pitch of the corrugation of the outer pipe.

11. A conduit according to claim 10, wherein the winding is of heat-shrinkable or shrunk plastics tapes.

12. A conduit according to any of claims 1 to 11, wherein the inner pipe is a helically corrugated metal pipe, and wherein the corrugation of the inner pipe corresponds to a metric thread.

13. A conduit according to claim 1, substantially as described with reference to any of Figures 1 to 4 of the accompanying drawings.

14. A conduit according to any of claims 1 to 13, when engaged in a terminal or through connection substantially as described with reference to Figure 5 or Figure 6, respectively, of the accompanying drawings.