The present invention relates to a heat insulated pipe system, in particular a subterranean district heat¬ ing pipe system, and the invention is to be considered in close association with the applicants European Patent No. 310,190, which relates to the use of conductor pipes of annealed copper in consideration of the fact that just such pipes exhibit a special relaxation effect for neutralizing axial stresses occurring in the pipe in response to heating and cooling thereof.

According to the present invention it has been found that there is another material, which, surprising¬ ly, exhibit corresponding properties, namely aluminium. For other fields of application aluminium pipes have been developed, coated at the inside and the outside by a plastic material, the so-called Alu-Pex-pipes, which have been found surprisingly well usable for the purpose here considered. It is necessary to still keep the in¬ side and the outside of the aluminium pipe coated by a plastic layer, but it has been found that also these layers are relaxating, such that the said particular type of pipe is very well usable for the present field of application.

While copper pipes can be joined by hard soldering so as to be stiff against both compression and extrac¬ tion, corresponding joints of the pipes here referred to are more difficult to work out, inter alia because it is considered essential that the aluminium pipe is not exposed at neither the inner nor the outer side, while this pipe should also, for anti-corrosion purposes, be kept electrically insulated from the applied coupling means. These, in their turn, will probably have to con¬ sist of metal, especially for being able to resist the very high tensile forces that will occur in the pipes when these are cooled after being heated during a long

period of time.

On this background, a special coupling device has been developed for the joining of the pipes, as descri¬ bed in more detail below.

It should be mentioned that the pipes are very well suited for use as conductor pipes in prefabricated heat insulated pipes of large lengths. For easy handling and transportation it will be a condition that the insulated pipes can be wound up in reels with a reasonably small diameter, and the conductor pipe is well suited for that, as it may well be made of soft aluminium as far as the large size pipes are concerned. It is corresponding¬ ly possible to use for the outer protection or mantle tube a soft and flexible plastic, while the intermediate insulation material should not exactly be soft , because it is still preferred to operate with a so-called rigid system , in which the foam insulation keeps the conduc¬ tor pipe effectively centred in the mantle tube and transfers axial forces between the conductor pipe and the mantle tube; however, even such a rigid insulation may well be flexible.

When the pipes may thus be produced in relatively large lengths, correspondingly fewer pipe joints will have to be made. .Another and quite significant circum¬ stance is that it will then also be possible, in prac¬ tice, to make use of a laying technique, by which the pipe is advanced through the sub soil just as with known pipe shootings across road beds. These techniques are by now so highly developed that that they are well usable for a subterranean laying out of the pipes also along roads, in stretches of e.g. 50 , whereby the digging of long trenches can be avoided, as it is sufficient to dig well holes for each 50 m, in which the pipe joints may be established.

It is from practice well known to use plastic pipes as the inner conductor pipes in district heating pipes,

and that also these pipes may be produced and transpor¬ ted in long lengths in a reeled up condition. These pipes are relatively inexpensive, but particularly in their simpler form they present marked qualitative draw¬ backs. From the surroundings oxygen diffuses inwardly through the outer plastic mantle tube and through the insulation into the conductor pipe, which is thereby exposed to accelerated aging, and further through the conductor pipe, whereby the district heating water is enriched with oxygen, which acts corrosively on metal parts in the district heating pipe system. Moreover, water vapours will diffuse through the wall of the con¬ ductor pipe out into the surrounding insulation, where the vapour condensates into water that will act to- break down the insulation effect. These drawbacks have already been recognized, and it has been attempted to coat the conductor pipe with a diffusion barrier, but without this having been particularly successful.

When considered on this background the invention can be viewed in the manner that it provides for a par¬ ticularly effective diffusion barrier, viz. in the form of a real aluminium pipe, which is furthermore arranged such that it is kept contact insulated both inwardly and outwardly for neutralization of the aggressive proper¬ ties or effects of this material. Additionally, it is recognized that all this is possible without associated expansion or contraction problems being encountered in connection with long pipe stretches, as the material has been found to have suitable properties with respect to resiliency and relaxation. It is not commonly known that the material possesses just these properties.

In the following the invention is explained in more detail with reference to the drawing, in which:-

Fig. 1 is a schematic view illustrating the laying of a branchfree district heating pipe e.g. along a road,

Fig. 2 is a longitudinal sectional view thereof,

Fig. 3 is a longitudinal sectional view of an asso¬ ciated pipe joint in the inner conductor pipe, shown in an initial mounting phase,

Fig. 4 the same in a finally mounted condition, and

Figs. 5 and 6 are views corresponding to Figs. 3 and 4 of a modified embodiment of the pipe joint.

In Fig. 1 is shown a ground stretch, along which there is to be laid out a branchless district heating pipe 2. As already mentioned and in more detail shown in Fig. 2 this pipe consists of an inner conductor pipe 4 that is made of an aluminium pipe 6 having an inner plastic coating 8 and an outer plastic coating 10. The pipe has been developed as a cold water pipe, but as mentioned, according to the invention it has been-found that the pipe has sufficient relaxation properties in connection with both heating and cooling to make it well usable as or in district heating pipes being laid with¬ out any axial compensators. The pipe is surrounded by an insulation layer 12 which in the usual way has a sur¬ rounding mantle tube 14 of plastic and which adheres to both the outside of the coating 10, which, itself, adhe¬ res sufficiently to the aluminium pipe 6, and to the inner side of the mantle pipe 14.

Preferably the mantle tube 14 is made of a relati¬ vely soft plastic material, such that a long pipe length can be rolled up in a reel 16 having a diameter that is realistic for conventional transportation.

The pipe can be laid in the usual manner, but it has been found particularly advantageous, due to possi¬ ble large and transportable production length, to mount the pipe by shooting it through the ground, from a dig¬ ged well hole 18 to a following hole 20 perhaps some 50 m or more ahead. The required laying technique is alrea¬ dy highly developed in connection with the laying of smooth conductor pipes, e.g. crosswise under roads, and care should just be taken that the mantle tube 14 is

made with a smooth and sufficiently resistant outer surface.

The pipe lengths can be joined in the wells, and the joints, which are required to be extremely tight and durable, require special precautions, as it is impossi¬ ble to just weld the pipe ends together. It is inevi¬ table to use intermediate coupling members which should fulfil more different conditions, first of all an un¬ breakable fixation of the pipe ends in the special situ¬ ation where very strong tensile forces occur in the pipes. This situation does not arise in connection with usual water pipes, inasfar as it is specific for the heating pipes here considered, when these pipes after heating through longer periods of time have assumed a relaxed condition for neutralization of the compression forces created by the heating, such that the same pipes in case of a following and rather sudden cooling, e.g. if repair works are to be done, are caused to contract and thereby build up the strong tensile force, which will be neutralized by renewed relaxation only if the pipe is thereafter kept unheated for quite a long period of time, this rarely being actual. The cooling takes place relatively rapidly, and the tensile forces will be at maximum immediately thereafter, i.e. for an effective maintaining of the joints it will be unimportant whether the cool period is long or short. The joints, in every case, will have to be designed such that they can take up the largest occurring tensile forces in the pipes.

In practice this implies that the coupling parts should preferably be made of iron or steel, i.e an elec¬ trically conducting material, and an associated electri¬ cal connection with the aluminium pipe 6 easily gives rise to problems with respect to cathodic corrosion. Consequently, the coupling parts should be connected with the pipe ends without any electrical connection with the aluminium pipes.

Moreover, the difficulty arises that it is diffi¬ cult to secure complete tightness by an abutting engage¬ ment of clamping means against the surface coatings present on the pipe 6. The reason is that the plastic relaxates at high temperature and that the strain cor¬ responding to the residual tension will be less than the contraction taking place in the plastic owing to the cooling. This implies that a pressure seal may be un¬ tight in cold condition after having been kept warm for a long time, and it is just this situation which, in connection with the invention, should be anticipated to create problems.

Thus, there are quite a few problems in establish¬ ing a coupling which is advantageous in these different respects and is reasonably easy to work with, but the invention also comprises a solution to this problem, confer the illustrations in Figs. 3 and 4.

The coupling device shown therein will resemble a hose connector with an intermediate flange 22 having a projecting, cylindrical inner portion 24, which is in- troduceable into an end portion of the conductor pipe 4 and is provided with outer bent-in portions 26 between outstanding portions 28, in which annular grooves for rubber resilient O-rings 30 are provided.

Against the intermediate flange 22 there is provi¬ ded a groove 32 having an outer, projecting limitation portion 34, in front of which there is mounted an O-ring 36, which can receive the end of the the pipe 6 in an electrically insulated manner.

In connection with this joining element is provided an outer joining bushing 38, which can be crimped or pressed against the outside 10 of the inner conductor pipe 4. This bushing has some annular out-bulgings 40 which, when crimped by means of a straight cylindrical tool, will cause the formation of the bent-in portions shown in Fig. 2, just outside the bent-in portions 26 in

the inner bushing portion 24, whereby the pipe 4 will be deformed correspondingly. Thereby the pipe will be me¬ chanically locked to the bushing, but the locking will be further secured by virtue of the innermost of the said out-bulgings, here designated 42, pressing the adjacent end portion of the outer bushing 38 into the groove 32 in the rigid part of the bushing, without this very strong engagement being able to effect any such damage to the pipe 4 which could lead to a contact for¬ mation between the aluminium pipe 6 and the joining membe .

After this joining and, optionally, an associated pressure testing, a conventional muff coverage and in¬ sulation of the space around the joint can be effected, the joints being located slightly protruding from the ends of the respective insulation layers 12 and outer mantle tubes 14.

In connection with the invention it has been found that for a really efficient joining of the pipe ends against excessive pulling forces it is a necessity that the pipes are anchored in the end bushings with a plura¬ lity of bendings of the pipe wall, this not being possi¬ ble with the use of conventional joining bushings, and that the tightening up of the bendings shall be very tight or strong. This is achievable with the use of the bushing according to Figs. 3 and 4, but to a still high¬ er degree with the embodiment shown in Figs. 5 and 6, where certain reference numbers designate similar parts as in Figs. 3 and 4. Hereafter, Figs. 5 and 6 are des¬ cribed in more detail as illustrating a joining method with the use of a particularly designed joining bushing:

As a first operation there is inserted on the pipe 6 an outer bushing 38 having a head portion 43 fitting loosely around the pipe, while this bushing is otherwise made as a relatively thin-walled cylindric member 44, the inner diameter of which is noticeably larger than

the outer diameter of the pipe 6.

Thereafter the outermost portion 46 of the pipe 6 is drifted so as to adopt an outer diameter correspond¬ ing to the inner diameter of the cylindric member 44. Thereafter is inserted in the pipe mouthing an insert member 24, which, on the coupling member designated 47 as a whole, projects forwardly from an abutment collar 48 with an outer diameter corresponding to the inner diameter of the drifting of the pipe, though with an outer end portion 50 fitting into the non-drifted pipe 6. The drifting can be effected separately or directly by the driving in of the insert member 24. As shown, the design is such that the base rounding of the drifting 46, designated 52, abuts the transition between the parts 50 and 24, concurrently with the free end of the drifting abutting the abutment collar 48.

Thereafter the outer bushing 38 is pushed out so as to cover the drifting 46 and further to surround the part of the coupling member 47 positioned between the abutment collar 48 and a still further projecting poly¬ gonal collar 22. When the cylindrical member 44 is pres¬ sed against this collar, also the transition 54 between the parts 43 and 44 will be pushed against the base area 52 of the drifting.

Approximately midways on the insert bushing 24 there is provided a broad groove 56, and a corresponding groove 58 is cut in the bushing just outside the abut¬ ment collar. This implies that with the use of a suita¬ ble tool equipment it is possible to work out a pressing-in of partial areas of the cylindrical member 44, as shown at 60 and 62 in Fig. 6. By the pressing in of the areas 60 the wall of the drifting 46 will be correspondingly deformed into the groove 56, and it should be endeavoured or ensured that the relevant nar¬ rowing portions 64 of the pipe wall are pressed firmly against the sides and the bottom of the groove 56, to

make the filling-out complete.

At the pressed in areas 62 an independent anchoring of the outer bushing 38 to the coupling member 47 will be obtained, this providing a strong stabilization of the total holding engagement.

It will be noted that on the length section of the aluminium pipe there are no less than six changes of direction and that the diameter differences encountered correspond approximately to the wall thickness of the pipe. The transitions between the different sections take place through rounded angles of some 45°, and all in all it is possible to hereby establish a pipe fasten¬ ing, the strength of which is so close to the ultimate stress of the pipe that it will be superfluous to fur¬ ther strengthen the fixation. However, there may occur special circumstances for different pipe materials and dimensions, and it may then be relevant to modify de¬ tails of the coupling, e.g. by adding a further groove 56.

In the main portion 43 of the outer bushing 38 is provided an annular groove, just as in the polygonal flange 22. These grooves may be used for the gripping of the coupling parts during the joining operation.

Preferably the coupling parts are made of iron, and since the pipe is electrically insulated both internally and externally the formation of galvanic corrosion will thus be counteracted. However, as shown in Fig. 6, an insulation ring 36 should be added at the place of abut¬ ment of the pipe end against the flange 48, as the alu¬ minium will be exposed at the cut pipe end. For the same reason this area should be kept free of moisture, so there is also mounted a pair of sealing rings 30 at the outside of the insertion part 24.

The mounting of the coupling may well be done in situ, e.g. in connection with a short cutting of the pipe, but generally it will be natural that the pipe

elements are delivered with factory mounted couplings, whereby the joining work in the field can be reduced to a minimum.

It has been mentioned that the diffusion tight pipe portion is of aluminium, but of course the invention will also comprise other, similar materials which might be found to have the discussed relevant properties.