Technical Field of the Invention

The present invention relates to plumbing fittings for multilayer pipes and to methods of forming such fittings. In particular the invention relates to plumbing fittings for multilayer pipes formed in several parts.

Background to the Invention

Multilayer tube that is used for plumbing is constructed from 3 layers. In one example, the inner layer is plastic (cold cross linked polyethylene, PEX-Xc), this is covered in a layer of aluminium which gives the pipe its superior rigidity and strength compared to typical polybutylene (PB) and cross linked polyethylene (PEX) tube, and then there is an outer layer of PEX-Xc to provide a high quality finish and to protect the aluminium from external corrosion. These layers are bonded together with adhesive to prevent the pipe delaminating.

Plastic push fit fittings that are designed to work with multilayer tube have a requirement to prevent water from reaching the cut end of the pipe. This is because if water can get to the cut end of the pipe it will contact the aluminium layer exposed by the cut. This layer will potentially corrode away, delaminating the pipe and causing the joint to fail. Additionally there are health concerns if the corroded aluminium contaminates the drinking water in the pipe. An exemplary prior art push- fit pipe fitting for multilayer pipes is shown in figure

1. The fitting of figure 1 is an "elbow", having a corner and two mouths, one of which will serve as an inlet and the other as an outlet, for other purposes other shapes of fittings, e.g. "tees" with three mouths will be used. To prevent water reaching the cut end of the multilayer pipe, the fitting 1 is inserted into the pipe (not shown), rather than vice versa as is conventional, and has a seal (typically an o-ring) 2 that sits in a circumferential groove 3 in the outer wall of the fitting and seals against the inner wall of the pipe. Accordingly, the thickness of the pipe fitting will increase inwardly from the mouth 4, so as to provide sufficient space to accommodate the groove 3, and the diameter of the bore 5 through the fitting 1 will decrease commensurately.

Typically, manufacturers will injection mould the main body of the fittings 1 as a single moulding and will then fit the retaining mechanism to the main body via a snap fit, thread or weld. To mould a fitting 1 according to this method, the cores that form the waterways have to be able to be withdrawn from the fitting after the plastic is injected into the tool, therefore there can be no undercuts inside the body beyond the seal groove 3. This means that most of the waterway 5 has to be reduced in diameter to accommodate the seal groove 1. For example, in a 15mm elbow fitting, this reduces the cross section of the waterway from 95mm ² to 38.5mm ² . This causes a large pressure drop through the fitting and results in reduced flow rate through the system.

The present invention seeks to provide a pipe coupling that at least partially overcomes or alleviates this problem.

Summary of the Invention

According to one aspect of the invention, there is provided a pipe coupling for multilayer pipes comprising a body adapted for insertion into a pipe; the body having a through passage extending inwardly from at least two mouths and on its outer surface a groove for receiving a sealing element for forming a fluid-tight seal between an internal surface of the pipe and the body; wherein the cross sectional area of the through passage increases inwardly of the groove.

The increase in cross sectional area of the through passage inwardly of the groove means that the entire waterway inward of the groove is not reduced in cross section, so the pressure drop and consequential reduction in flow-rate is not so marked.

"Cross sectional area" is to be understood as the area of the pipe coupling in the plane perpendicular to the direction of flow through the pipe.

The pipe coupling may comprise a bend, the through passage extending inwardly from at least one mouth to the bend and extending outwardly from the bend to at least one other mouth.

The pipe coupling may be an elbow. The pipe coupling may be a tee, with the through passage extending inwardly from at least one mouth to the bend and extending outwardly from the bend to at least two other mouths. Alternatively, the pipe coupling may not comprise a bend in the through passage and may instead be straight The cross sectional area of the through passage which increases inwardly of the groove may increase gradually inwardly of the groove. This ensures that the flow remains laminar and avoids turbulence. The cross sectional area of the through passage which increases inwardly of the groove may increase smoothly inwardly of the groove.

The cross sectional area of the through passage which increases inwardly of the groove may comprise a frustoconical section along which the cross sectional area increases. The cone angle of the frustoconical section may be no more than 20 degrees. The cross sectional area of the through passage may increase outwardly of the groove, to at least one mouth. The cross sectional area of the through passage which increases outwardly of the groove may increase gradually outwardly of the groove.

The cross sectional area of the through passage which increases outwardly of the groove may comprise a frustoconical section along which the cross sectional area increases and the cone angle of the frustoconical section may be no more than 20 degrees.

The body of the pipe coupling may be formed of at least two tubular parts, joined so as to form the through passage therethrough. The at least two tubular parts may each have the features set out above. As set out in the background of the invention, manufacturers typically injection mould the main body of the fittings as a single moulding and will then fit the retaining mechanism to the main body via a snap fit, thread or weld. To mould a fitting according to this method, the cores that form the through passage have to be able to be withdrawn from the fitting after the plastic is injected into the tool, therefore there can be no undercuts inside the body beyond the seal groove. By forming the body of the pipe from a plurality of tubular parts, an e.g. frustoconical core can be used to produce the increasing cross section inwardly of the groove (and another core can be used to produce the increasing cross section outwardly of the groove) in one part, which can include the mouth and the groove. If the connector is for connecting a multi-layer pipe with a pipe of a larger bore, the second part (and any further parts) need not necessarily be of the same construction, having a part for insertion into a pipe with a groove for a seal thereon and a cross section increasing inwardly. On the other hand, if the connector is for connecting two multilayer pipes, at least two parts may both comprise a portion adapted for insertion into a pipe, and a mouth, with the through passage extending inwardly from the at least two mouths; and both parts may have on their outer surface a groove for receiving a sealing element for forming a fluid-tight seal between an internal surface of the pipe and the body; with the cross sectional area of the through passage increases (preferably gradually, and/or smoothly) inwardly of one or both of the grooves (and optionally the cross sectional area of the through passage increases, preferably gradually and/or smoothly outward of one or both grooves). The body of the pipe may be formed of at least three tubular parts joined so as to form the through passage therethrough.

At least two of the three tubular parts may each be adapted for insertion into a pipe and comprise a mouth through which fluid may flow between the pipe coupling and the respective pipe. The at least two tubular parts each comprising a mouth may each comprise a groove on their outer surface for receiving a sealing element for forming a fluid-tight seal between an internal surface of the pipe and the body. The cross sectional area of the through passage through each of the at least two tubular parts may increase inwardly of their respective grooves.

A third part of the at least three tubular parts may be located inwardly of the at least two tubular parts and comprise a bend. The third part may have a cross sectional area corresponding to at least the cross sectional area of the at least two tubular parts at the join. The body of the pipe may be formed of at least four tubular parts joined so as to form the through passage therethrough.

At least three of the four tubular parts may each be adapted for insertion into a pipe and comprise a mouth through which fluid may flow between the pipe coupling and the respective pipe; wherein the at least three tubular parts each comprising a mouth each comprise a groove on their outer surface for receiving a sealing element for forming a fluid-tight seal between an internal surface of the pipe and the body; and wherein the cross sectional area of the through passage through each of the at least three tubular parts increases inwardly of their respective grooves. Each of the at least three tubular parts adapted for insertion into a pipe may comprises any of the optional features set out above.

The fourth part of the at least four tubular parts may be located inwardly of the at least three tubular parts and comprise a bend, defining a branch. Thus, the four-part pipe coupling may be a "tee". The fourth part may have a cross sectional area corresponding to at least the cross sectional area of the at least three tubular parts at the join.

In a second aspect of the invention, there is provided a method of manufacturing a pipe coupling for multilayer pipes comprising: providing a plurality of tubular parts, wherein a first tubular part comprises a portion adapted for insertion into a pipe, having on its outer surface a groove for receiving a sealing element for forming a fluid-tight seal between an internal surface of the pipe and the first tubular part; the first tubular part having a through passage extending inwardly from a mouth and the cross sectional area of the through passage increasing inwardly of the groove; and wherein a second tubular part comprises a passage therethrough having a bend; and joining the second tubular part to the first tubular part.

The method may comprise providing at least three tubular parts, wherein a first tubular part comprises a portion adapted for insertion into a pipe, having on its outer surface a groove for receiving a sealing element for forming a fluid-tight seal between an internal surface of the pipe and the first tubular part; the first tubular part having a through passage extending inwardly from a mouth and the cross sectional area of the through passage increasing inwardly of the groove; wherein a second tubular part comprises a passage therethrough having a bend, and wherein a third tubular part comprises a portion adapted for insertion into a pipe, having on its outer surface a groove for receiving a sealing element for forming a fluid-tight seal between an internal surface of the pipe and the third tubular part; the third tubular part having a through passage extending inwardly from a mouth and the cross sectional area of the through passage increasing inwardly of the groove; and joining the second tubular part to the first tubular part and the second tubular part to form a body having a through passage extending between the two mouths.

The second tubular part may be joined to the first tubular part and optionally to the third tubular part by welding, or by screwing, or by a snap fit.

The method may comprise injection moulding the first tubular part. The method may comprise injection moulding the third tubular part. The method may comprise injection moulding the second tubular part.

The method may comprise attaching a gripping mechanism to the first tubular part and optionally to the third tubular part, to grip the outer surface of a pipe. The method may comprise introducing a sealing element into the or each groove.

Detailed Description of the Invention

In order that the invention may be more clearly understood embodiments thereof will now be described, by way of example only, with reference to the accompanying drawings, of which:

Figure 1 shows a lateral cross section through a prior art pipe coupling;

Figure 2 shows a lateral cross section through a first embodiment of a pipe coupling according to the invention;

Figure 3 shows an isometric exploded cross sectional view of the pipe coupling of figure 2;

Figure 4 shows an isometric cross sectional view of the pipe coupling of figures 2 and 3;

Figure 5 shows an isometric part-exploded cross sectional view of the pipe coupling of figures 2 to 4 including a gripping mechanism; Figure 6 shows an isometric part-exploded cross sectional view of a pipe coupling according to a second embodiment of the invention;

Figure 7 shows an isometric part-exploded cross sectional view of a pipe coupling according to a third embodiment of the invention;

Figure 8 shows an isometric exploded cross sectional view of a pipe coupling according to a fourth embodiment of the invention; and

Figure 9 shows an isometric cross sectional view of the pipe coupling of figure 8. With reference to figures 2 to 5, a pipe coupling 10 is provided to couple two multilayer pipes, for example for use in domestic water systems, in a fluid tight manner. The pipe coupling 10 in this first embodiment is an "elbow", having a first mouth 11 and a second mouth 12 at right angles to each other, with a through passage 13 extending inwardly between them.

The pipe coupling 10 comprises a body formed of three tubular parts each injection moulded from plastics material. The first tubular part 14 is a substantially cylindrical straight tube. The second tubular part 15 is connected to the first tubular part at the opposite end to the mouth 11 and an annular cross section with an internal dimension corresponding to the cross section of the through passage at the end of the first tubular part 14, but defines the bend of about 90 degrees in the through-passage through the pipe coupling. The third tubular part 16 is identical to the first tubular part 14, and is joined to the other end of the second tubular part 15, so as to define a through passage between the mouths 11, 12.

The first tubular part 14 and third tubular part 16, both comprise a portion 17, 19 adapted for insertion into a pipe, having a diameter slightly smaller than that of the pipe it is intended to be inserted into, the portion 17, 19 adapted for insertion into a pipe extending from the mouth 11, 12 and terminating at a stop 20, 21. The stop 20, 21 is in the form of an annular projection proximal to the end of the tubular part 14, 16 opposite its mouth 11, 12.

The first tubular part 14 is provided with a circumferential groove, 22, between its stop 20, and its mouth 11 on the outer surface of the portion 17 adapted for insertion into a pipe. A corresponding groove 23 is provided on the outer surface of the portion 19 of the third tubular part 16. The grooves 22, 23 each receive sealing elements 24, 25 in the form of O-ring seals to form a fluid-tight seal between an internal surface of the pipe and the body of the pipe coupling 10. The portion 17, 19 adapted for insertion into a pipe, of each of the first tubular part 14 and the third tubular part 16 has a substantially constant outer diameter (apart from the groove). However, the internal diameter varies. The internal diameter of the first tubular part 14 and the third tubular part 16 is at its smallest in the region of the groove 22, 23, in order to provide sufficient thickness to maintain rigidity despite the wall being thin in that area to accommodate the groove 22, 23. Accordingly, the cross sectional area of the first and third tubular parts 14, 16 is at a minimum in the region radially internally of the groove 22, 23. Outwardly of the groove 22 of the first tubular part 14, i.e. in the direction of its mouth 11, the internal diameter of the first tubular part 14 gradually and smoothly increases, such that the through passage 13 comprises a frustoconical section 26. Of course, a similar frustoconical section 27 with the diameter decreasing inwardly is provided in the same area of the third tubular part 16. The cone angle of these frustoconical sections in this embodiment is 20 degrees (i.e. 10 degrees each side).

Similarly, inwardly of the groove 22, i.e. in the direction away from its mouth 11, the internal diameter of the first tubular part 14 increases gradually and smoothly. Accordingly, a second frustoconical section 28 is provided in the first tubular part 14, whereby the cross sectional area of the through passage inwardly of the groove 22 increases. Once again, as it is identical, the third tubular part 16 also includes a corresponding frustoconical section 29 in which the cross sectional area of the through passage increases inwardly of the groove 23. In this embodiment the cone angle of the second frustoconical sections are 10 degrees.

In laboratory tests, with the cone angles maintained at less than 20 degrees, pressure loss caused by the reduction in diameter for the groove can be recovered, to the extent that with an elbow joint as described above, the pressure loss can be reduced by 50% compared to a conventional elbow joint with the same reduced internal diameter. The angles are kept small and the angles of the first frustoconical sections 26, 27 and the second frustoconical sections are similar to each other. This keeps flow through and after the restriction laminar and prevents turbulence which reduces flow rates. As set out above, second tubular part 15 of the body of the pipe coupling 10 is connected to the first tubular part 14 and the third tubular part 16 at their inward ends and has an annular cross section with an internal dimension corresponding to the cross section of the through passage at the end of the first tubular part, but defines the bend of about 90 degrees in the through-passage through the pipe coupling. At each end, the second tubular part 15 comprises an annular shoulder 30, 31, into which the inward ends of the first tubular part 14 and third tubular part 16 respectively are inserted. The shoulders 30, 31 each comprise a step portion 32 which abuts the end face of the internal end of the first or third tubular part 14, 16, a sleeve portion 33 which extends around the circumference of the first or third tubular part 14, 16 in the region between its end and the stop 20, 21 and a stop face 34 which abuts the stop 20, 21. The step portion 32 is provided with an axially extending circumferential rib 35, shown in figure 3 to form a good seal when the first and third tubular parts 14, 16 are joined to the second part 15. To form the body of the pipe coupling 10 of the first embodiment of the invention, the first tubular part 14 and third tubular part 16 may be manufactured by injection moulding, by providing a tool having an internal surface corresponding to the external surface of the first/third tubular part 14, 16. A pair of cores may be introduced, one from each end (the mouth 11, 12 and the inward end) each having a tapered shape and at least one having a substantially constant diameter section to correspond to the area internally of the groove 22, 23. The cores meet in the middle and molten plastics material is introduced into the tool around the cores. Because of their tapered shape, with no undercuts, once the tubular parts 14, 16 have solidified, the cores may then be removed, leaving the first and third tubular parts 14, 16 having the frustoconical portions discussed above. A similar injection moulding technique with a pair of cores each corresponding to one half of the second tubular part 15 may be used in its manufacture.

In the first embodiment, the inward ends of the first tubular part 14 and the second tubular part 16 are then introduced into the shoulders 30, 31 of the second tubular part and joined by adhesive, or ultrasonic welding to form the body of the pipe coupling 10. The O-ring seals 24, 25 may then be introduced into the grooves 22, 23 and a mechanism for gripping the external surface of the multilayer pipe can be attached to the first tubular part 14 and the third tubular part 16 of the body.

In the example of the first embodiment shown in figure 5, the mechanism for gripping the multilayer pipe comprises a two-part collar 36 which sandwiches a grab ring 37 between the two parts, is coaxial with the respective first or third tubular part 14, 16 that it surrounds and defines an annular recess 38 into which the pipe may be inserted. The rear part of the two-part collar 36 is provided with snap-fit formations 39 on its inner surface, such that it can be assembled onto the body of the pipe coupling 10 by snap- fitting over a respective stop 20, 21.

In use, the free ends of multilayer pipes are inserted into the annular recess 38 between the two-part collars 36 and respective first or third tubular part 14, 16, such that the respective first or third tubular part 14, 16 fits tightly into the pipe, the pipe extends over the respective sealing element 24, 25, which forms a fluid-tight seal and its free end abuts the respective stop 20, 21. The teeth of the grab ring 37 will then grip the outer surface of the pipe, to retain the pipe in position. Fluid will flow in through one of the mouths, e.g. the mouth 11, of the first tubular part 14 of the pipe coupling 10 and out through the other mouth 12, as fluid flows inwardly, the flow will be gradually restricted by the reducing diameter of the frustoconical section 26, but once past the seal, the restriction to the flow will be lifted, as the diameter gradually and smoothly increases - the increased diameter remains throughout the bend, and is only reduced again, gradually as when the fluid flows outwardly through the frustoconical section 29 of the second tubular part 16 which has a cross sectional area which increases inwardly. Again, the flow is only briefly restricted in the region of the groove 23, before the diameter again increases gradually to achieve good pressure loss recovery.

Figure 6 shows a second embodiment of the invention which includes many of the same features of the first embodiment. These therefore retain the same numbering as the first embodiment, but with the prefix "2", but will not be described in detail. Hence, the second embodiment is a pipe coupling 210 comprising a body formed of a first tubular part 214, a second tubular part 215 and a third tubular part 216, each of which have the same general shape and configuration as in the first embodiment. The second embodiment differs in terms of the joints between the first and third tubular parts 214, 216 and the second tubular part 215. Specifically, in the region of the shoulders 230, 231 and the corresponding ends 240, 241 of the first and third tubular parts, which, in the second embodiment are provided with corresponding threads 242, 243. The ends, 240, 241 each comprise an external thread 243, whilst the sleeves 233 of the shoulders 231 are each provided with internal threads. Moreover, a sealing element 244 (an O-ring) is provided in a groove in the end 240, 241, between the thread 243 and the stop 220, 221, for sealing engagement with the distal interior surface of the sleeve 242. Providing a thread, which may be connected by a user, can obviate the manufacturing step of adhering/welding the first and third tubular parts 214, 216 to the second tubular part 215. A further alternative to the welding/adhering technique is shown in the third embodiment, illustrated in figure 7, and which again includes many of the same features of the first embodiment. These therefore retain the same numbering as the first embodiment, but with the prefix "3", and will not be described in detail.

Once again, the difference in the third embodiment is in terms of the joints between the first and third tubular parts 314, 316 and the second tubular part 315. Specifically, in the region of the shoulders 330, 331 and the corresponding ends 340, 341 of the first and third tubular parts, which, in the third embodiment are provided with formations for snap fit engagement. The ends, 340, 341 each comprise an external bead 343, whilst the sleeves 333 of the shoulders 331 are each provided with corresponding circumferential recesses 334. Moreover, a sealing element 344 (an O-ring) is provided in the shoulder 330, 331 adjacent the internal face of the sleeve 333 and the step portion 332 for sealing engagement against the distal exterior surface of the end 420, 341. Just as the thread, a snap-fit bead may be connected by a user, and can obviate the manufacturing step of adhering/welding the first and third tubular parts 314, 316 to the second tubular part 315.

Figures 8 and 9 show a fourth embodiment of the invention. In the fourth embodiment a pipe coupling 410 is provided to couple three multilayer pipes. Accordingly, the pipe coupling 410 in this first embodiment is a "tee", having a first mouth 411 and a second mouth 412 in line to each other, with a through passage 413 extending inwardly between them. A third mouth 450 is also provided on a branch at right angles to the first mouth 411 and the second mouth 412, with the through passage 413 extending inwardly from the third mouth.

The pipe coupling 410 comprises a body formed of four tubular parts each injection moulded from plastics material. The first tubular part 414 is a substantially cylindrical straight tube. The second tubular part 415 is connected to the first tubular part at the opposite end to the mouth 411 and an annular cross section with an internal dimension corresponding to the cross section of the through passage at the end of the first tubular part 414, but defines a T-shape, having three openings one at about 90 degrees from the other two. The third tubular part 416 and the fourth tubular part 451 are identical to the first tubular part 414, and are joined to the two other ends of the second tubular part 415, so as to define a through passage between the coaxial mouths 411,412, and the branch mouth 450.

The first tubular part 414, third tubular part 416, and fourth tubular part 451 all comprise a portion 417, 419, 452 adapted for insertion into a pipe, having a diameter slightly smaller than that of the pipe it is intended to be inserted into, the portion 417, 419, 452 adapted for insertion into a pipe extending from the mouth 411, 412, 450 and terminating at a stop 420, 421, 453. The stops 420, 421, 453 are in the form of annular projections proximal to the end of the tubular parts 414, 416, 451 opposite their mouths 411, 412, 450. The first tubular part 414 is provided with a circumferential groove, 422, between its stop 420, and its mouth 411 on the outer surface of the portion 417 adapted for insertion into a pipe. Corresponding grooves 423, 454 are provided on the outer surfaces of the portions 419, 452 of the third and fourth tubular parts 416, 451. The grooves 422, 423, 454 each receive sealing elements 424, 425, 455 in the form of O-ring seals to form a fluid-tight seal between an internal surface of the pipe and the body of the pipe coupling 410.

The portion 417, 419, 452 adapted for insertion into a pipe, of each of the first, third and fourth tubular parts 414, 416, 451 has a substantially constant outer diameter (apart from the groove). However, the internal diameter varies. The internal diameter of the first, third and fourth tubular parts 414, 416, 451 is at its smallest in the region of the groove 422, 423, 451 in order to provide sufficient thickness to maintain rigidity despite the wall being thin in that area to accommodate the groove 422, 423, 454. Accordingly, the cross sectional area of the first, third and fourth tubular parts 414, 416, 451 is at a minimum in the region radially internally of the groove 422, 423, 454. Outwardly of the groove 422 of the first tubular part 414, i.e. in the direction of its mouth 411, the internal diameter of the first tubular part 414 gradually and smoothly increases, such that the through passage 413 comprises a frustoconical section 426. Of course, a similar frustoconical section 427, 456 with the diameter decreasing inwardly is provided in the same area of the third and fourth tubular parts 416, 451. The cone angle of these frustoconical sections in this embodiment is 20 degrees.

Similarly, inwardly of the groove 422, i.e. in the direction away from its mouth 411, the internal diameter of the first tubular part 414 increases gradually and smoothly. Accordingly, a second frustoconical section 428 is provided in the first tubular part 414, whereby the cross sectional area of the through passage inwardly of the groove 422 increases. Once again, as they are identical, the third and fourth tubular parts 416, 451 also include a corresponding frustoconical section 429, 457 in which the cross sectional area of the through passage increases inwardly of the groove 423. In this embodiment the cone angle of the second frustoconical sections are 10 degrees.

In laboratory tests, with the cone angles maintained at less than 20 degrees, pressure loss caused by the reduction in diameter for the groove can be recovered, to the extent that with a "tee" joint as described above, the pressure loss can be reduced by 35%, when the flow path is via the branch compared to a conventional tee joint with the same reduced internal diameter. The angles are kept small and the angles of the first frustoconical sections 426, 427, 456 and the second frustoconical sections 428, 429, 457 are similar to each other. This keeps flow through and after the restriction laminar and prevents turbulence which reduces flow rates.

As set out above, second tubular part 415 of the body of the pipe coupling 410 is connected to the first tubular part 414, the third tubular part 416 and the fourth tubular part 451 at their inward ends and has an annular cross section with an internal dimension corresponding to the cross section of the through passage at the end of the first tubular part, but defines a T-shape having a branch in the through-passage through the pipe coupling.

At each end, the second tubular part 415 comprises an annular shoulder 430, 431, 458, into which the inward ends of the first tubular part 414, third tubular part 16 respectively are inserted. The shoulders 430, 431, 458 each comprise a step portion 432 which abuts the end face of the internal end of the first, third or fourth tubular part 414, 416, 451 a sleeve portion 433 which extends around the circumference of the first, third or fourth tubular part 414, 416, 451 in the region between its end and the stop 420, 421, 453 and a stop face 434 which abuts the stop 420, 421, 453. The step portion 432 is provided with an axially extending circumferential rib 435, shown in figure 8 to form a good seal when the first, third and fourth tubular parts 414, 416, 451 are joined to the second part 415.

Just as in the previous embodiments, to form the body of the pipe coupling 410 of the first embodiment of the invention, the first tubular part 414, third tubular part 416 and the fourth tubular part 451 may be manufactured by injection moulding, by providing a tool having an internal surface corresponding to the external surface of the first/third/fourth tubular part 414, 416, 451. A pair of cores may be introduced, one from each end (the mouth 411, 412, 450 and the inward end) each having a tapered shape and at least one having a substantially constant diameter section to correspond to the area internally of the groove 422, 423, 454. The cores meet in the middle and molten plastics material is introduced into the tool around the cores. Because of their tapered shape, with no undercuts, once the tubular parts 414, 416, 451 have solidified, the cores may then be removed, leaving the first and third tubular parts 414, 416, 451 having the frustoconical portions discussed above. A similar injection moulding technique with three cores each corresponding to one inlet of the second tubular part 415 may be used in its manufacture.

In the fourth embodiment, the inward ends of the first tubular part 414, the third tubular part 416 and the fourth tubular part 451 are then introduced into the shoulders 430, 431, 458 of the second tubular part 415 and joined by adhesive, or ultrasonic welding to form the body of the pipe coupling 410. The O-ring seals 424, 425, 455 may then be introduced into the grooves 422, 423, 451 and a mechanism for gripping the external surface of the multilayer pipe can be attached to the first tubular part 414, the third tubular part 416 and the fourth tubular part 451 of the body, in the same manner as the first embodiment discussed above with reference to figure 5.

In use, the free ends of multilayer pipes are attached to respective first, third and fourth tubular parts 414, 416, 451 such that the respective first, third or fourth tubular part 414, 416, 451 fits tightly into the pipe, the pipe extends over the respective sealing element 424, 425 455, which forms a fluid-tight seal and its free end abuts the respective stop 420, 421, 453. The teeth of a grab ring or the like can grip the outer surface of the pipe, to retain the pipe in position. Fluid will flow in through one or more of the mouths, e.g. the mouth 411, of the first tubular part 414 and the mouth 450 of the fourth tubular part 451 of the pipe coupling 410 and out through one or more the other mouths, e.g. the mouth 412 of the third tubular part 416, as fluid flows inwardly, the flow will be gradually restricted by the reducing diameter of the frustoconical sections 426, 456 but once past the seal, the restriction to the flow will be lifted, as the diameter gradually increases - the increased diameter remains throughout the path of the flow, whether round the bend in the T-section, or straight through the pipe coupling, and is only reduced again, gradually as when the fluid flows outwardly through the frustoconical section 429 of the second tubular part 416 which has a cross sectional area which increases inwardly. Again, the flow is only briefly restricted in the region of the groove 423, before the diameter again increases gradually to achieve good pressure loss recovery. The above embodiments are described by way of example only. Many variations are possible without departing from the scope of the invention as defined in the appended claims.