Apparatus and method for fusion joining a pipe and fittings

Abstract: An apparatus and method for fusion joining a pipe either directly to or with respect to a fitting (10) or another pipe. An induction heating element (20) is sandwiched between the external surface of a portion of one pipe (15) and an internal surface of a fitting socket or flange (10), prior to fusion joining. Alternating current is induced through a conductive material of the induction heating element, resulting in softening or melting of preferably polyethylene material of the one pipe and the fitting. As a result of the softening or melting, the materials fuse together.

Description: This invention relates to an apparatus and method for fusion joining a pipe with respect to a fitting, and more particularly to an improved apparatus and method wherein a novel heating element is insertably positioned between a fitting and a portion of the pipe. Various conventional apparatus and methods exist for fusion joining polyethylene pipes, such as those that employ heat fusion, electrofusion or induction fusion techniques. Electrofusion uses a conventional resistive heating method wherein a wire spiral coil is typically internally moulded within the thermoplastic fitting. An electrical current is applied directly to the wire coil via terminals on the fitting which results in the coil heating the thermoplastic material to a temperature which softens the thermoplastic material enough to fuse the pipe to the fitting. In GB1149716 there is described an electrical resistance heating coil for use as a welding sleeve for joining plastics pipes to fittings wherein the pipe and fitting describe an annulus within which the welding sleeve is mounted. The coil comprises an electrical resistance wire embedded in a sheath of plastics material of precisely defined dimensions in order, it is said, to provide adequate welding efficiency. The proposed design complicates manufacture and it is difficult to connectorise the coil to an external power source. In addition, the existing wires provide potential leak paths from the joint region. In a conventional induction fusion method for joining a pipe and a fitting, a heating element is moulded within the thermoplastic fitting. Current flow is induced through the heating element by a primary induction coil that is positioned around the portion of the fitting that contains the heating element. The temperature of the heating element is raised enough to soften or melt the fitting material and the pipe material, thereby resulting in fusion between the fitting and the pipe. Examples of induction fusion methods and apparatus are described in DE-A-2136656, EP-A-026191 and EP-A-513391. Such apparatus require a fitting with a moulded-in healing element. Thus, such methods cannot be used with existing conventional plastics fittings that do not contain a moulded-in heating element. GB 2272663 describes a plastics pipe having an enlarged diameter portion at one end, which is provided at its inner surface or in the vicinity of its inner surface with a heating unit. A second plastics pipe to be joined is inserted into the enlarged portion and the heating unit energised. In manufacturing the pipe, having the enlarged diameter portion, an end of the pipe is heated and softened, the enlarged socket formed, and the heating generating unit incorporated, in one embodiment, by pressure-inserting the heat generating unit on a core mould into the socket. The heat generating unit can be an electrofusion coil embedded in a plastics material, or a bare induction fusion element. The electrofusion heat generating unit of GB2272663 is inconvenient, due to the necessity of providing a large flange to accommodate the terminals of the coil, difficult to overmould and expensive to manufacture. The bare induction fusion element, even when pressure inserted into the socket, is not completely surrounded by plastics material and is liable to overheat and be subject to thermal runaway. Once thermal runaway has started in one part of the element, its resistance changes dramatically and that area of the element draws in current (voltage, power) such that the rest of the element will not heat. The overheated region of the element can also cause local degradation of the thermoplastic material. In such cases usually the melt region cannot be contained, and thus a good fusion joint cannot be made. In DE930203 there is described a tapping fitting that can be welded to a plastic pipe. The fitting is provided with a resistance heating wire which is embedded in a plastic joint face of the fitting and is arranged spirally starting from an internal bore of the plastics part. It is apparent that there is a need for an apparatus and method that can be used to fuse a fitting with respect to a pipe, using induction techniques and a conventional fitting that does not contain a moulded-in heating element. CH-A-336980, CH-A-349404 and CH-A-576326 each disclose an electrofusion fitting in which a separate collar of plastics material and embedded resistance wires is insertable between a fitting (eg a sleeve coupler) and a pipe prior to fusion. CH-A-532996 discloses a variation of this in the form of a mat of electrical conductors coated with plastics material and inserted between the mating surfaces of a saddle fitting for subsequent fusion. It is one object of this invention to provide an electrofusion joining apparatus and method that can be used with conventional fittings to join a pipe either directly to or with respect to a fitting or another pipe. It is another object of this invention to provide a novel induction heating element which can be quickly and easily field-installed within a conventional thermoplastic fitting or within an electrofusion fitting from which the moulded-in heating element has been omitted. The above and other objects of this invention are accomplished with an apparatus that employs a novel induction heating element. In accordance with the present invention, there is provided an induction heating element according to claim 1. Preferably, said induction heating element comprises a flat mat deformed into a cylindrical shape. Said induction heating element may comprise preformed conductive wire. The induction heating element is preferably wholly encased or coated with a fusible thermoplastic polymeric material provided with peripheral regions of plastics material which do not include the cage and so that cold zones are formed which help to contain the melt during the fusion step. The invention also provides an apparatus for fusion joining a pipe with respect to a fitting comprising an element as defined above and a fitting. The fitting may be provided with a flange, the induction heating element being mountable on said flange, such that the fitting and the element together define a throughbore. In another aspect, the present invention provides apparatus for fusion joining a pipe with respect to a fitting comprising: an induction heating element, the element being wholly encased or coated with a fusible thermoplastic polymeric material, a fitting provided with a flange, the induction heating element being mountable on said flange, such that the fitting and the element together define a throughbore, and a portion of the pipe being mountable on said flange whereby the induction heating element can, when energised, provide a fusion joint between the fitting and the said portion of the pipe, characterised in that said induction heating element comprises an induction heating cage, in series with an induction loop mounted outside of said fitting. In a further aspect, the invention provides a method of fusion joining a pipe with respect to a fitting employing apparatus as defined above, the method including the steps of: mounting the induction heating element on the flange of the fitting, such that the fitting and the element together define a throughbore, mounting a portion of the pipe on said flange, and inducing an alternating current in the induction loop and which flows through the induction heating cage to provide a fusion joint between the fitting and the said portion of the pipe. The apparatus and method according to this invention can be used to fusion join standard, off-the-shelf thermoplastic fittings either directly to or with respect to a pipe. As used throughout this specification and in the claims the word fitting is intended to relate to any coupling member or other suitable fitting that forms a socket or a flange for joining with a pipe or similar member. The heating element portion of an induction heating element may comprise any one or combination of a preformed conductive wire, a conductive solid having a tubular shape, a conductive grid having a tubular shape or any other suitable shape apparent to those skilled in the art. Suitable shapes for the induction heating element are also described, for example, in GB 2272643 and in EP-A-572546. Any suitable method can be used for manufacturing the induction heating element of the invention. However, in a particularly preferred method according to the invention, the conductive material of the induction heating element, which may comprise a coil, cage, or other suitable shape of conductive material, is overmoulded with a suitable thermoplastic polymeric material. It is particularly preferred to use a cage of conductive material, since this allows the conductive material cage to be made separately and then overmoulded with a thin layer (preferably from 1.0 to 10.0 mm) of thermoplastic polymeric material. The thermoplastic material is preferably polyethylene, although other polyolefins, and other thermoplastic polymers, may also be used. It is a common and conventional practice to shave or otherwise remove an outer layer of material of pipe prior to forming a fusion joint. In another preferred embodiment of the invention, the induction heating element can be factory installed in the fitting prior to shipping the fitting to the customer. Such installation can comprise, for example, fusing the induction heating element to the fitting by energising the induction heat element in a first heating step. Such fusion can ensure, with proper factory control, that the induction heating element is fused to the fitting in a gas tight fashion. Current flow through conductive material can, for example, be generated by inducing an alternating current in a primary coil which is positioned about or around the heating element. Suitable induction generators and other suitable induction methods and apparatus for inducing alternating current are known to those skilled in the art, for example, as disclosed in EP-A-566316 and EP-A-566317. In a preferred method according to the invention, the induction generator comprises a high frequency generator as described in EP-A-566316. Polymeric materials for fitting pipes, and the casing or coating applied to the heating element are well known to those skilled in the art and any suitable material can be used in combination with or in lieu of polyethylene. The polyethylene, or other polymeric materials, can be wholly or partly cross-linked, provided that sufficient fusible polymeric material is available to form a fusion joint. Also, materials and configurations for the heating element are well known to those skilled in the art and can be interchanged without departing from the results desired and achieved according to the apparatus and method of this invention. These and other objects and features of this invention will be better understood from the following detailed description taken in conjunction with the drawings wherein: Figures 1, 2 and 3 show respectively in plan view, side elevation and end elevation an induction heating cylindrical mat element according to the invention; and Figures 4 and 5 show respectively a view from below and a perspective view from one side of a tapping tee fitting provided with an induction heating cylindrical mat element according to the invention. Figures 1 to 5 illustrate the use of an induction heating element comprising a flat disc or cylindrical mat. Referring firstly to Figures 1 and 2, the induction heating element 60 comprises a flat induction heating cage 61 which comprises sets of conductive wires 62 which are linked together and connected to terminals 63, 64. The terminals are in turn connected to an upstanding induction loop 65 as best seen in Figure 3. The entire heating element is encased in a fusible thermoplastic polymeric material which is formed into "cold zone" regions 66, 67 around the inner and outer circumferences of the heating element, and into snap-fit projections, the first 68, mounted on a resilient tongue 69 on a tab 70, and the second pair, 71, mounted on the upstanding induction loop 65. Figures 4 and 5 illustrate the use of the induction heating element of Figures 1 to 3 with a tapping tee fitting. The fitting, illustrated generally at 80, is provided with a saddle region 81, having an internal surface 82 adapted to be fused to a pipe surface, an upstanding hollcw body 83, and a branch pipe 84. At the top of the body 83 is a threaded cap 85 which may incorporate a threaded cutter which can be screwed down through the body 83 in order to cut a hole in the pipe body (not shown). The fitting also has a recess 86 which can accommodate the first snap-fit projection 68, and an opposed edge 87 around which the projections 71 can be located. The assembly of the fitting and the induction heating element is best seen in Figure 4, where the cage 61 is illustrated in position mounted on the underside of the fitting 80. The cage is sufficiently flexible to follcw the contour of the internal surface 82 of the saddle 81, and is held in position by the snap-fit projections 68 and 71 prior to fusion. In operation, an alternating current is induced in the induction loop 65 and flows through the induction heating cage 61, melting or softening the surrounding thermoplastic plastics material and forming a fusion joint between the saddle region 81 and the pipe surface.

Claims: An induction heating element (60) adapted to form a fusion joint between a fitting (80) and a pipe by a fusion step, which is provided with means (68, 71) for locating and maintaining its position relative to the fitting or to the pipe, or both, prior to the fusion step, characterised in that the element comprises an induction heating cage (61) connected to an upstanding induction loop (65) adapted to have an alternating current induced therein to flow through said cage. An element according to claim 1, wherein said induction heating element comprises flat mat (60) deformed into a cylindrical shape. An element according to claim 1 or 2, wherein said induction heating element comprises pre-formed conductive wire (62). An element according to any of the preceding claims, wherein the induction heating element (60) is wholly encased or coated with a fusible thermoplastic polymeric material, provided with peripheral regions (66, 67) of plastics material which do not include the cage and so that cold zones are formed which help to contain the melt during the fusion step. An element according to claim 4, wherein said regions of plastics material are moulded with the plastics material encasing the induction heating element. An element according to any of the preceding claims, wherein the locating and maintaining means comprises one or more projections (68, 71) which are a snap-fit on either the fitting or the pipe, or both. An apparatus for fusion joining a pipe with respect to a fitting comprising an element as claimed in any of the preceding claims and a fitting (80). An apparatus as claimed in claim 7, wherein the fitting (80) is provided with a flange (81), the induction heating element (6C) being mountable on said flange, such that the fitting and the element together define a throughbore. An apparatus for fusion joining a pipe with respect to a fitting (80) comprising: an induction heating element (60), the element being wholly encased or coated with a fusible thermoplastic polymeric material, a fitting (80) provided with a flange (81), the induction heating element being mountable on said flange, such that the fitting and the element together define a throughbore, and a portion of the pipe being mountable on said flange whereby the induction heating element can, when energised, provide a fusion joint between the fitting and the said portion of the pipe, characterised in that said induction heating element comprises an induction heating cage (61) in series with an induction loop (65) mounted outside of said fitting. An apparatus according to claim 9, wherein said induction heating element is as claimed in any of claims 1 to 6. A method of fusion joining a pipe with respect to a fitting employing apparatus as claimed in claim 9 or 10 the method including the steps of: mounting the induction heating element on the flange of the fitting, such that the fitting and the element together define a throughbore, mounting a portion of the pipe on said flange, and inducing an alternating current in the induction loop (65) and which flows through the induction heating cage (61) to provide a fusion joint between the fitting and the said portion of the pipe. A method according to claim 11, wherein an external layer of material of the portion of the pipe in contact with the induction heating element is removed prior to being brought into contact with the induction heating element.

Inventors: SMITH CIN, SMITH, CIN
Applicants: UPONOR ALDYL COMPANY
Application year: 1997
Main classification: F16L47
IPC codes: F16L47