Weldable pipe fittings and pipe joints formed therewith

Abstract: A weldable pipe fitting (1) includes a sleeve member (2) of thermoplastic material, the sleeve member (2) having a stress resistant portion (10) of greater wall thickness, a terminal portion (11,12) of greater wall thickness, and a fusion accommodating portion (8,9) of lesser wall thickness between the stress resistant portion (10) and the terminal portion (11,12), wherein the stress resistant portion (10) is arranged in use to overlie the end of the pipe (17,18) to be connected to the pipe fitting; the stress resistant portion of the sleeve has a standard dimensional ratio within the range of no less than 8:1 and no more than 12:1, the terminal portion of the sleeve has a standard dimensional ratio within the range of of no less than 8:1 and no more than 12:1, and the fusion accommodating portion has a standard dimensional ratio within the range of no less than 14:1 and no more than 19:1; and the internal wall of the sleeve is provided with an electric heating wire (3,4) or wires along and within the wall and disposed circumferentially thereof adjacent the radially inner surface of the wall, said heating wire or wires being disposed over a length of the sleeve at least the majority of which is within the fusion accommodating portion thereof.

Description: This invention relates to weldable pipe fittings, by which is meant hollow thermoplastic fittings connectable to thermoplastic pipes in which a part of an inner peripheral wall of the fitting is arranged to be joined by fusion or "welding" to a part of an outside peripheral wall of a plastics pipe. The invention relates more particularly to the production of such weldable fittings adapted to be fused to the outside periphery of a pipe by means of an electric heating wire embedded within the fitting. It is to be understood that the expression "pipe" as used herein includes pipes and tubes as such and also pipe-like members and fittings for use in pipework such as bends, elbows, "T" connectors, flange adaptors and parts and devices adapted to be fitted by pipe-like parts to the pipework such as valves and pumps. It has already been proposed to provide a hollow plastics fitting of the kind referred to in which a helical groove is formed around a part of the inner wall of a sleeve-like part of a fitting within which is located an electrically conducting wire, so that when in use the fitting is placed in close proximity about a thermoplastic pipe or pipe-like member and an electric current passed through the electic wiring, melting of the adjacent plastics surfaces of the fitting and the pipe occurs and fusion or "welding" between the two bodies takes place. Such arrangements are disclosed in our United Kingdom Patents 2135746B and 2135747B. Another such arrangement is disclosed in EP-A-183189. EP-A-183189 is concerned only with shrinkage problems (or volume contractions) during the cooling phase of the production of electrofusion joints utilizing an electrofusion plastics sleeve. The particular problem addressed by EP-A-183189 is that of cavities, from the shrinkage factor during cooling of electrofusion sleeve joints, which can impair the strength of the sleeve and which occurs at the centre of the melting zone or zones. EP-A-183189 solves this problem by providing one or more recesses in the "edge" areas of the sleeve, such as slot 14 of Figure 3 of EP-A-183189 in the outer circumference of the sleeve member 1. This reduces the sleeve stiffness in the edge zones of the sleeve in that the bottoms of the recesses can collapse radially inwardly which thereby compensates for the volume contraction during solidification of the melting zone so as to avoid the occurrence of cavities. One of the critical factors in design of such fittings is ensuring the provision of consistently successful welds meeting professionally specified performance requirements. A most important aspect in this is achieving the correct conditions at the welding interface between the fitting and the pipe. Two such conditions comprise obtaining a suitable heating profile for satisfactory welding, and, perhaps more difficult to achieve, correct pressure within the molten plastics material during the welding process. With the development of large diameter fittings for large diameter pipes, of 180mm above, for example, the performance pressure requirements necessitate the use of thicker walled fittings and from this the achievement of the correct amount of heat and pressure during welding presents increasingly difficult problems with increase in size. Thus, for example, the thickened walls of such large fittings can cause excessive melt pressure during the welding process resulting in molten polymer, and sometimes part of the heating wires, being extruded from between the pipe and the pipe fitting. Such movement of heating wires can cause, in extreme cases, overheating by contacting and short circuiting with each other, whilst at the other extreme, premature joint failure can be caused due to the chilling effect the large cold pipe has on the melt, resulting in crack propagation and joint failure. Although theoretically a reduction in pressure at the weld melt could be achieved by reducing the wall thickness of the fitting, this can seriously compromise the stress performance under pressure in use of the fitting, whilst, in some situations still leading to exudation of molten polymer, and even at times voids along the weld interface due to the melt pressure producing a radially outward bending along the length of the reduced thickness and weakened fitting. It is an object of the present invention to overcome or at least substantially reduce the above mentioned difficulties and problems. In accordance with the present invention there is provided a weldable pipe fitting including a sleeve member of thermoplastic material, the sleeve member having a stress resistant portion of greater wall thickness, a terminal portion of greater wall thickness, and a fusion accommodating portion of lesser wall thickness between the stress resistant portion and the terminal portion, wherein the stress resistant portion inclines to the fusion accommodating portion by means of a tapered portion and wherein the stress resistant portion is arranged in use to overlie an end of a pipe to be connected to the pipe fitting; the stress resistant portion of the sleeve has a standard dimensional ratio of outside diameter to wall thickness within the range of no less than 8:1 and no more than 12:1, the terminal portion of the sleeve has a standard dimensional ratio within the range of of no less than 8:1 and no more than 12:1, and the fusion accommodating portion has a standard dimensional ratio within the range of no less than 14:1 and no more than 19:1; and the internal wall of the sleeve is provided with an electric heating wire or wires along and within the wall and disposed circumferentially thereof adjacent the radially inner surface of the wall, said heating wire or wires being disposed over a length of the sleeve at least the majority of which is within the fusion accommodating portion thereof. The standard dimensional ratio is the ratio of the outside diameter to the wall thickness of the fitting sleeve member. Preferably, the fusion accommodating portion of the sleeve has a standard dimensional ratio in the range of no less than 15:1 and no more than 17:1. Again, the terminal portion of the sleeve preferably has standard dimensional ratio in the range of no less than 9:1 and no more than 11:1. Yet again, preferably, the stress resistant portion of the fitting preferably has a standard dimensional ratio in the range of no legs than 9:1 and no more than 11:1. In one embodiment of the invention the heating wire or wires extend axially of the sleeve such that a minority thereof lie within the stress resistant portion of the sleeve, and this portion in its axial dimension may incorporate up to 40% of the heating wire or wires. In another embodiment of the invention, the fitting sleeve is double-ended, and is, therefore, provided with two terminal portions two fusion accommodating portions, and two sets of electric heating wire or wires associated therewith, a single generally axially central stress resistant portion arranged in use to overlie two pipes to be connected to the fitting. The sets of heating wire or wires may be seperate from each other or may be connected in series, for example by helical wire continuations running, at wide pitch, around the inner perifery of the sleeve across the stress resistant portion. The stress resistant portion of the sleeve may reduce by a conical incline to the or each fusion accommodating portion so that the change in stress performance axially along the fitting is not abruptly changed. The invention includes within its scope a pipe joint incorporating a fitting as hereinabove defined. In order that the invention may be more readily understood, one embodiment thereof will now be described by way example with reference to the accompanying drawings in which:- Figure 1 illustrates a fitting in accordance with the invention loosely placed about two pipes to be connected thereby, prior to electrofusion welding; and Figure 2 shows the pipes and fitting of Figure 1 during welding. As can be seen in Figure 1, the pipe coupling 1 comprises a sleeve member 2 having helically wound heating elements or wires 3,4 at each end embedded just below the internal surface 5 of the sleeve, the helical heating elements being set in from the axial end walls 6,7 of the sleeves and ending, axially inwardly of the sleeves well short of the centre of the sleeve 2. The sleeve is provided around its outer periphery with two substantial fusion accommodating portions 8,9 of reduced diameter radially overlying the majority of the heating elements 3,4 at each end of the sleeve 2, and thereby defining a central thicker stress resistant portion 10 overlying the ends of the two pipes 17,18 to be joined and two thicker terminal portions 11,12. The standard dimensional ratio of the stress resistant portion 10 is typically 10:1, that of the fusion accommodating portions 8,9 is typically 18:1, and that of the terminal portions 11,12 is 11:1. As can be seen from Figure 2, during the passage of electric current through the heating elements 3,4 at each end of the sleeve during the welding cycle for welding the pipes 17,18 to the sleeve 2, the heating and consequent pressure adjacent the heating elements 3,4 causes the fusion accommodating portions 8,9 (and the pipes 17,18 thereadjacent) to bulge radially outwardly thereby reducing excessive pressure along the line of weld between the pipes and the sleeve. Additionally, this bulging expansion reacts in a lever-like manner on terminal portions 11,12, causing them to diverge at the radially outer extremities 13,14 of the sleeve end walls 6,7 from each other so that their radially inner edges 15,16 move inwardly reducing the clearance between the pipe and the radially inner extremities of the sleeve, thus reducing risk of exudation of melt therefrom. During this time the central stress resistant portion 10 of the fitting extending over the ends of the two pipe: being connected is not significantly affected by the heating and bulging of the fusion accommodating portions 8,9 on either side. After completion of the welding process, and upon cooling of the now welded pipes and sleeve, the bulged fusion accommodating portions 8,9 contract once more, causing the terminal portions 11,12 of the sleeve 2 to regain their original alignment. It has been found that by means of the arrangement illustrated, satisfactory welds can be produced in which, because excessive melt pressure is limited by the radial movement of the fitting sleeve, axial movement of molten polymer and exudation thereof is severely limited reducing to a very large extent the risk of unsatisfactory welding. The invention as illustrated thereby provides a significantly improved electrofusion welding fitting for large diameter pipes. In a typical fitting of the kind illustrated in Figures 1 and 2 for use on 250mm outside diameter pipe, the sleeve is formed of medium density polyethylene and has an overall axial length of 240mm of which 25mm comprises at each end the terminal portions of the fitting, 84mm comprising the central stress resistant portion, tapering at each side at an angle of 30° for approximately 20mm to reduced diameter fusion accommodating portions approximately 32mm long. The central stress resistant portion and the tapered sides thereof extend over some 40% of the heating elements at each end, the remainder of which are disposed beneath the fusion accommodating portions. A taper may additionally be provided between the fusion accommodating portions and the terminal portions of the fitting. The wall thickness of the fitting at the terminal portions and at the central stress resistant portion is 30mm and at the fusion accommodating portion approximately 17mm.

Claims: A weldable pipe fitting (1) including a sleeve member (2) of thermoplastic material, the sleeve member (2) having a stress resistant portion (10) of greater wall thickness, a terminal portion (11, 12) of greater wall thickness, and a fusion accommodating portion (8, 9) of lesser wall thickness between the stress resistant portion (10) and the terminal portion, wherein the stress resistant portion (10) inclines to the fusion accommodating portion(8, 9) by means of a tapered portion, and wherein the stress resistant portion (10) is arranged in use to overlie an end of a pipe (17, 18) to be connected to the pipe fitting (1); the stress resistant portion (10) of the sleeve (2) has a standard dimensional ratio of outside diameter to wall thickness within the range of no less than 8:1 and no more than 12:1, the terminal portion (11, 12) of the sleeve (2) has a standard dimensional ratio within the range of no less than 8:1 and no more than 12:1, and the fusion accommodating portion (8, 9) has a standard dimensional ratio within the range of no less than 14:1 and no more than 19:1; and the internal wall of the sleeve (2) is provided with an electric heating wire or wires (3, 4) along and within the wall and disposed circumferentially thereof adjacent the radially inner surface (5) of the wall, said heating wire or wires (3, 4) being disposed over a length of the sleeve (2) at least the majority of which is within the fusion accommodating portion (8, 9) thereof. A weldable pipe fitting (1) as claimed in Claim 1 characterised in that the fusion accommodating portion (8, 9) of the sleeve (2) has a standard dimensional ratio in the range of no less than 15:1 and no more than 17:1. A weldable pipe fitting (1) as claimed in Claim 1 or 2 characterised in that the terminal portion (11, 12) of the sleeve (1) has a standard dimensional ratio in the range of no less than 9:1 and no more than 11:1. A weldable pipe fitting (1) as claimed in any one of Claim 1 - 3 characterised in that the stress resistant portion (10) of the sleeve (2) has a standard dimensional ratio in the range of no less than 9:1 and no more than 11:1. A weldable pipe fitting (1) as claimed in any one of the preceding claims characterised in that the heating wire or wires (3, 4) extend axially of the sleeve (2) such that a minority thereof lie within the stress resistant portion (10) of the sleeve (2). A weldable pipe fitting (1) as claimed in Claim 5 characterised in that said minority of the heating wire or wires (3, 4) within the stress resistant portion (10) of the sleeve (2) incorporates up to 40% of the heating wire or wires (3, 4). A weldable pipe fitting (1) as claimed in any one of the preceding claims characterised in that the stress resistant portion (10) of the sleeve reduces by a conical incline to the fusion accommodating portion (8, 9). A weldable pipe fitting (1) as claimed in any one of the preceding claims characterised by being double ended and provided with two terminal portions (11, 12), two fusion accommodating portions (8, 9) and two sets of electric heating wire or wires (3, 4) associated therewith, and a single generally axially central stress resistant portion (10) arranged in use to override two pipes (17, 18) to be connected to the fitting. A weldable pipe fitting (1) as claimed in Claim 8 characterised in that the sets of heating wire or wires (3, 4) are connected in series by means of helical wire continuations running, wide pitch, around the inner periphery of the sleeve (2) across the stress resistant portion (10). A weldable pipe fitting (1) as claimed in any one of the preceding claims characterised in that the tapered portion is at an angle of 30° to the axis of the fitting. A pipe joint characterised by incorporating a weldable pipe fitting (1) as claimed in any one of the preceding claims.

Inventors: BEECHEY ROBERT EDWARD, WALL DAVID JOHN, BEECHEY, ROBERT EDWARD, WALL, DAVID JOHN
Applicants: RUTLAND PLASTICS, VICTAULIC PLC, RUTLAND PLASTICS LIMITED
Application year: 1992
Main classification: F16L47
IPC codes: F16L47, B29C65, B29L31