British Gas Plc

Trevor G Stafford

Alan J Dickinson

Leslie Maine

Andrew Smart

Lindsay Ewing

A method of lining a pipeline 24 intended to carry gas, oil, water, sewage or chemicals in which a cross-linked polymeric liner pipe 10, which has been reduced from its original diameter to a lesser diameter or to a shape of reduced maximum dimension, is pulled through the pipeline 24. The lesser diameter is less than the least inner diameter of the pipeline 24. After the liner pipe 10 has been fully pulled through, the pipe liner 10 is heated by means of a source 60 of visible or infrared radiation forming part of a pig assembly 50 to cause it to revert to the pipeline. The pig assembly or assemblies are stationary at zones 70 where service connections are to be made. Subsequently, the remaining liner pipe 10 is reverted by pulling the source 60 through the liner pipe 10.

Method of lining a pipeline

A coupler 10 consists of a length of cross-linked polyethylene pipe having a spiral groove 16 cut in its outer surface and another spiral groove cut in its inner surface. A heating element 18 is wound in the groove. The coupler is expanded mechanically so as to have an easy fit over the ends of polyethylene pipes 12, 14. The heater element 18 is energized and the coupler shrinks and forms fusion welded joints between itself and the pipes. No preparation of the pipe ends is necessary because the serrated inner surface cuts through the oxidized skin and bonds to material beneath the skin. The coupler is especially advantageous in large sizes of 500 mm or above. Instead of an electric heater, radiant energy, e.g. infra red or light is used to heat the coupler. For example, an array of reflector lamps can be used circumferentially located around the coupler.

Polyolefin coupler for joining polyolefin pipes

A polyethylene pipe 10 is joined by a welded joint to a cross-linked polyethylene coupler 14 which has been expanded to make the coupler heat recoverable. Radiant energy (eg light in the visible spectrum or infra-red light) illuminates the outer surface of the coupler 14 supplied by an array of 15OW reflector lamps 20. The coupler 14 shrinks and is forced into engagement with the pipe 10 and the pipe 12 to join the pipes. A fusion welded joint is formed between the coupler 14 and each pipe, 10, 12. In an alternative a pipe is drawn down and inserted in an aperture through the wall of another pipe. A lamp is inserted in the drawn down pipe and the radiant energy causes the pipe to expand into engagement with the edge of the aperture and to be joined to it by a welded joint (FIG. 2).

Method of forming a welded joint between polyolefinic members

A liner or other elongated duct is made by joining lengths of thin-walled polyethylene fusion. Typical thickness 2-4 mm, diameter 200 mm. Pipe ends are clamped in clamps which restore circularity. The ends are trimmed leaving aligned ends normal to pipe length. The clamps force the ends against a heated body, which is retracted and the ends are then forced together. The total heating period, say 15 seconds, and the periord between separation from the body and engagement of the ends, say 1 second, are very short. Good joint strengths and small beads result. The liner can be pulled into burst cast-iron gas main before a new main is passed through the liner; or the liner is a seal in a pipe. The method is applicable to other polyolefins e.g. polyropylene.

Method for joining polyolefin pipes by fusion

Members especially hollow members 10, 12 such as pipes and pipe fittings one member 12 of cross-linked polyethylene say and the other member 10 of medium density polyethylene say are joined by fusion using a butt fusion machine. End surfaces of the members are pressed at higher pressure against a hot plate 18 to upset only the medium density material to form beads 20, 22. No cross-linked material is upset to form beads. The pressure is reduced during a soak stage then the plate is removed and the surfaces are mutually engaged at the higher pressure. The beads formed are adjacent the interface between the members and both beads are offset towards the medium density material, member 10. Careful alignment will avoid faults caused when the beads do not cover exposed parts of end surfaces. Other materials include polypropylene, copolymer or monopolymer types. Natural gas distribution systems can be fabricated using the invention.

Joining polyolefinic members by fusion

Pipes, typically of medium density polyethylene for use in underground distribution systems for natural gas are joined by fusion using a very short period (25 seconds) for heating the pipe ends and a very short period (3 seconds) between separation of ends from the heater body and engagement of the ends. Those periods are such that the beads formed by upset of the softened end material of the pipes are relatively small, e.g. beads have a cross-sectional area of 10mm.sup.2 for pipe for 250 mm outside diameter and wall thickness 15 mm. Typically such a bead extends only some 2.5 mm away from the adjacent pipe wall surface. Tensile, bending and impact tests show joint strength equivalent to the parent wall strength. In accelerated lifetime tests at high pressure and temperature the joints are at least equal to conventionally made joints.

First and second members (e.g. pipes, fittings) of cross-linked polyolefinic material such as polyethylene having a melt flow index (MFI) of zero are joined using a third member of polyolefinic material such as polyethylene having a higher MFI e.g. 0.3. The first member 10 and the third member gripped by clamps 18, 20 of a butt fusion machine are pressed against opposite faces of a heater plate which is then withdrawn and the members are pressed together by the clamps. Only the third member is upset to form beads 24, 26. Part of the third member is removed leaving a stub part 16. The second member is joined to the stub's face 36 using the heater plate. The part 16 is upset leaving a thin wafer 48. The two interfaces 30, 50 limit the growth of cracks in the wafer to prevent their propagation through the pipe thickness under hoop stress. The beads are preferably removed at each stage.

The apparatus increases the hydraulic pressure in rams 16 until one pipe moves towards a heater plate 14. The movement is detected by a limit switch 26 and a microprocessor-based control system 28 adds a pre-programmed value of pressure to that pressure which caused the movement (drag pressure). The total pressure is applied to force the pipe ends against a heater plate 14 to form end beads (bead-up). After heating for a pre-programmed period the control system automatically initiates plate retraction and forces the pipe ends together (fusion) for another programmed period. The applied pressure is the sum of the recorded drag pressure and a pre-programmed added pressure. Operator judgement of bead size, pressures and time periods is eliminated. Pressurised fluid is supplied to the rams 16 from a pump and accumulator combination. The sense of ram motion is determined by changeover valves controlled by the control system 28.

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Whitley Bay, United Kingdom

Trevor G Stafford