Pipes made of plastics material are used extensively in industry and in construction. Many applications now use pipes made from plastics materials such as polyethylene and polypropylene rather than metal. One such application is the pipe work used in petroleum installations such as garage forecourts.
In the design of petroleum forecourt installations for example, it is regarded as increasingly important to contain and detect any leaks of petrol or diesel fuel from subterranean pipes which connect one or more storage tanks to dispensing pumps in the installation. To that end, many current designs of forecourt installation utilise secondary containment. This involves containing each supply or primary pipeline in a respective secondary containment pipeline which is optionally sealed at its ends to the fuel supply pipelines. The secondary containment pipeline prevents leaks from the fuel supply pipeline from being discharged into the environment, and also can convey leaked petrol to a remote-sensing device.
Whilst every effort is made to avoid having joints in an underground supply pipeline, other than inside manhole chambers, these joints sometimes cannot be avoided. Such joints are conventionally made using special fittings and the connections are made using a chemical-based jointing compound or by electrofusion welding. The latter technique is preferred in many applications.
In conventional single containment plastic piping systems, successive lengths of plastic pipe are joined end to end using so-called electrofusion couplings, sockets or welding muffs, which typically comprise short plastic sleeves providing sockets at either end having internal diameters of a size to receive the ends of the respective pipes as a close fit and incorporating electrical resistance heating windings. Thus two adjoining pipe lengths can be connected end to end by inserting the adjoining pipe ends into such an electrofusion coupling from opposite ends thereafter passing electric current through the heating windings in order to fuse the internal surfaces of the electrofusion coupling and the adjacent external surfaces of the inserted pipe ends, thereby welding the pipe ends to the electrofusion coupling to form a fluid tight joint.
In the context of this invention the term welding socket will be used to encompass any coupling which can be used to join two or more pieces of pipe end to end using the process described above.
It follows that these electrofusion-welding sockets must have at least two terminals to enable them to be connected to an electrical supply or welding unit as it is termed.
The most common solution is to use terminal pins which protrude proud of the socket body and usually at right angles to the longitudinal axis of the socket and to the pipework which is being welded. These pins are typically small metal pin connectors and need protection during welding for safety reasons. They are therefore usually formed within a shroud moulded from the same material as the welding socket. All in all, the protrusion caused by these terminals is substantial since the terminal pins project outwardly from the welding socket by a distance greater than the thickness of the socket wall. Typically the terminals project outward from the surface of the socket body by an amount equivalent to 2.5 to 4 times the thickness of the socket wall.
This causes a number of problems. Firstly, it prevents a secondary pipe from passing smoothly over the welding socket region of a primary pipe. The secondary pipe tends to snag or foul on the protruding terminals. Secondly, the secondary pipe has to be formed with a larger diameter than would otherwise be necessary. This results in a larger than ideal interstitial space between the primary and secondary pipes. It also increases the cost of the installation because the larger the diameter of the secondary pipe, the more plastics material there is in it and the more it costs.
Matters are further complicated if a joint is being formed in the latest composite primary/secondary pipe. In this type of pipe, an example of which is described in GB9824955.6 and PCT/GB98/03422 (PetroTechnik Ltd), the primary and secondary pipe are formed as one with a relatively thin insulating layer in-between. Conventional welding sockets cannot be used at all to weld this type of pipe because there is insufficient space between primary and secondary pipes to accommodate the inevitable terminal pins on the welding socket used on the primary pipe.
Accordingly, it is an object of the present invention to overcome or at least mitigate one or more of the problems outlined above.