The present invention relates to a method of leading static electricity away from spliced or mutually joined plastic pipes of the kind intended for the transportation of gasoline or other hydrocarbons and including an inner barrier layer on their inner wall surfaces. The invention also relates to an arrangement for use in carrying out the method.

Pipes of this kind may suitably be comprised of polyethylene or polypropylene, e.g. MD-polyethylene, and the barrier layer may also include polyethylene or polypropylene, such as HD- polyethylene, in combination with butyl rubber, polyamide or polyester. Such pipes are described in WO 93/09948.

The purpose of the barrier layer is to prevent or substantially reduce the leakage of hydrocarbons by diffusion through the pipe wall. For cost reasons, it is desired to use the thinnest possible barrier layer. A layer thinner than 0.3 mm is difficult to achieve in practice, and it is preferred that the layer has a thickness of 1-2 mm. A barrier layer whose thickness corresponds to 10-15% of the total material thickness is estimated to reduce leakage to l/lOth of that experienced typically in plastic pipes.

Another problem encountered with insulating plastic pipes resides in the static electricity that can be generated by passage of fluid through the pipe. In this respect, it is known to include an electrically conductive material in the barrier layer material, such as finely-divided carbon powder, carbon black, metal particles, metal fibres or like material therewith making the barrier layer electrically conductive.

Prior publications US-A 5,081,326, DE-A1-1775006, DE-A1-2103394 and DE-A1-2543811 all describe different methods and arrange- ments for conducting static electricity away from plastic

pipes. These solutions either involve work being carried out on the actual pipes and/or causing the outer shape or dimen- sions of the pipes to deviate from standard shapes and dimensions, resulting in problems in respect of pipe connec- tions, pipe couplings and the like.

US-A 5,081,326 describes a pipe which is made of polytetrafluoroethylene and which has crimped thereon an outer plastic pipe that contains carbon powder so as to provide the requisite electrical conductivity. Connection of the pipe re- quires the use of a specially manufactured end-sleeve of complicated design that includes a slotted flange. The non- conductive inner layer is inserted into the slot in the flange, whereas the conductive outer layer is passed over the outer surface of said flange. Connection of the pipe assembly to such a sleeve is a relatively complicated and laborious task. The known sleeve cannot be used to splice or join two pipes end-to- end and affects the outer contours and dimensions of the pipes, therewith preventing its combination with other known splicing or jointing sleeves of standard types.

A particular problem arises when splicing such pipes wherewith it is necessary to establish an electrical connection between the pipes to be spliced. This can require the use of special splicing couplings, which further complicate the work involved with pipe installations and adds further to the cost of such work.

An object of the present invention is to provide a method and an arrangement for conducting away static electricity from spliced plastic pipes of standard types that will have no affect on the outer contours and dimensions of the pipes, and therewith enable the pipes to be spliced or joined together and connected with the aid of mass-produced couplings and jointing pieces of standard designs.

This object is achieved in accordance with the invention by rendering the whole of the barrier layer, or longitudinally extending strings of said layer, electrically conductive such as to enable said barrier layer to be utilized as an electri- cally conductive path along which static electricity will be led away.

In this regard, carbon black or metal particles, for instance, can be admixed with the barrier layer material prior to extruding the barrier layer, or by co-extruding strings of material that includes such additives.

According to the present invention, the conductive layers on respective sides of a splice or join are connected by means of an inner sleeve-like body of electrically conductive material fitted in the pipe. This enables the outer surface of the pipe to remain unaffected and therewith to be spliced and connected with the aid of standard components.

In accordance with the invention, a method of the kind defined in the first paragraph of this document is mainly characterized by mixing an electrically conductive substance in the inner barrier layers of respective pipes, so as to make the whole of these layers or longitudinally extending strings thereof electrically conductive; placing an electrically conductive sleeve-like body in the ends of two pipe parts to be spliced or joined end-to-end, such as to fully bridge the intermediate joint; and generating contact pressure between the sleeve-like body and the inner surfaces of respective barrier layers at the ends of the two pipe parts, such as to provide an internal, electrically conductive connection between the electrically conductive barrier layers of the mutually spliced or joined pipe parts.

This method can be carried out easily and cheaply and does not affect the pipe laying costs to any appreciable extent, since

standard components can be used for splices, joints and connec- tions.

When the plastic pipes are conveniently joined together with the aid of an external welding sleeve that bridges over the joint between the pipes and that is adapted to cause the welding sleeve material to fuse together with the pipe material on respective sides of the joint when the welding sleeve is heated, a contact pressure will be generated between the sleeve-like body fitted in the pipes and the barrier layers of respective pipe as a result of part of the fusion pressure that occurs between the welding sleeve and the tube walls when fusion-welding propagating to the inner surfaces of the pipes and therewith pressing said surfaces against the sleeve-like body.

The sleeve-like body may consist of an essentially rigid sleeve made of electrically conductive metal or plastic.

In order to also generate contact pressure in a direction away from the sleeve and towards the barrier layers, there can also be used a sleeve that has resilient properties.

Other features of the method and features of an arrangement for use when practicing the method will be apparent from the following Claims.

The invention will now be described with reference to exem- plifying embodiments thereof and also with reference to the accompanying drawings.

Fig. 1 is a sectioned view showing the adjacent ends of two pipes that have been mutually joined in accordance with the invention.

Fig. 2 illustrates a jointing sleeve used in the joint shown in Fig. 1.

Fig. 3 is a sectioned view of two pipe ends that have been joined together in accordance with the invention while using a different type of sleeve.

Fig. 4 illustrates the jointing sleeve used in the joint shown in Fig. 3.

Fig. 5 is a sectional view taken through a pipe of a different configuration that can be joined or spliced in accordance with the present invention.

Fig. 1 shows respective ends 1 and 2 of two pipes that are joined end-to-end. The pipes may be intended for the transpor- tation of gasoline in a gasoline station or service station and are therewith suitably made of polypropylene or polyethylene, preferably MD-polyethylene. In order to prevent or substan- tially reduce leakage through the pipe walls, the inner surfaces of the pipes are provided with a respective barrier layer 3, which may also consist of polypropylene or polyethylene, preferably HD-polyethylene, in combination with butyl rubber, polyamide or polyester, for instance.

With the intention of enabling any static electricity that may be generated as the liquid flows through the pipes to be led away, without needing to modify the outer shape or dimensions of the pipes, which would make the use of standard components for joining and connecting the pipes impossible, a fine carbon powder, carbon black, metal particles or some other electrical- ly conductive material has been mixed with the carrier layer 3, such as to make the carrier layer conductive to the extent required. This layer will therewith fulfil two functions at one and the same time. The cost of imparting to the pipe electri- cally conductive properties in this way is very low, since no additional extrusion operation or any other modification or alteration of the pipe is required.

Alternatively, the pipe may have the cross-sectional form illustrated in Fig. 5. In this embodiment, longitudinally extending strings 5 of electrically conductive material are co- extruded, as an alternative to making the whole of the barrier layer 3 conductive. The material present in the strings 5 may be identical to the material in the remainder of the barrier layer 3, with the addition of conductive substances, such as carbon black or metal particles. These strings will therewith be embedded in the inner wall of the finished pipe.

When joining pipes of the kind according to Fig. 1 or Fig. 5, it must be ensured that a conductive path is also established- between the pipes to be joined. There is used to this end a sleeve 4 (see also Fig. 2) which is made of a conductive material, suitably a conductive plastic material. Alternative- ly, the sleeve may be made of steel, for instance. As will be seen from the Figure, the sleeve 4 includes a central, circumferentially extending flange 6.

Reference numeral 7 designates a conventional welding sleeve with built-in heating coils 8 and 9 made of kanthal wire or the like. Reference numerals 10 and 11 designate connection pins for the heating coils.

The pipe ends are joined together by first fitting a welding sleeve 7 over the end of one pipe. The sleeve 4 is then fitted halfway on one pipe end, and the other pipe-end inserted into the outwardly projecting, remaining part of said sleeve. The flange 6 forms, in this respect, an effective stop that ensures that the sleeve is definitely centred correctly over the join between the pipe-ends.

In order to provide a tight joint between the pipe-ends 1 and 2, the pin connectors 10, 11 on the welding sleeve 7 are connected to a voltage source. The coils 8 and 9 will then heat the material in the welding sleeve 7 and in respective pipe- ends 1 and 2, therewith resulting in local fusion of the

materials on opposite sides of the join. Since the material primarily melts at the positions of the coils 8, 9, the molten material will be enclosed by the cold material that lies adjacent the coils, therewith preventing lateral seepage of the molten material.

When using MD-polyethylene, the temperature is raised typically to about 2000C, resulting in a volumetric expansion of about 20%. This expansion gives rise to an elevated pressure in the molten material on both sides of the joint, due to the fact that said molten material cannot be pressed-out laterally. The pressure of the melt quickly rises to a peak in the order of 6 bar. The melt pressure can be still higher, when using welding sleeves that have a very fine fit on the pipes.

Part of the resultant melt pressure propagates through the pipe walls 1 and 2 to their inner surfaces and produces a correspon- ding pressure against the conductive jointing sleeve 4, which may be essentially rigid. This results in an highly effective contact pressure between the jointing sleeve 4 and the barrier layers 3 at respective pipe-ends 1 and 2.

The sleeve 4 thus provides an effective electrical connection between the conductive barrier layers 3 or the conducting strings 4 in the barrier layers of the mutually joined pipes that enables static electricity generated in the pipes to be effectively led away. The use of the sleeve 4 does not affect the outer dimensions of the pipes and enables the pipes to be mutually joined and connected with the aid of mass-produced couplings of a standard type.

Figs. 3 and 4 illustrate the use of a modified jointing sleeve 12. This sleeve has been given certain spring properties with the aid of axially extending slots 13. Reference numeral 14 designates ring-shaped edges that improve the engagement and contact with the surrounding barrier layer 3 in the pipes 1,

2 when the sleeve 12 is inserted into said pipes, as illu- strated in Fig. 3.

When using such a sleeve, which is suitably made of steel, there is obtained as a result of the engagement of the sleeve with the barrier layer in respective pipe-ends, an effective internal, conductive connection between the barrier layers 3 of the two pipes 1, 2 without requiring an outer compressive force to be applied to the pipe ends.

Although the invention has been described with reference to exemplifying embodiments illustrated in the accompanying draw- ings, it will be understood that several modifications and variations can be made within the scope of the following Claims. For instance, the precise form of the jointing sleeve and the material in said sleeve can be varied as desired.