The traditional materials used for pipework systems for conveying fuels, water, hazardous gases and fluids, operating at high pressures has been metals. This is primarily due to an immunity to permeation through the pipe wall, mechanical strength of metals, and welding jointing techniques. Metal pipework systems also offer the advantage of being electrically conductive throughout. Electrical continuity is becoming increasingly important and is specified in many cases because a static charge can build up due to friction generated between the pipe and the medium being transported. If this charge cannot be dissipated then a situation can result where a spark can occur, this could potentially ignite the medium being transported. The main disadvantage with metal pipework systems is susceptibility to corrosion and the systems, therefore, require some form of protection.

Polymer pipework systems offer the advantage of excellent resistance to corrosion.

When the polymer is thermoplastic, they offer the facility to be fully welded. Polyethylene, for example, is widely used for transporting water and gas and welding techniques of electrofusion and butt fusion are firmly established. However, fuels can permeate through the polymers and into the surrounding area. Also, polymer systems cannot offer the same levels of internal pressure resistance as metal pipework systems due to the mechanical characteristics of polymers. Conductivity throughout the polymer pipework system is difficult to achieve due to the electrical insulating properties of polymers.

There are also applications where permeation can occur from the outside of the pipe through to the inside and can contaminate the medium being transported. An example being water supplies piped through contaminated land where leachates are present.

There are, at present, systems manufactured that utilise combinations of metal and plastic layers to gain the advantages of each material ie. polymer corrosion protection, metallic permeation resistance, metallic resistance to high hoop stresses imposed by high operating pressures, fully welded. None of the above mentioned systems provide jointing

techniques that can offer the same advantages/properties as the pipe. Many of the joints rely solely on the thermoplastic electrofusion fitting. Therefore, the thermoplastic electrofusion fitting is subjected to the medium being transported and can potentially result in permeation through the wall of the fitting. Also, the fitting is exposed to the internal pressure of the medium being transported and, therefore, places limitations on maximum operating pressures. Metal continuity throughout the pipework system is not achieved therefore there is the possibility of static charge build up.

British Patent Publication 2,324,132 provides a considerable improvement in relation to the connection of pipes comprising an outer host and an inner guest pipe, it advocating the employment of an outer host pipe, an inner guest pipe, each of a material to provide a particular required attribute, and a pipe insert of a material weld-compatible with the material of the outer host pipe, the pipe insert being inserted in the end of the inner guest pipe and having an outermost flange to overlie the outer edge of the inner guest pipe. Whilst generating a welded joint that is leak free, by way of relatively conventional fusion welding techniques, it does not attend the problem of electrical continuity.

The object of the present invention is to improve still further the connection between composite pipes.

According to the present invention, a pipe joint for pipes formed by an outer host plastics pipe and an inner guest metallic pipe, comprises a hollow metal spigot connector in sealing engagement with the inner wall of the ends of abutting guest metallic pipes and an electrofusion sleeve surrounding the ends of adjacent outer plastics host pipes. Preferably, the spigot has a generally centrally disposed outer flange to lie between and be abutted by the ends of the inner metallic guest pipes, and the electrofusion sleeve has an inner generally centrally disposed flange or central stops to lie between and be abutted by the ends of the outer host plastics pipes.

Further preferably, the outer surface of the spigot connector can utilise annular projections to each side of its centre and/or metallic attachments such as circlips, spring

washers, springs, to bear against the inner wall or edge of the guest metallic pipe to ensure electrical continuity across the joint. Still further preferable, each end of the spigot has a means of sealing against the inner metallic guest pipes, for example, by means of o-rings, lip-seals, quad-seals, interference fit.

By overlaying the ends of the outer plastics host pipes, the electrofusion sleeve allows the creation of an effective seal between the host pipe ends. The weld between the sleeve and outer host plastic pipe also enables the pipework system to resist all loads placed on it due to, for example, expansion, contraction, end loads, ground movement.

By providing a metal spigot bridging the ends of the guest pipes of metal, electrical continuity is provided at the joint between two pipe lengths, and the sealing engagement of the ends of the spigots in the ends of the metal guest pipes ensures that fluid flowing through the guest pipes cannot reach and hence penetrate through the plastic material of the joint. In addition to this, the metal spigot bridging the guest pipes eliminates any weakness at the joint with respect to resisting the hoop stresses imposed on the joint by the fluid/gases being transported under high pressures.

Depending on a host of variables (examples of which could include o-ring compression, o-ring cross section tolerances, spigot connector machining tolerances, inner guest metallic pipe dimensions), the inner bore of the guest pipe may need re-rounding or expanding to a fixed diameter for a specific length to accept the spigot connector.

The invention therefore allows the provision of an outer plastics host pipe and inner metallic guest pipe system where there is a considerable security against leakage at all connections of pipe to pipe and to bends, tees and elbows and indeed at all polymer/metal transition points. The provision of the metallic spigot connector ensures the sealing together of metallic guest pipes or inner metallic elements of bends, tees and elbows, with the surety of electrical continuity across a joint and the provision of a considerable ability to withstand high hoop stresses imposed on the pipe system by the pressure in the medium being transported.

Two embodiments of the invention will now be described with reference to the accompanying drawings in which:- Figure 1 is a schematic cross-sectional view of one embodiment of pipe joint in accordance with the invention; Figure 2 is a side elevation of an alternative spigot to that of Figure 1; and Figure 3 generally corresponds to Figure 1, but shows a second embodiment of pipe joint.

In Figure 1 is illustrated a joint between adjacent ends of plain pipes 1, each of which are composed of an outer host plastics pipe 2 and an inner guest metallic pipe 3. A hollow metal spigot connector 4 is inserted into the adjacent pipe ends, the spigot having a centrally located external flange 5 against which the pipe ends abut and tapered ends 6 to assist in the insertion of the spigot into the pipe ends. To provide for sealing engagement of the spigot with the pipes, sealing rings 7 are provided, and the spigot has circumferential ribs 8, to engage with the inner surface of the guest pipe 3.

Surrounding the abutting ends of the pipes 1 is an electrofusion coupling member 9 of a compatible plastics material, having an internal centrally located flange 10 against which the pipe ends abut. On the inner surface of the coupling member are two wire windings 11 to each side of the centrally located flange 10, the inner ends of the windings being connected together, and the respective outer ends each connected to a terminal 12 in a socket 13 formed on the coupling member.

With the joint assembled as illustrated, and electrical power applied to the winding, the interface as between the plastics coupling member and the plastics host pipe to cause the melting of the interface that on cooling generates a substantially leak free joint between the pipes 1.

Figure 2 illustrates an alternate form of spigot, where in place of circumferential ribs, a circular spring 14 is provided to each side of the central flange 5, to provide a mechanical

contact as between the spigot and the inner guest pipe, to supplement the sealing action of the sealing rings 7.

In Figure 3 is illustrated the different circumstance of pipes 1 composed of host pipes 2 and guest pipes 3 having a prepared length 15 to receive the spigot 4. Although not shown, abutting sockets will be inserted into an electrofusion coupling member as is illustrated in Figure 1. Depending on a host of variables (examples of which could include o-ring compression, o-ring cross section tolerances, spigot connector machining tolerances, inner guest metallic pipe dimensions), the inner bore of the guest pipe may need re- rounding or expanding to a fixed diameter for a specific length to accept the spigot connector.

An example of this is detailed in Figure 3 where the pipe bore is increased to provide a prepared diameter X for a length Lp where Lp is greater than the spigot connector insertion length Lsc.