This invention relates to pipes for transporting fluids, and also to pressure vessels and other fluid-containing enclosures, all of which are referred to herein for convenience as"pipes".

In the field of hydrocarbons and petrochemicals, it is common to use steel pipes which are lined with a plastics material to prevent chemical degradation of the pipe by the product. The lining may for example be of fluoropolymer.

Such linings give excellent protection in normal use. However, problems can arise when the product within the pipe is, or contains, a gas. The gas permeates slowly through the lining and a significant gas pressure can build up at the lining/steel interface. In normal use this causes no harm, but if the pipe is suddenly depressurised the gas pressure at the interface can cause bursting and splitting of the lining.

One object of the present invention is to provide a pipe construction in which the foregoing problem is obviated or mitigated.

Accordingly, the present invention provides a pipe or pressure vessel comprising a metal pressure containment having a plastic liner bonded within it; in which a porous substrate is interposed between the pressure containment and the plastic liner, and one or more bleed holed are provided through the pressure containment to vent gas from the porous substrate to the environment.

From another aspect, the invention provides a method of making a pipe or pressure vessel comprising: providing a metal pressure containment, providing a porous substrate on an internal face of the pressure containment, arranging a layer of a thermoplastic material on said porous substrate, and applying heat and pressure to the interior of the pipe or pressure vessel thereby causing the thermoplastic layer to partially extrude into the adjacent part of the porous substrate to form a plastic liner bonded to the porous substrate.

Preferred features and advantages of the present invention will be apparent from the following description and claims.

Embodiments of the invention will now be described, by way of example only, with reference to the drawings, in which: Fig. 1 is a schematic cross-sectional perspective view of one form of pipe embodying the invention; Fig. 2 is a similar view showing one form of end portion of the pipe; Fig. 3 is a cross-sectional side view illustrating a method of making a modified form of pipe; and Figs. 4 and 5 are detail views, to an enlarged scale, of parts of the pipe of Fig. 3.

Referring to Fig. 1, a pipe comprises a metal tube 10 which will typically be of steel of a grade suited to the application. The tube 10 is lined with a plastic liner 12 which is secured in place by a bonding layer 14. The liner 12 will typically be of fuoropolymer, but may be of polyethylene or other plastic material according to the fluid to be transported or contained.

A porous substrate 16 is interposed between the bonding layer 14 and the tube 10. The porous substrate 16 may be of sintered metal or ceramic and may be formed for example by applying heat and pressure to metal or ceramic particles in situ. The particles may be in the form of powder; alternatively metal balls may be used, such as steel

balls 0.5 mm in diameter. A further alternative is the use of metal mesh or metal wire.

Bleed holes 18 are provided through the tube 10 at intervals.

The exterior of the tube 10 may be bare, except for conventional protective finishes, or may be protected by a porous cover 20, suitably of porous rubber.

In use, when gas is present under pressure within the pipe a proportion of the gas permeates slowly through the plastic liner 12 and bonding layer 14.

This gas can move freely within the porous substrate 16, from which it can pass through the bleed holes 18 into the environment. Where a porous cover 20 is used, the gas permeates through this quickly relative to the rate of permeation through the plastic liner 12.

Fig. 2 shows a portion of pipe where the plastic liner 12 terminates, for example to provide an unlined portion suitable for welding. The porous substrate 16 is terminated short of the pipe end 22.

The liner 12 and bonding layer extend to a position intermediate the porous substrate 16 and the pipe end 22, and in this region the liner 12 is formed with an angled face 24. The angled face 24 is received in a corresponding annular recess in the tube 10, and pressure activation holes 26 are provided to communicate internal pipe pressure to

the angled faces 24, whereby the ends of the liner 12 effectively form active seals preventing internal pipe pressure from reaching the porous substrate 16.

Figs. 3 to 5 illustrate one method of producing a pipe in accordance with the invention, and also illustrates certain modifications to the foregoing embodiment.

In the arrangement of Figs. 3 to 5 there is no separate bonding layer. Instead, the liner 12 is bonded to the porous substrate 14 by the liner 12 being extruded into the porous substrate 14. To assist in this, the porous substrate 14 in this example comprises a first layer 14a 1 mm thick and formed of very porous sintered metal to allow the liner material to fully penetrate, a second layer 14b 1 mm thick of medium porosity metal to allow the liner material to partially penetrate, and a third layer 14 c, also 1 mm thick, into which the liner material does not significantly penetrate, to allow passage of gas. This is shown in more detail in Fig. 4.

As seen in Fig. 3, this construction may be achieved by inserting a sleeve of the liner material and then positioning a mandrel 30 having end seals 32 engaging the pipe and a gas inlet 33 and outlet 34.

Hot gas is supplied to the mandrel 30 at a temperature and pressure sufficient to extrude the liner material into the porous substrate in the manner described above. Typically, the liner material will be a fluoropolymer, and the applied

temperature and pressure will be about 300°C and 50 bar.

The right hand end of the pipe as seen in Fig. 3 has a solid end portion 36 to provide a seat for the end seal 32. After the pipe is manufactured, the end portion may be cut off by machining at the line 38.

The left hand end as seen in Fig. 3 is shown in more detail in Fig. 5. The end of the liner 14 is machined to form a chamfer 40. A metal sleeve 42 is engaged with the end of the tube 10 by screw threading. The sleeve 42 has a tapered portion 44 forming a matching chamfer, and provided with pressure activation holes one of which is seen at 46. Thus, a self activating seal similar to that described above is formed. A gas-tight seal is formed between the tube 10 and the sleeve 42 is ensured by welding the end of the thread at 48.