Tubular sections are often rigidly joined together end to end by welding or otherwise bonding together the adjacent ends of the sections. This provides a strong, fluid tight joint between the sections, but, particularly in the case of full penetration welding, which is necessary for some applications, a weld bead is formed on the inside of the joint, and thus the inner wall surface is left with discontinuities at the joints. There are also problems in accurately aligning the internal passages of adjacent sections. These considerations are important in circumstances where a continuous, smooth inner wall surface is essential, such as in the case of vortex separator frustocones. These are typically a metre long a_nd have a finely defined tapered or curved inside wall surface and a wall thickness in excess of 3mm. It is not economically viable to machine such a long section, particularly where materials such as stainless steels, plastics, composite materials, titanium or stellite are utilised. It has been found that investment casting can be used to produce shorter sections, but due to the casting limits on the length, the shorter sections need to be permanently joined.

In accordance with a primary aspect of the invention, a method of fixing together end to end two tubular sections so that their internal passages are coaxial and lead smoothly one into the other, comprises providing annular recesses in the adjacent end faces of the sections to divide the ends of the sections into coaxial inner and outer annular portions, at least the inner annular portions of the two sections having the same accurately dimensioned internal and external diameters, juxtaposing the ends of the two sections with a tubular connector in the cavity provided by the annular recesses, the connector closely surrounding the two inner annular portions and thus maintaining the internal passages in alignment, and bonding

together the adjacent ends of the two outer annular portions.

Depending on the material from which the sections are made, the bonding may involve placement of weld material, electron beam or other high energy welding or the use of a solvent or adhesive.

When the sections are, for example, metal castings, or carbon or glass fibre reinforced plastics mouldings, it will usually be necessary to machine the end faces of the inner and outer annular portions; and to machine the radially outer surfaces of the inner annular portions, coaxially with the passages, which terminate at the respective ends of the sections, to receive the tubular connector. However such machining may not be necessary if the sections are, e.g. accurately injection moulded from an engineering plastics material. The connector then ensures smooth passage transition from section to section and protects the passage from weld spatter or other excess bond waste. When welding together certain materials, a radial bleed hole may be incorporated through the outer annular portion of at least one of the sections, in order to evacuate any gases present during the weld procedure. This bleed hole may then be plugged after the welding of the joint has been completed.

A tubular section having at least one end with inner and outer coaxial annular portions separated by an annular recess, for being bonded to a similar end of another section by a method according to the primary aspect of the invention, forms a secondary aspect of the invention.

The invention will now be described by way of example with reference to the accompanying diagrammatic drawings, in which:-

Fig. 1 is a perspective view showing the ends of two sections to be joined together;

Fig. 2 is an axial section through the two sections and a tubular connector;

Fig. 3 is a section taken through the ends of the sections after they have been welded together; and.

Fig. 4 is a section similar to Figure 3 but showing a modification. The illustrated joint is between two sections 5, 6 of a hydrocyclone separator. Typically there will be three or four sections to be connected end to end and each section may have a length of the order of 350-400 mm and a tapering internal passage with a diameter in the range 15-40 mm. Each cast tubular steel section 5, 6 has an enlarged end where it is to be connected to the other. Each enlarged end comprises an inner annular portion 7 which surrounds the essential internal passage 8 and an outer annular portion 9, which is spaced from the inner portion 7 by an annular recess 10, opening into the end face of the section. As shown in Figure 2, at least the radially outer surface of the inner portion 7 is machined, to provide a cylindrical surface 11 which is accurately coaxial with the passage 8 and the external diameter of which is substantially the same as the internal diameter of a cylindrical tubular connector 12. ^* The end faces of the portions 7 and 9 are also machined flat, sufficient material being removed such that the diameter of the end of the passage 8 is increased (at the convergent end of the taper) or decreased (at the divergent end of the taper) to the required diameter which will exactly match that of the adjacent end of the adjacent section to which it is to be juxtaposed.

Upon initial assembly, the two sections are pushed together so that the connector 12 slides onto the two surfaces 11, with the adjacent ends of the inner portions 7 of the two sections abutting one another. Welding is then applied to the opposed faces of the outer portions 9 to complete a full penetration weld 13 as shown in Figure 3. The connector avoids any possibility of weld spatter from passing between the ends of the portions 7 and into

the passage 8 and is of course permanently sacrificed within the cavity 14 provided by the two recesses 10.

If it is necessary to vent any gases produced during the welding from the cavity 14, a bleed hole 15 may be formed through one of the outer portions 9, as shown in Figure 4, and possibly subsequently closed by a plug 16.