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A method of covering the inside surface of a steel connection part having an empty inside volume defined by the inside surface, with a liner constituted by a layer of substantially uniform thickness of a thermoplastic material. The connection part including at least two open tubular ends suitable fo

A method of covering the inside surface of a steel connection part having an empty inside volume defined by the inside surface, with a liner constituted by a layer of substantially uniform thickness of a thermoplastic material. The connection part including at least two open tubular ends suitable for being connected respectively to at least two steel pipe elements, each including the same internal liner. A rotary molding method is implemented in which the connection part acts as a mold. A connection part obtained by the rotary lining method is provided, and methods of assembling a connection part, as obtained by rotary lining, with a likewise lined pipe element.

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1. A method of covering the inside surface of a steel connection part having an empty inside volume defined by said inside surface, with a liner constituted by a layer of substantially uniform thickness of a thermoplastic material, said connection part having at least two open tubular ends suitable

1. A method of covering the inside surface of a steel connection part having an empty inside volume defined by said inside surface, with a liner constituted by a layer of substantially uniform thickness of a thermoplastic material, said connection part having at least two open tubular ends suitable for connecting respectively to at least two steel pipe elements having the same internal liners, the method being characterized in that the following steps are performed: a) closing said open ends of said connection part with covers, at least one of said covers presenting a filler orifice with an isolation valve suitable for enabling granules of said thermoplastic material to be introduced into said inside volume, and preferably also a ventilation or purge orifice with an isolation valve, and all other openings, if any, in said part are plugged;b) introducing said granules of said thermoplastic material into said inside volume through said filler orifice, in an amount that is necessary and sufficient for obtaining a layer of said thermoplastic material over all of said inside surface with a substantially uniform desired thickness;c) heating the walls of said connection part defining said inside volume to a temperature enabling said granules to be melted;d) causing said connection part to turn simultaneously about at least two different axes of rotation for a length of time that is sufficient and at a speed that is sufficiently low to enable said thermoplastic material to melt on said inside surface and to be distributed in substantially uniform manner over all of said inside surface; ande) while maintaining said rotation, cooling said connection part or allowing said connection part to cool prior to removing said covers. 2. The method according to claim 1, wherein the internal pressure is maintained against said walls covered in said material within said inside volume during said cooling. 3. The method according to claim 1, wherein the pressure inside said volume is increased before cooling by pressurizing said inside volume with a gas or water under pressure that is introduced via ventilation or purge orifice, and said increased pressure inside said volume is maintained during cooling, preferably at a value of not less than 106 Pa. 4. The method according to claim 1, wherein, in step c), said walls are heated by placing said part in an oven. 5. The method according to claim 1, wherein in step d), said connection part is caused to rotate with the help of a multidirectional rotation device for rotary molding. 6. The method according to claim 1, wherein use is made of granules of polyethylene, polypropylene, polyamide, and polyvinyl. 7. The method according to claim 1, wherein said connection part is a bend pipe element suitable for enabling two pipe elements that extend in different directions to be connected together by being connected to respective ends of said bend element constituting said connection part. 8. The method according to claim 1, wherein said connection part is a T-shaped element suitable for enabling three pipe elements to be connected to its respective ends, the elements extending in two directions making between them an angle lying in the range 0 to 90°, and preferably of 90°. 9. The method according to claim 1, wherein said connection part is a tapering tubular sleeve presenting two tubular ends of circular cross-sections that are of different inside diameters, suitable for enabling two pipe elements of diameters corresponding respectively to the inside diameters of the two tubular ends of said tapering tubular sleeve to be connected thereto. 10. The method according to claim 1, wherein said inside surface is a surface that is not entirely cylindrical. 11. The method according to claim 1, wherein during the cooling step, said inside volume is pressurized with water at high temperature that is cooled by being caused to flow through a heat exchanger enabling said water to lose heat while maintaining the desired level of pressure until all of said connection part and said thermoplastic material applied as a layer thereon is completely cooled. 12. The method according to claim 1, wherein said connection part comprises walls with zones of smaller thicknesses, said zones of smaller thickness being thermally insulated. 13. The method according to claim 1, wherein said covering comprises a first said covering and a second said covering, and wherein: said first covering is made to obtain a said liner comprising a first layer of a first thermoplastic material; andsaid second covering is made to obtain a second layer of a second thermoplastic material different from said first material, said second thermoplastic material being contained in a different reservoir and being introduced into said connection part via a said cover once said first material has melted completely and has bonded to said walls, said second material covering said first material and being melted to obtain the desired final thickness. 14. The method according to claim 1, wherein said connection part comprises, at at least one of its tubular ends, a first connecting flange suitable for co-operating with a second connecting flange of one of said two pipe elements for connection to said connection part, and the covering is made continuously over the entire combined inside surface of the connection part and over said connecting flange with a layer of said thermoplastic material, after said first flange has been fastened by welding to the end of said connection part and the open end of said first flange has been closed with one of said covers. 15. The method according to claim 14, wherein said first flange comprises an external bearing face bearing against said second flange and whereby said first flange bears against said second flange, said cover presenting a shape and means for fastening to said flange that enable said layer of thermoplastic material to be made continuously over said inside surface of the connection part and said inside surface of said first connecting flange, and also over said external bearing face whereby said first flange bears against said second flange.