CORROSION-RESISTANT PIPE COUPLING STRUCTURES

Abstract: A corrosion-resistant pipe coupling made of carbon steel has on its inner surface an intermediate annular projection di­ rected radially inward. At least an innermost part of the annular projection is formed of corrosion-resistant material such as stainless steel. The annular projection is covered by a corro­ sion-resistant annular member which is secured by friction welding, electron beam welding, an adhesive or mechanical engagement. The annular member prevents corrosion of the coupling by corrosive fluids flowing in pipes.

Description: This invention relates to a corrosion-resistant pipe structure, and more particularly to a pipe coupling structure used for piping in chemical and other plants, oil and gas pipelines, well tubes, and like piping. In the piping and pipelines referred to above, multiple-wall composite pipes are used for transporting fluids such as those containing corrosive substances. A typical multiple-wall composite pipe is a dual or doublewall pipe in which the inner wall is a liner tube made of a corrosion-resistant material for effective conducting of corrosive fluid and the outer wall or pipe is designed to provide strength to withstand internal pressures of the corrosive fluid and external forces. For example, there is a double-wall pipe comprising a stainless-steel inner pipe and a carbon-steel outer pipe in which the inner pipe is interference-fitted. There are certain limitations to the length of such multiple-wall pipes due to restrictions in the manufacture of the pipes and to conditions in which the pipes are installed in site. Therefore, pipe couplings are used to obtain a desired length of piping as is well known in the art. As a matter of course, the pipe couplings are also required to have an internal corrosion resistance as well as a mechanical strength to withstand internal pressures and external forces. A pipe coupling structure as defined in the precharacterizing portion of the claim is known from GB-A-1 428 433 or US-A-3 870 351. The radially inner surface of the annular projection is made to have the same diameter as the inner diameter of the two pipes to be coupled whereby a smooth, continuous or uninterrupted cylindrical inner surface is formed from one pipe to the other. This means that the annular projection of the coupling member is exposed to the corrosive fluid or fluids flowing in the pipes. As a consequence it has been customary to make the coupling member of a corrosion-resistant material such as stainless steel. This gives the coupling sufficient resistance to corrosive fluids but is disadvantageous in that the coupling is very expensive and lacks sufficient strength, thereby resulting in increased thickness and weight and in interference with other elements and devices in and along pipe line. DD-A-130 591 discloses a pipe coupling structure having a tubular coupling member including on its inner surface an intermediate annular projection the diameter of which is greater than the inner diameter of the two pipes to be coupled. A lining may be provided on the inner surfaces of the pipes, on the actually opposing end parts of the pipes and on the inner surface of the annular projection. However, in constrast to the corrosion-resistant pipe coupling structure defind in the precharacterizing portion of the claim, in this coupling structure the annular projection is not abuttlingly interposed between the opposing end parts of the pipes and corrosive fluids may enter into the space between the end parts of the tubes and the annular projection. Applying the linings to said parts involves a plurality of manufacturing steps which renders the prior art structure expensive. It is an object of the invention to provide a pipe coupling structure, wherein the strength of the coupling member is sufficient to withstand internal brushes and external forces, while the intermediate annular projection may be provided with means which is resistant to corrosive fluids and which may easily be secured to the annular projection. In accordance with the present invention, in a pipe coupling structure defind in the precharacterizing portion of the claim, this object is achieved by the features defind in the characterizing portion of the claim. The detailed nature, utility, and further features of this invention will be more clearly apparent from the following detailed description when read in conjunction with the accompanying drawings, briefly described below. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a fragmental longitudinal section of a pipe coupling structure according to an embodiment of this invention; FIG. 2 is a view explanatory of a padding overlaid to form an annular projection shown in FIG 1; FIG. 3 is a fragmental longitudinal section of a coupling member of a second embodiment of the invention and FIG. 4 is a fragmental longitudinal section showing a known pipe coupling structure. Referring to FIG. 4 there is illustrated a known pipe coupling structure comprising two dual or doublewall composite pipes 1 to be coupled, and a screwed coupling member 2 having a substantially cylindrical body. Each of the double-wall composite pipes 1 comprises an outer wall or pipe 3 of carbon steel, for example, and an inner wall or pipe 4 of stainless steel, for example. The end portion of each pipe 1 is provided with outer or male screws 5. The coupling member 2 has a pair of screw-threaded parts 6 with inner or female screws 7 which are in engagement with the male screws 5 of the pipes 1. The coupling member 2 has intermediate annular ridge or projection 8 directed radially inward between the screw parts 6 so as to be abuttingly interposed between the opposing end surfaces of the pipes 1. The annular projection 8 has annular undercut portions 9 so that an annular overhang portion 8 diverging in radially inward direction is formed. The end surfaces of the pipes 1 are shaped complementarily to the undercut portions 9 of the projection 8, as shown at 10. When the pipes 1 are screwed into the coupling member 2, the end surfaces 10 of the pipes are brought into tight, abutting, sealing contact with the undercut portions 9, respectively. However, with such a construction, the annular projection 8 is exposed to corrosive fluid or fluids flowing in the pipes and subject to corrosion by the fluid or fluids, so that there arise the disadvantages mentioned hereinbefore. FIG. 1 illustrates a first embodiment of the invention wherein the annular projection 8 entirely made of corrosion-resistant material is along its outer periphery embedded in an annular groove 46 of the coupling member 2. In order to form the projection 8, a cylindrical main body of the coupling member 2 is first formed with an annular groove 46 as illustrated in FIG. 2, and then an annular padding 8a of stainless steel, for example, is overlay-welded in the groove 46. Thereafter, the padding 8a is machined into the shape having an inner cylindrical surface 47 and a pair of undercut portions 9. In the second embodiment of the invention, shown in Fig. 3, the annular projection 8 has its outer diameter equal to the outer diameter of the coupling member 2. The annular projection 8 is prepared separately, and welded to cylindrical members 49 at 50 by electron beam welding, for example, as indicated by arrows.

Claims: Claims 1. A corrosion-resistant pipe coupling structure comprising: a tubular coupling member (2) including an inner surface and axially opposite ends having internal screw threads (7) for engaging male screw threads provided on axially opposing end parts of two pipes (1, 1) to be coupled, the internal surface of each of the pipes having secured thereto an inner lining pipe (4) resistant to corrosive fluids, said coupling member (2) including an intermediate annular projection (8) extending radially inwardly from said inner surface to be abuttingly engaged by the opposing end parts of the pipes (1, l),characterized in that said annular projection (8) is formed entirely of a corrosion-resistant annular member (8) embedded in the coupling member (2) at the radially outermost part thereof. (FIGS. 1 and 3 2. A pipe coupling structure as claimed in claim 1, wherein said annular member (2) is embedded in an annular groove (46) formed in the radially inner surface of the coupling member (2). (FIG. 1 3. A pipe coupling structure as claimed in claim 2, wherein said annular member (2) is formed of a machined padding of said corrosion-resistant material. (FIG. 2 4. A pipe coupling structure as claimed in claim 1, wherein said coupling member (2) comprises a pair of axially spaced cylindrical members (49) having therein said internal threads (7), and said annular member (8) is positioned between and fixed to said cylindrical members (49). (FIG. 3 5. A pipe coupling structure as claimed in claim 4, wherein the outer diameters of said annular and cylindrical members (8, 49) are equal. (FIG. 3 6. A pipe coupling structure as claimed in claim 4, wherein said annular member (8) is welded to said cylindrical members (49). (FIG. 3 7. A pipe coupling structure as claimed in claim 1, wherein said annular member (8) includes undercut portions (9) on axially opposite sides thereof. (FIGS. 1 and 3

Inventors: ATSUTA, TOSHIO, MATSUI, SHIGETOMO, YOSHIDA, TOSHIO
Applicants: KAWASAKI JUKOGYO KABUSHIKI KAISHA
Application year: 1982
Main classification: F16L15
IPC codes: E21B17, F16L15, F16L58