JPS5933452B2 - Double pipe manufacturing method - Google Patents

Description

[Detailed Description of the Invention] <Technical Classification/Field> The disclosed technology belongs to the technical field of manufacturing by hydraulic expansion of double pipes for transporting corrosive fluids, such as oil country tubular goods.

<Explanation of the gist>The present invention involves inserting and layering an inner tube made of stainless steel or the like having a smaller diameter by a set amount into an outer tube made of carbon steel or the like, and applying hydraulic pressure inside the inner tube. This invention relates to a method for manufacturing a double-layered pipe, in which the inner pipe is expanded, the outer pipe is brought into contact with the outer pipe, and the outer pipe is caused to self-tighten by releasing pressure through the elastic return difference between the two pipes, In particular, when applying axial compressive force in the tube expansion process to reduce circumferential stress, the pressure is applied at a level lower than the sufficient axial compressive force that is applied together with the circumferential tube expansion liquid pressure to expand the tube and increase elasticity. This invention relates to a method of manufacturing a double-walled pipe in which the return difference is increased to ensure a sufficient interference margin, and a large compressive residual stress is created in the inner pipe.

<Prior art> As is well known, corrosive fluid transportation piping such as oil country tubular goods has an outer pipe such as a carbon steel pipe that provides pressure and heat resistance functions, and a stainless steel pipe that is attached to the inside of the outer pipe and provides corrosion resistance. A double pipe consisting of an inner pipe is used,
A type in which both pipes are tightened with a large interference margin in order to prevent the possibility of displacement, buckling, or destruction of the interface during operation due to differences in physical properties such as differences in expansion coefficients between the two pipes. has been devised, and various fastening-type double pipe manufacturing methods have been adopted.

Among these, the so-called hydraulic pipe expansion method has become commonly used due to its advantages such as uniform pressure distribution, ease of installation of pressurizing means, and no metallurgical deterioration. When using water, cost, incompressibility,
Coupled with the advantages of cooling, etc., it is becoming easier to use.
As shown in Fig. 1, the method of manufacturing a self-contained double pipe using hydraulic pipe expansion, which has been commonly used in the past, involves, for example, inserting and layering a stainless steel inner pipe 2 into a carbon steel outer pipe 1, and then inserting a stainless steel inner pipe 2 into a carbon steel outer pipe 1, and then inserting a stainless steel inner pipe 2 into a carbon steel outer pipe 1, and then inserting a stainless steel inner pipe 2 into a carbon steel outer pipe 1, and then inserting a stainless steel inner pipe 2 into a carbon steel outer pipe 1, and then inserting a stainless steel inner pipe 2 into a carbon steel outer pipe 1, and then inserting a stainless steel inner pipe 2 into a carbon steel outer pipe 1, and then inserting a stainless steel inner pipe 2 into a carbon steel outer pipe 1, and then inserting a stainless steel inner pipe 2 into a carbon steel outer pipe 1, and then inserting a stainless steel inner pipe 2 into a carbon steel outer pipe 1. The seal frames 3, 3 are clamped in a fixed state, and the frames 3,
3 is zero, that is, with no external force applied, water for expansion is supplied inside the tube and internal pressure P is applied to perform hydraulic expansion. A tensile stress P is applied in both the circumferential direction and the axial direction, and as a result, the relationship between the diameter D of the inner and outer tubes 2 and 1 and the tube expansion stress P is shown in FIG. ), the stress of the elastic expansion and contraction C2 of the inner tube 2 becomes higher than that of the elastic expansion and contraction C1 of the outer tube 1, and the yield point becomes higher. Therefore, the difference in elastic return after pressure release after yielding increases. The potential was small, and as shown in the figure, there was a drawback that only a very small interference margin ΔD1 could be obtained.

To deal with this, as shown in FIG. 3, the supports 5, 5 are brought into contact with the outer end surfaces of the plugs 3, 3 to take the reaction force T of the axial compressive force, that is, P= An axial force balance method that accelerates yielding as T is also considered, but in this embodiment, as shown in Fig. 4, a slightly larger difference in elastic return is obtained, and the tightening margin ΔD2 is larger than the tightening margin ΔD1 of the above-mentioned embodiment, which is ΔD1 - ΔD2. Although this was achieved, there was still a problem in that it was difficult to create a sufficient tightening allowance.

As a result, the compressive residual stress on the inner tube 2 cannot be removed, and there is a risk that stress corrosion cracking may occur during operation.

Purpose of the Invention The purpose of the present invention is to solve the problems of the double pipe manufacturing method by hydraulic pipe expansion based on the above-mentioned prior art, and
By applying a larger axial force to counter the axial stress caused by the pipe expansion liquid pressure, the yield point is lowered and the binding force is increased.
It is an object of the present invention to provide an excellent method for manufacturing double-layered pipes that can increase compressive residual stress and obtain highly durable double-layered pipes, thereby benefiting piping applications in various industries.

Structure of the Invention> The structure of the present invention, which is full of the above-mentioned objects and whose gist is the scope of the above-mentioned patent claims, is to solve the above-mentioned problems by inserting an inner tube into an outer tube, supplying a tube expansion liquid, pressurizing it, and expanding the tube. At this time, a sufficient axial compression force is applied to the end of the inner tube in advance, and a tube expansion liquid pressure is applied so as to apply a circumferential tube expansion liquid pressure that is lower than the compression force to cause the inner tube to yield early and expand. By performing concomitant expansion of the tube, then releasing the expansion pressure with a predetermined increase in diameter and releasing the axial compression force, it is possible to significantly increase the difference in elastic return between the inner and outer tubes, thereby achieving a high degree of fit. This is a technical measure.

Embodiments Next, embodiments of the present invention will be described below with reference to FIG. 5 and the following drawings.

Note that the same parts as in FIGS. 1 to 4 will be described with the same reference numerals.

The embodiment shown in FIG. 5 is a principle embodiment, in which an inner tube 2 made of stainless steel is inserted into an outer tube 1 made of carbon steel or the like, and a stopper 3,
3 are fitted on both ends of the inner tube 2, water 4 as a tube expansion liquid is supplied into the inner tube 2, and jack plates 6, 6 are fitted on both outer ends of the plugs 3, 3 of the inner tube 2. When the inner tube 2 is brought into contact with the inner tube 2 and a set axial compressive force T that is sufficiently larger than the tube expansion pressure P is applied at the same time as the tube expansion pressure P is applied, the stress strain curve q of the inner tube 2 as shown in FIG. As a result, the inner tube 2 begins to yield early, and the stress in the circumferential direction of the inner tube 2 in the plastically deformed state becomes slight, and the inner tube 2 expands integrally with the outer tube 1 following the stress strain line C1, and reaches a predetermined level. When the post-expansion stress is released at the time when the diameter is increased, the elastic return difference is made to obtain an extremely large degree of fitting ΔD2 as shown in the figure.

Of course, the axial compressive force T is finally released, but both pipes 1 and 2
Since a sufficient degree of fitting is obtained between the two, no misalignment occurs. Moreover, the compressive force T is internalized as a large compressive residual stress.

As an example, as shown in FIG. 7, for example, when a stainless steel inner tube 2 is inserted into a carbon steel outer tube 1, a jack plate 6 having a sealing function is attached to the end of the inner tube 2.
', 6' are brought into contact with each other and connected to the jack plates 7, 7 which are connected to a hydraulic jack (not shown), and the expansion water as pipe expansion liquid is supplied into the inner pipe 2 from the water supply channel 8 provided on one jack plate 6'. Water supply is charged.

At the same time, a hydraulic jack (not shown) applies a compressive force T in the axial direction of the inner tube 2 via the jack plates 6', 6'.
At the same time, a circumferential pipe expansion force P that is lower than the axial compressive force T by a set amount is applied to the pipe expansion water by a booster pump via an appropriate detection control mechanism.

Therefore, due to the auxiliary force of the axial compressive force T applied as described above, the circumferential direction yields early, the inner tube 2 is plastically deformed by the small tube expansion stress, comes into contact with the outer tube 1, and expands integrally. The diameter is increased to a predetermined diameter, and then the pressure is reduced, and both the inner and outer tubes are elastically reduced in diameter.

As shown in FIG. 6, the plastic deformation expansion stress of the inner tube 2 is small, so the difference .DELTA.D3 between the inner and outer tubes 2 and 1 in elastic recovery is significantly large.

Therefore, as in the principle described above, the degree of fitting between the two tubes 1 and 2 is large, and the degree of self-tension is large.

Therefore, even when the axial compressive force by the hydraulic jack is finally released, no axial displacement of the inner tube 2 with respect to the outer tube 1 occurs, as described above.

Further, as described above, a large amount of axial compressive residual stress is present.

In addition, FIG. 8 clearly shows that by combining FIGS. 2, 4, and 6, a large degree of fit can be obtained due to the large effect of the difference in elastic return of the present invention, and an axial compressive residual stress can be obtained. has been done.

It goes without saying that the embodiments of this invention are not limited to the above-mentioned embodiments, and various embodiments can be adopted.

In addition, although it is based on target & round pipes, it can also be applied to double pipes such as triple pipes and quadruple pipes, and is applicable not only to oil country tubular goods but also to oil pipes and plant pipes.

Effects of the Invention> According to the present invention, in the method for manufacturing double pipes by hydraulic pressure expansion, when the inner pipe inserted into the outer pipe is expanded by applying hydraulic pressure in the circumferential direction, compressive force is not applied in the axial direction. By applying a sufficiently large axial compression force and applying a circumferential expansion pressure that is lower than the large axial compressive force, the inner tube yields early, and the inner tube yields at an early stage. The plastic deformation stress after yielding of the inner tube is small, so by releasing the pressure after the accompanying expansion of the outer tube to the inner tube reaches a predetermined diameter, the difference in elastic return between the two tubes, that is, the degree of fit, can be increased. This results in an excellent effect of being able to manufacture self-contained double pipes with a sufficient tightening margin.

Further, as described above, since the tube expansion stress in the inner tube plastic deformation process is small, the power required for tube expansion is correspondingly small, which also has the effect of contributing to a reduction in manufacturing costs. Furthermore, even after the axial compressive force is released after pipe expansion, a strong fit is obtained, so there is no misalignment and it acts as a large compressive residual stress, which prevents stress corrosion cracking during operation of the formed double pipe. This not only improves the durability of the double pipes, but also has the excellent effect of requiring less plant maintenance due to piping maintenance.

[Brief explanation of the drawing]

Fig. 1 is a schematic sectional view for explaining the conventional fluid pipe expansion, Fig. 2 is a graph for explaining the stress strain curve, Fig. 3 is a sectional view for explaining the improved pipe expansion, and Fig. 4 is a graph for explaining the stress strain curve in Fig. 3. , 5th
The following figures are explanatory diagrams of embodiments of the present invention: FIG. 5 is a cross-sectional view explaining the principle aspect, FIG. 6 is a graph diagram explaining the stress strain curve, and FIG. Figure 8 is the second
Figures 4 and 6 are composite explanatory graphs. 1...Outer pipe, 2...Inner pipe, P...
...Hydraulic pressure, T...Compression force, ΔDl, ΔD2, Δ
D3...Elastic return difference.

Claims (1)

[Claims] 1 Inserting an inner tube with a slightly smaller diameter over the outer tube, applying hydraulic pressure to the inner tube to expand the inner tube, and enlarging the outer tube as well, bringing both tubes into close contact and releasing the pressure. In the method of manufacturing a double-walled tube, a compressive axial force larger than the axial force due to the tube expansion pressure is applied in the axial direction of the inner tube, and hydraulic pressure is also applied to expand the tube to increase the difference in elastic return between the inner and outer tubes. A method for manufacturing a double-layered pipe, characterized in that a self-stressing double-layered pipe is obtained which has a high fitting stress and has a compressive residual stress formed in the inner pipe.