FR2844587A1 - Heat exchanger tube lengthwise traction pre-stressing procedure, for heat exchanger used in nuclear reactor, has level regulated by adjusting initial radial clearance before expanding - Google Patents

Abstract

The lengthwise traction pre-stressing procedure for a heat exchanger tube (4) with at least one straight section fixed by its ends (4a, 4b) in the holes (5) of a tube plate (3a, 3b) uses diametrical hydraulic expansion. The prestressed value of each tube is set by adjusting the initial radial clearance (6) between the outer surface of a tube end and the inner surface of a plate hole before the tube end is expanded. The tube end is then expanded hydraulically along the whole of its length inside the plate hole.

Description

i The invention relates to a method of prestressing in tension

  in a longitudinal direction of tubes of a heat exchanger with

  precise adjustment of the tube preload.

  Heat exchangers are known which are used in particular as steam generators for nuclear reactors which comprise a very large number of tubes which are fixed at their ends in at least one plate.

  tubular heat exchanger.

  Such steam generators can have very large

  dimensions, for example a height greater than 20 meters and a diameter 10 slightly less than 4 meters.

  Such heat exchangers may in particular be heat exchangers with straight tubes (designated by the English term "oncethrough"), each of the tubes of the bundle being fixed, at a first end, in a first tubular plate disposed in the upper part 15 of the steam generator in the operating position and at a second end,

  in a second tubular plate disposed at the bottom of the steam generator in its operating position.

  In such generators with straight tubes, the tube bundle can comprise more than fifteen thousand tubes with a length greater than 15 meters. 20 The ends of the tubes are fixed in holes passing through the plates

  tubular over their entire thickness in the axial direction of the bundle and open at their ends in water boxes delimited by a generally hemispherical wall and by the corresponding tubular plate.

  Steam generators of another type have a bundle of 25 tubes bent into a U-shape which are fixed at their two ends in two

holes in the same tube plate.

  It is known to fix the ends of the tubes of the steam generators in the holes passing through the tubular plates, by crimping and welding inside the tubular plates.

  It is also known to produce, on the end parts of the tubes of the heat exchangers as described above, a diametrical expansion inside the holes passing through the tube plate, by hydraulic means, the interior surface of the tube being subject to pres-

  sion of a hydraulic fluid, generally water at very high pressure.

  This diametrical expansion operation which is generally carried out on an end portion of the tube extending along the entire length of the hole passing through the tubular plate can be used to crimp the tube into the hole or to ensure perfect plating. of the wall of the tube against the wall of the hole, in particular on the side of the internal face of the tube plate, in order to avoid the presence of any gap between the tube and the tube plate which could be responsible for corrosion in crevice , during the operation of the steam generator, the external surface 10 of the tubes coming into contact with one of the exchange fluids whose power

  corrosive can be significant at the interstices between the tube and the tube plate.

  In all cases, the diametrical expansion of a tube produces a retraction of the tube in the axial or longitudinal direction, so that, if the tube is fixed at its two ends in tubular plates whose

  the spacing is fixed, the tube is prestressed in longitudinal traction, due to the hydraulic diametral expansion.

  In heat exchangers such as straight tube exchangers, it is generally desirable to impose a certain longitudinal tensile preload on the tubes, in order to limit the buckling in service due to differential expansions of the shell of the exchanger. of heat and / or tubular plates and tubes as well as the vibrations produced

  by the circulation of exchange fluids in contact with the walls of the tubes.

  The tubes of straight tube heat exchangers are also held by transverse spacer plates distributed in the longitudinal direction of the heat exchanger. The buckling of the tubes can then lead to a deterioration of the tubes by contacting with

  the opening edges of the spacers.

  The longitudinal tensile preload of the tubes must be adjusted to a precise value determined to limit buckling and vibrations and

  possibly to limit and balance the axial direction forces exerted by the tubes on the tube plates of the heat exchanger with straight tubes.

  To adjust the value of the induced tensile preload in the

  tubes by hydraulic diametral expansion, in US 6, 357,114, it has been proposed to adjust the length of the end portion of the tube to which the hydraulic expansion is applied, in order to obtain the desired axial shrinkage and pre-stress. of the tube subjected to diametrical expansion. In the implementation of this method, the depth of introduction of the expansion tool is adjusted inside at least one of the end parts of the tube, from the external face of the plate. tubular. The diametrical expansion is generally carried out at the two ends of the tube, previously fixed on the tubular plates, so that the two hydraulic expansion operations contribute to the prestressing of the tube. The preload adjustment can be achieved by adjusting the length of hydraulic expansion of only one of the two ends of the tube, in one of the tubular plates.

  In all cases, it remains, at least at one end of the

  tube, a part of the tube wall which is not pressed against the inner surface of the hole in the tube plate, which results in the presence of a crevice corrosion-generating gap, during the operation of the steam generator .

  In addition, the implementation of the hydraulic expansion operation is

  complex, because it is necessary to adjust the depth of introduction of the diametral expansion tool into at least one of the ends of the very large number of tubes of the heat exchanger before carrying out their diametral expansion.

  The object of the invention is therefore to propose a method of prestressing the longitudinal traction of tubes of a heat exchanger comprising at least one rectilinear part and fixed at their ends in holes passing through at least one tube plate, in which the tensile preload is obtained by hydraulic diametral expansion of at least one end part of each of the tubes of the heat exchanger 30 fixed at its two ends in the at least one tubular plate, so as to induce a deformation by longitudinal shrinkage of the tube by subjecting the inner surface of the tube to hydraulic pressure and in such a way that the preload of the tube is adjusted to a precise value, this process

  making it possible to avoid any risk of corrosion in the crevice of the tubes in the heat exchanger in service and to facilitate the set prestressing operation of the tubes.

  For this purpose, the value of the prestress in each of the 5 tubes of the heat exchanger is adjusted by adjusting the initial radial clearance between the external surface of the end portion of the tube and the internal surface of the hole passing through the tube plate. in which is engaged the end portion of the tube before performing the diametrical expansion of the end portion, and the hydraulic diametral expansion of the tube is carried out along the entire end portion of the tube which extends substantially over the entire length of the

  hole passing through the tube plate.

  The invention also relates, in general, to a method of prestressing longitudinal traction of tubes of small diameter and of great length comprising at least one rectilinear part, in order to increase the buckling margin and reduce the vibrations induced and fixed at their ends in holes passing through at least one tubular plate, in which the tensile preload is obtained by hydraulic diametrical expansion of the ends of each of the tubes so as to induce a deformation by longitudinal retraction uniformly distributed over the length of the tube in subjecting the inner surface of the tube to hydraulic pressure and in such a way that the preload of the tube is adjusted to a precise value, by predetermined diametral expansion of the part

  end, and that the hydraulic diametral expansion of the tube is carried out.

  In order to clearly understand the invention, several embodiments of the method according to the invention of prestressing tubes of a steam generator with straight tubes will be described, by way of example.

of a nuclear reactor.

  Figure I is a vertical axial sectional view of a generator

  straight tube steam from a nuclear reactor.

  Figure 2 is a plan view of one of the tubular plates of

the heat exchanger.

  Figures 3 and 4 are enlarged details of part

  of the tube plate shown in Figure 2.

  FIG. 3 is a partial view of the internal face of the tube plate, before diametrical expansion of the tubes of the steam generator.

  FIG. 4 is a partial view of the internal face of the tube plate after diametrical expansion of the end parts of the tubes of the steam generator.

  Figures 5A, 5B, 5C, 5D and 5E are partial sectional views

  axial, during five successive stages of a method of fixing and prestressing a tube of the steam generator with straight tubes, according to the invention and according to a first embodiment.

  FIGS. 6A, 6B and 6C are views in partial axial section of a tube of the steam generator, during three successive stages of a method of fixing and prestressing the tube, according to the invention and

  according to a second embodiment.

  In Figure 1, there is shown, schematically, a steam generator 15 straight tubes generally designated by the reference 1 which has a generally cylindrical casing 2 connected at a first end or upper end to a tube plate upper 3a and at a second end or lower end at a plate

lower tubular 3b.

  The tubular plates 3a and 3b are pierced with a very large number of through openings 5 arranged in a triangular mesh network, as can be seen in particular in FIGS. 2, 3 and 4. The openings 5 could also be arranged according to a square mesh network. The straight tubes 4 of the steam generator bundle are fixed, by a first end part, inside the upper plate 3a and, by a second end part, in a through opening of the

bottom plate 3b.

  The plates 3a and 3b have an internal face directed towards the inside of the steam generator enclosing the bundle of tubes 4 and an opposite external face 30 constituting a wall delimiting a water box of the steam generator for the distribution or recovery of an exchange fluid.

  In Figure 2, there is shown the internal face of a tubular plate of generally circular shape 3a (or 3b) which has a very large number of through holes (for example more than fifteen thousand holes). The holes 5 are distributed uniformly between a central zone and a peripheral zone of the tube plate 3a (or 3b). The holes 5 passing through the tubular plate along its entire thickness and arranged along the triangular mesh network 5 are visible in particular in FIG. 3 which shows the end parts of tubes 4 engaged in the openings 5 with a certain play

radial (or diametral) 6.

  FIG. 3 represents the internal face of a tubular plate in which the ends of the tubes are engaged, in a step of mounting the bundle or casing prior to carrying out the expansion step

  diameter of the process according to the invention.

  FIG. 4 represents a partial view, similar to the view of FIG. 3, when the diametrical expansion operation according to the invention has been carried out, the clearance 6 being then eliminated and the tubes being pressed against the interior surface corresponding holes 5.

  We will now describe an operation for mounting a tube of a steam generator during which the tube is prestressed according to the invention and according to a first embodiment,

  with reference to Figures 5A to 5E.

  In a first step of the casing process, the tubes are introduced into the tubular plates and the envelope of the steam generator which have been pre-assembled. At the end of this first step of the method, as shown in FIG. 5A, the respective end portions 4a and 4b of the tubes 4 are introduced into the openings 5 of the respective tubular plates 3a and 3b. It should be noted that the mounting clearance 6, between the

  external surface of the end part of the tube 4 and the surface of the hole 5 passing through the tubular plate 3a (or 3b), makes it possible to easily introduce the tube through the openings of the tubular plates, without injuring the external surface of the tube which must present, at the end of the operation, a perfect surface condition.

  According to the invention, the amplitude of the diametral (or radial) clearance 6 is very precisely adjusted in order to obtain a prestressing of the tube with a prestressing of a predetermined value which is extremely precise; this prestress results from a longitudinal retraction uniformly distributed over

the length of the tube.

  To do this, first of all, the radial clearance necessary to obtain the desired prestressing value in the tube 4 is determined. This value of the radial clearance is obtained by calculations and by tests on a model comprising fastening parts of test tubes similar to tubular plates. We were able to establish a very good correlation between the results obtained

  by the calculation and the results of the measurements obtained from the model.

  The desirable effective radial clearance 6 between the tube and the through holes 5 of the tubular plates in which it is housed is obtained by adapting the drilling diameter of the hole 5 to the external diameter of the tube, taking into account the manufacturing tolerances of the tubes and by reserving the game

6 wanted.

  As will be explained below, the mounting clearances 6 of the different tubes can be provided to obtain an identical prestress in each of the tubes of the bundle, whatever the position of the tube, or, on the contrary, to modulate the value of the prestressing as a function of the position of the tube in the bundle or other special conditions for mounting the tubes.

  As shown in FIG. 5B, after having put the tube in place inside the steam generator, the end parts 4a and 4b of the tube being inside the openings 5 of the corresponding tubular plates 3a and 3b , one carries out, inside the opening 5 of a tubular plate 25 (for example the plate 3a), a diametrical expansion of

  end 4'a of the tube in an area adjacent to the external face of the tube plate 3a. The diametrical expansion of the end 4'a of the tube 4 can be carried out by swaging, for example using a roller bladder.

  At the end of the expansion operation, the fixing end 4'a of the tube 4 is pressed against an external layer 3'a of the plate 3a in a material chemically compatible with the material of the tube 4a which is example a nickel alloy. After the expansion of the end 4'a of the tube 4, a circular weld 7 is produced, for example by TIG welding without filler metal to fix the end of the tube 4 permanently and tightly, to the

  level of the external face of the tube plate 3a.

  As can be seen in FIG. 5C, a diametrical expansion is then carried out of the end part 4a of the tube 4 which extends over the entire length of the hole 5 passing through the tube plate 3a. This operation is carried out by inserting a hydraulic pressurizing tool inside the end part of the tube 4, so that the internal surface of the tube 4 is subjected to the hydraulic pressure of a fluid, for example of water, carrying out its hydraulic diametral expansion along the entire axial length of hole 5. At the end of the diametral expansion operation in

  the first tubular plate 3a, the end part 4a of the tube 4 is perfectly pressed against the surface of the hole 5, so that there is no gap remaining on the internal face of the tubular plate 3a, between the exterior surface of the tube and the opening 5 of the tube plate 3a.

  During the diametrical expansion operation of the end part 4a of the tube, the retraction of the tube in its axial direction 8 only results in a slight displacement of the lower end 4b of the tube which is free inside the opening 5 of the second tube plate 3b. Any

  prestressing is then introduced into the tube.

  As can be seen in FIG. 5D, in a subsequent tubing step, the expansion of an attachment end portion 4'b of the tube 4 is carried out in the vicinity of the external face of the second tube plate 3b, then circular welding 9 of this end 4'b of the tube 4 in the vicinity of

  the external face of the tubular plate 3b.

  As can be seen in FIG. 5E, an expansion is then carried out

  diametral of the second end part 4b of the tube 4 inside the opening 5 of the second tube plate 3b, over the entire axial length of the opening 5. The part 4b of the tube is perfectly pressed against the wall of the opening 5 and there remains no gap between the outer surface of the tube and the hole 5, in the vicinity of the internal face of the tube plate 3b.

  During this second hydraulic diametral expansion operation during which a hydraulic expansion tool is used, as in the case of the first end portion 4a of the tube inside the first tubular plate 3a, a deformation by retraction of the tube in the axial direction, this deformation of the tube fixed at its ends on the tubular plates 3a and 3b producing a tensile preload distributed along the entire length of the wall of the tube 4 between the tubular plates 3a and 3b. Furthermore, since the initial clearance 6 between the outer surface of the

  end part 4b of the tube 4 and the opening 5 of the tube plate 3b has been adjusted to obtain a deformation by retraction of a desired amplitude, the preload in the tube is itself at a perfectly determined value which has was obtained by calculation and verified by model tests.

  According to the invention, a prestressing of a predetermined precise value is therefore obtained by checking the clearance and carrying out the radial expansion over the entire length of the holes passing through the tubular plates, so that no crevice-generating gap is not found

present at the ends of the tube.

  In FIGS. 6A, 6B and 6C, three successive stages have been shown.

  of an operation of placing a tube during the casing of the steam generator during which a prestressing of the tube is carried out according to the method of the invention and according to a second embodiment.

  Figure 6A is similar to Figure 5A and will not be described again. At the end of the operation shown in FIG. 6A, the tube 4 is in place inside the steam generator, its end parts 4a and 4b being engaged inside the openings 5 of the tubular plates 25 correspondents 3a and 3b.

  The mounting clearance 6 of the ends 4a and 4b of the tube 4 inside

  openings 5 of the tubular plates 3a and 3b have been determined and fixed so as to produce, during the subsequent diametral expansion operations which will be described below, the desired deformation of the tube in axial retraction and a desired predetermined level of prestressing .

  As shown in FIG. 6B, in a second step of the casing process, the ends 4'a and 4'b of the tube are expanded in the vicinity of the external faces of the tubular plates 3a and 3b, so as to press the ends of the tube 4 against the outer layers of the tube plates. The circular connection welds 7 and 9 are then produced between the respective ends 4'a and 4'b of the tube 4 and the external faces.

respective plates 3a and 3b.

  The tube 4 is then fixed at its two ends to the plates 3a and 3b, the spacing of which is fixed due to the mounting of the plates on the structure of the

steam generator.

  As shown in FIG. 6C, a final step is carried out

  of the method, the hydraulic diametrical expansion of the two end portions 10 4a and 4b of the tube 4, in the corresponding openings 5 of the tubular plates 3a and 3b, over the entire axial length of the openings 5, so that at the after the hydraulic expansion operations, the parts 4a and 4b are perfectly flattened over the entire length of the holes 5, no residual interstice being present at the level of the internal faces of the tubular plates 3a and 3b.

  During the diametrical expansion operation of the end part 4a

  at the tube plate 3a and the end portion 4b at the tube plate 3b, the tube 4 which is fixed at its two ends undergoes a deformation by retraction in the axial direction 8, so that the tube is prestressed during both diametrical expansion operations.

  The effects of prestressing the tube 4 during the first diametrical expansion operation of the first end portion 4a of the tube 4 and the second diametrical expansion operation of the second end portion 4b of the tube 4 combine, so that the preload

  final results from the two additive operations of diametrical expansion.

  The initial assembly clearance 6 of the ends of the tube 4 in the openings of the tube plates is determined, by calculation and by tests on a model, so that the total prestressing obtained by the two diametrical expansions is, precisely, the prestressing. sought.

  In the case of the first embodiment, only the second diametral expansion operation generates the deformation by retraction of the tube and the prestressing whereas in the case of the second embodiment the

  two diametral expansion operations work together to generate the prestress.

  The method according to the invention therefore makes it possible to adjust very

  specifies the level of axial prestressing of the tubes of a heat exchanger bundle while avoiding the presence of gaps at the internal faces of the tubular plate or plates.

  In addition, the required prestressing of the tubes or the distribution of the prestresses of the tubes in the bundle can be obtained in a simple manner by drilling the holes of the tubular plates to a diameter or diameters perfectly defined and programmed, for example according to the position of the holes in the plane of the tube sheets, taking into account the manufacturing tolerances of the batches of tubes used for casing

of the heat exchanger ,.

  The method according to the invention makes it possible in particular to easily obtain a perfectly constant prestress for all of the tubes.

of the heat exchanger.

  The method according to the invention also makes it possible to modulate the prestresses of the tubes, in particular as a function of their position in the bundle determined by the position of the openings for receiving the tube in the tubular plates.

  An adequate distribution of the prestresses of the tubes in the bundle makes it possible in particular to limit the deformation of the tube plates under the effect of the tensile forces exerted by the tubes on the tube plates. Constant prestressing in all tubes of the beam can result in a bending deformation of the tube plates

  subjected to significant forces in their peripheral part. This effect can be corrected by adopting an adequate distribution of the prestresses in the bundle tubes. Likewise, the deformations in service of the different parts of the heat exchanger, for example under the effect of differential expansion, can be limited by adopting an adequate distribution of the prestresses in the tubes of the bundle.

  In addition, to limit the deformations of the tube plates during assembly of the bundle which can be carried out by successive assembly of the tubes from the central part towards the periphery or vice versa from the periphery towards the central part, it is possible to adopt a drilling plan for the different parts of the tubular plates making it possible to increase the levels of the prestresses and therefore the forces exerted by the tubes during assembly. This produces a preload of the tubular plates at a

  lower level during the first stages of assembly, which provides a certain stiffening of the tubular plates which are more able to support prestresses and higher forces exerted by the tubes assembled in later stages of casing.

  Such distributions of prestresses exerted in the tubes

  can be made by adjusting the drilling diameters of the holes passing through the tubular plates. In particular, the diameters of the holes passing through the tubular plates for mounting the tubes at the start of casing can generally be smaller than the diameters of the holes for mounting the tubes at the end of casing.

  In the case of circular tubular plates, drilling can be carried out in such a way that the holes passing through the tubular plates at their central part have diameters different from the holes passing through the

  tubular plates in their peripheral part.

  The invention is not strictly limited to the embodiments which

have been described.

  The assembly and the diametrical expansion of the tubes can be carried out

  using any type of tool suitable for the dimensions of the tubes and the characteristics of the steam generator harness.

  The invention can be applied to heat exchangers different from heat exchangers with straight tubes, from the moment when these tubes have at least one rectilinear part which can be subjected to a

longitudinal prestress.

Claims (10)

  1.- Method for prestressing longitudinal traction of tubes (4) of a heat exchanger (1) comprising at least one rectilinear part and fixed at their ends (4'a, 4'b) in holes (5 ) passing through at least 5 a tubular plate (3a, 3b), in which the tensile preload is obtained by hydraulic diametral expansion of at least one end part (4a, 4b) of each of the tubes (4) of the heat exchanger fixed at its two ends (4'a, 4'b) in the at least one tubular plate (3a, 3b), so as to induce a deformation by longitudinal retraction of the tube (4) 10 by subjecting the surface inside of the tube at hydraulic pressure and in such a way that the preload of the tube is adjusted to a precise value, characterized in that the value of the preload is adjusted in each of the tubes (4) of the heat exchanger ( 1) by adjusting the initial radial clearance (6) between the external surface of the part of e x end (4a, 4b) of the tube (4) and the inner surface of the hole (5) passing through the tube plate (3a, 3b) in   which is engaged the end part (4a, 4b) of the tube (4) before carrying out the diametrical expansion of the end part (4a, 4b), and that the hydraulic diametral expansion of the tube ( 4) along the entire end part (4a, 4b) of the tube which extends substantially over the entire length of the hole 20 (5) passing through the tubular plate (3a, 3b).   2.- Method according to claim 1, characterized in that a hydraulic diametral expansion is carried out of a first end part (4a) of the tube (4) introduced inside a through hole (5) a first tubular plate (3a) on which is fixed one end (4'a) of the tube (4), the second end (4'b) of the tube (4) being free inside a hole passing through (5) a second tubular plate (3b) and a second diametrical expansion of the second part of the tube (4b) introduced into the through hole (5) of the second tubular plate (3b), the tube (4 ) being fixed at its two ends (4'a, 4'b) respectively, on the first tube plate (3a) and on the second tube plate (3b), respectively, the deformation by longitudinal retraction of the tube and the prestressing being obtained only during the second expansion operation   diametral of the second end part (4b) of the tube (4).   3.- Method according to claim 1, characterized in that a first diametrical expansion is carried out of a first end part (4a) of the tube (4) introduced inside the opening (5) d '' a first tubular plate (3a) and a second diametrical expansion operation of a second part (4b) of the tube (4) inside the opening (5) of a second   tube plate (3b), the tube (4) being fixed at a first end (4'a) and at a second end (4'b), respectively, on the first tube plate (3a) and on the second tube plate ( 3b), the deformation by longitudinal retraction of the tube (4) and the prestressing being obtained additively by the first diametral expansion operation and the second diametral expansion operation.   4.- Method according to any one of claims 1, 2 and 3,   characterized in that the heat exchanger (1) is a straight tube heat exchanger comprising a first tubular plate (3a) and a second tubular plate (3b) fixed in spaced arrangements one   on the other in the longitudinal direction of the straight tubes (4).   5.- Method according to any one of claims 1 to 4, characterized in that the initial radial clearance (6) between the outer surface of the end portion (4a, 4b) of the tube (4) and the surface inside the hole (5) traver20 sant the tube plate (3a, 3b) is adjusted by adjusting the drilling diameter of the hole (5) passing through the tube plate (3a, 3b), taking into account the tolerances concerning the outside diameter of the tube ( 4).   6.- Method according to any one of claims 1 to 5, characterized in that the at least one tubular plate (3a, 3b) comprises a plurality of through holes (5) for mounting tubes (4) of a beam   of the heat exchanger and that the preload is adjusted in the different tubes (4) of the heat exchanger bundle by adjusting the diameters of   drilling holes (5) through the tube plate (3a, 3b).   7.- Method according to claim 6, characterized in that the heat exchanger (1) is a heat exchanger with straight tubes comprising two tubular plates (3a, 3b) of generally circular shape placed parallel to one the other at a certain distance and crossed by holes (5) distributed uniformly between a central zone and a zone pe   ripherical of the circular tubular plate (3a, 3b), characterized in that the initial radial clearance (6) between the end parts of the tubes (4) and the surface of the openings (5) is adjusted by drilling the openings (5) at diameters determined as a function of the desired clearance and preload of the tu5 bes fixed in the holes (5) distributed along the tubular plate (3a, 3b).   8.- Method according to claim 7, characterized in that one generally carries out a drilling of the holes (5) intended to receive the tubes by which one begins the assembly of the bundle of the heat exchanger (1), at a smaller diameter than the diameter of the holes in the holes (5) 10 intended to receive the tubes (4) of the heat exchanger bundle (1)   by which the assembly of the beam is finished.   9.- Method according to claim 8, characterized in that the   holes passing through the tubular plates (3a, 3b) have generally different diameters in the central part and in the peripheral part of the tubular plate of circular shape.   10.- Method for prestressing longitudinal traction of tubes (4) of small diameter and great length comprising at least one straight part, to increase the buckling margin and reduce the induced vibrations, and fixed at their ends (4 ' a, 4'b) in holes (5) through at least one tubular plate (3a, 3b), in which the tensile preload is obtained by hydraulic diametral expansion of the ends (4a, 4b) of each of the tubes (4 ), so as to induce a deformation by longitudinal retraction uniformly distributed over the length of the tube (4) by subjecting the inner surface of the tube to hydraulic pressure and in such a way that the preload of the tube is adjusted to a precise value, by predetermined diametral expansion of the end part (4a, 4b), and   that the hydraulic diametral expansion of the tube (4) is carried out.