Patent Publication Number: US-2004049915-A1

Title: Method for prestressing tubes of a heat exchanger with precise tailoring of the prestress

Description:
[0001] The invention relates to a method for prestressing tubes of a heat exchanger in tension in a longitudinal direction, with precise tailoring of the prestress in the tubes.  
       [0002] Heat exchangers used in particular as steam generators in nuclear reactors and which comprise a great many tubes which are fixed at their ends into at least one tube plate of the heat exchanger are known.  
       [0003] Such steam generators may be very large, for example in excess of 20 metres tall with a diameter barely smaller than 4 metres.  
       [0004] Such heat exchangers may, in particular, be heat exchangers with straight tubes (known as “once-through” exchangers), each of the tubes of the bundle being fixed, at a first end, into a first tube plate arranged in the upper part of the steam generator in the service position and, at a second end, into a second tube plate arranged at the lower part of the steam generator in its service position.  
       [0005] In such once-through generators, the tube bundle may contain over fifteen thousand tubes of a length in excess of 15 metres. The ends of the tubes are fixed in holes passing through the tube plates over their entire thickness in the axial direction of the bundle and open at their ends into water boxes delimited by a generally hemispherical wall and by the corresponding tube plate.  
       [0006] Steam generators of another type comprise a bundle of tubes bent into the shape of a U which are fixed at both ends into two holes in the same tube plate.  
       [0007] It is known for the ends of the tubes of the steam generators to be fixed into the holes passing through the tube plates by crimping and welding inside the tube plates.  
       [0008] It is also known for diametral expansion to be performed on the end parts of the tubes of heat exchangers as described above, inside the holes passing through the tube plate, by hydraulic means, the internal surface of the tube being subjected to the pressure of a hydraulic fluid, generally water under very high pressure. This diametral expansion operation, which is generally carried out on an end part of the tube stretching along the entire length of the hole passing through the tube plate, may be used to crimp the tube into the hole or to press the wall of the tube perfectly firmly against the wall of the hole, particularly on the interior face side of the tube plate, so as to prevent there being any gap between the tube and the tube plate as this gap could prove responsible for crevice corrosion while the steam generator is running, the external surface of the tubes coming into contact with one of the heat-exchange fluids the corrosive capability of which may be high at gaps between the tube and the tube plate.  
       [0009] In any case, diametral expansion of a tube produces contraction of the tube in the axial or longitudinal direction which means that if the tube is fixed at both ends into tube plates, the separation of which is fixed, the tube is prestressed in longitudinal tension as a result of the hydraulic diametral expansion.  
       [0010] In heat exchangers such as once-through exchangers, it is generally desirable to dictate a certain longitudinal tensile prestress for the tubes, so as to limit the buckling in service as a result of the differential expansions of the heat exchanger wrapper and/or of the tube plates and of the tubes and the vibrations produced by the circulation of the heat-exchange fluids in contact with the walls of the tubes.  
       [0011] The tubes of once-through exchangers are also held by transverse spacer plates distributed along the longitudinal direction of the heat exchanger. The buckling of the tubes may therefore give rise to damage to the tubes by bringing these into contact with the edges of the openings in the spacers.  
       [0012] The longitudinal tensile prestress of the tubes needs to be tailored to a precise value determined so as to limit the buckling and vibrations and also possibly to limit and balance the axial forces exerted by the tubes on the tube plates of the once-through heat exchanger.  
       [0013] To tailor the value of the tensile prestress introduced into the tubes by the hydraulic diametral expansion, U.S. Pat. No. 6,357,114 proposes altering the length of the end part of the tube to which hydraulic expansion is applied so as to obtain the desired contraction and therefore the desired axial prestress in the tube subjected to the diametral expansion. In carrying out this method, the depth to which the expansion tool is introduced into at least one of the end parts of the tube from the external face of the tube plate is tailored. The diametral expansion is generally performed at both ends of the tube, already fixed to the tube plates, which means that the two hydraulic expansion operations work together in prestressing the tube. The prestress can be tailored by altering the length of hydraulic expansion of just one of the two ends of the tubes in one of the tube plates.  
       [0014] In all cases, there remains, at one of the ends of the tube, a part of the wall of the tube which is not pressed firmly against the internal surface of the hole in the tube plate, and this manifests itself in the presence of a gap which generates crevice corrosion while the steam generator is running.  
       [0015] Furthermore, carrying out the hydraulic expansion operation is complex, because it is necessary to tailor the depth to which the diametral expansion tool is introduced into at least one of the ends of the very numerous tubes of the heat exchanger before diametrally expanding them.  
       [0016] The object of the invention is therefore to propose a method for prestressing, in longitudinal tension, tubes of a heat exchanger comprising at least one straight part and fixed at their ends in holes passing through at least one tube plate, in which the tensile prestress is obtained by hydraulically diametrally expanding at least one end part of each of the tubes of the heat exchanger fixed at its two ends in the at least one tube plate in such a way as to induce deformation through longitudinal contraction of the tube by subjecting the internal surface of the tube to a hydraulic pressure and in such a way that the prestress of the tube is tailored to a precise value, this method making it possible to avoid any risk of crevice corrosion of the tubes in the heat exchanger in service and to make the operation of effecting tailored prestressing of the tubes easier.  
       [0017] To this end, the value of the prestress in each of the tubes of the heat exchanger is tailored by altering the initial radial clearance between the external surface of the end part of the tube and the internal surface of the hole passing through the tube plate in which hole the end part of the tube is engaged before diametrally expanding the end part and the tube is hydraulically diametrally expanded along the entire end part of the tube which extends substantially over the entire length of the hole passing through the tube plate.  
       [0018] The invention also relates, in general, to a method for prestressing, in longitudinal tension, tubes of small diameter and long length comprising at least one straight part, so as to increase the buckling margin and reduce induced vibrations, and which are fixed at their ends in holes passing through at least one tube plate, in which method the tensile prestress is obtained by hydraulically diametrally expanding the ends of each of the tubes so as to induce deformation through longitudinal contraction distributed uniformly along the length of the tube by subjecting the internal surface of the tube to a hydraulic pressure and in such a way that the prestress of the tube is tailored to a precise value by predetermined diametral expansion of the end part and that the tube is hydraulically diametrally expanded. 
     
    
    
     [0019] In order to make the invention easy to understand, several embodiments of the method according to the invention for prestressing tubes of a once-through steam generator of a nuclear reactor will be described by way of example.  
     [0020]FIG. 1 is a view in vertical axial section of a once-through steam generator of a nuclear reactor.  
     [0021]FIG. 2 is a plan view of one of the tube plates of the heat exchanger.  
     [0022]FIGS. 3 and 4 are details on a larger scale of part of the tube plate depicted in FIG. 2.  
     [0023]FIG. 3 is a part view of the internal face of the tube plate, prior to diametral expansion of the tubes of the steam generator.  
     [0024]FIG. 4 is a part view of the internal face of the tube plate after diametral expansion of the end parts of the tubes of the steam generator.  
     [0025]FIGS. 5A, 5B,  5 C,  5 D and  5 E are part views in axial section, during five successive steps of a method for fixing and prestressing a tube of the once-through steam generator, according to the invention and according to a first embodiment.  
     [0026]FIGS. 6A, 6B and  6 C are views in axial part section of a tube of the steam generator during three successive steps of a method for fixing and prestressing the tube, according to the invention and according to a second embodiment. 
    
    
     [0027]FIG. 1 schematically depicts a once-through steam generator using the overall reference  1 , which steam generator comprises a wrapper of cylindrical overall shape  2  connected at a first end or upper end to an upper tube plate  3   a  and at a second end or lower end to a lower tube plate  3   b.    
     [0028] The tube plates  3   a  and  3   b  are pierced with a great many through-openings  5  arranged in a triangular-pattern network as can be seen in particular in FIGS. 2, 3 and  4 . The openings  5  could just as easily be arranged in a square-pattern network. The straight tubes  4  of the steam generator bundle are fixed, by a first end part, inside the upper plate  3   a  and, by a second end part, in a through-opening in the lower plate  3   b.    
     [0029] The plates  3   a  and  3   b  have an internal face facing toward the inside of the steam generator containing the tube bundle  4  and an opposite external face constituting a wall delimiting a water box of the steam generator for distributing or collecting a heat-exchange fluid.  
     [0030]FIG. 2 depicts the internal face of a tube plate of circular overall shape  3   a  (or  3   b ) which has a great many through-holes (for example over fifteen thousand holes). The holes  5  are distributed uniformly between a central zone and a peripheral zone of the tube plate  3   a  (or  3   b ). The holes  5  passing through the tube plate through its entire thickness and arranged in the triangular-pattern network are visible in particular in FIG. 3 which shows the end parts of tubes  4  engaged in the openings  5  with a certain radial (or diametral) clearance  6 .  
     [0031]FIG. 3 depicts the internal face of a tube plate in which the ends of the tubes are engaged, in a stage of the mounting of the bundle or tubing prior to the carrying-out of the diametral expansion step in the method according to the invention.  
     [0032]FIG. 4 depicts a part view, similar to the view of FIG. 3, when the diametral expansion operation according to the invention has been carried out, the clearance  6  having therefore been eliminated and the tubes being pressed firmly against the interior surface of the corresponding holes  5 .  
     [0033] An operation of mounting a tube of a steam generator, during which the tube is prestressed according to the invention and according to a first embodiment will now be described with reference to FIGS. 5A to  5 E.  
     [0034] In a first step of the tubing method, the tubes are introduced into the tube plates and the wrapper of the steam generator which have been preassembled. At the end of this first step of the method, as depicted in FIG. 5A, the respective end parts  4   a  and  4   b  of the tubes  4  are introduced into the openings  5  of the respective tube plates  3   a  and  3   b . It must 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 tube plate  3   a  (or  3   b ) allows the tube to be introduced with ease through the openings in the tube plates, without damaging the exterior surface of the tube which, at the end of the operation, needs to have a perfect surface finish.  
     [0035] According to the invention, the size of the diametral (or radial) clearance  6  is tailored very precisely so as to achieve prestressing of the tube with a prestress of an extremely precise predetermined amount; this prestress results from a longitudinal contraction distributed uniformly over the length of the tube.  
     [0036] For that, first of all, the radial clearance needed in order to obtain the desired prestress value in the tube  4  is determined. This value of the radial clearance is obtained by calculations and by tests on a model comprising test tube fixing components similar to tube plates.  
     [0037] It has been possible to establish a very good correlation between the results obtained by calculation and the results of measurements obtained from the model.  
     [0038] The desirable actual radial clearance  6  between the tube and the through-holes  5  in the tube plates in which it is housed is obtained by matching the diameter with which the hole  5  is pierced to the outside diameter of the tube, taking account of the manufacturing tolerances on the tubes and reserving the desired clearance  6 .  
     [0039] As will be explained later on, the mounting clearances  6  for the various tubes may be designed to obtain an identical prestress in each of the tubes of the bundle, regardless of the position of the tube or, on the other hand, to alter the value of the prestress according to the position of the tube in the bundle or according to other particular conditions involved in the mounting of the tubes.  
     [0040] As depicted in FIG. 5B, having fitted the tube inside the steam generator, the end parts  4   a  and  4   b  of the tube being inside the openings  5  in the corresponding tube plates  3   a  and  3   b , diametral expansion is carried out inside the opening  5  of a tube plate (for example plate  3   a ) on an end  4 ′ a  of the tube in a zone adjacent to the external face of the tube plate  3   a . Diametral expansion of the end  4 ′ a  of the tube  4  may be carried out by expansion rolling, for example using a roller-type expander. At the end of the expansion-rolling operation, the fixing end  4 ′ a  of the tube  4  is pressed firmly against an external layer  3   a ′ of the plate  3   a  made of a material which is chemically compatible with the material of the tube  4   a  which is, for example, a nickel alloy. After the expansion rolling of the end  4 ′ a  of the tube  4 , a circular weld  7 , for example using fillerless TIG welding, is used to fix the end of the tube  4  in a permanent and sealed fashion to the external face of the tube plate  3   a.    
     [0041] As can be seen from FIG. 5C, diametral expansion is then performed on the end part  4   a  of the tube  4  which extends over the entire length of the hole  5  passing through the tube plate  3   a . This operation is carried out by introducing a tool for exerting hydraulic pressure 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 water, carrying out its hydraulic diametral expansion along the entire axial length of the hole  5 . At the end of the operation of a diametral expansion in the first tube plate  3   a , the end part  4   a  of the tube  4  is perfectly firmly pressed against the surface of the hole  5 , which means that no gap remains on the internal face of the tube plate  3   a  between the external surface of the tube and the opening  5  in the tube plate  3   a.    
     [0042] During the diametral expansion operation on the end part  4   a  of the tube, the contraction of the tube in its axial direction  8  is manifested only in a slight displacement of the lower end  4   b  of the tube which is free inside the opening  5  in the second tube plate  3   b . No prestress is therefore introduced into the tube.  
     [0043] As can be seen in FIG. 5D, in a later step of the tubing, a fixing end part  4 ′ b  of the tube  4  is expansion-rolled near the external face of the second tube plate  3   b , then this end  4 ′ b  of the tube  4  is welded with a circular weld  9  near the external face of the tube plate  3   b.    
     [0044] As can be seen in FIG. 5E, the second end part  4   b  of the tube  4  is then diametrally expanded inside the opening  5  in the second tube plate  3   b , over the entire axial length of the opening  5 . The part  4   b  of the tube is perfectly firmly pressed against the wall of the opening  5  and no gap remains between the external surface of the tube and the hole  5 , near the internal face of the tube plate  3   b.    
     [0045] During this second operation of hydraulic diametral expansion during which use is made of a hydraulic expansion tool, as in the case of the first end part  4   a  of the tube inside the first tube plate  3   a , deformation is produced by contraction of the tube in the axial direction, this deformation of the tube, which is fixed at its ends to the tube plates  3   a  and  3   b , producing a tensile prestress distributed along the entire length of the wall of the tube  4  between the tube plates  3   a  and  3   b.    
     [0046] Furthermore, because of the initial clearance  6  between the external surface of the end part  4   b  of the tube  4  and the opening  5  in the tube plate  3   b  has been tailored to obtain deformation by contraction of a desired amount, the prestress in the tube is itself at a perfectly determined value which has been obtained by calculation and checked by tests on models.  
     [0047] According to the invention, a prestress of a predetermined precise amount is therefore obtained by controlling the clearance and carrying out radial expansion along the entire length of the holes passing through the tube plates, which means that no gaps that generate crevice corrosion are present at the ends of the tube.  
     [0048]FIGS. 6A, 6B and  6 C depict three successive steps in an operation of fitting a tube during the tubing of the steam generator during which the tube is prestressed according to the method of the invention and according to a second embodiment.  
     [0049]FIG. 6A is similar to FIG. 5A and will not be described again. At the end of the operation depicted in FIG. 6A, the tube  4  is in place inside the steam generator, its end parts  4   a  and  4   b  being engaged in openings  5  in the corresponding tube plates  3   a  and  3   b.    
     [0050] The clearance  6  with which the ends  4   a  and  4   b  of the tube  4  are mounted inside the openings  5  of the tube plates  3   a  and  3   b  has been determined and set in such a way as, during subsequent diametral expansion operations which will be described hereinbelow, to produce the desired deformation of the tube in axial contraction and a desired predetermined level of prestress.  
     [0051] As depicted in FIG. 6B, in a second step of the tubing method, the ends  4 ′ a  and  4 ′ b  of the tube are expansion-rolled near the external faces of the tube plates  3   a  and  3   b , so as to press the ends of the tube  4  firmly against the external layers of the tube plates. The circular connecting welds  7  and  9  connecting the respective ends  4 ′ a  and  4 ′ b  of the tube  4  and the respective external faces of the plates  3   a  and  3   b  are then performed.  
     [0052] The tube  4  is therefore fixed at both ends to the plates  3   a  and  3   b , the separation of which is fixed because of the mounting of the plates on the structure of the steam generator.  
     [0053] As depicted in FIG. 6C, in a final step of the method, the two end parts  4   a  and  4   b  of the tube  4  are hydraulically diametrally expanded in the corresponding openings  5  in the tube plates  3   a  and  3   b  over the entire axial length of the openings  5  so that, at the end of the hydraulic expansion operations, the parts  4   a  and  4   b  are perfectly firmly pressed over the entire length of the holes  5 , no residual gap being present at the internal faces of the tube plates  3   a  and  3   b.    
     [0054] During the operation of diametral expansion of the end part  4   a  at the tube plate  3   a  and of the end part  4   b  at the tube plate  3   b , the tube  4 , which is fixed at both ends, undergoes deformation in contraction in the axial direction  8 , which means that the tube is prestressed during both of the diametral expansion operations.  
     [0055] The effects of the prestressing of the tube  4  during the first diametral expansion operation performed on the first end part  4   a  of the tube  4  and the second diametral expansion operation performed on the second end part  4   b  of the tube  4  combine so that the final prestress is the result of the cumulative effect of the two diametral expansion operations.  
     [0056] The initial clearance  6  with which the ends of the tube  4  are mounted in the openings in the tube plates is determined, by calculation and by tests on models, so that the total prestress obtained by the two diametral expansions is precisely the desired prestress.  
     [0057] In the case of the first embodiment, only the second diametral expansion operation generates the deformation by contraction of the tube and the prestress, whereas in the case of the second embodiment, the two diametral expansion operations combine to generate the prestress.  
     [0058] The method according to the invention therefore makes it possible very precisely to set the level of axial prestress on the tubes of a heat exchanger bundle while at the same time avoiding the presence of gaps at the internal faces of the tube plate or plates.  
     [0059] Furthermore, the required prestress for the tubes or the distribution of the prestresses on the tubes in the bundle can be obtained in a simple way by piercing holes in the tube plates with a perfectly defined and programmed diameter or with perfectly defined and programmed diameters, for example dependent on the position of the holes in the plane of the tube plates, bearing in mind the manufacturing tolerances on the sets of tubes used for tubing the heat exchanger.  
     [0060] The method according to the invention makes it possible in particular easily to obtain perfectly constant prestress for all of the tubes of the heat exchanger.  
     [0061] The method according to the invention also makes it possible for the prestresses of the tubes to vary, particularly according to their position in the bundle determined by the position of the openings accommodating the tube in the tube plates.  
     [0062] A suitable 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 prestress in all of the tubes of the bundle may manifest itself in flexural deformation of the tube plates subjected to high forces in their peripheral part. This effect can be corrected for by adopting an appropriate distribution of prestresses in the tubes of the bundle. Likewise, in-service deformations of the various parts of the heat exchanger, for example under the effect of differential expansion, may be limited by adopting a suitable distribution of the prestresses in the tubes of the bundle.  
     [0063] Furthermore, to limit the deformations of the tube plates during the mounting of the bundle, which may be performed by mounting tubes in succession from the central part towards the periphery or vice-versa from the periphery towards the central part, it is possible to adopt a plan for piercing the various parts of the tube plates to allow the levels of prestress and therefore the forces exerted by the tubes during mounting to be increased. The tube plates are thus preloaded at a lower level during the initial steps of mounting, and this affords a certain stiffening of the tube plates which become better able to withstand the prestresses and forces exerted by the mounted tubes in later steps of the tubing process.  
     [0064] Such distributions of the prestresses exerted in the tubes may be achieved by tailoring the diameters with which the holes passing through the tube plates are pierced. In particular, the diameters of the holes passing through the tube plates for mounting tubes at the beginning of tubing may be generally smaller than the diameters of the holes for mounting tubes at the end of tubing.  
     [0065] In the case of circular tube plates, the piercing may be achieved in such a way that the holes passing through the tube plates at their central part have different diameters from the holes passing through the tube plates in their peripheral part.  
     [0066] The invention is not strictly restricted to the embodiments which have been described.  
     [0067] The mounting and diametral expansion of the tubes may be performed using any type of tool suited to the dimensions of the tubes and to the characteristics of the steam generator bundle.  
     [0068] The invention can be applied to heat exchangers other than once-through heat exchangers, provided that the tubes have at least one straight part which can be subjected to longitudinal prestressing.