EP0027092B1 - Spacing grid for tubular bundles - Google Patents

Description

The present invention relates to a spacer grid for tubular bundles, more particularly intended by way of example for tubular bundles of steam generators of nuclear power plants.

The steam generators of pressurized water nuclear power plants comprise a long, straight or U-shaped tubular bundle, traversed by the primary fluid coming from the reactor core, and bathed by the secondary fluid which enters in liquid form to exit in the form of steam after having taken a significant part of the calories from the primary fluid.

For a correct and efficient operation of such an exchanger, it is necessary that, whatever the thermal constraints, the long and fine lubes of the bundle are well maintained at their optimal spacing, and that at the same time the circulation of the fluid secondary around the beam can be carried out without significant pressure drop.

For this, and more particularly still in the case of tabular U-shaped beams, the tubes are generally held radially at several points by maintein and spacing grids.

The most conventional grids are constituted by a simple flat sheet metal pierced with holes, at the pitch of the bundle of tubes and of diameter slightly greater than the outside diameter of the latter, to take account of the inevitable misalignments during assembly of the generator. These grids further comprise a mesh of additional orifices for the circulation of the secondary fluid; of course, an effort is made to keep sections sufficient to withstand the radial forces between the orifices, so that the fluid passage sections remain relatively small and restrict its circulation. In addition, this solution facilitates the appearance of corrosion in the transition zones between the actual bearing zone of the tube on the spacer and the crescent zone where the clearance appears due to the differences in diameters.

We have been able to improve the perforated plane grids by making trifoliated or quadrifolated orifices, for example, in which each tube passage orifice has only three or four incomplete circular bearings of diameter slightly greater than the outside diameter of the tube, and between them Ci are formed of the bloomings which constitute zones of passage of the secondary fluid. Such embodiments improve the circulation of the secondary fluid and limit the risks of corrosion, but require broaching machining, which is expensive and requires the use of special machine tools.

Complex grids were also produced, composed of several networks of parallel bars superimposed and crossed to form a mesh enclosing the tubes. The networks must then be assembled together by mechanical assembly or by welding. A system is thus obtained allowing good conditions for the circulation of the secondary fluid. But the need for many delicate assemblies increases the difficulty of assembly and its cost; furthermore, these systems are not all entirely satisfactory for resuming lateral forces.

The present invention allows the production of spacer grids of low manufacturing cost thanks to simple machining, without tight tolerances, and achievable on conventional machines, while ensuring good rigidity of the assembly, good support of the tubes and a large section. secondary fluid passage.

According to the invention, the spacer grid is constituted by a flat sheet having on each face a series of parallel grooves, of different direction for each face, with depths of grooves, respectively relative to each face, such that their sum is greater than l sheet thickness, the number, the pitch and the width of the grooves being determined as a function of the corresponding characteristics of the tube bundle.

• La figure 1 est une vue en perspective cavalière d'une portion de grille entretoise réalisée selon l'invention.
• La figure 2 est une coupe selon II-II de la figure 1.
• La figure 3 est une coupe selon III-III de la figure 1.
• La figure 4 est une coupe selon IV-IV de la figure 1.
• La figure 5 est une vue partielle homologue de la figure 1 dans laquelle les tubes sont représentés en place dans l'entretoise.
• Les figures 6, 7 et 8 représentent plus schématiquement des variantes de réalisation.

The invention will be better understood by referring to embodiments given by way of examples and represented by the appended drawings.

• Figure 1 is a perspective view of a portion of the spacer grid produced according to the invention.
• Figure 2 is a section on II-II of Figure 1.
• Figure 3 is a section on III-III of Figure 1.
• Figure 4 is a section on IV-IV of Figure 1.
• Figure 5 is a partial homologous view of Figure 1 in which the tubes are shown in place in the spacer.
• Figures 6, 7 and 8 show more schematically alternative embodiments.

We will first refer to all of Figures 1 to 4, where Figure 1 shows the angle of a thick sheet machined according to the invention to form a spacer grid. It is to facilitate the drawing and the description that a part of a rectangular sheet has been shown here, but of course, in particular for use in a steam generator, the outline of the sheet before machining will be more often circular.

On its upper face the sheet 1 is here hollowed out by a series of parallel grooves 2 with a U-shaped profile. On the lower face it is also hollowed out by a series of parallel grooves 3 with inverted U profile, the directions of the grooves 2 and 3 being perpendicular. The depths of grooves 2 and 3 are here greater than the half-thickness of the sheet metal, so that there is interpenetration of the grooves and appearance of lights at their intersections. If, as here, the grooves 2 and 3 are equidistant, of the same width and perpendicular, a regular network of orifices 5 is obtained, the profile of which is square when the sheet is viewed perpendicularly to one or other of its faces.

The sheet thus perforated remains a single piece, therefore particularly rigid, and is presented as the superposition with offset of two crossed networks of parallel bars 7 and 8, rigidly joined by connecting parts 9. It will be noted that the two series of grooves can be made on entirely conventional machine tools such as planers or milling machines, and with completely standard tools, that is to say under very economical conditions of production.

There has thus been produced, at low cost, a spacer grid which functionally behaves like the complex grids mentioned above, and which, like these, allows good flow of the secondary fluid. We can now refer to Figure 5, which corresponds to part of Figure 1, but with tubes 11 engaged in the network 5 of the plate. In the upper part of the plate, the tubes 11 are held between the parallel bars 7; in the lower part they are held between the bars 8. It can be seen that in the plane of the upper face of the spacer grid, the free cross-section of the secondary fluid around a tube corresponds to the surface between the rectangle 13 traced in phantom, and the circle 14, also in phantom and which represents the outer contour of the tube. It is the same in the plane of the lower face, where the rectangle would then be of the same surface but oriented at 90 ° relative to the rectangle 13. It can be seen that this surface is relatively large, and considered to be sufficient for the two networks bars 7 and 8 do not constitute an obstacle to the circulation of the secondary fluid. In the area between the two faces of the grid, the passage section is also important since it corresponds to the perforated parts in the side walls of each groove. It will also be noted that the passage sections between bars on the lower and upper faces of the spacer only depend on the characteristics of the tubes, that is to say on their pitch and their diameter; on the other hand the passage section inside the spacer depends on the relative values of the groove depths and the thickness of the raw sheet, so that the choice of these various parameters will depend on the residual section that is will want to leave the bars 7 and 8, that is to say the importance of the radial forces that we want to make them bear.

Of course, the invention is not strictly limited to the embodiment which has been described by way of example, but it also covers the embodiments which would differ from it only in details, in variant embodiments, or by the use of equivalent means.

Thus it is not strictly necessary that the two series of grooves are of the same depth, and it is sufficient to make the lights necessary for the passage of the tubes appear that the sum of the depths of each series is greater than the initial thickness of the sheet. . An inequality in the depths of grooves would lead to an inequality in the thickness of the bars 7 and 8, which can be sought if the transverse forces are likely to be predominant in a certain direction.

Among the various possible variants of detail, it would also be possible to provide small complementary simple machining operations, intended for example to better guide the tubes as in FIG. 6, or else to slightly increase the cross-sections of the fluid as in FIG. 7.

We could still imagine many variations by playing on the direction, the pitch or the relative widths of the grooves. FIG. 8 schematically illustrates an embodiment, for a triangular mesh of the tubes, where the two series of grooves instead of being perpendicular are inclined at 60 °; the tube passage orifices then have, seen from above, a diamond shape.

One could also plan to create more rigid zones by locally eliminating some grooves, or to further facilitate the circulation of the secondary fluid by profiling the external edges of the bars 7 and 8.

Finally, and without this limiting the possible variants, one could still imagine using two superimposed grids, each with half as many bars, and with alternating supports for each tube, which would further increase the cross section of the secondary fluid.

Claims (1)

1. Bracing structure for the tubular bundle of a heat exchanger for maintaining the transverse position of the tubes (11) whilst allowing longitudinal circulation of the fluid which contacts the outer surface of the tubes, characterised by the fact that it comprises a flat metal sheet (1) each face of which includes a series of parallel grooves (2, 3) having different directions for each face, the depths of said grooves (2, 3) respectively, relative to each face being such that 'their sum is greater than the thickness of said sheet (1), the number, spacing and width of the said grooves (2, 3) being determined as a function of the corresponding characteristics of the tubular bundle.

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