Patent Number: 053902190
Section: description

DETAILED DESCRIPTION OF AN EMBODIMENT OF THE INVENTION FIG. 2 shows the intermediate part of the steam generator of FIG. 1. The upper part 10 of the tube bundle envelope 8 surrounds the upper part 9 of the bundle of tube 3. It can be seen that at this level the pressure envelope 7 widens, i.e., it has a conical shape, leaving a space 14, known as the water return, between the pressure envelope 6 and the outside of the envelope 8. It is in this space that the water distributed by the supply ring mixed with the recirculated water is piped into the lower part of the steam generator and that the device according to the invention can be located. FIG. 2 shows circular sectors of grids 16 placed on the outer surface 17 of the upper part 10 of the nest bundle envelope 8 and extending through the water return 14 in order to be flush with the inner surface 18 of the pressure envelope 7. The grids 16 are shown in the horizontal position, but this is only an example. The functional aspect of the position of the grids 16 is that they must extend over virtually the entire cross-section of the water return 14 in order to have an optimum efficiency. The grids 16 are preferably positioned in the water return 14 by means of brackets 19, which are preferably fixed to the outer surface 17 of the upper part 10 of the tube bundle envelope 8. The width of the grids 16 is very slightly less than the width of the water return 14 at this location leaving, a space or clearance 20 between the grids 16 and the inner surface 18 of the pressure envelope 7. The function of this clearance 20 is to permit expansions due to temperature differences between the pressure envelope 7, the tube bundle envelope 8, the grids 16 and the brackets 19. FIG. 2 also shows the outer strips 21 fixed to the outer periphery of the grids 16 to prevent the reintroduction by the secondary water of objects trapped on the grids 16, between the latter and the pressure envelope 7. Inner strips 22 are positioned horizontally between the grids 16 and the outer surface 17 of the upper part 10 of the tube bundle envelope 8. Thus, the clearance left between grids 16 and the envelope 8, which can be conical, cylindrical or the like, is filled. In the same way, outer strips 30 fixed to the inner surface of the pressure envelope 7 fill the clearance left between the grids 16 and the pressure envelope 7. Assuming that the tubes of the bundle 3 (FIG. 1) are spaced by a distance, the meshes of the grids 16 are dimensioned in such a way as not to permit the passage of migrating bodies or objects liable to jam or become wedged between two tubes of the bundle 3. In other words, the meshes of the grids 16 must not permit the passage of objects whose size exceeds the distance separating the tubes. Thus, the maximum width of the meshes of the grids 16 is less than the said distance. The grids 16 are shown with a certain thickness E, i.e., height. Bearing in mind that these grids 16 can be made of metal, they can therefore have a sufficient mechanical strength to withstand the various stresses, in particular top to bottom vertical stresses. Thus, the grids 16 must firstly withstand the hydraulic stresses imposed by the mixture of the water from the supply ring and the recirculated water descending between the pressure envelope 7 and the tube bundle envelope 8, during both normal and emergency operation. Moreover, during the installation of the steam generator, the grids 16 can carry one or more operators, thus facilitating the installation of the various steam generator components, together with inspection and maintenance operations. In this case, the grids 16 form a type of all-round path around the upper part 10 of the tube bundle envelope 8. Without damage, i.e., breaking or deformation, grids 16 must also be able to withstand the impacts of migrating bodies. As shown in FIG. 3, the mesh for the grids 16 may be square. Without taking into account the scale in which these meshes are shown, the diagonal of the square of each of the meshes must be smaller than the distance separating the tubes of the bundle. This is an order of magnitude for indicating that the size of the grid meshes of the device according to the invention must be such that they do not permit the passage of objects liable to jam or become wedged between the tubes of the bundle. This square mesh grid can be constituted by a first series of parallel metal sheets 23, defining two opposite sides of the meshes of the same column or line, and a second series of metal sheets 24 perpendicular to sheet 23 and fitted in the latter in order to define the two other opposite sides of the meshes. The fitting can be achieved by providing slots 29 on half the height in each of the metal sheets of each series at the point of intersection of the sheets 23 and 24. FIG. 4 shows grids having triangular meshes. It is thus possible to construct a thick grid formed by a series of bent or undulated metal sheets welded on either side of two planar sheets. The largest side of the triangle must be smaller than the distance separating the tubes of the bundle. FIG. 5 shows a third embodiment of the grids constructed in a rigid plate 26, in which holes 27 are made, e.g., in a staggered manner. The diameter of the holes 27 must obviously be smaller than the distance separating the tubes of the bundle. The embodiments of the meshes described in FIGS. 3, 4 and 5 are in no way limitative with respect to the construction and shape of the meshes forming the grids 16. FIG. 6 shows in detail a way of attaching the grids 16 to their brackets 19, which have a T-shaped profile. The bracket 19 extends radially with respect to the steam generator tube bundle envelope and can be welded to the latter. The grids 16 can be fixed to the brackets 19 by screwing, e.g., using bolts 28, which makes it possible to dismantle them. They can also be welded to the brackets 19. Thus, it is possible to fix to each bracket 19 two adjacent grids 16, with or without leaving a space between them. The T-shape of the brackets and in particular the upper part of the T makes it possible to close any radial clearances left between the grids 16. Therefore the grids 16 make it possible to form a type of trapping or filtering grating for migrating bodies from the upper inner circuits of the steam generator, the feed water system of the secondary circuit or which are simply circulating within the steam generator. The invention is not limited to the embodiments described hereinbefore. Any system equivalent to a grid and fulfilling the same function can be used. The supporting procedure can also differ and can involve welding to other parts or using other means (e.g., screwing). The height position of the device within the steam generator can be at a random level between the secondary water supply ring and the tube plate.