Patent Number: 047284790
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 illustrates the prior art construction of a high pressure seal formed at a seal table 11 between a guide tube stub 13 and a thimble 15. Guide tube stub 13 is the terminal portion of a guide tube 17 which penetrates the seal table 11 and is sealed thereto by a weld 19. Guide tube 17 and thimble 15 disposed therein extend in the direction of arrow 21 toward the bottom of a nuclear reactor vessel (not shown). Guide tube 17 and thimble 15 both penetrate the bottom of the vessel, with the guide tube being welded to the vessel and the thimble extending up through the core in the usual manner. The space between the outer surface of thimble 15 and the inner surface of guide tube 17 is subjected to the normal operating pressure of the reactor vessel and therefore the end 23 of the guide tube stub must be sealed to the thimble by means of a high pressure seal. Heretofore, the seal has been formed as shown in FIG. 1 by a standard compression seal comprised of a reducer union 25 having a main body 26 with a throughbore 27 through which the thimble 15 passes, and oppositely directed projections 28 and 29. Projection 28 forms a socket for receiving the end region of the guide tube stub 13 and has exterior threads 30 and a compression surface 31 which is inclined with respect to the surface of the guide tube stub. A lock nut 33 is positioned on the guide tube stub 13 for engaging the threads 30 of projection 28 for compressing a conventional ferrule 35 between the guide tube stub 13 and compression surface 31 of projection 28 to form a high pressure seal in a manner well understood by those skilled in the art. Similarly, at the other end of reducer union 25, projection 29 has exterior threads 37 which cooperate with a lock nut 39 for compressing another conventional ferrule 41 between thimble 15 and a corresponding compression surface 43 of projection 29 to form a further high compression seal. During a refueling operation, it is necessary to retract the thimble a sufficient distance to remove it from the nuclear core (not shown) disposed in the reactor vessel (not shown). In the past this has been accomplished by lowering the water in the vessel to a level below that of seal table 11 so that there is no water pressure at the seal table, loosening lock nut 33, retracting thimble 15, forming a low pressure seal as will be described in connection with FIG. 2, and raising the water level in the refueling canal to the required level for the refueling operation. Referring to FIG. 2 wherein the components corresponding to those in FIG. 1 are given like reference numerals, the low pressure seal heretofore employed has been formed by placing a split rubber ring 45 over thimble 15 so as to abut against the end 23 of guide tube stub 13. The outside diameter of rubber ring 45 substantially corresponds to the outside diameter of guide tube stub 13 and the inner diameter of rubber ring 45 substantially corresponds to the outside diameter of thimble 15. Thereafter, a retaining cap 47 having an axial slot (not shown) and a projection 49 corresponding to projection 28 in FIG. 1, is slipped over guide tube 15 and cooperates with lock nut 33 to compress rubber ring 45 against the end of the guide tube stub and against the surface of thimble 15 to form a low pressure seal. The ferrule 35, which has a tight friction fit about thimble 15 as a result of its initial compression in the high pressure seal formed according to FIG. 1, is used as an anchor point for lock nut 33 in pulling retaining cap 47 in against rubber ring 45. After the refueling operation is completed, the water level must again be temporarily dropped to below the level of the seal table so that the low pressure seal of FIG. 2 may be disassembled and the high pressure seal of FIG. 1 reassembled. The use of two separate seal arrangements for effecting the high and low pressure seals, respectively, is not only time consuming in that it requires a temporary reduction of the water level while one seal is being disassembled and the other seal installed, but also there is the risk that in disassembling and assembling the seals, the metal to metal seal provided by the ferrule 35 against the guide tube stub 13 may become imparied. Generally, when this happens, the surface of the guide tube stub beneath the ferrule is no longer suitable for effecting a compression seal and therefore a portion of the guide tube stub may have to be cut off. This presents a problem because the guide tube stub extends beyond the seal table only by several inches and therefore it is conceivable that the entire guide tube may have to be replaced in order to provide a sufficient length of guide tube stub to accommodate a new high pressure compression seal. The present invention overcomes these problems by the provision of a high pressure seal fitting body which incorporates a low pressure seal arrangement as shown in the embodiment of the invention illustrated in FIGS. 3 and 4, respectively, and wherein components common to those illustrated in FIGS. 1 and 2 are identified by the same reference numeral. Referring first to FIG. 3, there is provided in accordance with the invention a fitting body 51 interposed between reducer union 25, which is sealed to thimble 15 as in FIG. 1, and lock nut 33 and ferrule 35 which are disposed on the guide tube stub also as in FIG. 1. The fitting body 51 comprises two component 51a and 51b, which are welded together at 53. Component 51a comprises a socket 55 which is configured to receive the end region of guide tube stub 13 and to cooperate with lock nut 33 and ferrule 35 for forming a high pressure compression seal in the usual manner. At the other end of fitting body 51, component 51b is provided in the region of its free end with an integral projection 57 which projects outwardly around the entire circumference of component 51b and is shaped to simulate a conventional ferrule. Accordingly, projection 57 has a radial surface 58 which serves as a stop for a lock nut 59 and an inclined surface 60 which is compressed against the corresponding compression surface 31 of reducer union 25 to form a high pressure seal therewith. Fitting body 51 has an inner surface 61 defining a through passage for slidably accommodating the thimble 15. Inner surface 61 is provided in the region of the free end of component 51b with a first annular recess 63 for accommodating a rubber seal ring 65 which has a U-shaped axial cross section, and a second annular recess 67 which has a greater diameter than the first recess 63 for accommodating a stainless washer 69. Between annular recess 67 and the free end of component 51b the inner surface 61 is provided with threads for accommodating a seal nut 71. Seal ring 65 is a conventional U-shaped cup seal which may be made for example from Buna N material, having a durometer hardness 90, such a seal being commercially available from Buckeye Rubber & Packing Company, Cleveland, Ohio. Seal nut 71 is provided with a suitable slot (not shown) in its axial end facing reducer union body 26 for receiving an appropriate tool for tightening nut 71 against washer 69, which in turn presses against the adjacent axial end of U-shaped seal 65. Recess 63 in which U-shaped seal 65 is disposed defines a radial surface 73 which presents an axial abutment for the other end of U-shaped seal 65, so that seal 65 is essentially held between axial abutment 73 and washer 69. Additionally, the annular recess 67 in which washer 69 is seated defines a radial surface 75 which presents an axial abutment with respect to washer 69 so that washer 69 is prevented from compressing cup-shaped seal 65 beyond its design limits. Use of the fitting body 51 in the manner illustrated in FIG. 3 thus permits the guide tube stub 13 to be sealed to the thimble 15 with respect to high operating pressures by virtue of the high pressure compression seals formed by lock nut 33, ferrule 35 and socket 55; by lock nut 59, protrusion 25; and lock nut diameter projection 28 of reducer union 25; and lock nut 39, ferrule 41 and the small diameter projection 29 of reducer union 25. Further, the seal fitting 51 has a low pressure seal built into its end adjacent reducer union 25 so that, when lock nut 59 is loosened and the thimble 15 retracted, the U-shaped cup seal 65 maintains a low pressure seal between the inner surface 61 of component 51b and the thimble 15. It may be noted that the high pressure compression seal formed at the guide tube stub 13 by lock nut 33, ferrule 35 and socket 55 need never be disassembled when releasing the high pressure seal, thus avoiding the possibility of marring the surface of the guide tube stub 13. If by chance the projection 57 becomes damaged requiring the replacement of component 51b, the weld 53 may simply be broken down in order to permit removal and replacement of component 51b after which the weld 53 is again made. It may be noted that the weld 53 should be made only after the lock nut 59 is slipped over the component 51b, otherwise it would not be possible to install the lock nut 59 in the position shown. When it is necessary to reassemble the high pressure seal for normal operating pressures of the reactor, the thimble is slid through the fitting 51 toward the reactor until the reducer union 25 is in the correct position juxtaposed the projection 57 so that lock nut 59 can be secured to re-create the high pressure compression seal at that point. FIG. 4 illustrates an embodiment of the invention which is similar to FIG. 3 except that the component 51a of FIG. 3 which is adapted to form a compression seal has been replaced by a socket 77 which is welded to the guide tube stub 13 as shown by weld joint 79. The embodiment of FIG. 4 would again permit the component 51b to be replaced by simply breaking the weld 53 in order to release component 51b from the end of the guide tube stub. A new component 51b could then be installed on the guide tube stub via a new weld joint 53. It will be understood that the above description of the present invention is susceptible to various modifications, changes and adaptations, and the same are intended to be comprehended within the meaning and range of equivalents of the appended claims.