Patent Number: 051679051
Section: summary

BACKGROUND OF THE INVENTION This invention generally relates to apparatus for sealing conduits and more particularly relates to a foldable nozzle dam having a foldable extrusion-resistant seal or gasket for sealing conduits, such as the primary nozzles of a nuclear steam generator. Before discussing the current state of the art, it is instructive first to briefly describe the structure and operation of a typical nuclear steam generator. In this regard, a typical nuclear steam generator generally comprises a shell and a plurality of U-shaped heat transfer tubes disposed in the shell, the U-shape of the tubes defining a first tube leg portion and a second tube leg portion interconnected by a U-bend tube portion. The shell defines an inlet plenum and an outlet plenum therein such that the first tube leg portion of each U-shaped tube is in fluid communication with the inlet plenum and the second tube leg portion of each tube is in fluid communication with the outlet plenum. Also in fluid communication with the inlet plenum is an inlet primary nozzle attached to the shell for delivering a radioactive primary fluid into the inlet plenum. Moreover, in fluid communication with the outlet plenum is an outlet primary nozzle attached to the shell for allowing the primary fluid to exit the outlet plenum and thus exit the steam generator in the manner discussed hereinbelow. In addition, the steam generator shell also includes a plurality of relatively small diameter manway openings therethrough for allowing access to the inlet and outlet plena so that maintenance can be performed in the steam generator. In this typical nuclear steam generator, each manway opening has a diameter substantially less than the inside diameters of the inlet and outlet nozzles. During operation of the nuclear steam generator, the radioactive primary fluid, which is heated by the fission reaction of a nuclear reactor core, flows through the tubes as a nonradioactive secondary fluid of lower temperature circulates around the tubes. More specifically, the primary fluid flows from the nuclear reactor core where it is heated, through the inlet nozzle and into the inlet plenum. The primary fluid then flows into the first tube leg portion of each heat transfer tube, through the U-bend portion of each tube, out the second tube leg portion of each tube and then into the outlet plenum, whereupon it exits the outlet nozzle and the steam generator. Moreover, as the primary fluid exits the steam generator, it is returned to the nuclear reactor core to be reheated. Of course, as the primary fluid flows through the heat transfer tubes, it gives up its heat to the secondary fluid circulating around the tubes for producing steam in a manner well known in the art. Such a typical nuclear steam generator is more fully disclosed in U.S. Pat. No. 4,079,701 entitled "Steam Generator Sludge Removal System" issued Mar. 21, 1978 to Robert A. Hickman et al. Periodically, it is necessary to shut down the nuclear reactor core for refueling. At that time, it is cost advantageous also to simultaneously perform maintenance on the steam generator. During such maintenance activities, a reactor cavity, which encloses a reactor vessel containing the reactor core, is partially drained of primary fluid to a level that is below the elevation of the inlet and outlet nozzles of the steam generator. Of course, for safety reasons the nuclear reactor core is not uncovered when the reactor cavity is partially drained of primary fluid. Thus, it will be appreciated that this process of partially draining the reactor cavity to an elevation that is below the inlet and outlet nozzles of the steam generator also drains the heat transfer tubes as well as the inlet and outlet plena of the steam generator. After the steam generator tubes and the inlet and outlet plena are drained of primary fluid, nozzle dams are inserted through the relatively small diameter manways and installed in the mouths of the inlet and outlet nozzles to block the nozzles. Once these dams are installed, the reactor cavity can be refilled with primary fluid for the refueling operation, the reactor cavity being refilled to a level above the elevation of the inlet and outlet plena. Therefore, refilling of the reactor cavity with primary fluid can be accomplished without interfering with maintenance activities being performed in the steam generator plena because the nozzle dams which block the inlet and outlet nozzles prevent the radioactive primary fluid from rising into the inlet and outlet plena. Moreover, simultaneously performing reactor refueling and steam generator maintenance activities reduces the total time the nuclear reactor core is shut down, thereby recapturing revenue that would otherwise be lost when reactor refueling and steam generator maintenance are performed in seriatim. In addition, as stated hereinabove, the manway openings have a diameter smaller than the inside diameter of the inlet and outlet nozzles. Therefore, a problem in the art is to provide a nozzle dam that is not only capable of passing through the relatively small diameter manways but also capable of being disposed across the relatively larger inside diameters of the inlet and outlet nozzles to block the nozzles. Of course, once installed across the inlet or outlet nozzle, the nozzle dam should be fluid-tight so that primary fluid will not rise into the inlet and outlet plena to interfere with maintenance activities being performed in the steam generator. In this regard, the nozzle dam may include at least two parts sized to pass through the inlet or outlet nozzle, the two parts having a seal or gasket therebetween to seal the nozzle dam so that the nozzle dam is fluid-tight. The seal or gasket, which is intended to be clamped between the two parts for creating a seal therebetween, may have at least one aperture for passage of clamping means therethrough. However, applicants have observed that the aperture of the seal or gasket may extrude away from the clamping means when the two parts are tightly clamped together, thus enlarging the fluid flow path defined by the aperture surrounding the clamping means. This is undesirable because such enlargement of the flow path compromises the ability of the seal or gasket to perform its intended function of providing a nozzle dam that is fluid-tight. Therefore, another problem in the art is to provide a nozzle dam having a seal or gasket that resists extrusion away from such clamping means so that the nozzle dam is fluid-tight. Steam generator nozzle dams having seals are known. One such nozzle dam is disclosed in U.S. Pat. No. 4,637,588 entitled "Non-Bolted Ringless Nozzle Dam" issued Jan. 20, 1987 in the name of John J. Wilhelm et al. and assigned to the assignee of the present invention. This patent discloses a nozzle dam having one or more seal assemblies, each seal assembly including a foldable circular seal plate encircled with an inflatable seal which is disposable in frictional engagement with the nozzle wall. However, this patent does not appear to disclose a nozzle dam having an extrusion-resistant seal or gasket. Another nozzle dam having a seal assembly is disclosed in U.S. Pat. No. 4,671,326 entitled "Dual Seal Nozzle Dam and Alignment Means Therefore" issued Jun. 9, 1987 in the name of John J. Wilhelm et al. and assigned to the assignee of the present invention. This patent discloses a seal assembly including a foldable circular seal plate having a center section hingedly connected to two side sections. However, this patent does not appear to disclose a nozzle dam having an extrusion-resistant seal or gasket. An extrusion-limiting seal or gasket is disclosed in U.S. Pat. No. 4,181,313 entitled "Seals And Gaskets" issued Jan. 1, 1980 in the name of Edward F. H. B. Hillier et al. According to this patent, a seal or gasket, which has at least one aperture providing a fluid passageway and which is intended in use to be clamped by clamping means between surfaces of two parts to be sealed, comprises an elastically-compressible material and a relatively-incompressible material bonded therein, the relatively-incompressible material forming an extrusion-limiting barrier which extends at least partway around the aperture. However, this patent does not appear to disclose a foldable nozzle dam having a foldable extrusion-resistant seal or gasket. An anti-extrusion sealing device is disclosed in U.S. Pat. No. 4,468,042 entitled "Anti-Extrusion Sealing Device With Interlocked Retainer Portions" issued Aug. 28, 1984 in the name of Aaron J. Pippert et al. According to this patent, a relatively soft body, including a sealing portion, and a harder body, which serves as an anti-extrusion device for the soft body, are permanently joined together by mating mechanical interlock formations. However, this patent does not appear to disclose a foldable nozzle dam having a foldable extrusion-resistant seal or gasket. Thus, although the above-recited patents may disclose nozzle dams and anti-extrusion seal devices, these patents do not appear to disclose a foldable nozzle dam having a foldable extrusion-resistant seal or gasket, the nozzle dam being foldable for passing through the relatively small diameter steam generator manway and being unfoldable for placement across the larger diameter of the nozzle in combination with a foldable seal or gasket attached to the nozzle dam, the seal or gasket being extrusion-resistant for providing a nozzle dam that is fluid-tight. Therefore, what is needed is a foldable nozzle dam having a foldable extrusion-resistant seal or gasket for sealing conduits, such as the primary nozzles of a nuclear steam generator. SUMMARY OF THE INVENTION A typical nuclear power plant includes a nuclear reactor core for producing heat and a steam generator in fluid communication with the nuclear reactor core for generating steam. The steam generator includes inlet and outlet primary nozzles attached to the steam generator. At times it is necessary to perform maintenance in the steam generator. To safely and satisfactorily perform this maintenance, it is prudent first to seal or block the inlet and outlet primary nozzles of the steam generator. Therefore, disclosed herein is an apparatus for sealing or blocking conduits, such as the primary nozzles of a nuclear steam generator. In general, the apparatus comprises an annular bracket surrounding the open end of the conduit, the bracket having a circular opening transversely therethrough and a top surface thereon and having a periphery portion sealingly attached to the open end of the conduit, the periphery portion having a plurality of spaced-apart holes transversely therein; a cover plate mounted on the bracket for covering the opening of the bracket, the cover plate having a periphery portion having a plurality of spaced-apart bores transversely therethrough; a plurality of elongated fasteners engaging the holes of the bracket and the bores of the cover plate for tightly connecting the cover plate to the bracket; and extrusion-resistant seal means interposed between the bracket and the cover plate for providing a fluid-tight seal therebetween, the seal means having a periphery portion having a plurality of spaced-apart transverse apertures for receiving the fasteners therethrough, the seal means intimately engaging the top surface of the bracket for providing a fluid-tight seal between the cover plate and the bracket, the seal means being configured to resist extrusion away from the fasteners as the cover plate is tightly connected to the bracket. More specifically, the invention comprises an annular bracket sealingly attached to the rim of the open end of the primary nozzle, the bracket having a plurality of threaded holes therein. Mounted atop the bracket is a foldable nozzle dam that includes a generally arcuate-shaped first side section hingedly connected to a generally arcuate-shaped second side section. Each arcuate-shaped side section includes a cut-out centrally formed along the straight portion thereof, the cut-outs defining a generally rectangular opening when the hinged side sections are moved into the same plane with respect to each other. Covering the rectangular opening and removably connected to each side section is a generally rectangular center section. When the side sections are moved into the same plane with respect to each other and when the center section is connected to the side sections, the center section and the side sections define a nozzle dam having a generally circular configuration for covering the circular opening defined by the bracket. The periphery portions of the center section and of the side sections have bores therethrough capable of being coaxially aligned with the threaded holes in the bracket. A plurality of threaded bolts extend through the bores in the nozzle dam and into the threaded holes in the bracket for removably connecting the nozzle dam to the bracket. Interposed between the nozzle dam and the bracket is an elastomeric foldable extrusion-resistant seal member for providing a seal therebetween so that the primary nozzle may be suitably sealed. The periphery of the seal member has a plurality of apertures for receiving each bolt therethrough. The seal member is attached, such as by contact cement, to the side sections of the nozzle dam. In one embodiment of the seal member, the seal member comprises in transverse section a plurality of layers bonded or laminated together, such as by a suitable adhesive. In this embodiment of the seal member, a generally annular first layer is sealingly attached to the bottom of a generally circular second layer. The first layer has an outside diameter that is substantially the same as the outside diameter of the nozzle dam. The second layer has a diameter that is also substantially the same as the outside diameter of the nozzle dam and covers the full surface of the nozzle dam. The first layer is of a material that is softer than the second layer. In this regard, the first layer may be EPDM (ethylene propylene diene monomer) rubber having a Shore A durometer hardness of between 40 and 60. The second layer may be EPDM rubber having a Shore A durometer hardness of between 60 and 80. The dual hardness of the seal member allows it to be extrusion-resistant and also allows it to intimately engage the top surface of the bracket for creating an effective seal between the nozzle dam and the bracket, which nozzle dam and/or bracket may have surface imperfections that could otherwise lead to leakage. In another embodiment of the seal member, the seal member comprises in transverse section a plurality of regions fused or molded together rather than laminated layers bonded together. In this embodiment of the seal member, a generally annular first region is molded with a generally circular second region. The first region has an outside diameter that is substantially the same as the outside diameter of the nozzle dam. The second region has a diameter that is also substantially the same as the outside diameter of the nozzle dam. The first region is of a material that is softer than the second region. In this regard, the first region may be EPDM rubber having a Shore A durometer hardness of between 40 and 60. The second region may be EPDM rubber having a Shore A durometer hardness of between 60 and 80. The molded configuration of the molded seal member allows it to be mounted on the nozzle dam assembly more easily than the laminated seal member. The molded seal member is formed in a shop-fabricated, press-cure process. The laminated seal member, on the other hand, requires that the annular first layer be formed separately from the second layer. The next step for assembling the laminated seal member is to mat and glue the two layers together. This step for producing the laminated seal member requires that the first layer be precisely matted and glued to the second layer. The gluing operation must result in laminated layers that are not buckled and that do not contain air bubbles therebetween; that is, the laminated layers must be substantially uniformly glued to each other across their entire interface. If the laminated layers are not precisely matted and glued, then the layers must be separated, the gluing removed and the operation repeated. Thus, the laminated seal member requires more labor-intensive assembly steps than the molded seal member and is therefore more costly to produce. During installation in the primary nozzle, the nozzle dam is drawn toward the bracket as the bolts threadably engage the holes of the bracket. As the nozzle dam is drawn toward the bracket, a compressive force will act perpendicularly on each opposing face or side of the seal member because the seal member is interposed between the nozzle dam and the bracket. This compressive force acting perpendicularly against each side of the seal member will tend to cause the seal member to extrude laterally outwardly away from each bolt. Such lateral extrusion of the seal member as the nozzle dam is tightly clamped or connected to the bracket will tend to enlarge the aperture that defines the annular gap or fluid flow path around each bolt. This is undesirable because creation of such an enlarged flow path will compromise the ability of the seal member to perform its intended function of providing a nozzle dam that is fluid-tight. Thus, according to the invention, the seal member is configured to be extrusion-resistant so that the seal member will not laterally extrude away from each bolt in a manner that adversely enlarges the fluid flow path around each bolt. An object of the present invention is to provide an apparatus for sealing or blocking the open end of a conduit. Another object of the present invention is to provide a foldable extrusion-resistant seal or gasket for sealing the primary nozzles of a nuclear steam generator. A feature of the invention is the provision of an extrusion-resistant laminated seal member having in transverse cross-section a plurality of bonded layers of differing hardnesses. Another feature of the invention is the provision of an extrusion-resistant molded seal member having in transverse cross-section a plurality of fused or molded regions of differing hardnesses. An advantage of the invention is that the configuration of the seal member allows it to be extrusion-resistant so that the seal member will not substantially laterally extrude away from bolts passing therethrough so that the fluid flow path surrounding the bolts is not adversely enlarged. These as well as additional objects, features and advantages of the present invention will become apparent from the following detailed description of the preferred embodiments with reference to the several figures.