Patent Number: 059354399
Section: description

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to the drawings, in which like reference characters indicate like parts throughout the several views, FIG. 1 illustrates a suction system 9 in accordance with a first preferred embodiment of the present invention. The suction system 9 includes an elongated suction strainer 40 formed by connecting two suction strainer sections 10 end-to-end in series, and includes system suction piping 36 to which the strainer 40 is connected. FIG. 2A illustrates a first preferred embodiment of the suction strainer section 10 of the present invention. However, because of the number of different types of BWR and PWR nuclear power plants, and therefore the number of different suppression pool and/or containment area configurations, it is to be understood that no one elongated suction strainer 40, 40', 40", or 40'" and no one suction strainer section 10 or 10' can be said to be the preferred embodiment. For this reason, the present invention will be described by referring to several preferred embodiments. Reference numeral 10 of the various Figs. illustrates a first embodiment of a suction strainer section of the present invention. As seen most clearly in FIG. 2A, the suction strainer section 10 extends along a longitudinal axis 38 and comprises an internal core tube 12 and an exterior filtering structure 20. The internal core tube 12 includes a core wall 14 that defines a generally cylindrical core chamber 16. The core wall further includes a plurality of fluid inlets 18 which provide access to the core chamber 16 from the exterior filtering structure 20. The exterior filtering structure 20 is attached to and encircles the entire length of the core wall 14. A plurality of perforations 22 spaced throughout the exterior filtering structure 20 permit water from a fluid reservoir (for example, stored within the suppression pool of a BWR nuclear power plant) to pass through the exterior filtering structure 20, and together with the fluid inlets 18, place the fluid reservoir in fluid communication with the core chamber 16. The perforations 22 are sized and spaced so as to allow the passage of water and other fluids while preventing the passage of solids and other particulate matter. The suction strainer section 10 of FIG. 2A further comprises a core tube extension 15 extending from each end of the internal core tube 12. FIG. 2B shows a second preferred embodiment of a suction strainer section 10'. The core tube extension 15' of this section 10' includes a flange 17 which will be described more fully below. The core tube extensions 15 and 15' are not encircled by the exterior filtering structure 20, thus the core tube extensions 15 and 15' are solid structures and facilitate the connection of one suction strainer section 10 or 10', to another suction strainer section 10 or 10', respectively. The core tube extensions are also adapted for connection to other structures, as well. As shown in FIGS. 2A, 2B, only a portion of the perforations 22 are depicted in the exterior filtering structure 20. Likewise, only a portion of the fluid inlets 18 are depicted in the core wall 14. In both instances, this is done in an effort to clarify the view. The perforations 22 and fluid inlets 18 are actually spaced throughout the entire surface areas of the exterior filtering structure 20 and core wall 14, respectively, as described in detail in U.S. Pat. No. 5,696,801, which has been incorporated herein by reference. As seen more clearly in FIGS. 1, 3 and 4, the exterior filtering structure 20 includes a plurality of concentric plate assemblies 24 spaced sequentially along the length of the core tube 12. As shown in FIG. 3, the suction strainer sections 10 (of FIG. 2A) are aligned in series along a common longitudinal axis 38 and connected to form a first embodiment of an elongated suction strainer 40 used in the suction system 9 of the present invention. The core tube extensions 15 are aligned end to end and welded together at the connection point 28. It is to be understood that a clamp or other fastening device can be used to make this connection. FIG. 4 shows a pair of suction strainer sections 10' (of FIG. 2B) aligned and connected in series along a common longitudinal axis 38 to form a second embodiment of an elongated suction strainer 40'. The suction strainer sections 10' of FIG. 4 differ from the suction strainer sections 10 of FIG. 3 in that the core tube extensions 15' of FIG. 4 include flanges 17. Thus, as shown in FIG. 4, adjacent flanges 17 of adjacent core tube extensions 15' are aligned along a connection point 29 to facilitate connection of the suction strainer sections 10'. The adjacent flanges 17 are typically bolted together to form the elongated suction strainer 40', but they can be welded or attached by use of other mechanisms commonly known in the art. A seal (not shown) is, alternately, provided between the flanges 17 to prevent fluid from entering the core tube at the connection point 29; however, such a seal is not required. In FIG. 1, the elongated suction strainer 40 is shown supported at its ends by the suction pipe 36 of the suction system 9. This configuration represents a first preferred embodiment of the suction system of the present invention. The suction pipe 36 extends through the reservoir wall 34 (for example, the suppression pool wall of a BWR nuclear power plant), and is welded to the core tube extensions 15 remote from the connection point 28. In this embodiment, the elongated suction strainer 40 is suspended at its ends within the water of the suppression pool. Due to the tensile strength of the internal core tube 12, the elongated suction strainer 40 can withstand the hydrodynamic forces following a LOCA while supported in this manner. The length of the elongated suction strainer 40 of this embodiment is limited only by the load limits of the suction pipe 36 supporting the system 9 and the dimensions and construction of the suppression pool itself. In FIG. 5, a second embodiment of the suction system 9' of the present invention is shown. In this embodiment, an elongated suction strainer 40' is formed by connecting the flanges 17 of the core tube extensions 15' of adjacent suction strainer sections 10' as described above with respect to FIG. 2B. One end of the elongated suction strainer 40' is then connected to an end cap 42, while the other end of the suction strainer 40' is connected to a pipe extension 44. As shown in FIG. 5, one end of the end cap 42 and one end of the pipe extension 44 are sized and shaped to be bolted or otherwise attached to the flange 17 of the core tube extension 15' on each end of the elongated suction strainer 40'. The other ends of the end cap 42 and pipe extension 44 are connected to structural members 46 extending into the pool from the wall 34 of, for example, the suppression pool of a BWR nuclear reactor. The suction pipe 36 of the suction system 9' extends through the wall 34 and is connected to the elongated suction strainer 40' by 90.degree. connections in the pipe extension 44. While the suction strainer 40' of this embodiment of the suction system 9' is still supported at its ends, it is not supported at its ends by the suction pipe 36. Instead, it is supported at its ends by structural members 46 (known as ring girders) within the suppression pool. In this case, ring girders are the structural members 46 supporting the elongated suction strainer 40'. Their function is to transfer hydrodynamic forces applied to the elongated suction strainer 40' as a result of a LOCA, to the suppression pool wall 34. This minimizes or prevents the transfer of hydrodynamic loads to the suction pipe 36. An elongated suction strainer 40' having increased length results. FIG. 6 shows a third embodiment of the suction system 9" of the present invention within a toroidal-shaped suppression pool 30 of a Mark I BWR nuclear power plant. FIG. 6 further shows the proximity of the downcomers 32 and elongated suction strainer 40". During a LOCA, high pressure steam is discharged from the downcomers 32 toward the elongated suction strainer 40". The hydrodynamic forces resulting from the super-turbulent water caused by this discharge is delivered to the elongated suction strainer 40", the suction pipes 36, the suppression pool wall 34 and other structural members within the suppression pool 30. The elongated suction strainer 40" of FIG. 6 represents a third embodiment of the elongated strainer of the present invention and is shown in more detail in FIG. 7. In this embodiment, the elongated suction strainer 40" is supported at both ends by structural members 46. The ends of the elongated suction strainer 40" are shown welded to structural members 46, but other attachment methods are possible. In this embodiment, the elongated suction strainer 40" is again formed by aligning several suction strainer sections 10 end-to-end in series along a common axis. However, a T-connection 50 is placed between and aligned with each of the adjacent suction strainer sections 10 to facilitate connection of the elongated suction strainer 40" to the suction pipe 36. As shown in FIGS. 8A and 8B, each T-connection 50 has a core tube portion 52 and a suction pipe portion 54. The core tube portion 52 includes a core wall surface 56 defining a bore 58 therethrough. The suction pipe portion 54 includes a solid suction wall surface 60 defining a channel 62 that opens into the bore 58. The core wall surface 56 further includes a plurality of spaced apertures 64 along the portion of the surface opposite the suction pipe portion. A plurality of partial plate assemblies 26 are spaced sequentially along and eccentrically connected to that portion of the core wall surface 56 having the apertures 64 therethrough. The partial plate assemblies 26 include a plurality of spaced perforations 27 similar to the perforations 22 in the concentric plate assemblies 24 of the suction strainer sections 10 and 10' described above. Thus, what is formed is a T-connection 50 that acts as a suction strainer section. Only a portion of the apertures 64 and perforations 27 are shown in FIGS. 8A and 8B in an effort to clarify the view. The perforations 27 are actually spaced throughout the entire surface area of the partial plate assemblies 26, and the apertures 64 are actually spaced throughout the entire surface area of the core tube portion 52 bounded by the partial plate assemblies 26. As seen in FIGS. 8A and 8B, the preferred embodiment of the T-connection 50 has apertures 64 along one-half or 180.degree. of the core wall surface 56. Thus, for example, the partial plate assemblies 26 cover a corresponding 180.degree. portion of the core wall surface 56. It is to be understood that the amount of core wall surface 56 having apertures 64 therethrough is purely a matter of design choice. Thus, the partial plate assemblies 26 could cover the entire core wall surface 56 with the exception of that portion of core wall surface 56 having the suction pipe portion 54 extending therefrom. As seen in FIG. 7, the ends of the suction strainer sections 10 and T-connections 50 are aligned and welded along weld lines 48. The result is an elongated suction strainer 40" having multiple suction pipe portions 54 extending transversely along its length. Thus, several suction pipes 36 communicating with the nuclear power plant can be connected to and staggered along the length of one elongated suction strainer 40". Several ECCS pumps can thus be connected to the same elongated suction strainer 40". With reference to all embodiments, the elongated suction strainer 40, 40', 40", 40'" and its associated extensions 15, caps 42 and pipe extensions 44 when occupying a supported position within the suppression pool suction system, 9, 9', 9", define, in accordance with the preferred embodiments of the present invention, a segment (of which there may be more than one in some embodiments) which will be referred to herein as the "spanning suction strainer segment" being that segment of the suction system which is supported at two displaced support members 46 and spans a considerable free-span length between the displaced support members. The spanning suction strainer segment (as well as the "free span length" thereof) is represented by the dimension "L" in FIGS. 1, 5 and 7 (and other Figs.). The "free-span length" ("L") is a length of the suction system 9, 9', 9" that is not interrupted by a member which transfers weight or force to the walls 34 or other supporting structure. By virtue of the present invention, and in accordance with preferred embodiments thereof, suction strainer systems 9, 9', 9" within the unique environment of nuclear power plants define free span lengths ("L") in excess of seven (7) feet, and, more preferably, in excess of ten (10) feet. Spanning suction strainer segments of shorter lengths are within the scope of the present invention and the inventiveness of such smaller length strainers are themselves novel and unobvious both alone and as part of the overall suppression pool suction system 9, 9', 9", 9'" of the present invention. The maximum free-span length for a given embodiment is limited in practical application by factors such as the location of suppression pool structural supports, the shape of the suppression pool, the internal dimensions of the suppression pool, and the location of other structural members within the suppression pool. The ability of the elongated suction strainers 40-40'" to span the desired free-span length in a given installation is provided by the load bearing strength of the core tube 12. For example, for longer free-span lengths, the core tube 12, core tube extensions 15, and core tube portions 52 (for example in embodiments utilizing T-connections), and related pipe extensions 44 are constructed of stronger metals or thicker materials, or are reinforced with longitudinally extending ribs. By way of example, a free-span length is represented by the dimension L in FIG. 7. There, the T-connections 50 extend the overall length of the elongated suction strainer 40", but the connection of the suction pipe portions 54 to the ECCS pipe penetration (not shown) in the suppression pool wall does not transfer or support appreciable weight or force to the suppression pool wall. Thus, the free-span length (as defined herein) of the spanning suction strainer segment "L" is from support 46 to support 46. Additional exemplary embodiments of suction systems (labeled 9a-9e) are depicted in FIGS. 9-13. FIG. 9 depicts an exemplary suction system 9a which includes an elongated suction strainer 40a comprised of three suction strainer sections 10' connected to one ECCS pipe penetration 36. Each of the strainer sections 10' is supported at its ends by ring girders 46 mounted to the reservoir wall 34. Each of the suction strainer sections 10' is itself "elongated" such that each defines a spanning suction strainer segment ("L"). In one example, and without limitation, the free-span length of each of these spanning suction strainer segments "L" of FIG. 9 is acceptably, approximately 150 inches. FIG. 10 depicts an exemplary suction system 9b which includes an elongated suction strainer 40b comprised of two identical suction strainer sections 10' symmetrically installed on each side of a "ram's head" tee 70, with the entire elongated suction strainer 40b supported at two ends by ring girders 46 connected to the reservoir wall 34. The ram's head tee 70 interconnects with the suction pipe 36 of the system through a "slip-fit connection" in a manner that does not transfer meaningful weight to the suction pipe. Thus, the spanning suction strainer segment "L" is that considerable length shown in the drawings. One example, without limitation, of an acceptable free-span length associated with this embodiment is approximately 210 inches. FIG. 11 depicts an exemplary suction system 9c which includes an elongated suction strainer 40c, comprised of three strainer sections 10' bolted end-to-end to one another to form a single elongated suction strainer supported at two ends by ring girders 46, and then connected by elbows to an ECCS pipe penetration. The dimensions shown are intended as examples only. FIG. 12 is an exemplary embodiment of the suction system 9d which includes an elongated suction strainer 40d supported from ring girders 46. The suction system 9d further includes additional cantilevered suction strainers 80, of a type known in the prior art. The elongated suction strainer 40d and the two cantilevered strainers 80 are all connected to one ECCS pipe penetration 36. The dimensions shown are intended as examples only. FIG. 13 depicts an exemplary suction system 9e which includes an elongated suction strainer 40e comprised of a single, elongated suction strainer section 10' supported at its two ends by ring girders 46 and which elongated suction strainer 40e is connected to suction pipe 36 through an end connection. By way of example, and without limitation, an exemplary free-span length "L" acceptable for this embodiment is approximately 150 inches. FIG. 14 is provided as a schematic representation of a reservoir (e.g. suppression pool of a BWR) outfitted with a plurality of suction systems, each including an elongated suction strainer 40 supported by structural members to reservoir walls 34, each elongated suction strainer 40 being connected to one of a plurality of suction pipe penetrations 36. FIG. 15 shows a plan view of a PWR nuclear power plant retrofitted with a suction system 9'" of the present invention. As mentioned, unlike a BWR nuclear power plant, a PWR nuclear power plant does not utilize a suppression pool. Rather, a PWR nuclear power plant incorporates a containment area 91 which remains dry until an accident occurs. (The containment area 91, when flooded with water, functions as a fluid reservoir for purposes of this description). As shown in FIG. 15, the elongated suction strainer 40' of the present invention provides a means of increasing the overall surface area and straining capacity for the ECCS. As shown in FIGS. 15 and 16, an elongated suction strainer 40', comprised of a plurality of suction strainer sections 10' is positioned above the floor 93 of the containment area 91 between the containment wall 94 and the shield wall 92 of the PWR nuclear power plant. Through a series of flange connections and angled pipe sections (and supported, as necessary, at intervals by support members), the elongated suction strainer 40' is directed throughout the containment area 91 to maximize the overall straining surface area of the suction system 9'" of this embodiment. A pipe extension 95 is connected to the drain orifice within the sump pit 96 (the trash rack and debris screen being removed) and extends protruding out of the sump pit into the containment area 91 where, in the depicted embodiment, it is connected by the necessary pipe fittings to the two halves of the elongated suction strainer 40', in a manner that will be understood in light of the discussion of earlier suction system embodiments. It is to be understood that the elongated suction strainer 40' embodiment of FIG. 4 has been shown in FIGS. 15 and 16 to describe the suction system 9'" of this embodiment for use in PWR nuclear power plants. However, it is anticipated that any embodiment of elongated suction strainer 40, 40', 40", 40'" of this invention could be used as well. In operation (in connection with, for example, BWR nuclear power plants), the suction system of the present invention is designed for installation in new BWR nuclear power plants and/or retrofit into existing BWR nuclear power plants. The suction system embodiment 9, 9', 9" includes generally an elongated suction strainer 40, 40', 40", respectively, which can be formed from a single suction strainer (not shown) or from a plurality of suction strainer sections 10, 10' connected end-to-end in series. The suction system further comprises a suction pipe 36 that protrudes into the suppression pool of a BWR nuclear power plant through a penetration in the suppression pool wall 34. The elongated suction strainer 40, 40', 40" is connected to the suction pipe 36 in any of the number of ways discussed hereinabove. The suction pipe 36 is tied to the recirculation system of the BWR nuclear power plant to provide fluid for the recirculation pumps. When the ECCS or other recirculation pumps are engaged, fluid from the suppression pool of the BWR nuclear power plant is drawn into the suction system 9, 9', 9" through the exterior filtering structure 20 (and partial plate assemblies 26 if T-connections 50 are used), and then through the fluid inlets 18 of the internal core tube 12 (and apertures 64 of the core tube portion 52 if T-connections 50 are used) into the core chamber 16 (and bore 58 if T-connections 50 are used) of the elongated suction strainer 40, 40', 40". The fluid, thus strained of solids and other particulate matter, is then pumped out of the suppression pool through the suction pipe 36 to the reactor (not shown). The fluid is used to cool the core of the reactor and is then recirculated back to the suppression pool. In the event of a LOCA, the core tube 12 of the elongated suction strainer 40, 40', 40" provides sufficient strength and stability so that the suction system 9, 9', 9" can withstand the extreme post-LOCA hydrodynamic forces discharged into the suppression pool of a BWR nuclear power plant. The result is an end-supported suppression pool suction system 9, 9', 9'" having a greater filtering surface area than heretofore known in the art. It is to be understood that the suction system 9'" incorporating elongated suction strainer 40' shown in FIGS. 10 and 11 operates as described above. However, the suction system 9'" operates within the containment area of a PWR nuclear power plant rather than a suppression pool. Accordingly, the suction strainer 40' will only be exposed to water in the event of an accident. The containment area of a PWR nuclear power plant, unlike the suppression pool of a BWR nuclear power plant, remains dry until an accident occurs. In the event of an accident, the containment area is partially filled with water, sufficiently that the strainer's core tubes become submerged, and the suction system 9'" is then activated as described hereinabove. While several preferred embodiments of the invention have been disclosed in the foregoing specification, it is to be understood by those skilled in the art that variations and modifications thereof can be made without departing from the spirit and scope of the invention as set forth in the following claims. In addition, the corresponding structures, materials, acts, and equivalents of all means or step plus function elements in the claims below are intended to include any structure, material or acts for performing the functions in combination with other claimed elements as specifically claimed herein.