Abstract:
A suction strainer includes a hollow internal core tube and an external filtering surface built around the internal core tube. A plurality of openings are defined through the side wall of the internal core tube core. In some cases the openings through the side wall are constructed and arranged such that there is somewhat less open area near the downstream end than the upstream end of the internal core tube, and the amount of open area tapers between the upstream end and the downstream end. As a result, when liquid is drawn into the internal core tube through the plurality of openings, a substantially uniform inflow distribution may be defined along substantially the entire length of the internal core tube. The internal core tube functions as a rigid structural support for the external filtering surface, enabling the apparatus to withstand post-LOCA hydrodynamic forces. The size of the filtering surface is enlarged by virtue of the fact that the filtering surface defines a plurality of filtering disk assemblies that are connected to and extend radially from the internal core tube. A separation distance is defined between neighboring disk assemblies, and filtering inner walls connect between neighboring disk assemblies and extend around the internal core tube at a radius less than the outermost radius of the disk assemblies. The total flow surface area is increased within a limited geometric profile. This serves to maximize surface area while minimizing post-LOCA reactive forces on attachment ECCS piping in a BWR suppression pool.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The present invention relates generally to the field of suction strainers, and more particularly to the field of suction strainers employed in the suppression pools of boiling water reactor (BWR) nuclear power plants.  
           [0002]    A suction strainer employed in a suppression pool removes solids from a flow of liquid (e.g., water) being drawn into an emergency core cooling system (ECCS) pump. The flow of water is drawn through the suction strainer and then into the suction line of the ECCS pump. Employment of suction strainers is desirable because solid debris drawn into the suction line of a pump can degrade pump performance by accumulating in the pump or its suction or discharge lines, or by impinging upon and damaging internal pump components.  
           [0003]    While almost any pump degradation can be characterized as being costly, the degradation of ECCS pump performance at BWR nuclear plants can be detrimental to safe plant shutdown following a loss of coolant accident (LOCA). At a BWR nuclear power plant following a LOCA, it is critical for the ECCS pumps to operate for an extended period of time in an undegraded fashion. In one mode of operation, the ECCS pumps are operated to recirculate water from the suppression pool back to the reactor core for the purpose of core cooling. A LOCA results from a high pressure pipe rupturing with such great force that large quantities of debris, such as pipe and vessel insulating material, and other solids, may be washed into the suppression pool. Conventional ECCS suction strainers currently installed in BWR plants would have a tendency to become clogged by such debris due to their small size and poor design. Also, when the large pressure pipes rupture with great force, suction strainers in the suppression pool are subjected to great hydrodynamic forces that can damage the suction strainers as well as subject the attachment recirculation piping to large reactive forces. These structural considerations, and space constraints, limit the size and shape of suction strainers in suppression pools.  
           [0004]    Conventional BWR plant suction strainers are typically constructed and arranged in a manner such that, under full flow conditions, localized high entrance velocities are established through that portion of the suction strainer that is most proximate to the suction line of the pump, while low entrance velocities are established through that portion of the suction strainer that is more distant from the suction line of the pump. The high entrance velocities may draw more solid debris into contact with the suction strainer causing the portions of the suction strainer experiencing the high entrance velocities to experience higher head loss. As the portion of the suction strainer most proximate to the suction line collects debris, high entrance velocities are established at the portion of the suction strainer that is next closest to the suction line causing that portion to collect debris. This process often continues until the entire suction strainer has collected debris in varying quantities, resulting in a non-uniform build-up of debris on the outer surface of the strainer.  
           [0005]    Localized high entrance velocities can be detrimental even when solids are not present in the liquid being pumped. For example, high entrance velocities can result in turbulent flow which tends to create greater pressure losses than laminar flow. Any such pressure losses reduce the net positive suction head (NPSH) available to a pump. As the NPSH available decreases, pump cavitation may occur. Similarly, localized high entrance velocities can cause vortexing. When a suction strainer is not sufficiently submerged, the vortexing can cause air ingestion which can severely degrade pump performance.  
           [0006]    Attempts have been made to resolve certain of the problems associated with suction strainer-like devices in other applications. For example, cylindrical suction flow control pipes have been encircled with screen material and employed in water wells. Such wells typically employ a well pump above the ground surface and a riser pipe extending from the well pump to the water table. The suction flow control pipe is connected to the end of the riser pipe and extends further below the water table. Openings are defined through the side wall of the suction flow control pipe such that there is somewhat less open area near the riser pipe and somewhat more open area distant from the riser pipe. As a result, when water is drawn into the flow control pipe through the openings, a substantially uniform inflow distribution is defined along the length of the flow control pipe. While such suction flow control pipes offer some advantages, they are not suitable for all applications.  
           [0007]    Attempts have been made, totally separate from flow control pipes, to increase filtering surface areas of BWR ECCS suction strainers in an effort to decrease pressure losses and thereby prevent pump cavitation. For example, such suction strainers may include a plurality of spaced, coaxial, stacked filtering disks. More particularly, such stacked disk suction strainers typically include an annular flange for attachment to the corresponding flange on the pump suction line. The stacked disk suction strainer provides an enhanced surface area and defines a longitudinal axis that is encircled by the attachment flange. A first disk is attached to the attachment flange. The first disk includes a pair of a radially extending, circular, disk walls, each of which encircle the longitudinal axis, and define a central hole. A first disk wall of the pair of disk walls is connected to the attachment flange. The first and second disk wall of the pair of disk walls face one another and are separated by a slight longitudinal distance. The first disk further includes an outer annular wall that encircles the longitudinal axis. The outer annular wall includes an annular first edge and an annular second edge. The entirety of the annular first edge of the outer annular wall is connected to the entire peripheral edge of the first perforated disk wall; and the entirety of the annular second edge of the outer annular wall is connected to the entire peripheral edge of the second perforated disk wall such that the pair of disk walls are connected at their periphery.  
           [0008]    The stacked disk suction strainer further includes a plurality of inner annular walls that encircle the longitudinal axis, each of which includes an annular first edge and an annular second edge. The annular first edge of one of the inner annular walls is connected around the periphery of the central hole of the second disk wall. The annular second edge of that inner annular wall is connected around the periphery of the central hole of a disk wall of a second disk. The first and second disk walls, and the outer and inner annular walls are perforated and comprise the filtering surface of the stacked disk suction strainer. Additional perforated disks and inner annular walls are attached to one another in the above manner until the last disk is attached, wherein the outer disk wall of the last disk does not include a central hole. The stacked disk suction strainers may incorporate separate structural members to maintain the structural integrity of the stacked disk suction strainer. However, the conventional stacked disk suction strainers do not incorporate an internal core tube and related components, whereby the conventional stacked disk suction strainers are difficult to structurally reinforce and are susceptible to vortexing and the detrimental non-uniform localized entrance velocities discussed above.  
           [0009]    There is, therefore, a need in the industry for an improved suction strainer.  
         SUMMARY OF THE INVENTION  
         [0010]    Briefly described, the preferred embodiments of the of the present invention include a suction strainer that includes a filtering device with a strategically enlarged filtering surface and an internal core. The internal core is preferably in the form of an internal core tube, which is preferably an internal pipe with flow openings. In accordance with the preferred embodiments of the present invention, the internal core tube structurally reinforces the filtering device.  
           [0011]    In accordance with the preferred embodiments of the present invention, the structural reinforcement provided by the internal core tube is enhanced by reinforcing structural members that extend radially from the internal core tube. The reinforcing structural members are preferably connected to and extend radially from and angularly around the internal core tube to structurally support the filtering surfaces of the external filtering structure. The internal core tube, in conjunction with the structural members, seeks to prevent air ingestion and vortexing. The suction strainer preferably extends away from the suction line of an ECCS pump to define a length, and in accordance with certain examples the preferred embodiments of the present invention, the internal core tube seeks to promote controlled inflow along the length to preclude the establishment of non-uniform localized entrance velocities through the filtering surface. In accordance with other examples of the preferred embodiments of the present invention, the internal core tube is not constructed to specifically promote such a uniform inflow along the length.  
           [0012]    In accordance with the preferred embodiments of the present invention, the suction strainer is constructed in a manner that seeks to enlarge the filtering surface while minimizing the projected area of the suction strainer. The minimization of the projected area as well as structural reinforcement of the suction strainer enables the suction strainer to withstand high levels of hydrodynamic impact loading following a LOCA. The suction strainer also serves to minimize the bending moment and other reactive forces on the attachment ECCS piping in the BWR suppression pool.  
           [0013]    In accordance with the preferred embodiments of the present invention, the filtering surface is defined by an external filtering structure that is attached to, extends from, and is built around the internal core tube and the reinforcing structural members. When the suction strainer is connected to the suction of a pump and submerged, a liquid flow path is established through the internal core tube and external filtering structure. The liquid originates exterior to the external filtering structure and is drawn through the filtering surfaces of the external filtering structure. The filtering surfaces separate solids from the liquid. The size of the filtering surface is enlarged by virtue of the fact that the filtering surface defines protrusions such that the distance that the filtering surface extends from the internal core tube alternates. The resulting enlarged filtering surface seeks to decrease average flow velocities through the filtering surface and thereby spread the collected solid debris in thinner layers, thereby decreasing overall pressure losses associated with the suction strainer. Once the liquid flows through the filtering surface, the liquid is drawn through the internal core tube and into the suction of the pump.  
           [0014]    In accordance with the preferred embodiments of the present invention, the protrusions of the external filtering structure are in the form of a plurality of filtering plate assemblies that are connected to and extend radially from the internal core tube. Each plate assembly includes a pair of plate walls that face one another, define a distance therebetween, and are connected at their peripheries by an outer wall that surrounds the internal core tube. A separation distance is defined between neighboring plate assemblies. Inner walls connect between neighboring plate assemblies and extend around the internal core tube at a radius less than the radius of the outer walls. The outer and inner walls as well as the plate walls are perforated and comprise the filtering surfaces of the suction strainer. In accordance with first and second preferred embodiments of the present invention, the plurality of plate assemblies are preferably in the form of stacked disks that are spaced to defined troughs therebetween. In accordance with other embodiments, the plate assemblies are in other forms that increase the surface area of the suction strainer.  
           [0015]    In accordance with preferred embodiments of the present invention, the internal core tube has a downstream end for connection to the pump suction flange and an upstream end distant from the downstream end. The internal core tube defines a longitudinal axis extending between the upstream and downstream ends. In accordance with the preferred embodiments of the present invention, a plurality of openings are defined through the side wall of the internal core tube. In accordance with certain examples of the preferred embodiments, the openings are constructed and arranged such that there is somewhat less open area near the downstream end than the upstream end, and the amount of open area tapers between the upstream end and the downstream end. As a result, when water flows into the internal core tube through the openings, a substantially uniform flow rate distribution is defined along substantially the entire length of the internal core tube.  
           [0016]    It is therefore an object of the present invention to provide an improved BWR ECCS suction strainer.  
           [0017]    Another object of the present invention is to increase safety by improving the operability of the ECCS of a BWR nuclear plant following a LOCA.  
           [0018]    Yet another object of the present invention is to structurally reinforce a suction strainer sufficiently so that it can withstand the hydrodynamic forces following a LOCA in the suppression pool at a BWR nuclear plant.  
           [0019]    Still another object of the present invention is to minimize reactive forces on the attachment ECCS piping following a LOCA.  
           [0020]    Still another object of the present invention is to maximize the total strainer surface area within a limited geometric profile while providing a maximum strength strainer.  
           [0021]    Still another object of the present invention is to simultaneously minimize both the thickness of collected debris on the strainer and the average entrance velocities to minimize the resultant NPSH of the ECCS following a LOCA.  
           [0022]    Still another object of the present invention is to maximize the amount of time required to reach a particular head loss across the strainer.  
           [0023]    Still another object of the present invention is to control the distribution of fluid flow over the strainer so as to collect debris uniformly, from disk to disk or from trough to trough, to allow scaling of the strainer for other flow rates with similar, but different size, strainers with different water flow rates.  
           [0024]    Still another object of the present invention is to prevent vortexing and air ingestion.  
           [0025]    Other objects, features and advantages of the present invention will become apparent upon reading and understanding this specification, taken in conjunction with the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0026]    [0026]FIG. 1 is a schematic, downstream end, perspective view of a suction strainer with an internal core tube in accordance with one example of the first preferred embodiment of the present invention.  
         [0027]    [0027]FIG. 2 is a schematic, partially cut-away, upstream end, perspective view of the suction strainer of FIG. 1.  
         [0028]    [0028]FIG. 3 is a schematic, side elevational view of a suction strainer with an internal core tube in accordance with one example of the second preferred embodiment of the present invention.  
         [0029]    [0029]FIG. 4 is a schematic, upstream end, elevational view of the suction strainer of FIG. 3.  
         [0030]    [0030]FIG. 5 is an isolated, plan view an internal core tube of the suction strainer of FIG. 3, wherein the internal core tube is in an unrolled and flattened configuration.  
         [0031]    [0031]FIG. 6 is an isolated, schematic, elevational view of a wall of a filtering portion of the suction strainer of FIG. 3.  
         [0032]    [0032]FIG. 7 is an isolated, plan view of a structural member of the filtering portion of the suction strainer of FIG. 3.  
         [0033]    [0033]FIG. 8 is an isolated, schematic, plan view an outer wall of the filtering portion of the suction strainer of FIG. 3, wherein the outer wall is in an unrolled and flattened configuration.  
         [0034]    [0034]FIG. 9 is an isolated, schematic, plan view an inner wall of the filtering portion of the suction strainer of FIG. 3, wherein the inner wall is in an unrolled and flattened configuration.  
         [0035]    [0035]FIG. 10 is a schematic representation of portions of a BWR nuclear power plant, wherein the suction strainer of FIG. 1 is connected the ECCS of the power plant.  
         [0036]    [0036]FIG. 11 is a schematic, side elevational view of a suction strainer with an internal core tube in accordance with an alternate embodiment of the present invention.  
         [0037]    [0037]FIG. 12 is a schematic, upstream end, elevational view of a suction strainer with an internal core tube in accordance with another alternate embodiment of the present invention.  
         [0038]    [0038]FIG. 13 is a schematic, side elevational view of the suction strainer of FIG. 12. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0039]    Referring now in greater detail to the drawings, in which like numerals represent like components throughout the several views, FIG. 1 is a schematic, perspective view of a suction strainer  20  with a core, in accordance with the first example of the first preferred embodiment of the present invention. The core is in the form of an internal core tube  26 , and the suction strainer  20  further includes an upstream end  22 , an opposite downstream end  24 , and an exterior filtering structure  28  connected to and at least partially bounding the internal core tube  26 . In accordance with the first preferred embodiment of the present invention, the internal core tube  26  is preferably in the form of a cylinder that structurally reinforces exterior filtering structure  28 . The internal core tube  26  extends between the ends  22 , 24  and protrudes from the filtering structure  28  at the downstream end  24 . The internal core tube  26  includes a core wall  30  that encircles and defines a core chamber  32 . The core wall further defines a primary opening  33  that provides access to the core chamber  32 , and the longitudinal axis  34  of the suction strainer  20 . The portion of the core wall  30  that is internal to the filtering structure  28  preferably defines a plurality of openings therethrough (for example, see openings  74 ,  76 ,  78  defined through core wall  30 ′ in FIG. 5), as will be discussed in greater detail below. The portion of the core wall  30  that extends from the filtering structure  28  at the downstream end  24  is preferably not perforated.  
         [0040]    The filtering structure  28  encircles a majority of the internal core tube  26  and includes an exterior filtering surface  40 . FIG. 1 is schematic in nature because, as will be discussed in greater detail below, the entire filtering surface  40  is preferably perforated (i.e., the filtering surface  40  defines a plurality of openings therethrough). The perforations are not depicted in FIG. 1 in an effort to clarify the view. The filtering surface  40  further defines a plurality of protrusions such that the contour of the filtering surface  40  is varied to uniquely maximize the effective filtering area of the filtering surface  40  within a limited geometric profile described by the length and outer diameter of the suction strainer  20 . The filtering structure  28  includes a plurality of spaced protrusions which cooperate to define peaks and valleys. In accordance with the first preferred embodiment of the present invention, the protrusions are in the form of plate assemblies  42   a - c  which are preferably in the form of circular disks, and the valleys defined between the disks are in the form of annular troughs. In accordance with the first preferred embodiment of the present invention, the disk and troughs are preferably arranged in a uniform and consistent pattern. While the suction strainer  20  is constructed in a manner that seeks to maximize its filtering surfaces  40 , that construction also seeks to reduce the projected area of the suction strainer  20  such that the suction strainer  20  can withstand both high levels of hydrodynamic impact loading and minimize bending moments on the attachment piping  102  (FIG. 10).  
         [0041]    An annular connection flange  38  encircles and is connected to the core wall  30  at the downstream end  24 . The connection flange  38  is preferably constructed and arranged for attachment to a corresponding flange (not shown) in the suction line  102  (FIG. 10) of a pump  104  (FIG. 10). The connection flange  38  is depicted in simplified form in FIG. 1 in an effort to clarify the view. The connection flange  38  preferably includes a plurality of bolt holes (not shown) therethrough that facilitate connection to the corresponding flange, as should be understood by those reasonably skilled in the art. In accordance with the preferred embodiments of the present invention, the connecting flange  38  refers, for example and not limitation, to a standard bolted flange connection. In accordance with alternate embodiments of the present invention a flange  38  is not employed, and the suction strainer  20  is connected to the suction line  102  by virtue of threading, welding, or other conventional fastening techniques or devices.  
         [0042]    When the suction strainer  20  is connected to the suction line  102  (FIG. 10) of an ECCS pump  104  (FIG. 10), liquid is drawn into the suction strainer  20  through the perforations defined through the filtering surface  40 . The filtering surface  40  functions to collect solids (not shown) on the suction strainer  20 . Once liquid is drawn through the filtering surface  40 , the liquid is drawn through the openings (for example, see openings  74 ,  76 ,  78  in FIG. 5) defined through that portion of the core wall  30  that is internal to the filtering structure  28 .  
         [0043]    Referring back to the plate assemblies  42   a - c , they preferably encircle the longitudinal axis  34 , and the exposed surfaces of the plate assemblies  42   a - c  constitute a substantial portion of the filtering surface  40 . The plate assemblies  42   a - c  include perforated plate walls  44   a - f  and perforated outer annular walls  46   a - c  that preferably encircle the longitudinal axis  34 . More particularly, and representative of the construction of the plate assemblies  42   b,c , the plate assembly  42   a  includes the plates walls  44   a,b  which face one another and are separated by a longitudinal distance. The plate walls  44   a,b  each define a peripheral edge (as an example, see peripheral edge  47  of plate wall  44 ′ in FIG. 6), and the outer wall  46   a  spans and is connected between the peripheral edge of the plate wall  44   a  and the peripheral edge of the plate wall  44   b . Each of the plate walls  44   a - e  define a plate hole (as an example, see plate hole  48  of plate wall  44 ′ in FIG. 6) therethrough, and the plate holes are preferably circular and centered with respect to their respective plate wall  44   a - e . The internal core tube  26  extends and is connected through the plate holes (as an example, see plate hole  48  of plate wall  44 ′ in FIG. 6). The filtering structure  28  further includes a plurality of inner walls  43   a,b . The inner walls  43   a,b  are preferably annular. The inner walls  43   a,b  are also preferably perforated such that they constitute the remainder of the filtering surface  40 . The inner walls  43   a,b  preferably encircle the longitudinal axis  34 , and the inner wall  43   a  is connected between the plate assemblies  42   a,b  while the inner wall  43   b  is connected between the plate assemblies  42   b,c.    
         [0044]    [0044]FIG. 2 is a schematic, partially cut-away, upstream end, perspective view of the suction strainer  20 , in accordance with the first example of the first preferred embodiment of the present invention. Portions of the plate assemblies  42   b,c  and the inner wall  43   b  are cut-away to expose the portion of the internal core tube  26  that is proximate to the upstream end  22 . FIG. 2 is schematic in nature by virtue of the fact that perforations are not shown extending through the plate assemblies  42   a - c  or the inner walls  43   a,b , and the openings (for example, see openings  74 ,  76 ,  78  in FIG. 5) that extend through the core wall  30  are not depicted in an effort to clarify the view. As mentioned above, the plate walls  44   a - e  each define a plate hole (for example see plate hole  48  in FIG. 6) therethrough, through which the internal core tube  26  extends and is connected. Conversely, the plate wall  44   f , which is partially cut-away in FIG. 2, does not define such a plate hole such that the plate wall  44   f  functions to cover the upstream end  22  of the internal core tube  26  and the core chamber  32 . Except for that difference between the plate wall  44   f  and the other plate walls  44   a - e , the plate assembly  42   c  is representative of the plate assemblies  42   a,b.    
         [0045]    In accordance with the first preferred embodiment of the present invention, the internal core tube  26  functions as a structural member that supports the filtering structure  28 . The filtering structure  28  and the internal core tube  26  are interconnected in a manner that synergistically strengthens the suction strainer  20 . In accordance with the first preferred embodiment of the present invention, the strengthening is enhanced by a plurality of structural members  50  that are preferably rectangular and planar. The structural members  50  are disposed within and are effectively part of each of the plate assemblies  42   a - c . Some of the structural members  50  are cut-away in FIG. 2 to clarify the view. In accordance with the first preferred embodiment of the present invention, the strengthening is also enhanced by a plurality of shorter structural members  52  that are preferably rectangular and planar. Structural members  52  are associated with each of the inner walls  43   a,b . In accordance with the first preferred embodiment of the present invention, the structural members  50 ,  52  are solid. In accordance with alternate embodiments of the present invention, holes or other perforations are defined through the structural members  50 ,  52 . In accordance with the first preferred embodiment of the present invention, the solid structural members  50 ,  52  function to both structurally reinforce the filtering structure  28  and prevent vortexing and air ingestion.  
         [0046]    More particularly, and representative of the construction of the plate assemblies  42   a,b , the structural members of plate assembly  42   c  extend radially from the core wall  30  and are angularly displaced about the longitudinal axis  34 . Each of the structural members  50  includes an inner edge (for example, see inner edge  54  of structural member  50 ′ in FIG. 7) connected to the core wall  30 , an opposite outer edge (for example, see outer edge  56  in FIG. 7) connected to the outer annular wall  46   c , a side edge (for example, see side edge  58  in FIG. 7) connected to the plate wall  44   f , and an opposite side edge (for example, see side edge in FIG. 7) connected to the plate wall  44   e . Further, and representative of the plate assemblies  42   a,b , the plate assembly  42   c  defines an annular plate chamber  62  that is bound by the core wall  30 , the outer annular wall  46   c , the plate wall  44   f , and the plate wall  44   e.    
         [0047]    The inner wall  43   b  is representative of the inner wall  43   a  . The inner wall  43   b  is connected to a plurality of the structural members  52 , wherein the structural members  52  extend radially from the core wall  30  and are angularly displaced about the longitudinal axis  34 . Each of the structural members  52  is similar to but sized differently from the structural members  50 . Each of the structural members  52  associated with the inner wall  43   b  include an inner edge connected to the core wall  30 , an opposite outer edge connected to the inner wall  43   b , a side edge connected to the plate wall  44   e , and an opposite side edge connected to the plate wall  44   d . (For example, see the inner edge  54 , outer edge  56 , side edge  58 , and side edge  60  of the structural member  50 ′ in FIG. 7.) Further, and representative of the inner wall  43   a  , the inner wall  43   b  defines an intermediate chamber  64  that is bounded by the core wall  30 , the inner wall  43   b , inner portions of the plate wall  44   e , and inner portions of the plate wall  44   d . In accordance with the first preferred embodiment of the present invention, each of the plate assemblies  42   a - c  define plate chambers  62  and each of the inner walls  43   a,b  define intermediate chambers  64 , and all of the chambers  62 ,  64  comprise a filter chamber  66 . Stated differently, the filter chamber  66  is defined between the core wall  30  and the filtering surface  40  of the filtering structure  28 .  
         [0048]    As discussed in greater detail below with reference to a first example of the second preferred embodiment of the present invention, in accordance with the first example of the first preferred embodiment of the present invention, the openings defined through the core wall  30  (for example, see openings  74 ,  76 ,  78  defined through core wall  30 ′ in FIG. 5) are uniquely constructed and arranged so that a substantially controlled inflow distribution, which is preferably uniform, is defined along the length of the internal core tube  26  that is internal to the filtering structure  28 . A uniform inflow distribution seeks to, among other things, collect the debris on the filtering surface  40  uniformly, from disk to disk or trough to trough, and thereby allow scaling of test results to strainers with different flow rates and different surface areas. This flow pattern control also seeks to assist in preventing vortexing and air ingestion at the suction strainer  20 . In accordance with other examples of the first preferred embodiment, the openings defined through the core wall  30  are constructed and arranged so that the internal core tube  26  does not seek to control the inflow distribution.  
         [0049]    [0049]FIG. 3 is a schematic, side elevational view of a suction strainer  20 ′ in accordance with a first example of the second preferred embodiment of the present invention. The suction strainer  20 ′ of the first example of the second preferred embodiment is very similar, in general terms, to the suction strainer  20  (FIGS. 1 and 2) of the first example of the first preferred embodiment. Thus, except where specific differences between the suction strainer  20 ′ and the suction strainer  20  are noted or apparent, the following disclosure of the suction strainer  20 ′ should be considered as a supplement to the foregoing disclosure of the suction strainer  20 , and visa versa.  
         [0050]    In accordance with the second preferred embodiment of the present invention, the internal core tube  26 ′ of the suction strainer  20 ′ is preferably cylindrical and longer than the internal core tube  26  (FIGS. 1 and 2) of the first embodiment. Further, the suction strainer  20 ′ includes more inner walls  43 ′ a - e  and plate assemblies  42 ′ a - f  than the suction strainer  20  (FIGS. 1 and 2) of the first preferred embodiment. The plate assemblies  42 ′ a - f  include plate walls  44 ′ a - l . The inner walls  43 ′ a - e  and plate assemblies  42 ′ a - f  preferably encircle the internal core tube  26 ′, as is indicated by the broken line showing of the internal core tube  26 ′. FIG. 3 is schematic in nature because, while the inner walls  43 ′ a - e  and plate assemblies  42 ′ a - f  preferably define a multiplicity of apertures therethrough, those apertures are not depicted in FIG. 3 in an effort to clarify the view. In accordance with the second preferred embodiment of the present invention, the internal core tube  26 ′ is preferably in the general form of a cylinder that structurally reinforces exterior filtering structure  28 ′.  
         [0051]    In accordance with the second preferred embodiment of the present invention, the suction strainer  20 ′ defines an overall length that is represented by the dimension “a”. The core wall  30 ′ defines a first length that does not define openings  74 ,  76 ,  78  (FIG. 5) therethrough, and that first length is represented by the dimension “b”. The core wall  30 ′ further defines a second length, represented by the dimension “c”, that does define openings  74 ,  76 ,  78  (FIG. 5) therethrough and that is surrounded by the plate assemblies  42 ′ a - f  and the inner walls  43 ′ a - e . A separation distance, represented by the dimension “d”, is defined between each of the plate assemblies  42 ′ a - f . Also, each of the plate assemblies  42 ′ a - f  individually define a thickness that is represented by the dimension “e”. The internal core tube  26 ′, inner walls  43 ′ a - e , and plate assemblies  42 ′ a - f  define diameters that are represented by the dimensions “f”, “g”, and “h”, respectively. In accordance with one acceptable example of the second preferred embodiments, and in approximation, the dimension “a” is acceptably 36.0 inches, the dimension “b” is acceptably 3.375 inches, the dimension “c” is acceptably 29.875 inches, the dimension “d” is acceptably 2.0 inches, the dimension “e” is acceptably 3.313 inches, the dimension “f” is acceptably 24.0 inches, the dimension “g” is acceptably 26.0 inches, and the dimension “h” is acceptably 42.0 inches.  
         [0052]    As suggested by the broken line showing of the core wall  30 ′ in FIG. 3, in accordance with the second preferred embodiment of the present invention, each of the plate walls  44 ′ a - k  defines a centered plate hole  48  (FIG. 6) therethrough. The internal core tube  26 ′extends through the plate holes  48 . As an exception, however, the plate wall  44 ′ l  does not define such a hole  48  and therefore covers the upstream end  22 ′ of the internal core tube  26 ′ and the core chamber (for example see the core chamber  32  of FIGS. 1 and 2). The plate wall  44 ′ l  is more fully depicted in FIG. 4, which is a schematic, upstream end, elevational view of the suction strainer  20 ′, in accordance with the first example of the second preferred embodiment of the present invention. The fact that the plate wall  44 ′ l  covers the upstream end  22 ′ of the internal core tube  26 ′ is indicated by the broken line showing of the core wall  30 ′ in FIG. 4.  
         [0053]    In accordance with the second preferred embodiment of the present invention, each of the plate assemblies  42 ′ a - f  (FIG. 3) include a plurality of structural members  50 ′ therein; and the arrangement and configuration of the structural members  50 ′ within the plate assembly  42 ′ f  is representative of the configuration and arrangement of the structural members  50 ′ within each of the plate assemblies  42 ′ a - e . As indicated by the broken line showing of the structural members  50 ′ in FIG. 4, the structural members  50 ′extend radially from and are angularly displaced about the internal core tube  26 ′. In accordance with the second preferred embodiment, an identical angle “φ” is defined between each adjacent structural member  50 ′, and one acceptable example of the angle “φ” is 60 degrees. In accordance with the second preferred embodiment of the present invention, the inner walls  43 ′ a - e  are not reinforced by structural components that correspond to the structural members  52  (FIG. 2) of the first preferred embodiment. However, in accordance with alternate embodiments of the present invention the inner walls  43 ′ a - e  are reinforced by structural components that correspond to the structural members  52  (FIG. 2) of the first preferred embodiment.  
         [0054]    [0054]FIG. 5 is an isolated, plan view of the core wall  30 ′ in accordance with the first example of the second preferred embodiment of the present invention, wherein the core wall  30 ′ is in an unrolled and flattened configuration. The core wall  30 ′ includes opposite wall edges  68 ,  70  that are preferably joined together in a manner that creates the cylindrical internal core tube  26 ′ (FIG. 3). The internal core tube  26 ′ further includes an upstream edge  71  to which the plate wall  44 ′ l  (FIGS. 3 and 4) is affixed. Broken lines  72   a - k  are included in FIG. 5 for explanatory purposes only. The lines  72   a - k  represent the points at which the inner peripheral edges  73  (FIG. 6) of the plate walls  44   a - k  (FIG. 3), respectively, contact the core wall  30 ′ when the plate walls  44   a - k  are properly installed on the cylindrical internal core tube  26 ′ (FIG. 3).  
         [0055]    In accordance with the first example of the second preferred embodiment of the present invention, the openings  74 ,  76 ,  78 , only a few of which are specifically pointed out in FIG. 5 in an effort to clarify the view, are in the form of circular holes which extend through the core wall  30 ′. Also, in accordance with the second preferred embodiment of the present invention, open areas are defined through the core wall  30 ′ by virtue of the openings  74 ,  76 ,  78 . Each individual opening  74 ,  76 ,  78  represents or defines an open area. The open area of an individual opening  74 ,  76 ,  78  is representative of the capacity of that individual opening  74 ,  76 ,  78  to pass liquid. Thus, the open area can acceptably be measured perpendicular to the direction of liquid flow through the most restrictive portion of an opening  74 ,  76 ,  78 . In accordance with the first example of the second preferred embodiment of the present invention, the openings  78  are the only type of openings defined between the lines  72   h  and the upstream end  22 ′ of the core wall  30 ′, the openings  76  are the only type of openings defined between the lines  72   d,h , and the openings  74  are the only type of openings defined between the lines  72   a,d.    
         [0056]    Unit areas consisting of a portion of the core wall  30 ′ can be considered to define open areas, wherein the open area of a unit area is the summation all of the individual open areas defined within that unit area. Accordingly, and for example and not limitation, a plurality of core units can be defined as extending sequentially along the length of the internal core tube  26 ′ (FIG. 3). In accordance with one acceptable example of such a sequential arrangement of core units, a first unit of the plurality of units can acceptably and hypothetically be identified as being that portion of the core wall  30 ′ defined between the lines  72   b,d . That first unit defines a first unit open area equal to the summation of the open areas of each of the openings  74  defined between the lines  72   b,d . Similarly, a second unit of the plurality of units can acceptably and hypothetically be identified as being that portion of the core wall  30 ′ defined between the lines  72   d,f . That second unit defines a second unit open area equal to the summation of the open areas of each of the openings  76  defined between the lines  72   d,f . In accordance with the first example of the second preferred embodiment of the present invention, the first unit open area is smaller than the second unit open area such that when a pump draws liquid through the wall  30 ′ of the internal core tube  26 ′(FIG. 3), the flow (i.e., inflow) of liquid through the first unit is substantially similar to the inflow of liquid through the second unit. The lines  72   a - k  are used in the foregoing example only because they aid in the explanation of the concept of core units. The lines  72   a - k  are not intended to and should not limit the hypothetical configuration of core units. Core units can be conceptualized and defined without any reference to the lines  72   a - k . For example and not limitation, it is acceptable for the edge of a core unit to be located at any location between neighboring lines  72 .  
         [0057]    As evidenced by the foregoing, in accordance with the first example of the second preferred embodiment of the present invention, the openings  74 ,  76 ,  78  are constructed and arranged such that less open area is defined near the downstream end  24 ′ (FIG. 3) than the upstream end  22 ′ (FIG. 3). Thus, when liquid is drawn into the core chamber (for example see the core chamber  32  of FIGS. 1 and 2) by a pump or the like, a substantially uniform flow (i.e., inflow) distribution is defined along the length of the internal core tube  26 ′ (FIG. 3). In accordance with the first example of the second preferred embodiment of the present invention, the variation in open area of the core wall  30 ′ (e.g., the variation in the open area of the internal core tube  26 ′) is achieved by varying the sizing of the openings  74 ,  76 ,  78 . In other words, in accordance with the first example of the second preferred embodiment of the present invention, the pattern of the openings  74 ,  76 ,  78  does not vary significantly along the length of the core wall  30 ′. More particularly, the opening pattern defined between the lines  72   a,b  is repeated between the lines  72   c,d , the lines  72   e,f , the lines  72   g,h , the lines  72   i,j , and the line  72   k  and the upstream end  22 ′ of the core wall  30 ′. Similarly, the opening pattern defined between the lines  72   b,c  is repeated between the lines  72   d,e , the lines  72   f,g , the lines  72   h,i , and the lines  72   j,k . Accordingly, in accordance with the first example of the second preferred embodiment of the present invention, the variation in open area is achieved by virtue of the fact that the openings  78  (which are preferably the only type of openings defined between the lines  72   h  and the upstream end  22 ′ of the core wall  30 ′) are larger than the openings  74 ; and the openings  74  (which are preferably the only type of openings defined between the lines  72   d,h ) are larger than the openings  72  (which are preferably the only type of openings defined between the lines  72   a,d ). In accordance with first example of the second preferred embodiment of the present invention, each of the openings  74  acceptably defines a diameter of approximately 0.875 inches, each of the openings  76  defines a diameter of approximately 1.063 inches, and each of the openings  78  defines a diameter of approximately 1.25 inches. Also in accordance with the first example of the second preferred embodiment of the present invention, the core wall  30 ′ is acceptably constructed from a sheet of steel such as, but not limited to, a piece of quarter inch coated A36 carbon or uncoated stainless steel.  
         [0058]    As mentioned above, in accordance with the first example of the second preferred embodiment of the present invention the variations in open area along the length of the internal core tube  26 ′ (FIGS. 3 and 4) are established by repeating the pattern of openings but varying the size of the openings defined through the core wall  30 ′. However, in accordance with alternate embodiments of the present invention all of the openings defined through the core wall  30 ′ are the same size, and the variations in open area along the length of the internal core tube  26 ′ (FIG. 3) are established by varying the pattern of the openings through the core wall  30 ′. In other words, in accordance with alternate embodiments of the present invention, the variation in open area exists by virtue of the fact that more openings are defined through the core wall  30 ′ proximate to the upstream end  22 ′ (FIG. 3) of the internal core tube  26 ′. Also, in accordance with other examples of the second preferred embodiment, the open areas through the core wall  30 ′ are substantially uniform along the length of the internal core tube  26 ′, whereby uniform inflow along the length of the internal core tube  26 ′ is not provided.  
         [0059]    [0059]FIG. 6 is an isolated, schematic, elevational view of a plate wall  44 ′ that is representative of the plate walls  44 ′ a - k  (FIG. 3), in accordance with the second preferred embodiment of the present invention. The plate wall  44 ′ includes an outer peripheral edge  47  and an inner peripheral edge  73  that encircles and defines a central plate hole  48 . The internal core tube  26 ′(FIG. 3) extends through the central plate holes  28  of the plate walls  44 ′ a - k  (FIG. 3). The plate wall  44 ′depicted in FIG. 6 is not representative of the plate wall  441  (FIG. 3) because the plate wall  441  does not define a plate hole  48  therethrough, as mentioned above.  
         [0060]    [0060]FIG. 6 is schematic in nature because each of the plate walls  44 ′ a - l  (FIG. 3) define a multiplicity of perforations therethrough that are preferably evenly distributed. The perforations are not clearly shown in FIG. 6 in an effort to clarify the view, however a sampling of the perforations is schematically represented by dots. In accordance with the second preferred embodiment of the present invention, an acceptable size of each individual perforation is within the range of approximately 0.0625 inches to approximately 0.250 inches. In accordance with one acceptable example, the perforations are so sized and so numerous that each of the plate walls  44 ′ a - l  is approximately forty percent open area. Also, the plate walls  44 ′ a - l  are acceptably constructed from a material such as, but not limited to, eleven gauge carbon or stainless steel. In accordance with an alternate embodiment of the present invention, the portion of the plate wall  44 ′ l  (FIG. 3) that covers the upstream end of the core chamber (for example see core chamber  32  in FIG. 2) is not perforated.  
         [0061]    The term “percent open area” as used within this disclosure can be explained with reference to the plate wall  44 ′. For example, before the plate wall  44 ′ is perforated it is zero percent open, whereas after the plate wall  44 ′ is perforated it includes a multiplicity of perforations such that the plate wall  44 ′ defines an open area. Each individual perforation represents or defines an open area which is representative of the capacity of that perforation to pass liquid, as discussed above. Further, unit areas consisting of a portion of the plate wall  44 ′ can be considered to define an open area, wherein the open area of a unit area is the summation all of the individual open areas defined within that unit area. For example, if a one square inch surface area of the plate wall  44 ′ is identified before the plate wall  44 ′ is perforated, that square inch surface area is zero percent open. After the plate wall  44 ′ is perforated and that one square inch surface area includes a plurality of perforations therethrough, that square inch surface area is some percent open. If the sum of all of the open areas in that one square inch surface add up to an area of 0.4 square inches, then that square inch surface area is forty percent open. If the entire plate wall  44 ′ is perforated in a manner substantially similar to that one square inch, the plate wall  44 ′ is forty percent open.  
         [0062]    [0062]FIG. 7 is an isolated, elevational view of a structural member  50 ′ in accordance with the second preferred embodiment of the present invention. In accordance with the second preferred embodiment of the present invention, the structural members  50 ′ are solid. In accordance with the second preferred embodiment of the present invention, the solid structural members  50 ′ function to both structurally reinforce the filtering structure  28 ′ (FIG. 3) and prevent vortexing. In accordance with alternate embodiments of the present invention, a plurality of holes are defined through the structural members  50 ′. The holes seek to equalize flow within the filter chamber (for example, see the filter chamber  66  in FIG. 2).  
         [0063]    In accordance with the second preferred embodiment of the present invention, structural members  50 ′ are incorporated into each of the plate assemblies  42 ′ a - f  (FIG. 3). Each of the structural members  50 ′ extends radially from the core wall  30 ′ (FIG. 3). Further, structural members  50 ′ within a single plate assembly  42 ′ are, in accordance with the second preferred embodiment, angularly displaced about the longitudinal axis  34 ′ (FIG. 3) as depicted in FIG. 4. In accordance with various alternate embodiments of the present invention, each plate assembly  42 ′ preferably includes six or more structural members  50 ′ angularly displaced about the longitudinal axis  34 ′. Each of the structural members  50 ′ includes an inner edge  54  connected to the internal core tube  26 ′ (FIG. 3), an opposite outer edge  56  connected to the respective outer wall  46 ′ (FIG. 3), a side edge  58  connected to the one plate wall  44 ′ (FIGS. 3 and 6), and an opposite side edge  60  connected to another plate wall  44 ′. In accordance with the second preferred embodiment of the present invention the structural members  50 ′ are acceptably constructed from a sheet of steel such as, but not limited to, a piece of eighth-inch carbon or stainless steel. In accordance with the second preferred embodiment of the present invention, structural members  52  (FIG. 2) are not employed. In accordance with an alternate embodiment of the present invention, structural members  52  are employed.  
         [0064]    [0064]FIG. 8 is an isolated, schematic, plan view of an representative outer wall  46 ′ of a plate assembly  42 ′ (FIG. 3), wherein the outer wall  46 ′ is in an unrolled and flattened configuration in accordance with the second preferred embodiment of the present invention. The outer wall  46 ′ includes opposite short edges  80 ,  82  that are preferably connected such that the outer wall  46 ′ encircles the longitudinal axis  34 ′ (FIG. 3). The outer wall  46 ′ further includes opposite elongated side edges  84 ,  86 . In a representative plate assembly  42 ′(FIG. 3), the elongated edge  84  of the outer wall  46 ′is connected to the outer peripheral edge  47  (FIG. 6) of one of the plate walls  44 ′, and the other elongated edge  86  of the outer wall  46 ′ is connected to the outer peripheral edge  47  of the other plate wall  44 ′.  
         [0065]    [0065]FIG. 9 is an isolated, schematic, plan view a representative inner wall  43 ′ of the suction strainer  20 ′ (FIG. 3), wherein the inner wall  43 ′ is in an unrolled and flattened configuration in accordance with the second preferred embodiment of the present invention. The inner wall  43 ′ includes opposite short edges  88 ,  90  that are preferably connected such that the inner wall  43 ′ encircles the longitudinal axis  34 ′(FIG. 3). The inner wall  43 ′ further includes opposite elongated edges  92 ,  94 . With respect to an exemplary inner wall  43 ′, the elongated edge  92  is connected to one plate wall  44 ′ (FIG. 3) while the other elongated edge  94  is connected to another plate wall  44 ′.  
         [0066]    [0066]FIGS. 8 and 9 are schematic in nature because each of walls  43 ′,  46 ′ define a multiplicity of perforations therethrough that are preferably evenly distributed. The perforations are not clearly shown in FIGS. 8 and 9 for the sake of clarity, however a sampling of the perforations is schematically represented by dots. In accordance with the second preferred embodiment of the present invention, and with respect to the inner walls  43 ′ a - e  (FIG. 3) and outer walls  46 ′ a - f  (FIG. 3), an acceptable size for each individual perforation is within the range of approximately 0.0625 inches to approximately 0.125 inches. In accordance with one acceptable example, the perforations through the walls  43 ′ a - e ,  46 ′ a - f  are so sized and so numerous that each of the walls  43 ′ a - e ,  46 ′ a - f  are approximately forty percent open. In accordance with the second preferred embodiment of the present invention, the walls  43 ′,  46 ′are acceptably constructed, for example and not limitation, from a sheet of metal such as, but not limited to, a piece of eleven gauge carbon or stainless steel. In accordance with alternate embodiments of the present invention, the walls  43 ′ a - e ,  46 ′ a - f  are acceptably constructed, for example and not limitation, from wire wrapped well screen.  
         [0067]    Referring to FIG. 10, which is a schematic representation of portions of a BWR nuclear power plant, in accordance with the preferred embodiments of the present invention, the suction strainer  20  (FIGS. 1 and 2) and the suction strainer  20 ′ (FIGS. 3 and 4) are preferably connected to an ECCS of a BWR nuclear power plant. FIG. 10 depicts a portion of an ECCS following a LOCA. The suction strainer  20  is submerged in a suppression pool  100 , where the suction strainer  20  is connected to the downstream end of a suction line  102  through which an ECCS pump  104  draws water  105  from the suppression pool  100 . In one mode of operation (which is generally depicted in FIG. 10), the ECCS pump  104  discharges through a discharge line  106  to a reactor core  108 . In another mode of operation, the ECCS pump  104  discharges back to the suppression pool  100 . The suction strainers  20 ,  20 ′ are uniquely constructed and arranged, and operate, such that they are capable of being readily incorporated into, optimize the operation of, and are capable of withstanding the rigors associated with, ECCSs.  
         [0068]    It should be understood that the specific construction and arrangements of the suction strainer (FIGS. 1 and 2) and the suction strainer  20 ′ (FIGS. 3 and 4) are provided as acceptable examples only. The broad concepts disclosed with respect to the suction strainer  20  and the suction strainer  20 ′ lend themselves to a variety of differently configured suction strainers, and configurations will vary depending upon desired flow rates, space constraints, and the hydrodynamic forces that a particular suction strainer and the attachment ECCS piping will be potentially subjected to. For example, FIG. 11 is a schematic, side elevational view of a suction strainer  20 ″ in accordance with an alternate embodiment of the present invention. The suction strainer  20 ″ depicted in FIG. 11 is substantially similar to the suction strainer  20 ′ of the second preferred embodiment, except that it includes less plate assemblies  42 ″ a - d , and the diameters of the plate assemblies  42 ″ a - d  taper. As another example, FIG. 12 is a schematic, upstream end, elevational view of a suction strainer  20 ′″, in accordance with another alternate embodiment of the present invention. FIG. 13 is a schematic, side elevational view of the suction strainer  20 ′″ of FIG. 12. The suction strainer  20 ′″ depicted in FIGS. 12 and 13 is substantially similar to the suction strainer  20 ′ of the second preferred embodiment, except that the suction strainer  20 ′″ includes less plate assemblies  42 ″ a - d , and the plate assemblies  42 ′″ a - d  are starshaped. The plate assemblies  42 ′″ encircle the internal core tube  26 ″, as is indicated by the broken line showing of the internal core tube  26 ″ and core wall  30 ″ in FIG. 12.  
         [0069]    As an additional example, another embodiment of the present invention includes a convertible suction strainer (not shown) that includes a 170 square foot filtering surface (for example see the filtering surface  40  in FIGS. 1 and 2), 13 disks (for example see the plate assemblies  42  in FIGS. 1 and 2) that are 40 inches in diameter, and a 24 inch flange (for example see the connection flange  38  of FIG. 1), wherein the convertible suction strainer is 48 inches long from the first to the last disk. The internal core tube (for example see the internal core tube  26  in FIGS. 1 and 2) of the convertible suction strainer has large evenly spaced holes therethrough that are not constructed and arranged to control inflow through the internal core tube. The internal core tube of the convertible suction strainer is constructed and arranged to structurally support a filtering structure (for example see the filtering structure  28  in FIGS. 1 and 2) that is connected to and extends radially from the internal core tube. The downstream end of the internal core tube of the convertible suction strainer is covered with a perforated plate (for example see the plate wall  44 ′ l  in FIG. 3) that can be temporarily opened to provide an opening to the core chamber that is defined by the internal core tube (i.e., the first internal core tube). A second internal core tube is capable of being inserted through the provided opening so that it is installed within the core chamber of the first internal core tube. The second internal core tube is constructed and arranged to control the inflow of water, in the manner discussed above, such that substantially even inflow is established along the length of the first internal core tube.  
         [0070]    While the embodiments of the present invention which have been disclosed herein are the preferred forms, other embodiments of the method and apparatus of the present invention will suggest themselves to persons skilled in the art in view of this disclosure. Therefore, it will be understood that variations and modifications can be effected within the spirit and scope of the invention and that the scope of the present invention should only be limited by the claims below. It is also understood that any relative relationships and dimensions shown on the drawings are given as preferred relative relationships and dimensions, but the scope of the invention is not to be limited thereby.