Abstract:
The present invention relates to filters used to remove debris from water being sucked into a piping system. It has particular application use in nuclear power plants, which, after a loss of coolant accident, must pump cooling water back into the reactor core from a collection sump. This water may contain various types of debris that must be removed before the water is sent back into the reactor cooling system. There are restrictions on the allowable pressure drop across the strainer and the space available for installing this equipment. The finned strainer of the present invention addresses these issues while maximizing the quantity of debris filtered from the water.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention pertains to the field of filters used to remove debris from water being sucked into a piping system. More particularly, the present invention relates to the field of filters used in nuclear power plants. 
       BACKGROUND 
       [0002]    Nuclear plants have various safety systems to ensure that the nuclear fuel in the reactor core remains cooled in all credible accident scenarios. One such scenario is a “loss of coolant accident,” in which an external pipe is postulated to break, allowing a large amount of water to escape from the reactor cooling system. This water may dislodge solid debris from neighbouring pipes or other reactor structures. The water, along with some of the dislodged debris, will flow to the lowest parts of the reactor building into a sump. Plants are equipped with safety systems that pump water from the sump back into various reactor cooling systems. Strainers on the pump intakes ensure that any debris large enough to clog equipment in these systems is prevented from entering. Depending on the type of debris, the first layer to deposit on the strainer may form a finer filter than the underlying screen, and catch many smaller particles. 
         [0003]    Strainers must have enough screen area that the debris layer on the strainer is not too thick to cause unacceptably high restriction to flow. Strainers must also be as small as possible to fit into the available space. Therefore compactness, i.e., accommodating the most screen area in the smallest volume, is important. 
         [0004]    Conventional strainers in many nuclear plants are simple box-type devices mounted over the pump intakes. Newer more advanced strainers often have an irregular surface to increase the surface area. 
         [0005]    This background information is provided for the purpose of making known information believed by the applicant to be of possible relevance to the present invention. No admission is necessarily intended, nor should be construed, that any of the preceding information constitutes prior art against the present invention. 
       SUMMARY OF THE INVENTION 
       [0006]    In accordance with one aspect of the invention there is provided a strainer for filtering debris from a fluid comprising an elongated header defining an enclosed flowpath, having an outlet in fluid communication with a suction source and a plurality of inlet apertures disposed along the length of said flowpath, said flowpath exhibiting a pressure drop in the direction of fluid flow; a strainer element disposed in each said inlet aperture for straining debris from fluid entering said flowpath; and a flow controlling device for maintaining substantially uniform fluid flow through strainer elements located at different positions along said flowpath. 
         [0007]    In accordance with another aspect of the invention, the flow controlling device comprises an orifice for producing a pressure drop between an inlet aperture and the flowpath located at a position closer to said suction source that is greater than the pressure drop between an inlet aperture and said flowpath located at a position farther from said suction source. The orifice can be in the form of a nozzle for accelerating the fluid entering said flowpath in a direction substantially parallel thereto and can be formed in a baffle disposed in said header, the baffle defining a collection channel enclosing a plurality of apertures. 
         [0008]    In accordance with another aspect of the present invention, the header has a generally planar side-wall and the inlet apertures are a series of substantially parallel slots formed in the side-wall in a direction transverse to said flowpath. The strainer elements can be in the form of flat-surface fins projecting outwardly from the apertures in the planar side-wall. 
         [0009]    In accordance with another aspect of the present invention, there is provided a strainer for filtering debris from a fluid comprising a header defining an enclosed volume and having an outlet in fluid communication with a suction source, said header having a plurality of inlet aperture slots formed therein, a fin-like strainer element projecting outwardly from each aperture slot for straining debris from said fluid, each said strainer element comprising a perimeter frame and a pair of fluid permeable screens fixed thereto in opposed spaced relation, and at least one fluid flow channel therebetween in fluid communication with said enclosed volume through a marginal side edge of said frame and said aperture slot. The fluid permeable screens can be formed of perforated metal sheet or mesh. A corrugated metal spacer can be disposed between the fluid permeable screens for maintaining said fluid permeable screens in spaced relation, and a plurality of flow channels can be defined between said corrugated metal spacer and said fluid permeable screens. The perimeter frame can be impermeable to fluid except at said one marginal side edge. 
         [0010]    In accordance with another aspect of the present invention, the fluid permeable screens are each formed of corrugated metal mesh having a plurality of parallel peaks and valleys, said screens being maintained in opposed spaced relation by contact at alternating peaks and defining a plurality of said flow channels therebetween. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES  
         [0011]      FIG. 1  is an isometric view of a strainer module for connection to a pump intake in accordance with an embodiment of the present invention. 
           [0012]      FIG. 2  is an exploded view of the strainer module shown in  FIG. 1 ; 
           [0013]      FIG. 3  is a cut-away isometric view of a strainer module mounted directly onto a sump in accordance with an embodiment of the present invention; 
           [0014]      FIG. 4  is a cut-away isometric view of a section of a flat-surface fin in accordance with an embodiment of the present invention; 
           [0015]      FIG. 5  is a cut-away isometric view of a section of a corrugated-surface fin in accordance with an embodiment of the present invention; 
           [0016]      FIG. 6  is an exploded view of a corrugated-surface fin in accordance with an embodiment of the present invention; 
           [0017]      FIG. 7  is an isometric section view of flow equalization devices in accordance with an embodiment of the present invention; and 
           [0018]      FIG. 8  depicts a corrugated-surface fin in accordance with an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0019]    Referring to  FIGS. 1 and 2 , the strainer module of the present invention comprises elongated header  3  that defines an internal fluid flowpath that is in fluid communication with a suction source through pump intake pipe  2  which may be located in the floor or wall through one or more connection(s)  1 . Header  3  has a generally planar sidewall with a plurality of inlet apertures  9  in the form of a series of substantially parallel elongated slots disposed along the length of the header to accommodate fins  4 . The inlet apertures are oriented in a direction transverse to the fluid flowpath within header  3 . Strainer elements in the form of hollow flat-surface fins  4  may be mounted on the sides (as shown in  FIG. 1 ), top, or bottom of header  3  and project outwardly from inlet apertures  9 . Fins  4  may have a uniform or variable spacing and are located by mounting frames  5  and braces  6 . In a preferred embodiment, fins are easily removable, using a pins  10  and bolts  11 , but they may also be permanently attached to header  3 . 
         [0020]    Water enters the strainer through fluid permeable screens  7  on the surface of the fins  4 , leaving debris on the screens. Water then flows through the fluid flow channel in hollow core  8  of the fin  4  towards header  3 . Various portions of header  3 , particularly the portion between fin slots  9 , may be made using fluid permeable material to increase the filtration area. Header  3  may have one or more baffle plates  12  to provide structural support for the sides to resist high suction pressures. Baffle plates  12  have large holes  13  to ensure flow velocity in the fluid flow channel in header  3  is the same above and below the baffle. 
         [0021]    The ends of each header  3  have flanges  14  that allow adjoining modules to be attached together. Modules may be attached together, or they may be independently mounted with seals between modules. Mounting frames  5  may be provided under the module. Mounting frames  5  have adjustable-height mounts  15  that allow the device to be installed on floors that are not level. 
         [0022]      FIG. 3  shows an alternative embodiment of the present invention. This embodiment is useful for situations where there is a pre-existing sump  45  with a cover  46  (which could be pre-existing or installed specifically to support the strainer modules) and a pump intake  47  inside the sump  45 . Strainer module  48  comprises fins  49  mounted in a frame  50  with appropriate bracing. For simplicity, only one module has been shown in  FIG. 3 . If required, a plurality of strainer modules  48  can be mounted in a similar manner over sump  45 . 
         [0023]    Flow enters fins  49  in the same manner as described above in relation to the embodiment shown in  FIGS. 1 and 2 , but then flows directly into sump  51 , and then into the pump intake  52 . Pump intake  52  may be modified to reduce inlet losses. No separate collection header is required for this arrangement, because sump  51  itself performs this function. Undesirable bypass flow between the sump cover  46  and the module frame  50  can be prevented using close tolerances between mating parts, or using wire mesh gaskets or any other suitable type of seal, such as item  25  shown in  FIG. 5  on the edge of the fin. Suitable portions of the module frame  50  or sump cover  46  may be made from perforated metal sheet to increase the filtration area. To make use of some of the volume of sump  51 , the strainer module can be recessed, either partially or completely, into the sump below the level of the floor. In such a case, the frame  50  of the module would extend down from the floor level to the bottom of the module in order to prevent flow from bypassing the filtration elements of the strainer. 
         [0024]    Air ingestion can be prevented by ensuring that there is a sufficient height of water above the strainer. In the alternative, a horizontal cover (not shown) can be added over the fins. This cover allows the fins to be closer to the water surface without ingesting air or causing hollow-core vortices. 
         [0025]    Various types of bracing, such as those shown in  FIGS. 1 ,  2  and  3 , are used to ensure that the strainer is sufficiently rigid to resist the predicted seismic and pressure loads. In addition, external bracing, such as indicated by reference numeral  6  in  FIG. 2 , may also be placed between fins. 
         [0026]    For all applications, it is desirable to optimize the design for the type and quantity of debris that the strainer is required to handle. Two basic factors need to be considered: the filtration area required, and the potential volume of debris that must be accommodated within the strainer. The number of fins is determined by the required filtration area, and then fin spacing can be varied to ensure that there is sufficient space between fins to accommodate the potential debris volume. The strainer module is advantageously fabricated in a size that is easily manageable and can be moved into place without interference with surrounding equipment. In addition, a complete strainer assembly may contain as many strainer modules as necessary. 
         [0027]    Two types of fins that may be incorporated in the apparatus of the present invention are discussed below in relation to  FIGS. 4 ,  5 ,  6  and  8 . 
       Flat-Surface Fin 
       [0028]    Referring now to  FIG. 4 , in accordance with one aspect of the present invention, flat-surface fins, have a pair of planar fluid permeable screens  16  of perforated metal sheet on each side of the fin in opposed spaced relation. Screens  16  filter debris from the water entering the strainer. The two screens  16  are separated by a corrugated metal spacer  17 . Spacer  17  provides stiffness and strength, and also forms flow channels between screens  16  to header  3 . The edges of screens  16  are enclosed by perimeter frame  18 . The flow channels between screens  16  are open to fluid communication with the enclosed flowpath in header  3  through the marginal side edge of frame  18  that fits into aperture  9  of header  3 . Frame  18  also adds significantly to the structural strength of the fin. 
         [0029]    If the application requires smaller filtration holes than are achievable using standard perforated metal mesh, a layer of fine wire mesh may be laminated onto the surface of perforated metal screens  16  of the fin. 
         [0030]    The advantages of the fin construction shown in  FIG. 4  include simplicity of manufacture and minimal internal volume. 
       Corrugated-Surface Fin 
       [0031]    Referring now to  FIGS. 5 ,  6  and  8 , in accordance with another aspect of the present invention, corrugated-surface fins are constructed of two layers of perforated metal screen  19  that have been corrugated to increase their exposed surface area. Debris filtered out of water entering the strainer is deposited on these corrugated surfaces. 
         [0032]    The corrugations provide a number of advantages. The large increase in filtration surface area over a flat screen is a very significant advantage for thin debris layers, which often pose more of a problem than more porous thick debris layers. The increased area reduces the restriction to flow entering the strainer by decreasing the water velocity through the screen and reducing the thickness of debris (because it is spread over a larger area). The “peaks” of the corrugations also reduce the pressure drop by tending to encourage a locally non-uniform debris bed. Even with layers of debris that are thicker than the height of the corrugations, there can be a significant benefit, because fine particulates often migrate through the debris bed and concentrate near the filtration surface, causing a thin relatively impermeable layer at the surface. The resistance of this thin layer to flow entering the strainer is reduced with the larger screen area achieved by the corrugations. 
         [0033]    Another important feature of this design is that the corrugated screens can be made strong enough to be relied on as the sole structural element in the fin. Moreover, the screen can also be formed using relatively thin gauge material. This minimizes the amount of material required to make a fin, saving cost and making the fins easier to handle because of their reduced weight. 
         [0034]    The corrugated metal mesh screens have a plurality of parallel “peaks” and “valleys” and are positioned in opposed spaced relation such that alternating peaks in one screen are in tip-to-tip contact with alternating peaks in the opposed screen. This configuration forms hollow internal channels for fluid entering the strainer to flow towards the collection header. These flow channels are unobstructed and can be made large enough to offer minimal restriction to flow. The internal volume of the design is minimized, therefore maximizing the space outside the strainer to collect debris. 
         [0035]    As shown in  FIG. 5 , the perimeter frame about the fluid permeable screens can comprise flat bar  20  to seal the edges parallel to the corrugations and to provide strong attachment points  21  for bracing  6  and fin attachment hardware  10  and  11 . These edges can also be sealed with perforated metal screen  22  to further increase the filtration surface area. 
         [0036]    The perimeter frame about the fluid permeable screens can also comprise perforated metal caps  23  to seal the ends of the corrugations. The advantage of this type of end cap is that it adds to the perforated screen area and does not restrict flow access to the space between the fins. In the embodiment shown in  FIGS. 6 and 8 , end cap  40  is formed from a channel, which is then welded over the end of the fin. The advantage of this type of end cap is that it is simple to manufacture and adds significantly to the strength of the fin. It can be fully or partially perforated if extra screen area is needed. 
         [0037]    The marginal side edge of the perimeter frame at the edge of the fin fitting into the header is adapted to a rectangular cross-section to fit the rectangular slots  9  in the header. This can be done using a toothed strip of perforated metal  24 , with the edge sealed into the collection header with a flexible metal strips  25  as shown in  FIG. 5 .  FIGS. 6 and 8  also shows a simpler design of a cap  41  for the portion of the perimeter frame at the header end of the fin. Cap  41  is formed from a channel that is welded over the ends of the corrugated metal mesh sheets. Cap  41  has large openings through which flow  43  from the channels between the corrugations communicates with apertures  9  and the flowpath enclosed in header  3 . The sides of the end cap provide surfaces to attach seals  42 , which ensure a good fit of the fin into the header. 
       Flow Equalization 
       [0038]    Reasonably uniform flow is desirable to prevent formation of hollow-core vortexes and to ensure that debris depositing onto the strainer is not packed too densely. If the flow concentrates at one spot, debris will quickly build up at this spot in a very dense mat, raising the flow resistance enough that flow will enter at an adjacent spot, causing a dense bed to build up there also. If unrestrained, this can progress throughout the whole strainer, causing a much higher pressure loss than if the debris had built up uniformly. 
         [0039]    In a further embodiment of the present invention shown in  FIG. 7 , the flow entering at various points along the length of the header is controlled using flow-balancing devices that boost the pressure inside the header. As fluid flows along the header, frictional pressure drop and acceleration pressure drop cause decreasing pressures closer to the suction end of the header (i.e., in the direction of fluid flow). This would normally provide more driving pressure for flow entering the fins, causing somewhat non-uniform flow (more flow entering fins closer to the pump intake, or suction end). To ensure that water entering any fin is subjected to the same driving pressure differential, calibrated flow-balancing devices are added, which provide greater restriction for fins closer to the suction end of the strainer than at the far end. 
         [0040]    In accordance with a further preferred embodiment of the present invention, the flow-balancing devices provide flow restriction in a partially reversible fashion. Thus, the energy required to accelerate the flow through the flow-balancing devices is converted first to kinetic energy as a jet of water in the header pointed in the direction of flow towards the suction end. The momentum of this jet is used to boost the pressure in the header in a manner that partially offsets the upstream friction and acceleration losses. This pressure boost reduces the amount of pressure imbalance along the length of the header. A substantially uniform flow can be achieved at the same time as providing a lower overall pressure loss. 
         [0041]      FIG. 7  shows the details of this embodiment in which all but one fin have been removed for clarity. Flows at various locations are illustrated with arrows. 
         [0042]    Fluid flow  25  enters fin-like strainer element  32  through perforated screens, passes through the internal channels formed by the corrugations which are in fluid communication with header  35  through end cap  41  (see  FIGS. 6 and 8 ) and slot  26 . Flow from neighbouring fins similarly enters header  35  through slots  27  and  28 . A relatively narrow collection channel  38  inside header  35  is defined by vertical baffle  31  running inside outer wall  36  of the header  35 . Flow from the collection channel is accelerated through an orifice  33  forming a jet  39  that joins the flow  29  from upstream fins. Because the velocity of the jet  39  is substantially parallel to the velocity of the main flow  29 , the pressure at downstream flow  30  is raised over that if the water were injected perpendicular to the main flow. Furthermore, because the velocity of the jet  39  is greater than the velocity of the main flow  29 , momentum is added to the main flow, which boosts pressure at flow  30  over that that would exist if the flow velocity of jet  39  were the same as that of main flow at  29 . In addition, the orifice  33  provides a smooth contraction for the flow so that there is minimal energy loss in creating the jet  39 . 
         [0043]    The pressure in the main header  35  drops as one moves closer to the pump intake because of friction and acceleration pressure drops. The differential pressure across orifices closer to the pump intake are therefore greater than across orifices farther away. In order to balance the flows entering the main header, the width of each orifice  33 ,  34  is selected so that the pressures upstream of all orifices, e.g., through each of the collection channels  38 ,  37  are equal. Substantially equal pressure can be achieved by providing that orifices closer to the pump intake have smaller flow area than orifices farther from the pump intake with the result that substantially uniform fluid flow is maintained through strainer elements located at different positions along the flowpath in header  3 . 
         [0044]    A flow control device in the form of an appropriately sized and shaped orifice can be provided for individual collection channels each of which encloses a plurality of apertures (as shown in  FIG. 7 ), or in the alternative, can be provided for each individual inlet aperture. 
         [0045]    Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. 
         [0046]    The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of this application.