HK1112205B - Finned strainer - Google Patents

Description

Filter with fins

Technical Field

The present invention relates to the field of filters for removing debris from water drawn into a piping system. More particularly, the present invention relates to the field of filters used in nuclear power plants.

Background

Nuclear power plants have different safety systems to ensure that the nuclear fuel in the reactor remains cool in all credible accident situations. One such situation is the "loss of coolant accident" in which, assuming the outer tube breaks, a large amount of water escapes from the reactor cooling system. This water will dislodge solid debris from adjacent tubes or other reactor structures. The water flows to the lowest part of the reactor, together with some of the dislodged debris, to form a sump. Nuclear power plants equipped with safety systems draw water from the sump back into the various reactor cooling systems. A strainer at the suction inlet of the pump ensures that any debris that is sufficient to clog the devices in these systems is prevented from entering. Depending on the type of debris, the first layer deposited on the strainer may form a finer filter than the underlying screen and trap many smaller particles.

The strainer must have a sufficient screen area and the debris layer on the strainer is not so thick as to cause unacceptably high flow restriction. The filter must also be as small as possible in order to fit into the available space. Therefore, compactness, i.e., accommodating the most screen area in the smallest volume, is very important.

The conventional strainer is in many nuclear power plants the simplest box-shaped device mounted on the pump intake. Newer, more advanced filters often have irregular surfaces to increase surface area.

The purpose of this background information is to present what applicants believe is known to constitute a possible association with the present invention. Are not necessarily to be construed as admissions scope, and any of the foregoing information constitutes prior art with respect to the present invention.

Disclosure of Invention

According to one aspect of the present invention, there is provided a strainer for filtering fluid debris comprising an elongated header defining a closed flow path having an outlet in fluid communication with a suction source and a plurality of inlet apertures disposed along the length of the flow path, the flow path exhibiting a pressure drop in the direction of fluid flow; a filter element disposed in each of the inlet apertures for filtering debris of the fluid entering the flow path; and flow control means for maintaining a substantially uniform fluid flow through the filter element at different locations along the flow path.

According to another aspect of the invention, the flow control device comprises an orifice creating a pressure drop between the inlet orifice and the flow path located closer to the suction source that is greater than the pressure drop between the inlet orifice and the flow path located further from the suction source. The aperture may be in the form of a nozzle for accelerating fluid entering the flow path in a direction generally parallel thereto, and may be formed in a baffle disposed in the header, the baffle defining a collection channel surrounding the plurality of apertures.

According to yet another aspect of the invention, the header has a generally planar sidewall, and the inlet apertures are a series of generally parallel slots formed in the sidewall in a direction transverse to the flow path. The filter element may be in the form of a planar fin projecting outwardly from an aperture in the planar sidewall.

According to a further 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 plurality of fin-like strainer elements projecting outwardly from each aperture slot for straining debris from said fluid, each strainer element comprising a peripheral frame and a fluid permeable screen secured in opposed spaced relation, and at least one fluid passage therebetween in fluid communication with said enclosed volume through a marginal side edge of said frame and said aperture slots. The fluid permeable screen may be in the form of a perforated metal plate or mesh. Corrugated metal spacers may be disposed between the fluid permeable screens for holding the fluid permeable screens in a spaced relationship and a plurality of fluid passages may be defined between the corrugated metal spacers and the fluid permeable screens. The perimeter frame is impermeable to fluid beyond the one marginal side edge.

According to yet another aspect of the invention, each fluid permeable screen is formed from a corrugated metal mesh having a plurality of parallel peaks and valleys, the screens being maintained in opposed spaced relation by contact at alternating peaks and defining a plurality of said flow channels therebetween.

Drawings

Fig. 1 is an isometric view of a filter module connected to a pump intake in accordance with an embodiment of the present invention.

FIG. 2 is an exploded view of the strainer module shown in FIG. 1;

FIG. 3 is a cross-sectional isometric view of a strainer module according to an embodiment of the present invention mounted directly to a sump;

FIG. 4 is a cut-away isometric view of a portion of a planar tab according to an embodiment of the present invention;

FIG. 5 is a cross-sectional isometric view of a portion of a corrugated face fin according to an embodiment of the present invention;

FIG. 6 is an exploded view of a corrugated face sheet according to an embodiment of the present invention;

FIG. 7 is an isometric cross-sectional view of a flow equalization apparatus according to an embodiment of the present invention; and

FIG. 8 depicts a corrugated face fin according to an embodiment of the present invention.

Detailed Description

Referring to fig. 1 and 2, the strainer module of the present invention includes an elongated header 3 defining an internal fluid flow path that is in fluid communication with a suction source through a suction tube 2 of a pump, which suction tube 2 may be positioned in the ground or wall by one or more fittings 1. Header 3 has a generally planar side wall with a plurality of inlet apertures 9, the inlet apertures 9 being in the form of a series of generally parallel elongated slots disposed along the length of the header to receive the fins 4. The inlet aperture is oriented in a direction transverse to the fluid flow path within header 3. A filter element in the form of a hollow planar fin 4 may be mounted to the side (as shown in fig. 1), top or bottom of header 3 and project outwardly from inlet aperture 9. The fins 4 have a uniform or variable pitch and are positioned by the mounting frame 5 and the struts 6. In the preferred embodiment, the fins are easily removable using pins 10 and bolts 11, but may also be permanently attached to header 3.

Water enters the strainer through a fluid permeable screen 7 on the surface of the fins 4, leaving debris on the screen. The water then flows through the fluid flow passages in the hollow cores 8 of the fins 4 towards the header 3. Different parts of the header 3, in particular between the fin slots 9, may be made of a fluid permeable material to increase the filtration area. Header 3 may have one or more baffles 12 to provide structural support against the sides of high suction pressures. Baffle 12 has large holes 13 to ensure that the flow velocity in the fluid flow channels of header 3 is the same above and below the baffle.

The ends of each header 3 have flanges 14 that allow adjoining modules to be connected together. The modules may be connected together or mounted separately by seals between the modules. A mounting frame 5 may be provided under the module. The mounting frame 5 has a base 15 of adjustable height which allows the device to be mounted on a non-horizontal ground.

Figure 3 shows an alternative embodiment of the invention. This embodiment may be used in the case of a filter having a pre-existing sump 45 with a cover 46 (which may be pre-existing or specially installed to support the filter module), and a pump intake 47 inside the sump 45. The strainer module 48 includes fins 49 mounted within a frame 50 by suitable struts. For simplicity, only one module is shown in FIG. 3. If desired, a plurality of strainer modules 48 may be mounted on sump 45 in the same manner.

With respect to the embodiment shown in fig. 1 and 2, flow enters the fins 49 in the same manner as described above, then flows directly into the sump 51 and then into the pump intake 52. The pump intake 52 may be modified to reduce inlet losses. This arrangement eliminates the need for a separate collection header because the sump 51 itself performs this function. Close tolerances between fittings, or the use of wire mesh gaskets or any other suitable type of seal on the edges of the vanes, such as the components shown in fig. 5, may be used to prevent unwanted bypass flow between the sump cover 46 and the module frame 50. Appropriate portions of the module frame 50 or sump cover 46 may be made of perforated metal sheet to increase the filtration area. To take advantage of some of the volume of sump 51, the strainer module may be partially or fully recessed into the sump below the ground level. In this case, the frame 50 of the module will extend from the floor level down to the bottom of the module to prevent fluid from bypassing the filter elements of the strainer.

Air intake can be prevented by ensuring that the water on the filter is of sufficient height. In the alternative, a horizontal cover (not shown) may be added to the tab. The cover allows the fin to be closer to the water surface without drawing air or causing hollow core vortices.

Different types of struts are used as shown in figures 1, 2 and 3 to ensure that the filter is sufficiently rigid to resist the anticipated seismic and pressure loads. In addition, external struts, such as those indicated by reference numeral 6 in fig. 2, may also be placed between the fins.

For all applications, it is desirable to optimize the design for the type and amount of debris that the strainer needs to handle. Two basic factors to consider are: the required filtration area, and the potential volume of debris that must be contained within the strainer. The number of fins is determined based on the desired filter area and the fin spacing is then varied to ensure that there is sufficient space between the fins to accommodate this potential debris volume. The filter module is advantageously sized so that it is easy to manage and can be put into place without disturbing surrounding equipment. In addition, a complete filter assembly can contain the desired filter modules.

Two types of fins that may be incorporated into the device of the present invention are discussed below with respect to fig. 4, 5, 6 and 8.

Planar fin

Referring now to fig. 4, in accordance with one aspect of the invention, a planar fin has a pair of opposed spaced apart perforated metal sheet planar fluid permeable screens 16 on each side of the fin. The screen 16 filters debris from the water entering the strainer. The two screens 16 are separated by a corrugated metal spacer 17. The spacers 17 provide rigidity and strength and also form flow channels between the screens 16 to the header 3. The edges of screen 16 are sealed by perimeter frame 18. The flow channels between screens 16 are open to fluid communication with the closed flow path in header 3 through the marginal side edges of frame 18 that fit into apertures 9 of header 3. The frame 18 also significantly increases the structural strength of the tab.

If the application requires smaller filtration holes than would be obtained using a standard perforated metal mesh, a layer of fine wire mesh may be laminated to the surface of the perforated metal screen 16 of the airfoil.

The advantages of the airfoil shown in fig. 4 include simplicity of manufacture and minimal internal volume.

Corrugated surface fin

Referring now to figures 5, 6 and 8 of the drawings, in accordance with another aspect of the invention, the corrugated surface fin is formed of two layers of perforated metal screen 19 which is corrugated to increase its exposed surface area. Debris filtered from the water entering the strainer is deposited on the corrugated surfaces.

The corrugation provides a number of advantages. A larger increase in the area of the filtering surface compared to a flat screen is a very significant advantage of a thin layer of debris, which often causes more problems than a porous thick layer of debris. The increased area reduces restriction of flow into the strainer by reducing the water flow rate through the screen, and reduces the thickness of debris (as 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 bed of debris. Even with a debris layer that is thicker than the height of the corrugations, there can be significant benefit because fine particles often move through the debris bed and collect at the filtration surface, which results in a thin, relatively impermeable layer at the surface. The resistance of the lamina to flow into the strainer is reduced by the larger screen area obtained by the corrugations.

Another important feature of this design is: the corrugated screen can be made strong enough to be relied upon as the only structural element in the fin. Also, the screen may be formed using relatively thin gauge material. This minimizes the amount of material required to manufacture the airfoil, saves cost, and makes the airfoil easier to handle due to its reduced weight.

The corrugated metal mesh screen has a plurality of parallel "peaks" and "valleys" and is positioned in an opposing spaced relationship such that alternating peaks in one screen make tip-to-tip contact with alternating peaks in the opposing screen. This configuration creates a hollow internal channel for fluid entering the filter to flow to the collection header. These flow channels are unobstructed and can be made large enough to provide minimal restriction to flow. The internal volume of the design is minimized, thus maximizing the space outside the strainer to collect debris.

As shown in fig. 5, the perimeter frame around the fluid permeable screen may include flat bars 20 to seal the edges parallel to the corrugations and provide strong attachment points 21 for the struts 6 and fin attachment hardware 10 and 11. These edges may also be sealed by a perforated metal screen 22 to further increase the filtering surface area.

The perimeter frame around the fluid permeable screen may also include a perforated metal cap 23 to seal the ends of the corrugations. The advantages of this type of end cap are: which increases the perforated screen area and does not restrict flow to the space between the fins. In the embodiment shown in fig. 6 and 8, the end cap 40 is formed from a channel, which is then welded to the end of the fin. The advantages of this type of end cap are: which is simple to manufacture and which significantly increases the strength of the fin. If additional screen area is required, it may be perforated in whole or in part.

The marginal side of the perimeter frame at the edge of the fin fitting into the header is adapted to a rectangular cross-section to fit into a rectangular slot 9 in the header. This can be done using a toothed strip 24 of perforated metal, the edge being sealed into a collection header having a flexible metal strip 25 as shown in figure 5. Figures 6 and 8 also show a simpler design of the cap 41 for the peripheral frame portion of the fin header end. The cap 41 is formed by channels welded to the ends of the corrugated expanded metal sheet. Cap 41 has large openings through which flow 43 from the channels between the corrugations communicates with bore 9 and the flow path enclosed in header 3. The side of the end cap provides a surface to which the seal 42 is attached, which ensures that the fin is well adapted to the header.

Flow equalization

A reasonably uniform flow is desired to prevent the formation of hollow core vortices and to ensure that debris deposited onto the strainer is not packed too densely. If the flow is concentrated at one point, the debris will collect in a very dense mat at that point, raising enough flow resistance that the flow will enter an adjacent point, which causes a dense bed to also collect there. If not restricted, this can progress through the strainer, which can result in higher pressure losses than if the debris were to collect uniformly.

In a further embodiment of the invention shown in fig. 7, the flow entering at different points along the length of the header is controlled using a flow balancing device that raises the pressure inside the header. As fluid flows along the header, frictional pressure drops (frictional pressure drops) and acceleration pressure drops (acceleration pressure drops) cause a pressure decrease (i.e., in the direction of fluid flow) near the suction end of the header. This will generally provide more driving pressure for flow into the vanes, resulting in some non-uniform flow (more flow into the vanes nearer the pump suction or intake end). To ensure that water entering any of the fins is subjected to the same driving voltage differential, a calibrated flow balancing device is added which provides greater restriction to the fins closer to the suction end of the strainer than to the fins at the distal end.

According to a further preferred embodiment of the invention, the flow-balancing means provides the flow restriction in a partially reversible manner. In this way, the energy required to accelerate the flow by the flow accelerating device is first converted into kinetic energy of the water jet stream in the header which projects in the flow direction towards the suction end. The momentum of the jets is used to boost the pressure in the header in a manner that partially compensates for the friction and acceleration losses upstream. This increase in pressure reduces the amount of pressure imbalance along the length of the header. A substantially uniform flow may be obtained while providing a lower total pressure loss.

Fig. 7 shows a detail of this embodiment, where only one flap is left for clarity. Arrows indicate flow at different locations.

Fluid flow 25 enters the fin-like strainer element 32 through a perforated screen and passes through internal passages formed by the corrugations which are in fluid communication with header 35 through end caps 41 (see fig. 6 and 8) and slots 26. Flow from adjacent fins similarly enters header 35 through slots 27 and 28. The relatively narrow collection channels 38 inside the header 35 are defined by vertical baffles 31 acting inside the outer wall 36 of the header 35. The flow from the collecting channel is accelerated by the holes 33 forming the jets 39 connecting the flow 29 from the upstream fins. Because the velocity of the jets 39 is substantially parallel to the velocity of the primary flow 39, the pressure of the downstream flow 30 will rise above the case where water is injected perpendicular to the primary flow. Also, because the velocity of the jet 39 is greater than the velocity of the primary flow 29, kinetic energy is added to the primary flow, which raises the pressure of the flow 30 beyond what it would be if the velocity of the jet 39 were the same as the velocity of the primary flow 29. In addition, the holes 33 provide a smooth constriction to the flow so as to have a minimum energy loss in producing the jets 39.

When moving closer to the pump intake, the pressure in the primary header 35 drops due to friction and acceleration pressure drops. The differential pressure of the orifices closer to the pump intake is greater than the differential pressure of the orifices further away. To balance the flow into the primary header, the width of each orifice 33, 34 is selected so that the pressure upstream of all orifices is equal, for example, through each collection channel 38, 37. By providing apertures closer to the pump intake with a smaller flow area than apertures further from the pump intake, substantially equal pressures can be achieved, with the result that substantially uniform fluid flow is maintained through filter elements located at different positions along the flow path in header 3.

A separate collection channel may be provided for each surrounding plurality of apertures or, alternatively, a flow control device having appropriately sized and shaped apertures may be provided for each separate inlet aperture (as shown in fig. 7).

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.

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 the present application.

Claims (13)

1. A strainer for filtering fluid debris:

i) an elongated header defining a closed flow path having an outlet in fluid communication with a suction source and a plurality of inlet apertures disposed along the length of the flow path, the flow path exhibiting a pressure drop in the direction of fluid flow;

ii) a filter element disposed in each of the inlet apertures for filtering debris from fluid entering the flow path;

iii) flow control means for maintaining a substantially uniform fluid flow through the filter element at different positions along the flow path.

2. A filter as claimed in claim 1, wherein the flow control means comprises an aperture for creating a pressure drop between the inlet aperture and the flow path at a location closer to the suction source, which pressure drop is greater than the pressure drop between the inlet aperture and the flow path at a location further from the suction source.

3. A filter as claimed in claim 2, wherein the apertures are in the form of nozzles for accelerating fluid entering the flow path in a direction substantially parallel to the flow path.

4. A strainer according to claim 3, wherein said holes are formed in a baffle provided in said header, said baffle defining a manifold channel surrounding a plurality of said holes.

5. A strainer according to claim 4, wherein said header has a generally planar side wall, said inlet apertures being a series of generally parallel slots formed in said side wall in a direction transverse to said flow path.

6. A filter as claimed in claim 5, wherein the filter elements are in the form of tabs projecting outwardly from apertures in the planar side wall.

7. A strainer for filtering fluid debris 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 filtering said fluid debris, each said strainer element comprising a perimeter frame and a pair of opposed and spaced apart fluid permeable screens secured to said perimeter frame, and at least one fluid flow channel between said pair of fluid permeable screens, said fluid flow channel being in fluid communication with said enclosed volume through a marginal side edge of said perimeter frame and said aperture slots.

8. The strainer of claim 7 wherein said fluid permeable screen is formed from a perforated metal plate.

9. The strainer of claim 8 further comprising corrugated metal spacers disposed between said fluid permeable screens and holding said fluid permeable screens in spaced relation, a plurality of said flow channels being defined between said corrugated metal spacers and said fluid permeable screens.

10. The strainer of claim 9 wherein said peripheral frame is impermeable to fluid except at said one marginal side edge.

11. The strainer of claim 7, wherein each of said permeable screens is formed of a corrugated metal screen 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 between each pair of fluid permeable screens.

12. The strainer of claim 11 wherein the peripheral frame is impermeable to fluid except at the one marginal side edge.

13. The strainer of claim 11, wherein said corrugated metal screen is a corrugated metal screen.