Abstract:
A screen for separating solids and liquids is formed from parallel strips of wedge wire or similar material into a two-sided enclosure having the flat bases of the wedge wires oriented to contact dirty liquid. Unfiltered liquid containing solids contacts the screen surface on both sides of the enclosure, forming a passage for filtrate. A wedge wire screen may be bent back on itself to form a C-shape or may be a closed curve, still exposing only the flat sides of the wedge wires to the dirty liquid, and collection filtrate within the enclosure so formed. Filter units may be placed in a housing adapted to accommodate two or more in a substantially concentric relationship.

Description:
RELATED APPLICATION  
       [0001]     This is a continuation-in-part of application Ser. No. 11/374,234 filed Mar. 13, 2006, which claims the full benefit of Provisional Application 60/662,065 filed Mar. 14, 2005. 
     
    
     FIELD OF THE INVENTION  
       [0002]     Substantially concentric convex and concave wedge wire filtering surfaces are formed into an enclosure that will fit into a housing. Filtrate from both filtering surfaces passes to a common space in both dead-end and cross-flow modes. High throughputs and separation efficiency are obtained.  
       BACKGROUND OF THE INVENTION  
       [0003]     Good screening and filter throughput is desirable for many high volume fluid handling operations, such as filtering and screening of well completion and workover fluids, but has been difficult to sustain in the varied and generally hostile conditions of many well drilling and producing operations. Backwashing is also sometimes inefficient because of the design of the solids separation device.  
       SUMMARY OF THE INVENTION  
       [0004]     We have developed a new design for a filter or screen which overcomes to a large degree the difficulties recited in the background of the invention; namely the invention provides a sustainable throughput for large volumes of fluid, and the ability efficiently to backwash. The invention provides that two wedge-wire screens are formed into elongated substantially concentric or parallel shapes so that the flat sides of the wedge wires acting on the liquid to be filtered will form a common space for the filtrate. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0005]      FIG. 1  illustrates a prior art technique for manufacturing a cylindrical wedge wire screen.  
         [0006]      FIG. 2  is a simplified section of a cylindrical two-section concentric screen of the invention.  
         [0007]      FIG. 3  is a perspective of a construction similar to that of  FIG. 2 .  
         [0008]      FIG. 4  is an “exploded” view of the screen device, including the end units.  
         [0009]      FIG. 5  is an overhead view of the top plate of the reservoir which facilitates collection of the filtered fluid.  
         [0010]      FIG. 6  shows a C-shape screen in the configuration of a candle filter.  
         [0011]      FIG. 7   a  is a simplified sectional view of two oppositely oriented C-shaped filters, the C-shape being somewhat “squashed.” 
         [0012]      FIG. 7   b  shows two oppositely facing wedge wire screens attached to each other to form a portion of a variation of our invention.  FIG. 7   c  illustrates a further variation wherein a fabric may be used as an outer filtration surface.  
         [0013]      FIG. 8  is a view of a double filter having a closed oval shape.  
         [0014]      FIG. 9  illustrates the operation of a filter similar to  FIG. 7   a  or  8 .  
         [0015]      FIG. 10  is a detailed, somewhat idealized view of the operation of our filters.  
         [0016]      FIG. 11  is an overhead or sectional view of a filter of our invention made from a single sheet of wedge wire. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0017]     The invention is illustrated in terms of a wedge wire screen, but the C-shape enclosure to be described below is applicable to other types of screens and to filters, as will be explained.  
         [0018]      FIG. 1  is a detail of the construction of a wedge wire screen useful in the invention, which can be placed in various positions. As is known in the art, a screen can be made by winding a wedge wire  40 , an extruded, triangular section wire, around a cage of parallel ribs  41 , fixing them to form a space or slot  42  of a desired opening dimension between them, usually by welding. The ribs may lie on the outside of the unit instead of the inside as shown, and may be welded to either a flat side of wedge wire  40  or to an edge as shown. We may use the ribs in any of these variations, but generally, since we believe it is desirable to use the flat sides of the wedge wires for the retentate side of the filter surface, we prefer that the ribs be welded or otherwise fixed to the longitudinal edges of the wedge wires. They need not be helically wound as shown in  FIG. 1 , but can be made in flat sheets, of predetermined segments of wedge wires. It is not essential that the wedge wires used in our invention have a delta-shaped profile. We include in the term “wedge wire” wires or extrusions (metal or plastic) having a trapezoid profile; see, for example, the shapes illustrated in U.S. Pat. No. 5,476,588. We may refer to the flat side of the wedge wire which contacts the dirty fluid or retentate as the “base” of the wedge wire; in the case of a trapezoid form, the base is the widest side of the polygon—the sides next to the base, in profile, must be at acute angles from the base. Bases of the wedge wires form the outer, or retentate, sides of the filter.  
         [0019]      FIG. 2  is a simplified sectional view of the construction of a substantially cylindrical filter or wedge wire screen of my invention. Here, there are two C-shaped screen units  43  and  44  set substantially concentrically in a cylindrical housing  45 . Each screen unit  43  and  44  has a convex face  43   a  and  44   a , and a concave face  43   b  and  44   b , both of which are to be contacted by unfiltered fluid, represented here by the shaded areas. Each screen unit  43  and  44  also has end caps  43   c  and  44   c , which may be impervious—that is, it need not be of wedge wire or other screen material. Together with the concave and convex faces, and, together with the fact that I seal the C-shaped units at the top and bottom, the end caps form an enclosure. Unfiltered fluid enters the cylindrical housing  45  through inlets not shown (from anywhere through the housing  45 , or its top, provided it passes into a portion of the shaded area labeled “unfiltered fluid) and passes through the separator media (such as wedge wire screens) of both the convex and concave sides of the screen units, leaving solids of the undesired size behind. Filtered or screened fluid within the screen units may then be removed through outlets  52  as illustrated in  FIGS. 4 and 5 . It should be noted that both the convex faces  43   a  and  44   a , and the concave faces  43   b  and  44   b  of the screen units are constructed so that, if they are made of wedge wire, the flat side of the wedge wire contacts the unfiltered fluid. One of the features of wedge wire screens is that a solid particle is not likely to become lodged in a slot  42  because the anterior of the slot is divergent, i.e. the slot is between two triangular shapes opening to the interior of the filter surface. Thus the construction of the concave faces  43   b  and  44   b  is opposite the convex orientation shown in  FIG. 1 , the wedge wire being laid on the inside of ribs  41  rather than the outside; nevertheless, the unfiltered fluid contacts only flat surfaces defining the slots  42  ( FIG. 1 ). A generally C-shaped face  43   a  or  43   b  can be made by making a longitudinal cut in the wedge wire screen of  FIG. 1 . A concave filter or screen surface may be made by bending a cylindrical surface such as that made in  FIG. 1  so that the flat surfaces face inwardly. Of course, “sheets” of wedge wire screen can be made by welding or otherwise fixing precut lengths of wedge wire to parallel ribs on a plane or flat surface and then bending them to the desired form.  
         [0020]     As indicated above, such flat sheets may be made by welding or otherwise securing either a longitudinal edge or a flat side of the wedge wire to the ribs. It may be observed that if a sheet of wedge wires is bent over an axis perpendicular to the wedge wires, the spacing between them is unaffected, but if the axis around which the sheet is bent is parallel to the bending, the spaces will either enlarge or contract depending on the direction of the bend. See FIG. 2 of Nagaoka U.S. Pat. No. 5,858,235 for an example of a cylindrical shape with the wedge wires running parallel to the axis; also FIG. 3 or 4 of Norell et al U.S. Pat. No. 6,698,595. The wedge wires in these depictions are of precut lengths rather than a long helical strand in the present  FIG. 1 . We may use any such construction.  
         [0021]      FIG. 3  is a perspective of the two-enclosure, substantially concentric, configuration, without the housing  45 . Wedge wires form the entire convex ( 43   a  and  44   a ) and concave ( 43   b  and  44   b ) faces of the C-shaped screen units. Slots  42  of the desired dimension are established between wedge wires  40 . In the configuration of  FIG. 3 , C-shaped screen unit  43  is shown with its elongated opening  47  aligned with elongated opening  46  of C-shaped screen unit  44 , but this is not essential—that is, screen unit  44  could be turned, for example, 180 degrees so that opening  46  is oriented away from opening  47  of screen unit  43 .  
         [0022]     Referring now to  FIGS. 4 and 5 , the top plate  50  of reservoir  51  is seen to have outlets  52  for filtered fluid having passed through the wedge wire screens of screen units  43  and  44 . When assembled, housing  45  and the two screen units  43  and  44  are sealed to top plate  50 . Filtered fluid collects in reservoir  51  and is removed through pipe  54 . A cylindrical screen  55  constructed as in  FIG. 1  may reside in the center of inner enclosure  44 , providing additional volume for the collection of filtered fluid.  FIG. 4  is an exploded view of the top seal  53 , screen units  43  and  44 , reservoir  51  with its top plate  50 , and pipe  54 . Housing  45  and the inlet for the dirty fluid are not shown in this view.  FIG. 5  is an overhead view of top plate  50 , showing the deployment of outlets  52  for screen units  43  and  44 .  
         [0023]      FIG. 6  shows the use of my C-shaped wedge wire screen in a candle filter construction, in a more or less diagrammatic fashion. The C-shaped wedge wire screen  60  is viewed from its opening  61 . The screen  60 , made of wedge wires  69  in a manner similar to that of  FIG. 1 , is located and fixed next to a ledge  62  near the top of vessel  63 . C-shaped wedge wire screen  60  is essentially the same shape and structure as screen unit  43  or  44  in  FIG. 2  (having spaces not shown, similar to slots  42  in  FIGS. 2 and 3 ), but here we are looking directly at the opening  61  (equivalent to openings  46  and  47  in  FIG. 3 ), although the screen  60  is entirely enclosed in vessel  63 . Vessel  63  has an entrance  64  (which may preferably be oriented toward opening  61 ) for dirty fluid, an exit  65  for clean fluid, and a drain  66  for solids and concentrated dirty fluid. The lower end  72  of the vessel  63  has a shape similar to a funnel so that solids may collect and drop by gravity to drain  66 . The wedge wire screen  60  is constructed in a sense opposite to that of  FIG. 4  in that the clear filtered fluid is taken off the top and sent through exit  65  instead of through the bottom; solids and dirty fluid exit in the bottom. For these purposes, it should be noted that the top of wedge wire screen  60  may be completely open to the clean fluid collection chamber  68 ; on the other hand, the bottom of the wedge wire screen  60  should either be sealed or closed off with a screen material, so that solids and dirty water will not enter the wedge wire screen  60  from the bottom. Valves  70  and  71  may be used to control the flow out of the vessel  63 .  
         [0024]     A screen such as depicted in  FIGS. 1-6 , or any other effective screen, may advantageously be placed immediately upstream of a viscometer to protect the viscometer from solids, or just ahead of a filter, to remove solids larger than the filter is designed for. In addition to removing potentially damaging solids, the wedge wire screen can perform the function of breaking up “fish-eyes” or other localized gel blobs, as well as shearing a viscous fluid, sometimes delaying the point at which the fluid is diverted or at which the pump is shut down.  
         [0025]     Referring now to  FIG. 7   a , a section of a double C-shaped filter is shown ready for placement in a housing (not shown) of suitable shape. Here, both the larger filter  10  and the smaller filter  11  utilize wedge wires running lengthwise on the filter surface, covering the entire longitudinal surfaces of each of the filters. The wedge wires on both filters  10  and  11  run parallel to the axis of the “squashed” C-shaped unit. Wedge wires  12   a  and  12   b  of filter  10 , and  13   a  and  13   b  of filter  11 , are welded or otherwise secured to generally orthoganally placed ribs  14  and  15 . Ribs  14  and  15  can be on either the convex or concave sides of the filter face and either the retentate or filtrate side. Similar to the constructions of  FIGS. 2, 3 , and  4 , the filter units are closed beyond ends  16  and  17 , but need not be if a cap for the housing is to be used which will seal off the tops of the filter units. Wedge wires  12   a , the outer wedge wires, and  12   b , the inner wedge wires, are on opposite sides of filter  10 , and wedge wires  13   a , the outer wedge wires, and  13   b , the inner wedge wires, on opposite sides of filter  11 , are oriented with their flat sides facing the liquid to be filtered—the spaces labeled “IN.” The flat sides of the wedge wires thus form the “outer” or “retentate” sides of the filter as those terms are used herein. On the convex surfaces of each filter, the spaces between the wedge wires will be widened as the curvature of the surface increases, and on the concave surfaces the spaces between wedge wires will be narrowed. If it is desired to have the spaces on both the concave and convex sides of the filters of equal width, this can be accommodated by appropriately altering the spacings between the flats of the wedge wires before the wedge wires are shaped into the desired configuration. Filtrate passes to the spaces within the C-shaped filters  10  and  11  This double C-shaped filter can be utilized in either the dead-end or cross-flow mode. In either mode, incoming fluid can flow freely into all the spaces labeled “IN,” from any other space labeled “IN,” subject to the connections, pressure differences, and resulting planned flow patterns within a particular housing.  
         [0026]     In the configuration where the convex and concave wedge wires are perpendicular to each other (i.e. if one is horizontal and the other is longitudinal), then the ribs can be eliminated in some instances by fixing points where the apexes of the opposing wedge wires come into contact. This is illustrated in  FIG. 7   b , where a portion of a filter of our invention is shown.  FIG. 7   b  shows a filter surface made of the flat sides of parallel wedge wires  2  aligned to form slits  3 , with their triangular or delta profiles  4  forming immediately diverging channels  6  between the wedge wires. Parallel wedge wires  2  are fastened, such as by welding or sintering, at the intersections  7  to a similar plurality of parallel wedge wires  8 , also having slits  3  between them of a dimension capable of retaining solids of the desired size, and also forming diverging channels  6  between them. The two substantially parallel planes of wedge wires comprise a filter of our invention, able to receive dirty fluid and act as retentate surfaces on both outer surfaces—that is, both surfaces made up of the flat sides of the wedge wires. Filtrate passing through slits  3  will be able to flow in any direction (whatever the pressure differences and flow patterns of the filter dictate) through the diverging channels  6  and otherwise between the two joined, perpendicularly oriented, sheets of wedge wires. Slits  3  are not illustrated in the other figures herein except  FIG. 10  which illustrates the flow of filtrate through them, but are always present, in varying widths, between the wedge wires in our invention. It is not essential that the opposing, contacting, wedge wires be perpendicular to each other. Angles other than ninety degrees are possible and useful, so long as the opposing, and contacting, faces are not parallel. That is, the angle should be sufficiently greater than 0° so that the narrower portions of the wedge wires on the two sets will contact each other and can be secured in place.  
         [0027]     In  FIG. 7   c , a nonwoven fabric  9  may be seen covering the retentate surface of the parallel wedge wires configured as in  FIG. 7   b . Our invention contemplates the use of a woven or nonwoven fabric as an outer filtration surface; that is, a filter medium for contacting the dirty fluid before it contacts the wedge wire retentate surface in any of the configurations shown or contemplated herein. The fabric may completely enclose the wedge wire units. For example, a fabric cover may completely enclose C-shaped filters  10  and  11  in  FIG. 7   a . The wedge wire retentate surface thus not only serves as a filtering medium but as support for the fabric. Backwashing of such a fabric-covered filter will in some instances be more readily accomplished because the fabric will tend to flex away from the support, causing the filter cake to disperse. The fabric may be woven or nonwoven, synthetic or not, mono or multifilament, and of various permeability ratings. We intend for the term “fabric” to include all such possibilities.  
         [0028]      FIG. 8  demonstrates that our filter form can be a closed curve. As with the other variations of our invention, the flat faces of the wedge wires are oriented toward the liquid to be filtered on both sides of the space or enclosure formed to collect the filtrate. Here, longitudinal wedge wires  20  forming the outer shell or retentate surface are placed completely around the “squashed” tubular or oval shape of outer two-faced filter  27 , and are secured by annular ribs  21 . Wedge wires  22  on the concave side of two-faced squashed cylinder inner filter  28  also are shown running longitudinally and are secured to annular ribs  23 . Ribs  21  and  23  may be placed on the sides of the wedge wires opposite to those shown. The inner and outer squashed cylinder filters  27  and  28  can, but need not be, completely independent parts prior to placement in the housing, as the housing can hold them in a substantially concentric relationship or otherwise if desired. If it is desired to fix them in position with respect to each other prior to placement in the housing, this can be easily done with struts or other framework not shown. It should be observed that, because the oval forms of the squashed cylinders are closed—that is, they do not have a longitudinal opening as the C-shaped filter of  FIGS. 3 and 7 —liquid not yet filtered cannot move freely between or among retentate passages  24 ,  25  and  26  unless the cap and/or base of the housing (not shown) permits such flow or other connections are made to permit it.  
         [0029]      FIG. 9  is a simplified diagrammatic vertical section of a filter form similar to  FIG. 7   a  or  8  operating in a housing. Housing  90  surrounds the filter form. Cap  91  has an inlet  92  for the liquid to be filtered. Inlet  92  leads to manifold  93  which has openings  94  for exterior passage  24 , and  95  and  96  for interior passages  25  and  26 . Cap  91  otherwise seals off the top of the oval-shaped filter unit. At the lower end, as depicted, of housing  90  is base  32 , having openings  33  for retentate from exterior passage  24 ,  34  for interior passage  25 , and  35  for interior passage  26 . In the cross-flow mode, retentate will flow from passages  24 ,  25 , and  26  into manifold  37  and continue into conduit  36  for transport to another filter, to a disposal site, to be recycled, or to a system for recovering valuable components from the retentate. For these purposes, valve  38  will be open, but it may be closed, which will convert the operation of the filter into the dead end mode. In the dead end mode, solids may accumulate in manifold  37  and possibly in passages  24 ,  25 , and  26 . In either the dead end or cross flow mode, clean fluid or other filtrate will flow from passages  24 ,  25 , and  26  transversely into filters  27  and  28  and then to conduits  39  for collection or utilization as desired. As indicted elsewhere herein, the wedge wires (not shown in detail) forming filters  27  and  28  may run horizontally or vertically, in the same direction on all filter faces or not.  
         [0030]      FIG. 10  is an exemplary vertical sectional detail of portions of passages such as passages between wedge wires  13   a  and  13   b  in  FIG. 7   a , although here there is only one filter unit, and here horizontal wedge wires rather than vertical wedge wires are shown. Horizontal wedge wires may be either precut for the circumferential dimension or may be helically wound. Wedge wires  75  (corresponding to wedge wires  13   b  in  FIG. 7   a , in that they form an interior retentate surface) line the central passage  76 , generally similar to the central passage interior to filter  11  in  FIG. 7   a , as only one C-shaped filter is shown in  FIG. 10 . Wedge wires  78 , (corresponding to wedge wires  13   a  in  FIG. 7   a ), together with housing  90 , form exterior passage  79 , shown on both sides of the figure. Dirty fluid contacts only the flat sides of wedge wires  75  and  78 . The wedge wires are held in place by ribs not shown.  
         [0031]     A known virtue of a wedge wire screen is that the narrow entrances of the slots between the wedge wires are virtually two-dimensional—that is, the immediate divergence of the space between wedge wires encourages the flow of the filtrate. Dirty fluid entering the top, as depicted, of passages  76  and  79  is subjected to excess pressure relative to the pressure in the permeate passage  86 , which causes clean filtrate to pass through slots between wedge wires  75  and  78 , and flow as shown by the arrows, separated from the retentate. In the cross flow mode, the retentate continues through the passages  76  and  79 , while in the dead end mode, the only fluid exit is the permeate passage  86  formed by the substantially parallel or concentric wedge wires  75  and  78 , permeate passage  86  being accessible only by permeate passing between the wedge wires. Solids and other retentate material are thus accumulated in passages  76  and  79  or further downstream as in manifold  37  of  FIG. 9 . A few of the wedge wires  5  have been depicted optionally as having a trapezoidal profile. As the wedge wires are normally extruded in manufacture, this profile is not difficult to make using the appropriate die, and as indicated above the trapezoidal profile is included in the term wedge wire as used herein along with the more common deltoid form.  
         [0032]     As  FIG. 10  is a micro and idealized view of the operation of our invention, the wedge wire screens are shown in substantially parallel planes. We intend to include within the phrase “substantially parallel planes” both flat planes and curved planes, as can be seen from the figures herein—in particular, we consider substantially concentric planes, and the various “squashed” and bent forms of  FIGS. 7, 8 , and  11 , and other shapes which may not maintain constant dimensions between them, to be within the term “substantially parallel planes.” Generally, two opposing sheets of wedge wires, or opposing portions of the same sheet of wedge wires, are deployed to form an enclosure or passage for filtrate passing into it from both sides after the dirty fluid has contacted the bases of the wedge wires under pressure on both sides of the enclosure. Persons skilled in the art will recognize that the pressures on both sides (on the retentate sides) will normally be substantially equal, and the pressure within the enclosure will normally be lower than that on the retentate sides. The pressure differential between the retentate and permeate sides of these filters can also be achieved by pulling a vacuum on the permeate side of the filter as opposed to exerting excess pressure on the retentate side. Further, we do not intend for the invention always to be used by flowing the retentate and the filtrate in the same direction. Countercurrent flow is also contemplated within our invention.  
         [0033]      FIG. 11  shows how one configuration of a filter of our invention can be made from a single sheet of wedge wire for insertion in a housing  80 . As indicated above, the wedge wire screen may be made of plastic, such as polypropylene or any other conveniently extruded synthetic polymer; ribs may be plastic or metal. From this overhead or sectional view of the longitudinally oriented wedge wires, it can be seen that a single sheet beginning at end A can be bent as shown and then connected to a single point (the full length of the wedge wire screen) at X, Y, and Z so that the flat sides of the wedge wires are oriented as shown. The two ends A and B are also connected. The shape thus formed defines an external passage  82  and an internal passage  83 , each for incoming dirty fluid under pressure. In this particular configuration, filtrate passage  84  is fed from both passages  82  and  83  according to the precepts of our invention (the dirty fluid encountering the flat sides of the wedge wires on the filtering, retentate surfaces), while central filtrate passage  85  receives filtrate from only one transverse direction—that is, from internal passage  83 . Continued, additional, “wrap-arounds” using longer sheets can be employed to construct filters with additional filtrate passages fed from both sides as contemplated in our invention. If the central passage  85  is constructed with the flat sides of the wedge wires facing inwardly, and the rest of the wrap-arounds follow a pattern as shown, dirty fluid can be fed to the central passage and a further outer passage. Filters  10  and  11  in  FIG. 7   a  can be constructed from single sheets, as can filters  43  and  44  of  FIG. 4 , for example.  
         [0034]     Thus, our invention includes a filter having two retentate surfaces in substantially parallel planes, the retentate surfaces comprising the flat bases of a plurality of wedge wires. We mean by “retentate surface” the surface which will contact the dirty fluid to be filtered, sometimes herein called the outer surface. When in an appropriate housing, the two parallel planes will help to define an interior path for filtrate. Our invention also includes a filter comprising two sets of parallel wedge wires, the wedge wires having a substantially triangular, trapezoidal or deltoid profile including a substantially flat base and an apex or side narrower than the base opposite the base, each set of parallel wedge wires forming a filter surface (a retentate surface) comprising the substantially flat sides of the wedge wires, the two sets of parallel wedge wires being oriented and in contact with each other at an angle not parallel with each other, forming intersections with each other, and secured to each other at the intersections.