Abstract:
The present invention provides a vibratory screening apparatus ( 1 ) for use in removing solids from a liquid feed, and a basket ( 4 ) therefore. The apparatus comprises a static outer housing ( 2 ), and a floating basket vibratable by a vibrator device ( 10 ). The basket mounts a stack ( 7 ) of screen assemblies ( 8 ) provided with respective flow directing trays ( 9 ) for receiving filtrates from the screen assemblies. A flow distributor ( 15 ) divides the feed into at least first and a second feed streams and directs them onto respective screen assemblies, and receives from the flow directing trays, filtrates from respective screen assemblies.

Description:
FIELD OF THE INVENTION 
   The present invention relates to vibratory screening apparatus suitable for use with drilling fluids, mineral processing, classification, and dewatering, and the like. 
   BACKGROUND OF THE INVENTION 
   Vibratory screening apparatus is widely used in the oil drilling industry for removing drill cuttings from drilling fluids, and over the years various improvements have been made to the screens used therein, methods for mounting the screens etc to improve ease of use, reduce maintenance etc. A particular problem in offshore platform oil drilling is, however, that platform real estate is very restricted and extremely expensive. There is accordingly a need to improve the efficiency of vibratory screening apparatus in relation to the physical size thereof. 
   SUMMARY OF THE INVENTION 
   The present invention provides a vibratory screening apparatus for use in removing solids from a liquid and solids mixture feed, said apparatus comprising a static outer housing, at least one floating basket mounted so as to be vibratable, in use of the apparatus, by a vibrator device formed and arranged for vibrating said basket, said basket mounting a stack of screen assemblies, with superposed screen assemblies separated from each other by a respective flow directing tray, said apparatus being provided with a flow distributor formed and arranged for dividing said feed into at least a first feed stream and a second feed stream and directing said feed streams onto respective ones of first and second screen assemblies, and receiving filtrate from a respective screen assembly, from said respective flow directing tray(s). 
   With an apparatus of the present invention, the size of apparatus required to process a given volume of feed is substantially reduced compared with conventional apparatus, since a substantially increased effective screen surface area can be accommodated with relatively little or no increase in the size of the apparatus by means of stacking a plurality of screen assemblies within a single basket and using a flow distributor to route multiple flows in parallel through different screens in the stack. 
   Advantageously the distributor is formed and arranged so as to be switchable between a plurality of different flow directing configurations. Conveniently said plurality of flow directing configurations includes an intensive screening configuration in which the whole of the feed is directed onto said first screen assembly and the whole of the filtrate from said first screen assembly is directed onto said second screen assembly. Alternatively or additionally there is provided a restricted feed capacity configuration in which the whole of the feed is directed onto only one of said first and second screen assemblies, and the filtrate therefrom exhausted directly from the apparatus without passing through the other one said first and second screen assemblies. Such a configuration is useful for basic fluid processing where high efficiency or high volume processing are not required and a reduced number of screens in operation reduces operating cost for screens consumed. 
   Advantageously the mesh sizes of the various screens are selected to suit the particular distributor configuration being employed and/or the loading of the mixture (% solids content), the particle size of the solids, and/or the particle size distribution of the solids. Thus for example in a configuration where the feed is divided into one portion passing through the first screen and not the second, and another portion passing through the second screen and not the first, the first and second screens would normally have the same mesh size. On the other hand in a configuration where the whole of the feed is passed successively through both the first and second screens, then the second screen would normally have a finer mesh size than the first screen. 
   In general the distributor will comprise a plurality of passages provided with valves, typically flap valves, sleeve valves or plug valves, or closure plates etc, for selective opening or closing of different passages. The distributor may be mounted in either the static housing or on the floating basket. It is also possible, in principle, for part of the distributor to be mounted in the static housing and part on the floating basket. Where a greater or lesser part of the distributor is mounted in the static housing, then the distributor is generally provided with flexible conduit portions defining at least part of the passages, for coupling the passages from the static housing to the floating basket. 
   The passages of the distributor may be defined in various different ways. Conveniently they are defined by walls extending downwardly inside a downwardly extending chamber so as to provide a lateral subdivision of the chamber into individual passages providing predetermined proportions of the distributor flow capacity. Thus, for example, the distributor may be formed and arranged with one or more first flow passages for transmitting said first feed stream, and one or more second flow passages for transmitting said second feed stream. 
   It is generally preferred that vibratory screen apparatus should have a plurality of screen assembly stages with decreasing mesh size, i.e. meshes of successively finer cut. It will accordingly be appreciated that in addition to having first and second screen assemblies, with similar mesh size, formed and arranged for intercepting said first and second feed streams respectively, the vibratory basket may also have one or more further screen assemblies with different mesh size upstream and/or downstream of said first and second screen assemblies. Conveniently there is provided upstream of first and second screen assemblies, an initial, coarser mesh size, screen assembly and the vibratory screening apparatus is formed and arranged so that substantially the whole of the liquid and solids mixture feed is directed through said initial screen assembly, before being divided into said at least first and second feed streams. In such cases there would generally be used an initial screen assembly with a mesh size of around 10 to 80 (wires per inch), for example, about 20, and the first and second screen assemblies would have a mesh size of around 40 to 325, conveniently 100 to 250 for example about 200. In yet another possible distributor configuration which could also be provided, the feed is passed only through the initial coarse screen. 
   It will also be appreciated that, whilst in accordance with normal practice, all of the separated out solids are disposed of in one way or another, in certain cases it is advantageous to retain within the recycled drilling mud fluid, some solids within a particular size range. Typically these may comprise one or more of sized salt, sized calcium carbonate, and other suitable solids, which are selected to be of a size compatible with minimising formation damage during drilling of a specific formation such as an oil reservoir or a zone where fluid can be lost to the formation. In this instance solids above a specified size can be removed with a top screen and rejected, while solids of a smaller size but greater than a second size, can be separated with the second screen and subsequently returned to the drilling fluid mud system, with solids smaller than those removed by the second screen but larger than a third size, may be removed with a third screen and rejected. In other cases it may be desirable to return only the largest size particle fraction separated out at the first screen, for return to the drilling fluid where this is used in formations with particularly large pore size. 
   Various screen assemblies and screen mounting systems may be used in the apparatus and baskets of the present invention, including, for example, those described in our earlier patent publication WO 03/013690. 
   The floating basket may be mounted in any convenient manner known in the art. Typically there is used a resilient mounting such as a coil spring or rubber block mounting, and the basket vibrated with an eccentrically rotating weight drive. Other forms of resilient mounting may be more convenient with other forms of drive, for example, a leaf spring mounting, with the basket being vibrated with an electromagnetic displacement drive being used to displace the basket against the return force of the spring mounting. 
   In a further aspect the present invention provides a basket suitable for use in a vibratory screening apparatus, said basket mounting a stack of screen assemblies, with superposed screen assemblies separated from each other by a respective flow directing tray, and being provided with a flow distributor formed and arranged for dividing said feed into at least a first feed stream and a second feed stream and directing said feed streams onto respective ones of first and second screen assemblies, and receiving filtrate from a respective screen assembly, from said respective flow directing tray(s). 
   In another aspect the present invention provides a vibratory screening apparatus for use in removing solids from a liquid and solids mixture feed, said apparatus comprising a static outer housing, at least one floating basket mounted so as to be vibratable, in use of the apparatus, by a vibrator device formed and arranged for vibrating said basket, said basket mounting a stack of screen assemblies separated by flow directing trays, said apparatus being provided with a flow distributor formed and arranged for dividing said feed into at least a first feed stream and a second feed stream and directing said feed streams onto respective ones of first and second screen assemblies, and receiving from respective flow directing trays, respective filtrates from said respective screen assemblies. 
   In a yet further aspect the present invention provides a basket suitable for use in a vibratory screening apparatus, said basket mounting a stack of screen assemblies separated by flow directing trays, and being provided with a flow distributor formed and arranged for dividing said feed into at least a first feed stream and a second feed stream and directing said feed streams onto respective ones of first and second screen assemblies, and receiving from respective flow directing trays, respective filtrates from said respective screen assemblies. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Further preferred features and advantages of the invention will appear from the following detailed description given by way of example of preferred embodiments illustrated with reference to the accompanying drawings in which: 
       FIG. 1  is a schematic sectional elevation of a vibratory screening apparatus of the present invention; 
       FIGS. 2A to 4B  are schematic vertical sections illustrating different flow paths through the stacked screens with different configurations of the flow distributor set up for parallel and series operation; 
       FIGS. 5A-C  are schematic perspective end views of the basket of the apparatus also illustrating the flow paths in various different configurations of the flow distributor; 
       FIG. 6  is a side elevation of a modified apparatus with a static flow distributor connected to a floating vibratory apparatus; 
       FIG. 7  is a partly cut-away schematic perspective view of a further embodiment showing one module of a twin-module apparatus set up for parallel operation; 
       FIGS. 8A and 8B  are vertical sections of the apparatus of  FIG. 7  at A and B; 
       FIGS. 9-10  are corresponding views of the apparatus of  FIGS. 7-8 , set up for series operation; 
       FIGS. 11-12  are schematic general side elevations of vibratory screen apparatus of the invention showing the housing; and 
       FIG. 13  is a schematic perspective view of the apparatus of  FIG. 12 . 
   

     FIG. 1  shows schematically one embodiment of a vibratory screen apparatus  1  of the invention with an outer housing (indicated schematically)  2 , in which is mounted on springs  3 , a basket  4 . (See below for more detailed description of housing.) The basket is generally box shaped with pairs of circumferentially extending inwardly projecting flanges  5  height on the basket side walls  6 , for supporting respective ones of a stack  7  of screen assemblies  8  separated by flow directing trays  9 . A vibrator unit  10  is secured to the top  11  of the basket. (Alternatively, the vibrator  10  could be mounted on a side of the basket  4 , or incorporated into or within the structure of the basket  4 . The interior  12  of the basket  4  is divided into a series of levels  13  between neighbouring screen assemblies  8  and flow directing trays  9 . 
   FIGS.  2 A/B to  4 A/B show schematically a distributor  15  provided at one end  16  of the floating basket  4 . The distributor  15  is formed and arranged into inside and outside passages  17 ,  18  shown in  FIGS. 2A to 4A , and  2 B to  4 B, respectively, for connecting with the various levels  13  of the interior  12  of the basket  4  via openings  19  controlled by flap valves  20 . In some cases the flap valves  20  are additionally used to control openings  21  along the length of the passages  17 ,  18  in certain positions of said flap valves  20 , as further described hereinbelow. 
   FIGS.  2 A/B,  3 A/B and  4 A/B show different configurations of the distributor  15  for providing different feed flow arrangements through the screen assemblies  8 , which are indicated as A, B and C, respectively, in  FIGS. 2 to 5 . 
   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   In more detail  FIG. 2A  shows the inside passage  17  and interior  12  of the basket  14 , with an upper flap valve  20 ′ raised to open an upper connecting opening  19 ′ connecting the passage  17  and first level  13   1  above the upper flow deflector tray  9 ′. An intermediate flap valve  20 ″ is raised to close an intermediate connecting opening  19 ″ connecting the passage  17  and second level  13   2  between the upper and lower flow deflector trays  9 ′,  9 ″ whilst simultaneously opening an intermediate level opening  21 ′ in passage  17 . A lower flap valve  20 ′″ is lowered to open a lower connecting opening  19 ′″ connecting the passage  17  and a fourth level  134  below the lower flow deflector tray  9 ″. In this configuration it may be seen that a feed  22  of liquid and solids is passed through a coarse mesh, (typically mesh size 20) upper screen  8 ′ and the filtrate  23  passed along the upper flow deflector tray  9 ′ into passage  17  and thence, bypassing a first, mid-level, screen  8 ″, onto a second, low-level, screen  8 ′″. In this configuration the whole of the feed  22  is passed through the coarse screen  8 ′ and only one of the first and second screens  8 ″,  8 ′″. 
     FIG. 3B  shows the distributor  15  configured so that the upper flap valve  20 ′ is raised to open the upper connecting opening  19 ′, the intermediate flap valve  20 ″ is lowered to open the intermediate connecting opening  19 ″ whilst simultaneously closing the intermediate level passage opening  21 ′, and the lower flap  20 ′″ is lowered to open the lower connecting opening  19 ′″ whilst closing a bottom passage opening  21 ″ as before. In this configuration the whole of the feed  22  is passed through the coarse screen  8 ′ and then successively through each of the first and second screens  8 ″,  8 ′″ thereby providing a more progressively finer screening of the feed (by using a finer mesh size in the second screen than in the first screen). 
     FIG. 4A  shows the distributor in the inside passage  17  configured so that the upper flap valve  20 ′ is raised as before. The intermediate flap valve  20 ″ is lowered so as to open the intermediate connecting opening  19 ″ whilst simultaneously closing the intermediate level passage opening  21 ′ and the lower flap  20 ′″ is raised to close the lower connecting opening  19 ′″ whilst opening the bottom passage opening  21 ′″. In this configuration of the inside passage  17  in the distributor  15 , that part  23 ′ of the filtrate  23  from the coarse screen  8 ′ passing into the inside passage  17 , is directed onto the first screen  8 ″ and then out of the bottom opening  21 ″ of the inside passage  17 , by-passing the second screen  8 ′″. The outside passage  18  is configured as in  FIG. 2A  so that the remaining part  23 ″ of the filtrate  23  from the coarse screen  8 ′ passing into the outside passage  18 , is directed onto the second screen  8 ′″ by-passing the first screen  8 ″. It will be appreciated that in this configuration of the distributor  15 , the screen area available for screening of the feed  22  is effectively double that used in FIG.  2 A/B and that available in a conventional vibratory screening apparatus basket of similar footprint. 
     FIGS. 5A-C  are schematic perspective views of the end  16  of the basket  4  to which the distributor  15  is coupled but with the distributor  15  substantially removed for clarity, showing the flows in and out of the various openings  19  connecting the distributor  15  to the interior  12  of the basket  4 . 
     FIG. 6  shows schematically another embodiment in which there is used a distributor  24  mounted on the static housing  2  and with its connecting openings  19  coupled to the corresponding levels  25  inside the floating basket  4  by flexible conduits  26 . 
     FIG. 7  shows a further embodiment of a screening apparatus  27  of the invention which has identical twin modules  28 ,  29  (only one shown in detail). Each module has a first, coarse mesh, upper, scalping, deck  30  with a first, coarse mesh, screen  31  above a flow back tray  32 . Fluid  33  to be screened is retained on the screen  31  by an end wall  34 . 
   Below the first deck tray  32  is disposed a second deck  35  comprising a second screen  36  above a respective flowback tray  37 . A certain amount of fluid  38  is retained on the second screen  36  by a weir  39  provided at the lower end  40  thereof. When the flow rate of the feed of fluid  33  to be screened, exceeds the capacity of the second screen, part  41  of the fluid  38  overflows the weir  39  either directly into one or other of two vertically extending conduits  42  at opposite sides of the module  28 , or onto one or other of two sloping deflector plates  43  which divert it into a respective one of the conduit  42 , as shown by the single headed fluid flow arrows in  FIGS. 7-8 . 
   At the bottom  44  of the vertical conduits  42  are provided rearwardly facing openings  45  through which the diverted fluid  41  is directed onto the screen  46  of a third deck  47  disposed below the second deck  35 . Thus this part  41  of the fluid flow  33  passes through the first deck screen  31  and the third deck screen  46 , by-passing the second deck screen  36  (see also  FIGS. 8A and 8B , in which  FIG. 8A  is a section through a central vertical plane at A, which extends through a central portion  48  of the module  28 , with the deflector plates  43 ; and  FIG. 8B  is a vertical section through one of the vertically extending side conduits  42 ). 
   That part  38  of the fluid  33  retained on the second screen  36  is passed through the second deck screen  36  (the solid particulate material  49  retained thereon being “walked up” the screen  36  in the usual way—see  FIG. 8B ), as indicated by the double headed arrows  50 . This part  50  of the fluid flow  33 , is then passed through a second deck end wall opening  51  and down a central vertically extending conduit  52  underneath the deflector plates  43 . A closure panel  53  seals a third deck end wall opening  54 , below the second deck end wall opening  51 , thereby preventing this part  50  of the fluid flow  33  from entering the third deck  47 . A bottom opening  55  in the central vertical conduit  52  allows this fluid flow  50  to pass into the sump  56  of the apparatus  28  where it rejoins the other part  38  of the fluid flow  33 , the respective parts  41  and  38 ,  50  of the fluid flow  33 , being passed through the first deck screen  31  and then, in parallel, through a respective one of the second and third deck screens  36 ,  46 . 
   The module  28  as described above, may be readily reconfigured for serial operation whereby the whole of the fluid is passes through each one of the first, second and third deck screens,  31 ,  36 ,  46 , as shown in  FIGS. 9-10 . In more detail the weir  39  is replaced by a high wall  57  which ensures that the whole of the fluid flow  33  is passed through the second deck screen  36 . As before, the fluid flow  58  then passes out through the second deck end wall opening  51  into the central vertical conduit  52 . In this configuration, the bottom opening  55  is sealed by a closure plate  59  whilst the closure panel  53  of the third deck end wall opening  54  is opened so that the fluid flow  58  is routed from the central vertical conduit  52  into the third deck  47  and passed through the screen  46  thereof into the sump  56 . 
   Each of the first and second modules  28 ,  29 , would normally be configured in the same way, but if desired they could be configured differently i.e. one for parallel (2 screen) operation and one for series (3 screen) operation. Also single screen operation is possible when required, by removing one or two screens from the or each module—depending on the configuration of the modules and the fluid feed arrangement. In addition the fluid feed to the apparatus can be arranged to be directed to either or both of the modules (see also further discussion hereinbelow with reference to  FIG. 13 ). With the significantly increased fluid processing capacity of the apparatus (in parallel mode) it will be appreciated that occasions will arise when the fluid feed is insufficient to maintain a high fluid level and short beach length on the screens, which can result in drying of the particulate solids on the beach portion of the screen and damage to the screens therefrom, and/or reduced efficiency of transportation of the particulate solids up the beach for discharge from the screen. In such circumstances damage to the screens can be minimized by restricting the fluid feed to only one of the twin modules. 
   A particular advantage of this type of embodiment is that, in its parallel configuration, a more even and controlled distribution of the fluid flow across the width of the module is obtained, thereby providing a more efficient screening. Another significant advantage is a significantly increased fluid screening capacity—which can approach almost 100% greater than with conventional screening apparatus of the same footprint. 
   It will also be appreciated that various parameters of the modules may be made further configurable. Thus, for example, the weir height could be configurable for a series of different heights. Also the relative proportions of the central and side, vertical conduits could be selected to accommodate particular desired flow capacity proportions for the different fluid flow parts in parallel mode operation. 
   It will further be appreciated that various modifications may be made to the above embodiments without departing from the scope of the present invention. Thus, for example, in place of a flow distributor system based on the use of closure plates and/or flap valves, there could be used one based on proportional valves and the like. 
     FIGS. 11 to 13  show a vibratory screening apparatus  1  of the invention with a generally conventional form of static outer housing  2 , in which is mounted on springs  3 , and a basket  4  with a vibrator device  10 . In more detail the static housing  2  has a base support  60  which includes a sump  61  for receiving filtrate  62  from the basket  4 , and a feed device support portion  63  mounting a feed device  64 . The feed device  64  comprises a header tank  65  for receiving a liquid and solids mixture feed  66 , and having a feed chute  67  extending out therefrom above the basket  4  so as to pass said feed  66  into the basket  4 . In the case of  FIG. 11 , there is provided a static flow distributor  24  mounted on the header tank portion  65  of the static housing  2 , and coupled to the floating basket  4  via flexible conduits  26 . In the case of  FIGS. 12 and 13 , the flow distributor  15  is incorporated in the floating basket  4 . 
   In the apparatus shown in  FIG. 13  it may be seen that the basket  4  has a lateral divider  68  separating the basket into two independently operable basket feed processing modules  69 , 70 , and the (common) housing  2  has two separate feed chutes  71 , 72  extending from the header tank  65  and formed and arranged for directing said liquid and solids mixture feed  66 A,  66 B to respective ones of said basket feed processing modules  69 , 70 . The chutes  71 , 72  are provided with respective control gates  73 , 74  for controlling supply of feed  66  from the header tank  65 , so that the user has the option of using only one or other, or both, of the modules  69 , 70 , when required—as discussed hereinbefore.