US20040238414A9

US20040238414A9 - Shale shaker

Info

Publication number: US20040238414A9
Application number: US10/169,484
Authority: US
Inventors: David Schulte; Charles Gridhar
Original assignee: Varco IP Inc
Current assignee: Varco IP Inc
Priority date: 2000-11-17
Filing date: 2001-10-19
Publication date: 2004-12-02
Also published as: GB0310348D0; AU2001295757A1; NO20032194L; WO2002040186A1; NO329180B1; US6412644B1; US6371301B1; GB2384732B; US20030047493A1; CA2428828C; US6715611B2; US6863183B2; CA2428828A1; US20020175111A1; NO20032194D0; GB2384732A

Abstract

A shale shaker for separating material, said shale shaker comprising a basket (100) for supporting a screen assembly and a collection receptacle, the basket comprising two side walls (102), an end wall (103) and an opening in the bottom of said basket (100), said basket (100) having means (109,110) to support screen assemblies for substantially covering said opening characterised in that said basket (100) further comprises separating means (104,105,106)in or on any of said walls (102,103) for separating material. Preferably, further comprising directing means for directing separated material therefrom into said collection receptacle. The invention also provides a method for separating material using the shale shaker of the invention, a basket of the shale shaker of the invention and screen assemblies used in the shale shaker of the invention.

Description

The present invention relates to a shale shaker, a basket therefor, screen assemblies therefor and a method for separating material using a shale shaker of the invention.

Vibratory separators are used in a wide variety of industries to separate materials such as liquids from solids or to grade particles. Typically such separators have a basket mounted in or over a receiving receptacle and vibrating apparatus for vibrating the basket. One or more screens is mounted in the basket. Material to be treated is introduced to the screen (s). Separated material (e.g. liquid and/or smaller solids) flows through the screen(s) into the lower receptacle and separated larger solids (with or without liquid) move down and off the screen(s).

The need for solids control in drilling fluid or “mud” used in hydrocarbon well drilling is well known in the prior art. Drilling mud, typically a mixture of clay and water and various additives, is pumped down through a hollow drill string (pipe, drill collar, bit, etc.) into a well being drilled and exits through holes in a drill bit. The mud picks up cuttings (rock) and other solids from the well and carries them upwardly away from the bit and out of the well in a space between the well walls and the drill string. At the top of the well, the solids-laden mud is discharged over a vibratory separator known as a shale shaker, a device which typically has one or a series of screens arranged in tiered or flat disposition with respect to each other. The prior art discloses a wide variety of vibrating screens, devices which use them, shale shakers, and screens for shale shakers. The screens catch and remove solids from the mud as the mud passes through them. If drilled solids are not removed from the mud used during the drilling operation, recirculation of the drilled solids can create weight, viscosity, and gel problems in the mud, as well as increasing wear on mud pumps and other mechanical equipment used for drilling.

Typically, the screens used with shale shakers are emplaced in a generally horizontal fashion on a generally horizontal bed or support within a basket in the shaker. The screens themselves may be flat or nearly flat (i.e. substantially two-dimensional); or, due to corrugated, depressed, or raised surfaces, are three-dimensional. The basket in which the screens are mounted may be inclined towards a discharge end of the shake shaker. The shale shaker imparts a rapidly reciprocating motion to the basket and hence the screens. Material from which particles are to be separated is poured onto a back end of the vibrating screen. The material generally flows toward the discharge end of the basket. Large particles are unable to move through the screen, remain on top of the screen, and move toward the discharge end of the basket where they are collected. The smaller particles and fluid flow through the screen and collect in a bed, receptacle, or pan beneath the screen.

Shale shaker or screen capacity is typically determined by the position of a “fluid-end point”. The fluid end-point is the point where the fluid pool stops on the screen surface near the shaker's discharge end. This is particularly noticeable when the basket is inclined to the discharge end. As the fluid-end point moves closer to the discharge end, discharged solids can become too wet, risking whole mud losses over the screen.

It is advantageous to increase the screen area in a predefined size shale shaker, in order to increase the amount of material to be separated in a given time. It is also advantageous to decrease the weight of material to be separated in the vibratory separator as quickly as possible in order to conserve energy used by the vibratory mechanism and to increase total throughput for the basket is increased.

According to the present invention, there is provided a shale shaker for separating material, said shale shaker comprising a basket for supporting a screen assembly and a collection receptacle, the basket comprising two side walls, an end wall and an opening in the bottom of said basket, said basket having means to support screen assemblies for substantially covering said opening characterised in that said basket further comprises separating means in or on any of said walls for separating material. Preferably, the shale shaker further comprises directing means for directing separated material therefrom into said collection receptacle.

Other aspects and features of the shale shaker of th invention are set out in claims 2 to 27.

The invention also provides a method for screening material using a shale shaker as claimed in any preceding claim, comprising the steps of introducing material into said basket, whereupon small particles and fluid of the material flows through said separating means in said walls and directed into said collection receptacle.

The invention also provides a screen assembly for a shale shaker comprising a rectangular frame having a first plane on which at least one layer of screen cloth is arranged, characterised in that said rectangular frame comprises a hole or channel at an angle to said first plane. In use, the hole or channel is used to direct separated fluid and/or particles into a collection receptacle.

The invention also provides a screen assembly comprising a perforated plate and at least one layer of screen cloth thereon characterised in that said perforate plate comprises at least one channel therein. In use, the channels are used to direct separated fluid and/or particles into a collection receptacle.

For a better understanding of the present invention, reference will now be made, by way of example, to the accompanying drawings, in which: [0012]
FIG. 1 is a perspective view of a prior art shale shaker; [0013]
FIG. 2 is a schematic view of a prior art system for separating and treating mud in a well drilling operation; [0014]
FIG. 3 is a schematic view of part of a prior art shale shaker; [0015]
FIG. 4 is a side cross-sectional view of a prior art basket for a shale shaker as shown in FIG. 1, in use; [0016]
FIG. 5A is a side view of a basket for a shale shaker in accordance with the present invention; FIG. 5B is a perspective view of a screen assembly of the basket of FIG. 5A; FIG. 5C is an end view of parts of the basket as shown in FIG. 5A incorporating the screen assembly shown in FIG. 5B; FIG. 5D is a top view of part of the basket of FIG. 5A; FIG. 5E is a side view of a second embodiment of a basket and screen assembly for a shale shaker in accordance with the present invention; [0017]
FIG. 6A is a perspective view of a plate for a screen assembly in accordance with the present invention; FIG. 6B is a top view of a perforated plate in accordance to the present invention; FIG. 6C is a cross-sectional view taken along [0018] line 6C-6C of FIG. 6B; FIG. 6D is a top view of a second embodiment of a perforate plate in accordance with the present invention;
FIGS. 7A to [0019] 7E are perspective views of respectively a third, fourth, fifth, sixth and seventh embodiment of a basket for a shale shaker in accordance to the present invention;
FIG. 8 is a perspective view of a plate or sheet for a basket in accordance with the present invention;[0020]
Referring now to FIG. 1, a prior [0021] art shale shaker 1 has a screen assembly 2 mounted in a vibratable screen mounting apparatus known as a “basket” 3. The screen assembly 2 may be any known screen or any combination of screen cloth, coarse mesh, perforate plate with a rigid frame or hook strip. The basket 3 is mounted on four springs 4, two of which are shown and the other two are on the opposite side of the basket 3. The four springs 4 are supported from a frame 6. The basket 3 is vibrated by a motor 5 and interconnected vibrating apparatus 8 which is mounted on the basket 3 for vibrating the basket and the screens. Elevator apparatus 7 provides for raising and lowering of the basket 3 at one end end. Typically the basket will be in an inclined, “climb the hill” position (see FIG. 4) so that a pool 9 of liquid is maintained at a first end within the basket. In use, large particles will “climb the hill” towards a discharge and of the shale shaker.
FIG. 2 discloses one example of a typical prior art shaker system, as shown in U.S. Pat. No. 5,90,645. The system A comprises a shale shaker K having a screen assembly or assemblies S. The screen assemblies S (only one shown) are mounted in a typical shaker basket B and one or more vibratory mechanisms V vibrates the basket B and the screen assemblies S, which are rigidly mounted therein. A collection receptacle CR is arranged beneath the basket B to collect separated mud, which will go on for further processing before being reused. The other components of the system A are as described in U.S. Pat. No. 5,190,645 incorporated fully herein for all purposes. [0022]
FIG. 3 shows schematically a prior art system C with a Mud Box M, also known as a Back tank or Possum Belly, to distribute the flow to a screen assembly S (only one shown). The screen assembly S is mounted rigidly in a basket (not shown) which is vibrated to assist the throughput of mud and movement of separated solids. The basket (or deck) rests on Vibration Isolation Members, such as helical springs or rubber mounts. The vibration isolation members rest on the support member, which is also used to divert flow as desired, and is called a Bed. There are many deck, basket configurations used depending on the design criteria. The deck, basket, may be flat, horizontal, or mounted at a slope. [0023]
On sloped deck units for example, cascade or parallel flow units, the screens may be continuous with one screen covering the entire deck length, or have a divided deck which has more than one screen used to cover the screening surface, or with individual screens mounted at different slopes. On multiple deck units, more than one screen layer may be employed. In a two or three deck unit, the mud passes through one screen before flowing through the second. [0024]
The motion of the shaker controls the efficiency of separation, rate of travel of cuttings on the screen, solids separation capacity and fluid capacity. The shape and axial direction of the vibration motion along the deck is controlled by the position of the vibrator relative to the deck and the direction of rotation of the vibrator. [0025]
Shale shakers have capacity limits. Exceeding a capacity limit means excessive mud will be discharged over the ends along with the solids and hence, wasted. Capacity limits are typically defined for non-blinded screens. Capacity limits of a shale shaker include: [0026]
1. The solids capacity limit is the maximum amount of solids that a device will remove; and [0027]
2. The liquid limit—the maximum GPM capacity for various drilling muds. [0028]
The solids capacity limit is encountered when drilling soft, gummy formations. Usually the liquid limit controls the minimum size screen that can be used for a given circulation rate. The thicker the mud, the lower the GPM capacity. Open area of the screens and usable area of the screening deck influence this limit. The solids capacity and/or liquid capacity varies with different shaker designs. In practice, the smallest screen size that can be employed without flooding a unit is used. [0029]
There is a need for an efficient vibratory separator system which is efficient and cost-effective with increased efficiency, and screen throughput. [0030]
Referring now to FIGS. 5A to [0031] 5D there is shown a basket 10 for supporting screen assemblies in a vibratory separator such as a shale shaker. The basket 10 has two spaced-apart side walls 12, 14 and an end wall 16 between them. Typical vibrator apparatus 20 is connectible to the basket 10. An end screen assembly 40 is mounted in channels 22, 24 on opposite sides of the end wall 16. Screen mounting apparatus 46 may be any such known apparatus.
In one aspect the [0032] channels 22, 24 are open at the top so the screen assembly 40 can be introduced into the space between the channels 22, 24. Optional removable blocks 26, 28 on the interior of the basket hold the screen assembly 40 in place in the channels; and optional crown bars 32 on the screen assembly's exterior provide support for the screen 40. In one aspect each crown bar 32 has a rubber part 34 that contacts the screen assembly 40. Screening material of the screen assembly 40 (as on any screen according to the present invention) may be any suitable known screening material, including, but not limited to one, two, three or more layers of known screening material and/or mesh, two-dimensional or three-dimensional, bonded, glued, connected or unconnected.
The [0033] screen assembly 40 may, optionally, have a curved lower portion 42 that corresponds to a curved-shape assumed by screens (not shown) mounted in the basket 10. Crown members (not shown) may be provided to support the screen in the basket 10 in the curved shape.
The [0034] screen assembly 40 is emplaced over an opening in the end wall 16. Optionally a sealing gasket 19 or other seal material or seal member is used around the opening 18.
A basket [0035] 10 (and any basket according to the present invention) may be used with any suitable known shaker or separator. FIG. 5E shows another embodiment 10 a of a basket 10 with multiple vertical end screens 40′, 40 a, and 40 b supported on lower part 22 a of the channel 22 and positioned beneath upper part 22 b of the channel 22. Alternatively, the screen assembly may have appropriate frames or other mounting members and/or structure so that they can be bolted in place. The screens 40 a and 40 b may be like the screen 40′ with the screening material as described for the screen assembly 40 (and like numerals in FIGS. 5A and 5E are for like items). Each screen may have similar or different screening material. Two, three, four or more such screens may be used at a basket end and/or side(s) with screens positioned within or outside the basket.
FIG. 6A shows part of a [0036] tubular frame 60 used as a screen support for screening material for a screen assembly according to the present invention for use in a vibratory separator or shale shaker. The frame 60 has a top member 61 and a bottom member 62. A plurality of cross members 63 (one shown) interconnect the top and bottom members. A screen or screen assembly (not shown) may be arranged vertically against top and bottom members 61 and 64. One, two, three, four, or more holes 64 (or a series of perforations through the bottom member 62) provide for fluid flow through the bottom member 62 (shown as hollow; solid bottom members with one or more holes or perforations may be used). In one aspect the top member also has such a hole or holes 64. Screening material may be adhered or otherwise attached to the front face of the frame and a back plate may be welded or otherwise attached to the rear face of the frame. In use, separated material flows through the screening material and falls or is diverted through holes 64 into a collecting receptacle, such as the one shown in FIG. 2 with reference CR, beneath the basket of the shale shaker.
FIGS. 6B and 6C show a [0037] perforated plate 200 for supporting screening material (not shown) for a screen assembly for a vibratory separator, such as a shale shaker, particularly for an end wall or side wall of the basket. The perforated plate 200 has a plurality of spaced-apart openings 201 or holes extending through the plate from one surface to the other. A second series of spaced-apart openings 202 or holes extend from the topmost row of openings 201 (as viewed in FIG. 6B) to a bottom side 203 of the plate 200. Openings 202 intersect openings 201 so that fluid flowing into an opening 201 can flow down (as viewed in FIG. 6B) and out from the plate 200, e.g. into a collecting receptacle of a vibratory separator or shale shaker beneath a basket in which is mounted a screen assembly with such a plate.
It is within the scope of this invention for any known perforated plate or perforated member used as a support for a screen assembly which has a known series of spaced-apart perforations, holes, openings, etc. to have a second series of perforations, openings or holes (e.g. but not limited to like the holes [0038] 202) at any desired angle to the perforations, holes or openings, etc. through the plate (including, but not limited to at a right angle as shown in FIG. 6B) to conduct fluid through the plate as is done by the plate 200.
FIG. 6D shows a [0039] screen assembly 210 for a vibratory separator, such as a shale shaker, according to the present invention, with a perforated plate 211 for supporting screening material 212 on the plate 211, particularly for an end screen or side basket screen according to the present invention as described herein. The perforated plate 211 has a plurality of spaced-apart openings 213 or holes extending through the plate from one surface to the other. A series of spaced-apart grooves 214 or notches extend from one side of the plate 211 to the other and intersect the openings 213 so that fluid flowing into an opening 213 can flow down and out from the plate 211, e.g. into a receptacle of a vibratory separator or shale shaker beneath a basket in which is mounted a screen assembly with such a plate. There may be multiple holes 213 under each hole 213 as viewed in FIG. 6D across the plate's surface. The screening material 212 may be any disclosed herein and/or any known in the art.
It is within the scope of this invention for any known perforated plate or perforated member used as a support for a screen assembly which has a known series of spaced-apart perforations, holes, openings, etc. to have a second series of grooves or notches (e.g. but not limited to like the grooves [0040] 214) at any desired angle to the perforations, holes or openings, etc. through the plate (including, but not limited to at a right angle as shown in FIG. 6D) to conduct fluid down the plate as is done in the plate 211.
FIG. 7A shows a [0041] basket 70 with side walls 72, end wall 73, and cross-braces 79 on which screen assemblies for treating fluid introduced into the basket are to be mounted. End wall 73 has an end screen assembly 74 over an end opening 71 for treating fluid introduced to the basket 70. Side screen assemblies 75, 76 are mounted over side openings 77 in the side walls 72. An end brace 78 is interconnected between side walls 72 at an exit end of the basket 70 (opposite the other end at which fluid is introduced into the basket).
FIG. 7B shows a [0042] basket 80 with side walls 82, end wall 831, in which screen assemblies for treating fluid introduced into the basket are to be mounted. End wall 83 has an end screen assembly 84 over an end opening 81 for treating fluid introduced to the basket 80. Side screen assemblies 85, 86 are mounted over side openings 87 in the side walls 82. An end brace 88 is interconnected between side walls 82 at an exit end of the basket 80 (opposite the other end at which fluid is introduced into the basket).
FIG. 7C shows a [0043] basket 90 with side walls 92, end wall 93, and cross-braces 99 on which screen assemblies for treating fluid introduced into the basket are to be mounted. End wall 93 has an end screen assembly 94 over an end opening 91 for treating fluid introduced to the basket 90. Side screen assemblies 95, 96 are mounted over side openings 97 in the side walls 92. An end brace 98 is interconnected between side walls 92 at an exit end of the basket 90 (opposite the other end at which fluid is introduced into the basket).
FIG. 7D shows a [0044] basket 100 with side walls 102, end wall 103, and cross-braces 109. End wall 103 has an end screen assembly 104 over an end opening 101 for treating fluid introduced to the basket 100. Side screen assemblies 105, 106 are mounted over side openings 107 in the side walls 102. An end brace 108 is interconnected between side walls 102 at an exit end of the basket 100 (opposite the other end at which fluid is introduced into the basket) Mounting apparatus 110 (any known in the art) is used to mount screen assemblies in the basket 100.
FIG. 7E shows a [0045] basket 120 with side walls 122, end wall 123, and cross-braces 129. End wall 123 has end screen assemblies 124, 134 over end openings 121, 131 for treating fluid introduced to the basket 120. Side screen assemblies 125, 126 are mounted over side openings 127, 137 in the side walls 122. An end brace 128 is interconnected between side walls 122 at an exit end of the basket 120 (opposite the other end at which fluid is introduced into the basket). Mounting apparatus 130 (any known in the art) is used to mount screen assemblies in the basket 120.
FIG. 7F shows a [0046] basket 140 with side walls 142, end wall 143, and screen assemblies 162, 163 mounted on the basket for treating fluid introduced into the basket. End wall 143 has an end screen assembly 174 over an end opening 141 for treating fluid introduced to the basket 140. Side screen assemblies 144, 145, 146, 147, 148, 149 are mounted over side openings 154, 155, 156, 157, 158, 159 in the side walls 142. An end brace 168 is interconnected between side walls 142 at an exit end 182 of the basket 140 (opposite the other end at which fluid is introduced into the basket). A solid sheet or plate 180 is mounted in the basket at the fluid introduction end 184 opposite the solids exit end 182. The solid sheet or plate 180 performs no fluid treatment function of separating components of a fluid introduced into the basket 140. The end screen 174 and side screen assemblies and/or side screen portions adjacent the solid sheet or plate 180 do perform a fluid component separation function. It is within the scope of this invention for the solid sheet or plate to be sized so that it underlies a pond or pool (e.g. as shown in FIG. 4, item 9). It is also within the scope of this invention for the solid sheet or plate to extend from the end wall 143 toward the exit end 182 six inches, one foot, eighteen inches, two feet, thirty inches, three feet, forty two inches, four feet, or any desired amount. alternatively, the sheet or plate 180 may have a series of slits, slots, holes, openings and/or perforations therethrough so that the sheet or plate 180 does perform a fluid component separation function; and, in one such embodiment, the slits, slots, holes, openings and/or perforations are of a size like that of a finest screen mesh on top of a screen used for the screen assemblies 162, 163. Any screen or screen assembly may be used in the basket 140. Any sheet or plate 180 may be used in any known basket or any basket according to the present invention.
FIG. 8 shows a [0047] plate 180 with a plurality of perforations 185 therethrough.
Vertically “stacked” screen assemblies in the walls of the basket may all be mounted within the basket; outside the basket; or one or more of them may be mounted within the basket and one or more of them mounted outside the basket—with appropriate mounting members, gaskets, seals, seal members, and/or bolts and nuts so that fluid flows through the screens and then exits from an outermost screen. [0048]
For an opening not covered by a screen, a blocking plate or member is releasably and sealingly installed over the opening(s) to close off the opening to flow. [0049]
The shale shaker of the invention is one of any type of vibratory separator which may incorporate the features of the claims. [0050]

Claims

1. A shale shaker for separating material, said shale shaker comprising a basket (10;70;80;90;100;120;140) for supporting a screen assembly and a collection receptacle (C), the basket comprising two side walls (12,14;72;82;92;102;122;142), an end wall (16;73;83;93;103;123;143) and an opening in the bottom of said basket (1O;70;80;90;100;120;140), said basket (10;70;80;90;100;120;140) having means (46;79;99;109,110;129) to support screen assemblies for substantially covering said opening characterised in that said basket (10;70;80;90;100;120;140) further comprises separating means (40;200;74,75,76;84,85,86;94,95,96;104, 105,106;124,134,125,126;144-149,174) in or on any of said walls (12,14,16;72,73;82,83;92,93;102,103;122,123;142, 143) for separating material and directing means (202,60) for directing separated material therefrom into said collection receptacle (C).

2. A vibratory shaker as claimed in any preceding claim, wherein said separating means is located on said end wall (16;73;83;93;103;123;143).

3. A shale shaker as claimed in any preceding claim, wherein said separating means is through at least one of said side walls (12,14;72;82;92;102;122;142).

4. A shale shaker as claimed in claim 1, 2 or 3, wherein said separating means comprises an aperture (18;71,77;81,87;91,97;101,107;121,131,127,137;141,154,155 ,156,157,158,159;201).

5. A shale shaker as claimed in claim 4, wherein said aperture is substantially covered by a screen assembly (40;200;74,75,76;84,85,86;94,95,96;104,105,106;124,134,12 5,126;144-149,174).

6. A shale shaker as claimed in claim 5, wherein said screen assembly comprises screening material.

7. A shale shaker as claimed in claim 7, wherein said screen assembly comprises two-dimensional screening material.

8. A shale shaker as claimed in claim 6 or 7, wherein said screen assembly comprises three-dimensional screening material.

9. A shale shaker as claimed in any of claims 6 to 8, wherein said screen material comprises wire mesh.

10. A shale shaker as claimed in any of claims 6 to 9, wherein said screening material comprises screening cloth.

11. A vibratory shaker as claimed any of claims 5 to 10, wherein said screen assembly comprises a perforated plate (200).

12. A shale shaker as claimed in claim 11, wherein said perforate plate (200) comprises channels (202) for directing separated material into said collection receptacle (C).

13. A shale shaker as claimed in claim 12, wherein said screen assembly comprises a frame (60).

14. A shale shaker as claimed in claim 13, wherein said frame (60) comprises directing means (64,61,62,63) for directing separated material therefrom into said collection receptacle (C).

15. A shale shaker as claimed in any of claims 5 to 14, wherein said walls (123;143;122;142) comprise a plurality of apertures for a plurality of screen assemblies (124,134,125,126;144-149,174).

16. A shale shaker as claimed in claim 15, wherein said plurality of apertures are arranged side-by-side.

17. A shale shaker as claimed in any of claims 5 to 16, wherein said screen assembly is held over said aperture by at least one of the following: welding; soldering; and brazing.

18. A shale shaker as claimed in any of claims 5 to 17, wherein said screen assembly is removably replaceable from said aperture.

19. A shale shaker as claimed in claim 18, wherein said aperture is provided with channels, into which said screen assembly is removably replaceably slide.

20. A shale shaker as claimed in claim 18 or 19, wherein said screen assembly is held in said aperture by at least one of the following: nuts and bolts; clamps; glue; other adhesive; releasably cooperating fastener material such as VELCRO material; screws; inflatable structure; bladder; releasable tensioning bolts.

21. A shale shaker as claimed in any preceding claim, wherein said collection receptacle (C) is beneath the basket (10).

22. A shale shaker as claimed in any preceding claim, further comprising screen assemblies mounted on said means to support said screen assemblies.

23. A shale shaker as claimed in any preceding claim, further comprising a solid plate (180) arranged in the bottom of said basket (140).

24. A shale shaker as claimed in claim 23, wherein said solid plate (180) has holes (185) therein.

25. A shale shaker as claimed in any preceding claim, wherein the separating means is arranged substantially at right angles to the bottom of the basket.

26. A basket of the shale shaker as claimed in any preceding claim.

27. A shale shaker as claimed in any preceding claim, wherein said separating means is arranged at the end of the basket at which, in use, material to be separated is introduced.

28. A method for screening material using a shale shaker as claimed in any preceding claim, comprising the steps of introducing material into said basket, whereupon small particles and fluid of the material flows through said separating means in said walls and directed into said collection receptacle.

29. A screen assembly for a shale shaker comprising a rectangular frame (60) having a first plane on which at least one layer of screen cloth is arranged, characterised in that said rectangular frame (60) comprises a hole or channel (64) at an angle to said first plane.

30. A screen assembly comprising a perforated plate (200) and at least one layer of screen cloth thereon characterised in that said perforate plate (200) comprises at least one channel therein (202).