Improved sifting screen

Abstract: A sifting screen is described comprising a rigid frame, a first woven cloth (10,14) of hard wearing metal wire, stretched thereacross and secured thereto, and a second woven cloth (12,16) having a coarser mesh than the first cloth (10,14) and being woven from an elongate material of greater cross-section than the first, also stretched across the frame, and secured thereto, below the first cloth (10,14), to support the latter against sagging. In accordance with the invention, the frame may be formed from heavy guage steel or more preferably glass reinforced gas blown polypropylene reinforced by elongate metal reinforcing elements or rods.

Description: Field of invention This invention concerns screens for sifting wet particulate materials such as drilling muds as are obtained by drilling operations for gas and/or oil. Background to the invention Typically a sifting screen is composed of a first woven cloth of stainless steel wires stretched within or across a frame and supported by a second woven cloth of stainless steel wires also stretched within or across the frame, the mesh pitch of the second woven cloth being much coarser than that of the first woven cloth. For example, GB-A-2175222 discloses a sifting screen comprising a frame; a first woven cloth of hard wearing metal wire, stretched across the frame and secured thereto; and a second woven cloth having a coarser mesh than the first cloth and being woven from an elongate material whose cross-sectional area is greater than that of the first cloth, the second cloth also being stretched across the frame and being secured thereto below the first cloth to support the latter against sagging. Such a sifting screen is referred to below as a screen of the type described. Since the second woven cloth is intended to prevent the first one from sagging and to assist in de-binding of the top cloth, particularly when loaded with material to be sifted, wire of considerably greater cross-section is normally employed for the second cloth. The failure of the wires of woven wire sifting screens can be attributed to two factors. The first type of wire failure is commonly referred to as fatigue and appears as breaks in the wires in high flexure regions of the woven cloth. Continued use of a screen after such breaks have occurred rapidly results in adjoining wires breaking at similar points along their length causing cracks in the cloth which widen and elongate until they appear as relatively large elongate openings or slits. These render the screen useless for continued filtration of particulate material since the latter can readily pass through these openings instead of remaining on the upper surface of the fine mesh screen. Whilst techniques which speed up and facilitate the repair of in-situ screens have their place, the condition of a screen after use typically when filtering water or oil based muds from drilling rigs, is such that a considerable amount of time has to be spent cleaning the screen to allow it to be inspected and damaged regions found and replaced by new inserts. Any downtime of a machine such as this when associated with a process such as sea bed drilling for oil and gas, is not only costly but because sometimes weather conditions and other factors limit the time available for drilling etc, such a failure at a crucial point in time can be critical to the success of the overall operation. It is therefore an object of the present invention to provide an alternative form of sifting screen construction which will have a longer operational life than that of types hitherto and which under ordinary operating conditions should have a predictable ordinary life span allowing replacement to be performed at set periods of time much in the same way as other components are replaced at regular servicing intervals. References herein to "wire" are not intended to mean exclusively metal wire but also wire of non metallic materials including plastics, carbon fibre and Kevlar (TM). Summary of the invention According to the present invention, a sifting screen of the type described is characterised in that: (1) the screen frame is of rectangular shape, (2) the frame is made of a plastics material reinforced by elongate metal reinforcing elements, (3) the frame includes a rectilinear matrix of moulded ribs extending transversely and longitudinally across the frame, (4) the screen cloths are secured to the upper edges of the frame and the ribs, and (5) the ribs include upper and lower elongate metal reinforcing elements to provide a rigidity selected to restrict overall flexure of the woven cloths, so as to reduce fatigue-producing movement of the cloths and extend the life of the screen before material fatigue damages either of the cloths. Preferably the sifting screen is formed from glass-reinforced gas blown polypropylene. Screen flexure reduction, which is closely linked to extending cloth life before material fatigue sets in, tends to introduce screen blinding which reduces the screening area and tends to reduce the speed at which solids can be transported over the screen when in use. This arises from the fact that flexure reduction normally requires a greater number of rigid reinforcing members typically of steel, to which the cloths must be firmly secured and, which reduce the area available for filtration. According therefore to a further feature of the invention, the cross section of the materials from which the cloths are woven and the shape and size of unsupported areas of screen cloths are selected having due regard to the nature of solids materials to be transported over the resulting screen and to the maximum force acting on the screen in a downward sense due to the weight of solids materials heaped thereon during use. Flexure of a screen mesh when in vibration will be determined by a number of factors but one which can have a significant influence is whether the unsupported lengths of tensioned material are likely to be activated into a resonant mode of vibration or a harmonic or sub-harmonic of their natural resonant frequency by the vibration imparted by the operation of the machine within which the screen is mounted. According therefore to another feature of the invention, the lengths of elongate material extending across the unsupported regions of the cloths and the tensions in those lengths of material, are selected having regard to the frequency at which the screen is to be vibrated when in use so as to ensure that the natural resonant frequency of the lengths of material making up the warp and weft of each cloth is not capable of being activated into resonance or into any harmonic or sub-harmonic of its resonant frequency. Whilst this will possibly reduce the amplitude excursions of the cloths during vibration and possibly reduce the transportation characteristics of the screen, the likelihood of failure due to material fatigue in the thin smaller cross section material forming the upper cloth will be significantly reduced. A further step in reducing resonance effects can be achieved by utilising slightly different tensions in the warps and wefts of each cloth so that whatever the natural frequency in one direction, it is different in another. However it is important that the difference in frequency is sufficiently great as not to introduce a low frequency beating effect, which may outweigh the advantage. The invention will now be described with reference to the accompanying drawings in which: Figure 1 illustrates to an enlarged scale and partly in cross section the warp and weft wires of two woven mesh cloths of a sifting screen embodying the invention; Figure 2 illustrates the similar wires of another embodiment of the invention; Figure 3 is a plan view to a reduced scale of a screen support frame to which cloths constructed in accordance with the invention can be secured; Figure 4 is a cross section at AA in Figure 3; Figure 5 is an end view at B; Figure 6 is a part section at one end on AA; (to an enlarged scale); Figure 7 is a part section at the other end on AA (to an enlarged scale); and Figure 8 is a section on CC (to an enlarged scale). In the drawings Figures 1 and 2 show different cloth constructions. In Figure 1 the upper cloth 10 is formed from woven stainless steel wire in the range 0.19mm to 0.036mm diameter and 60-325 mesh, (ie number of strands per inch) while the lower cloth 12 is formed from woven phosphor bronze wire in the range 0.45mm to 0.19mm diameter and 20-40 mesh. In Figure 2 the upper cloth 14 is formed in a similar manner to cloth 10 in Figure 1 but the lower cloth is woven from stainless steel wire having a nominal diameter in the range 0.20 to 0.45mm diameter and typically 30 mesh, and is coated with an epoxy based material, or Molybdenum Disulphide, or Teflon (Registered Trade Mark), to a thickness in the range 5 to 50 microns typically 20 to 40 microns. Multiple passes of the wire through a coating process or through a succession of such processes may be necessary to achieve the desired coating thickness. The wires 18, 20, 22 are shown in cross section to show the outer material coatings 19, 21, 23 (albeit not to scale). The wire 24 is shown with the coating scraped of one end. Figures 3 to 8 show various views of an improved screen support frame which is formed from gas blown polypropylene with added glass fibre and reinforced with steel rods, each being of the order of 2.5mm diameter. Figure 3 is a plan view of the support frame and Figures 4 and 5 are edge views with the longer edge view shown in cross section along the line "AA" in Figure 3. In known manner one such screen frame is adapted to be joined to another in the lengthwise direction and to this end the right hand edge (in Figure 1) is formed with a male knuckle 26 and the left hand end is formed with two female jaws 28 and 30 (see Figures 3 and 4) which permit a knuckle edge 26 to be fitted therein. The join seals the two frame edges together. Steel reinforcing rods extend lengthwise and widthwise as shown in Figures 6, 7 and 8. These are denoted by reference numerals 32 to 42 in Figure 6. At their opposite ends, the rods 34 are bent in a downward sense and then in an outward sense, to enter the knuckle edge and reinforce same. Rods 44, 46 extend widthwise above and below the knuckle 26. Pairs of rods 36, 38 and 36', 38' extend at the top and bottom of widthwise extending reinforcing ribs 48, 50 which are located at regular intervals along the length of the frame, as at 52, 54 etc (in Figure 4 up to 62). Similar orthogonal reinforcing ribs 64, 66 etc (see Figure 5) extend lengthwise at regular intervals across the width of the frame. The rectilinear matrix of rods and moulded polypropylene reinforcing ribs (both longitudinal and transverse) can be seen in the top left and right hand corners of the plan view of the frame shown in Figure 3. Figure 8 shows how the ends of the top layer of widthwise rods 38 (36) enter the upper flange 68, of which there is a similar one 70 along the other lengthwise edge. The pairs of transverse rods in the section on CC in Figure 8 are denoted by reference numerals 73, 74; 76,78; and 80, 82. Two cloths such as shown in Figures 1 to 2 are laid across the frame shown in Figure 3 and after being tensioned are secured in position by a suitable adhesive along the side flanges 68, 70, along the two end flanges 84, 86 and to the upper edges of the matrix of reinforcing ribs.

Claims: 1. A sifting screen comprising a frame; a first woven cloth (10; 14) of hard wearing metal wire, stretched across the frame and secured thereto; and a second woven cloth having a coarser mesh than the first cloth and being woven from an elongate material whose cross-sectional area is greater than that of the first cloth, the second cloth (12; 16) also being stretched across the frame and being secured thereto below the first cloth (10; 14) to support the latter against sagging; characterised in that: (1) the screen frame is of rectangular shape, (2) the frame is made of a plastics material reinforced by elongate metal reinforcing elements (40, 42, 44, 46, 72, 74), (3) the frame includes a rectilinear matrix of moulded ribs (48 to 66) extending transversely and longitudinally across the frame, (4) the screen cloths are secured to the upper edges of the frame and the ribs, and (5) the ribs (48 to 66) include upper and lower elongate metal reinforcing elements (32 to 46) to provide a rigidity selected to restrict overall flexure of the woven cloths, so as to reduce fatigue-producing movement of the cloths and extend the life of the screen before material fatigue damages either of the cloths. A sifting screen according to claim 1, wherein the screen frame is formed from glass-reinforced gas blown polypropylene. A sifting screen according to claim 1 or claim 2, wherein the cross-section of the materials from which the cloths are woven and the shape and size of unsupported areas of screen cloths are selected having due regard to the nature of solids materials to be transported over the resulting screen and to the maximum force acting on the screen in a downward sense due to the weight of solids materials heaped thereon during use. A sifting screen according to claim 1 or claim 2, wherein the lengths of elongate material extending across the unsupported regions of the cloths and the tensions in those lengths of material are selected having regard to the frequency at which the screen is to be vibrated when in use, so as to ensure that the natural resonant frequency of the lengths of material making up the warp and weft of each cloth is not capable of being activated into resonance or into any harmonic or sub-harmonic of its resonant frequency. A sifting screen according to claim 4, wherein different tensions are imparted to the warps and wefts of each cloth so that whatever the natural frequency in one direction, it is different in another. A sifting screen according to any one of the preceding claims, in which both cloths (10, 12, 14, 16) are tensioned before being secured to the frame. A sifting screen according to claim 6, wherein the tension in the material forming the upper cloth (10; 14) is different from that in the material forming the lower cloth (12; 16). A sifting screen according to any one of claims 1 to 7, when fitted in a vibratory cradle of a shale shaker.

Inventors: COOK GORDON JAMES, HUGHES ANDREW, COOK, GORDON JAMES, HUGHES, ANDREW
Applicants: UNITED WIRE LTD, UNITED WIRE LIMITED
Application year: 1995
Main classification: B07B1
IPC codes: B07B1