Field of the disclosure Embodiments disclosed herein relate generally to a screen support frame comprising a perimeter, particularly but not exclusively for forming part of a screen e.g. for use in a shaker to separate solids from a liquid/solid mixture.

Background

Efficiently separating solids from liquids is a widespread technical problem. One of the most practical and robust methods of achieving this remains the use of a sieve, or screen, to sift the solids from the mixture of liquid and solid. When drilling for oil and/or gas, synthetic drilling fluids, or muds, are used. As these muds are relatively expensive to manufacture, once used they are typically recovered in a process including sifting rock, shale and other debris from the mud. This involves the use of a so- called shaker which has fitted, one or more sifting screens, made up of a screen frame with one or more sheets of woven wire mesh, or screen, stretched over and secured to it. In use, the shaker vibrates the sifting screen or screens, to aid the sifting process.

To be able to withstand the rigours of this sifting process, sifting screens must have a certain rigidity and be very hard-wearing. This has resulted in a design of sifting screens having a screen frame which has a plurality of reinforcing "ribs". A typical design of a screen frame is rectangular comprising an outer rectangular perimeter with each side connected to its opposing side by a plurality of ribs. Such a design results in a plurality of rectangular openings. Typically the screen is attached not only to the rectangular perimeter but also to the ribs, to provide better adhesion of the screen to the frame and prolonging its lifetime. In view of the fact that sifting screens are man-handled into position, such screen frames have for some time been made from plastics material to reduce weight. A typical design of plastic screen frame is reinforced by including a metal wire structure, embedded within the plastics rectangular perimeter and rib arrangement. However, it has been found that such wires can lack the required stiffness, especially when extending between longer distances for large screen frames, and sag under gravity reducing their effectiveness as reinforcing structures.

It has been proposed, in e.g. GB 2461725, to use strengthening ribs between the upper and lower arrays of wires to improve the overall rigidity of the screen cage and frame. However using such ribs requires modification of the manufacturing process and associated tooling and increases material costs and complexity.

Thus further ways to improve rigidity of such screen frames without introducing significant weight to the screen frame would be highly desirable.

Brief description of the drawings

The disclosure will now be described, by way of example, and with reference to the accompanying drawings in which:

Figure 1 is an exploded perspective view of a part of a known screen;

Figure 2 is a perspective view of a screen support frame forming part of such a screen;

Figure 3 is a perspective view of a screen support frame according to the disclosure;

Figure 4 is a detailed perspective view of part of the screen support frame of Figure 3;

Figure 5 is an enlarged partial cross-section of the frame structure shown in Figures 3 and 4; Figure 6 illustrates mathematical sections used to model the stiffness of the frame of Figure 2; and

Figure 7 illustrates mathematical sections used to model the stiffness of the frame of Figure 3. Detailed Description Figure 1 shows a known screen (10) comprising a frame (12) to which are attached three layers of woven wire mesh (14), shown in exploded view for ease of reference. The frame (12) comprises an orthogonal array of cells formed from intersecting plastics ribs (16) moulded over upper and lower arrays of structural wires (18), (20). Figure 2 shows a wire structure or subframe (22) which will be encased in plastics material, such as thermoplastics material, to form a screen frame as in Figure 1. Structure (22) comprises a rectangular perimeter frame (24) from opposing sides of which run a plurality of steel wires (25), (25'), (26), (26') welded together to form upper array (18) and lower array (20) of orthogonally intersecting wires, the two arrays being spaced from each other. Array (18) is formed from upper orthogonally intersecting wires (25), (26) and array (20) formed from lower orthogonally intersecting wires (25'), (26'). Where desired, and is known in the art, spacers (27) are welded between selected wires of the upper and lower arrays to maintain a desired separation distance.

The screen frame typically has a length of between 60 to 1300cm and a width of between 60 to 100cm. During moulding to create the plastics ribs shown in Figure 1, wires (25), (25'), (26), (26') experience a reduction in stiffness due to the length over which they are unsupported. This leads to the wires flexing and contacting the mould tool, causing the wires to break through the plastics encapsulation once moulded. This is particularly so for those wires (26), (26') running the length of the screen frame which are unsupported over a greater distance.

In accordance with the disclosure and as shown in Figures 3, 4 and 5, a screen frame having a rectangular perimeter (24) is provided. The perimeter is reinforced by a first upper arrangement (18) of reinforcing wires and a lower second arrangement (20) of reinforcing wires.

As can be seen in detail in figure 5, structural wires (26) provide the first array of structural wires for the upper arrangement (18) and structural wires (26') provide the first array of structural wires for the second lower arrangement (20). Also shown in Figure 5 is a wire from the second array, which is orthogonal to the wires of the first array (26), (26'). As can be seen, the wires of the first and second arrays in both the first (18) and second (20) arrangements, are in contact. Also provided are additional structural wires (30) for the upper arrangement (18) and additional structural wires (30') for the lower arrangement (20). As can be seen the additional structural wires (30), (30') are parallel and vertically spaced from a wire in the first array (26) in the first and second arrangement respectively. It will also be noted that only some of the wires (26) in the upper arrangement (18) and lower arrangement (20) have a corresponding additional structural wire (30), (30'). The secondary wires (30), (30') stiffen alternate pre-existing longitudinal wires (26), (26') which helps prevent the pre-existing wires flexing on moulding and improves the overall stiffness of the cage structure (22).

As shown in Figure 3, these supporting secondary wires run along the length of the frame co- linear and proximal to selected wires (26), (26') although if desired they can be used to reinforce wires running along the width of the frame.

If desired, additional structural wires can be provided for all wires (26), (26') running the length of the frame but generally it will be sufficient to provide secondary wires for every other wire running the length of the frame, as shown.

By having two proximal wires, the wire pairs (30), (26) and (30'), (26') effectively provide a beam structure that is equivalent to their total diameter plus the diameter of the wire from the second array between them. Thus as shown in Figure 4, if wires (30), (26) both have a cross- sectional diameter of 2.5mm with a gap of 1.5mm between them, then they act as a beam of 6.5mm.

By pressing the reinforced cage of Figure 3 and the unreinforced cage of Figure 2 with the same amount of force, a significant increase of stiffness of the reinforced cage was observed. To quantify the amount of improvement, finite element analysis was undertaken in ANSYS Workbench modelling software (available from ANSYS, Inc., of Canonsburg, Pensylvannia, USA) to compare the stress and deflection in the respective frames. For a common load, a traditional cage as shown in Figure 2 exhibited a maximum deflection of 0.94mm, with a reinforced cage as shown in Figure 3 exhibiting a maximum deflection of 0.53mm. Thus when the respective structures were loaded and constrained in an identical way, the reinforced structure deflected 43% less. The present disclosure provides a substantial improvement on the stiffness encountered with single wires as shown in the prior art frame of Figure 2, with this achieved for less material cost than using a rigid bar as the wire is cheaper and with less complexity as the secondary wires can be incorporated readily into the existing manufacturing techniques.

Theoretical modelling illustrates the improvements achieved using the disclosure. For a frame as shown in Figure 2 when moulded into a screen, calculation of the second moment of area can give an indication of the stiffness of the structure. Figure 6 shows diagrammatically how the second moment of area for the frame of Figure 2 can be viewed. Figure 6(a) represents the frame (12) as polypropylene with two strengthening steel wires equivalent to the wires in the upper and lower arrays (18), (20), those wires (25) having a circular cross-section of 2.5mm diameter. To simplify calculation of the second moment of area, the round wires can be converted to a square section of an equivalent second moment, see Figure 6(b), where the equivalent square wire dimension is 2.19 x 2.19mm.

Keeping the height constant, the width of the steel section when multiplied by the modular ratio gives the equivalent width of the square steel wire as polypropylene.

Young's modulus (mild steel) =E _S = 210GPa

Young's modulus (polypropylene) E _pp = 0.896GPa

Equivalent width = 2.19 x (E E _pp ) = 513mm

This is shown in Figure 6(c) where (c) represents an equivalent transformed polypropylene section having the same properties as the composite section of 6(a).

The second moment for the transformed section = Ιχχ TOTAL = I AREAI + (I AREA ₂ ) + (I AREA ₃ ) where:

I AREAI IS Ixx

12 and I AREAI, I AREA ₃ are Ιχχ Ah ²

b=width, d=height, A=area, h=distance from neutral axis to centroid. This gives second moments as shown below:

Analysis Result I AREA1 14634

I AREA2 225592

I AREA3 7352

I TOTAL (mm ⁴ ) 236710

Using the same principle, the second moment of area can be found for the reinforced structure of Figure 3. First the model is transformed into an equivalent polypropylene section, see Figure 7 where 7(a) shows the model with paired steel wires and 7(c) shows the polypropylene equivalent.

The second moment of area equals:

I TOTAL = I AREAi + (I AREA2) + (I AREAS) + (I AREA4) + (I AREAS) where I AREA4 and I AREAS are calculated as for I AREA2 and I AREA3 - This gives a total second moment of area as below:

The higher I, the stiffer a beam is and the more load that is required to generate deflections. The reinforced structure exhibits a higher I and so is better than the frame of Figure 2.

From finite element analysis using ANSYS Workbench, it was observed that a known deflection (0.2254mm) occurred at the centre of a RM3 industrial the screen when a 60m/ s ² acceleration was applied to the screen, using the deflection equation

F l ³

Deflection (mm) = δ =

48E I Rearranging the above equation, it is possible to calculate what force is required to generate a known deflection for the different second moments of areas calculated above. _{π Λη Γ} 8 E I 48

Force (Ν) = F =

Working with these values and rearranging the deflection equation to calculate force, it was found that the reinforced frame was 1.3 times stiffer than the frame of Figure 2 (2.37N as compared to 1.77N).

In accordance with one aspect of the present disclosure, there is provided a screen support frame comprising, a perimeter disposed in a horizontal plane, defining a vertical direction normal to said plane, said perimeter reinforced by an arrangement of reinforcing wires, said arrangement comprising a first array of substantially parallel structural wires extending between opposing regions of the perimeter in a first horizontal plane, a second array of substantially parallel structural wires extending between opposing regions of the perimeter in a second horizontal plane, said first and second arrays of structural wires being aligned at an angle to each other and in contact with each other thus forming a plurality of contact points between structural wires of the first and second array respectively, the arrangement further comprising at least one additional structural wire, said additional structural wire extending between opposing regions of the perimeter and being positioned parallel to, and substantially vertically spaced from, a wire in the first array of structural wires, and in contact with wires of the second array.

The apparatus of the present disclosure thus provides an arrangement of two parallel wires above and below and in contact with the wires in the second array, said arrangement providing a particularly stiff contact and much greater stiffness than if the at least one additional structural wire were not present.

Typically the additional at least one structural wire is spaced from a wire in the first array of structural wires by a distance between 1.0 and 2.5mm. The wires forming the first array are generally evenly spaced apart, i.e. that the distance between adjacent wires in the array is substantially fixed. Likewise the wires forming the second array are also generally evenly spaced apart. Typically the perimeter is rectangular comprising two parallel short sides and two parallel long sides. In this case it is preferable that the first array of substantially parallel structural wires extends between the short sides of the perimeter and the second array of substantially parallel wires extends between the long sides of the perimeter. This ensures that the wires extending for the longest distance are reinforced by a parallel at least one additional structural wire. In this case, the first and second arrays of structural wires are aligned at right angles to each other.

In one embodiment, each of the wires in the first array is reinforced by a respective additional structural wire. However, it has been found that the majority of the improvements in overall stiffness can be achieved when not all of the wires in the first array are reinforced by an additional structural wire.

Preferably the additional structural wires have a circular cross-section, which may be an identical circular cross-section to the wires of the first and/ore second array. Typically, the cross-sectional diameter of the additional structural wires may range from 10mm to 1mm and more preferably 5 mm to 2mm.

Thus, it will be understood that the first array of structural wires, the second array of structural wires and the additional structural wires, although being in different but parallel planes, are all in contact, and thus form an arrangement whereby the contacts provide the increased stiffness.

In a further preferred embodiment, in addition to the arrangement disclosed, there is provided a second arrangement of such wires, said second arrangement lying in a plane parallel to but spaced apart from, the first arrangement. This duplication of the arrangement of the present disclosure provides a further increase in stiffness.

As discussed above, the screen frame according to the present disclosure is intended to have woven wire mesh attached to the perimeter, which woven wire mesh carries out the screening function. In general, the screen support frame the wires of both the first and second arrays are encased in plastic material, thereby forming respective arrays of plastic wire-reinforced ribs extending between the perimeter. Such ribs preferably provide an upper surface so that the woven wire mesh can attach, not only onto the perimeter, but also to the top surface of the plastic ribs.

In another aspect, the disclosure relates to a method of improving the stiffness of a screen support frame, said support frame comprising a perimeter disposed in a horizontal plane, defining a vertical direction normal to said plane, a first array of substantially parallel structural wires extending between opposing regions of the perimeter in a first horizontal plane, a second array of substantially parallel structural wires extending between opposing regions of the perimeter in a second horizontal plane, said first and second arrays of structural wires being aligned at an angle to each other and in contact with each other thus forming a plurality of contact points between structural wires of the first and second array respectively, the improvement in stiffness being provided by providing at least one additional structural wire, said additional structural wire extending between opposing regions of the perimeter and being positioned parallel to, and substantially vertically spaced from, a wire in the first array of structural wires, and in contact with wires of the second array, thereby providing an

arrangement of two parallel wires on either side and in contact with the wires in the second array, said arrangement providing an increased stiffness to the screen support frame.