WO2000069573A1

WO2000069573A1 - Screening fabric and method of producing same

Info

Publication number: WO2000069573A1
Application number: PCT/NO2000/000149
Authority: WO
Inventors: Thor BJØRNSTAD
Original assignee: Bjoernstad Thor
Priority date: 1999-05-14
Filing date: 2000-05-04
Publication date: 2000-11-23
Also published as: NO310100B1; NO992330L; NO992330D0; AU4439200A

Abstract

A two-ply screening fabric (12, 14) and a method of producing such screening fabrics, that comprise two glued-together fabric layers, a first fine-meshed fabric (12) decisive of the screening grade, and a coarse-meshed supporting fabric (14) which meets the requirements as to rigidity, strength and consistency, have been described. The coarse-meshed fabric (14), which is plaited or woven, is subjected to rolling and exhibits - relative to known technique - a much greater surface smoothness on the side that the fine-meshed fabric (12) will rest on, which increases the supported contact area of the fine-meshed fabric (12) on the coarse-meshed fabric (14), which allows the two-ply screening fabric increased time in operation before holes are ground in the fine-meshed fabric (12). In the production of such two-ply screening fabrics, the two different layers (12 and 14) of fabric are withdrawn from respective supply rolls (24 and 26, respectively) and on to a motorized receiving roll (28). Upstream of the receiving roll (28) bonding glue is applied between the two layers (12 and 14). A braking moment is applied to each supply roll (24 and 26) during the unrolling, and thus, the two fabric layers are pre-stressed as they are glued together.

Description

SCREENING FABRIC AND METHOD OF PRODUCING SAME

This invention relates to a multi-ply screening fabric comprising a fine-meshed screening fabric whose meshes are decisive of the screening itself, and a somewhat coarser- meshed screening fabric, which serves to reinforce the fine- meshed screening fabric, the multi-layer screening fabric moreover being of the kind indicated in the preamble of claim 1.

Likewise, the invention relates to a method of producing screening fabrics formed in accordance with claim 1.

The invention particularly addresses screening fabrics for use in vibrating screens, for example the kind of screening devices that are used to separate cuttings from drilling fluid/mud in oil drilling.

By said vibrating screens, for example, which serve to remove cuttings and rock fragments from the drilling mud, the screening fabric is clamped tight across a horizontal or slightly sloping frame which is vibrated by means of an engine. Mass, mud or liquid that is to be screened, is introduced on the top surface of the screen, and the vibrating motion of the screening fabric results in transportation of particles that are larger than the mesh width of the screening fabric, across the screening fabric and beyond one lateral limit thereof, whereas liquid and particles that are smaller than the mesh size, pass through the screening fabric. Such screening is often carried out in several steps, so that liquid and progressively smaller particles pass through several screening fabrics arranged one underneath the other with successively decreasing mesh width.

Fine-meshed screening fabric has both low rigidity, low strength and thus insufficient consistency. Therefore, it is necessary to support such fine-meshed screening fabric with a coarser-meshed, and consequently stronger, more rigid and more consistent screening fabric.

It is known from earlier to support a fine-meshed screening fabric with a coarser-meshed screening fabric. The coarse- meshed screening fabric is stretched within the screen, and a fine-meshed screening fabric will rest, in its position within the screen, on the coarse-meshed screening fabric, which is sized to withstand the weight of the material being screened and the tensile force which is necessary to keep the screening fabric in position within the frame.

Thus, two-ply screening fabrics are known, each consisting of two layers/fabrics, a first coarse-meshed layer bonded together with a second fine-meshed layer by means of glue. A convenient distribution of the glue is parallel, narrow lines, so that the meshes of the screens are open between the lines of glue. Such a two-ply screening fabric is clamped at each lateral limiting edge between two strips or m a longitudinal groove of a rail. Said strips or rails are secured inside a frame which is arranged to carry and tighten the two-ply screening fabric.

This known embodiment of a two-ply screening fabric enables simple an practical replacing of the screening fabric, but it has turned out that the fme-meshed section of the fabric, which constitutes one layer of the two-ply screening fabric, is liable to damage after a short time m use. The vibrations of the screen results m holes being ground m the fme- meshed fabric section, or the fabric rupturing due to fatigue .

One known measure to remedy said weaknesses is to avoid tightening of the fme-meshed fabric, which constitutes one layer or ply of the composite fabric, when it is being glued to the coarse-meshed fabric, which constitutes the second layer or ply of the composite fabric. By allowing some slack m the fme-meshed fabric, the screening fabric can be tightened withm the screen frame, without correspondingly overstraining the relatively weak fme-meshed fabric, as the tensile load is absorbed by the coarse-meshed fabric. However, this known solution has not resulted m any particular increase m achievable time m operation.

The object of the invention has been to provide an improved screening fabric of the kind indicated m the preamble of claim 1, wherein said defects, drawbacks and limitations of use of known two-ply screening fabrics of the kind m question are avoided. Likewise, it is an object of the invention to prescribe a rational and convenient method of producing such screening fabrics.

Said object is realized by forming the screening fabric or by proceeding as stated in the characterizing parts of the claims.

A screening fabric according to the invention comprises, in a manner known in itself, two layers of fabric, a first layer of coarse-meshed fabric and a second layer of fine-meshed fabric, which fabrics are bonded together - also in a manner known in itself - by glue. The glue penetrates the two layers of fabric and is restricted to the local adhesion areas, which cover only a fraction of the area of the resultant structure with two layers of fabric representing the screening fabric in that form in which it is used, whereby the fine meshes of the one layer or ply of fabric are the ones that decide the screening grade, whereas the fabric layer with the coarser meshes provides the two-ply screening fabric with the necessary strength, rigidity and consistency.

According to the invention, the coarse-meshed layer of fabric of the screening fabric, which layer has first been plaited or woven, is subjected to a subsequent rolling to a greater surface smoothness than that of known coarse-meshed fabric layers included in composite fabrics of the kind in question.

By rolling the coarse-meshed component layer of the fabric to a great surface smoothness, at least on that side where the surface of the fine-meshed, non-self-supporting section of the fabric comes to rest supportingly, the surface contact area of the fine screening fabric supported on the coarse- meshed screening fabric is increased. The supporting surface contact area, which can be offered by a non-rolled layer of plaited/woven coarse-meshed fabric, and on top of which, the layer of fine-meshed screening fabric has the possibility to rest, is considerably smaller than by the arrangement according to the invention.

Thereby a screening fabric formed in accordance with the invention can endure a longer time in operation before holes are ground in the fine-meshed layer of the screen, than what is the case of known two-ply screening fabrics for vibrating screens. Due to its surface smoothness, the rolled coarse- meshed layer of the fabric, which layer exhibits at least one smooth surface (facing the fine-meshed layer of the fabric) and constitutes the supporting fabric layer of the two-ply screening fabric, will not cause any breakage marks in the fine-meshed, screening-grade-deciding fabric layer.

According to the invention it is advantageous to position lines of glue so that the edges of clamping rails or strips that are used to clamp the screening fabric to a frame, grip across areas where the screening fabric (fabric layers) is (are) provided with glue. Further it is an advantage if said edges of the clamping rails or strips are rounded, so that the edge portions will not give rise to any undue concentrations of tension in the screening fabric when the vibrating screen is working.

Further, the fine-meshed fabric layer of the screening fabric according to the invention should be under tensile strain in the position of use. This is achieved by pre-stressing the fabric layers held together, and gluing them together while they are both being stretched. In use, the layers of fabric "welded" or glued together, under pre-tensioning, into a composite or two-ply structure, behave in the way that the fine-meshed layer of fabric moves minimally on top of the underlying layer of supporting coarse-meshed fabric, and wear of threads in the mesh material is thereby prevented.

Conveniently, a type of glue is used, which is preferably a hot -melt adhesive based on plastic, which will provide advantages in terms of strength in the complete screening fabric with the two layers of fabric glued/welded together, while, at the same time, the adhesive material can withstand paraffins, heat etc. better than conventional screening fabrics of the relevant two-ply composite screening fabrics for vibrating screens. By the choice of adhesive, the aim has also been to avoid corrosion attacks.

Non-limiting examples of embodiments and arrangements of screening fabrics and methods of/installations for producing such screening fabrics are illustrated in the accompanying drawings, in which

Fig. 1 shows a two-ply screening fabric according to the invention, seen in top plan view;

Fig. la shows the same two-ply screen in a somewhat modified embodiment ;

Fig. 2 shows, on a considerably larger scale, an edge- enclosing rail with rounded free longitudinal edges, which receives a glued-together edge portion of an upper fine- meshed screen and a lower coarse-meshed screen; Fig. 3 shows, in a side view on a considerably smaller scale than the preceding figures, a schematically suggested installation for the production of two-ply screening fabrics according to the invention; and

Fig. 4 shows the installation in Fig. 3, in a top plan view.

A two-ply screening fabric, generally identified by the reference numeral 10, comprises, Fig. 2, a first upper fine- meshed layer (12) of fabric, which is decisive of the screening grade, and a second underlying layer 14 of coarse- meshed fabric, which forms the supporting layer for the first fine-meshed layer 12 of fabric.

The two fabric layers 12, 14 are glued together by means of lines 20 of glue, for example hot-melt glue. In two rails with cross-sections like U-frames, which extend along opposite edges, and which are generally identified by 18, Fig. 2, the glue lines 20 extend only partially into the cavity of the rails 18, cf. dotted lines in Fig. 1, which show the respective inner edge of the glue lines.

The intermediate glue lines 20 are equally spaced and in the same direction as the longitudinal direction of the edge rails 18. Glue lines 20A can also be positioned transversally to the longitudinal direction of the edge rails 18, cf . Fig. 1A.

The outer edge rails 18 (or corresponding strips) are used to clamp the two-ply screening fabric within a frame, preferably by means of a U-shaped securing portion 18A. In order for the parallel, outer edge rails 18 not to rub, in particular against the fine-meshed fabric layer 12 of the two-ply screening fabric 10 at the adjacent free rail edge, this rail edge and preferably also the other one are provided with a distinct rounding 22 in the form of some kind of flange portions (folded curl edge portions) . The rounded or folded free rail edge portions prevent the occurrence of undue concentrations of tension in the screening fabric when the vibrating screen is working.

The coarse-meshed layer 14 of fabric, Fig. 2, which was originally plaited or woven, has subsequently been subjected to rolling in order for it to have a greater surface smoothness. It is thereby the surface smoothness of the surface 14a of the coarse-meshed layer 14 of fabric facing the fine-meshed layer 12 of fabric, and on top of which the latter is resting supportedly, which is of importance, because the smoothness allows the fine-meshed fabric layer 12 to rest then on a larger area of the underlying supporting fabric 14 than if the latter was not rolled and did not exhibit the advantageous smoothness properties.

Earlier have been explained the advantages of letting the fine-meshed fabric layer be stressed in the position of use, which is effected during the production of such two-ply screening fabrics by pre-stressing the layers of fabric and gluing them together while they are both stressed.

A first fine-meshed fabric and a second coarse-meshed fabric are each arranged in the form of individually supported supply rolls, and are drawn from these rolls on to a common, motorized receiving roll, whereby the fabrics in the area from the respective supply roll to the receiving roll are kept at a pre-determined stress by a brake moment being applied to each supply roll.

Here, reference is made to Figs. 3 and 4, which show a schematically visualized installation for the production of two-ply screening fabrics of the kind indicated.

According to the side view of Fig. 3, an upper supply roll 24 with fine-meshed fabric 12 and a vertically aligned underlying supply roll 26 with coarse-meshed fabric 14, have a common, motorised receiving roll 28 arranged thereto, whereby fine-meshed fabric from the supply roll 24 and coarse-meshed fabric from the supply roll 26 are withdrawn therefrom and wound on the receiving roll 28 in the form of the two-ply screening fabric 10 described, comprising two locally glued-together fabric layers 12, 14 in sheet form.

From the supply rolls 24, 26 and up to the receiving roll 28 the fabric layers 12 and 14 are guided and supported by means of two opposed turning and supporting rolls 30 and 32.

Upstream of the supply rolls 24, 26 and vertically between the two, are arranged rolls with strip-shaped hot-melt glue 34 which is drawn in the shape of longitudinal lines in between the two fabric layers 12, 14 by the receiving roll 28, and are brought to melt by means of two opposed heating rolls 36 and 38.

In order to reinforce the screening fabric further, the two fabric layers 12,14 may be glued together along their short sides in a subsequent operation, by means of lines 20A of glue, cf. Fig. 1A. As earlier mentioned, a braking moment is applied individually to the two supply rolls 24 and 26.

The respective braking moment may with advantage be adjusted so that the respective fabrics, 12 and 14, respectively, receive a desired portion of the overall tensile force provided by the receiving roll 28.

Merely as an example; by adjustment of the braking moment, the layer of fine-meshed fabric may be brought to receive about ten per cent of said overall tensile force exerted by the receiving roll 28, the coarse-meshed fabric layer receiving the remaining tensile strain.

Claims

C L A I M S

1. A two-ply screening fabric intended in particular for incorporation in a vibrating screen of the kind that serves to separate and remove cuttings and stone

5 fragments from drilling mud, comprising a fine-meshed layer (12) of fabric, which is decisive for the screening grade, and a coarser-meshed layer (14) of fabric, which supports the fine-meshed fabric layer (12), said two fabric layers (12, 14) being bonded o together for the formation of said two-ply screening fabric (12, 14), preferably by locally implemented gluing together, which gluing may be distributed in a line-pattern, consisting of, for example, parallel lines (20) , and whereby opposite outer edges of the two-ply s screening fabric may be provided with clamping rails (18) or strips which are used to clamp the two-ply screening fabric (12, 14) within a frame to be mounted into the vibrating screen, c h a r a c t e r i z e d i n that said coarser-meshed layer (14) of supporting o fabric exhibits a smooth surface (14a) at least on the side facing the fine-meshed layer (12) of fabric.

2. A two-ply screening fabric according to claim 1, c h a r a c t e r i z e d i n that from a plaited/woven structural condition the coarser-meshed layer (14) of 5 fabric is rolled to a smooth surface at least on one side.

3. A two-ply screening fabric according to claim 1 or 2 , c h a r a c t e r i z e d i n that each of the free edge portions of said clamping rails (18) or strips grip across at least a transversal portion of a longitudinal area of the screening fabric (12,14) containing glue, which penetrates the thickness of the screen fabric.

4. A two-ply screening fabric according to claim 3 , c h a r a c t e r i z e d i n that across the area of the two-ply screening fabric (12,14), the glue is localized in a pattern of distribution comprising lines of glue (20) parallel with the clamping rails (18) or strips, of which the outermost two, in the areas of the opposite clamping rails (18), are partly inside and partly outside the clamping rails (18) .

5. A two-ply screening fabric according to claim 4, c h a r a c t e r i z e d i n that lines of glue (20A) are positioned transversely to the longitudinal direction of the clamping rail (18) .

6. A two-ply screening fabric according to claim 3 or 4 , c h a r a c t e r i z e d i n that each of the clamping rails (18) has the form of a mainly elongate frame profile with a cross-section in the shape of a mainly right-angled lying U, and that the free longitudinal edges of each U-rail (18) are rounded (22) , for example by being folded over, whereas the opposite free longitudinal edges are provided with a U-shaped securing portion (18A) .

7. A two-ply screening fabric according to any one of the preceding claims, c h a r a c t e r i z e d i n that each of said two fabric layers (12 and 14) is prestressed in the two-ply screening fabric.

8. A two-ply screening fabric according to claims 4 and 7, c h a r a c t e r i z e d i n that the two fabric layers (12 and 14) are prestressed in a common direction perpendicular to the longitudinal direction of the glue lines (20) and the clamping rails (18) .

9. A method of producing two-ply screening fabrics formed in accordance with one or more of the preceding claims, whereby a first fine-meshed layer of fabric (12) , decisive of the screening grade, and a second coarser- meshed supporting layer (14) of fabric are withdrawn from respective supply rolls (24 and 26, respectively) by means of a motorized receiving roll (28) and are joined together one another upstream of the receiving roll (28) preferably by means of glue which is applied locally, i.e. over a fraction of the area of the resulting two-ply screening fabric produced in sheet form and wound on the receiving roll (28) for subsequent cutting into suitable rectangular pieces of screening fabric (12,14) of the kind used in vibrating screens, c h a r a c t e r i z e d i n that in the area between the respective supply rolls (24 and 26, respectively) and the receiving roll (28) , each of the fabric layers (12 and 14) are kept under stress loads, predetermined for the respective fabric layers, a braking moment applied to each of the supply rolls (24 and 26) .

10. A method as claimed in claim 9, c h a r a c t e r i z e d i n that each braking moment is adjusted so that the respective fabric layer (12, 14) absorbs a desired amount of the total tensile force exerted by the receiving roll (28) .

1. A method as claimed in claim 10, c h a r a c t e r i z e d i n that the braking moments are adjusted relatively, so that the fine-meshed fabric layer (12) absorbs in the order of ten per cent of the prevailing tensile force, the coarse-meshed fabric layer absorbing the rest of the overall tensile force from the receiving roll (28) .