CA2210020C - Improved sifting screen - Google Patents
Improved sifting screen Download PDFInfo
- Publication number
- CA2210020C CA2210020C CA002210020A CA2210020A CA2210020C CA 2210020 C CA2210020 C CA 2210020C CA 002210020 A CA002210020 A CA 002210020A CA 2210020 A CA2210020 A CA 2210020A CA 2210020 C CA2210020 C CA 2210020C
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- Prior art keywords
- cloth
- screen
- frame
- cloths
- woven
- Prior art date
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- Expired - Lifetime
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- Combined Means For Separation Of Solids (AREA)
- Filtering Materials (AREA)
Abstract
A sifting screen is described comprising a rigid frame, a first woven cloth of hard wearing metal wire, stretched thereacross and secured thereto, and a second woven cloth having a coarser mesh than the first cloth 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, 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
Improved Siftinct Screen This is a division of copending Canadian patent Application Serial No. 2,157,276 which was filed internationally on February 28, 1995 and which entered the National Phase in Canada on August 30, 1995.
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.
Backctround 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.
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.
Vhilst 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 cased muds from drilling rigs, is such that a considerable amours' of time has to be spent cleaning the screen to allow it to be inspected and damaged regions found and replaced by neTrr irserts. Any downtime of a machine such as this when associated with a process such as sea bed dril l i ng for oil and gas, is not only costly but because sometimes weather conditions and other factors limit the time available for drillinc etc, such a failure at a crucial point in time can be cri~i cal to the success of the overall operation.
It is therefore an object of the present inventi on to provide an alternative form of sifting screen construction which will have a longer operational lif a than that of types hitherto and which under ordinary operating conditions s~culd have a predictable ordinary life span allowing replacement to be performed at set periods of time much in the sane way as other components are replaced at regular servicing intervals.
Summary of the invention According to the present inver_tion there is prov_ded a sifting screen comprising a frame; a first woven cloth o~ hard wearing metal wire, stretched across the frame and secures thereto; and a second woven cloth having a coarser mesh than the first cloth and bei ng woven from an elongate material whose cross-sectional area is greater than that of the fi rst cloth, the second cloth being also stretched across the frame and being secured thereto below the first cloth to supoc rt the latter against sagging;
wherein the screen frame is of rectangular shape made of a plastics material and having a rigidity selected to restrict overall flexure of the wo~.~en 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.
The tension in the material forming the upper cloth may be different from that in the material forming the lo~rrer cloth.
The screen may be constructed from heavy gauge steel and/or include structural support members, or from glass reinforced plastics, or resin based materials, with metal rei~:forcing such as glass reinforced gas b1 o:~~n polypropylene with metal reinforcing elements.
Screen flexure reduction, whi c:z is closely 1i nked ~.o 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 transpor;.ed over the screen when in use. This arises from the fact that flexure reduction normally requires a greater r_umbe= of rigid reinforcing members typically of steel, to which the cloths must be firmly secured and, which reduce the area available for filtration.
The cross section of the mater=als from which t~.~ cloths are woven and the shape and size of unsupported areas of screen cloths are preferably selected having due rega=d ~o the nature of solids materials to be transDOrted over the =es~~l ting screen and to the maximum force ac ring on the screen i~: 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 or.e which can have a significant influence is whether the unsupported lengths of tensioned material are likely to be ac tivated into a rason ant mode of vibration or a harmonic cr sub-harmonic of their natural resonant frequency by the vibration imparted by the operation of the machine cvithin which t.e screen is mounted.
The lengths of elongate n aerial extending across the unsupported regions of the c'c~hs and the tensicns in those lengths of material, are preferably 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.
Brief description of the drawings The present invention, taken in conjunction with the parent copending Canadian Patent Application Serial No. 2,157,276 which was filed internationally on February 28, 1995 and which entered the National Phase in Canada on August 30, 1995 will be described in detail hereinbelow with the aid of 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 two woven cloths in another embodiment of the invention;
Figure 3 is a plan view to a reduced scale of a screen support frame to which the 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 AF. ( to an enlarged scale);
Figure 7 is a part section at the other en~ cn ~.~; ( to an enlarged scale); and Figure 8 is a section on CC ( to an enlarged seal a .
Detailed description of the dra~.rincts Figures 1 and 2 show different c loth constructions.
In Figure 1 the upper cloth 10 i s formed from :coven stainless steel wire in the range 0.19mm to 0.036mm diameter and 60-325 mesh, ( ie number of strands per inch or per 25 . ~n.~~) while the lower cloth 12 i s formed from :coven phosphor bronze wire in the range 0.45<i to 0.19mm diameter and 20-40 mess:.
In Figure 2 the upper cloth 1 ~-_ is formed in a s=:ail ar manner to cloth 10 in Figure 1 but the lower cloth is woven from stainless steel wire having a nominal diameter -n the range 0.20 to 0.~5mm diameter and typically 30 mesh, and is coated with an epoxy based material, o r Molybdenum Disulphide, or Teflon (Registered Trade Mark), to a thickness in the range 5 to 50 microns typically 20 to ~0 microns. rlultiple 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 sho~rrn in cross section to show the outer material coatings 19, 21, 23 (albeit not to scale).
The wire 2s is shown with the coating scraped of one end.
Figures 3 to 8 show various vie:as of an improved screen support frame which is formed from gas blown polypropylene with added glass fibre and reinforced with steel rods, e~~h being of the order of 2.5mm diameter.
Figure 3 is a plan view of tha support frame a::d F figures 4 and are edge vi ears with the longer edge vie~r sown in cross section along the line "~~" in Figure 3.
In known manner one such screen frame is adapted to be joined to another in the lengthc,rise direction and t~ this end the right hand edge ( in Figure 1 ) is formed with a :gale knuckle 26 and the left head 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.
S reel reinforc_ng rods extend lengthwise and wi dthTrrise 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 downT~Jard sense and then in an outward sense, to enter the knuckle edge and reinforce same. Rods ee~ a6 extend wi dthwise abo~;e and below the knuckle 26.
Pairs o f rods - o , 38 and 3 6 ' , 3 8 ' extend at the top and bottom of widthcaise extending reinforcing ribs 48, 50 which are located at regular intervals along the length o~ the frame, as at 52, 54 etc (in Figure 4 up to 62 ) .
Similar orthogonal reinforcing ribs 64, 66 etc (see Figure 5) extend length:~ri se at regular i nter~rals across tae width of the f rame .
The rectilinear matrix of r:,ds and moulded polypropylene reinforcing ribs ( both longi tudinal and transverse ) can be seen in the top lef t and right hand corners of the plan vie~fr of the frame shown in Figure 3.
.7 Figure 8 shows haw the ends of the tap layer of widthwise rods 38 (36) enter the upper flange 68, of which there is a similar one 70 along t=he otherlength wiseedge. The pairs of transverse rods in t:.he sectian .in Figure 8 are denoted on CC
by reference numerals 73, 74; 76, and 80, 82.
78;
Two cloths such as shown in Figurev 1 to 2 are laid across the frame shown in figure 3 and a:~t:er being tensioned are secured in position by a suitable adhesive along the side flanges 68, 70, along the two end f:Lanc~es 84, 86 and to the upper edges of the matrix of reinfaxo:ing ribs.
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.
Backctround 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.
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.
Vhilst 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 cased muds from drilling rigs, is such that a considerable amours' of time has to be spent cleaning the screen to allow it to be inspected and damaged regions found and replaced by neTrr irserts. Any downtime of a machine such as this when associated with a process such as sea bed dril l i ng for oil and gas, is not only costly but because sometimes weather conditions and other factors limit the time available for drillinc etc, such a failure at a crucial point in time can be cri~i cal to the success of the overall operation.
It is therefore an object of the present inventi on to provide an alternative form of sifting screen construction which will have a longer operational lif a than that of types hitherto and which under ordinary operating conditions s~culd have a predictable ordinary life span allowing replacement to be performed at set periods of time much in the sane way as other components are replaced at regular servicing intervals.
Summary of the invention According to the present inver_tion there is prov_ded a sifting screen comprising a frame; a first woven cloth o~ hard wearing metal wire, stretched across the frame and secures thereto; and a second woven cloth having a coarser mesh than the first cloth and bei ng woven from an elongate material whose cross-sectional area is greater than that of the fi rst cloth, the second cloth being also stretched across the frame and being secured thereto below the first cloth to supoc rt the latter against sagging;
wherein the screen frame is of rectangular shape made of a plastics material and having a rigidity selected to restrict overall flexure of the wo~.~en 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.
The tension in the material forming the upper cloth may be different from that in the material forming the lo~rrer cloth.
The screen may be constructed from heavy gauge steel and/or include structural support members, or from glass reinforced plastics, or resin based materials, with metal rei~:forcing such as glass reinforced gas b1 o:~~n polypropylene with metal reinforcing elements.
Screen flexure reduction, whi c:z is closely 1i nked ~.o 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 transpor;.ed over the screen when in use. This arises from the fact that flexure reduction normally requires a greater r_umbe= of rigid reinforcing members typically of steel, to which the cloths must be firmly secured and, which reduce the area available for filtration.
The cross section of the mater=als from which t~.~ cloths are woven and the shape and size of unsupported areas of screen cloths are preferably selected having due rega=d ~o the nature of solids materials to be transDOrted over the =es~~l ting screen and to the maximum force ac ring on the screen i~: 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 or.e which can have a significant influence is whether the unsupported lengths of tensioned material are likely to be ac tivated into a rason ant mode of vibration or a harmonic cr sub-harmonic of their natural resonant frequency by the vibration imparted by the operation of the machine cvithin which t.e screen is mounted.
The lengths of elongate n aerial extending across the unsupported regions of the c'c~hs and the tensicns in those lengths of material, are preferably 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.
Brief description of the drawings The present invention, taken in conjunction with the parent copending Canadian Patent Application Serial No. 2,157,276 which was filed internationally on February 28, 1995 and which entered the National Phase in Canada on August 30, 1995 will be described in detail hereinbelow with the aid of 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 two woven cloths in another embodiment of the invention;
Figure 3 is a plan view to a reduced scale of a screen support frame to which the 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 AF. ( to an enlarged scale);
Figure 7 is a part section at the other en~ cn ~.~; ( to an enlarged scale); and Figure 8 is a section on CC ( to an enlarged seal a .
Detailed description of the dra~.rincts Figures 1 and 2 show different c loth constructions.
In Figure 1 the upper cloth 10 i s formed from :coven stainless steel wire in the range 0.19mm to 0.036mm diameter and 60-325 mesh, ( ie number of strands per inch or per 25 . ~n.~~) while the lower cloth 12 i s formed from :coven phosphor bronze wire in the range 0.45<i to 0.19mm diameter and 20-40 mess:.
In Figure 2 the upper cloth 1 ~-_ is formed in a s=:ail ar manner to cloth 10 in Figure 1 but the lower cloth is woven from stainless steel wire having a nominal diameter -n the range 0.20 to 0.~5mm diameter and typically 30 mesh, and is coated with an epoxy based material, o r Molybdenum Disulphide, or Teflon (Registered Trade Mark), to a thickness in the range 5 to 50 microns typically 20 to ~0 microns. rlultiple 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 sho~rrn in cross section to show the outer material coatings 19, 21, 23 (albeit not to scale).
The wire 2s is shown with the coating scraped of one end.
Figures 3 to 8 show various vie:as of an improved screen support frame which is formed from gas blown polypropylene with added glass fibre and reinforced with steel rods, e~~h being of the order of 2.5mm diameter.
Figure 3 is a plan view of tha support frame a::d F figures 4 and are edge vi ears with the longer edge vie~r sown in cross section along the line "~~" in Figure 3.
In known manner one such screen frame is adapted to be joined to another in the lengthc,rise direction and t~ this end the right hand edge ( in Figure 1 ) is formed with a :gale knuckle 26 and the left head 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.
S reel reinforc_ng rods extend lengthwise and wi dthTrrise 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 downT~Jard sense and then in an outward sense, to enter the knuckle edge and reinforce same. Rods ee~ a6 extend wi dthwise abo~;e and below the knuckle 26.
Pairs o f rods - o , 38 and 3 6 ' , 3 8 ' extend at the top and bottom of widthcaise extending reinforcing ribs 48, 50 which are located at regular intervals along the length o~ the frame, as at 52, 54 etc (in Figure 4 up to 62 ) .
Similar orthogonal reinforcing ribs 64, 66 etc (see Figure 5) extend length:~ri se at regular i nter~rals across tae width of the f rame .
The rectilinear matrix of r:,ds and moulded polypropylene reinforcing ribs ( both longi tudinal and transverse ) can be seen in the top lef t and right hand corners of the plan vie~fr of the frame shown in Figure 3.
.7 Figure 8 shows haw the ends of the tap layer of widthwise rods 38 (36) enter the upper flange 68, of which there is a similar one 70 along t=he otherlength wiseedge. The pairs of transverse rods in t:.he sectian .in Figure 8 are denoted on CC
by reference numerals 73, 74; 76, and 80, 82.
78;
Two cloths such as shown in Figurev 1 to 2 are laid across the frame shown in figure 3 and a:~t:er being tensioned are secured in position by a suitable adhesive along the side flanges 68, 70, along the two end f:Lanc~es 84, 86 and to the upper edges of the matrix of reinfaxo:ing ribs.
Claims (9)
1. 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 being also stretched across the frame and being secured thereto below the first cloth to support the latter against sagging; wherein the screen frame is of rectangular shape made of a plastics material and having 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.
2. A sifting screen according to claim 1, wherein the frame is formed from glass-reinforced gas blown polypropylene and is reinforced by elongate metal reinforcing elements or rods.
3. A sifting screen according to claim 1 or 2, wherein the screen includes structural support members.
4. 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.
5. 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.
6. A sifting screen according to claim 5, 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.
7. A sifting screen according to any one of the preceding claims, in which both cloths are tensioned before being secured to the frame.
8. A sifting screen according to claim 7, wherein the tension in the material forming the upper cloth is different from that in the material forming the lower cloth.
9. A sifting screen according to any of claims 1 to 8, when fitted in a vibratory cradle of a shale shaker.
Applications Claiming Priority (3)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| GB9404071A GB9404071D0 (en) | 1994-03-03 | 1994-03-03 | Improved sifting screen |
| GB9404071.4 | 1994-03-03 | ||
| CA002157276A CA2157276C (en) | 1994-03-03 | 1995-02-28 | Improved sifting screen |
Related Parent Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA002157276A Division CA2157276C (en) | 1994-03-03 | 1995-02-28 | Improved sifting screen |
Publications (2)
| Publication Number | Publication Date |
|---|---|
| CA2210020A1 CA2210020A1 (en) | 1995-09-08 |
| CA2210020C true CA2210020C (en) | 2003-04-22 |
Family
ID=25678129
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA002210020A Expired - Lifetime CA2210020C (en) | 1994-03-03 | 1995-02-28 | Improved sifting screen |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA2210020C (en) |
-
1995
- 1995-02-28 CA CA002210020A patent/CA2210020C/en not_active Expired - Lifetime
Also Published As
| Publication number | Publication date |
|---|---|
| CA2210020A1 (en) | 1995-09-08 |
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Legal Events
| Date | Code | Title | Description |
|---|---|---|---|
| EEER | Examination request | ||
| MKEX | Expiry |
Effective date: 20150302 |