Note: Descriptions are shown in the official language in which they were submitted.
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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.
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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.
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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
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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;
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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
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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.
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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.
