Vibrating screen for fine screening of liquids

(74) Agent: Simonnot, Rinuy, Santarelli.

54) Vibrating screen.

72) Invention of:

33) (32) (31

Conventional priority: Patent application filed in the United States of America ie February 2, 1970, n. 7,869 on behalf of Robert Alfred Krynock and Robert. William Ruhe, Jr.

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The present invention relates to improved vibrating screens with a flat bottom, in particular those which are used for the fine sieving of drilling muds from oil wells. A drilling mud is an aqueous solution of certain clays and other elements, the composition of which can vary within wide limits according to the various functions to be fulfilled. This sludge is intended mainly and in particular to provide lubrication, cooling and sealing making it possible to maintain the pressure in the drilled hole, as well as to constitute a hydraulic support which conveys the waste from the bottom to the surface during the rotary drilling of oil wells. As it is very expensive, this sludge is generally reconditioned and put back into circulation. In the first drilling techniques, this waste was removed by decantation. But as the depths of the oil wells increased, the increasing solids content of the mud contained them kept them in suspension, and they were put back into circulation. It was then necessary to abandon the settling devices and replace them with sieving devices, desanders and centrifuges which are therefore now essentially part of any rotary drilling installation.

The density, viscosity and coagulation force of a sludge are directly determined by the type and volume of the solid matter it contains. The earlier solids from drilling, i.e. drilling waste, sand and shale, are removed from the mud, the lower the likelihood that they will be recirculated and reduced by erosion. and abrasion in particles likely to cause viscosity and coagulation problems, and to increase the wear of the pump

3θ and other tools. These factors have a considerable effect on the cost price of the mud and the operating costs of the well, the speed of drilling, hydromechanics, etc. Since clays and other chemicals are soluble it is desirable to remove as much of the solids from the sludge as possible, regardless of their size. Determining the proportion of solids therefore becomes one of the most important phases of the operation of adjusting the composition of the sludge. It is generally less expensive to mechanically remove these solids, for example by sifting, than to try to adjust the proportion with water,

03310 barite and chemicals. The operation of determining the proportion of solids begins in the vibrating screen or sieve, the effectiveness of which is proportional to the fineness of the meshes. Maintenance costs for pumps and other tools in the circuit are also greatly reduced. We therefore agree that it is important to have a sieve whose mesh size is between 0.177 and 0.149 mm.

Fine sieving presents different difficulties from those encountered in the screens or sieves previously used. The thickness of the metal wires constituting the sieves with large meshes is relatively large (about 0.40 mm and above) and the opening surface of these sieves is about 50%, while the sieves with 0.177 rom opening of meshes have much finer wires (approximately 0.01 mm in diameter) and an opening area of ​​only 31.4%. Consequently, the treatment of the same volume of fluid with the same efficiency requires for a sieve of 0.177 rom of mesh opening an area almost twice as large. The greater fineness of the threads of these sieves also makes them very fragile and it is much more difficult to give them an acceptable longevity. Coarse sieving, as previously done, allows fine products and water to pass quickly and easily through the tarais, so that a relatively short and steep floor is satisfactory. On the other hand, in fine sieving, the mud contains a much higher proportion of fine products whose size approaches that of the mesh of the sieve at 0.177mm of mesh opening and these particles of irregular and triangular shape tend to pile up on the sieve and block the mesh. The viscosity of a surfacing mud is much higher than that of water, and this, adding to the natural surface tension presented by any liquid on a fine sieve, the difficulty becomes even greater.

Two types of sieving apparatus have been used in the past: rotary or cylindrical screens or sieves, and vibrating screens with a flat bottom or floor or vibrating screens.

These two types include sieves of the same fineness, but the vibrating sieve is more effective, in particular for the high-throughput treatment of viscous sludge.

A classic type of vibrating screen consists of a single

03310 bottom or floor, mounted in a shallow box inclined downwards at approximately 10 ° and comprising a two-level shaking device with unbalanced masses, of which an eccentric central part of the shaft is mounted transversely at the top and in the middle of the body in the direction of its length. The shaking device operates at high speed (1,750 rpm) and its stroke is reduced (1.59 mm) so as to produce at the ends of the body relatively low acceleration and an elliptical vibratory movement. The shaking device is driven by a V-shaped belt connecting its shaft to an independent electric motor mounted on the support frame. The body is mounted on a separate support frame by means of a suspension with four attachment points comprising leaf springs, coil springs or mounting elements made of rubber. The assembly of the supply box and the collecting tank is generally carried out independently depending on the particular installation. The assemblies are usually installed in pairs to ensure proper operating flow. The maximum fineness of this kind of sieve is 0.59 mm of mesh opening, but more often only 1.41 to 2 mm of opening. They are generally about 1.20 m long by 0.90 m wide or 1.50 m long by 1.20 m wide. These assemblies are therefore only suitable for removing large-scale waste.

2c It is after having carried out numerous tests to overcome the difficulties encountered, to improve the efficiency of the sieve and to give it a reasonable longevity that we have managed to produce this special and improved vibrating sieve according to the invention intended for fine sieving. . It has been found that it is possible to achieve the desired results by means of a relatively high acceleration shaker or vibrator, operating at least at 1120 rpm and having a vibratory stroke of at least 8 mm. The expression vibratory stroke represents the total amount of movement or the diameter of the circular movement, and is equal to twice the amplitude of vibration measured between the average position and the point corresponding to the maximum displacement. This arrangement provides the movement necessary to lift the particles and impact them against the screen. Knowing also that the vibrator located above the single floor must transmit an elliptical movement to a particle

03310 being at the ends of the box, the main axes of these ellipses meeting at a point located above the vibrator, we preferred to have the latter slightly beyond the middle of the box, in the direction of its length.

The best performance of the movement described above is obtained by means of a two-part floor, the one closest to the feed end being horizontal and the other having a slope inclined at 5 °. It has been found that the vertical component of the vibratory movement at the end of the material puts the clay slurry in suspension, while the fine solids collect in a layer closer to the surface of the sieve. At this point, the horizontal component of the movement, added to the initial speed of the liquid coming from the feed box and falling on the sieve, determines their advance.

As they move forward, these fine products exert a brushing action on the screen fabric and prevent it from clogging, thus allowing the falling mud to pass more easily through the released mesh. Between the two parts of the floor, a total nominal drop of 7.6 cm is split in two by an inclined intermediate plate, flow and connection, which changes the direction of fall of the material, tends to dissociate the largest drops and promotes the release of moisture. The material passing over the second inclined part is a little drier and contains more solid elements, since a large part of the retained moisture has already passed through the first part.

The vertical component of the vibratory movement is greater on this second part and therefore projects the material into the air, so that the surface of the screen capable of allowing the remaining liquid to pass is larger. The horizontal component of the movement prevents flow and therefore promotes the retention of the material on the floor and, consequently, further separation. The slope of 5⁰ ensures, however, by gravity, sufficient displacement of the remaining solids of larger dimensions, which are entrained up to the end of the sieve. It has also been discovered that during the displacement of the material on the second part of the floor, these larger solids tend to pick up and trap those whose dimension is smaller than the mesh of the sieve, thus achieving an effect of 'very clear agglomeration. This particular advantage does not exist in coarse sieving and therefore increases the efficiency of this new sieve.

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One test showed that about 6c% of the fine sands from 0.044 to 0.0 / 4 mm were thus removed from the sample.

A new supply box has been designed in order to reduce as much as possible the speed and the jolts of the flow of the fluid as it enters the box, and to allow its transverse spreading over the entire width of the screen. This feed box has a short horizontal trough advancing above the feed end of the screen and surmounting it by about 27 cm, so as to communicate a certain potential energy at the inlet ^ which promotes the passage of unrestrained moisture through this sieve. This trough also comprises, in the direction of its width, adjustable blades of. flow adjustment to distribute the fluid evenly over the feed end of the screen and to promote flow in a direction parallel to the flow of material flowing over the screen.

All of the improvements described above meet the aims of the invention, that is to say the production of a vibrating screen having fine meshes of 0.177 mm opening intended for the sieving of a mud of drilling, and whose longevity and maintenance difficulties are acceptable.

The present invention therefore relates to an improved vibrating sieve with meshes of 0.149 to 0.17 mm i¹ opening, intended for the fine sieving of a drilling mud and similar liquids, capable of effectively determining their properties and composition by retaining a large proportion of waste and suspended solids, and having a long service life.

Other characteristics and advantages of the present invention will emerge from the description which follows, given with reference to the appended drawings, and giving by way of explanation, but in no way limiting, an embodiment of the invention.

In these drawings:

Figure 1 is a schematic side elevation of the improved vibrating screen;

Figure 2 is a schematic plan view and partially broken away of the screen of Figure 1;

Figure 7 is a schematic elevation of the feed end of the screen of Figure 1;

Figure 4 is a schematic cross section along the

4 ^ · line 4-4 of figure 1;

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Figure 5 is a longitudinal section on the line 5-5 of Figure 2;

Figure 6 is a cross section on a larger scale of Figure 4, showing the support elements and the system ensuring the rigidity of the screen;

Figure 7 is an enlarged perspective view of the rubber cushioning strips mounted between the wire mesh of the screen and its longitudinal support bar;

Figure 8 is an end elevation of the spring weight control assembly mounted on the shaft of the vibrator shown in Figure 4;

FIG. 9 is a diagrammatic representation of the movements of each end and of the center of the sieve box relative to the side plate of this box, and of the location of the vibrator situated under a sieve according to the invention in operation; and Figure 10 is a partial plan view in the direction of the arrows 10-10 of Figure 5 and shows the assembly elements of the first and second parts of the sieve floor on one side of this floor.

Ge vibrating sieve 12 according to, the invention intended for the fine sieving of a drilling mud from an oil well or the like, is in its embodiment represented in FIGS. 1 and 2, consisting of a portable and autonomous assembly comprising a base 15 forming a reservoir, a box 16, a floor 17 and a drive 18. The base 15 has a L-shaped side elevation, as shown in FIG. 1.

The floor 17 is about twice as long as it is wide, so that the sieve seen in plan is substantially rectangular

OJ as shown in Figure 2. The screen floor also consists of two parts; a horizontal supply part 19 (FIG. 5) and a discharge part 20, inclined downwards by 5⁰ about. The body 16, resiliently supported on the base 15, supports the floor 17 and a vibrator 21. The drive 35, shown in Figure 4 consists of a V-shaped belt 22 connecting a pulley 23 attached to one end a shaft 24 of the vibrator and a pulley 25 fixed to an electric motor 26 mounted on the top of the body 16. As the body directly supports the motor 26, the whipping of the belt 22 is reduced and its longevity is increased.

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The base 15 forming a tank consists of two vertical side plates 27, two transverse tubes 28, a supply box 29 (FIG. 3) and a collecting tank 30. Cnaque plate 27, formed of reinforced welded elements , eom5 carries horizontal spacers 31 and two reinforced elements 32 supporting a spring. The upper edge 33 of the side plate is inclined towards the oas between the supply end and the discharge end, and its slope corresponds substantially to the average slope of the floor 17, two tubes 28 acting as' 0 bracing bars connect the lower part of the plates 2? at each end of the base. These tubes extend outwardly beyond the side plates and horizontal struts to allow the use of a hoisting sling around each of their ends 28a.

At the feed end of the screen, a feed box 29 (Figures 1 and 3), fixed to the plates 27 just above one of the tubes 28, has a lower part '29a and an upper part 29b. The lower part, of triangular cross-section, receives the material arriving via an admission tube 20 on 35 and Ta directs upwards towards the substantially rectangular part 29b which communicates with a trough 37 surmounting the top of the part 19 of the body . The volume of this feed box is relatively large so as to reduce the speed of the liquid passing through the inlet and to distribute it equally over the entire width of the screen.

Adjustable feed vanes 4C arranged in the feed trough (Figures 2, 3 and 5) direct and regulate the flow. Each of them consists of a pivoting vertical plate 41 secured to a threaded rod 42. The width of these plates is substantially equal to the distance separating the threaded rods, and their height corresponds to that of the walls 43 of the trough. Oriented perpendicular to the flow, they can interrupt it, but, once open, their large surface allows you to adjust its direction. Each of the threaded rods is fixed to a plate 44 forming the bottom of the trough and to a transverse angle 45 arranged across its upper opening. Nuts 4-6 retain the rods in the angle iron and normally maintain the angular orientation, determined manually, of the plates 41 relative to the current of the liquid. The position of the bottom 44 of the trough is such that there is between it and a

Ί \ 03310 surface 47 of the sieve a vertical drop substantially equal to the height of the blades, this giving the water a potential energy which promotes its rapid passage through the sieve and allows better drying during the first 5 phases of the sieving operation.

The collecting tank JO (FIGS. 1 and 4) is one with the base 15 and constitutes a sealed container receiving the mixture of water and small solids passing through the screen 47. The central and lower parts of each plate 27 includes discharge orifices 48 intended for the evacuation of these small solid materials. Vertical doors 49, sliding on guides 50 in these orifices and actuated by hand, make it possible to regulate the flow. A profiled bottom plate 51, welded to the plates 27, extending from the feed end to the openings 48 and straightening vertically at the discharge end, channels the mixture of water and small solids up to '' at ports 48.

The body 16 (FIGS. 1, 2 and 4) is essentially constituted by two vertical lateral plates 55 substantially trapezoidal, on each of which is welded, substantially to the sixth of their total length from each of their ends, a support element 56 of a spring. Coil springs 57, housed between these supports 56 and the supports J2, separate the vibrating box 16 from the base 15. The floor 17 of the sieve, which will be described later, is bolted to these plates 55 and connects them transversely so as to constitute a rigid structure. A plate 58, fixed at the top of one of the plates 55 and slightly in front of the longitudinal axis of the latter supports

1θ motor. A transverse horizontal tube 52 connects the plate 58 to a counterweight 53 mounted on the opposite side plate. This counterweight 53 balances the weight of the drive mounted on the body, so as to obtain a more uniform movement. A similar transverse tube 54 also connects the tops of the plates

55 at the feed end. As shown in FIG. 4, the assembly 21 of the vibrator, which will also be described later, is bolted to each side plate-55 by fastening elements 59 so that its axis is approximately five centimeters in front of the longitudinal axis of the side plate and

4o about twenty-eight centimeters above the extended plane of the

03310 part 19 of the floor 17. The shaft of the vibrator 24 has an extension 61 for driving, which extends beyond a counterweight 62 situated on the same side of the body as the plate 58. A conventional automatic lubricator 63 supplies in grease, the vibrator 21 through the tubing 64, as shown in FIGS. 1 and 3. The body 16 and its floor 17, the drive 18, the assembly 21 of the vibrator and the sieve 47 move as a unit under the action of the vibrations produced by the vibrator 21.

The floor 17 of the screen (FIGS. 2, 4 and 5) has a horizontal feeding part 19 which is followed by a discharge part 20 inclined downward at 5 ° in the direction of flow. To avoid unwanted whipping, better support the wire mesh 47 of the screen and provide it with a longer life, the floor 17 is made very rigidly and the surface supporting the wire screen is as smooth as possible. Each part of the floor consists of two longitudinal angles 65 and a series of six parallel longitudinal bars 66, flat, laid vertically and spaced transversely from each other by about fifteen centimeters over the width of the screen. The feed end of the first part 19 comprises a transverse end plate 6, U-shaped, the ends of which are fixed to the side plates 55 and the upper flange of which extends under the lower plate 44 of 1 '. feed trough. Spacers 68 and 69, welded to the bars 66 and to the angles 65, ensure the rigidity of the floor at this feed end and comprise at their junction with the bars 66 notches allowing passage beyond the plates 68 and 69 of rubber elements' 9 mounted on the bars 66. At the end of the part 19, an end angle O, jq disposed transversely to the floor, connects the longitudinal angles 65 and the bars 66. In order to rigidly assemble the part 20 to the part 19 (FIG. 10), a bent vertical plate 71 is welded at each end of the angle iron 70 and has a flange 71a projecting inwards towards the middle

-τ, ς and the discharge end of the screen. A second plate ”2, inclined by 5 ° with respect to the vertical, provides better rigidity of this assembly by connecting the angle iron 70 and the flange 71a. A horizontal junction plate 73 connects the tops of the plates 71 and 72 and forms a short compartment at each end of the angle iron 70. Another transverse plate 74, parai71 03310 connected to the part 20 of the floor and constituting the beginning, protrudes beyond the stiffening plates 72 and is welded between the plates 71 to the vertical branch of the angle iron 70, at about four centimeters below the top of the angles 65 of part 19 and about four centimeters above the corresponding angles 65 of part .20. This plate 74 thus ensures not only the flow of liquid between the two parts 19 and 20 of the floor, but also the transition corresponding to the slight vertical drop separating these two parts from one another, as well as the rigidity of the connection by giving the angle iron 70 a U-shaped profile. At the end of the part 20, a profiled transverse plate 75 is welded to the longitudinal angles 65 and to the bars 66. The lower flange of plate 75 is bent about 30 ° downwards and the feed end (Figure 1), to pass over the terminal vertical flange of plate 51 of the tank and force small solid waste to flow by themselves in the direction of orifices 4-8. A second profiled plate 76, welded near the top of the vertical branch of the plate 75 and on its outer face, forms a slide to allow large particles arriving from the top of the screen to reach the point where they are discharged. This plate is inclined approximately 55 ° with respect to the horizontal and has a vertical lower flange facing downwards.

The parts 19 and 20 of the floor have substantially in their middle in the longitudinal direction of the tubular transverse support elements 77 welded to the vertical branches of the angles 65. Oven to improve the rigidity of the part 19 and of the junction of the parts 19 and 20, two angles 78 are welded diagonally under the bars 66 and between the transverse angle 70 and the longitudinal angles 65, as shown in FIG. 2.

As^at shown in Figure 4, the vertical arrangement of the bars 66 is such that their upper ends rise gradually towards the longitudinal axis of the screen. The camber of the floor provided by this convex surface allows the establishment of the metal fabric 4? sieve, stretch it and avoid the formation of pockets or uneven wrinkles. However, unlike previous errors, this camber is considerably reduced to allow distribution

U331U uniform flow and keep it parallel to the longitudinal axis over the entire length of the screen. In an embodiment comprising a sieve approximately 1.20 m wide, the level of the longitudinal bar situated on the transverse axis is approximately 22 mm higher than that of the end bars, this being compared to the 46 mm of the standard. anterior. A special element 79, extruded in U-shaped closed cell foam vinyl synthetic rubber, of the BUNA-N type of relatively low hardness and density between 415 and 830 grams per cnP and shown in Figure J covers the edges bars 66. This material is manufactured by the company known as RUBATEX CORP., Bedford, Virginia, United States of America, under the designation Type R-1885-H. This element has a convex upper surface about 2.5 cm wide and a lower groove tightly enclosing the upper edge of the bar 66 with which it is associated. These elements 79 are also cut so as to be a little longer than the bars on which they adjust; they are then compressed lengthwise and glued to the bars 66 using a silicone adhesive, for example the product sold by GENERAL ELECTRIC COMPANY under the designation Type RTV Silastic Cernent. It is necessary to very carefully perform the welding and finishing of the upper surfaces of the screen floor so that it is smooth and free from any surface irregularity liable to cause premature rupture of the fabric 47 of the screen under the effect of vibrations.

The vibrator 21 shown in Figure 4 consists of a tubular casing 80 having at each of its ends a flared portion 81 in the shape of a bell and bolted to the side plates 55 of the sieve body. Each of the flared parts 81 contains a circular, removable block 82 of bearing comprising flanges and mounted in a sealed manner. To ensure the longevity of the blocks 82 in spite of the strong accelerations resulting from the operating conditions of the vibrator 21, an automatic lubricator 65 supplies them with grease via the pipe 64. These blocks support the shaft 24 of the vibrator which comprises each of its ends a concentric extension 85, external with respect to the blocks and on which counterweights 62 are mounted. The end 61 of the vibrator shaft has an additional extension supporting the

03310 pulley 25 of drive 18 around which the V-belt passes

As shown in FIG. 8, each of the counterweight assemblies 62 comprises a hub 84 fixed to the extension 85 of the shaft by a shim 85. This hub comprises a flange 86 oriented downwards and comprising two holes 87 spaced from the axis of the shaft and used for mounting the pivoting counterweights 88, as well as two additional holes 89 to which are fixed connecting rods 90. These counterweights 88 are plates, substantially bean-shaped, articulated on the flange 86 by an axis 91 .

To modify the counterweight effect, it is possible to fix additional plates 92 to the counterweights 88 by means of bolts 95 ^et 94. The upper surface of the hub 84 comprises a seat 95 on which is mounted a coil spring 96. A cap 97 covers the top of the spring 96 and has on each side of the legs 98> projecting parallel to the flange 86 and comprising holes 99 · Head screws 100 pass through holes 99 and are screwed into the upper end of a connecting element 101. The lower end of each element 101 com20 carries a yoke intended to accommodate the upper end of a rod 90. A key 102 connects the upper end of the rod 90 to the yoke and the tab 98, while the lower end of this rod 90 is fixed to the flange 86 by means of a key 105.

In the rest position, and during. starting and stopping phases, when the vibrator shaft has not reached its normal speed, the pressure of the spring forces the counterweights to remain in their closed position shown in solid lines in FIG. 8. The counterweights are then dynamically balanced and no vibration occurs. Once exceeded the critical speed of resonance, the counterweights 88 begin to behave in a manner analogous to that of the ball regulator of a steam engine. The weights start to rotate away from the axis of the shaft 24 under the action of the centrifugal force caused by the rotation of the flange 86, and their dynamic equilibrium is broken. At normal operating speed, the counterweights assume the maximum open position shown in dashed lines in FIG. 8, and their center of gravity deviates from the axis of the shaft 24 of the vibrator. The counterweights are thus dynamically unbalanced, and the vibrateupfirovo71 03310 that a continuous and uniform vibration of the sieve body.

The unequal elliptical movements produced at the ends of the body 16 by the asymmetrical position of the vibrator 21 are shown in FIG. 9, as regards a form of embodiment according to the invention. This position is offset forward relative to the longitudinal axis of the side plate 55 by the distance E, that is to say about 5 cm, and the vibrator is located above the floor 17 of the screen. On the side of the feed end, the ellipse corresponding to the vibratory movement presents in di10 rection of the vibrator an inclination towards the front and upwards corresponding to the angle X, that is to say approximately 52 ° with respect to the horizontal. On the side of the discharge end, this ellipse is inclined backwards in the direction of the vibrator at angle Y, that is to say approximately 58 ° with respect to the horizontal. Part 20 of the floor tilts down the angle Z, about 5 °. At the level of the vibrator, the movement is circular, its diameter is approximately 8 mm and its stroke therefore has the same value. The speed of rotation of the shaft 24 is approximately 1,130 revolutions / minute and produces a relatively rapid acceleration of the vibrator. Due to the inequality of the angles of the ellipses and their resultants Fp and Dp, the vertical component F of the movement on the side of the feed end is slightly less than that of the vertical component on the side of the end of evacuation. On the supply end side, the horizontal laying eom25 is directed in the direction of flow, while on the discharge end side the horizontal component D "is directed in the other direction. On the supply end side, the vertical component P ^. is also slightly larger than the horizontal component Fp and combines with the relatively large amplitude of vibration, so as to produce an elevating action greater than the translational action and to effectively stir the material so as to promote rapid separation. This action tends to suspend the sludge and collect the fine material in a layer closer to the surface of the sieve. As the horizontal component acts in the direction of flow, it increases the initial speed of the liquid flowing towards the supply part and causes an appropriate displacement of the thin products towards the second part 20 of the planener. This layer of moving material also tends

AC to exert a brushing action on the sieve fabric and to em / IUQ 3 IU fish to clog, thus allowing the mud to pass more easily through the released meshes. When passing from the first part 19 to the second part 20, the vertical drop, interrupted by the plate 74, modifies the direction of fall of the material, and the shock tends to dissociate all the large particles. On the second part 20, the material is a little drier and contains more solid products. At the discharge end, the largest vertical component projects the solid products on top so as to present the remaining liquid with a larger passage surface through the screen. The horizontal component D ^. directed opposite the current also prolongs the retention of solid products on the floor and allows the separation to be completed. However, the slope of 5 ° ensures by gravity a translational movement sufficient to cause the remaining large waste to pass over the end of the screen. The length of the retention time, ensured by the greater length of the combined sections of the sieve, makes it possible to remove the maximum quantity of muddy solution capable of being recirculated, while nevertheless allowing the separation of undesirable solids.

To carry out the assembly, the base 15 is put in place, the springs 57 being mounted on their supports 52. The box 16 is then mounted on the springs so that the supports 56 are properly aligned with the springs and that the par25 tie horizontal 19 of the floor is under the feed trough. As shown in Figures 4 and 6, the fabric 47 of the screen is carefully placed on the parts 19 20 of the floor. The tension plates 110 are then put in place on the hooked end 111 of the edges of the screen fabric. Bolts 112 are passed through the holes made on the plates 110, a washer 113, placed under each bolt head, and a block 114 of foam rubber, placed between the internal wall of the plates 110 and the side plates 55, serving as sealing elements. The diameter of the hole in block 114 is smaller than that of the pull bolt so that this block tightly grips the bolt during assembly and prevents any passage of liquid along the bolt. On the outer wall of the side plates, tubular spacers 115, the outer face of which is parallel to the main surface of the plate 110, are welded to the body and cover the bolt holes. As

03310 as we push the bolts 112 into the tubular liners, we first add a washer 116, then a spring 117, then a second washer 118 and finally a nut 119. Once the placement of all the plates is complete tension with their seemingly bolting, the bolts are carefully screwed, starting with the middle of the sieve and going towards each end, carefully smoothing the metallic fabric 47 to make all the wrinkles or unevenness disappear. Each bolt is then tightened until complete compression of the coil springs.

The fabric of the screen lengthens during use, the springs catch up on the play. The combination consisting of the washer 113, the elastic block 114 and the arrangement of the flanges of the plates 110 ensures, once the bolts are fully screwed, tight seal with the liquid content of the box which tends to escape through the bolt holes. The deflector plate 71, located on each side of the screen between the parts 19 and 20, also tends to channel towards the second part 20 the liquid which tends to penetrate laterally behind the ends of the stiffness plates, and to improve the sealing against 'water.

It goes without saying that the present invention has only been described and shown for explanatory purposes, but in no way limiting, and that it is capable of various variants without going beyond its ambit.

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