EP0707109B1 - Straining screen, in particular for paper pulp - Google Patents

Abstract

A sieve structure, for paper pulp, has at least one U-shaped profile formed into a circle, in a stack of identical circles (1). They are locked between two rings (2,3) by a number of the rods (4). The side walls of the stacked circles lie against each other without additional mountings, with elastic distortion through the locking action.

Description

In patent EP 0 354 846 filed on August 9, 1989, we describes a pulp purifier and classifier characterized by the juxtaposition of section elements U-shaped cross with a flat bottom fitted with perforations and two side walls. According to a first embodiment we make circles arranged the on each other; according to a second mode of realization we wrap the U profile on itself in spiral.

In both cases it is necessary to weld one to the others the ends of the side walls of the U in order to avoid that under pressure they separate, even very slightly, from each other.

When the screen is to operate in centrifugal mode, the side walls of the spirally wound U are facing outwards; so it is possible to weld them together. However, when the sieve must operate in centripetal regime, the walls sides of the U, wound in a spiral, are turned towards the interior and it is then very difficult to proceed welds inside the cylinder thus produced.

In addition, the installation of a weld all over the spiral path and an expensive operation.

However, it is essential to provide a fixation: for avoid partial displacement of the U under the effect of the moving organ (commonly called a "foil") which scrolls along the wall of the screen to cause pressure variations; to ensure a perfect parallelism between the two faces of the screen; for get a good stiffness of the sieve in the direction transverse.

In patent application WO90 / 12147, a sieve consisting of a stack of cylindrical elements in ceramic material between which are interposed interlayer crowns, this stacking being maintained assembled by tie rods fitted with elastic washers.

However despite the presence of these washers elastic, because the filter elements are ceramic and that the intermediate crowns are massive, such a sieve has no elasticity.

Now it turns out that polish the paper pulp screens which are fitted with a mobile unclogging device by pressure variation (commonly called "foil") it it is preferable that these sieves have a certain elasticity.

The present invention relates to a sieve consisting of a stack of circular elements stacked on top of each other the others and held together by a plurality of tie rods parallel to the generatrices of the cylinder, in which each circular element is constituted by a U-shaped curved so as to form a circle.

Because the circular elements are made to from U-shaped elements it will occur, under the effect of the traction exerted by the tie rods, on the one hand a slight bending of the two branches of the U, which slightly approach each other; on the other hand a slight deformation of the base of the U which bulges very slightly.

This gives a stack having an elasticity sufficient to absorb pressure variations caused by the foil, and to help determine at will the effect, of pre-tension which is given by the tie rods.

In addition, the U-shaped elements are fixed without have recourse, as was the case previously to the soldering or placing a jumper on the ends branches of the U.

This elasticity and this pre-tension, which are sought in order to have a good resistance to the use of sieves, however, have a drawback for sieves with large diameters, i.e. having a diameter greater than 1 meter because it occurs then sliding of the U-shaped circles on each other others and the stack of U-shaped circles is deformed.

According to an alternative embodiment of the invention the tie rods are associated with two upper crowns and lower to form a cylindrical framework rigid on which the U-shaped circles are threaded; the pre-tensioning said circles being carried out by tightening on the stack of an additional crown.

Figure 1, une vue schématique d'un tamis de type connu;

Figure 2, une vue en coupe de profilé en U au moyen duquel est réalisé le tamis la figure 1;

Figure 3, un premier mode de réalisation de l'invention;

Figure 4, une vue partielle à plus grande échelle de la figure 3.

Figure 5, une vue de dessus du tamis de la figure 3;

Figure 6, un deuxième mode de réalisation de l'invention;

Figure 7, une vue de dessus du tamis de la figure 5.

Figure 8, une vue partielle illustrant le montage et la mise en tension des cercles en U pour un tamis centripète de grandes dimensions.

By way of nonlimiting example and to facilitate understanding of the invention, the following figures show:

Figure 1, a schematic view of a known type of sieve;

Figure 2, a sectional view of a U-shaped section by means of which the screen is made Figure 1;

Figure 3, a first embodiment of the invention;

Figure 4, a partial view on a larger scale of Figure 3.

Figure 5, a top view of the screen of Figure 3;

Figure 6, a second embodiment of the invention;

Figure 7, a top view of the screen of Figure 5.

Figure 8, a partial view illustrating the mounting and tensioning of the U-shaped circles for a large centripetal sieve.

FIGS. 1 and 2 represent a sieve of known type, as described in patent No. 88.10863 of 08/12/1988.

Referring to these figures, we see that for make a sieve, use a profile whose section is a U whose base is pierced either by slots either through holes.

The U-profile thus prepared is then bent, which after which either we spirally wrap it or else cut it into pieces of the same length so that make circles that are stacked on top of each other other.

When the sieve must operate in centrifugal mode (i.e. when the material to be filtered is inserted inside the cylinder), the U-profile is arranged so that the two side walls of the U are facing outwards. In contrast, when the sieve must operate in centripetal regime (i.e. when the material to be filtered circulates outside to inside), the side walls of the U are turned inward.

In both cases, it is imperative to fix the ones to others the convolutions of the U-shaped profile.

In the first case, when the side walls of the U are facing outwards, it is possible to fix to each other by a stirrup or by a weld bead; but this additional operation is expensive.

In the second case, it is very difficult to make a weld bead inside the cylinder thus formed and it is practically impossible to put in place a stirrup.

According to the present invention, the profile is produced U-shaped circles of the same diameter. For that or else bend the U-section so as to obtain several windings, then cut sections which allow to form circles with the desired diameter; or else cut the U-shaped section into sections of the same length and we bend them to form a circle.

When the circle is shaped, the two ends of the section are welded to each other; so that we get a rigid circle.

Depending on the direction in which each section is curved, we get a circle 1 in which the side walls of the U are turned inward, as shown in Figure 3 or a circle 1 'in which the walls side of the U are facing outwards, as shown in Figure 6.

The circles thus obtained are then stacked one on others.

To keep them tight against each other, we has a crown 2 at the base of the stack and top another crown 3.

Between crowns 2 and 3, there is a plurality of tie rods 4.

These tie rods 4 are threaded at their two ends of so as to be able to receive nuts 6 which allow tighten large enough for the circles be kept clear of each other others in operation.

In the example shown in Figure 3, each tie 4 is screwed into the lower crown 2 and is fitted a nut 6 its upper part; but he is possible to put a nut 6 at each end, the two crowns 2 and 3 then being identical.

In the case of Figures 3 and 4 which represent a sieve to operate in centripetal regime, it is preferable to have as few obstacles as possible to the outer surface of the screen: the tie rods 4 are then placed inside the sieve.

In the case of Figures 5 and 6, it is the internal wall a sieve which must not contain any obstacle; of so that the tie rods 4 'are then outside.

In both cases, so that the stacking of the circles is stable, it is preferable to interpose regularly crowns 5.

• il s'avère plus facile et moins onéreux de fabriquer des cercles qu'une spirale;
• il y a beaucoup moins de soudure à faire, puisqu'il n'y a qu'une soudure par cercle au lieu d'une soudure tout le long de la spirale;
• il est possible de faire aussi facilement des tamis centrifuges ou des tamis centripètes;
• la suppression des soudures évite les problèmes de déformation et de fragilisation locale dus à la soudure;
• cela permet une réduction très importante de la durée d'assemblage et rend possible le regroupement des opérations d'assemblage et de contrôle dimensionnel final du tamis.

This arrangement is particularly advantageous. Indeed :

• it is easier and less expensive to make circles than a spiral;
• there is much less welding to be done, since there is only one welding per circle instead of one welding all along the spiral;
• centrifugal sieves or centripetal sieves can also be made easily;
• the elimination of welds avoids the problems of deformation and local embrittlement due to the weld;
• this allows a very significant reduction in the assembly time and makes it possible to group together the assembly operations and final dimensional control of the sieve.

But above all it allows to obtain a stacking of U which be rigid while having a certain elasticity.

Indeed as already mentioned previously under the effect of the traction exerted by the tie rods 4 the U's will tend to deform elastically, the two branches of each U approaching slightly each other and the bottom of each bulging U slightly.

This elasticity allows to precisely apply a determined pre-tension, which is not possible with the stacking described in the prior patent WO90 / 12147 and we can therefore adapt each sieve to pressure variations it will undergo during operation.

This relative elasticity is a big advantage, but it has a disadvantage for large sieves diameters, i.e. for sieves whose diameter is greater than 1 meter.

In fact, there inevitably occurs sliding U-shaped circles on top of each other.

Such slippages cannot occur with elements of the prior patent WO90 / 12147 because these elements have notches which fit into grooves; but it is not possible to realize such notches on the walls of the U-shaped profiles.

According to the invention and as illustrated in the figure 8 we use the tie rods, not to tighten the U against each other but to constitute, in cooperation with two crowns, a framework rigid cylindrical on which the U are threaded the one after the other. Elastic tightening with pre-tension U is then done by means of a crown additional, called tightening crown which is screwed precisely on said rigid frame.

Referring to Figure 8 we see that the framework rigid cylindrical is produced by bars 14 which are, by means of screws 15 fixed securely to a upper crown 10 and lower crown 12. So the two crowns 10-12 and the bars 14 form a kind of solid cage. To improve the rigidities of the assembly, the ends 14a and 14b of the bars 14 are fitted in housing correspondents in crowns 10 and 12.

The U-shaped circles 11 are then threaded onto the cage thus formed. Obviously the dimensions of crowns 10-12 and bars 14 are determined from so as to correspond to the internal diameter of the circles 11 in U.

A crown 13 is then placed on the circle in a U which is the first of the stack and this crown 13 is tightened by a plurality of screws 17 on the upper crown 10. The lower face 13a of the crown 13 thus exerts a clamping pressure on stacking circles 11.

Preferably, there is between the lower face 13a of the tightening ring 13 and the first circle 11 one or more shims 16.

Preferably also there is between the crown of tightening 13 and the upper crown of the shims 18 under shape of washers.

Thanks to the shims 18 we can determine with precision the tightening stroke of the crown 13 on the crown 10 and therefore, given the thickness of the shims 16, the exact value of the pre-tension at which is subject to the stacking of the circles 11 in a U-shaped profile.

Obviously the invention is not limited to the mode as described in FIG. 8 which is not given as an example; any other mode of realization of a rigid structure on which the circles 11 in U profile are threaded entering the part of the present invention.

Claims (13)

1. Sieve with slots or holes, made from a U-shaped section, constituted by a stack of identical circles, each circle (1, 1') being made from a tapered U-shaped section of which the two ends are welded or mechanically connected to one another, characterized in that this stack of circles (1, 1') is maintained tightly between two rings (2-3, 2'-3') by a plurality of tie rods (4, 4') and in that the lateral walls of the circles in the stack rest on one another without any further fastening together, so that the walls of the sections can undergo elastic deformation under the effects of the tightening.
2. Sieve as claimed in claim 1, wherein at least one intermediary ring (5) is disposed between two circles in the stack.
3. Sieve as claimed in either claim 1 or claim 2, wherein the lateral walls of the "U" are turned inwards and the tie rods (4) are disposed on the inside, for centripetal operation.
4. Sieve as claimed in either claim 1 or claim 2, wherein the lateral walls or wings of the "U" are turned outwards and the tie rods (4') are disposed on the outside, for centrifugal operation.
5. Sieve as claimed in any one of the previous claims, characterized in that the tie rods (4) are threaded at both their ends and receive nuts (6) enabling sufficient tightening to maintain the circles against one another without play during operation.
6. Sieve as claimed in any one of the previous claims, characterized in that the tie rods (4) are threaded at both their ends, are screwed into a lower ring (2) and are fitted with a nut (6) at their upper end.
7. Sieve as claimed in claim 2, wherein rings (5) are inserted at regular intervals between the circles in the stack.
8. Sieve as claimed in claim 1, characterized in that it comprises a rigid cylindrical structure onto which the U-shaped circles (11) are slid one on top of the other.
9. Sieve as claimed in claim 8, wherein the rigid cylindrical structure is comprised of an upper ring (10) and a lower ring (12) connected rigidly together by a series of bars (14), the dimensions of the rings (10-12) and bars (14) being determined so as to correspond to the inside diameter of the circles (11).
10. Sieve as claimed in claim 9, wherein the stacking of the circles (11) onto the rigid structure (10, 12, 14) is obtained by a further nip ring (13) which rests, by means of its lower side (13a), on the top circle 11 and is tightened onto the rigid structure (10, 12, 14) by a plurality of fastening screws (17).
11. Sieve as claimed in claim 10, in which adjusting wedges (16) are interposed between the nip ring (13) and the first circle (11) made from a U-shaped section.
12. Sieve as claimed in claim 11, wherein the adjusting wedges (18) are interposed between the nip ring (13) and the upper ring (10) of the rigid structure.
13. Sieve as claimed in claim 12, wherein rigid circles are interposed between circles spread over the height so as to improve the rigidity of the assembly.

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