The present invention relates to a method and apparatus for treating pulp of pulp and paper industry. The method and apparatus in accordance with the present invention are especially applicable for treating fiber suspension to be fed to the headbox of a paper machine.

So called headbox screens form a special group of their own among the screeners of the pulp and paper industry by setting special requirements both to the operation of the screener and its construction. One of the most significant features having influence on the operation of the headbox screen and their construction is the requirement of the paper manufacturers to keep the fiber flow from the headbox of the paper machine to the wire as uniform as possible. In other words the paper manufacturers require that there should not be any pulses in the suspension flowing from the headbox to the wire and thus ensure an as homogeneous quality of the suspension as possible. The pulses might cause, for example, profile changes in the paper being manufactured. Both the headbox manufacturers and the manufacturers of the apparatuses preceding the headbox strive for fulfilling said requirement, since everywhere, where the fiber suspension flows, turbulence is generated and thus also pressure pulses, which may, if not controlled, multiply in the system.

It is generally known that pressure pulses are generated both when pumping and when screening pulp. It is also known that attempts have been made to damp pressure pulses in the headbox screen. Various methods have been used so far, but none of those in the market has been completely pulseless or effective in damping the pulses. One known method is shown, for example, in FI patent application 753214, in which the pulp being fed to the

headbox is axially brought into a large screener, having a screen cage and a rotatable rotor arranged therein. The pulp to be screened is fed into the screen cage, from which the accept flows through the openings of the screen cage to the large space between the casing of the screener and screen cage, the upper part of which is pressurized. The purpose is to damp the pressure pulses caused by both the pump and the rotor feeding pulp to the screen, due to both the large space and the pressure affecting the space through the free liquid surface. The screened pulp is discharged from the apparatus to the side thereof.

CH patent 564 638 discloses a similar arrangement, in which pulp is axially fed to the screener, but this time to the outside of the screen cage, from which the accept flows through the openings of the screen cage to the inside of the cage to the same space, where the rotor rotates. The screened pulp is discharged from the apparatus axially, whereby the purpose is, of course, that the pressure pulses at different stages damp each other. Yet another embodiment in the patent specification is an arrangement, in which a space separated with foils is arranged inside the screen cage, whereby it is possible to lead pressure medium to said space for damping the pulses.

Yet another embodiment close to the above mentioned arrangements is disclosed in US patent 5,009,774, which practically does not differ from said CH patent at all.

Also many other arrangements to solve the pulse problem have been presented, but none of them has been efficient and thus one of the most significant problems of the paper manufacture is still unsolved. A survey through the prior art arrangements has shown that in most cases a solution offered to the pulse problem is the use of one

single large space, from which the suspension is discharged to the inlet pipe leading to the headbox of a paper machine. It is believed that in a large space the pulses damp on their own and that a large space may have so many pulses of different size and at different stages that the pulses in practice damp almost seizing to exist. However, there is a risk of the pulses getting larger in an uncontrolled manner and to be even more difficult to control than the initial pulses.

The purpose of the present invention is to bring an efficient solution to the above described problem. The purpose is to damp the pressure pulses generated to the fiber suspension both by the headbox screen itself and by the pump preceding said screen.

According to the present invention the pulses are damped in such a way that at least a substantial portion of said accept flow is brought to hit a special damping member and to flow substantially in the first flow direction parallel to the surface of the screen cage, said portion of the accept flow is forced to turn to the second flow direction and the accept flow is discharged from the apparatus.

A characterizing feature of the apparatus in accordance with the present invention is that the accept space inside the screen cage is provided with a damping member at a distance from the screen cage and extending along at least a portion of the length of the screen cage.

An additional advantage of the apparatus in accordance with the present invention worth mentioning is the easy finishing in manufacturing a power screen which requires high smoothness of the surface, leading again to the fact that the surfaces of the screen are polished at least starting from the accept side of the screen cage. In the

00358

embodiment in accordance with the present invention all the parts inside the screen cage as well as the parts outside the screen cage are rotationally symmetric, whereby their polishing is simple and relatively inexpensive.

The present invention is described below more in detail, by way of example, with reference to the accompanying drawings, in which Fig. 1 illustrates an apparatus in accordance with a preferred embodiment of the present invention,

Fig. 2 illustrates an apparatus in accordance with a second embodiment of the invention,

Fig. 3 illustrates an apparatus in accordance with a third embodiment of the invention.

Fig. 4 illustrates an apparatus in accordance with a fourth embodiment of the invention.

Fig. 5 illustrates an apparatus in accordance with a fifth embodiment of the invention, and Fig. 6 illustrates an apparatus in accordance with a sixth preferred embodiment of the invention.

Fig. 1 illustrates a so called vertical version of the screener in accordance with the present invention, in other words an alternative, from which the accept is taken axially upwards. A screener 10 preferably comprises a cylindrical outer casing 12, conduits 14 and 16 therein for the pulp to be screened and the reject, respectively and a bottom end 18, which is provided with bearings 20 for a shaft 24 of the rotor 22 and an end cover 26, to which a screen cage 28 and a uniform, i.e. solid damping member 30 and a discharge conduit 32 for accept is attached. It is typical of the apparatus in accordance with the present invention that pulse members 34 of the rotor 22, most usually the so called foils, are preferably located on the side of the pulp flowing to the screen cage, in other words untreated suspension, and are

supported by a supporting ring 36, that the screen cage 28 comprises a closed bottom part 38, which in the embodiment of the drawing surrounds the shaft 24 and the bearings 20 of the rotor 22, and that the screener is of so called in-flow type, in other words the accept flows from the outside of the screen cage 28 to the inside thereof. It is also typical of the apparatus that the damping member 30 is arranged inside the screen cage close to the screen cage 28, said member extending along at least a portion of the axial length of the screen cage (in the drawing about to the middle of the axial length of the screen cage 28). The construction of the damping member is so solid that the pressure pulses of the suspension cannot make it to resonance. Also the damping member is preferably smooth and uniform on the surface, i.e. it is not perforated. The damping member 30 is attached from one end, i.e. axially from one edge either to the end cover 26 of the apparatus or like, possibly together with the screen cage 28. From the other end, i.e. from the edge opposite to the first edge the damping member 30 is attached to the discharge conduit 32 for accept extending to the inside of the apparatus as a preferably widening cone. The damping member may be of its construction bent from a plate, cast or otherwise appropriately produced. It is typical of the damping member 30 that it receives the flow from the openings of the screen cage 28 in such a way that the pulses of the flow reflect back from the wall of member 30 and damp the thus oncoming pulses and that at least a portion of the accept flow substantially turns to the first flow direction parallel to the surface of the screen cage, which in the embodiment of the drawing is downwards. It is also substantial to try to maintain the inner volume of the screen relatively low so that a large amount of suspension cannot generate strong pulses. One preferable alternative worth mentioning, which is shown in the drawing is to arrange the damping member 30 as a conical

surface tapering towards the first flow direction so that the surface area between the member 30 and the screen cylinder 28 homogeneously increases to the first flow direction of the suspension, whereby it is possible to maintain the flow velocity constant.

The apparatus in accordance with the present invention operates in such a way that the fiber suspension to be screened is supplied from conduit 14 to the space between the casing 12 of the screener and the screen cage 28. There the suspension is brought into a rotational movement by means of the foils 34 of the rotor 22. Due to a pressure difference a portion of the suspension is pressed through the screen cage 28 practically speaking radially to a so called accept space. Said accept is also affected by the pressure pulses generated by the pump preceding the screening, but also by the rotor 22 of the screener 10 mainly close to the conduit 14, which in practice cause temporary fluctuation in the flow velocity through the openings of the screen cage 28. A damping member 30 is arranged inside the screen cage 28 at a distance thereof to receive the accept flow, in other words the accept flow hits the damping member, from which the pressure pulses reflect back causing the mixing of the pulses and their damping. At the same time the accept flow is forced to turn to the first flow direction, which in the embodiment of Fig. 1 is downwards along the wall of the damping member 30 and/or screen cage 28. When the flow reaches the other, free edge of the damping member 30 the flow is forced to turn due to the bottom of the screen cage to the second, practically speaking opposite, direction to the center of the apparatus 10, whereby all separate accept flows join to one uniform axial flow, which is practically speaking pulseless. However, it is possible to affect the damping of the pressure pulses by the design of the bottom part 38 of the screen cage 28 by letting said pulses to reflect to many different

directions preferably from the rotationally symmetric surfaces, whereby pulses with different directions effectively damp each other.

Fig. 2 illustrates an apparatus corresponding to the embodiment of Fig. 1 except that the position of the inlet conduit 14' of the apparatus and the discharge conduit 16' for the reject has slightly been changed and that the damping member 30' extends throughout the whole effective length of the screen cylinder 28.

The operational difference between the above described embodiments worth mentioning is, for example, the fact that in the embodiment of Fig. 1 the majority of the accept comes from the upper half of the screen cylinder. A characterizing feature of the apparatus in accordance with the present invention is that the damping member 30 extends along a significant portion of the length of the screen cylinder 28 seen from the conduit 14 for the pulp entering the apparatus. Thus a characterizing feature of the embodiment in accordance with Fig. 2 is that the damping member 30' extends along the whole effective length of the screen cylinder 28.

Fig. 3 illustrates yet another embodiment, in which the damping member 30'' is cylindrical being located throughout its whole length to a certain constant distance from the screen cylinder 28. In the embodiment of the drawing the damping member is also shown as a cast. If the cross-sectional flow area is desired to be increasing as in the previous drawings it is possible to arrange the screen cylinder conical in such a way that the flow area in the space between the screen cylinder and the damping member grows as in the other embodiments to the first flow direction for the accept.

Fig. 4 illustrates a screen arrangement corresponding to Figs. 1 and 2 except that the accept in this embodiment is not axially discharged, but through a tangential or even radial conduit 40. The idea is, however, that the accept is discharged from the actual screening zone, in other words from the zone axially defined by the screen cage, from the axial conduit 32 through the intermediary cover 26' of the apparatus, whereafter the direction of the accept flow may freely be changed without a risk of the pulses of the rotor in the apparatus affecting the accept. The apparatus in the embodiment of the drawing is provided with a discharge space 42 for the accept above the intermediary cover 26' and the discharge space 42 is preferably provided with a tangential discharge conduit 40. It may also be noted that in the embodiment of Fig. 4 the damping member is shown as solid, for example, as a casting, which ensures that the damping member will not vibrate even in the hardest conditions with the pulses of the fiber suspension.

Fig. 5 illustrates yet another construction embodiment typical of the vertical alternative of the screener in accordance with the invention, in other words a method of joining the screener 10 to an approach pipe 100. Said joining comes into question in the installation of the apparatus, but also always when the screener must be opened for some reason, for example, when the screen plate is worn out or is clogged. In such cases in the conventional screeners with a tangential discharge, the end cover of the apparatus is opened, whereby it is possible to directly reach the screen cylinder. In some embodiments of the apparatus in accordance with the invention (Figs. 1, 2, 3, 5 and 6) the screen cage 28 is attached in such a way that it is impossible to replace it without touching the approach pipe 100 of the paper machine. In this case it has been proven most convenient to arrange the joint between the approach pipe 100 and

the accept conduit 32' telescopic, whereby it is possible to open the end cover 26 of the apparatus, lift the cover with the conduit 32' and screen cage 28 (if the cage is attached to the cover 26) or without them so that the loosening of the screen cage and the replacement may be done successfully. The accept conduit 32' is preferably arranged to correspond of its outer diameter to the inner diameter of the approach pipe 100. It is, of course, also possible to arrange the attachment of the accept conduit in such a way that a portion of the piping, which is a little longer than the screen cage is attached from its flanges to the approach pipe and the end cover, whereby by loosening said portion from the flanges and removing it first, it is possible to open the end cover of the screener, remove it and replace the screen cage. The embodiment in accordance with the drawing is especially advantageous, because the direct approach pipe of the paper machine is significantly less expensive than the conventional curved pipe to produce and polish.

Fig. 6 illustrates a similar screen arrangement, but now a so called horizontal version 50, in which the discharge of the accept is carried out following the horizontal shaft of the apparatus. All embodiments described above in connection with the vertical version may be applied as such also to the horizontal version, so there is no need to go into them in detail at this point.

As is obvious from the above description, a completely new kind of method and apparatus for treating pulp eliminating the disadvantages of the prior art has been developed. The above description only exemplifies the invention with a number of variations from the invention with reference to a number of drawings, although in some of the drawings also details of the invention are shown. Therefore it must be born in mind that the invention contains such variations, which may be combined from the

details of different drawings, although it is not directly said either in the description of the invention or in the drawings. Thus the invention is limited merely by the enclosed claims, which alone define the scope of invention.