GAP FORMER IN A PAPER MACHINE

Abstract: The invention concerns a gap former in a paper machine, comprising a twin-wire zone, which is composed of a first forming-wire loop (10) and of a second forming-wire loop (20) and which starts from the forming gap (G), into which the slice part (31) of the headbox (30) feeds a pulp suspension jet. The forming gap (G) is defined, from one side, by the first forming wire (10), which runs over the forming roll (11), and, from the opposite side, by the substantially straight run of the second forming wire (20) starting from the breast roll (21) to the forming roll (11). Draining equipment (40...44) is fitted inside the first forming-wire loop (10) after the forming gap (G) and the forming roll (11), which draining equipment is provided with a deflector duct (12) and with a deflector rib (12a) placed against the first forming wire (10) in the lower part of said deflector duct (12), and, inside the second wire loop (20), a forming shoe is arranged, which is provided with a ribbed deck consisting of forming ribs (22) and which is loaded against the second forming wire (20). In the invention, the first forming rib of the forming shoe or a unified forming-rib construction (22) composed of two or more forming ribs is mounted so that it can be loaded adjustably, so that the free run of the forming wires (10, 20) from the forming roll (11) to the first forming rib is substantially little.

Description: Gap former in a paper machine The invention concerns a gap former in a paper machine, comprising a twin-wire zone, which is composed of a first forming- wire loop and of a second forming- wire loop and which starts from the forming gap, into which the slice part of the headbox feeds a pulp suspension jet, and which forming gap is defined, from one side, by the first forming wire, which runs over the forming roll, and, from the opposite side, by the substantially straight run of the second forming wire starting from the breast roll to the forming roll, draining equipment being fitted inside the first forming- wire loop after the forming gap and the forming roll, which draining equipment is provided with a deflector duct and with a deflector rib placed against the first forming wire in the lower part of said deflector duct, and, inside the second wire loop, a forming shoe is arranged, which is provided with a ribbed deck consisting of forming ribs and which is loaded against the second forming wire. With increased running speeds of paper machines, several problems in the web formation have been manifested with increased emphasis. In the former section of a paper machine, the phenomena that act upon the fibre mesh and upon the water, which is still relatively free in connection with the fibre mesh, such as centrifugal forces, are, as a rule, increased in proportion to the second power of the web speed. The highest web speeds of some existing paper machines are already of an order of 1500 metres per minute. However, machines, in particular newsprint machines, are being planned in which a web speed of up to about 2000 m/min or even higher web speeds are aimed at. With respect to the prior art related to the present invention, reference is also made, e.g. , to the applicant's FI Patent Application No. 885609 and to the FI Patent Applications Nos. 885606 and 885607 of Messrs. Valmet Ahlstrom Inc. , in which formers marketed by the applicant under the trade mark "MB-former" are described. Further, reference is made to the applicant's FI Patent Applications Nos. 904489, 911281, 913112, 920228, 920863, 932264, and 932365, in which various combina¬ tion assemblies of the above MB-former units and of hybrid and gap formers are described. In the roll-gap formers mentioned above, the pulp suspension is passed from the headbox into a gap formed by a forming roll and by two wires, after which gap the two wires and the pulp layer placed between them follow the curve form of the forming roll, most of the water being drained through the wires. The necessary dewatering pressure is produced by means of the tension of the outer wire, which tension produces a pressure in the pulp layer, which pressure is proportional to the wire tension and inversely proportional to the curve radius of the forming roll. In the gap, part of the kinetic energy of the pulp jet is converted to pressure energy, and the jet speed is lowered accordingly. The dewatering through the outer wire away from the forming roll is intensified by the centrifugal force. Dewatering towards the forming roll can be intensified by means of negative pressure in a zone of the forming roll. With increasing running speeds of paper machines, the amounts of water to be drained per unit of time in the twin-wire zone have also become larger. This has caused difficulties in how to accommodate draining chambers of sufficiently large section inside the upper-wire loop, for which reason, in order to prevent flooding, it has been necessary to provide the draining chambers with drain ducts passing both to the driving side and to the tending side of the machine. The ducts at the tending side make the other operations more difficult, and they are also problematic because they must be passed across the paper machine back to the driving side, where the drain ducts are connected to the wire pit. The problems described above are, for its part, caused or made worse by the fact that, as is known from the prior art, between the former roll, which is placed at the beginning of the twin-wire zone and which guides, for example, the upper wire, and the suction-deflector duct placed in the forward part of the set of draining chambers, there has been a considerable intermediate space, in which a drain trough has been placed so as to collect the water drained in connection with said former roll and to pass said water to the side of the paper machine. With increasing speeds of paper machines, attempts have also been made to avoid, or at least to reduce, the free draws of the web and/or the wires in order to improve the stability of the web and the wires. However, in the current gap former solutions, between the former roll and the subsequent ribbed shoe, as a rule, there has be quite a considerable free distance, in which case, with increasing running speeds, it has not been possible to keep the stability of the wires good in this area. The current solutions have not permitted a shortening of said distance. Since, in the prior-art solutions, the ribbed shoes following after the former roll have been installed on the former frame stationarily, it has been necessary to keep the thickness of the layer of pulp suspension fed out of the headbox into the former gap as unchanged and invariable as possible. Thus, it has not been possible to regulate the thickness of the layer in any way. Such a stationary rib is, however, very well suitable for use when the conditions in the web formation remain unchanged. The stability of the wires and the runnability of the former in different running situations have, however been found to be a problem in the current constructions, and the present invention aims in particular at solving this problem. In view of avoiding the problems described above and in view of achieving the objectives of the invention stated above and those that will come out later, the present invention is mainly characterized in that the first forming rib of the forming shoe or a unified forming-rib construction composed of two or more forming ribs is mounted so that it can be loaded adjustably, so that the free run of the forming wires from the forming roll to the first forming rib is substantially little. By means of the present invention, a number of remarkable advantages are obtained over the prior art, and, of these advantages, for example, the following can be stated. The principal idea of the invention, i.e. that the first or the first two forming ribs are mounted on the former resiliently, provides the significant advantage that said rib can be brought very close to the former roll, even so that the rib can be placed facing the former roll, in which case there is no free distance between the former roll and the rib. This improves the stability of the wires and the runnability of the former in different running situations to a substantial extent. Besides the fact that said rib is mounted resiliently, it can also be mounted adjustably, in which case such an adjustable rib permits running with a number of grammages, thicknesses, etc. When the rib is mounted resiliently in accordance with the present invention, the rib is placed automatically in the correct position when the layer thickness is changed. The solution in accordance with the present invention is particularly usable for different grades of fine paper. Further, it can be stated as a remarkable addi- tional advantage that, in the invention, the deflector rib of the MB-unit can be placed facing the resilient ribbed shoe to improve the draining. In such a case, the deflector rib does not have to be loaded or adjustable, but it may be stationary. The further advantages and characteristic features of the invention come out from the following detailed description of the invention. In the following, the invention will be described in detail with reference to some exemplifying embodiments of the invention illustrated in the figures in the accom¬ panying drawing, the invention being by no means strictly confined to the details of said embodiments. Figure 1 is a schematic side view of a gap former in accordance with the invention. Figure 2 shows an enlarged detail of the gap former as shown in Fig. 1 from the area of the former roll, the resilient forming rib, and the deflector rib. Figure 3 is a schematic sectional view of the installation of the resilient former rib in relation to the former roll. The former shown in Fig. 1 is a so-called gap former, in which the slice part 31 of the headbox 30 feeds a pulp suspension jet into the forming gap G between the forming wires 10,20. The geometry of said forming gap G is determined from above by the forming roll 11 and from below by the breast roll 21 , which guides the second forming wire 20, i.e. , in the illustration in Fig. 1 , the lower wire. Inside the upper-wire loop, i.e. the first forming-wire loop 10, the forming roll 11 is followed by the MB-unit 40. For fine adjustment of the gap geometry of the forming gap G, the breast roll 21 has been arranged so that its position can be regulated in the direction of the arrow E, i.e. , in the exemplifying embodiment shown in Fig. 1 , substantially in the vertical direction. The geometry of the forming gap G is prefer¬ ably such that, on the forming roll 11 , the forming wires 10,20 have a common curve sector a. The first forming wire 10 has been formed into a wire loop by means of the forming roll 11 and the guide rolls 14, and, as was already stated above, inside said first forming- wire loop 10, the MB-unit 40 is fitted. In the exemplifying embodiment of Fig. 1 , the MB-unit 40 comprises two draining chambers 41,42, in which a vacuum has been arranged to be effective. Each draining chamber 41,42 is provided with a drain duct 43,44, through which the water that has been passed into the draining chamber 41,42 with the aid of said vacuum is drained away from the draining chamber. Into each of the draining chambers 41 ,42, the water enters along the deflector duct 12, 13. As is shown in connection with the first deflector duct 12, at the lower end of said deflector duct 12, there is a deflector rib 12a placed against the first forming wire 10. Thus, the deflector rib 12a defines said deflector duct 12, and the water, which is guided by the deflector rib 12a, is drained through the first forming wire 10 along the first deflector duct 12 into the first draining chamber 41. In a corresponding way, the second forming wire 20 has been formed into a wire loop by means of the breast roll 21 and the guide rolls 21a. Inside the second wire loop 20, after the forming gap G and the common curve sector of the wires, a first forming rib 22 of a forming shoe is arranged, which is placed against the second forming wire 20 and which is followed by a number of forming ribs 23, which are placed at a distance from one another and which have been arranged as a set or ribs or as a ribbed deck. The set of ribs or the equivalent ribbed deck has been integrated as a common box 24, which can be subjected to a vacuum if necessary, by means of which vacuum the draining through the second forming wire 20 is intensified. As is shown in Fig. 1 , the first set of ribs or the equivalent ribbed deck is substantially straight or slightly curved. After the first set of ribs 23, there is a second set or ribs 25 or an equivalent ribbed deck, which is fitted inside the second forming- wire loop 20 and which has been integrated similarly as a box 26. According to the illustration of Fig. 1 , the open ribbed deck of the second set of ribs 25 has been formed as curved so that, owing to this curve form, a compression is applied to the stock web running between the wires 10,20. whereby this compres¬ sion, together with the centrifugal force produced by the curve form of the ribbed deck 25 , produces draining of water out of the stock web through the first forming wire 10. This water that has been drained out of the stock is passed along the second deflector duct 13 into the second draining chamber 42, being aided by a vacuum that is preferably effective in said chamber 42. The draining is intensified and promoted further by the deflector rib (not shown) placed in the lower part of the second deflector duct 13. Further, inside the second forming-wire loop 20, a number of suction boxes 27 have been arranged, which are provided with a ribbed deck and by whose means draining of water out of the stock web through the second forming wire 20 is produced, and by means of which suction boxes 27 it is achieved at the same time that the stock web follows along with the second forming wire 20 as the runs of the wires 10,20 are separated from one another. After the suction boxes 27, there follows the second forming roll 28, which is provided with a suction zone 29 for further removal of water out of the stock web and so as to guarantee that the stock web remains on the second forming wire 20. After this, it is shown that, inside the second forming-wire loop 20, a further suction box 27a is fitted, after which the run of the second forming-wire loop 20 and the formed paper web W placed on said wire are passed to the detaching or pick-up point (not shown), from which the paper web W is taken to the press section. 7 Fig. 2 shows an enlarged detail of the former as shown in Fig. 1 from the area of the forming gap G and of the first forming rib or the equivalent rib construction. According to Fig. 2, the first forming-rib construction 22 comprises two successive forming ribs 22a, 22b, which are interconnected by means of a fastening member 22c or equivalent. This forming-rib construction 22 is supported on the frame 22d of the former or the box 24 by means of adjustable loading members, preferably loading hoses 22e, so that, by feeding a pressure medium into these loading hoses 22e in the desired way, the rib construction 22 can be made to be positioned in the desired position in relation to the wires 10,20 and to the forming roll 11. When compressed air is used as the pressure medium, the installation of the forming-rib construction 22 is resilient, in which case the ribs 22a, 22b are placed automatically in the correct position if, for example, the thickness of the pulp suspension layer fed into the forming gap G is changed. The pressure of the pressure medium in the loading hoses 22e can be regulated even during running. Besides the possibility of regulating the pressure of the ribs against the second forming wire 20 by means of the loading hoses 22e, by means of said loading hoses 22e it is also possible to regulate the angle of the ribs 22a,22b in relation to the forming wire 20. It is of even greater significance, than just a resilient mode of installation of the ribs 22a, 22b or possibility of regulation during running, that owing to the construction in accordance with the present invention the first forming ribs 22a, 22b can be brought substantially closer to the forming roll 11 than in the prior art, even so that there is no free distance between the forming roll 11 and the first forming rib 22a. Attempts have been made to illustrate this in particular in Fig. 3, in which, in constructions of conventional technology (denoted with dashed-dotted lines), between the forming roll 11 ' and the first forming rib 22a, there is always a free distance a, which has been generally of an order of 200 mm or more. As is shown further in Fig. 3, the forming rib 22a in accordance with the invention can be placed fully facing the forming roll 11, in which case there is no such free distance a at all. The elimination of this free distance improves the stability of the wires 10,20 and the runnability of the former in different running situations essentially. In the present invention, the free run from the forming roll 11 to the first forming rib 22a is preferably in the range of 0...70 mm. Such an adjustable, either resilient or non- resilient installation is particularly favourable when grades of fine paper are run, in which case the common curve sector of the wires 10,20 on the forming roll 11 is very little and the streaks occurring in the fibre stock are more common than in cases in which a larger curve sector a is used. Such a rib 22a, 22b or equivalent rib construction 22 whose position can be adjusted can be pulled apart from the forming wire 20 when the machine is being started or run down. Also otherwise, if necessary, the rib can be pulled apart from the wire 20. In the construction, for these adjustable ribs, there may be fixed stops, against which the rib is loaded for the time of running, so that during running the position of the rib is fixed. In such a case, it is possible to use a hydraulic pressure medium as the pressure medium, and the supply of the pressure into the loading hoses 22e can also be carried out manually. Above, the invention has been described by way of example with reference to the figures in the accompanying drawing. The invention is, however, not confined to the embodiments illustrated in the figures alone, but the invention can be modified within the scope of the inventive idea defined in the accompanying patent claims. Thus, even if the exemplifying embodiments shown in the illustrations in the drawing are concerned with a so-called "horizontal former", the invention is equally well suitable for use in vertical gap formers and in equivalent constructions.

Claims: Claims 1. A gap former in a paper machine, comprising a twin- wire zone, which is com¬ posed of a first forming- wire loop (10) and of a second forming-wire loop (20) and which starts from the forming gap (G), into which the slice part (31) of the headbox (30) feeds a pulp suspension jet, and which forming gap (G) is defined, from one side, by the first forming wire (10), which runs over the forming roll (11), and, from the opposite side, by the substantially straight run of the second forming wire (20) starting from the breast roll (21) to the forming roll (11), draining equipment (40...44) being fitted inside the first forming-wire loop (10) after the forming gap (G) and the forming roll (11), which draining equipment is provided with a deflector duct (12) and with a deflector rib (12a) placed against the first forming wire (10) in the lower part of said deflector duct (12), and, inside the second wire loop (20), a forming shoe is arranged, which is provided with a ribbed deck consisting of forming ribs (22,22a,22b) and which is loaded against the second forming wire (20), characterized in that the first forming rib (22a) of the forming shoe or a unified forming-rib construction (22) composed of two or more forming ribs (22a, 22b) is mounted so that it can be loaded adjustably, so that the free run of the forming wires (10,20) from the forming roll (11) to the first forming rib (22a) is substantially little. 2. A gap former as claimed in claim 1, characterized in that the first forming rib (22a) or the equivalent forming-rib construction (22) has been mounted so that it can be loaded adjustably, so that said forming rib (22a) or forming-rib construction (22) can be pulled apart from the second forming wire (20). 3. A gap former as claimed in claim 1 or 2, characterized in that the first forming rib (22a) is mounted, in the running direction of the forming wires (10,20), substan¬ tially at the point at which the forming wires (10,20) depart from the forming roll (11) at the end of their common curve sector (α). 4. A gap former as claimed in claim 1 or 2, characterized in that the first forming rib (22a) or the equivalent forming-rib construction (22) is mounted substantially facing the deflector rib (12a), which is placed directly after the forming roll (11). 5. A gap former as claimed in any of the preceding claims, characterized in that the first forming rib (22a) or the equivalent forming-rib construction (22) is loaded against the second forming wire (20) by means of pressure-medium operated loading members, such as loading hoses (22e) or equivalent. 6. A gap former as claimed in claim 5. characterized in that the first forming rib (22a) or the equivalent forming-rib construction (22) can be loaded adjustably by means of a hydraulic pressure medium at least to a running position and to a free position. 7. A gap former as claimed in claim 5 or 6, characterized in that the first forming rib (22a) or the equivalent forming-rib construction (22) can be loaded by means of a hydraulic pressure medium to the desired position between the running position and the free position. 8. A gap former as claimed in claim 5, characterized in that the first forming rib (22a) or the equivalent forming-rib construction (22) is loaded resiliently by means of a pneumatic pressure medium. 9. A gap former as claimed in claim 8, characterized in that the first forming rib (22a) or the equivalent forming-rib construction (22) is loaded pneumatically so that said forming rib (22a) or the equivalent forming-rib construction (22) is fitted to be positioned automatically in accordance with the thickness of the layer of fibrous pulp. 10. A gap former as claimed in any of the preceding claims, characterized in that the run of the forming wires (10,20) from the forming roll (11) to the first forming rib (22a) is substantially nil. 11. A gap former as claimed in any of the claims 1 to 9, characterized in that the run of the forming wires (10,20) from the forming roll (11) to the first forming rib (22a) is in the range of 0...70 mm.

Inventors: EVAESOJA PEKKA, POIKOLAINEN ANTTI, EVAESOJA, PEKKA, POIKOLAINEN, ANTTI
Applicants: VALMET CORP, VALMET CORPORATION
Application year: 1995
Main classification: D21F1
IPC codes: D21F1, D21F9