Sealing arrangement for pulp dewatering arrangement

Abstract: Arrangement 1 for washing and dewatering a fibre pulp suspension, which arrangement comprises two hollow, circular-cylindrical screen members 2a, 2b delimited by envelope surface and end walls. The screen members rotate towards each other to form a nip 10, at least one of the said screen members 2a, 2b being arranged in a vat 6 which partially encloses the envelope of the screen member and which, in the direction of rotation of the screen member, converges towards the envelope of the screen member. The invention relates to a sealing arrangement 8 at the end wall of the screen member. By retracting the seal a distance X from the envelope surface of the screen member in a limited area a near the nip 10, the seal is relieved of the very high hydraulic and mechanical pressures which occur in the nip. Wear between the end wall of the drum and the seal at the press nip is reduced. A simple pressurized seal can thus be used with the same low pressure applied along the entire circumference.

Description: The present invention relates to a sealing arrangement according to the pre-characterizing clause of Claim 1. PRIOR ART When producing paper pulp from cellulose-containing fibre material, it is necessary to wash and dewater the paper pulp at a number of stages in the process. A previously known and commonly used arrangement for washing and dewatering paper pulp, called a wash press, is disclosed in SE-C-380,300, SE-C-501, 710, US 5,488,900 and SE-C-504, 011. The arrangements disclosed in these documents comprise two cylindrical, rotatable screen members arranged in an essentially converging vat. Other examples of known arrangements are disclosed in US 4,543,161 and in US 5, 667, 642, the latter constituting an arrangement in which the screen members rotate in the opposite direction to the conventional one, that is to say, as seen from the short end, the right-hand screen member rotates counter-clockwise and the left one rotates clockwise. A problem encountered in washing and dewatering with wash presses of the abovementioned type is that a very high local pressure is built up at the end of the screen drum, which acts on the sealing arrangement at the end wall. US-A-4,543,161 discloses two alternatives for handling the high pressures in the nip. In a first variant, shown in Figures 3 - 4, the ends of the drums are given a slightly smaller dimension, for which reason the pulp dewatered in the nip can be pressed out into a collection shaft which is formed in the area of reduced diameter of the drum. In this way, the high pressure at the end wall of the screen drum is reduced. A second variant, shown in Figures 5 - 7, is a variant with low-friction shields which bear against the end walls of the drum in the area of the nip and prevent the pulp from being pressed out to the sides in the nip. This type of seal becomes greatly worn because the very high local hydraulic and mechanical pressures against the ends of the drum in the smallest cross section of the nip are maintained. These solutions reveal two extremes of the sealing problems, one solution resulting in a lower dry substance content of the wash press, and the other solution still resulting in very high pressures at the end wall of the screen drum. OBJECT OF THE INVENTION An object of the present invention is to permit an improved sealing of the end walls of the wash press by means of controlled pressure relief at the end walls of the screen drum. A further object is to make available a wash press with improved sealing of the end walls, which sealing allows fibre residues, which may be pressed into the pressure relief zone, to be continuously returned to the level of the envelope surface downstream of the nip. In this way, it is possible to avoid successive build-up of an increasingly hard dewatered fibre plug. The wash press can in this way be operated for longer periods of time without unnecessary shutdown for cleaning, or without the need for complicated cleaning arrangements. Yet another object is to make available a wash press with improved sealing of the end walls, which sealing consists of a sealing profile, preferably with a pressurized seal, which affords a cost-effective sealing construction. Yet another object of a preferred embodiment with pressurized sealing is that the pressure can be kept at a constant level in the whole sealing strip, while at the same time achieving an optimum sealing capacity around the entire circumference of the end wall. The pressure relief in the area of the nip means that one and the same pressure can be applied in the seal and still be sufficient in the area of the nip where the press forces on the pulp web are extremely high. At the same time, a contiguous sealing element can be used with just one pressurizing chamber. The seal does not therefore need to be divided into parts, pressurized at different levels, and this reduces the costs of the seal and of the pressurizing system for the sealing strip. BRIEF DESCRIPTION OF THE FIGURES The invention will be described below with reference to the figures, of which: Fig. 1 is a diagrammatic view of a wash press; Fig. 2 shows a cross section of the wash press, as seen from below in Figure 1; Fig. 3 shows an overview of a sealing arrangement according to the invention, arranged on an end wall of a drum in a wash press; Fig. 4 shows the sealing arrangement according to the invention, seen in cross section B-B from Figure 3; and Fig. 5 shows the sealing arrangement according to the invention, as seen in cross section A-A from Figure 3. DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS The type of wash press in which the sealing arrangement according to the invention is used is shown in Figures 1 and 2 and comprises two hollow, circular-cylindrical screen members 2a, 2b which include a number of evacuation chambers (not shown) under the envelope surface of the screen member for the purpose of leading off evacuated liquid. The pulp suspension WP is fed into the inlet boxes 3a and 3b and transported, under rotation of the screen members 2a, 2b, into a converging gap between a vat 6 and the surface of the screen members 2a, 2b, and onwards to a nip 10 between the screen members. The dewatered pulp DP is then withdrawn upwards after it has passed the nip 10. The two screen members form a press nip 10 between each other and are arranged to rotate towards each other, so that, as seen from the short end (Figure 2), the right-hand screen member rotates clockwise and the left-hand screen member rotates counter-clockwise. The wash press 1 preferably has a diameter of 1.0 - 2.5 metres. Its envelope 3 is also perforated in order to allow liquid to be evacuated from a web of fibre pulp lying against the envelope surface. In the preferred embodiment shown in Figures 1 and 2, a pulp inlet box 3a, 3b is arranged on each drum 2a, 2b, respectively. Each pulp inlet box 3a, 3b is arranged at 0° on the wash press, where 0° represents the highest/uppermost point of the wash press and the degrees increase positively in the direction of rotation of the screen member. Incoming paper pulp, which normally has a concentration of about 1 - 12%, preferably 3 - 10%, is distributed uniformly across the length of the screen member by means of the inlet box. The screen members are arranged to rotate at a speed of 5 - 20 rpm by means of a suitable drive device. Paper pulp thus follows the rotation of the screen members in the gap between the perforated envelope surface 2a, 2b and the walls of the vat 6, where it forms a web of fibre pulp which is dewatered by virtue of the fact that the gap converges in the direction towards the nip. The liquid which is pressed out from the web of fibre pulp is led off (not shown) from the arrangement. At wash zones, where the gap can diverge slightly, wash liquid can be introduced into the web of fibre pulp, and the latter is washed. Finally, the web of fibre pulp is dewatered, by means of the pressure in the nip 10, to a concentration which is about 5 - 20 times higher than the concentration of the incoming paper pulp, for example 1 - 12% on admission and 25 - 40% downstream of the nip. The web of fibre pulp is torn off from the envelope 2a, 2b and led off from the arrangement by means of suitable shredders/conveyor screws. According to the invention as shown in Figure 3, a sealing arrangement comprises a seal 8 which seals between the end wall 5a, 5b of the screen member and a housing 13 surrounding the screen member, in the area of the end wall 5a, 5b. The seal 8 is arranged at a first distance from the circumferential plane of a screen member along at least a predetermined part of the periphery of the screen member, but excluding the area at the nip 10, and the sealing element is arranged at a second distance X from the circumferential plane of the screen member near the nip 10, the second distance being greater than the first distance. The distance X from the plane of the envelope surface in the area of the nip corresponds to a measurement in the range of 10 - 1000 millimetres, preferably 20 - 60 millimetres, more preferably 25 millimetres. This retracted position of the seal from the plane of the envelope surface results in a pressure relief in the nip in the vicinity of the seal, which greatly reduces the load on the seal. The first distance at which the seal is arranged from the plane of the envelope surface corresponds essentially to a 0 distance, meaning that the sealing arrangement touches the plane of the envelope surface. In this way, it is possible to achieve complete sealing against those areas of the pulp web where the pressures are not as high as in the press nip 10. This prevents fibre becoming packed in the gap between the end wall of the drum and the seal holder. The gap is necessary for manufacturing tolerances and heat expansion. The sealing arrangement is retracted towards the centre of rotation of the screen member, in the area of the nip, across an angle range in the range of 15 - 60 degrees, preferably 25 - 50 degrees, more preferably 25 - 35 degrees. In the preferred embodiment, the seal thus lies in such a way that it touches the plane of the envelope surface over the remaining parts of the turn. The first distance for the seal, resulting in the seal touching the plane of the envelope surface, is formed over an angle in the range of 300 - 345 degrees, preferably 310 - 335 degrees, more preferably 325 - 335 degrees. To achieve a good transition between the retracted position of the seal and the position in which it touches the plane of the envelope surface, it is advantageous for the seal to converge seamlessly to the first distance in the angle range by which the seal is retracted relative to the envelope surface of the screen member. This provides a symmetrical retraction of the seal 8 around the press nip 10, i.e. identical upstream and downstream of the press nip 10. The preferred seal is a pressurized seal corresponding to that shown in Figures 4 and 5. The seal comprises a pressurizing chamber arranged in the housing 13, which pressurizing chamber acts on the sealing surfaces 8 of the seal against the end walls 5a, 5b of the drum. Exchangeable wear rings 14 are preferably arranged on the end walls. The seal also preferably consists of a sealing element 8 which is contiguous in the circumferential direction, with identical sealing profile, which sealing element is arranged in a groove in a seal holder 9 arranged on the housing 13. The seal 8 can in turn be provided with sealing rings 12 which prevent impurities from penetrating into the pressure chamber and, if appropriate, permit pressurizing directly in the groove for the seal 8. The pressurizing chamber 11 of the seal preferably consists of a pressurizing chamber 11 which is contiguous in the circumferential direction, which pressurizing chamber 11 is arranged in the bottom of the groove in the seal holder 9. The division of pressure chamber and seal part is not absolutely necessary, and the pressurizing chamber 11 may preferably be made integral with the sealing part of the seal 8 in the same way as with "CEFILAIR®" pneumatic seals. The arrangement according to the invention is not limited to the embodiments described above, and instead it can be varied within the scope of the attached patent claims. For example, the seal can be retracted asymmetrically around the press nip 10. It can be retracted starting immediately in front of the press nip and it can more quickly reach the maximum retracted distance, which distance is maintained for a length after the press nip. The so-called "0 distance" between the seal and the plane of the envelope surface, excluding the area around the press nip 10, can of course be given a minimum drawing-in corresponding to a fraction of the drawing-in at the press nip 10. The essential feature of the inventive concept is that pressure relief in the press nip is permitted by the fact that the pressure can be temporarily led off across the end surface of the drum. Alternatively, a local drain or pressure outlet can be arranged upstream or downstream of the press nip, radially outwards from the retracted seal. An extra drain in the holder 9 can facilitate evacuation of liquid which is pressed out at the press nip. An outlet for compressed air directly downstream of the press nip can prevent the build-up of fibre plugs. A number of wash zones, where liquid is also added to remove the liquid in the fibre web, can be introduced between the inlet boxes and the press nip 10. In certain applications, the wash zones can also be completely omitted, in which case only pressing of the pulp web is carried out.

Claims: Arrangement (1) for washing and dewatering a fibre pulp suspension, which arrangement comprises two circular-cylindrical screen members (2a, 2b), each with an envelope surface and end walls, which circular-cylindrical screen members are arranged to rotate towards each other to form a nip (10), at least one of the said screen members being hollow to permit evacuation of liquid radially inwards in the screen member, at least the hollow screen member (2a, 2b) being arranged in a vat (6) which partially encloses the envelope of the screen member and which vat, in the direction of rotation of the screen member, converges towards the envelope of the screen member, and where at least one pulp inlet box (3a, 3b) is arranged adjacent to the screen member for introducing pulp between the envelope of the screen member (2a, 2b) and its vat (6) in order to form a web of fibre pulp on the screen member, wherein the arrangement comprises a sealing arrangement having a seal (8) which seals between the end wall (5a, 5b) of the screen member and a housing (13) surrounding the screen member in the area of the end wall, where the seal (8) is arranged at a first distance from the circumferential envelope plane of the screen member along at least a predetermined part of the periphery of the screen member, but excluding the area adjacent to the nip (10), and wherein the sealing element is arranged at a second distance (X) from the circumferential envelope plane of the screen member in theareaofthenip (10), characterized in that the seal is retracted from the circumferential envelope plane of the screen member towards the centre of rotation of the screen member, in the area of the nip such that the second distance (X) being greater than the first distance. Arrangement according to Claim 1, characterized in that the sealing arrangement is arranged at the distance X from the plane of the envelope surface in the area of the nip, and in that the distance X corresponds to a measure in the range of 10 - 1000 millimetres, preferably 20 - 60 millimetres, more preferably 25 millimetres. Arrangement according to Claim 1 or 2, characterized in that the first distance at which the sealing arrangement is arranged from the plane of the envelope surface corresponds essentially to a 0 distance, meaning that the sealing arrangement touches the plane of the envelope surface. Arrangement according to any of the preceding claims, characterized in that the sealing arrangement is retracted towards the centre of rotation of the screen member, in the area of the nip, across an angle range in the range of 15 - 60 degrees, preferably 25 - 50 degrees, more preferably 25 - 35 degrees. Arrangement according to Claim 4, characterized in that the first distance of the seal, meaning that the seal is touching the plane of the envelope surface, is formed over an angle in the range of 300 - 345 degrees, preferably 310 - 335 degrees, more preferably 325 - 335 degrees. Arrangement according to Claim 4, characterized in that the seal (8) is retracted towards the centre of rotation of the screen member, in the area of the nip, at the distance X, and in that the seal continuously changes over to the first distance in the angle range by which the seal is retracted relative to the envelope surface of the screen member. Arrangement according to any of the preceding claims, characterized in that the seal comprises a pressurizing chamber (11) arranged in the housing (13), which pressurizing chamber acts on the sealing surfaces (8) of the seal against the end walls (5a, 5b) of the drum, which end walls preferably comprise exchangeable wear rings (14). Arrangement according to Claim 7, characterized in that the seal consists of a sealing element (8) which is contiguous in the circumferential direction, and which sealing element is arranged in a groove in a seal holder (9) arranged on the housing (13). Arrangement according to Claim 8, characterized in that the pressurizing chamber (11) of the seal consists of a pressurizing chamber (11) which is contiguous in the circumferential direction; which pressurizing chamber (11) is arranged in a groove in a seal holder (9) arranged on the housing (13). Arrangement according to Claim 9, characterized in that the pressurizing chamber (11) is integrated with the sealing part of the seal (8).

Inventors: BROETTGARDH GOERAN, ERIKSSON JOHAN, LAEMAS AXEL, BROETTGARDH, GOERAN, ERIKSSON, JOHAN, LAEMAS, AXEL, BRÖTTGARDH, Göran, LÄMAS, Axel
Applicants: KVAERNER PULPING TECH, KVAERNER PULPING AB
Application year: 2000
Main classification: D21D1
IPC codes: D21D1, D21C9, D21F1