WO1998046823A1

WO1998046823A1 - Control flow system (method and device) in the headbox of a paper machine

Info

Publication number: WO1998046823A1
Application number: PCT/FI1998/000295
Authority: WO
Inventors: Antti Vaahto; Erkki Aho
Original assignee: Vaahto Oy
Priority date: 1997-04-14
Filing date: 1998-04-02
Publication date: 1998-10-22
Also published as: FI101726B; FI101726B1; AU6733598A; FI971561A0

Abstract

The objective of the invention is a control flow system in a paper machine head box to guarantee a controlled even thickness of the fibre suspension over the whole width of the head box at the stage when the fibre suspension discharges through the slice to the paper machine wire. The method is characterized in that the control dilution water is mixed with the fibre suspension flowing through the head box inside the head box before its slice chamber, and that all fibre pipes of the head box belonging to the manifold-system extend as such straight through the head box intermediate chamber close to the turbulence generator, and that at least part of the distributor pipes are double-cased for the adjustment of the length of the distributor pipes in relation to the turbulence pipe.

Description

Control flow system (method and device) in the headbox of

The objective of the invention is a control flow system in a paper machine head box for the control of the fibre suspension flow discharging therefrom.

The formation of a continuous web on a paper machine contains the following steps: - the fibres coming from the pulp mill or pulp mill storage tank are led as a mushy pulp to the paper machine, to its feed pump ;

- the required paper additives or fillers can in this connection be dosed to the pulp; - in the feed pump the pulp is mixed into the process white water coming from the paper machine so that a suspension with a solids content of e.g. 0,3 - 0,8% is obtained;

- the fibre-water suspension thus obtained is fed, generally through a pipe with a big diameter, to the head box and via its slice to the paper machine wire.

The task of the of head box is to spread the fibres and other suspension components on the paper machine wire at a speed approximately corresponding to the speed of the paper machine, and in a right thickness as an uniform jet extending over the whole width of the wire, so that the desired grammage (g/m²) is obtained for the web formed.

The most important properties especially for printing paper are the uniformity of its grammage over the whole width of the web and also in the longitudinal direction, as well as the so called formation, which means the uniform distribution of the paper fibres. A bad formation appears as "cloudiness" when inspecting the paper sheet against the light.

General means of affecting the grammage and the thickness uni- formity of the manufactured paper is to locally adjust the height of the head box slice with a special profile bar in the slice beam. If, however, the suspension flow coming to the slice is instable due to uncontrolled cross-flows in the head box, the adjustment of the slice does not provide a satisfac- tory result. Moreover, if the profile bar is bent too much, e.g. to lower the slice in a certain area, this results in a detrimental flow-disturbance on both sides of said adjustment area. Further, if the profile bar is adjusted often and too much, permanent deformations can be caused thus preventing the providing of an even slice.

One of the objectives if the subject invention is to provide a head box in which the- control mechanism of the slice can be avoided.

Another objective of the invention is to provide a head box in which the fibres of the suspension flowing to the wire are evenly dispersed and do not form such fibre deposits which cause bad formation of the paper.

The main objective of the subject invention is to provide in the head box such a fibre suspension flow system which guarantees the control of an even suspension thickness over the whole width of the head box at the stage when the fibre suspension discharges through the slice to the paper machine wire.

In order to achieve the objectives of the subject invention, the Fl-application 156/65, already presents, at least partly, a method for controlling the eveness of the paper so that several separate dilution water flows are led to the head box close to its discharge opening over the whole machine width in order to even local thickness variations in the suspension. This method cannot, however, eliminate the varying cross-flows in the head box itself, which make the slice flow instable and difficult to control .

The newest technique contains, therefore, solutions in which the suspension flow coming to the head box is divided into longitudinal zones by partitions or alike over the whole width of the head box. The principle of the reference invention is applied in each one of these, however so that the control flow is mixed into the actual basic suspension corresponding to the fi- bre web to be formed long before the slice area.

These new solutions of adjusting and controlling the suspension flow via the head boxes are mainly useful in so called hydraulic head boxes, the development work of which started in the beginning of 1970. These are characterized in a very compact and low construction with a small volume. They comprise generally four main parts: the manifold, the distributing chamber, the turbulence generator and the slice chamber. The manifold is part of the so called manifold-system, which is known from e.g. the US-publication 5,196,091 (fig. 2), and the objective of which is to secure an as even as possible distribution of the fibre suspension from the feed pump over the whole width of the head box. The task of the distributing chamber, which is missing from the reference publication, is to uniform and calm the flow coming from the separate distributor pipes before the turbulence generator, i.e. the turbulence part. The head of the turbulence generator comprises in relation to the flow direction almost similar pipes as the distributor pipes, but these expand in their ends into e.g. squares so that they fill the flow area corresponding to the whole slice chamber. The objective of the turbulence generator is to disperse the fibre suspension as efficiently as possible in order to provide a good formation for the paper manufactured.

Regarding the technical level of the invention reference is made to the following publications.

According to the arrangement disclosed in the patent FI-90260, the circulation water is led through several, tightly adjacent feed pipes to the distributing chamber close to the turbulence tubing. The amount of circulation water i.e. "dilution fluid" according to the arrangement can be adjusted in a desired way over the whole width of the machine. Although it is preferable that the control flows are brought as close as possible to the head box slice chamber, there is the risk that the actual fibre suspension flow coming from the distributing chamber to the turbulence generator is disturbed to that extent that the ef- feet aimed at is not achieved in respect of the thickness nor the quantitative control of the flow. The fibre suspension flow in the distributing chamber can be partly uncontrolled, when it is difficult to affect even with additional flow tubes.

The construction according to the US-patent 5,549,792 corresponds to the solution of the FI-90260 publication to that extent that the control flows are led to the head box distributing chamber, however so that it has been divided laterally into several compartments. This enables, compared to the FI- publication, considerably bigger possibilities to control the amount and thickness of the main suspension flow in the cross direction. Although there are several parallel compartments, the control of the cross flow profile aimed at might not sue- ceed as the control parameters, i.e. the flows C_x and C₂ as well as their entering angles to the distributing chamber offer a countless amount of alternatives which are difficult to control.

The method according to the application FI-94 6181 requires that the "additional liquid flow", which means the circulation water from the paper machine diluting the main pulp flow, is brought to be led to the head box already before the distribu- tor pipes of the manifold-system. Although the dilution water flow at that stage is directed at a certain area of the head box width, nothing guarantees that the so achieved change in the suspension thickness would effect the fibre distribution of the suspension jet discharging from the head box. After the manifold there are no partitions or alike to prevent possible cross-flows .

The objective of the invention is to provide in the paper machine head box such an apparatus and method for the local con- trol of the fibre suspension thickness in cross direction that the disadvantages observed in the head boxes of the reference publications can be avoided. In order to realized this objective, the apparatus according to the invention is mainly characterized in that all fibre suspension distributor pipes of the head box manifold-system extend as such, without thereto related water units, straight through the head box intermediate chamber close to the turbulence generator, and the thickness of the fibre suspension flowing in the distributor pipes is the one determined in the head box feed pump system, and that the water required for the control dilution of the fibre suspension is obtained from the head box intermediate chamber. The method according to the invention is characterized in that the control dilution water is mixed with the fibre suspension flowing via the head box inside the head box before its slice chamber, and that all fibre suspension distributor pipes of the head box manifold-system extend as such, without thereto related water units, straight through the head box intermediate chamber close to the turbulence generator, and that at least part of the distributor pipes are double-cased to enable the external casing to slide there in relation to the internal cas- ing in order to adjust the length of the distributor pipes in relation to the turbulence pipe.

The apparatus and the suspension flow control method according to the invention are described below with reference to the en- closed drawings, in which

Fig. 1 presents schematically the head of the paper machine wire portion including the head box and the related fibre suspension and process water flow arrangements,

Fig. 2 shows a schematic profile section of the fibre sus- pension manifold and the turbulence generator with its turbulence pipes according the invention,

Fig. 3 shows a top view of the head box manifold and the turbulence generator according to the invention, Figs. 4a, 4b and 4 c show different alternative locations of the fibre suspension distributor pipes in relation to their axially corresponding turbulence pipes .

Fig. 5 shows one construction of a double-cased distributor pipe head in relation to its corresponding turbulence generator turbulence tube used in one embodiment of the invention.

Fig. 1 illustrates the embodiment of the operating principle of the paper machine head box and wire portion presented in the beginning of the specification of the invention. The paper fi- bres come e.g. from the pulp mills storage tank (not shown) as a mushy pulp via the M-pipe 12 to the paper machine head box feeding pump 13 , which is connected with the pipe 14 to the circulation water basin 15 of the paper machine wire portion. The feeding pump 13 sucks circulation water from the basin 15 and simultaneously also pulp M coming via the pipe 12 with the valves 16 in a adjustable ratio so that e.g. a 0,3 - 0,5 % suspension F is obtained, and feeds it to the head box 1 and from there further to the paper machine wire in an as such known way. Another pump 17 is connected to the circulation water basin 15, which pump 17 feeds dilution water W to the system 18 providing the thickness control in the head box 1. The head box can either be fully hydraulic or provided with an air-bag.

Fig. 2 shows a partial profile drawing of the head box part to which the fibre suspension initially is led through the manifold-system main pipe 5. The reference number 6 refers to the distributor pipes branching from the main pipe, which distributor pipes are several parallel and on top of each other at even distances over the whole width of the head box. The suspension flows through the straight distributor pipes 6 close to the head box turbulence chamber and further to the turbulence pipes 8, which are coaxial with distributor pipes and of equal amount as the distributor pipes. The intermediate chamber 4 is an es- sential part of the apparatus, to which chamber 4 the pump 17 shown in fig. 1 continuously feeds circulation water required for the dilution of the fibre suspension through the pipe 18, which extends transversely over the head box width and has several branches provided with the valves 19. The amount of branch pipes depends on the width of the head box.

According to fig. 3, which shows a top view of the same distributor pipes 6 and turbulence pipes 8 as in fig. 2, the in- termediate chamber 4 is provided with partitions 10. These partitions divide the intermediate chamber into compartments, which are equally many as the branches exiting from the dilution water pipe 18. With the valves 19 shown in fig. 2, the amount and pressure of the dilution water can be controlled in each compartment of the intermediate chamber.

Figs. 4a, 4b and 4c show principal alternative solutions for the internal installation of the distributor pipes 6 and the turbulence pipes 8. It is essential for the arrangement of the subject invention that the diameter of the distributor pipes 6 is smaller than the diameter of the corresponding turbulence pipes 8. This enables the dilution water surrounding the distributor pipes to mix with the fibre suspension F coming from the distributor pipe in a desired way, which fibre suspension thereafter proceeds thickness controlled in the corresponding turbulence pipes to the head box slice chamber and further to the wire. Factors that effect the so embodied control of the suspension thickness are i.a. the diameter ratio between the turbulence pipes and distributor pipes, as well as very essentially the distance of the distributor pipe head from the corresponding turbulence pipe opening. This distance can even be "negative", i.e. the distributor pipe head can to some extent be pushed into the turbulence pipe, as shown in fig. 4a.

Fig. 5 shows one possible embodiment of the invention using a distributor pipe of special construction provided with a double-casing 11a, lib. The external casing 11a is accommodated sliding in relation to the internal casing lib, whereby a suit- able mechanism can be connected to the external casing enabling thus the adjustment of the distributor pipe length. This arrangement does not require the distributor pipe to be double- cased throughout its whole length. Different forms of embodiment of the invention have been presented in the specification above. The invention is, however, not to be considered as so restricted, but the different fea- tures of the invention can be modified within the scope of the invention.

Claims

1. An apparatus in a paper machine head box (1) , with which apparatus the grammage cross profile of a web produced of a fibre suspension (F) is evened so that the control dilution water (W) is mixed with the fibre suspension (F) flowing through the head box (1) inside the head box before its slice chamber (3) in such places in which corresponding areas occurs disturbances affecting the grammage profile of the ready web, c h a r a c t e r i z e d in that,

- the water (W) required for the control dilution of the fibre suspension (F) is obtained from the head box intermediate chamber (4), that all fibre suspension distributor pipes (6) belonging to the manifold-system (5) extend as such, without thereto related water units, straight through the head box intermediate chamber (4) close to the turbulence generator (7) , and the thickness of the fibre suspension -(F) flowing in the distributor pipes (6) is as defined in the paper machine head box (9) feed pump sys- tern,

- that the amount of the turbulence pipes (8) of the turbulence generator (7) are equal to the amount of distributor pipes (6) ,

- that the heads of the fibre suspension distributor pipes (6) are either in the direction of the distributor pipes' (6) axle or perpendicular to it at a measurable distance (a,b) from the opening of the turbulence pipes (8) , that the turbulence pipes (8) are at least at their initial part coaxial with the distributor pipes (6) .

2. An apparatus according to claim 1, c h a r a c t e r i z e d in that the heads of the distributor pipes (6) feeding the fibre suspension (F) to the turbulence generator (7) are all essentially at the same level (x-x) , which is perpendicular in relation to the fibre suspension (F) flow direction.

3. An apparatus according to claim 1, c h a r a c t e r i z e d in that the downstream wall (y-y) of the intermediate chamber (4) , which is essentially perpendicular in relation to the fibre suspension (F) flow direction, is a level provided with openings for the turbulence pipes (8) of the turbulence generator (7) .

4. An apparatus according to claim 1, c h a r a c t e r i z e d in that the outer diameter of the fibre suspension distributor pipes (6) close to the level of the turbulence generator (7) is smaller than the opening of the turbulence pipes (8) or than the inner diameter of the turbulence pipes (8) .

5. An apparatus according to claim 1 or 4 , c h a r a c t e r i z e d in that at least part of the fibre suspension distributor pipes (6) are adapted to extend partly inside the turbulence pipes (8) of the turbulence generator.

6. An apparatus according to claim 1 or 4 , c h a r a c t e r i z e d in that at least part of the fibre suspension distributor pipes (6) are adapted to extend partly even with the heads of the turbulence pipes (8) of the turbulence generator .

7. An apparatus according to claim 1 or 4 , c h a r a c t e r i z e d in that at least part of the fibre suspension distributor pipes (6) are adapted to extend partly outside the turbulence pipes (8) of the turbulence generator.

8. An apparatus according to claim 1, c h a r a c t e r i z e d in that at least part of the distributor pipes (6) are double-cased (11a, lib) so that the external casing (11a) can slide in relation to the internal cas- ing (lib) in order to adjust the length of the distributor pipes in relation to the turbulence pipe.

9. A method for evening the grammage profile of a web formed of a fibre suspension (F) in a paper machine, characterized in that the control dilution water (W) is mixed with the fibre suspension (F) flowing through the head box (1) inside the head box before its slice chamber (3), that all fibre suspension distributor pipes (6) belonging to the manifold-system extend as such, without thereto related water units, straight through the head box intermediate chamber (4) close to the turbulence generator (7) , and that at least part of the distributor pipes (6) are double-cased (11a, lib) so that the external casing (11a) can slide in relation to the internal casing (lib) in order to adjust the length of the distributor pipes in relation to the turbulence pipe, c h a r a c t e r i z e d in that the required grammage profile of the web to be formed is obtained by adjusting the location of the distributor pipe heads in relation to the heads of the turbulence pipes and by keeping the pressure difference be- tween the fibre suspension (F) and the control dilution water (W) essentially low at the point where the heads of the distributor pipes and the heads of the turbulence pipes are closest to each other.