FI111391B - Process and process arrangement in the short circulation of a paper machine - Google Patents

Abstract

A process arrangement in the short circulation of a paper machine comprises devices (10-13) for dilution of thick stock, centrifugal cleaners (14), and a headbox fan pump (16), as well as a pipe system connecting these. There is a direct flow connection from the centrifugal cleaners (14) along a pipeline (30) to the headbox fan pump (16), through which pipeline (30) a centrifugally cleaned stock flow (F6) is passed direct to the headbox fan pump (16) without any deaeration or dilution stages in between. A secondary line (32) is connected to the pipeline (30) for passing through it part of the centrifugally cleaned stock flow as a return flow (F7) back to the thick stock dilution devices (10-13). By this arrangement it can be ensured that the pressure on the suction side of the fan pump (16) remains substantially constant.

Description

111391

BACKGROUND OF THE INVENTION The present invention relates to a paper machine short cycle process stage comprising devices for diluting a thick mass, vortex cleaning devices and a headbox feed pump and piping connecting them.

The invention also relates to a method of keeping the pressure 10 constant at the suction side of the headbox feed pump in a short cycle of a papermaking machine, wherein the pulp is diluted to headbox consistency, cleaned by vortex cleaners and pumped through the headbox to the wire section of the papermaking machine.

In a short rotation of a papermaking machine, the pulp is fed to the headbox preceded by steps of diluting the thick pulp to the headbox consistency in either one or two steps and removing various impurities from the pulp during the vortex cleaning, air removal and / or sorting steps. It is important for the stability and trouble-free operation of the entire papermaking process that the pressure at the inlet side of the headbox feed pump can be kept constant. If the pressure is not constant, the flow rate produced by the feed pump will vary, causing pressure changes in the headbox lip jet, which are reflected as the basis weight variation in the web.

The venting tank is usually provided with an overflow or leveling function, which serves not only to de-air the rigid system but also to ensure an even pressure on the suction side of the headbox feed pump. There are also known solutions where the main line deaeration tank is missing. For example, in a short rotation of a paper or board machine comprising a vortex cleaning step, it is desired to omit the main line deaeration tank, a double dilution system has generally been the solution. In this case, the thick mass is first diluted to the consistency required by the vortex cleaning and only after the vortex cleaners is the final dilution made to the headbox consistency. Dilutions are accomplished by introducing 2111391 viscous masses into a constant backpressure tap water flow.

In this way, it is possible at the same time to ensure that the pressure remains constant on the inlet side of the stern feed pump. However, dual dilution cannot be used for paper grades requiring high headbox consistency 5 such as SC, LWC, and newsprint.

The object of the invention is to solve in a new way the maintenance of a constant pressure at the inlet side of the headbox feed pump. The aim is to find an alternative solution to the double pulp dilution and main line deaeration tank, which has been used for this purpose on a slope.

The process arrangement according to the invention is characterized by what is set forth in the characterizing part of claim 1. Correspondingly, the method according to the invention is characterized by what is stated in the characterizing part of claim 6.

In the short circuit of the invention, a pipeline is provided to the accept side of the vortex cleaning step, which leads directly to the headbox feed pump without the normally vented and / or dilution steps. Connected to this pipeline between the vortex purifiers and the feed pump is a side line through which part of the mass flow from the acceptor side of the vortex purifiers is returned as a return flow to the thick pulp diluent so that the return flow to the suction side of the mixing pump The recovery site is thus selected to avoid interference. A bypass line is designed to reduce the vortex cleaning of the accept flow and pressure variations. If it is desired to increase the head box feed pump inlet pressure, the secondary flow can be used for the valve. However, the valve is not required.

The process layout according to the invention is simple and the number of containers and devices required therein is smaller than in conventional arrangements. Solution 3111391 also reduces the precipitation of calcium compounds in the headbox approach piping in neutral processes, which results from the release of some carbonate-containing carbon dioxide into the air vent through the filler calcium carbonate. When the air is already removed from the tap water, the distance to the approach pipe 5 is longer and the filler flow through the deaerator is smaller.

In the following, the invention will be described with reference to the accompanying drawings, in which details, however, are not intended to be limited solely by the invention.

Figure 1 schematically shows a portion of a conventional paper machine short rotation.

Figure 2 schematically shows a process layout in accordance with the invention.

Figure 3 shows a modification of the process layout of Figure 2.

15

Figure 1 is a simplified illustration of a portion of a short rotation of a papermaking machine, and in particular of a conventional headbox approach system. The viscous fresh mass M is introduced into the mixing zone 11 of the wire well 10 where it is diluted with tap water to a consistency required for vortex cleaning, which in this case is almost equal to the consistency of the pulp fed to the headbox. The diluted mass flow rate Fi is passed through a mixing pump 12 to a vortex cleaner 14 where heavy impurities such as sand are removed from the mass. The purified pulp stream F2 is led to an air outlet tank 15 having a vacuum air above the free surface of the pulp. From the bottom of the venting tank 15, a mass flow F3 is supplied to the suction side of the headbox feed 25 pump 16 and thereafter to the machine connections 18. From the accept side of the machine screens 18, The recovered water F10 from the wire section is led to the wire well 10 for use in the dilution of the thick mass M.

The surface height in the deaeration tank 15 is kept constant by means of an overflow 15a which is conducted as a return flow F5 to the wire well 10 for use in the dilution of a thick mass M 111191. The wire well 10 also has an overflow 10a. The wire well 10 with level control 10a maintains a constant pressure on the inlet side of the mixing pump 12 and the corresponding air outlet tank 15 with a level control 15a maintains a constant pressure on the inlet side of the feed pump 16.

5

Figure 2 illustrates a short rotation of a paper machine with vortex cleaning and single dilution according to the invention, with no main line air vent container. The thick mass M is diluted at the bottom of the wire well 10 and the diluted mass flow rate Fi is pumped by a mixing pump 12 into the vortex cleaners 14.

Most of the first stage acceptor of the vortex cleaners 14 is fed as a mass flow Fg through the pipeline 30 directly to the headbox feed pump 16, which further pumps it to the machine connections 18. The first stage acceptor flow F4 of the machine screens 18 is then led to the headbox 20 and further.

15

From the accept side of the vortex purifiers 14 to the pipeline 30 leading to the feed pump 16 is provided a side line 32 through which a portion of the accept mass is returned as a return flow F7 to the wire well 10 for use in the dilution of the thick mass M. Thanks to the free bypass circuit, the pressure remains constant in the pipeline 30 at the suction side of the 20 feed pumps. Other process flows smaller than the main line (not shown) can also be introduced into the side line 32.

The solution of Figure 3 is substantially similar to the solution of Figure 2, in that the diluted mass flow Fi is also pumped through vortex cleaners 14 to a feed pump 25 that pumps the mass flow F6 further through machine passages 18 to the stern 20 and a portion of the vortex purified mass back to mass dilution. The solution differs from Fig. 2 in that the thick mass M is diluted without a wire well in a closed mixing volume 13 by a so-called tube dilution. The thick mass M, the tap water F10, and the vortex-purified mass return stream F7 are led to the mixing volume 13 on the suction side of the feed pump 12 in parallel flows such that the pulp return stream F7 and the most outflow stream are surrounded. The use of pipe dilution further reduces the tank and flow volume required for a short cycle.

The solution shown in Figures 2 and 3 can ensure a steady pressure on the inlet side of the feed pump 16 and thus improve system stability and trouble-free operation. At the same time, the pressure control of the headbox 20 can be improved by providing a common speed control or feed-in control for the mixing pump 12 and the headbox feed pump 16. It controls both parts of the process 10 simultaneously and in parallel, allowing for better anticipation of change situations.

The process flow according to the invention is particularly suitable for paper or board machines that require vortex cleaning, but due to the high humidity of the headbox, double dilution cannot be used and due to process technical or economic reasons, it is not desirable to use de-aeration. The method is also suitable for use in multilayer stacking, where pulp suspensions for different pulp layers are separately treated prior to forming the multilayer web.

20

The process flow according to the invention is considerably simpler than the known solutions, since it does not require a main line venting tank and double pulp dilution, which have previously been necessary to ensure uniform pressure on the inlet side of the headbox feed pump.

25

Claims (6)

A process arrangement for a short circulation in a paper machine, comprising means (10-13) for diluting a thick mass, vortex cleaning means (14) and a feed pump (16) for an inlet carriage and a pipe system joining them and in which from the whirlpools (14) a direct flow connection along a pipe line (30) to the feed pump (16) to the inlet carriage, wherein a whirlpool pulp flow (Fo) is conducted via this pipe line (30) directly to the feed pump (16) to the inlet carriage without intermediate venting or dilution steps, characterized in that said side line (30) is joined by a lateral line (32) through which a portion of the vortex-cleaned mass flow is conducted as a return flow (F7) back to the devices (10- 13) for diluting the thick mass, thereby ensuring by this arrangement that the pressure is kept substantially constant on the suction side of the feed pump (16). 15 Process arrangement according to Claim 1, characterized in that the devices for diluting the thick mass comprise a mixing pump (12), wherein an inlet flow (M) of thick mass, a flow (Fio) of viral water and a return flow (F7) of swirled pulp are led to a mixing space (11,13) on the suction side of the pump. Process arrangement according to claim 2, characterized in that the return flow (F7) is centrally and parallel in relation to the flow (Fio) of the wirewater to the suction side of the mixing pump (12). 25 Process arrangement according to any of the preceding claims, characterized in that the mixing pump (12) and the feed pump (16) of the inlet carriage have a common pressure control which controls the speed, i.e. a control with positive feedback. 30 111391 Process arrangement according to any of the preceding claims, characterized in that other process flows smaller than the main line are joined to the side line (32). 5 6. A method of short circulation in a paper machine to constantly apply pressure to the suction side of a feed pump (16) to an inlet carriage, at which the process mass is diluted to inlet carriage thickness, cleaned with vortex cleaner (14) and pumped via the inlet tray ( 20) out on a wire portion of the paper machine and at which a mass flow (Fo) is conducted from the acceptance side of the vortex rings (14) directly to the feeder pump (16) to the inlet drawer without intermediate venting or dilution steps, characterized in that the pressure in a pipe line (30) ) which leads from the swirls (14) to the feed pump (16) is kept substantially constant by passing a portion of the mass coming from the swirls (14) as a return flow (F7) back to the dilution of the thick mass. 15