FI105834B - Process for reducing consistency fluctuations in wire water - Google Patents

Abstract

The invention relates to a process for reducing consistency fluctuations in wire water in a papermaking machine comprising a double-wired web-forming section 15, in which a selvage-loading unit 20 serves as a web- forming and water-removing element, and in which web- forming section 15 the majority of the water removed through the wires 27, 28 is conducted via a wire well 10 for use firstly for diluting thick pulp M1 conveyed to the papermaking machine and secondly for regulating a transversely directed consistency profile of a pulp flow fed from a flow box 14 to the web-forming section 15. The process comprises steps in which an upper limit is established for the flow of dry substance conducted from the selvage-loading unit 20 to the wire well 10, the dry substance content of the filtrate FM coming from the selvage-loading unit 20 is measured, on the basis of this measured dry substance content, the water flow FMV conducted from the selvage-loading unit 20 to the wire well 10 is regulated such that the established limit value for the dry substance flow is not exceeded, and the surplus element FMK of the filtrate FM from the selvage- loading unit is conducted to the circulation water system in the papermaking machine. <IMAGE>

Description

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Method for reducing fluctuations in consistency of tap water Förfarande för minskning av consistensväxlingar i viravatten%

The invention relates to a method for reducing the variations in the consistency of tap water, in particular on a paper machine comprising a dilution headbox and using a strip loading unit as one of the dewatering elements.

10 The forming part of the papermaking machine removes water from the pulp suspension by various web forming and dewatering elements, and the water drained from the web through the wire (s) is collected in a wire well. The tap water that has passed the forming wire contains high amounts of fiber and fillers, which are returned to the pulp using the water from the well to dilute the thick pulp fed to the headbox.

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By using a dilution headbox, the transverse basis weight profile of the papermaking machine can be influenced by feeding the headbox to a pulp suspension at different widths of the papermaking machine to obtain a uniform consistency of pulp consistency through the lip opening of the headbox. Dilution headboxes are the most common type of diluent water containing fiber and filler containing tap water. The quality of the dilution water taken out of the wire well can vary within wide limits depending on the process conditions, the quality to be prepared and the raw material, whereby sudden variations in the solids content of the tap water are essential to the success of headbox consistency profiling.

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To solve this problem, U.S. Patent Application No. 941619 proposes the use as a headbox diluent fraction of a recovered filtrate from a recirculating water fiber with a solids content less than the solids content of the tap water. U.S. Patent Application No. 973389 discloses dilution water to be taken from a subject on a paper machine having a water solids content of 5 g / l or less. In U.S. Patent Application No. 971126, the dilution water taken from the wire well is led through a water purification apparatus which separates e.g. fibers and fillers to provide enhanced • <•. , 2 105834 dilution water flow. All of the systems listed above are complex and require additional tanks and / or piping.

In the prior art solutions, essentially all the fibrous filtrates 5 of the wire section are led to a common wire well which overflows into the circulating water system of the papermaking machine. Measurements made with a fine paper reformer have shown that the variation in the load of the strip loading unit used as the dewatering element causes a significant variation in the consistency of the tap water. In addition, many other factors related to the driving behavior of the forming unit affect the solids content and variability of the tap water discharged from the web. The wire well compensates to some extent for short-term variations in the consistency of the tap water, but its effectiveness is limited.

The strip loading unit comprises a plurality of overlapping dewatering and guide strips stacked against each other, between which the web passes under the control of two wires. The upper wire generally has fixed moldings and is associated with at least one dewatering chamber connected to a vacuum source, to which water discharged by the dewatering strips from the web is led. On the underside of the wire are the actual load strips, which are pressed against the wires by means of pressure medium, and a dewatering box, which may also be connected to a vacuum source. The strips placed on either side of the web generate pressure pulses within the web which improve the formation of the web and promote the removal of water from the web. On the other hand, strong pulsation also promotes the removal and removal of fines with water, which is reflected in the high solids content of the filtrates collected from the strip loading unit.

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One example of known strip loading units is the applicant's MB unit «4l. And an example of fine paper formers with strip loading unit is the applicant's SpeedFormer MB ™.

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The object of the invention is to reduce the variation in the consistency of tap water in the wire well and thus to improve the conditions for successful consistency profiling when using tap water as a dilution head dilution fraction.

In order to achieve this and the following objects, the method according to the invention is characterized in what is stated in the characterizing part of claim 1.

In the method according to the invention, the aim is to reduce the variations in the consistency of the wire well by directing part of the filtrates of the strip loading unit instead of the wire well, for example, to a circulating water tank and thence through a solid recovery step. Initially, an upper limit is set for the solid flow from the strip loading unit to the wire well. The solids content of the filtrate from the strip loading unit is then measured and, based on the measured solids content, the water flow from the strip loading unit to the wire well is adjusted so that the set solids flow limit is not exceeded. The remainder of the filtrate in the strip loading unit is discharged to the paper machine's circulating water system.

The reduction in the consistency of the wire well facilitates the adjustment of the transverse profile 20 of the paper web and thereby the desired basis weight profile. Of particular importance is this controlled division of the thick filtrate from the strip loading unit into two partial streams when species change or other change in former or headbox running parameters causes sudden changes in the strip loading unit dewatering volume or solids content.

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In the following, the invention will be described in detail with reference to the accompanying drawing *. however, the invention is not intended to limit the details thereof.

Figure 1 schematically illustrates a prior art short-circuit flow arrangement in a paper machine with a dilution headbox.

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Fig. 2 shows, by way of example, a two-wire formatter with a strip loading unit.

Fig. 3 schematically illustrates an arrangement according to the invention to reduce variations in wire well consistency 5.

According to Figure 1, the thick pulp Mj is introduced into the paper machine for a short circulation through a wire well 10, where the pulp is diluted with tap water, for example from about 3% consistency to about 1% consistency. The pulp M2 diluted in the headbox consistency is pumped through conventional vortex cleaning, deaerating and machine screening steps, generally designated by reference 12, to the headbox 14 for supply to a papermaking wire section 15, which may be a flat wire section or a twin wire reformer. The dilution water Fj used to adjust the transverse consistency profile of the mass flow is also taken from the wire well 10 and, after the venting and machine screening steps 13, is fed to headbox 15 to size 14, where dilution water Fj is supplied at various desired locations in the web width direction. The wire portion 15 is dewatered from the web by various dewatering elements and the tap water F2 drained through the wire (s) is led to the wire well 10 for further dilution of the mass fed to the headbox 14 and the consistency profile adjustment 20 in the headbox 14. The wire well 10

One purpose of the short cycle is to return the fibrous material and filler leached with the tap water F2 from the pulp web back to papermaking. The retention of the wire section 25 and thus also the consistency of the tap water can vary greatly depending on, among other things, the composition of the pulp suspension and the running conditions of the web forming section 15. When the tap water taken from the wire well 10 is used as the dilution-water fraction Fj of the consistency-adjusted headbox 14, the variation of retention on the wire portion 15 strongly affects the solids content of the dilution water Fj, making the consistency profiling of the header 14 difficult. Variations in the operation of up to 3 0 individual dewatering elements can have a decisive effect on the consistency of the tap water taken from the wire well, as shown below.

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Fig. 2 shows an example of a twin-wire form 15 having a strip loading unit 20, suitable for e.g. for the production of fine paper. Said web forming section 15 comprises, as dewatering elements, a suction forming roll 16, a forming shoe 17, a suction deflector unit 18, a strip loading unit 20, a suction box 21, a forming roll 22, a plurality of suction boxes 26 a forming roller 16, a suction deflector assembly 18, and an overhead dewatering chamber 20b, with all other dewatering elements located inside the bottom wire loop 28. Fig. 2 also illustrates a water collecting basin 29 for collecting drainage water from a web through a forming section for introduction into a wire well.

From the headbox 14, a pulp suspension jet is injected into the friction between the forming wires27 and 28, and the forming member 15 is dewatered from the web so that the dry solids content of the web after the forming member is greater than 20%. The largest amounts of water from the pulp suspension are removed by the first three web forming elements of the forming member 15, which are a suction forming roll 16, a forming shoe 17 and a suction deflector unit 18, which scrapes free water from the upper surface of the upper wire 27. Measurements carried out have shown that the amount of water removed from the web by these three first dewatering elements 16-18 per unit time is more than 80% of the amount of pulp suspension fed from the headbox simultaneously and the consistency of the filtrates removed is about 30-40% of the consistency of the headbox mass. In the measurement situation, former Ilia produced fine paper with a basis weight of 80 g / m2 and a paper machine running speed of 18 m / s.

The drip loading unit 20 comprises a lower dewatering chamber 20a associated with a pressure medium dewatering strip, the so-called drip tray, and an upper dewatering chamber 20b also covered by a duct cover. The dewatering and guiding strips disposed overlapping on both sides of the web cause pressure pulsation in the web passing through the strip loading unit 20, which improves the formation of the web and promotes dewatering. At the same time, pressure pulses increase the mobility of the finishing material 6 105834, which results in relatively large solids losses. In the above-mentioned measurement situation, the filtrates of the strip loading unit 20 had a consistency of more than 70% of the headbox mass, but the filtrate volume flow was only 12% of the headbox mass flow rate. In the same measurements, it was found that the amounts of water and solids removed from the web by the forming and dewatering elements 21 to 26 following the 5 strip loading units were so small as to be of minor importance to the solids content of the tap water.

Although the volume flow rate of the filtrate removed from the pulp web by the strip loading unit 20 is relatively small, the filtrate is high in solids compared to the filtrates from other dewatering elements and thus significantly influences the total solids flow from the former 15 to the well 10.

After examining the effect of various factors on the solids content of the filtrates 15 of the strip loading unit 20, it has been found that as the loading strip loads are added to the drainage chambers 20a, 20b, the solids content of the collected filtrates increased. Many other control parameters of headbox 14 and former 15 have also been found to affect the amount of solids discharged from the web by the strip loading unit 20. Conventional fine paper formers can be used to produce grades of paper having a basis weight ranging from 40 to 120 g / m 2 and a filler content of 10 to 25%. A change of species on such a paper machine easily results in major changes in the operating conditions of the headbox 14 and web forming section 15, resulting not only in retention level changes on the wire section 15 but also in a sharp variation in the discharge water of the strip loading unit 20, which can result in transverse

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Figure 3 illustrates a solution according to the invention which aims to reduce the variation in the consistency of tap water. The filtrates from the dewatering chambers above and below the dummy loading unit 20 are led to a common drainage channel 32.

Flow passage 32 is provided with a three-way valve 33 at which flow fm 30 is divided into two branches FMV and FMK, the first of which is conducted to a wire well 10 and the latter to a circulating water system for use in other papermaking processes.

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7 105834 steps. A flow sensor 32 is provided with a consistency sensor 34 to measure the solids content of the water flow FM exiting the strip loading unit 20. Further, in the flow branch leading to the wire well 10, the FMV has a flow sensor 35, which signals the position of the three-way valve 33 based on the signals it provides.

5 In order to effect the adjustment, an upper limit Kset is initially set for the flow of solids from the strip loading unit 20 to the wire well 10. Based on the consistency information C obtained from the consistency sensor 34, the three-way valve 33 is adjusted so that the solid flow K entering the well 10 via the water flow Fmv remains below the set limit Kset, ίο volume flow (from sensor 35) and C is the consistency of tap water (from sensor 34).

An additional portion of the discharge water, i.e. flow fMK, of the strip loading unit 20 is directed from the valve 33 to, for example, a circulating water tank (not shown) and from there through fibers or inlet to a long circulation of the papermaking machine.

If there is a sudden change in the operation of the strip loading unit 20 or the entire forming unit 15 resulting in a sharp increase in the consistency C and / or the volume flow K of the water flow from the strip loading unit 20, the arrangement according to the invention

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conductive water flow FMK increases. On the other hand, if the actual solid flow K from the strip loading unit 20 is less than the set limit Kset, the entire flow FM is led to the wire well 10. Fig. 3 also shows the flow Fv that introduces the other tap water into the wire well 10.

«*. When a portion of the water flow fM of the strip loading unit 20 is led directly into the circulating water system, the overflow F3 of the wire well 10 into the circulating water system naturally decreases. In this context, it should be noted that the overflow F3 of the wire well is normally at-30, that is, without dividing the flow FM into two partial streams FMV and FMK, greater than the strip load unit 20 The total amount of water removed from the dough F ^.

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The details of the invention may differ from the foregoing and vary within the scope defined by the following claims.

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Claims (2)

A method for reducing consistency changes in wire water in a paper machine, comprising a double wire web forming portion (15), in which a mold loading unit (20) serves as a web forming and dewatering element, and wherein the web forming portion (15) is the main part of the water dewatered by the wires (27,28) is passed through a wire well (10) for use partly for dilution of thick mass (Mj) conveyed to the paper machine and partly for controlling a transverse consistency profile of a pulp flow measured from an inlet tray (14) to the web forming portion (15), characterized in that the method comprises steps in which an upper limit of the flow of dry substance conducted from the yeast loading unit (20) to the wire well (10) is set, the dry matter content of the filtrate (FM) which will be measured from the list load unit (20), on the basis of the measured dry matter content, the water flow (Fmv) which is conducted from the list load unit is controlled ethylene (20) to the wire well (10) so that the set limit value for the dry matter flow is not exceeded, and the excess part (Fmk) of the filtrate (FM) from the yeast loading unit is led to the circulation water system in the paper machine. 2. A method according to claim 1, characterized in that the set point value of the Upper 2. limit of the dry matter flow is changed if necessary in changing driving conditions in; the web forming part (15), for example in connection with quality change or speed change. in