Method for determining retention of wire section in paper machine

Abstract

A method and apparatus for determining the retention of a wire section in a paper machine. The mass flow of a paper web on the wire section is determined by means of web properties measured after the wire section. In addition, the mass flow of a headbox and/or white water is determined. The retention of the wire section is determined on the basis of the mass flows of the paper web and the headbox and/or the mass flow of the white water.

Description

FIELD OF THE INVENTION

The invention relates to a method for determining the retention of a wire section in a paper machine.

The invention further relates to an apparatus for determining the retention of a wire section in a paper machine.

The invention further relates to a method for adjusting the retention of a wire section in a paper machine, in which method the retention of the wire section is adjusted by controlling the flow rate of a retention agent.

The invention further relates to an apparatus for adjusting the retention of a wire section in a paper machine, the apparatus comprising means for adjusting the retention of the wire section by controlling the flow rate of a retention agent.

BACKGROUND OF THE INVENTION

In a paper or board machine, the retention of the wire section expresses which proportion of the solids released from the headbox to the wire section remains on the wire, and thus on the formed paper web, after the filtration process. Since fibres are usually about 2 to 4 mm long, their retention is intrinsically high, because they are considerably longer than the 0.2 mm diameter of the openings in the wire of the wire section and thus they remain easily on the wire after mechanical filtration. The retention of fibres is usually good, almost 100%. Since the diameter of fines and filler components is typically 0.1-10 μm, they cannot attach to the wire after mechanical filtration until a fibre network is formed on the wire. The retention of fines and fillers on the wire is therefore intrinsically very low. Fines and fillers are therefore bonded to the paper web by means of different chemical retention agents, which allow retention of fillers to be increased to a level of 20-40%.

A current method that is widely used for determining the retention of the wire section in a paper or board machine is to calculate a total retention R and a filler, or ash, retention R_A of the wire section using the following formulae: $\begin{array}{cc}R=\left(1-\frac{C_{\mathrm{ww}}}{C_{\mathrm{hbx}}}\right)·100\%\mathrm{and}& \left(1\right)\\ R_A=\left(1-\frac{C_{\mathrm{Aww}}}{C_{\mathrm{Ahbx}}}\right)·100\%,& \left(2\right)\end{array}$
where C_ww is the total consistency [g/l] of white water, C_hbx the total consistency [g/l] of a headbox, C_Aww the filler, or ash, consistency [g/l] of white water, and C_Ahbx the filler, or ash, consistency [g/l] of the headbox. The determining of retention according to formulae (1) and (2) is based on the idea that the amount of water to be removed from the wire section to white water circulation is approximately the same as the amount of water in the headbox. Since in reality about 95% of the water coming from the headbox to the wire section is removed in the wire section, formulae (1) and (2) may be modified to take this error into account. Assuming that K % of the water coming from the headbox is removed in the wire section, formula (1), for example, changes as follows: $\begin{array}{cc}R=\left(1-K\frac{C_{\mathrm{ww}}}{C_{\mathrm{hbx}}}\right)·100\%.& \left(3\right)\end{array}$

If water is removed from the wire section in two directions, i.e. a gap former is used or the paper machine is provided with a dilution 20 headbox, formula (1) and (2) take another form. For example, in a paper machine comprising a dilution headbox, the total retention of the wire section is $\begin{array}{cc}R=\left(1-\frac{C_{\mathrm{ww}}}{\left(1-d\right)·C_{\mathrm{hbx}}+d·C_{\mathrm{ww}}}\right)·100\%& \left(4\right)\end{array}$
and the filler retention of the wire section is $\begin{array}{cc}{R}_A=\left(1-\frac{C_{\mathrm{Aww}}}{\left(1-d\right)·C_{\mathrm{Ahbx}}+d·C_{\mathrm{Aww}}}\right),& \left(5\right)\end{array}$
where d is the dilution ratio, which expresses the ratio of the dilution water flow to the total flow at the headbox slice.

In this specification, the expression “the retention of the wire section” is used, generally and without making a distinction, both for the total retention of the wire section, which influences the entire stock to be fed to the wire section, and for the fibre retention, filler retention and fines retention of the wire section, which relate to the sub-components, i.e. fibres, fillers and fibre-based fines, of the stock. Where a specific reference is made to the total retention of the wire section or to the retention of a specific sub-component of the stock, the exact name of the retention is used. The total retention of the wire section thus consists of the fibre, fines and filler retentions.

The retention of the wire section in a paper machine is currently controlled indirectly by controlling the total consistency of white water. The total consistency of white water is controlled by the dosage of a retention agent. This solution is based on the fact that white water contains most of the poorly retaining components of the short circulation, i.e. fines and fillers, and thus by controlling white water consistencies, a significant indicator of the state of short circulation can be controlled at the same time. An increase in the amount of the retention agent increases retention, which is shown in a decrease in the white water consistency. A decrease in white water consistency reduces the return of fines and fillers to the headbox and thus decreases headbox consistency as well. The solution is thus based on stabilizing the white water consistency, or, when a dilution headbox is concerned, the dilution water consistency to a desired level.

The use of formulae (1) to (5) for determining retention is based on the idea that the measurements of the total consistency and the filler consistency of the headbox and the white water are error-free and represent the entire mass volume of the headbox and the white water. In practise it is extremely difficult to take a representative sample from white water in particular, because the consistencies of the water fractions leaving the wire section may vary significantly. White water consistency therefore almost always represents only a specific fraction of the waters in the wire section and thus it is biased in relation to the mean value of the actual consistencies. The accuracy of white water consistency measurement can be improved by measuring the consistency from a water fraction coming from a former roll, where the flow of the fraction, and thus its impact on retention, is the greatest. Another alternative for improving the accuracy of white water consistency measurement is to measure the consistency of white water at a point, such as a dilution water line of the dilution headbox, where the different water fractions are already mixed. However, even by selecting the location of the white water consistency measurement it is not possible to avoid the dynamic phenomena relating to the measurement and making it difficult to determine a momentary retention of the wire section with sufficient precision to allow retention control, for example, to be carried out in an optimal manner, taking into account the high requirements set to the quality and efficiency of both the paper to be manufactured and the manufacturing process.

EP 1,054,102 discloses a method for controlling the basis weight of a paper or board web in a paper or board machine comprising a dilution headbox in which method the basis weight of the web is measured and the wire retention of the machine is determined and, on the basis of these, the basis weight of the paper or board web is adjusted by changing the flow of the mass and the dilution water to the headbox. In the disclosed solution wire retention is determined, in a manner known per se, on the basis of the consistency of white water, mass flow, or the consistency of the mass in the headbox.

DE 10,043,142, in turn, teaches a method for adjusting the dry mass of paper or board in a paper or board machine provided with a dilution headbox. In the method, a model representing the dry mass of the web is used for determining a calculated value for the dry mass of the web, the calculated value being compared with a set value of the dry mass. The difference between the set value and the calculated value is used to control the flow of the mass to be fed to the headbox and/or the flow of the dilution water or additive to be added to the mass to thereby regulate the dry mass of the web. According to an embodiment of the solution disclosed in the publication, the additive to be added to the mass consists of a retention agent, the flow of which is controlled in an attempt to adjust the retention degree of the wire section to a constant level for controlling the dry mass of the web. The calculated dry mass of the web is determined on the basis of the mass consistency, dilution water consistency and flow, and the speed of the machine, the measured basis weight of the web being used for calibrating the model representing the dry mass of the web.

FI applications 974,327 and 974,328 also describe methods for adjusting paper properties by the dosage of the retention agent to be added to the stock. In the method disclosed in FI 974,327, a variable representing the amount of fillers is measured from white water and, further, the ash content of the paper and/or of the headbox is measured, the amount of fillers in the white water being adjusted by controlling the flow of the retention agent on the basis of the variable representing the amount of fillers in the white water and the ash content of the paper being adjusted by controlling the flow of the fillers on the basis of the ash content of the paper and/or of the headbox. According to this solution, a model is devised of the impact of the retention agent flow on the amount of the fillers in the white water and on the ash content of the paper and of the impact of the filler flow on the ash content of the paper and on the amount of fillers in the white water, this modelling being used for adjusting the amount of fillers in the white water and the ash content of the paper by simultaneously controlling the retention agent flow and the filler flow. In the method disclosed in FI 974,328, the basis weight of paper is measured and then adjusted by controlling the flow of the machine stock on the basis of the measurement. The method comprises devising a model of the impact of the retention agent flow and the machine stock flow on the basis weight of the paper, the modelling being used for adjusting the basis weight of the paper by simultaneously controlling the retention agent flow and the machine stock flow.

With all the above solutions there is, however, still the problem that the measured total or filler consistencies of white water or the headbox are assumed to represent the total mass volume of the headbox and the white water, which may prevent the precise determining of the retention of the wire section and also the adjustment of the retention of the wire section with sufficient precision in different production conditions.

BRIEF DESCRIPTION OF THE INVENTION

It is an object of the invention to provide a method and an apparatus for determining the retention of a wire section with greater precision than before.

The method of the invention for determining the retention of the wire section is characterized by determining the mass flow of a paper web on the wire section by means of paper web properties measured after the wire section; determining the mass flow of a headbox and/or the mass flow of white water; and determining the retention of the wire section on the basis of the mass flow of the paper web and the mass flow of the headbox and/or the mass flow of the white water.

Further, the apparatus of the invention for determining the retention of a wire section is characterized in that the apparatus comprises: means for determining the mass flow of a paper web on the wire section by means of paper web properties measured after the wire section; means for determining the mass flow of a headbox and/or the mass flow of white water; and means for determining the retention of the wire section on the basis of the mass flow of the paper web and on the basis of the mass flow of the headbox and/or the mass flow of the white water.

Still further, the method of the invention for adjusting the retention of a wire section is characterized by determining the mass flow of a paper web on the wire section by means of paper web properties measured after the wire section; determining the mass flow of a headbox and/or the mass flow of white water; determining the retention of the wire section on the basis of the mass flow of the paper web and the mass flow of the headbox and/or on the basis of the mass flow of the white water; and adjusting the retention of the wire section by controlling the flow rate of the retention agent on the basis of the determined retention of the wire section.

Still further, the apparatus of the invention for adjusting the retention of the wire section is characterized in that the apparatus further comprises: means for determining the mass flow of a paper web on the wire section by means of paper web properties measured after the wire section; means for determining the mass flow of a headbox and/or the mass flow of white water; and means for determining the retention of the wire section on the basis of the mass flow of the paper web and the mass flow of the headbox and/or the mass flow of the white water, and that the retention of the wire section is arranged to be adjusted by controlling the retention agent flow on the basis of the determined retention of the wire section.

According to a basic idea of the invention, the retention of the wire section is determined by: determining the mass flow of the paper web on the wire section by means of web properties measured after the wire section; determining the mass flow of the headbox and/or the mass flow of the white water; and determining the retention of the wire section in the paper machine on the basis of the mass flow of the paper web and the mass flow of the headbox and/or the mass flow of the white water. According to an embodiment of the invention the wire section retention to be determined represents the total retention of the wire section, the mass flow of the paper web thus representing the total mass flow of the paper web and, correspondingly, the mass flows of the headbox and the white water representing the total mass flows of the headbox and the white water. According to another embodiment of the invention, the wire section retention to be determined is the filler retention of the wire section, the mass flow of the paper web representing the filler mass flow of the paper web and, correspondingly, the mass flows of the headbox and the white water representing the filler mass flows of the headbox and the white water. According to a third embodiment of the invention, the total mass flow of the headbox is determined on the basis of the total mass flow of the paper web and the total mass flow of the white water; the total consistency of the headbox is determined on the basis of the total mass flow of the headbox; and the total retention of the wire section is determined on the basis of the total consistency of the headbox. In a similar manner, also the total mass flow of the white water can be determined on the basis of the total mass flow of the paper web and the total mass flow of the headbox; the total consistency of the white water is determined on the basis of the total mass flow of the white water and further the total retention of the wire section on the basis of the total consistency of the white water. Further, the filler consistencies of the headbox and the white water and, on the basis of these, the corresponding filler retention can be determined in a similar manner on the basis of the mass flows corresponding to the consistencies in question.

The solution of the invention allows the retention of a wire section of a paper machine to be determined with precision on the basis of the mass flow of the headbox and/or the mass flow of the white water and the mass flow of the paper web on the wire section, the mass flow of the paper web being determined on the basis of paper web properties measured after the wire section. The solution of the invention is easy to implement in a corresponding manner for determining both the total retention and the filler retention of the wire section. When necessary, the total mass flow of the paper web and the total mass flow of the white water can also be used for specifying the measurements of the total consistency of the headbox, which allows the total retention of the wire section to be determined with precision also on the basis of the consistencies of the white water and the headbox, if the consistency measurement is suspected to be erroneous. In a similar manner, the measurement of the filler consistency of the headbox and the total consistency and filler consistency of the white water can be specified on the basis of the mass flows corresponding to these consistencies. Further, because of the precise value of the wire section retention, the retention of the wire section can be adjusted with precision, taking into account both the quality properties of the paper to be manufactured and production efficiency.

In this specification, the term “paper” refers not only to paper but also to board and soft tissue and thus the disclosure relating to paper machines or paper manufacture is also valid for board or soft tissue machines and for the manufacture of board and soft tissue.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, the invention will be described with greater precision with reference to the accompanying drawings, in which

FIG. 1 is a schematic view of a paper machine and a short circulation of the paper machine;

FIG. 2 is a schematic block diagram illustrating the determining of total retention and, further, of adjusting total retention on the basis the total retention obtained;

FIG. 3 is a schematic block diagram illustrating a second embodiment for determining total retention and, further, for adjusting total retention on the basis of the determined total retention;

FIG. 4 is a schematic block diagram illustrating the adjusting of the total consistency of white water in a situation where a target value is determined for the total consistency of white water on the basis of a target value determined for the total retention of the wire section; and

FIG. 5 is a schematic block diagram illustrating the adjusting of the filler consistency of white water in a situation where a target value is determined for the filler consistency of the white water on the basis of a target value determined for filler retention of the wire section.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a schematic view of a paper machine 1 and a short circulation thereof. For the sake of clarity, FIG. 1 does not show the actual stock preparation department of the paper machine 1, where the stock to be supplied to the paper machine 1 is prepared from one or more fibre raw materials and different fillers and additives. The paper machine 1 comprises a headbox 2 from which stock is fed to a wire section 3 to form a paper web 4 onto a wire 3′. The paper web 4 is led to a press section 5 and further to a drying section 6. From the drying section 6, the web is led to a reel 7. FIG. 1 also shows a measuring frame 8 placed upstream of the reel 7, the measuring frame 8 being provided with a back and forth moving measuring carriage having measurement means for measuring quality properties of a finished paper web 4. The paper machine 1 may comprise other parts as well, such as a size press, calendar or a coating unit, which are not shown in FIG. 1, for the sake of clarity. Further, since the operation of the paper machine 1 is known to a person skilled in the art, it will not be discussed in greater detail here.

The stock coming from the stock preparation department of the paper machine I is fed to the headbox 2 through a wire pit silo 9. Stock fed to the wire pit silo 9, or in general to the short circulation of the paper machine 1, is also called machine stock KM. In the wire pit silo 9, water is added to the machine stock KM to adjust the consistency of the machine stock KM to the headbox consistency. The water to be added to the machine stock KM is white water that has filtrated through the wire 3′ and although it contains some fibres, it mostly contains fillers and fines, whose retention to the paper web 4 is poor. If the headbox 2 is a dilution headbox, the stock to be fed to the wire section 3 can be diluted by adding white water to the mass not only in the wire pit silo 9 but also in the headbox 2. The collection of white water in the wire pit silo 9 and its mixing with the stock to be fed into the headbox 2 forms the short circulation of the paper machine 1, the purpose of which is to return the water leaving the wire section 3 and attach the solids it contains back to the paper web 4, and to attenuate and even out interference impulses arriving to the headbox. For the sake of clarity, FIG. 1 does not show the different cleaners, air discharge tanks and screens, which also belong to the short circulation of paper machine and which are responsible for cleaning impurities and gases from the stock to be fed to the headbox 2. Since a person skilled in the art is familiar with the operation of a short circulation in a paper machine, it is not discussed in greater detail here.

The total retention R of the wire section 3 is determined on the basis of the mass balance of the wire section 3, i.e. the proportions of the total mass flow Q_hbox to be fed from the headbox 2 to the wire section 3, the total mass flow Q_pap moving from the wire section 3 to the press section 5, and the total mass flow Q_ww filtrating from the wire section 3 to the short circulation, which can be established when two out of the three total mass flows are known. On the basis of the mass balance of the wire section 3, the total retention R can be calculated. Since the total mass flow Q_pap or the filler mass flow Q_Apap moving from the wire section 3 to the press section 5 cannot, in practise, be to determine on the wire section 3, the total retention R and/or the filler retention R_A of the wire section 3 is determined on the basis of the total mass flow Q_papRL [kg/s] or the filler mass flow Q_ApapRL [kg/s] of the paper web 4 determined after the wire section 3. The total mass flow Q_pap [kg/s] of the paper web 4 on the wire section 3 is thus determined on the basis of the total mass flow Q_papRL of the paper web 4 that is at the dry end of the paper machine 1, and the filler mass flow Q_Apap [kg/s] of the paper web 4 on the wire section 3 is determined on the basis of the filler mass flow Q_ApapRL of the paper web 4 at the dry end of the paper machine 1. The dry end of the paper machine 1 refers to the portion of the machine that is downstream of the wire section 1, i.e. it includes the press section 5 and the subsequent parts of the paper machine 1. The total mass flow and the filler mass flow of the paper web 4 at the dry end of the paper machine 1 can thus be determined also at another location that at the measuring frame 8 placed just before the reel 7.

The total retention R of the wire section 3 can be determined with precision when the total mass flow Q_pap of the paper web 4 on the wire section 3 and either the total mass flow Q_hbox of the headbox 2 or the total mass flow Q_ww of the white water, or both, are known. The same principle can be applied in a corresponding manner to determine the filler retention R_A of the wire section 3. The following example illustrates the determining of the total retention R of the wire section 3 on the basis of the total mass flow Q_ww of the white water and the total mass flow Q_pap of the paper web 4.

The total mass flow Q_ww [kg/s] of the white water can be calculated from the following formula: $\begin{array}{cc}{Q}_{\mathrm{ww}}=\frac{f_{\mathrm{ww}}·C_{\mathrm{ww}}}{1000},& \left(6\right)\end{array}$
where f_ww represents the white water flow [l/s] and C_ww the total consistency of the white water. In practice the white water flow f_ww is approximately equal to the headbox 2 flow f_hbx [l/s], which can be determined for example directly by measuring the flow from the feed conduit 10 leading to the headbox 2 or indirectly on the basis of the pressure at the headbox 2 slice opening. The total mass flow Q_papRL [kg/s] of the paper web 4 at the dry end of the paper machine can be calculated using formula (7). It is assumed that only water is removed from the web as it proceeds in the paper machine from the press section 5 to the reel 7, i.e. Q_press=Q_papRL, where Q_press is the total mass flow of the paper web 4 on the press section 5. $\begin{array}{cc}{Q}_{\mathrm{papRL}}=\frac{m_{\mathrm{papRL}}}{1000}·l_{\mathrm{papRL}}·v_{\mathrm{papRL}},& \left(7\right)\end{array}$
where m_papRL represents the average dry weight [g/m²] of the paper web 4, v_papRL the velocity [m/s] of the paper web 4, and l_papRL the width of the paper web 4 at the dry end of the paper machine 1. The dry weight m_papRL of the paper web 4 is determined on the basis of the basis weight BW and moisture Moi of the paper web 4, which are usually measured with measuring devices provided in the measuring carriage that traverses the paper web 4 back and forth in the measurement frame 8. The velocity v_papRL of the paper web 4 can be determined on the basis of the speed of the closest operational group. The total width l_pap of the paper web 4 on the wire section 3 is
l_pap=l_press+l_trimm,  (8)
where l_press is the width of the paper web 4 running from the wire section 3 to the press section 5 and l_trim is the width of the edge trimmings removed from the paper web 4 in the wire section 3 during the production of the web, i.e.
l_press=l_hbx−l_trim,  (9)
where l_hbx is the width of the headbox 2 slice opening. The width l_hbx of the headbox 2 slice opening and the width l_trim of the edge trimmings can be determined in a manner known per se to a person skilled in the art. The total mass flow Q_papRL of the paper web 4 at the dry end is scaled to correspond to the uncut or untrimmed total mass flow Q_hbx of the headbox 2 by applying the following formula:
Q_pap=Q_papRL·(l_hbx/l_press)  (10)
where Q_pap is the total mass flow of the paper web 4 on the wire section 3. Assuming that the mass losses taking place in the press section 5 and the drying section 6 are insignificant, the following estimate can be generated for the headbox 2 mass flow Q_hbx:
{circumflex over (Q)}_hbx=Q_ww+Q_pap,  (11)
which can then be used for determining an estimate {circumflex over (R)} for the total retention R: $\begin{array}{cc}\hat{R}=\frac{Q_{\mathrm{pap}}}{Q_{\mathrm{hbx}}}=\frac{Q_{\mathrm{pap}}}{Q_{\mathrm{ww}}+Q_{\mathrm{pap}}}·100\%.& \left(12\right)\end{array}$

The disclosed solution allows the total retention R of the wire section 3 to be determined with precision on the basis of two known total mass flows, one of which is the total mass flow Q_pap of the paper web 4 on the wire section 3, which is determined on the basis of the paper web 4 properties measured after the wire section 3. The precise estimate {circumflex over (R)} of the total retention R allows the total retention R of the wire section 3 to be adjusted under any production circumstances by controlling the retention agent flow f_ra to be added to the stock to be fed to the headbox with precision in such a way that the quality and efficiency requirements set for the paper to be manufactured and to the manufacturing efficiency can be attained.

FIG. 2 is a schematic block diagram illustrating, in accordance with the above disclosure, the calculation of the estimate {circumflex over (R)} of the total retention R and the adjusting of the total retention R based on changing the dosage of the retention agent, when the retention is being calculated directly from the mass balance of the wire section 3 by applying the measurements taken at the dry end of the paper machine 1. The total retention estimate {circumflex over (R)} and a set value or target value R_SP of the total retention are used for determining a set value f_rasP for the retention agent flow to control the retention agent flow f_ra. The retention agent flow f_ra can be changed for example by changing the opening of the valve 11 shown in FIG. 1.

The estimate {circumflex over (R)} of the total retention R according to formula (12) is typically updated at intervals of 30 to 60 seconds. The update interval depends on how fast the measuring carriage traveling in the measuring frame 8 traverses the paper web 4, i.e. how often a new value is obtained for the dry weight m_papRL of the paper web 4. If a faster, i.e. more frequently determined, retention estimate is needed for adjusting the total retention R, the estimate can be determined on the basis of an estimate of the total mass flow Q_hbx of the headbox 2 and an estimate of the total consistency C_hbx of the headbox 2, which also allow the accuracy of the consistency measurement of the headbox 2 to be improved. Between the total mass flow Q_hbx of the headbox 2 that corresponds to the precise total consistency C_hbx of the headbox 2 and a mass flow {tilde over (Q)}_hhx calculated on the basis of a consistency measurement deviating from the precise value there may be a difference equal to a term ΔQ_hhx:
ΔQ_hbx={circumflex over (Q)}_hbx−Q_hbx.  (13)

Assuming that the headbox 2 mass flow {circumflex over (Q)}_hbx estimated according to formula (11) comes very close to the actual flow, i.e. {circumflex over (Q)}_hbx≈Q_hbx, the deviation ΔQ_hbx caused by the consistency measurement can be estimated as follows:
Δ{circumflex over (Q)}_hbx={tilde over (Q)}_hbx−{circumflex over (Q)}_hbx,  (14)
from which a consistency correction estimate ΔĈ_hbx can be calculated: $\begin{array}{cc}\Delta {\hat{Q}}_{\mathrm{hbx}}=\frac{f_{\mathrm{hbx}}·\Delta {\hat{C}}_{\mathrm{hbx}}}{1000}\iff \Delta {\hat{C}}_{\mathrm{hbx}}=\frac{\Delta {\hat{Q}}_{\mathrm{hbx}}}{f_{\mathrm{hbx}}}·1000.& \left(15\right)\end{array}$

On the basis of the consistency correction estimate ΔĈ_hbx of formula (15), the consistency estimate of the headbox 2 is
Ĉ_hbx={tilde over (C)}_hbx−ΔĈ_hbx,  (16)
where {tilde over (C)}_hbx is the measured consistency of the headbox 2 and ΔĈ_hbx is a correction term of the total consistency C_hbx estimated on the basis of the mass balance. Assuming that the term ΔĈ_hbx changes slowly, depending on paper grade, for example, the accuracy of the headbox 2 consistency to be used for calculating a fast estimate {circumflex over (R)} of the total retention R can be improved. The retention estimate {circumflex over (R)} can then be calculated in the following manner, either according to formula (1) $\begin{array}{cc}\hat{R}=\left(1-\frac{C_{\mathrm{ww}}}{C_{\mathrm{hbx}}}\right)·100\%& \left(17\right)\end{array}$
or by deducing from formula (1) $\begin{array}{cc}\hat{R}=\frac{f_{\mathrm{hbx}}·{\hat{C}}_{\mathrm{hbx}}-f_{\mathrm{ww}}·C_{\mathrm{ww}}}{f_{\mathrm{hbx}}·{\hat{C}}_{\mathrm{hbx}}}·100\%.& \left(18\right)\end{array}$

FIG. 3 is a block diagram schematically illustrating the adjusting of the total retention R when the retention is calculated using the total consistency of the headbox 2 determined as disclosed above.

The total retention R of the wire section 3 can be determined and the accuracy of the measurement of the total consistency C_ww of the white water can be improved in a corresponding manner on the basis of the total mass flow Q_pap of the paper web 4 and the total mass flow Q_hbx of the headbox 2. Further, the total retention R of the wire section 3 can be determined and the accuracy of the measurement of both the total consistency C_hbx of the headbox 2 and the total consistency C_ww of the white water can be improved in a corresponding manner on the basis of the total mass flow Q_pap of the paper web 4, the total mass flow Q_hbx of the headbox 2, and the total mass flow Q_ww of the white water.

The disclosed solution is fully applicable when, instead of the total retention R of the wire section 3, the filler retention R_A of the wire section 3 is to be determined, in which case the wire section 3 mass flows to be considered are the headbox 2 filler mass flow Q_Ahbx, white water filler mass flow Q_Aww, and paper web 4 filler mass flow Q_Apap. The corresponding consistencies are: the headbox 2 filler consistency C_Ahbx and the white water filler consistency C_Aww, and the amount of fillers, i.e. ash, determined on the basis of the dry weight m_papRL of the paper and its filler, or ash, content A_papRL.

According to an embodiment, the disclosed solution for determining the total retention R or the filler retention R_A of the wire section is used for calculating a target value C_hbxSP, C_wwSP, C_AhbxSP or C_AwwSP for the total headbox 2 or white water consistencies C_hbx, C_ww or for the headbox or white water filler consistencies C_Ahbx, C_Aww. Since white water consistency is a typical adjustable variable currently used in retention adjustment, the following example illustrates the calculation of the target values for the total and filler consistencies of white water. The target values for the total consistency and filler consistency of the headbox 2 can be calculated in a corresponding manner.

The paper to be manufactured is given a dry weight target value m_papSPRL[g/m²] and filler content target value A_papSPRL [%]. Assuming that the width of the paper web 4 at the measurement point is l_papRL [m] and its velocity is v_papRL [m/s], the target[kg/s] for the total production at the dry end is $\begin{array}{cc}{Q}_{\mathrm{papSPRL}}=\frac{m_{\mathrm{papSPRL}}}{1000}·l_{\mathrm{papRL}}·v_{\mathrm{papRL}},& \left(19\right)\end{array}$
which is scaled to correspond to a production target set to an untrimmed total mass flow Q_pop of the wire section 3:
Q_papSP=Q_papSPRL·(l_hbx/l_press)  (20)

Correspondingly, the production target [kg/s] relating to the amount of fillers or ash in the paper is
Q_ApapSP=A_papSPRL·Q_papSPR.  (21)

Similarly as in formula (12), when a target value R_SP is to be attained for the total retention R, the following formula must be valid: $\begin{array}{cc}{R}_{\mathrm{SP}}=\frac{Q_{\mathrm{papSP}}}{Q_{\mathrm{papSP}}+Q_{\mathrm{ww}}}·100\%.& \left(22\right)\end{array}$

From formula (22), the following can be solved: $\begin{array}{cc}{Q}_{\mathrm{ww}}=\frac{100·Q_{\mathrm{papSP}}-R_{\mathrm{SP}}·Q_{\mathrm{papSP}}}{R_{\mathrm{SP}}}=\frac{Q_{\mathrm{papSP}}}{R_{\mathrm{SP}}/100}-Q_{\mathrm{papSP}}.& \left(23\right)\end{array}$

Since it is still true that $\begin{array}{cc}{Q}_{\mathrm{ww}}=\frac{f_{\mathrm{ww}}·C_{\mathrm{wwSP}}}{1000}=\frac{Q_{\mathrm{papSP}}}{R_{\mathrm{SP}}/100}-Q_{\mathrm{papSP}},& \left(24\right)\end{array}$
where C_wwSP is the target value for the total consistency [g/l] of white water, the following target value is obtained for the white water total consistency: $\begin{array}{cc}{C}_{\mathrm{wwSP}}=\frac{\left(\frac{Q_{\mathrm{papSP}}}{R_{\mathrm{SP}}/100}-Q_{\mathrm{papSP}}\right)·1000}{f_{\mathrm{ww}}}.& \left(25\right)\end{array}$

If, instead of the target retention R_SP of the total retention R, a target value R_ASP for the filler retention R_A is to be set, an equation can be deduced in the above manner for calculating a target value C_AwwSP [g/l] for the white water filler consistency C_Aww: $\begin{array}{cc}{C}_{\mathrm{AwwSP}}=\frac{\left(\frac{Q_{\mathrm{ApapSP}}}{R_{\mathrm{ASP}}/100}-Q_{\mathrm{ApapSP}}\right)·1000}{f_{\mathrm{ww}}},& \left(26\right)\end{array}$

• • where R_ASP is the target value [%] for the filler retention RA. FIG. 4 is a block diagram schematically illustrating the adjusting of the white water total consistency C_ww and FIG. 5 the adjusting of the white water filler consistency C_Aww, the target values C_wwSP and C_AwwSP for the consistencies being calculated using the retention targets R_SP and R_ASP.

In FIG. 1 and in the above examples, the quality properties of the paper web 4 are measured with measuring devices or means arranged at the measurement frame 8 located immediately before the reel 7. It is of course evident that the necessary quality properties of the paper web 4 may be measured at any point of the paper machine 1 after the wire section 3. They may therefore be measured immediately after the wire section 3, on the press section 5, on the drying section 6 or just before the reel 7, as shown in FIG. 1.

The functionalities shown in the block diagrams of FIGS. 2 to 5 are carried out using equipment belonging to the automation system of the paper machine 1, such as computers or other separately operated calculation and/or control and adjustment equipment. Functionalities requiring calculation are preferably implemented by means of software. Each functionality of FIG. 2 to 5 involving calculation can naturally be executed in separate calculation units or by separate devices, but preferably all the necessary calculation functionalities are carried out in a single, central computing unit or device.

The drawings and the related specification are only meant to illustrate the idea of the invention. The details of the invention may vary within the scope of the claims. The method disclosed above and its embodiments can therefore be applied in a corresponding manner to a fibre-based fines material, although the examples only discuss the determining of the total retention and the filler retention and the use of these retentions for adjusting retention. In addition to actual retention adjustment, an accurate retention estimate enables to improve the operation of other applications requiring retention data.

Claims

1. A method for determining the retention of a wire section in a paper machine, comprising:

determining the mass flow of a paper web on the wire section by means of paper web properties measured after the wire section;

determining the mass flow of a headbox and/or the mass flow of white water; and

determining the retention of the wire section on the basis of the mass flow of the paper web and the mass flow of the headbox and/or the mass flow of the white water.

2. A method according to claim 1, wherein the retention of the wire section to be determined in the paper machine represents a total retention of the wire section.

3. A method according to claim 2, wherein the mass flow of the paper web to be determined represents a total mass flow of the paper web on the wire section, the mass flow being determined on the basis of production corresponding to the dry weight of the paper web.

4. A method according to claim 3, comprising: determining the mass flow of the white water, the mass flow of the white water to be determined representing a total mass flow of the white water determined on the basis of a headbox flow rate or a white water flow rate and a total consistency of the white water.

5. A method according to claim 4, further comprising:

determining a total mass flow of the headbox on the basis of the total mass flow of the paper web and the total mass flow of the white water;

determining a total consistency of the headbox on the basis of the total mass flow of the headbox; and by

determining a total retention of the wire section on the basis of the total consistency of the headbox.

6. A method according to claim 3, comprising: determining the mass flow of the headbox, the mass flow of the headbox to be determined representing the total mass flow of the headbox determined on the basis of the flow rate of the headbox and the total consistency of the headbox.

7. A method according to claim 6, further comprising:

determining the total mass flow of the white water on the basis of the total mass flow of the paper web and the total mass flow of the headbox;

determining the total consistency of the white water on the basis of the total mass flow of the white water; and by

determining the total retention of the wire section on the basis of the total consistency of the white water.

8. A method according to claim 3, comprising: determining both the mass flow of the headbox and the mass flow of the white water, the headbox mass flow to be determined representing the total mass flow of the headbox and the white water mass flow to be determined representing the total mass flow of the white water.

9. A method according to claim 8, comprising: determining the total mass flow of the white water on the basis of the headbox flow rate or the white water flow rate and the measured total consistency of the white water and by determining the total mass flow of the headbox on the basis of the headbox flow rate and the measured total consistency of the headbox.

10. A method according to claim 8, further comprising:

determining the total mass flow of the headbox on the basis of the total mass flow of the paper web and the total mass flow of the white water;

determining the total consistency of the headbox on the basis of the total mass flow of the headbox;

determining the total mass flow of the white water on the basis of the total mass flow of the paper web and the total mass flow of the headbox;

determining the total consistency of the white water on the basis of the total mass flow of the white water; and by

determining the total retention of the wire section on the basis of the total consistency of the headbox and the total consistency of the white water.

11. A method according to claim 2, further comprising: determining a target value for the total consistency of the headbox on the basis of a total production target set for the production after the wire section or the total production after the wire section and a target value for the total retention of the wire section.

12. A method according to claim 2, further comprising: determining a target value for the total consistency of the white water on the basis of a total production target set for the production after the wire section or the total production after the wire section and a target value for the total retention of the wire section.

13. A method according to claim 1, wherein the retention of the wire section to be determined in the paper machine represents a filler retention of the wire section.

14. A method according to claim 13, wherein the mass flow of the paper web to be determined represents the filler mass flow of the paper web on the wire section and it is determined on the basis of the amount of filler in the paper web.

15. A method according to claim 14, comprising: determining the mass flow of the white water, the mass flow of the white water to be determined representing a filler mass flow of the white water determined on the basis of the headbox flow rate or the white water flow rate and the filler consistency of the white water.

16. A method according to claim 15, further comprising:

determining a filler mass flow of the headbox on the basis of the filler mass flow of the paper web and the filler mass flow of the white water;

determining a filler consistency of the headbox on the basis of the filler mass flow of the headbox; and by

determining a filler retention of the wire section on the basis of the filler consistency of the headbox.

17. A method according to claim 14, comprising: determining the mass flow of the headbox, the headbox mass flow to be determined representing the filler mass flow of the headbox determined on the basis of the headbox flow rate and the filler consistency of the headbox.

18. A method according to claim 17, further comprising:

determining the filler mass flow of the white water on the basis of the filler mass flow of the paper web and the filler mass flow of the headbox;

determining the filler consistency of the white water on the basis of the filler mass flow of the white water; and by

determining the filler retention of the wire section on the basis of the filler consistency of the white water.

19. A method according to claim 14, comprising: determining both the mass flow of the headbox and the mass flow of the white water, the headbox mass flow to be determined representing the filler mass flow of the headbox and the white water mass flow to be determined representing the filler mass flow of the white water.

20. A method according to claim 19, comprising: determining the filler mass flow of the white water on the basis of the headbox flow rate or on the basis of the white water flow rate and the measured white water filler consistency and by determining the headbox filler mass flow on the basis of the headbox flow rate and the measured filler consistency of the headbox.

21. A method according to claim 19, further comprising:

determining the filler mass flow of the headbox on the basis of the filler mass flow of the paper web and the filler mass flow of the white water;

determining the filler consistency of the headbox on the basis of the filler mass flow of the headbox;

determining the filler mass flow of the white water on the basis of the filler mass flow of the paper web and the filler mass flow of the headbox;

determining the filler consistency of the white water on the basis of the filler mass flow of the white water; and by

determining the filler retention of the wire section on the basis of the filler consistency of the headbox and the filler consistency of the white water.

22. A method according to claim 13, further comprising: determining a target value for the filler consistency of the headbox on the basis of a production target set for the filler production after the wire section or on the basis of the filler production after the wire section and the target value for the filler retention of the wire section.

23. A method according to claim 13, further comprising: determining a target value for the filler consistency of the white water on the basis of the production target set for the filler production after the wire section or on the basis of the filler production after the wire section and the target value for the filler retention of the wire section.

24. A method for adjusting the retention of a wire section in a paper machine, in which method the retention of the wire section is adjusted by controlling the flow rate of a retention agent, comprising:

determining the mass flow of a paper web on the wire section by means of paper web properties measured after the wire section;

determining the mass flow of a headbox and/or the mass flow of white water;

determining the retention of the wire section on the basis of the mass flow of the paper web and the mass flow of the headbox and/or on the basis of the mass flow of the white water; and

adjusting the retention of the wire section by controlling the flow rate of the retention agent on the basis of the determined retention of the wire section.

25. An apparatus for determining the retention of a wire section in a paper machine, the apparatus comprising:

means for determining the mass flow of a paper web on the wire section by means of the paper web properties measured after the wire section;

means for determining the mass flow of a headbox and/or the mass flow of white water; and

means for determining the retention of the wire section on the basis of the mass flow of the paper web and on the basis of the mass flow of the headbox and/or the mass flow of the white water.

26. An apparatus according to claim 25, wherein the apparatus is configured to determine the total retention of the wire section.

27. An apparatus according to claim 26, wherein the mass flow of the paper web to be determined represents the total mass flow of the paper web on the wire section, the headbox mass flow to be determined represents the total mass flow of the headbox and/or the white water mass flow to be determined represents the total mass flow of the white water.

28. An apparatus according to claim 27, wherein the apparatus is configured to determine the total mass flow of the paper web on the basis of the production corresponding to the dry weight of the paper web.

29. An apparatus according to claim 27, wherein the apparatus is configured to determine the total mass flow of the headbox on the basis of the flow rate of the headbox and the total consistency of the headbox and/or the apparatus is configured to determine the total mass flow of the white water on the basis of the headbox flow rate or the white water flow rate and the total consistency of the white water.

30. An apparatus according to claim 29, wherein the apparatus further comprises

means for determining the total mass flow of the headbox on the basis of the total mass flow of the paper web and the total mass flow of the white water;

means for determining the total consistency of the headbox on the basis of the total mass flow of the headbox; and

means for determining the total retention of the wire section on the basis of the total consistency of the headbox.

31. An apparatus according to claim 29, wherein the apparatus further comprises

means for determining the total mass flow of the white water on the basis of the total mass flow of the paper web and the total mass flow of the headbox;

means for determining the total consistency of the white water on the basis of the total mass flow of the white water; and

means for determining the total retention of the wire section on the basis of the total consistency of the white water.

32. An apparatus according to claim 29, wherein the apparatus further comprises

means for determining the total mass flow of the headbox on the basis of the total mass flow of the paper web and the total mass flow of the white water;

means for determining the total consistency of the headbox on the basis of the total mass flow of the headbox;

means for determining the total mass flow of the white water on the basis of the total mass flow of the paper web and the total mass flow of the headbox;

means for determining the total consistency of the white water on the basis of the total mass flow of the white water; and

means for determining the total retention of the wire section on the basis of the total consistency of the headbox and the total consistency of the white water.

33. An apparatus according to claim 26, wherein the apparatus further comprises means for determining a target value for the total consistency of the headbox on the basis of a total production target set for the production after the wire section or the total production after the wire section and a target value for the total retention of the wire section.

34. An apparatus according to claim 26, wherein the apparatus further comprises means for determining a target value for the total consistency of the white water on the basis of the total production target for the production after the wire section or the total production after the wire section and the target value for the total retention of the wire section.

35. An apparatus according to claim 25, wherein the apparatus is configured to determine the filler retention of the wire section.

36. An apparatus according to claim 35, wherein the mass flow of the paper web to be determined represents a filler mass flow of the paper web on the wire section, the headbox mass flow to be determined represents a filler mass flow of the headbox and/or the mass flow of the white water to be determined represents a filler mass flow of the white water.

37. An apparatus according to claim 36, wherein the apparatus is configured to determine the filler mass flow of the paper web on the basis of the amount of the filler in the paper web.

38. An apparatus according to claim 36, wherein the apparatus is configured to determine the filler mass flow of the headbox on the basis of the headbox flow rate and the filler consistency of the headbox and/or that the apparatus is configured to determine the filler mass flow of the white water on the basis of the headbox flow rate or the white water flow rate and the filler consistency of the white water.

39. An apparatus according to claim 38, wherein the apparatus further comprises

means for determining the filler mass flow of the headbox on the basis of the filler mass flow of the paper web and the filler mass flow of the white water;

means for determining the filler consistency of the headbox on the basis of the filler mass flow of the headbox; and

means for determining the filler retention of the wire section on the basis of the filler consistency of the headbox.

40. An apparatus according to claim 38, the apparatus further comprising

means for determining the filler mass flow of the white water on the basis of the filler mass flow of the paper web and the filler mass flow of the headbox;

means for determining the filler consistency of the white water on the basis of the filler mass flow of the white water; and

means for determining the filler retention of the wire section on the basis of the filler consistency of the white water.

41. An apparatus according to claim 38, the apparatus further comprising:

means for determining the filler mass flow of the headbox on the basis of the filler mass flow of the paper web and the filler mass flow of the white water;

means for determining the filler consistency of the headbox on the basis of the filler mass flow of the headbox;

means for determining the filler mass flow of the white water on the basis of the filler mass flow of the paper web and the filler mass flow of the headbox;

means for determining the filler consistency of the white water on the basis of the filler mass flow of the white water; and

means for determining the filler retention of the wire section on the basis of the filler consistency of the headbox and the filler consistency of the white water.

42. An apparatus according to claim 35, the apparatus further comprising means for determining a target value for the filler consistency of the headbox on the basis of a production target set for the filler production after the wire section or on the basis of the filler production after the wire section and the target value for the filler retention of the wire section.

43. An apparatus according to claim 35, the apparatus further comprising means for determining a target value for the filler consistency of the white water on the basis of the production target set for the filler production after the wire section or on the basis of the filler production after the wire section and the target value for the filler retention of the wire section.

44. An apparatus for adjusting the retention of a wire section in a paper machine, the apparatus comprising:

means for determining the mass flow of a paper web on the wire section by means of paper web properties measured after the wire section;

means for determining the mass flow of a headbox and/or the mass flow of white water;

means for determining the retention of the wire section on the basis of the mass flow of the paper web and the mass flow of the headbox and/or the mass flow of the white water, and

means for adjusting the retention of the wire section by controlling the retention agent flow on the basis of the determined retention of the wire section.