The invention concerns a method as defined in the preamble of claim 1 for regulation of the basis weight of paper or board in a paper or board machine.

Regarding its principal features, the stock feed at a paper machine is, as a rule, as follows. The stock components are stored at the paper mill in separate storage towers. From the storage towers the stocks are fed into stock chests, and from them further into a common blend chest, in which the stock components are mixed with each other. From the blend chest the stock is fed into a machine chest, and from the machine chest there is an overflow back into the blend chest.

From the machine chest the stock is fed into a dilution part of the wire pit, in which the stock is diluted with white water recovered from the wire section. From the wire pit the stock is fed through centrifugal cleaners into a deaeration tank, from which the stock free from air is fed through a machine screen into the headbox and through the slice opening of the headbox to the wire part. A bypass flow of the headbox is fed back into the deaeration tank, and the white water recovered from the wire part is fed into the wire pit.

The basis weight and the ash content of the paper are measured on-line right before reeling from a ready, dry paper, as a rule, by means of measurement apparatuses based on beta radiation and x-radiation. Based on this measurement, the basis weight of the paper is regulated, for example, by means of a what is called basis weight valve, by whose means the stock flow after the machine chest is controlled. A second possibility is regulation of the speed of rotation of the pump that feeds stock from the machine chest into the wire pit. The ash content is controlled by dozing of fillers. The basis weight profile of the paper in the

cross direction is obtained when the measurement apparatus is installed to move back and forth across the web.

In prior-art solutions of regulation in a paper machine, the metering of component stocks usually takes place with the aid of the surface level in the blend chest, of the consistency of component stock, and of a pre-determined stock proportion reference. The ash contents of component stocks are not used for controlling the metering of component stocks. The measurement values obtained by means of measurement of basis weight are used in the control of the basis weight valve fitted after the machine chest, but said values are not used for controlling the metering of component stocks.

In prior-art regulation of basis weight, exclusively the overall consistency and the overall flow rate of the stock are controlled. Regulation of basis weight taking place by means of the flow departing from the machine chest is disturbed, among other things, by disturbance in the consistency of the machine stock, which disturbance arises from incomplete mixing of the stock in the blend chest and in the machine chest. The volumes of the blend chest and of the machine chest are considerable, so that the regulation of their surface levels readily starts oscillating, which results in disturbance in the regulation of the basis weight. From the recovery of fibres, ash disturbance arises in the blend chest. The dynamics of the recovery of fibres produce different dynamics and dwell times for a part of the stock. Owing to the large volumes of the machine chest and of the blend chest, a long stilling time is required before the basis weight can be set at the desired level. This is why a change of paper grade is slow.

The principal characteristics of the method in accordance with the invention are disclosed in claim 1.

The method in accordance with the present invention for regulation of the basis weight by means of metering of component stocks is particularly well suited for

process arrangements in which there is no blend chest/machine chest solution that equalizes the pumping and the consistencies. For precise regulation of basis weight, in the method in accordance with the invention the following solutions have been discovered: 'the dilution of component stocks to metering consistency takes place before the component stock stock chest, * the regulation of the basis weight takes place from the component stock stock chest by means of regulation of the flows of component stocks, * the diluting to the headbox consistency takes place in two stages, of which in the first stage there is an invariable flow, and in the second stage the flow is regulated by means of a control signal received from the headbox pressu- re regulation.

The method in accordance with the invention for regulation of the basis weight by means of metering of component stocks can also be used in conventional process arrangements in which a machine chest/blend chest solution is applied. In such a case, the basis weight regulation circuit controls, in parallel, both the traditional basis weight valve or the regulations of the flow of machine stock and the regulation of the metering of component stocks in accordance with the invention.

To the regulation of component stocks in accordance with the invention, the change in surface level computed by the surface level controller of the blend chest is fed as a correction signal, which change in surface level compensates for any disturbance caused by a flow coming from the recovery of fibres and for calibrati- on errors of measurement apparatuses.

The method in accordance with the invention for regulation of the basis weight by means of metering of component stocks permits a considerably simpler process solution, as compared with conventional process solutions. The novel process solution permits very quick change of paper grade, and precise metering of the desired quantity of each component stock is possible. Moreover, by means of the

method in accordance with the invention, more precise control of fibre length, more precise control of ashes, uniform mixing, and easier measurement operations are achieved. Also, the regulations of flow and consistencies of the component stocks can be made precise more readily, because there are fewer regulations of flow and consistency that interfere with each other.

With respect to a novel process arrangement related to the method in accordance with the present invention, reference is made to the applicant's FI Patent Appli- cation No. 981327.

With respect to metering of component stocks applicable in the novel process arrangement related to the method in accordance with the present invention, reference is made to the applicant's FI Patent Application No. 981328.

In the following, some preferred embodiments of the present invention will be described with reference to the figures in the accompanying drawing, the inventi- on being, yet, not supposed to be confined to the details of said illustrations alone.

Figure 1 is a schematic illustration of a prior-art process arrangement for the feed of stock in a paper machine.

Figure 2 is a schematic illustration of a stock feed arrangement, in which the method in accordance with the present invention for regulation of the basis weight of paper by means of metering of component stocks can be applied.

Figure 3 shows a modification of the process arrangement shown in Fig. 2, in which the method in accordance with the invention can also be applied.

Figure 4 shows a second modification of the process arrangement shown in Fig.

2, in which the method in accordance with the invention can also be applied.

Figure 5 is a schematic illustration of regulation of the basis weight of paper by means of metering of component stocks in accordance with the present invention.

Fig. 1 is an illustration of a conventional prior-art process arrangement of the stock feed in a paper machine. In the figure, just one component stock is shown. In the figure, the recovery of fibres, the regulation of the flow of the component stock, or the regulation of the surface level in the stock chest of the component stock have not been illustrated.

In Fig. 1, the component stock Mi is fed from a storage tower 10 by means of a first pump 11 into a stock chest 20. To the component stock, a dilution water flow is passed through a regulation valve 18 into connection with the first pump 11. Further, the component stock is diluted in the bottom portion of the storage tower 10 by means of a dilution water flow 9 passed into said bottom portion. From the stock chest 20, the component stock M1 is fed by means of a second pump 21 through a regulation valve 22 and through a feed pipe 23 to the main line 60 of the process, which passes into a blend chest 30. From the blend chest 30 the stock is fed by means of a third pump 31 into a machine chest 40. From the machine chest 40 the machine stock MT is fed by means of a fourth pump 41, through a second regulation valve 42, into the short circulation. Moreover, from the machine chest 40, there is an overflow 43 passing back to the blend chest 30. The blend chest 30 and the machine chest 40 form a stock equalizing unit, and in them the stock is diluted to the ultimate metering consistency. Further, by their means, a uniform metering of the machine stock is secured.

The metering of the component stocks Mi into the blend chest 30 takes place so that attempts are made constantly to keep an invariable surface level in the blend chest 30. Based on changes in the surface level in the blend chest 30, which changes are measured by a surface level detector LT, the surface level controller computes the total requirement Qtot of stock to be metered, which information is fed to the component stock metering-control block 25. Also, a pre-determined

stock proportion value KQi of the component stock Mi and a consistency value Csi of the component stock Mi are fed to the metering-control block 25.

Based on the total requirement Qtot of stock MT and on the pre-determined proportions KQi of component stocks, the metering-control block 25 computes the requirement Qi of feed of component stock. Based on the component stock feed requirement Qi and on the data Csi on the consistency of the component stock Mi, the component stock metering-control block 25 computes the flow target Fi of the component stock Mi. Based on this flow target Fi, the regulation valve 22 is controlled so as to produce said flow Fi into the blend chest 30. The flow Fi of the component stock Mi is also measured constantly by means of a flow detector FT, whose measurement signal is fed through the flow controller FC to the component stock control valve 22.

From the blend chest 30, the stock is fed at an invariable flow velocity by means of the third pump 31 into the machine chest 40. At this pumping stage, the consistency of the stock is also regulated to the desired target consistency of the machine stock MT. This is accomplished by means of dilution water, which is fed through the regulation valve 32 to the outlet of the blend chest 30 to the suction side of the third pump 31. By means of the dilution water, the stock present in the blend chest 30, which is, as a rule, at a consistency of about 3.2 %, is diluted to the ultimate metering consistency of about 3 %. To the dilution water regulation valve 32, the measurement signal of a consistency detector AT is fed, which detector AT has been connected to the pressure side of the pump 31. To the basis weight controller, the measurement signal CsT of the consistency detector AT is fed, measured either after the third pump 31 or after the fourth pump 41.

The regulation of the basis weight takes place so that the basis weight controller 50 controls a regulation valve 42 placed after the fourth pump 41. By means of this regulation valve 42, the flow of the stock to be fed into the short circulation is regulated, which flow again affects the basis weight of the paper web obtained

from the paper machine. When the flow is increased, the basis weight becomes higher, and when the flow is reduced, the basis weight becomes lower.

In the basis weight controller 50, changes in the machine speed, and possibly also changes in the consistency of the machine stock, changes in metering of ashes, and changes in retention are taken into account. Based on these parameters, the basis weight regulation computes a target value for the flow of machine stock.

In prior-art solutions, as a rule, it is assumed that, from the area of the short circulation, no disturbance comes that affects the basis weight of the paper web.

In this connection it is assumed that, in the operation of the centrifugal cleaners, the deaeration tank, and of the machine screen, no such changes occur as a result of which stock components of the machine stock would depart from the process.

Likewise, it is assumed that the consistency of the dilution water pumped from the wire pit remains invariable.

Fig. 2 is a schematic illustration of a stock feed arrangement in which the regula- tion of the basis weight of paper by means of metering of component stocks in accordance with the present invention can be applied. Each component stock Mi is fed from its stock chest 20i by means of a pump 21i through a component stock feed pipe 23i into a feed line 100 between the deaeration tank 200 and the first pump 110 in the main line of the process. The first pump 110 in the main line feeds the stock through a screen 115 and through a centrifugal cleaner 120 to the suction side of the second pump 130 in the main line. The second pump 130 in the main line feeds the stock through the machine screen 140 into the headbox 150. The white water recovered from the wire part 160 is fed by means of a circulation water pump 170 into the deaeration tank 200. Any excess white water is passed by means of an overflow F40 to atmospheric pressure.

The component stocks Mi are metered from component stock stock chests 20i precisely to the mixing volume of the stocks in the dilution water feed pipe 100

coming from the deaeration tank 200. A precise invariable pressure of the compo- nent stock to be metered is produced so that the surface level and the consistency in the component stock stock chest 20i are kept invariable and so that an invariab- le back pressure is arranged at the mixing point of the component stocks Mi. A precise invariable pressure in the mixing volume is produced so that a sufficient reduction in pressure occurs between the nozzle of the component stock Mi and the mixing volume, in which case changes of pressure in the mixing volume do not interfere with the metering.

In Fig. 2, the diluting of the stock is carried out in two stages. The dilution of the first stage is carried out at the suction side of the first pump 110 in the main line when the component stocks Mi are fed into the feed line 100 between the deaerati- on tank 200 and the first pump 110 in the main line. In the deaeration tank 200 the surface level is kept invariable by means of a surface level controller of the primary side (not shown in the figure), which controls the speed of rotation of the circulation water pump 170. The flow into the feed line 100 takes place with a ram pressure at an invariable pressure, in which case the feed pressure of the dilution water flow Fio remains invariable. This secures an invariable back pressure for the component stocks Mi when they are fed into the feed line 100. By means of the first pump 110 in the main line, an invariable volume is pumped constantly to stock cleaning 115,120 and to the dilution of the second stage.

The dilution in the second stage is carried out at the suction side of the second feed pump 130 in the main line, to which suction side a second dilution water flow F20 of invariable pressure is passed with a ram pressure from the deaeration tank 200. The regulation of the pressure in the headbox 150 controls the speed of rotation of the second feed pump 130 in the main line.

Further, a third dilution water flow F30 is fed into the dilution headbox 150 from the deaeration tank 200 by means of a dilution water feed pump 180 through a screen 190. By means of this third dilution water flow F30 passed into the dilution

headbox 150, the stock consistency is profiled in the cross direction of the machine.

Fig. 3 illustrates a modification of the process arrangement shown in Fig. 2, in which modification the deaeration tank 200 is placed below the wire section 160. In such a case, the white water can be passed from the wire section 160 directly by means of ram pressure into the deaeration tank 200. From the deaeration tank 200, the dilution water is fed by means of the circulation water pump 170 into the first Flo and second F20 dilution stage in the main line of the process. Further, into the dilution headbox 150, a third dilution water flow is fed by means of a dilution water feed pump 180 through a screen 190. In the first Flo and second F20 dilution water flow, an invariable pressure can be maintained by means of regulation of the speed of rotation of the circulation water pump 170 and/or by means of throttles in the feed lines 100,101. Also in this case, there is an overflow F40 between the wire part 160 and the deaeration tank 200, from which overflow any excess white water is passed to atmospheric pressure. From the deaeration tank 200, the surface level is measured at the point A, and by means of the surface level controller LIC the flow controller FIC is controlled, which controls the valve 201 provided in the line passing from the wire part 160 to the deaeration tank 200. In this way, the surface level in the deaeration tank 200 is kept at an invariable level.

Fig. 4 shows a second modification of the process arrangement shown in Fig. 2, in which modification the deaeration tank 200 has been removed completely. In such a case, the headbox 150 and the wire section 160 must be closed so that the stock does not reach contact with the surrounding air. The white water collected from the closed wire section 160 is then fed directly, by means of the circulation water pump 170, into the first Flo and second F20 dilution stage in the main line of the process.

The method in accordance with the invention can, of course, also be applied in connection with the process arrangements illustrated in Figs. 3 and 4.

In Fig. 2, the feed pipes 23i of the component stocks Mi have been passed directly to the dilution water feed pipe 100. In Figs. 3 and 4, the component stock feed pipes 23i have been passed first into a common pipe, which common pipe has then been passed to the dilution water feed pipe 100. From the point of view of the present invention, the coupling between the component stock Mi feed pipes 23i and the first dilution water feed pipe 100 can be of any kind whatsoever, provided that the mixing together of the component stocks and the mixing of the component stocks with the dilution water can be made efficient.

In Figs. 2 to 4, no bypass flow of stock or dilution water at the inlet header of the headbox 150 has been illustrated. These bypass flows are arranged here by means of short feed-back connections.

Figs. 2 to 4 illustrate a situation in which a dilution headbox is employed, but the invention can also be applied in connection with a headbox of a different sort. In such a case, a second circulation water pump 180 and a related screen 190 are not needed at all.

The main line screen 115 and the centrifugal cleaner 120 shown in Figs. 2 to 4 can comprise one or several stages.

The first feed pump 110, the screen 115, and the centrifugal cleaner 120 shown in the main line in Figs. 2 to 4 can be omitted completely in a situation in which the component stocks Mi have already been cleaned to a sufficiently high level of purity before the stock chests 20i. In such a case, in the main line of the process, just the feed pump 130 and the following machine screen 140 are needed.

Fig. 5 is a schematic illustration of regulation of the basis weight of paper by means of metering of component stocks Mi in accordance with the invention. Where applicable, the reference denotations in the figure correspond to those used in Figs. 2,3 and 4. The figure illustrates the feed of a component stock M1 as a flow F1 by means of a component stock feed pump 211 into the feed line 100 between the deaeration tank (Figs. 2,3) and the first feed pump 110 in the main line of the process. Of the other component stocks M2, M3 just the connections to the feed line 100 are shown. The invention is not confined to three component stocks M1, M2, M3 of which the stock MT is formed, but the number of com- ponent stocks Mi can be Z, wherein Z is a positive integer number 2 2.

It is the starting point of the metering of the component stocks in accordance with the present invention that the volume and the consistency of each component stock Mi are kept constantly invariable in the stock chest 20i. In this respect, reference is made to the applicant's FI Patent Application No. 981328, in which applicati- on a possibility is described for keeping the surface level and the consistency of a component stock Mi in a stock chest 20i at an invariable level.

In the first stage in the regulation process related to the metering of component stocks Mi, the stock proportions Ki of the component stocks Mi are optimized on the basis of fibre lengths FLi in a fibre length optimizing block FLO. A pre- determined target value FLT of fibre length of the machine stock MT and a pre- determined stock proportion reference KQi of one or several component stocks Mi are fed into the fibre length optimizing block FLO. Further, the fibre lengths FLi of component stocks measured from the component stock feed lines 23i are fed into the fibre length optimizing block FLO.

The target value FLT of fibre length of the machine stock MT can be given as one discrete numerical value, or it can be given as the desired distribution of the fibre length in the machine stock MT. In both cases, of course, the fibre length target FLT of the machine stock MT must be such that it can be carried into

effect in general with the distributions of fibre lengths in the available component stocks Mi.

If the target value FLT of the fibre length of the machine stock MT has been given as one discrete numerical value, the average value of the fibre length FLi in the component stock Mi concerned can be calculated from the individual fibre length values FLij obtained from measurement of fibre length of the component stock. From a specimen material comprising, e. g., 10,000 individual fibre length measurements xm, the sample average can be calculated as an arithmetic average from the equation:

wherein N is the number of specimens xm.

The specimen material can also be first classified into classes, e. g. into 144 fibre length classes, after which, from the classified specimen material, the sample average of fibre length of the component stock can be calculated from the equati- on:

wherein y 1,..., yk are mean points of the class gaps, and fi,..., fi. are corres- ponding class frequencies.

Out of the classified specimen material, the weighted sample average of the fibre length of the component stock can also be calculated from the following equation: This arithmetic average, sample average, or weighted sample average determined for each fibre length of a component stock is then used for optimizing the fibre length.

If the target value FLT of the fibre length in the machine stock MT has been given as a distribution, for example, out of the mean points ym of the classified specimen material and out of the class frequencies fm, a distribution of the fibre lengths LFi in the component stocks Mi is formed, which distribution is then used for optimizing the fibre length.

Based on the fibre lengths FLi in the component stocks Mi and on the fibre length target FLT of the stock MT, it is possible to determine an optimal proportion Ki of each stock Mi in the stock MT. With these data, it is possible to form two basic equations: Thus, here the fibre lengths FLi in the component stocks Mi and the fibre length FLT in the machine stock MT can be discrete numbers or distributions of fibre length. If a distribution of fibre length is concerned, an arithmetic summing is, of

course, not possible, but in such a case the fitting is carried out on the basis of areas.

In the case of three component stocks M 1 9 M2 9 M3 illustrated in Fig. 5, we have two equations and three unknown quantities K1, K2, K3, so that a third equation is needed further in order that the unknown quantities could be solved. This third equation can be formed, e. g., on the basis of the prices of the component stocks Mi so that more expensive component stocks Mi are used to a lower extent, and less expensive component stocks Mi are used to a higher extent. The third equation can also be based on availability of the component stocks Mi so that component stocks Mi that are most poorly available are used to a lower extent, and more readily available component stocks Mi are used to a higher extent. The third equation can also be based on the idea that a certain amount of broke must be used, etc. Combined optimizing of cost and availability can also be concerned, etc.

In order that the proportions of component stocks Mi could be solved in a closed form, the number of equations needed is always equal to the number of unknown quantities.

In addition to the limitations mentioned above, each component stock Mi also has a pre-determined minimal value Kimin of stock proportion Ki, below which the regulation circuit cannot go, and a maximal value KimaXX which the regulation circuit cannot surpass.

In cases in which the optimizing in respect of cost or of other parameters cannot be solved for some reason or other, a pre-determined stock proportion KQi of one or several component stocks Mi is employed.

The stock proportion target Ki of each component stock Mi determined in the fibre length optimizing block FLO is, after this, fed into the component stock computing block MQ.

Into the component stock computing block MQ, the stock target On of the stock MT is also fed, which target has been formed at the end of the machine from dry paper based on basis weight measurement. The stock target On determines the amount of fibres desired for the stock MT per unit of time, e. g. kilograms per second (kg/s). When the stock target On of the stock MT and the stock proportion Ki of each component stock are known, the metering target QiT (kg/s) of each component stock can be calculated from the equation: QiT = Ki Q0 After this, the metering target QiT of each component stock Mi is fed to the computing block MFTi of the component stock concerned. The figure shows the computing block MFT1 of the flow target of one component stock M1 only.

Further, the consistency Csi and the ash content RMi of the component stock Mi concerned, measured from the feed line 23i after the feed pump 21i, are fed to the computing block MFTi of the flow target of the component stock Mi. In the computing block MFTi of the flow target of the component stock Mi, it is now possible to compute the flow target Fi of the component stock Mi. First, the fibre proportion CsiFibre of the component stock Mi is determined from the equation: CsiFibre-Csi*100-RMi)/100 and after this the flow target FiT of the component stock Mi (1/s = litres per second) is determined from the equation: FiT = Ri*QiT*100/CsFibre-

Ri is a correction coefficient, by whose means any calibration errors and similar scaling errors are corrected.

The flow target Fi of the component stock Mi is fed to the flow controller FICi, which again controls the rev. controller SIC. of the feed pump 21i of the com- ponent stock Mi. The regulation of flow can be accomplished in the way described above, by directly regulating the speed of rotation of the feed pump 21i of the component stock Mi, or by means of a regulation valve (not shown in the figure) placed after the feed pump 21i, or by means of a combination of these modes. In a pure regulation valve control, the speed of rotation of the feed pump 21i of the component stock Mi is kept invariable, and the regulation of flow takes place exclusively by means of the regulation valve by throttling the flow. In combinati- on regulation, both the speed of rotation of the feed pump 21i of the component stock Mi and the throttle of the regulation valve are regulated.

The ash content RMi and the consistency Csi measured from the component stock Mi are also fed into the control circuit of the machine.

In the embodiment of the method in accordance with the invention described above, the stock proportions Ki of the component stocks Mi are optimized on the basis of the fibre lengths FLi measured from the component stocks Mi. Further, from each feed line 23i of a component stock Mi, both the consistency Csi of the component stock concerned and the ash content RMi of the component stock concerned are measured. By means of this arrangement, the essential parameters related to the stock and affecting the quality of the paper are controlled.

In the method in accordance with the invention, it is not necessary to measure the fibre lengths Fli from the component stocks Mi, but the metering targets QiT of the component stocks Mi can be computed on the basis of the stock proportion target Qn of the basis weight controller and on the basis of pre-determined stock proportions Ki of component stocks Mi. In such a case, of course, some of the

precision of the regulation of the component stocks is lost. In such a case, the fibre lengths of the component stocks are not controlled, which may result in disturbance in the quality of the paper.

Nor is it necessary, in the method in accordance with the present invention, to measure the ash content Csi of each component stock Mi, but from each feed line 23i of component stock Mi, it is possible to measure the consistency Csi of the component stock Mi concerned only. In such a case, in the stock target Qn of the stock MT received from the basis weight controller, the ash content of the stock MT has already been taken into account, in which case the flow target FiT of each component stock Mi can be computed directly on the basis of the metering target QiT and of the measured consistency Csi of the component stock. Also in this alternative, some of the precision of regulation is lost. In such a case, the ash contents of the component stocks are not controlled, which may result in distur- bance in the quality of the paper.

In the following, the patent claims will be given, and different details of the invention can show variation within the scope of the inventive idea defined in said claims and differ from what has been stated above by way of example only.