The invention concerns a method as defined in the preamble of claim 1 for regulation of the surface level and the consistency in a tank for metering of a component stock.

Regarding its principal features, the stock feed at a paper machine is, as a rule, as follows. The component stocks 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 component stocks 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 tank is controlled. A second possibility is regulation of the speed of rotation of the

pump that feeds stock from the machine tank 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.

By means of the method in accordance with the present invention, attempts are made to maintain an invariable surface level in the metering tank constantly and to maintain the stock constantly at the desired invariable consistency throughout the whole of the metering tank.

The principal characteristics of the method in accordance with the invention have been stated in claim 1.

In process solutions in which no blend chest/machine chest solution is employed, the component stocks are fed directly into a mixing volume placed in the main line of the process. In such a case, it is required that, in the component stock stock chest, there are an invariable consistency and an invariable pressure all the time. By means of the method in accordance with the present invention, an invariable consistency and an invariable pressure are secured in the stock chest.

The method in accordance with the invention can also be used in conventional process solutions for stock feed in which a blend chest/machine chest solution is used.

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 regulation of the basis weight 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. 981329.

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 conventional process arrangement for the feed of stock in a paper machine, in connection with which arrangement it is possible to use the method in accordance with the present invention for keeping the surface level and the consistency in a stock chest at invariable values.

Figure 2 is a schematic illustration of a second process arrangement for the feed of stock in a paper machine, in which the method in accordance with the present invention for keeping the surface level and the consistency in a stock chest at invariable values can be applied.

Figure 3 shows a modification of the process arrangement shown in Fig. 2.

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

2.

Figure 5 is a schematic illustration of a process arrangement in accordance with the present invention in which the surface level in the stock chest and the consis- tency in the stock chest can be kept at invariable values.

Fig. 1 is a schematic 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 Mi 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, 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 metering 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 centifugal 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 second process arrangement for the feed of component stocks, in which it is possible to apply the method in accordance with the invention for keeping the surface level and the consistency in the stock chest at invariable levels. 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 section 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. The precise invariable pressure of the component 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 invariable 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 M1 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 F1o 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 to 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 F1o 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 section 160 and the deaeration tank 200, from which overflow any excess wire 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 section 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 Fio and second F20 dilution stage in the main line of the process.

The method in accordance with the invention for keeping the surface level and the consistency in the stock chest at invariable values can, of course, also be applied in connection with the process arrangements illustrated in Figs. 3 and 4.

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 a process arrangement in accordance with the invention, by whose means the stock surface level S20 in the stock chest 20 and the stock consistency Cs20 in the stock chest 20 can be regulated. The component stock Mi is fed from the bottom portion 10a of the storage tower 10 by means of a first pump 11 as a flow Fi 1 into the stock chest 20. From the stock chest 20, component stock is fed by means of a third pump 21 into the main feed line 100 passing into the headbox (Fig. 2,3 and 4). From the stock chest 20, there is an overflow F13 to a pumping tank 20a, from which the component stock M1 is fed by means of a second pump 12, as a flow F12, back into the bottom portion 10a of the storage tower 10.

A first dilution water flow F15 is fed from the bottom portion of the storage tower 10 into the first outlet line 13a passing to the suction side of the first pump 11, by means of which dilution water flow F15 the stock flow F11 fed by means of the first pump 11 from the outlet line 13a into the stock chest 20 along the first feed line 13b is diluted to the desired consistency. On the other hand, a second dilution water flow F16 is fed into the second feed line 14b passing from the pressure side of the second pump 12 into the bottom portion 10a of the storage tower 10, by means of which dilution water flow F16 an invariable consistency CslOa is maintained in the bottom portion 10a of the storage tower 10.

The stock column of the component stock Mi is a large storage tower 10 of, for example, about 1000 cubic metres, in which the consistency Csl0b in the upper portion 10b of the column is typically 10... 14 %. New stock is fed (not shown in the figure) into the upper portion 10b of the storage tower 10, and the consistency CslOa in the bottom portion 10a of the storage tower 10 is lowered to a level of 4 % by means of recirculation of stock and addition of dilution water (not shown in the figure). In the bottom portion of the storage tower 10, there is also a first mixing equipment Solo, by whose means the stock present in the bottom portion 10a of the storage tower 10 is kept at an invariable consistency.

The quantity of the stock flow F11 pumped by means of the first pump 11 is measured in the first feed line 13b at the point C, and said amount is regulated to the desired level by means of a second flow controller FIC2 connected with the first pump. This second flow controller FIC2 obtains its set value in a way which will be described later. The second flow controller FIC2 computes the speed of rotation of the first pump 11, and the rev. controller SIC2 regulates the speed of rotation of the first pump 11 to the desired level.

In the first feed line 13b, at the point B, the consistency of the stock that is fed from the storage tower 10 by means of the first pump 11 into the stock chest 20 is measured. By means of a first consistency controller QIC1, it is possible to control the first flow controller FIC1 directly, by means of which flow controller the first dilution water flow F15 to be passed to the suction side of the first pump 11 is regulated. Here it is also possible to employ a more efficient method, in which the first consistency controller QIC1 regulates the ratio of the first dilution water flow Fie to the stock flow F i i measured in the first feed line 13b at the point C and fed by the first pump 11. When the stock flow F11 fed by the first pump 11 is changed, the set value of the first flow controller FIC1 is also changed, and the first flow controller FIC1 changes the first dilution water flow F15 quickly. Thus, the first consistency controller QIC1 can be tuned to eliminate any variations in consistency coming from the storage tower 10.

The first flow controller FIC1 receives the flow data F15 concerning the first dilution water from the measurement point D placed in the feed line of the first dilution water flow and regulates the flow to the desired level by means of the first regulation valve SV1. This regulation eliminates any pressure disturbance occurring in the dilution water line and any problems arising from wear of the first regulation valve SV1 partially.

In the stock chest 20, the stock is stirred intensively by means of a second mixing equipment S20 in order that a uniform consistency could be achieved for mete- ring. By means of a third pump 21, the component stock Mi is fed, in the situations shown in Figs. 2,3 and 4, into the pipe for mixing of component stocks. In particular a process arrangement in accordance with Figs. 2,3 and 4 requires precise metering of the component stock Mi from the stock chest 20. In such a case, the whole of the stock chest 20 must have a uniform consistency, and the feed pipe 21a departing from the stock chest 20 to the third pump 21 must be at a uniform feed pressure.

The stock level L20 can be kept at an invariable level in the stock chest 20 by means of surface level regulation alone. In such a case, the suction side of the second pump 12 is connected directly to the stock chest 20, and the measurement point F of the fourth level controller LIC4 is placed in the stock chest 20, in which case a pumping tank 20a is unnecessary. In such a situation, the fourth level controller LIC4 controls the fourth flow controller FIC4 connected to the second pump 12, which flow controller FIC4 again controls the fourth rev. controller SIC4 connected with the second pump 12. In such a case, the return flow F12 from the stock chest 20 is regulated directly in compliance with the stock surface level L20 in the stock chest 20.

In Fig. 5, the regulation of the surface level in the stock chest 20 has been taken care of in a different way. From the stock chest 20, there is an overflow F13 to the pumping tank 20a, from which stock is fed by means of the second pump 12

back into the bottom portion 10a of the storage tower 10. The stock surface level L4 in the pumping tank 20a is measured at the point F in the pumping tank 20a, and the measurement result can be fed to the fourth surface level controller LIC4, which controls the fourth rev. controller SIC4, by whose means the speed of rotation of the second pump 12 is regulated. In such a case, the surface level L4 of the stock present in the pumping tank 20a can be kept invariable.

If the surface level L4 of the stock present in the pumping tank 20a is allowed to vary within a certain range, said fourth surface level controller LIC4 can be formed in the novel way described in the following.

The set value SP4 of the fourth flow controller FIC4 is computed from the formula: SP4 = KO + K1*L4 (1) wherein L4 is the surface level measured in the pumping tank 20a, and KO and K1 are constants. When the stock level L4 present in the pumping tank 20a rises, the exhaust flow is increased correspondingly. The stock flow F12 produced by the second pump 12 is measured in the second feed line 14b at the point I. These measurement data are also fed to the fifth flow controller FFIC 5, which will be described later.

Into the second feed line 14b passing into the bottom portion 10a of the storage tower 10, dilution water is additionally fed at the point G in order to bring the consistency of the stock present in the bottom portion 10a of the storage tower 10 to the desired level. This second dilution water flow F16 is regulated by means of the second flow controller FIC6 connected with said flow, which controller regulates the sixth regulation valve SV6. The set value SP6 of the sixth flow controller FFIC6 can be computed based on the flow data concerning the first

dilution water flow F15 and measured at the point D and based on other characte- ristics representing the process.

The set value SP6 of the sixth flow controller FFIC6 can also be determined in an alternative way by using a ratio control as an aid. If the consistency of the stock pumped by means of the first pump 11 from the bottom portion 10a of the storage tower 10 is increased, the first consistency control QIC1 increases the amount of the first dilution water flow F15. In order that the consistency in the bottom portion 10a of the storage tower 10 could be lowered to the desired level, the second dilution water flow F16 must also be increased.

Based on this fact, the set value of the sixth flow controller FIC6 related to the second dilution water flow F16 can be computed from the equation: SP6 = K1*F (E) + K2*F (D) (2) wherein K1 and K2 are empiric constants depending on the point of operation, F (E) is the flow at the point E, and F (D) is the flow at the point D.

The term K2*F (D) helps the first flow controller FIC1 to remain constantly in the range of operation, and by means of the term K1*F (E), consideration is given to the difference between the amount of water departing from the circulation in the stock metering flow F1 and the amount of water entering into the circulation from the bottom portion 10a of the storage tower 10 in the outward stock flow F11, the dilution waters included.

The computing of the set value of the second flow controller FIC2 takes place in the fifth flow controller FFIC5 in the way described in the following:

The set value SP2 of the stock flow Fi 1 fed by means of the pump 11 from the bottom portion 10a of the storage tower 10 into the stock chest 20 at the point C is computed by means of the equation: SP2 = Kl + F (E) wherein F (E) is the metering flow F1 measured at the point E, and K1 is a correction term. Kl can be invariable, in which case the outward flow Fil produced by the first pump 11 into the stock chest 20 is constantly by said invariable higher than the metering flow F1 removed by the third pump 21 from the stock chest 20. In this situation, the second pump 12 returns any excess stock into the storage tower 10.

The correction term K1 mentioned above can also be defined, for example, in accordance with the following equation: Kln = Kln l + K2* (FSP (In)-F (In)) wherein FSP (I) is the set value of the return flow F12 at the point I, and F (I) is the factual measured return flow F12 at the point I. In a situation in which the measured flow value of the stock flow F12 produced by the second pump 12 is lower than the corresponding set value, the set value SP2 of the first pump 11 is increased, and in a contrary case it is reduced. By means of this arrangement, it is possible to take into account an increase or reduction of stock flow occurring in the outward stock flow Fll, for example, in connection with recovery of fibres, which increase or reduction is unknown from the point of view of the control circuit, so that the stock return flow F12 fed by the second pump 12 remains at the desired value. If the return flow F (In) of the second pump 12 measured at the point I is higher than the set value FSP (In) of the return flow of the second pump 12, the correction term K1 reduces the stock flow Fi l fed by the first pump 11 until an equilibrium is reached, and the other way round.

In the embodiment described above, at the pumps 11,12 and 13, regulation of the speed of rotation has been employed in order to regulate the stock flows Fl 1, F12 and Fi produced by said pumps. In stead of regulation of the speed of rotation, for regulation of the stock flows, it is possible to use a regulation valve fitted in connection with each pump. In such a case, the pump revolves at an invariable speed, and the stock flow is regulated by means of a regulation valve, by whose means the stock flow can be throttled. It is also possible to employ both regulation of the speed of rotation of a pump and a regulation valve in order to regulate the stock flows.

In Fig. 5, an allusion has also been made to a possible connection of the outward flow F 11 with grinding JAU and recovery of fibres KTO. In grinding, a com- ponent stock that is supposed to be ground is passed through a grinder, after which it returns to the first feed line 13b. The same flow that passes to the grinders returns from the grinders. In recovery of fibres, a component stock, e. g. cellulosic pulp, circulates in recovery of fibres, in which it can be bound with fibres, ashes and fines recovered from zero water by means of a disk filter. In such a case, the flow passing to the recovery of fibres and the flow returning from the recovery to the first feed line 13b are not necessarily equally large.

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.