Patent Publication Number: US-6210529-B1

Title: Method for regulating the surface level and consistency in a tank for metering component stock

Description:
FIELD OF THE INVENTION 
     The present invention relates to a method for regulating the surface level and consistency in a stock chest for metering a component stock wherein the component stock is fed as an outward flow out of the bottom portion of a storage tower by a pump into the stock chest, a first dilution water flow is passed into the outward flow to thereby regulate the consistency of the component stock fed into the stock chest to a desired level, the component stock is stirred in the stock chest in order to obtain a uniform consistency and the component stock is then fed as a metering flow from the stock chest by another pump into the short circulation of the paper or board machine. 
     BACKGROUND OF THE INVENTION 
     Regarding its principal features, the stock feed at a paper machine is generally 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 the stock chests 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 and serving as dilution water. From the wire pit, the stock is fed through one or more centrifugal cleaners into a deaeration tank. From the deaeration tank, stock free from air is fed through a machine screen into the headbox, i.e., into the inlet header thereof, and through the slice opening of the headbox to the wire section. A bypass flow of the headbox is fed back into the deaeration tank, and the white water recovered from the wire section 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, usually 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 so-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 dosing 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. 
     OBJECTS AND SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a new and improved method for regulating the surface level and consistency in a stock chest for metering a component stock. 
     It is another object of the present invention to provide a method for regulating the surface level and consistency in a stock chest for metering a component stock in which attempts are made to maintain a substantially constant surface level in the stock chest constantly and to maintain the stock constantly at the desired constant consistency throughout the entire stock chest. 
     In order to achieve these objects and others, a method for regulating a surface level and consistency of stock in a stock chest in accordance with the invention comprises the steps of directing a flow of component stock from a bottom portion of a storage tower into the stock chest, directing a first flow of dilution water into the flow of component stock before the flow of component stock enters into the stock chest to mix with the component stock, controlling the surface level of stock in the stock chest by directing an adjustable amount of stock removed from the stock chest as a return flow into the bottom portion of the storage tower to mix with the component stock in the storage tower, and regulating the consistency of the component stock in the bottom portion of the storage tower by directing a variable second flow of dilution water into the return flow of stock from the stock chest. The component stock in the bottom portion of the storage tower is preferably mixed to thereby provide the component stock with a uniform consistency in the bottom portion of the storage tower. 
     To control the surface level in the stock chest, a pumping tank may be arranged to receive overflow from the stock chest, and the adjustable amount of stock pumped from the pumping tank into the storage tower via a pump. The surface level in the stock chest may be controlled to be substantially constant. 
     The flow of component stock may be directed from the bottom portion of the storage tower in the stock chest by passing the component stock from the bottom portion of the storage tower into an outlet line, and arranging a pump to receive the component stock from the outlet line and direct the component stock through a feed line into the stock chest. The first flow of dilution water is thus directed into the outlet line. 
     In some embodiments, the consistency of the mixed component stock and first flow of dilution water is measured before the stock chest and the first flow of dilution water being directed into the flow of component stock is regulated, e.g., its flow rate or quantity, based on the measured consistency. 
     A pump may be arranged to direct the mixed flow of component stock and first flow of dilution water into the stock chest, the consistency of the mixed flow of component stock and first flow of dilution water measured after the pump and before the stock chest and a flow property of the mixed first flow of component stock and first flow of dilution water, e.g., flow rate or quantity, measured after the pump and before the stock chest. A flow property of the first flow of dilution water is also measured before the first flow of dilution water is directed into the flow of component stock, and then, the first flow of dilution water being directed into the flow of component stock may be regulated based on the measured consistency and flow property of the mixed flow of component stock and first flow of dilution water and the measured flow property of the first flow of dilution water. 
     In another embodiment, a flow property of the first flow of dilution water is measured before the first flow of dilution water is directed into the flow of component stock, and the first flow of dilution water into the flow of component stock is regulated based at least in part thereon. A pump can be arranged to pump stock from the stock chest to a short circulation of a paper machine and the flow of the stock being pumped from the stock chest measured. The second flow of dilution water can then be regulated based on the measured flow property of the first flow of dilution water and the measured flow property of the stock being pumped from the stock chest to the short circulation of the paper machine. Optionally, the second flow of dilution water is also regulated in consideration of any difference between an amount of water in the stock being pumped from the stock chest to the short circulation of the paper machine and an amount of water entering into the stock chest in the mixed flow of component stock and first flow of dilution water. 
     In another embodiment, a pump pumps stock from the stock chest to the short circulation of a paper or board machine and this flow of the stock is measured. The flow of component stock from the bottom portion of the storage tower is regulated to be larger than the measured flow of stock from the stock chest by a substantially constant amount. Optionally, a flow property of the return flow is measured and the flow of stock from the storage tower is regulated by a flow controller in accordance with a set value based on the measured flow of stock from the stock chest to the short circulation of the paper machine and the measured flow property of the return flow. 
     In yet another embodiment, the surface level of stock in the stock chest is controlled by arranging a pumping tank to receive overflow from the stock chest, the return flow of stock from the stock chest being directed from the pumping tank into the storage tower, measuring the surface level of stock in the pumping tank, and regulating the return flow of stock from the pumping tank into the bottom portion of the storage tower based on the measured surface level of stock in the pumping tank such that the surface level of stock in the pumping tank is maintained substantially constant. Optionally, a pumping tank is arranged to receive overflow from the stock chest, the return flow of stock from the stock chest being directed from the pumping tank into the storage tower and the return flow of stock from the pumping tank into the bottom portion of the storage tower regulated by means of a flow controller in accordance with a set value based on the measured surface level of stock in the pumping tank such that when the surface level of stock in the pumping tank rises, the return flow increases and when the surface level of stock in the pumping tank decreases, the return flow is reduced. 
     In process solutions in which a blend chest/machine chest arrangement is not 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 is a constant consistency and a constant pressure all the time. By means of the method in accordance with the present invention, a constant consistency and a constant pressure are reliably obtained in the stock chest. 
     The method in accordance with the invention can also be used in conventional process arrangements for stock feed in which a blend chest/machine chest arrangement is used. 
     With respect to a novel process arrangement related to the method in accordance with the present invention, reference is made to the current assignee&#39;s Finnish Patent Application No. 981327. 
     With respect to regulating the basis weight applicable in the novel process arrangement related to the method in accordance with the present invention, reference is made to the current assignee&#39;s Finnish Patent Application No. 981329. 
     The invention will be described in detail with reference to some preferred embodiments of the invention illustrated in the figures in the accompanying drawing. However, the invention is not confined to the illustrated embodiments alone. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Additional objects of the invention will be apparent from the following description of the preferred embodiment thereof taken in conjunction with the accompanying non-limiting drawings, in which: 
     FIG. 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 constant values; 
     FIG. 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 constant values can be applied; 
     FIG. 3 shows a modification of the process arrangement shown in FIG. 2; 
     FIG. 4 shows a second modification of the process arrangement shown in FIG. 2; and 
     FIG. 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 consistency in the stock chest can be maintained at constant values. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIGS. 1-5 wherein like reference numerals refer to the same or similar elements, FIG. 1 is a schematic illustration of a conventional prior art process arrangement of the stock feed in a paper machine. Only one component stock is shown in FIG.  1  and the recovery of fibers, 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, a component stock M 1  is fed from a storage tower  10  by means of a first pump  11  into a stock chest  20 . A dilution water flow is passed through a regulation valve  18  to mix with the component stock before a 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 to the bottom portion. From the stock chest  20 , the component stock M 1  is directed by means of a second pump  21  through a regulation valve  22  and through a feed pipe  23  to a main line  60  of the process, which passes into a blend chest  30 . From the blend chest  30 , the stock is directed by means of a third pump  31  into a machine chest  40 . From the machine chest  40 , the machine stock M T  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 enabled. 
     The metering of the component stocks M i  into the blend chest  30  takes place so that attempts are made constantly to keep a substantially constant 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 Q tot  of stock to be metered, which information is fed to the component stock metering-control block  25 . Also, a pre-determined stock proportion value K Qi  of the component stock M i  and a consistency value Cs i  of the component stock M i  are fed to the metering-control block  25 . 
     Based on the total requirement Q tot  of stock M T  and the pre-determined proportions K Qi  of component stocks, the metering-control block  25  computes the requirement Q i  of feed of component stock. Based on the component stock feed requirement Q i  and on the data Cs i  on the consistency of the component stock M i , the component stock metering-control block  25  computes the flow target F i  of the component stock M i . Based on this flow target F i , the regulation valve  22  is controlled so as to produce the flow F i  into the blend chest  30 . The flow F i  of the component stock M i  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 a substantially constant 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 chest. 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 typically 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 directed, which detector AT is connected to the pressure side of the pump  31 . The measurement signal Cs T  of the consistency detector AT, measured either after the third pump  31  or after the fourth pump  41 , is passed to a basis weight controller  50 . 
     The regulation of the basis weight takes place so that the basis weight controller  50  controls the 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 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 arrangements, generally 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 also 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 substantially constant. 
     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 constant levels. 
     In FIG. 2, each component stock M i  is fed from a respective stock chest  20   i  by means of a pump  21   i  through a component stock feed pipe  23   i  into a feed line  100  between the deaeration tank  200  and a first pump  110  in the main line of the process. The first pump  110  in the main line directs or 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 F 40  to atmospheric pressure. 
     In the deaeration tank  200 , there could be an air space subjected to a vacuum above the free surface of the stock to thereby cause the removal of air from the white water. Also, in the screen  115 , for example, shivers and debris can be removed from the stock, and in a centrifugal cleaner  120 , for example, sand and other particles heavier than fibers can be removed from the stock. 
     The component stocks M i  are metered from component stock chests  20   i  precisely to the mixing volume of the stocks in the dilution water feed pipe  100  coming from the deaeration tank  200 . The dilution water feed pipe  100  defines a closed space in which the component stocks M i  are mixed and diluted with the flow of dilution water from the deaeration tank  200  (the deaerated white water constituting the dilution water in this case). The precise, substantially constant pressure of the component stock to be metered is produced so that the surface level and the consistency in the component stock chest  20   i  are kept substantially constant and so that a substantially constant back pressure is arranged at the mixing point of the component stocks M i . A precise, constant pressure of the mixing volume is produced so that a sufficient reduction in pressure occurs between the nozzle of the component stock M 1  and the mixing volume, in which case, changes of pressure in the mixing volume do not interfere with the metering. The mixing volume is comprised of the dilution water feed pipe  100  passing to the first feed pump  110 , the feed pipes  23   i  of the metering pumps  21   i  and connection arrangements between them. 
     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 M i  are fed into the feed line  100  between the deaeration tank  200  and the first pump  110  in the main line. In the deaeration tank  200 , the surface level is kept substantially constant by means of a surface level controller of the primary side (not shown in FIG.  2 ), 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 a constant pressure, in which case, the feed pressure of the dilution water flow F 10  remains constant. This secures a substantially constant back pressure for the component stocks M i  when they are fed into the feed line  100 . By means of the first pump  110  in the main line, a substantially constant 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 F 20  of substantially constant 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 F 30  is fed from the deaeration tank  200  to the dilution headbox  150  by means of a dilution water feed pump  180  through a screen  190 . By means of this third dilution water flow F 30  passed into the dilution headbox  150 , the stock consistency is profiled in the cross direction of the paper machine. 
     FIG. 3 illustrates a modification of the process arrangement shown in FIG. 2, in which modification, the deaeration tank  200  is situated 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 (white water from which air is removed) is fed by means of the circulation water pump  170  into the first F 10  and second F 20  dilution stages in the main line of the process. Further, into the dilution headbox  150 , a third dilution water flow F 30  is optionally fed by means of a dilution water feed pump  180  through a screen  190 . In the first F 10  and second F 20  dilution water flows, a substantially constant 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 F 40  between the wire section  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 a valve  201  provided in the line passing from the wire section  160  to the deaeration tank  200 . In this manner, the surface level in the deaeration tank  200  is maintained at a substantially constant 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 come into 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 F 10  and second F 20  dilution stages in the main line of the process. 
     The method in accordance with the invention for maintaining the surface level and consistency in the stock chest at constant values can, of course, also be applied in connection with the process arrangements illustrated in FIGS. 3 and 4. 
     FIGS. 2-4 illustrate arrangements 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  would not be required. 
     The main line screen  115  and the centrifugal cleaner  120  in the embodiments shown in FIGS. 2-4 can comprise one or more stages. 
     The first feed pump  110 , the screen  115 , and the centrifugal cleaner  120  in the main line in the embodiments shown in FIGS. 2-4 can be omitted completely in a situation in which the component stocks M i  have already been cleaned to a sufficiently high level of purity before the stock chests  20   i . In such a case, in the main line of the process, only the feed pump  130  and the following machine screen  140  would be needed. 
     FIG. 5 is a schematic illustration of a process arrangement in accordance with the invention by whose means the stock surface level S 20  in the stock chest  20  and the stock consistency Cs 20  in the stock chest  20  are regulated. The component stock M 1  is fed from a bottom portion  10   a  of the storage tower  10  by means of a first pump  11  as a flow F 11  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 F 13  to a pumping tank  20   a , from which the component stock M 1  is fed by means of a second pump  12  as a flow F 12  into the bottom portion  10   a  of the storage tower  10 . 
     A first dilution water flow F 15  is fed into the first outlet line  13   a  passing to the suction side of the first pump  11 . By means of the dilution water flow F 15 , the stock flow F 11  fed by means of the first pump  11  from the outlet line  13   a  into the stock chest  20  along the first feed line  13   b  is diluted to the desired consistency. On the other hand, a second dilution water flow F 16  is directed into a second feed line  14   b  passing from the pressure side of the second pump  12  into the bottom portion  10   a  of the storage tower  10 . By means of the dilution water flow F 16 , a constant consistency Cs 10a  is maintained in the bottom portion  10   a  of the storage tower  10 . 
     The storage tower of the component stock M 1  is a large storage tower  10  of, for example, about 1000 cubic meters, in which the consistency Cs 10b  in the upper portion  10   b  of the column is typically about 10% to about 14%. New stock is fed (not shown in FIG. 5) an the upper portion  10   b  of the storage tower  10 , and the consistency Cs 10a  in the bottom portion  10   a  of the storage tower  10  is lowered to a level of about 4% by means of recirculation of stock and addition of dilution water (not shown. In the bottom portion of the storage tower  10 , there is also mixing means such as a first mixing equipment S 10 , by whose means the stock present in the bottom portion  10   a  of the storage tower  10  is maintained at a substantially constant consistency. 
     The quantity of the stock flow F 11  pumped by means of the first pump  11  is measured in the first feed line  13   b  at the point C, and this amount is regulated to the desired level by means of a second flow controller FIC 2  connected with the first pump  11 . This second flow controller FIC 2  obtains its set value in a way which will be described later. The second flow controller FIC 2  computes the speed of rotation of the first pump  11 , and the rev. (revolution) controller SIC 2  regulates the speed of rotation of the first pump  11  to the desired level. 
     In the first feed line  13   b , 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 QIC 1 , it is possible to control the first flow controller FIC 1  directly, by means of which flow controller the first dilution water flow F 15  to be passed to the suction side of the first pump  11  is regulated. It is also possible to employ a more efficient method in which the first consistency controller QIC 1  regulates the ratio of the first dilution water flow F 15  to the stock flow F 11  measured in the first feed line  13   b  at the point C and fed by the first pump  11 . When the stock flow F 11  fed by the first pump  11  is changed, the set value of the first flow controller FIC 1  is also changed, and the first flow controller FIC 1  changes the first dilution water flow F 15  quickly. Thus, the first consistency controller QIC 1  can be tuned to eliminate any variations in consistency coming from the storage tower  10 . 
     The first flow controller FIC 1  receives the flow data F 15  concerning the first dilution water from a measurement point D situated in the feed line of the first dilution water flow and regulates the flow to the desired level by means of a first regulation valve SV1. This regulation eliminates any pressure disturbance occurring in the dilution water line and any problems arising from partial wear of the first regulation valve SV1. 
     In the stock chest  20 , the stock is stirred intensively by mixing means such as a second mixing equipment S 20  in order that a uniform consistency could be achieved for metering. By means of a third pump  21 , the component stock M 1  is fed, in the arrangements 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 M 1  from the stock chest  20 . In such a case, all of the stock in the stock chest  20  should have a uniform consistency, and the feed pipe  21   a  departing from the stock chest  20  to the third pump  21  must be at a uniform feed pressure. 
     The stock level L20 can be maintained at a constant 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 a measurement point F of the fourth level controller LIC 4  is placed in the stock chest  20 , in which case a pumping tank  20   a  is unnecessary. In such a situation, the fourth level controller LIC 4  controls the fourth flow controller FIC 4  connected to the second pump  12 , which flow controller FIC 4  again controls the fourth rev. controller SIC 4  connected with the second pump  12 . The return flow F 12  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  is accomplished in a different way. To wit, from the stock chest  20 , there is an overflow F 13  to the pumping tank  20   a , from which stock is fed by means of the second pump  12  into the bottom portion  10   a  of the storage tower  10 . The stock surface level L4 in the pumping tank  20   a  is measured at the point F in the pumping tank  20   a , and the measurement result can be provided to the fourth surface level controller LIC 4 , which controls the fourth rev. controller SIC 4 , 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  20   a  can be maintained substantially constant. 
     If the surface level L4 of the stock present in the pumping tank  20   a  is allowed to vary within a certain range, the fourth surface level controller LIC 4  can be formed in the following novel manner. 
     The set value SP4 of the fourth flow controller FIC 4  is computed from the formula: 
     
       
         SP4=KO+K1*L4  (1) 
       
     
     wherein 
     L4 is the surface level measured in the pumping tank  20   a , and 
     KO and K1 are constants. 
     When the stock level L4 present in the pumping tank  20   a  rises, the exhaust flow increases correspondingly, The stock flow F 12  produced by the second pump  12  is measured in the second feed line  14   b  at a point I. This measurement data is also fed to the fifth flow controller FFIC  5 , which will be described later. 
     Dilution water is additional directed at a point G into the second feed line  14   b  passing into the bottom portion  10   a  of the storage tower  10  in order to bring the consistency of the stock present in the bottom portion  10   a  of the storage tower  10  to a desired level. This second dilution water flow F 16  is regulated by means of the second flow controller FIC 6  connected with the flow, which controller regulates a sixth regulation valve SV6. A set value SP6 of the sixth flow controller FFIC 6  can be computed or determined based on the flow data relating to the first dilution water flow F 15  and measured at the point D and based on other characteristics representing the process. 
     The set value SP6 of the sixth flow controller FFIC 6  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  10   a  of the storage tower  10  is increased, the first consistency control QIC 1  increases the amount of the first dilution water flow F 15 . In order that the consistency in the bottom portion  10   a  of the storage tower  10  could be lowered to the desired level, the second dilution water flow F 16  must also be increased. 
     Based on this fact, the set value of the sixth flow controller FIC 6  related to the second dilution water flow F 16  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 FIC 1  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 F 1  and the amount of water entering into the circulation from the bottom portion  10   a  of the storage tower  10  in the outward stock flow F 11 , the dilution waters included. 
     The computation or determination of the set value of the second flow controller FIC 2  takes place in the fifth flow controller FFIC 5  in the following manner: 
     The set value SP2 of the stock flow F 11  fed by means of the pump  11  from the bottom portion  10   a  of the storage tower  10  into the stock chest  20  at the point C is computed by means of the equation: 
     
       
         SP2=K1+F(E) 
       
     
     wherein 
     F(E) is the metering flow F 1  measured at the point E, and 
     K1 is a correction term. 
     K1 can be constant, in which case the outward flow F 11 , produced by the first pump  11  into the stock chest  20  is constantly higher by the constant than the metering flow F 1  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 K 1  mentioned above can also be defined, for example, in accordance with the following equation: 
     
       
         K1 n =K1 n−1 +K2*(FSP(I n )−F(I n )) 
       
     
     wherein 
     FSP(I) is the set value of the return flow F 12  at the point I, and 
     F(I) is the factual measured return flow F 12  at the point I. 
     In a situation in which the measured flow value of the stock flow F 12  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 F 11 , for example, in connection with recovery of fibers, which increase or reduction is unknown from the point of view of the control circuit, so that the stock return flow F 12  fed by the second pump  12  remains at the desired value. If the return flow F(I n ) of the second pump  12  measured at the point I is higher than the set value FSP(I n ) of the return flow of the second pump  12 , the correction term K1 reduces the stock flow F 11  fed by the first pump  11  until an equilibrium is reached, and vice versa. 
     In the embodiment described above, at the pumps  11 ,  12  and  13 , regulation of the speed of rotation is employed in order to regulate the stock flows F 11 , F 12  and F 1  produced by the pumps. Instead of regulation of the speed of rotation, for regulation of the stock flows, it is possible to use a regulation valve arranged in connection with each pump. In such a case, the pump revolves at a constant 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 fibers KTO. In grinding, a component stock that is supposed to be ground is passed through a grinder, after which it returns to the first feed line  13   b . The same flow that passes to the grinders returns from the grinders. In recovery of fibers, a component stock, e.g., cellulosic pulp, circulates in recovery of fibers, in which it can be bound with fibers, ashes and fines recovered from zero water by means of a disk filter. In such a case, the flow passing to the recovery of fibers and the flow returning from the recovery to the first feed line  13   b  are not necessarily equally large. 
     Above, some preferred embodiments of the invention have been described, and it is obvious to a person skilled in the art that numerous modifications can be made to these embodiments within the scope of the inventive idea defined in the accompanying patent claims. As such, the examples provided above are not meant to be exclusive. Many other variations of the present invention would be obvious to those skilled in the art, and are contemplated to be within the scope of the appended claims.