METHOD FOR CONTROLLING CONSISTENCY OF PAPERMAKING PULP

Abstract: A procedure for adjusting the consistency of paper pulp, in which procedure: allowed minimum and maximum values are defined for the consistency of head box pulp and/or for the consistency of the tail liquid coming from the wire dehydrators (14, 15), the consistency of the pulp entering the head box (13) is measured by means of measuring element (4) and/or the consistency of the tail liquid coming from the wire dehydrators (14, 15) is measured by means of measuring element (5). Based on the measurement data of at least one consistency measurement and controlled by a control apparatus (8), liquid or pulp of desired consistency is supplied via a consistency regulating liquid line (1) into the pulp line leading to the head box (13) at a point after or before the wire pit (12) or directly into the wire pit (12) until the desired consistency is obtained.

Description: Method for controlling consistency of paperma ing pulp The present invention relates to a procedure for adjusting the consistency of paper pulp, as defined in claim 1. Variations in pulp consistency in the so-called short circulation in a paper machine and the consequent changes in dehydration in the wire section and in the solid matter after the wire section as well as the operation of dehydration in the press section have a significant effect on the runnability of the paper machine. In a prior-art solution, the control of short-circulation pulp consistency is implemented using so-called retention chemicals, based on measuring the consistency of the so-called tail water removed from the wire section of the paper machine. This solution works well in the case of paper machines used to produce only a few paper grades. In paper machines designed for the production of many different paper grades, in which there are large variations in the weight per cm 2 or in the ash percentage of the paper, controlling the profiles and paper quality is particularly difficult due to the continual variations in the fines and ash percentages of the pulp. In conjunction with a change of the paper grade to be produced, depending on the process employed, a large amount of broke may be produced, thus leading to great losses in production. As the retention chemicals also have a pronounced effect on dehydration, runnability of the paper machine, tightness and solids content of the paper web and on paper properties, the amount of chemicals used cannot be allowed to change too fast. For this reason, the use of chemicals for the control of pulp consistency cannot be con- sidered an adequate control mechanism. The object of the present invention is to achieve a completely new type of solution for the adjustment of short-circulation pulp consistency. The invention is based on an idea of diluting the pulp in the head box in short circulation using consistency regulating water that is considerably weaker in consistency than the tail water and the head box pulp. In addition to the viscous pulp and tail water, either circulation water of the paper machine, clear or turbid filtrate or fresh water is supplied into the pulp line before the wire pit, into the wire well or into the pulp line after the wire well. The diluting water is dosed into the head box pulp based on measurement of the consistency of the head box pulp (or tail water). The excess of tail water displaced by the diluting water is passed into long circulation for fibre recovery. More explicitly defined, the invention is characterized by what is presented in the claims. The invention has numerous significant advantages. The procedure eliminates the variations in the consistency of head box pulp, thus allowing more stable dehydration in the wire section. The new control procedure makes it possible to adjust the fines content to a constant level in relation to the long fibre fraction, thus improving the strength and printability properties of the paper. Moreover, the new control procedure considerably reduces the variation in the dosage of chemicals. Because of the above-mentioned circumstances, the runnability and productive capacity of the paper machine are improved. In the following, the invention will be described by referring to the attached drawings, in which Fig. 1 presents a simplified diagram of an arrangement according to the invention, Fig. 2 presents a simplified diagram of another arrangement according to the invention, Fig. 3 presents a simplified diagram of yet another arrangement according to the invention. Of the parts comprised in the short circulation in a paper machine, Fig. 1 depicts the head box 13 and the wire section in a schematic representation. The thicker solid lines in the figure represent connections used to convey liquids or pulp while the thinner or broken lines represent the routes of transmission of the signals needed for data transfer and control. The dehydrators 14, 15 in the wire section are connected to the so-called wire pit 12 or a similar tank. The dehydration lines are provided with measuring devices 5 for the measurement of consistency. The pulp supply line leads from a mixing tank 16 or the like into the wire pit 12 and further into the head box 13. Disposed in the pulp supply line after the wire pit 12 are a chemical dosage system 11 and a consistency measuring device 4 for the measurement of the consistency of head box pulp. The equipment associated with the procedure comprises a consistency regu- lating water line 1 , which in the case illustrated by Fig. 1 is connected to the head box pulp line after the wire pit 12. Alternatively, the consistency regulating water line can be connected to the wire pit 12 of the paper machine or to the viscous pulp line before the wire pit. The consistency regulating liquid line 1 can also be e.g. a wire pit filling/level control pipe or a separate pipe leading into the wire pit 12 (Fig. 2) or into the viscous pulp line (Fig. 3), through which a supply of viscous pulp, head box pulp, filler or water of a lower consistency than tail water can be mixed with the pulp/water flowing into the head box 13. The aim is to achieve a desired con- sistency of head box pulp by combining liquids or pulps having different degrees of consistency. The equipment also comprises shutter/regulating elements in the consistency regulating water line 1 , such as an automatic valve 2, to regulate the water flow, and a measuring device, such as a flow meter 3, for measuring the liquid flow. Disposed in the pulp line leading into the head box is a consistency meter 4 to supply consistency measurement data to the consistency control system. In addition, the apparatus comprises a diluting water line 6 connected to the consistency regulating water line 1 , and a consistency me-ter 7 placed after the diluting water line. Via the diluting water line 6, it is possible to adjust the consistency of the consistency regulating liquid. The sys- tern comprises a control apparatus 8, such as the paper machine's control system, logic or a separate computer such as a PC, to control automatic valve 2 on the basis of consistency measurement 4 or 5 or automatic valve 9 in the diluting water line 6 on the basis of consistency measurement 7. Moreover, the wire pit 12 is provided with a spill pipe 10, through which a volume flow corresponding to the liquid quantity required for consistency control is passed out of the short circulation. The control apparatus 8 controls the use of diluting water as well as the chemical dosage system 11. In the procedure of the invention for adjusting the consistency of paper pulp, target consistency values (maximum and minimum) are defined for the head box pulp and/or the tail liquid coming from the dehydrators 14, 15, the consis- tency of the pulp entering the head box 13 is measured by means of measuring element 4 and/or the consistency of the tail liquid coming from the dehydrators 14, 15 is measured by means of measuring element 5, and, based on the measurement data of at least one consistency measurement and controlled by the control apparatus 8, liquid or pulp of desired consistency is supplied via the consistency regulating liquid line 1 into the pulp line leading to the head box 13 at a point after or before the wire pit 12 or directly into the wire pit 12 until the desired consistency is obtained. In addition to these steps, adjustment by means of chemicals is used. The flow of consistency regulating liquid is measured by means of a measuring device 3 connected to the consistency regulating liquid line 1. The consistency of the liquid used for consistency control is measured by means of another measuring device 7. Consistency control is effected using at least three components having different consistency values, such as viscous pulp, tail water and circulation water of the paper machine. The volume flow corresponding to the amount of material needed for consistency control is passed out of the short circulation e.g. via the spill pipe 10 of the wire pit 12. The primary controlled variable used for adjusting the consistency of head box pulp is a consistency regulating liquid consisting of liquid, pulp or filler having a consistency higher or lower than that of the head box pulp, and the secondary controlled variable used is retention chemical control. Controlled by the control apparatus 8, the dosage of chemicals is adjusted based on measurement of the consumption of consistency regulating liquid. The chemical dosage is not changed until the consumption of consistency regulating liquid has risen above/fallen below a very wide consumption range (e.g. 30 - 200 l/s). In the following, the invention will be described by the aid of a preferred embodiment. If consistency measurement 4 indicates that consistency of the head box pulp differs from the target values, an automatic valve 2 in the consistency regulating water line 1 is opened and, based on control data from the control apparatus 8, admits a flow of a liquid having a different consistency into the head box pulp. If the consistency of head box pulp exceeds the target value, then dilution is effected using a liquid with a consistency below that of the head box pulp, such as raw water, clear or turbid filtrate from filter devices such as disc filters or the white water of the paper machine. On the other hand, if the consistency of head box pulp is below the target value, the consistency can be adjusted by using so-called viscous pulp, which has a consistency higher than that of the head box pulp, or a filler. According to another preferred embodiment, consistency control can be implemented based on measurement 5 of tail water consistency. If the consistency measurement 5 indicates that the tail water consistency differs from the target values, an automatic valve 2 in the consistency regulating water line 1 is opened and, based on control data from the control apparatus 8 and on flow measurement, admits a flow of a liquid having a different consistency into the head box pulp. If the tail water consistency exceeds the target value, then dilution is effected using a liquid of a consistency below that of the tail water, such as raw water, clear or turbid filtrate from disc filters or the white water of the pa- per machine. On the other hand, if the consistency of tail water is below the target value, adjustment of the consistency can be effected using so-called viscous pulp, which has a consistency higher than that of the tail water, or filler. The control apparatus 8 preferably also controls the chemical dosage system 11 , such as e.g. a retention chemical dosage system. In a consistency control system according to the invention, the consistency regulating liquid is a primary controlled variable and the retention chemical is a secondary controlled variable. Dosage limits are defined for the consistency regulating liquid, e.g. minimum 0 l/s and maximum 250 l/s. In addition, chemical dosage limits are defined for the consistency regulating liquid, for instance minimum 30 l/s and maximum 200 l/s. When the flow of consistency regulating water as indicated by the flow measurement 3 is e.g. 30 l/s or below, the control apparatus 8 reduces the retention chemical dosage 11 e.g. by 20 g/t. As a result, as the solids retention deteriorates, the consistencies of tail water and head box pulp tend to rise and the consumption of consistency regulating water to rise again over 30 l/s. Correspondingly, if the consumption of consistency regulating liquid is above 200 l/s, the control program adds e.g. 20 g/t, causing the consistencies of tail water and head box pulp to fall and the consumption of consistency regulating liquid to fall under 200 l/s. When the consumption of consistency regulating liquid is above or below the chemical dosage limits, the control system increases or decreases the chemical dosage until the consumption of consistency regulating water returns to a level within the chemical dosage limits. If the consistency of the liquid used for consistency control in the consistency regulating water line 1 is to be adjusted, diluting water of a different consistency can be supplied from the diluting water line 6 and mixed with the consistency regulating water, controlled by the control apparatus 8, which controls an automatic valve 9 on the basis of measurement data obtained from a consistency meter 7 in the consistency regulating water line 1. It is obvious to the skilled person that the invention is not restricted to the em- bodiments described above, but that it may be varied in the scope of the claims presented below.

Claims: CLAIMS 1. Procedure for adjusting the consistency of paper pulp, in which procedure: allowed minimum and maximum values are defined for the consistency of head box pulp and/or for the consistency of the tail liquid coming from the wire dehydrators (14, 15), the consistency of the pulp entering the head box (13) is measured by means of measuring element (4) and/or the consistency of the tail liquid coming from the wire dehydrators (14, 15) is measured by means of measuring element (5), characterized in that, based on the measurement data of at least one consistency measurement and controlled by a control apparatus (8), liquid or pulp of desired consistency is supplied via a consistency regulating liquid line (1) into the pulp line leading to the head box (13) at a point after or before the wire pit (12) or directly into the wire pit (12) until the desired consistency is obtained. 2. Procedure as defined in claim 1 , characterized in that, in addition to the above-mentioned steps, adjustment by means of chemicals is used. 3. Procedure as defined in claim 1 or 2, characterized in that the flow of consistency regulating liquid is measured by means of a measuring device (3) in the consistency regulating liquid line (1). 4. Procedure as defined in any one of claims 1 - 3, characterized in that the consistency of the liquid used for consistency control is measured by means of a measuring device (7). 5. Procedure as defined in any one of claims 1 - 4, characterized in that consistency control is effected using at least three components having different consistency values, such as viscous pulp, tail water and circulation water of the paper machine. 6. Procedure as defined in any one of claims 1 - 5, characterized in that the volume flow corresponding to the amount of material needed for consistency control is passed out of the short circulation e.g. via the spill pipe (10) of the wire pit (12) or the like. 7. Procedure as defined in any one of claims 1 - 6, characterized in that, to adjust the consistency of head box pulp, a consistency regulating liquid consisting of liquid, pulp or filler with a consistency higher or lower than that of the head box pulp is used as a primary controlled variable, and retention chemical control is used as a secondary controlled variable. 8. Procedure as defined in any one of claims 1 - 7, characterized in that, controlled by the control apparatus (8), the dosage of chemicals is adjusted based on measurement of the consumption of consistency regulating liquid. 9. Procedure as defined in claim 8, characterized in that the dosage of chemi- cals is not changed until the consumption of consistency regulating liquid has risen above/fallen below a very wide consumption range (e.g. 30 l/s - 200 l/s).

Inventors: LOIPPO KIMMO, LOIPPO, KIMMO
Applicants: ECOPUMP OY, LOIPPO KIMMO, LOIPPO, KIMMO
Application year: 1998
Main classification: D21F1
IPC codes: D21F1