Method and arrangement for controlling short circulation in a paper machine or the like

Abstract

A method and arrangement for controlling short circulation in a paper machine, in which the control of at least one property of the diluted fiber-containing pulp conducted to the headbox is based on consistency measurement or determination in high-consistency fiber-containing pulp, diluted pulp and/or white water. The amount of fiber-based fines in high-consistency fiber-based pulp, diluted fiber-based pulp and/or white water is determined, and the result obtained is used for controlling at least one property of the fiber-containing pulp to be conveyed to the headbox.

1 . A method for controlling short circulation in a paper machine, the method comprising: 
 (a) diluting with white water of a high-consistency fiber-containing pulp to be conveyed to the short circulation;    (b) feeding the diluted high-consistency fiber-containing pulp diluted at stage (a) into a headbox and further onto a wire;    (c) separating white water from the diluted high-consistency fiber-containing pulp fed onto the wire;    (d) conveying the white water separated at stage (c) to stage (a),    (e) determining an amount of fiber-based fines in high-consistency fiber-containing pulp, diluted high-consistency fiber-containing pulp or white water, and    (f) controlling at least one property of the diluted high-consistency fiber-containing pulp to be conveyed to the headbox, in which the controlling is based on consistency measurement or determination carried out in paper, high-consistency fiber-containing pulp, diluted high-consistency fiber-containing pulp or white water, and the determined amount of fiber-based fines in high-consistency fiber-containing pulp, diluted high-consistency fiber-containing pulp or white water.    
   
   
       2 . The method as claimed in  claim 1 , wherein the amount of retention aid or other agent affecting retention fed to the diluted fiber-containing pulp is regulated in the course of the control at stage (f).  
   
   
       3 . The method as claimed in  claim 1 , wherein the amount of filler or other corresponding agent to be fed to the diluted fiber-containing pulp and affecting the properties of the paper being manufactured is regulated in the course of the control at stage (f).  
   
   
       4 . The method as claimed in  claim 1 , wherein the consistency of the white water is regulated in the course of the control at stage (f).  
   
   
       5 . The method as claimed in  claim 1 , wherein the addition of fiber-containing fines into high-consistency fiber-containing pulp, diluted fiber-containing pulp or white water is regulated in the course of the control at stage (f).  
   
   
       6 . The method as claimed in  claim 1 , in which fiber-containing pulp diluted with white water is conveyed to the headbox, to which pulp has been added an agent affecting retention, filler or other corresponding agent affecting the properties or runability of paper, wherein, at stage (f), 
 the amount of fiber-based fines in paper, high-consistency fiber-containing pulp, diluted high-consistency fiber-containing pulp or white water is determined by one of    measurements of the amount of fiber-based fines,    calculating from the total consistency and fiber consistency measurements for fiber-based material, and    calculating from the total consistency, fiber consistency and filler consistency measurements, and, at stage (f),    addition of an agent affecting retention is controlled according to the determined amount of fiber-based fines in paper, high-consistency fiber-containing pulp, diluted fiber-containing pulp or white water.    
   
   
       7 . The method as claimed in  claim 6 , wherein the addition of the agent affecting retention is changed at that moment when a substantial change takes place in the amount of fiber-based fines at the feed point of the agent affecting retention.  
   
   
       8 . The method as claimed in  claim 1 , wherein the addition of filler is changed at that moment when a substantial change takes place in the amount of fiber-based fines at the feed point of filler.  
   
   
       9 . The method as claimed in  claim 1 , wherein a total fiber consistency of the high-consistency fiber-containing pulp to be conveyed to the short circulation, including fiber fraction and fiber-based fines, is kept constant.  
   
   
       10 . The method as claimed in  claim 9 , wherein the consistency of the fiber-based fines is measured using a microwave-based consistency meter.  
   
   
       11 . The method as claimed in  claim 9 , wherein fiber consistency and consistency of the fiber-based fines are measured separately, and that total fiber consistency is calculated on the basis of the measurements obtained.  
   
   
       12 . The method as claimed in  claim 11 , wherein the fiber consistency is measured by measurement based on shear force.  
   
   
       13 . The method as claimed in  claim 9 , wherein the total fiber consistency is controlled to be constant by means of feedforward consistency control.  
   
   
       14 . The method as claimed in  claim 1 , wherein the total consistency of the high-consistency fiber-containing pulp to be conveyed to the short circulation, including fibers, fiber-based fines and other fines, is kept constant.  
   
   
       15 . The method as claimed in  claim 1 , wherein the determination of fiber-based fines in the white water takes place through measurement.  
   
   
       16 . The method as claimed in  claim 1 , wherein the determination of fiber-based fines in the white water is calculated from the difference between the total consistency of the white water and ash consistency, which represents the amount of fiber-based fines in the white water.  
   
   
       17 . The method as claimed in  claim 2 , wherein 
 the amount of filler in paper, high-consistency fiber-containing pulp, diluted high-consistency fiber-containing pulp or the white water is measured, and    the addition of the agent affecting retention is regulated according to the measured amount of filler.    
   
   
       18 . An arrangement for controlling short circulation in a paper machine, the arrangement comprising: 
 (a) dilution apparatus for dilution with white water of a high-consistency fiber-containing pulp to be conveyed to the short circulation;    (b) feed apparatus for feeding diluted high-consistency fiber-containing pulp into a headbox and further onto a wire;    (c) separation apparatus for separating white water from the diluted high-consistency fiber-containing pulp fed onto the wire;    (d) conveying apparatus for conveying the white water to the dilution apparatus,    (e) feed apparatus for feeding an agent affecting properties of paper to high-consistency fiber-containing pulp, diluted high-consistency fiber-containing pulp or white water, and control devices for regulating the feed of the affecting agent based on a determined amount of fiber-based fines in high-consistency fiber-containing pulp, diluted high-consistency fiber-containing pulp or white water, and    (f) control devices for determining the amount of fiber-based fines in paper, high-consistency fiber-containing pulp, diluted high-consistency fiber-containing pulp or white water,    wherein the control devices for determining the amount of fiber-based fines comprise measuring or calculation devices for determining the amount of fiber-based fines in high-consistency fiber-containing pulp, diluted high-consistency fiber-containing pulp or white water.    
   
   
       19 . The arrangement as claimed in  claim 18 , wherein to the control device is connected a multivariable controller for connecting two or more control parameters to the control device.