The invention relates to a method and a device for controlling the solids profile of a web in a paper/board machine in a wire section.

In papermaking, the function of the wire section is to remove water from a pulp suspension supplied by a headbox and form a web structure that is optimal from the point of view of the end product. The properties of an optimal web include, for example, a homogeneous distribution of solids, an optimal floc size (formation), minimizing faults (such as streaks), and forming of desired layer structures, in which connection the fibre material, fillers and chemicals are distributed in a desired manner. hi the headbox and the wire section of a paper/board machine, control of turbulence is an important factor from the point of view of paper structure and product quality. Measurement of turbulence in the headbox is known from the applicant's patent application FI 992016. In accordance with the method described in it, the headbox is provided with fibre mobility sensors, by means of which it is possible to determine the fibre mobility profile in the cross direction of the headbox, and the turbulence profile of the headbox is also obtained from this information.

The properties of the web formed in the wire section are also influenced by the location of the solids profile. With increasing solids of the web, the mobility of fibres decreases. In that connection, fibre mobility measurement can be used for determining the solids profile of the web in the cross direction of the machine.

Fibre mobility measurement enables the dry line to be found from the development of the solids profile in the wire section, in which connection the fibres, for the most part, are fixed in place and the mobility of fibres is low. The

dry line is understood to mean in fourdrinier machines a situation in which the shine of water disappears from the surface of the web. At that point, the solids content of the web is 5-8 %. In a twin-wire former, the dry line cannot be observed visually but its location is concluded from the solids profile.

As a result of a non-optimally located or uneven dry line, and thus also as a result of an uneven solids profile, the quality of paper deteriorates. In adjusting and correcting the solids profile, the following methods are known: profiling of vacuum in a suction box, profiling of loading pressure of loading elements, profiling of vacuum in a forming roll, control of the flow rate of a headbox, adjustment of a profile bar in a headbox, and profiling of additives feed in a headbox. The last-mentioned method is described in US Patent 5, 560, 807. All the above-mentioned control methods are based on the fact that the quantity of the water removed from the web in the cross machine direction can be affected by means of them.

In dry line measurement, a method is known in which the dry line is measured in a fourdrinier wire section by means of a video camera, from which image information is passed to a computer for analysis. Based on the measurement, the dry line is controlled by adjusting the slice opening. This method is known from the publicationProceedings of 27tSl EUCEPA Conference, Grenoble, October ll- 14, 1999.

It is an object of the present invention to create a method and a device for measuring the mobility of fibres in a wire section of a paper/board machine, in particular for wire sections in which the web is between two wires.

It is also an object of the present invention to create a method and a device for measuring and adjusting the solids profile in a wire section of a paper/board machine.

It is a further object of the present invention to create a method and a device which provide a more homogeneous fibre material distribution than previously in a wire section.

It is a still further object of the present invention to create a method and a device for eliminating faults that occur in the web in a wire section.

The method according to the invention is mainly characterized in that, in the method, the wire section is provided with fibre mobility sensors which are used for measuring the cross-direction fibre mobility profile in the wire section, and the solids profile of the web is determined based on the fibre mobility profile, said solids profile being adjusted based on the fibre mobility profile.

The device according to the invention is in turn characterized in that dewatering can be controlled by means of a dewatering profiling device in a cross direction substantially across the width of the wire section and that means for measuring fibre mobility are fitted in connection with the wire section.

In accordance with the present invention, the wire section of a paper/board machine is provided with sensors measuring fibre mobility and with means profiling dewatering in a cross-direction. The solids profile of the web is adjusted in the wire section based on the fibre mobility measurement carried out in the wire section.

Fibre mobility is measured across the entire width of the web in the wire section, for example, by a method which is based on ultrasonic measurement (PUD), the use of microwaves, optical methods, friction measurement, or pressure measurement. The solids profile of the web or the dry line location (level) and profile of the web are controlled in real time based on fibre mobility measurement by affecting, for example, dewatering in the wire section or by adjusting the slice opening of the headbox. The solids profile can also be controlled by adjusting the

level of turbulence, in which connection machine-direction control is provided by adjustment of the length of trailing elements and machine-and cross-direction control is provided by stepping the formation of turbulence in the slice opening in the manner described in the patent application FI 992133. By more precise adjustment of the solids profile it is possible to reduce faults in the web and get the properties of the web under better control than before.

The device in accordance with the present invention comprises fibre mobility measuring means and dewatering profiling means placed in the wire section of a paper/board machine, by which dewatering profiling means water is removed from the web, as well as a control unit for controlling dewatering based on a signal received from the fibre mobility measuring means.

In the following, the invention will be described in greater detail by means of the appended figures in which Figure 1 shows a state-of-the-art wire section and the dry line in the wire section.

Figure 2A shows a measuring system in accordance with the invention for measuring and controlling the dry line in a wire section.

Figure 2B shows measuring and controlling means of the device in accordance with the invention.

Fig. 1 shows a headbox 10 of a paper/board machine from which a slice jet is discharged through a slice channel 11 onto wires 16,20 of a wire section 12, in which a web W is formed. The wire section also comprises guide rolls 14,15,19.

A dewatering unit 13 is located in the region common to the wires 16,20, by means of which dewatering unit the structure of the web W is controlled and water is removed from the web. The dewatering unit 13 is advantageously a loading element and suction box unit which comprises fixed blades on the side of

the first wire and loadable or flexible blades on the side of the second wire. The loading element and suction box unit produces pressure pulses in the web in the twin-wire zone, by the action of which pressure pulses water is removed from the web. The pressure pulses also affect the structure of the web, so that, for example, the formation of the web can be controlled by means of this unit. Fig. 1 shows the dry line d, which is very uneven in the state-of-the-art implementation.

Fig. 2A shows an arrangement in accordance with the invention for controlling the dry line. A dewatering unit, for example, a loading element and suction box unit 13 is provided with fibre mobility sensors in accordance with the invention and a transverse dewatering profiling device 17 is placed in connection with the web W. By means of the dewatering profiling device 17 it is possible to adjust, for example, the deflection of the loading element in the cross machine direction or the vacuum level in the cross machine direction. A measuring signal ms is obtained from the fibre mobility sensors situated in connection with the loading element and suction box unit 13 and said signal is passed to a control unit 18 of the profiling device 17. The control unit 18 gives a control signal c to the profiling device 17. Based on the control signal c, the profiling device 17 adjusts the dewatering of the web in a wire section 12 such that different amounts of water are removed at different cross-direction locations to adjust the solids profile so that it becomes straight, whereby the dry line d is also straightened, as shown in Fig. 2A.

Fig. 2B shows the measuring and controlling means of the measurement and control system in accordance with the invention in greater detail. The wire section is provided with fibre mobility sensors S and the control unit 18 controls dewatering in the profiling device 17 based on the measuring signal in, obtained from said fibre mobility sensors. The fibre mobility sensors Sll,..., Snm measure indirectly the speed of the fibre phase, i. e. the solids, of the web. The machine direction MD is indicated in Fig. 2B.

In the example of Fig. 2B, the loading element and suction box unit 13 is provided with an array or a matrix of fibre mobility sensors for measuring the cross- direction fibre mobility profile. The number of the fibre mobility sensors Sil,....

Snm in one row is at least n and they advantageously form a matrix having n*m fibre mobility sensors. In one horizontal row there are, for example, 5-20 sensors per metre and there may be, for example, 1-6 rows m. The fibre mobility sensors are attached to, for example, foil blades 131,..., 13m of the loading element and suction box unit 13, as shown in Fig. 2B. The distance between the sensors Sn, ..., Snm in the sensor row or matrix is advantageously 60 mm. The more precisely it is desired to implement the control of the fibre mobility profile, the more close to one another must the sensors Sll,..., Snm be placed. As the fibre mobility sensor it is also possible to use one fibre mobility sensor traversing in the cross direction.

In the wire section 12, shear stresses and turbulence are applied to the web W, among other things, by means of various pulsation elements, whereby the fibre flocs of the web are broken up and the mobility of individual fibres increases.

Rapid dissipation of turbulence and a rapid increase in consistency caused by drainage give rise to re-flocculation, with the result that fibre mobility diminishes.

This phenomenon can be measured indirectly by using different measuring methods. Such measuring methods include, for example, ultrasonic measurement, microwave technique, optical methods, radioactive methods, measurement of surface friction, and measurement of pressure. Advantageously, pulsed ultrasound Doppler measurement is used for measurement, in which a measuring sensor applies ultrasonic pulses to the web and echoes reflected from the solids of the web are received. The scattering values and the position correlations of the measured velocity signal are proportional to the degree of fluidization of the suspension and thus to the turbulence prevailing in the web. Based on this, the local solids profile and thus also the MD location of the dry line are determined.

The dewatering profiling device 17 shown in Fig. 2B comprises profiling units Pi,..., Pk, the number k of which is, for example, 5-20 per metre. Each profiling unit Pl,..., Pk controls dewatering based on the control signal c coming from the control unit 18 of the profiling device. Cross-direction control of dewatering is thereby accomplished. The solids profile can also be controlled by adding water to the web.

The location of the dry line in the wire section can be concluded by measuring the fibre mobility profile of the web in the wire section. In accordance with the invention, the dry line is determined to the location where fibre mobility ceases or is very low. The method in accordance with the invention makes it possible both to adjust the solids profile and the dry line location in the machine direction and to straighten an uneven dry line.

The method in accordance with the invention for measuring fibre mobility and for adjusting the solids profile can be made use of in optimizing the running conditions of a paper/board machine, for example, when production, pulp composition, consistency, retention, or dosing of chemicals changes.

The claims are presented in the following and the various details of the invention may vary within the inventive idea defined by said claims and differ from the disclosure given above by way of example only.