A METHOD OF MEASURING THE FORMING PROCESS OF A FIBRE WEB WITHIN THE WEB FORMING SECTION OF A PAPER MACHINE

Abstract: A method of measuring the forming process of a fibre web by ultrasonic measuring. In the method, the thickness of the fibre web is determined by measuring variation in the thickness of the fibre web at one measuring point at a time and by counting the number of pulses obtained for each thickness value during a measuring period, whereafter a curve (T) describing the turbulence profile is formed of the measured values as an envelope of the number (n) of occurrence of each thickness value (h).

Description: A method of measuring the forming process of a fibre web within the web forming section of a paper machine The invention relates to a method of measuring the forming process of a fibre web within the web forming section of a paper machine, wherein the thickness of the fibre web is measured by an ultrasonic measuring device by sending ultrasonic signals to the fibre web in the direction of its thickness and by recording variation in the thickness of the fibre web by means of the reflected pulses. Within the wire section, i.e. the web forming section of a paper machine, there occurs turbulence when the fibre web is still at least partially in the form of a fibre suspension, that is, in flowing state, and this turbulence affects substantially the quality and forming of the fibre web to be formed. When the fibre suspension reaches the beginning of the wire section, the removal of water from it starts immediately, and so, in theory, the suspension gets evenly drier and when the fibres are bonded together, it is gradually felted into a solid web, which is then passed into the press and drying sections to achieve a dry web. However, when the wire moves on and the machine operates, there occurs vibration in the fibre suspension when water is removed from one side while the upper side is open, which causes turbulence in the fibre web and thus variation in the thickness of the fibre web positioned on the wire. In a way the turbulence is necessary in order that fibres contained in the fibre web could be more easily bonded together, thus forming a firm and fixed texture. In some cases, a strong .turbulence occurring too late during the process may cause disturbances and damage the texture of a web portion which has already felted, and attempts are made to prevent this by adjusting the machine. It is essential to monitor the web to get information about the turbulence at each specific point of the web and about the web forming process. In practice, only turbulence within the wire section has been monitored while the web forming process has not otherwise been paid attention to. Typically, turbulence has been measured by photographing, video-taping and ultrasonic measurements, which all indicate the occurrence of turbulence in some way. Photographing and video¬ taping show what the turbulence looks like • at a par¬ ticular point, thus giving information about the turbulence at this point. However, these methods fail to provide reliable information about the intensity and intensity variation of turbulence because they do not provide any measurable quantity on the basis of which comparisons between different points could be made. Accordingly, these methods are mainly suitable only for getting a general view. In ultrasonic measurements, in turn, an ultrasonic signal is applied to the web to be measured from below it, and this signal passes through the fibre suspension and is reflected from boundary surfaces, thus indicating their position and enabling the measurement of turbulence variation on the basis of these positions. If the reading of the indicator is observed and the numerical values are recorded, it is thereby possible to see the smallest and the greatest thickness of the web at this particular point, that is, the thickness variation caused by turbulence. A drawback of this method is that the minimum and maximum values do not necessarily tell what the turbulence is like and how the fibre suspension behaves at this point. The estimations are made on the basis of the extreme values, as a result of which the adjustment may worsen the situation instead of making it better. These measuring methods do not either provide any other information about the web forming process. The object of the present invention is to provide a method of measuring turbulence variation and web forming, by means of which it can be reliably determined what the turbulence is like at a pre¬ determined point of the fibre suspension and which gives reliable information about turbulence variation and, if desired, about the felting of the web. This is achieved according to the invention in such a way that the measurement is performed several times at each measuring point during a predetermined measuring period, that the number of measurements giving a thickness value indicating a particular boundary sur¬ face in the fibre web is counted, and that a web forming profile is determined at each measuring point on the basis of each measured thickness and the number of the measurements corresponding to each thickness. The basic idea of the invention is that the measurement is carried out similarly as a normal ultrasonic measurement but the different thickness values obtained by ultrasonic pulses from the fibre web while it is still at least partially in the form of a fibre suspension are utilized to classify the thickness or thickness variation of the uppermost surface of the fibre web or its separate portion forming a boundary surface, while counting the number of measuring results corresponding to a certain thickness value, thus providing information about the magnitude of turbulence variations at each measuring point and the frequency of occurrence of each thickness class, or correspondingly, about the thickness of a felted layer. In this way it is possible to draw a web forming profile at each measuring point on the basis of the thickness and its frequency of occurrence. The web forming profile preferably illustrates both the nature and intensity of the turbulence occurring at each particular point and the degree of felting of the web and possible disturbances occurring in it at the terminal end of the web forming section. In accordance with the present method, it is possible to measure turbulence variation and degree of felting within the entire web forming section and then adjust the water removal and other adjustable properties of the web forming section in such a manner that a web of optimal quality is obtained. The invention will be described in greater detail with reference to the attached drawings, in which Figure 1 illustrates schematically the web forming section or the former of a paper machine; Figure 2 illustrates schematically a measuring equipment according to the invention; Figure 3a and 3b illustrate schematically the measurement and the measuring principle; Figure 4 shows schematically a turbulence profile obtained by the measurement; and Figure 5 shows schematically one illustrative way of displaying the measuring values. Figure 1 shows schematically the inlet end of a paper machine or the like, comprising a headbox 1 from which a fibre web is fed in the form of a fibre suspension as a broad spray on to a wire 2 which turns around a roll 3 and then .moves away from the headbox 1 into a web forming section 4. Within the web forming section 4 water is removed from the fibre web by water removal means positioned below the wire 2, such as foils 5 and suction boxes 6. When water is removed from the web, its fibres gradually adhere together and the water content decreases, and so the end result in the terminal end of the web forming section 4 is a felted web. At the terminal end of the web forming section 4, the wire 2 is passed around rolls 7 and 9 and back to the headbox 1, while the formed web is transferred by means of a suction roll 9 on to a wire 10 of the press section and it is forwarded into the other sections of the paper machine. When the fibre web emerges from the headbox 1 on to the wire 2 and is still in substantially liquid form, there occurs turbulence in it. Both the foils 5 and the suction boxes 6 affect the turbulence, and the turbulence causes the fibres to both mix and adhere together, but too drastic a turbulence at the wrong place may break the bonds between the fibres, thus deteriorating the strength and other properties of a felted web portion. In order that the turbulence could be adjusted optimally in view of the production process and the properties, it is necessary to know what the turbulence is like at each particular point within the web forming section 4. Figure 2 shows a measuring equipment according to the invention, comprising an ultrasonic sensor 11 connected to a signal processing device 12, which determines on the basis of reflected ultrasonic pulses how thick the fibre web is at the measuring moment, and so the smallest thickness of the fibre web and, correspondingly, the greatest thickness of the fibre web as well as thickness variation within the fibre web at the measuring point can be determined on the basis of the measuring signals irrespective of whether the fibre web is partly or wholly in the form of a fibre suspension. The signal processing device 12 is further connected to a data acquisition computer 13 which determines from the measured web thicknesses during a predetermined period what thicknesses have been measured and counts how many times each thickness has occurred during the measuring period, thus obtaining a profile describing the behavior of the turbulence and possibly the thickness of a felted web at a certain measuring point. The data acquisition computer 13, in turn, is connected e.g. to a conventional microcomputer 14 which forms display curves on the basis of the measured values. These curves describe various features, such as the turbulence profile. Further, the microcomputer 14 can be connected to a printer 15 which outputs the curves made on the basis of the measurement and, if required, also a desired number of numeric values. Figure 3a is a schematic enlarged view of the measuring situation, and Figure 3b illustrates the principle of forming the measuring values in the signal processing device. A fibre web 16 at least partially in the form of a fibre suspension passes by the ultrasonic sensor 11 with the wire 2, whereby a measuring signal is applied to it and a thickness h χ of the fibre web at a particular moment is measured. The measurement is performed during a measuring period of predetermined duration, whereby a suf- ficient number of pulses are obtained for indicating variations on a statistical basis. Figure 3b shows how a certain measuring result is obtained, whereby the horizontal direction in Figure 3b represents the measuring time t and the vertical direction the measured web thickness hx. When the measuring starts, a web thickness between hO and hn is obtained during each measuring pulse pi to pn, whereby, for instance, the first pulse pi gives the web thickness h8, the second pulse p2 gives the web thickness h4, the third pulse p3 gives the web thickness h3, the fourth pulse p4 gives the web thickness h7, etc. In this way it can be determined on the basis of each measuring pulse to which height class the web thickness h χ belongs at the measuring moment and the occurrence of the different thickness classes is counted by the data acquisition computer 13 in such a way that the number of pulses falling into each thickness class during the measuring period can be determined. After all measuring points have been measured, which, in practice, means in most cases that the turbulence profile is measured at several successive points in the longitudinal direction of the web forming section 4, whereas only one point at a predetermined distance from the edge of the wire is measured in the transverse direction of the wire, the data are transferred from the data acquisition computer 13 to the microcomputer 14, in which a graphic representation is formed. Figure 4 shows schematically a web forming profile obtained as a measuring result and to be dis¬ played on the screen of the microcomputer. The horizontal axis h represents the thickness of the web beginning from the zero, and the vertical axis n indicates the number of pulses giving the thickness in question during the measuring period. The web forming profile indicates only turbulence variation at the beginning of the web forming section while it indicates both turbulence variation and the thickness of the felted web and variation in it at the terminal end of the web forming section. The curve is an envelope of values formed on the basis of the measured and counted numbers, and it describes clear¬ ly the occurrence of turbulence and thickness varia¬ tions in the web at each particular measuring point. Figure 5, in turn, shows a graphic representa¬ tion against the background of a dimensional sketch 17 of the web forming section to be measured and its foils and suction boxes as drawn to scale. Points at which the ultrasonic measurements are performed within the web forming section 4 are indicated perspectively by broken lines 18a to 18n. Further¬ more, Figure 5 shows perspectively envelopes Tl to Tn showing the web forming profile similarly as in Figure 4 at each measuring point. The envelopes describe the turbulence and its intensity and possibly the felting of the web at each measuring point. On the basis of this, the minimum and maximum values of the web thickness at each point can be determined, and on the basis of these values, a ruled surface area pattern 19 representing the turbulence variation of the web over the length of the web forming section and a similar ruled surface area portion 20 representing felting variation over the length of the web forming section are shown in the perspective basic plane of the envelopes, that is, in a plane parallel to the thickness h χ . It can thereby be seen, for instance, that the turbulence variations in the situation of Figure 5 decrease in the middle of the web forming section 4 and then increase drastically. This means that the turbulence is obviously too strong within the terminal portion of the web forming section, as a result of which bonds between the ibres contained in the web are loosened or broken, and so the felted web is damaged at some points and the strength of the web is deteriorated. On the basis of the result shown in the figure, the web forming section can be adjusted in such a manner that the turbulence pattern is of a desired shape as accurately as possible, and thus by measuring the strength values and other properties of the formed web, it is possible to determine an optimal shape for the turbulence pattern of a specific web forming section, and so the turbulence can be subsequently adjusted as close as possible to the optimal pattern obtained by measuring. Further in Figure 5, the reference numeral 21 indicates a curve which represents the moisture ratio of the fibre web, that is, a numerical value kg water/kg fibre. This curve also describes the web forming process and indicates when the felting of the web starts. The invention has been described and shown in the above description and the attached drawings only by way of example, and it is in no way restricted to this example. The data acquisition computer 13 may be a separate unit or a fixed unit connected to a normal computer, such as a microcomputer, and the computer 14 may be designed and constructed for this kind of measuring operation. The signal processing unit 12 may also be a separate device or it may be integral with the computer, whereby a simpler embodiment of the equipment is a single unit to which the measuring sensors 11 are attached. The data obtained from the measurings can be processed and converted into graphical form in different ways, and the result can be presented in a desired way, even though the perspective representation of Figure 5 in combination with the dimensional sketch of the web forming section is very advantageous and illustrative for the purpose. The web forming profile can be arranged to indicate only the turbulence or only the thickness of felted web or, if desired, both of them similarly as above. As used in this patent application and claims, the paper machine means a paper machine, cardboard machine or any other machine forming a fibre web in a similar manner and in which the web is fed onto the wire and water is removed within the web forming section merely from below the web.

Claims: Claims: 1. A method of measuring the forming process of a fibre web (16) within the web forming section of a paper machine, wherein the thickness (hx) of the fibre web (16) is measured by an ultrasonic measuring device (11) by sending ultrasonic signals to the fibre web (16) in the direction of its thickness and by recording variation in the thickness (hx) of the fibre web (16) by means of the reflected pulses, c h a r a c t e r i z e d in that the measurement is performed several times at each measuring point during a predetermined measuring period, that the number of measurements giving a thickness value indicating a particular boundary surface in the fibre web (16) is counted, and that a web forming profile (Tl to Tn) is determined at each measuring point on the basis of each measured thickness (hi to hn) and the number of the measurements corresponding to each thickness (hi to hn). 2. A method according to claim 1, c h a r a c ¬ t e r i z e d in that the measurement is carried out during a time period of 10 to 180 seconds. 3. A method according to claim 1 or 2, c h a r- a c t e r i z e d in that the measurement is carried out during a time period determined by a pre¬ determined number of measuring pulses. 4. A method according to any of claims 1 to 3, c h a r a c t e r i z e d in that the measuring is carried out at predetermined points within the web forming section (4) in the direction of travel of a wire (2). 5. A method according to any of the preceding claims, c h a r a c t e r i z e d in that the measuring result is defined as an envelope passing through the end points of line segments corresponding to the number (n) of measurements occurred for each measured thickness (hi to hn). 6. A method according to any of the preceding claims, c h a r a c t e r i z e d in that the measured thicknesses (hx) are divided into pre¬ determined classes and that the number of thickness measurements falling into each class is counted. 7. A method according to any of the preceding claims, c h a r a c t e r i z e d in that turbulence variation in the fibre web (16) is measured while the web is still in the form of a fibre suspension. 8. A method according to any of the preceding claims, c h a r a c t e r i z e d in that the thickness of the web (16) is measured after it has already felted. 9. A method according to any of the preceding claims, . c h a r a c t e r i z e d in that the measuring result within the entire web forming section (4) is given by a graphic representation which shows schematically the web forming section drawn longitudinally to scale, and that the web form¬ ing profile (Tl to Tn) measured at each particular point is drawn perspectively at the point in question in a sketch (17) of the web forming section. 10. A method according to claim 9, c h a r a c¬ t e r i z e d in that the turbulence variation and the thickness of the felted fibre web (16) are shown by separate surfaces between the minimum and maximum values of their variation in a perspective bottom plane of the web forming profile (Tl to Tn).

Inventors: KAIPILA JUKKA, VAAJANEN TIMO, VIRMO JARMO, KAIPILA, JUKKA, VAAJANEN, TIMO, VIRMO, JARMO
Applicants: TAMFELT OY AB
Application year: 1991
Main classification: G01S15
IPC codes: G01B17, G01N29, G01N33, G01S15