Better productivity by measuring web tension profile

Transcription

1 Better productivity by measuring web tension profile HANNU LINNA,* MARKKU PAROLA** and JORMA VIRTANEN *Research Group Manager, **Senior Research Scientist, VTT Information Technology, PO Box 120, FIN-020, Espoo, Finland, Product Manager, Metso Paper Automation Inc., PO Box 23, FIN-331, Tampere, Finland ABSTRACT The stability of a paper web as its runs in the paper machine and in the printing press determines the productivity of the whole chain - from headbox to printed product. Runnability management is becoming still more important when machine widths and production speeds are increasing year by year. One of the critical parameters is the web tension. An uneven tension profile causes slackness in the web, wrinkling and web breaks leading to poor runnability. Metso has now, as a result of an extensive developing work, IQTension for reliable monitoring of web tension profile. The system has been adapted in numerous cases of paper manufacturing processes. VTT Information Technology has been developing a new kind of data management and modelling technology for improving runnability and carried out several research projects thus showing the importance of the even tension profile. This paper presents the measuring system and proves web tension profile as a relevant indicator of paper machine runnability. Consequences of unstable tension have been seen in paper machines but also printing press runnability is strongly affected by tension characteristics. Practical cases are outlined for describing experiences in controlling web tension in paper making and its effect on machine performance. Thus, better capacity utilisation and overall productivity has been achieved. In all, more effective production means savings in raw materials, energy and the environment. is strong pressure to use natural resources more effectively and to pay greater attention to environmental aspects. The main objective of this study has been to improve the runnability of the paper web by controlling the crossdirectional (CD) web tension profile of the paper. Work was carried out in paper mills and printing houses. Measurements included tension profile and other on-line measurements in different machines. The measured data was processed by developed software and by FEM (Finite Element Method) analysis. The effects of the papermaking process on the web tension profile were studied. The available on-line data was collected in normal production conditions. The additional measurements taken included moisture profile measurements during paper making. Shrinkage and tensile stiffness profile measurements were taken on paper samples in the laboratory. IQTENSION THE MEASUREMENT SYSTEM FOR TENSION PROFILES Web tension in the paper industry and in printing presses is measured almost exclusively as the load caused by the web on a roller. This measurement is not without problems, as load cells are sensitive to temperature and vibration and tend to drift. Moreover, a relatively large wrap angle is required (more than 30 degrees), and the CD web tension profile cannot be measured in this way. The need to control the web better has therefore led to the search for new measuring concepts. The ideal solution for paper mill applications is a compact, robust, mechanically simple measurement system capable of adequate CD resolution. INTRODUCTION Poor stability of a paper web at high speed can contribute to slackness in the web, wrinkling and web breaks. In printing, stability problems can lead to colour register errors, resulting in excessive paper waste. It is also well known that reels cut from different cross-machine positions behave differently, and that runnability varies between different paper machines, calenders and winders. Statistical studies have shown that poor tension profiles cause runnability problems in printing presses (1). It has been difficult to define the factors that contribute to good runnability in printing and how to measure the characteristics of paper reels in a way that would predict press performance. This has been a question of special interest to both paper manufacturers and printers. There is huge economic value in good runnability. In addition, there Fig. 1 IQTension measuring apparatus.

2 Metso has developed a tension measuring equipment that meets all these requirements (Figure 1). Its basic idea is to measure the pressure of the air film between a curved surface and the web. A beam with a curved surface deflects the web. When the web speed is high enough, an air film is formed between the surface and the web. This air film prevents the web from contacting the beam. At the edges, however, the web seals in the formed air film and therefore touches the beam surface. The pressure of the air film increases from ambient pressure in the entry region to constant pressure in the central region. In the exit region the pressure decreases to the ambient pressure. The magnitude of the pressure in the central region correlates with the tension level of the web. The measuring equipment required for on-line measurements in a paper machine consists of a measuring beam, an instrumentation cabinet and the data acquisition equipment. The web makes a wrap angle of -30 degrees over or under the measuring beam. The air film pressure is measured by pressure sensors through orifices in the beam surface and the signals are converted to a tension profile. Since only static pressure is of interest, the pressure sensors can be placed away from the machine in an instrumentation cabinet. Field measurements show that the length of the pressure tubes does not noticeably affect the pressure reading. This allows a very robust construction. To ensure proper operation, the sensor orifices are cleaned periodically. This is done by blowing air through the orifices automatically during paper breaks or machine shutdowns, which has proven to be effective enough in a process environment. The calibration of the pressure sensors is also done automatically. During the calibration sequence, a known pressure difference is created over the pressure sensors by an I/P-converter. The portable IQTension Metso has also developed a portable tension measurement system based on the same principle described above. To ensure easy handling the portable beam consists of several equal parts with fibreglass body and aluminium orifice plate. To enable the system to be used in different locations with different sheet widths for example in the printing presses the portable beam is extendable; several parts can be consecutively connected together. The portable IQTension is designed for troubleshooting in paper machines or printing presses. The measuring points can be spaced as desired. The commercial model of IQTension uses mm for the edge regions and mm for the centre where the profile is generally flatter. The portable unit typically uses 2 mm spacing. RESULTS Experiences and results in paper production Several on-line tension profile measurement systems have recently been installed on paper machines. They measure the cross-machine profile and the average machine direction tension. They have proven to be more repeatable for machine direction tension control than load cell measurement systems. A newsprint paper manufacturer reports that, by using the measured profile average, i.e. tension level of an IQTension for reel drives control, they can make better jumbo reels. Earlier they were forced to decrease the tension level every time before the reel turn-up to ensure the successful operation. This procedure caused an excessive amount of wasted paper. After installation of IQTension system this kind of false practice proved to be unnecessary and they can now run the machine with a constant tension level at every turn. This has produced great savings in wasted paper related to reel turn-ups. An on-line LWC paper manufacturer has replaced the load cell measurement in their coating section inside the hood. The environment after the coating unit and between two infra-dryers was too hard and disastrous for load cells. Even the surface of the tension roll was repeatedly damaged or destroyed due to the difficult circumstances. After six months experiences the mill reports to be very satisfied with the system and especially pleased with its practically maintenance-free operation. In the near future they will focus on utilizing the tension profile measurement capabilities in the coating process. On the paper machine, the most promising actuators seem to be those used for CD moisture control, i.e. the steam box and remoisturizing sprays. Co-ordination of steam and remoisturizer to optimize both moisture and tension should be possible, because the tension profile depends strongly not only on the moisture level but also on the evolution of the moisture profile through the drying process. An uneven moisture profile after the press section always results in an uneven tension profile even though the final moisture is corrected later in the dryers through the use of the remoisturizing sprays. Ideally, the moisture profile after the press section should be measured and the steam box used to minimize moisture CD variability before entering the dryer section. The development of the moisture profile through the dryer affects the elastic properties, the frozen-in strain, and the CD shrinkage profiles that give rise to the tension profile. To reach an even tension profile, the moisture profile has to be even throughout the dryer section, particularly in the early drying stages when most shrinkage occurs. The shape of the web tension profile in the pressroom Typically, the tension profile in the paper machine is convex, and the shape of the tension profile inherits to the reel stand of the press, as can be seen in Figure 2. As a result the edge reels have a skewer profile than the reels cut in the middle positions. This may cause runnability problems in the press, such as fluttering and register problems, especially if the guiding rolls are out of balance. It is important to note that also the press may sometimes, for example when guiding rollers or nips are uneven, dominate the shape of the tension profile.

3 00 Web tension (N/m) 0 Front edge Web width in the paper machine Fig. 2 The web tension profile measured in a paper machine (solid line) and in a printing press (dashed line). The effect of printing on the tension profile was studied by making measurements simultaneously on the reel stand and after the printing units. It was found that printing did not have any remarkable effect on the shape of the tension profile, Figure 3, and that the tension profile measured in the paper mill could still be seen after the printing units. This is quite surprising especially in the case of coldsetoffset presses, where the paper absorbs a lot of damping water. The amount of water in the paper has a great effect on the elastic modulus and in this way it should affect the tension. This insignificant effect is presumably due to the high -colour content of the printed web, which may cause an even distribution of moisture. It is also possible that the water does not penetrate through the paper and there may be intact bearing layers inside the web. Reelstand tension (N/m) Tension after four units (N/m) Paper machine Printing press Back edge Fig. 3 The tension profiles measured simultaneously on the reel stand and after the printing units in a coldset-offset press. The figure shows the front edge reel (solid line) and the back edge reel (dashed line) cut from a machine reel. when the web is cut into narrow ribbons. An uneven tension profile results in different tension levels in different ribbons and this may cause problems in folding. The same applies to offset printing where in many cases several webs are printed, cut and combined. As reported, in the optimum case the different webs should run together in the folder with a 20 N/m tension difference between the ribbons (2). This target is quite hard to reach because of the differences in the tension profiles. The importance of the tension profile to pressroom runnability Pressroom runnability is regarded as the number of web breaks and the percentage of paper waste calculated by different methods. The tension profiles of SC and LWC papers, supplied by different paper mills, were measured on the rotogravure press using IQTension mounted on the reel stand. The reel widths varied between cm and 2 cm and the grammages ranged from to 0 g/m2. The web tension profile was measured on over reels during a period of two years. In the printing press, the main runnability problems were web breaks and register errors, which increased paper waste and decreased production efficiency. Most of the web breaks occurred after the printing units when the web was cut into narrow strips. The possible connections between runnability and the tension profiles were examined statistically. Different parameters were calculated from the tension profiles of each reel (key figures). The most important key figures were the maximum, minimum and average values as well as skewness of the tension profiles. In Figure the reels are divided into groups, depending on the key figure values. The columns show the web break and ribbon break rates in each group. The effect of the slack area (low minimum tension) on the break rate can be explained by the wrinkling and the instability of the web, especially when the slack area of the web turns into a narrow slack ribbon before the folder. The high tension causes web breaks presumably because of the limited strength of the paper, especially if there are defects on the web. The effect of skewness on the web break rate may be caused by the instability of the web in the press. The production control system of the printing press allowed to gather the amount of paper waste per reel. The percentage of waste increased when the minimum or the average values of the tension profile decreased. The passing of an uneven tension profile through the press may cause runnability problems especially in the superstructure and folding, for example, web displacements which at the worst may cause a web break. The most web breaks occur in rotogravure presses after the printing units

4 Breaks / reels Breaks / reels Tension profile minimum 131 / 1 31 / / 3 11 / 1 1 / Tension profile maximum 120 / 12 3 / 2 21 / 2 / Average tension In chapter The shape of the profile in the pressroom it was presented that the convex shape of the tension profiles is also seen in the customer reels in the pressroom. Since the tension profile is normally different with customer reels from different machine reel positions, some printers run one machine reel position at a time. In this way large, sudden changes in the tension profile, which may cause runnability problems can be avoided. One printing plant began to sort reels by position and discovered better operating practices, which helped to reduce the web break rate by 0 % (1). Control of web tension profile in paper machines Tests carried out in paper mills and printing presses included tension profile and other on-line measurements. These comprised moisture profile measurements in the wet and dry end, as well as dry weight and thickness profiles. The speed differences between drying groups and many setting profiles of different actuators were collected. The CD drying shrinkage profile and tensile stiffness orientation measurements were carried out from paper samples in the laboratory. Tensile stiffness was also defined on-line by carrying out tension measurements in different draw levels. Actually this means that stress-strain tests without breaking the sheet were performed in paper machine scale. The experiments carried out on paper machines were slice control, edge flow control, jet-wire speed, control of press section, moisturizing, control of draws, breaker stack and calendering trials. Breaks / reels 33 / 33 / 0 12 / / 10 1 / Tension skewness (max-min) Tensile stiffness, MD (knm/g) Press moisture (g/m2) W eb width (cm) Breaks / reels 11 / 10 2 / 21 / 22 / Fig. The effect of the tension profile on the web break and ribbon break rate. The total number of reels with a break and the amount of reels in each category are given in columns. Fig. The effect of steam box control on press section moisture, on-line tensile stiffness and tension profiles. The effect of moisture control could clearly be seen in tension profiles. As moisture increased in certain area, the tension dropped. The best actuator found to control the moisture profile was the steam box in the press section. The changes made by the steam box can be corrected later on with a remoisturizer. In this way the final tension profile could be manipulated without ruining the final moisture profile. The effect of moisture on tension is based on the changes occurring in the tensile stiffness. The moisture content has a straight effect on tension, as moisture increases the tensile stiffness of paper decreases (3). Further

5 on the adjustment made before the drying section has effects on the internal stresses, which are built up in paper sheet during drying. Increasing moisture decreases the internal stress and also this leads to decreased tensile stiffness. This makes it possible to control the tension profile by profiling the moisture in different parts of the machine. The effect of steam box control on web tension, tensile stiffness (on-line) and moisture in press section can be seen in Figure. The web was more dried in front middle area and less dried in back. The control made by steam box led to changes in moisture profile and the effects on on-line tensile stiffness and web tension are clear. Other above-mentioned experiments often generated changes in the moisture profile and changes in the tension could be explained by the moisture. Jet-wire speed trials caused changes in the orientation profile, leading to changes in the tensile stiffness and tension profile. Also the draw ratio in the drying section had impact on tension depending on the effect on the tensile stiffness profile. More load in machine calendering decreased the tension. The effect can be understood by increased strain rate. FORMATION OF THE WEB TENSION PROFILE The objective was to find the reasons behind the convex shape of the tension profile and to study how to adjust the profile with the existing control devices. As the amount of tests and gathered on-line measurement data was huge, software X-proFiles () capable of managing the profile data was developed. This software is based on multivariate methods (multiple linear regression and principal component regression) using a stepwise interactive interface which makes it possible to effectively process and make detailed analyses of the large amount of measured data. The developed software was used to find the key parameters affecting the tension profile. The best mathematical models for tension profile were achieved with cross directional shrinkage and moisture profiles and with the strain rate from the press section to the reel. In Figure the measured tension profile is modelled stepwise with the key parameters. In the upper part of the figure only shrinkage is used to model tension, in the middle moisture is included into the model and in the bottom also strain is added. Four test points are shown in each case. The shape of the web tension profile was best explained by the shrinkage profile, as it correlated strongly with tension profiles in every paper machine. The local changes in the tension profile were best explained by the moisture content variation. The role of strain in the model is purely to determine the average tension level. These kind of simple models cannot be used as an accurate model for every paper machine, but it can be used to find the interdependencies in one machine., trial 2(1) Trial (3) Trial 2(1) Trial (3) Trial 2(1) Trial (3) Web tension modelled by shrinkage and end moisture content RMSE= RMSE=3.3 RMSE= RMSE= RMSE= Trial (3) Trial (11) Fig. Tension profile is modelled first only with shrinkage profile (top), then moisture profile is included (middle) and finally the strain rate throughout the paper machine is added to the model (bottom). The solid line represents the measured tension and the dashed line the model. As mathematical modelling does not take into consideration the interactions occurring between neighbouring areas in the web and does not result in qualitative understanding, FEM (Finite element method) physical modelling was also applied. In FEM the web is RMSE=1.22 RMSE= Web tension modelled by shrinkage, end moisture content and strain from wet end to the reeler RMSE=20.11 Web tension modelled by shrinkage Trial (3) Trial (11) Trial (3) Trial (11) RMSE= RMSE= RMSE= RMSE=

6 constructed from continuum elements and the physical interactions of these elements can be defined by the Hooken law, for example. The stress-strain behaviour of paper has normally been described by different kinds of string models, usually exemplifying visco-elastic behaviour. In this way we can estimate the development of tension in the machine direction. As the paper web has a three-dimensional structure, we have to construct a 3-D network of springs and dashpots to simulate the behaviour of the paper under stress. Fortunately, there are many commercial software programs available to model various kinds of structures under dynamic conditions. The FE method is a general numerical method for differential and partially differential equations and integral equations. The method produces a discrete solution and only a finite number of values can be considered (). Applying the FE method, the paper web can be constructed from basic elements. The measured physical values, such as the elastic modulus and Poisson's ratio, may be used in the model. The simulated paper may be stretched in several stages, the elements may be pretensioned, and the simulated web can be in contact with the nips and the rollers. Figure shows the network of elements and the stresses formed in the web as a result of stretching. The physical parameters may be defined separately for each element. The simulated web is fixed on both edges and stretched to the right. The different grey scales in the bottom figure represent the different tension levels. The formation of the tension profile through the paper machine s drying section was modelled by the FE method. The different draw lengths and strain rates were measured in paper machine. Tension profile was measured in dry end. Tensile stiffness and CD drying shrinkage were measured in laboratory. Stretching in open draws in the machine direction (MD) causes a non-homogeneous stress field in the fixed web because the paper is subjected to mechanical shrinkage defined by its Poisson ratio. This typically leads to a situation where the edges of the web are slacker than the middle areas. The uneven stress fields in the web also lead to greater drying CD shrinkage in the edge areas which also decreases the tension due to the increasing MD relaxation phenomenon. As drying stresses have a remarkable effect on the elastic modulus of paper, the slacker edges become less stiff than the middle areas of the web. This also amplifies the slackness of the edge areas. Figure represents the results of the FE modelling and tension measurements in a paper machine Tension (MPa) group.-.group Last draw Measured tension Fig. Simulation of the paper web by FEM. The paper web is constructed from homogeneous elements (top figure) which conform to physical laws. The element network is fixed on both edges and the simulated web is stretched to the right. The bottom figure represents the tensions induced in the web. The darker areas are more tense. Fig. The modelling results and measured tension in a paper machine. Paper was modelled as an orthotropic material. The parameters used in the model were elastic modulus (MD > CD), Poisson value (0.), draws in drying section and shrinkage.

7 It can be seen that the basic shape of the tension profile can be modelled with quite a limited number of parameters. It is proposed that the draws in the drying section lead to the unwanted crying shape of the tension profile. CONCLUSIONS The web tension profile in the paper machine is normally convex in shape, which means that the edges of the web are slacker than the middle of the web. The main reason behind convex shape is the drying process. The web is stretched in several open draws, which cause slackening in edge areas because of stronger mechanical shrinkage. This leads to decrease in the tensile stiffness and increase in the CD drying shrinkage. These effects amplify the slackness of the edges. The most effective control is moisture profiling. In many paper machines tension profile can be adjusted by controlling the steam box in press section. The control made in the press section can be then evened out with remoisturizers so that the final moisture profile is not ruined. The shape of the tension profile inherits to the reel stand of the printing press. When the web is cut in the winder the typical crying shape of the tension profile normally turns into skew edge reel profiles and relatively even middle position profiles. Printers find edge reels troublesome at least in four colour offset printing where water is added into paper in every printing unit. This can lead to runnability problems in the press, where the typical tension related problems are register errors, web wandering, wrinkles and, in the worst case, web breaks. REFERENCES (1) Parola, M., Sundell, H., Virtanen, J. and Lang, D. - Pulp & Paper Canada 1(2): 2(0). (2) Glöckner, H. - Valmet Paper News (1): 1(11). (3) Salmen, L. - Temperature and water induced softening behaviour of wood fibre, Doctor Thesis: Kungliga Tekniska Högskolan, Stockholm (12). () Beletski, N., Lindqvist, U. and Linna, H. Proc. of the 2 th Int. Research Conf.,Graz, Austria, 0. () ABAQUS/Theory Manual: Hibbitt, Karlsson & Sorensen, Inc., Pawtucket, Rhode Island, USA (1).