Control Systems for Paper Machines

Analog process controls are mainly simple standard controllers. Examples are level controllers in chests, pressure controllers in pipes, etc Most process controllers act much more slowly than the MCS controls. Actions happen in seconds to minutes. The cycle time of typical DCS functions is therefore in the 1 to 

Some controls in the process are more complex than just PID control loops. One example is the wet end control which ensures uniform process conditions in the approach flow of the paper machine and in the forming section by maintaining constant consistencies, gas content, charge and other parameters. It involves e. g. special software tools (soft sensors) using wet end data instead of direct measurements to predict basis weight during start up of the machine, when the paper has not yet reached the reel or the quality measurements. Such, wet end controls not only reduce the start up time of the process, but also minimize the consumption of chemicals, i. e. only as much retention aid is used as is really needed to reach a required retention level in the forming section. 

The QCS has on-line sensors for basis weight, ash content, moisture content, gloss and other paper quality parameters. The sensors are mounted on measurement frames, which allow the sensors to move (scan) within 20 to 40 s across the paper width. These sensors measure the paper quality parameters in cross machine direction (CD) and machine direction (MD). Each scan typically gives about 1000 measurement values (databoxes), equally spaced across the web width. An array of CD measurement values is also called a CD quality profile or CD profile. 

The average measurement of each scan gives one MD quality value. A time series of MD measurement values is also called an MD quality profile or MD profile. 

MD measurement values and CD profile measurements are available only every 20 to 30 s. 

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Control systems for paper machines can be categorized in different ways. The following terms are those most commonly used ... 

Sodium chlorite has also been used for treatment and removal of toxic and odorous gases such as hydrogen sulfide and mercaptans. Chlorine dioxide from chlorite is also useful for microbial and slime control in paper mills and alkaline paper machine systems (164,165). The use of sodium chlorite in textile bleaching and stripping is well known. Cotton is not degraded by sodium chlorite because the oxidation reactions are specific for the hemiceUulose and lignin components of the fibers. 

Acidity. For many years the pH of the paper has been considered an important characteristic affecting peramnence. Permanency specifications exist which call for paper with a pH of 6.5 or higher. Other specifications say that the pH should not be lower than 5.5. There is no disagreement, however, about the poor permanency of paper when it is under pH 5.5 (4). In this context, paper pH is determined by the extraction procedure already described. The acidity that is present in the paper will promote acid hydrolysis of the glucosidic bonds of cellulose. To prevent this type of degradation and to produce paper with a high extract pH, the paper machine process water system must be controlled at a neutral to alkaline pH with a minimum of total acidity, preferably with an excess of alkalinity. The term total acidity is used to describe the concentration of all dissolved ions and particles in the process water that contribute to acidity in the system and a low extract pH in the paper. For example, papermaker s alum dissociates to alumi-... 

The neutral or alkaline pH of the paper machine system necessary for a high extract pH in the paper is not compatible with the rosin size-alum sizing system. To set rosin size properly on fiber, alum is required. Since total acidity is an approximate measure of the amount of alum in solution, it is often used as a control in connection with rosin sizing. The most effective size-alum precipitate forms at a papermaking pH between 4.2 and 4.8 and a total acidity between 90 and 150 ppm. The low pH of the process water, high total acidity, excess alum, and the size-alum precipitate all place a severe limit on the permanency properties of paper. 

Uses Effluent system defoamer for foam control in textile, paper, and municipal effluent systems in aeration ponds, flumes, and at river outfall paper machine defoamer esp. for recycled fiber furnish foam control agent for coatings based on starch, casein, and waxes Features General purpose exc. dispersing props. 

In 2005, he was leading a multidisciplinary engineering team responsible for the design and implementation of the Distributed Control System (DCS) of a fuUy operational pulp and paper mill with a complete paper machine in a new 11,000 m building on campus, representing a total investment of 80 million Canadian dollars, 10 % of which came directly from industry. This DCS manages over 3,000 I/Os and 500 control loops and its flexibility is still unique worldwide. 

If the product is being stripped from the carrier, the take-up system is a bit more complex. The speed control to provide constant speed during the cast is actually on the take-up roll for the carrier, and this speed is maintained by the tachometer feedback loop described previously The product, on the other hand, must be handled with a tensionless take-up system, since there is no carrier to provide the mechanical strength needed in a system under tension. This is accomplished by using a proximity sensor with a loop of tape where a servo-motor accelerates and decelerates the take-up roller, depending on where the loop is with respect to the sensor. This system provides a tensionless take-up system for the tape-cast product. Many different types of proximity sensors have been used in these systems. The main requirement is that the sensor detects the tape that is being speed controlled. At times, especially when the tape-cast product "blocks —that is, sticks to itself—a paper or other material must be interwoven between the layers on the roll. Machines have been built with all of these features added as options. 

This paper has presented the development of a dynamic, predictive and optimal control method for the wet end of a papermaking system. The control of this part of the papermaking process is difficult because of its complex, multivariable and non-linear nature with long time delays. The main objective of this work has been to develop a closed loop control strategy suitable for control of the wet end processes of a paper machine. This includes industrial implementation directed towards achieving optimal control of the wet end of a paper making system. 

To be able to produce a suspension having a controlled solids content, often of more than 70w/w%, special dispersants are required. Without such dispersants, a mixture containing a mineral, such as calcium carbonate, with only 30w/w% water is no longer flowable. Pigment and filler producers have developed highly active polymer systems, especially for the use of mineral slurries in the paper industry, which allow such concentrations to be achieved without destroying the complex chemistry of a paper machine. 

This paper describes a project covering the development of a program system for the failure mode and effects analysis of plant and machine control systems in several steps. The project leader was Mr. Demski of the TOV Rheinland. The result also takes account of the experiences gathered by Dr. Germer, BIA, and Mr. Six, Technische Universitat Braunschweig, when using PASS I in practice. 

Guide rolls give a stable run either to the paper web or to wires, fdts or belts. They are about 400 to 1000 mm in diameter and are covered with coatings (Section 5.2.4). They are also used in tension control devices to ensure that the fabrics operate at adequate tension in the machine direction and in wire and felt guide systems for continuous control of the fabric position in the cross machine direction. 

Automation by fast, stable and accurate control loops for consistency, flow, pressure and level is elementary for providing the necessary constancy in the approach flow system. Variations in pressure for example will mainly directly influence the MD profile whilst stock consistency deviations will affect both CD and MD basis weight profiles. In addition the retention on the paper machine and the chemical conditions of the water systems must be kept constant. Here a retention control loop is standard in many appHcations keeping the white water consistency constant by adapting the quantity of retention agent added. Controls for filler, color, air content, cationic demand or zeta potential are also available today. Combining the different controls to a total control concept will lead from purely functional controls to control systems which also address quality and production issues. 

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