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STABLE AND AGILE WET END CONCEPT
Based on case studies performed, a number of disturbance sources are commonly encountered on paper machines that severely affect the runnability of the machine and the quality of the paper produced. These disturbance sources arise from the process and automation equipment commonly used. In addition, there are strong interactions inherent in the forming process between the paper furnish components and the chemistry; for example the weight, paper ash content and white water consistency are all severely affected when anyone of the stock flow, filler flow or the retention aid flows are changed.
In view of the fact that there are severe disturbances coming from various sources, and the papermaking process is highly interactive, the disturbances in one part of the plant propagate to other process areas in many ways, some of them quite non-obvious. Part of the problem stems from the fact that a very complex, multivariable process is often controlled with single variable control loops resulting in loops fighting each other.
First, this paper explores some of the commonly encountered disturbances and their propagation paths in the interactive papermaking environment and reports some of the troubleshooting experiences and guidelines. While process re-design is the ultimate answer to some of these issues, there is much that can be done short of this extreme step.
Today, modern measurements and multivariable controls are providing superior performance both during a grade run and, more significantly, during highly upset conditions such as startups, grade changes and recovery from breaks where traditional methods have not performed well. The second goal of this paper is to present such a comprehensive solution for wet end management including disturbance handling, adaptive retention modelling, and white water target determination. We also explore what is needed to apply the papermakers' knowledge of how the process actually works to improve the design and tuning of the control system.
Examples from the PaperIQ Plus applications in the industry are used to demonstrate how stability and agility can be achieved in today's papermaking conditions.
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