Dryer Section Control in Paper Machines During Web Breaks

Abstract: Web breaks in the dryer section of a paper machine cause loss of production and quality problems. After a web break, the steam pressure in the cylinders must be reduced to avoid overheating. The goal of this project is to determine optimal steam pressure trajectories during web breaks, so that the production is restarted with the same drying properties of the cylinder as before the break.A detailed physical dynamic model of the drying cylinder has been developed. The model describes the relations between the steam valve position, the steam pressure, the cylinder temperature, and the papertemperature. The model is based on partial differential equations that describe heat conductivity for the cylinder and the paper web, and mass balances of water and dry material in the paper. The accuracy of the model has been verified through experiments made at the M-real paper mill in Husum, Sweden. Verifications are made both during normal operation and during web breaks.The dynamic model has been reduced in order to derive simple transfer functions between the steam pressure and the cylinder temperature, and between a logic signal that is active during web breaks and thecylinder temperature, respectively. The transfer functions obtained were used to find the optimal steam pressure trajectory during web breaks.A new feed-forward strategy for steam pressure control during web breaks is presented. The strategy has been tested on a paper machine with good results. The strategy is built on feed-forward compensation and has been well received at the mill.