Paper Technology Journal 19 - Voith

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48 Bernd Stibi Process Solutions bernd.stibi@voith.com Stefan Natterer Process Solutions stefan.natterer@voith.com 19/05 Voith Process Solutions – Wet End Process Analysis as the foundation of an optimization of paper quality and process stability Where is paper actually made? That question is often asked rhetorically to emphasize the complexity of the papermaking process. Fact is that the “wet end”, between the machine chest and the first dryer cylinder, forms the basis for good paper profiles. Disturbances and errors made here could hardly be changed further downstream. They often cause extensive quality and efficiency limitations of the whole line.
Fig. 1: Procedure for a WEP Analysis. Procedure for solving product quality problems, e.g. basis weight and ash content fluctuations Periodic fluctuations Stochastic variations Pressure pulsation Mechanical vibrations Speeds and speed variations Paper transmission and quality control system Analysis in frequency range (Fourier spectra) The approach and evaluation method of the Wet End Process Analysis explained here shows that it is a largely standardized service product developed by Voith in close partnership with the paper industry. Goals of Wet End Process Analysis Since the Wet End Process Analysis is largely standardized, it serves as a tool for various purposes: It can be used for troubleshooting problems such as bad sheet profile or wrinkles. Time signal analysis (statistics and distribution) Paper laboratory tests e.g. Tapio analyzer (Fig. 3) Pressure pulsation Stock/ash samples and stock consistency variations Process control system (raw signal registration) Gas content, temperature, pH-value, conductivity This tool can also be used as a condition analysis prior to rebuilds or for assessing optimization potential and investment, e.g. to increase runnability or operating speed or better profile quality. The results from a Wet End Process Analysis by Voith Process Solutions are the basis for answers to various questions. The analysis in the paper mill consists of on-site measurements of all parameters affecting the papermaking process, production output and quality. Thanks to sophisticated evaluation techniques, conclusions can then be drawn on widely varying aspects. Voith Process Solutions Wet End Process Analysis 49 Fig. 1 shows the basic procedure for a Wet End Process Analysis evaluation. Starting with the paper characteristics, such as basis weight or ash content stability, potential periodic or stochastic variation parameters are identified and evaluated. Interrelationships for example between pulsation and rotational frequencies, gas content and defoamers and sheet breaks are investigated thereby using all available information from the process. Typical aspects related to the papermaking process ● Hydraulic stability (freedom from pulsation) of the stock and water loops ● Inflow and outflow conditions as well as headbox pressure stability ● Stock mixing efficiency among the various flows in the approach flow section ● Efficiency-optimized operating ranges of the cleaner and screen sections ● Optimized positioning of pipe elbows and fittings ● Necessity and efficiency of mechanical degassing systems in relation to chemical defoamers ● Additive quality in relation to gas content and retention of fiber and ash ● Stability of the jet impingement line after the headbox; web pick-up deviations on the central press roll ● Drainage element and vacuum settings for optimal quality and water removal 19/05
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Paper Technology Journal 19 - Voith

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