High-frequency pressure measurements in the refining zone of a high-consistency refiner
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This thesis is based on an experimental study of the pressure conditions in the refining zone of high-consistency refiners used in paper making. The work presents the findings from two different mill-scale experiments in addition to results obtained from a pilot refiner study. The experiments have been performed using two different types of pressure sensors: • Fibre-optic pressure sensors based on extrinsic Fabry Perot interferometer (EFPI). • Piezoresistive temperature and pressure transducers. This study has shown that it is possible to establish high-frequency pressure measurements in the refining zone of high-consistency refiners. Data have been collected at rates up to 20 million samples per second. The fibre-optic sensors seem to be preferable to the piezoresistive transducers as the fibre-optic sensors were working well after long exposure to the harsh environment inside the refining zone. Reliable measurements were still obtained after 1000 operating hours. On the other hand, the piezoresistive sensors suffered from problems caused by the wet environment inside the refiner as well as from the electrical dependent circuits and transmission cables. However, in the pilot refiner under less harsh environment, the piezoresistive transducer functioned well. The main focus in this thesis has been related to the second mill-scale experiment which used fibre-optic pressure sensors. The results obtained from this experiment are considered to be more valuable than those from the first mill-scale experiment which used combined piezoresistive pressure and temperature transducers. Furthermore, pulp samples were collected during the second mill-scale experiment allowing an even more comprehensive analysis. However, the results from the first mill-scale experiment are shown as a comparison and in order to demonstrate the challenge of selecting the appropriate technology. Results from a successfully test performed in the pilot refiner, which also used a combined piezoresistive pressure and temperature transducer, are shown as well. In addition, as a supplement to the results from the mill-scale experiment with fibre-optic sensors, an experiment which employed external accelerometers for measuring the high-frequency vibrations in the mill refiner is discussed. Another supplement was made through an experiment measuring the deviation of the rotational speed of the refiner shaft. The main objective of this study has been to find out how to make high-frequency pressure measurements in the refining zone of a mill-scale high-consistency TMP refiner. The study has particularly focused on the signal analyses and the reliability of the pressure measurements. Since the investigations have been focused on the experimental issues, assessments are primarily made regarding the recorded data and the related process observations. One of the challenges was to find suitable technology for use in a harsh environment. Obtaining and selecting an appropriate sensor was extremely important. A fast responding sensor was prerequisite. It was of particular interest to investigate the pressure pulses generated from the squeezing of wood particles between the sensor surface and the bars on the opposite disc. In addition, the dimension of the sensor surface had to be small so that the peak pressures could be determined. The size should preferably be smaller than a width of a bar. Especially the fibre-optic sensors met this criterion as they had a surface diameter of approximately 1 mm. The piezoresistive sensors had a surface diameter of 3.8 mm. Mill-scale experiments were conducted to test the sensors in a realistic environment. The design of the experiments was important in the assessment of the performance of the sensors during different controlled conditions. Collecting of pulp samples to assess the pulp quality of the primary refining in comparison to the process behaviour and the pressure signals were matters of additional interest. The analyses of the experiment should give conclusions about the reliability of the sensors. A goal of the present work has also been to examine the behaviour and the properties of the pressure signals, and investigate the origin of the pressure pulses. Most of the results are from high-frequency measurements of pressure in the refining zone of a high-consistency TMP refiner. Combined temperature and pressure measurements are shown from the test in the pilot refiner. The fast development within sensor and computer technology has made it possible to achieve measurements that have not been performed earlier. The use of fibre-optic sensor technology in chip refiners has not been reported earlier. The high-frequency recordings using sample rates of up to 20 MSamples/s suffered from the lack of demodulation technology such that the accuracy of the absolute pressure readings is limited. However, this study indicates average pressures between 20 and 30 bar in the intermediate zone. Even more interesting is the fast pressure changes obtained using frequency analysis. Common frequencies indicating vibrations in the discs appearing from the pressure pulses when the pulp is squeezed between the bars of the stator and rotor disc have been observed. This observation was supported by vibration analysis using external accelerometers. It is claimed that there is a relationship between vibrations in the refiner discs and the pressure pulses that are generated from the squeezing of pulp between bars on the rotor and stator disc. However, nobody has investigated this particular relationship although this study strongly indicates that this relationship exists. Furthermore, it is shown that the pressure sensors were not affected by vibrations operating in resonance. This strongly supports the conclusion that the fibre-optic pressure sensors were solely measuring the activities in the refining zone. The frequency analysis of the different recordings gave firm evidence of the process related influences of the pressure readings. The reliability of the performance of the sensors was clearly visible through this analysis technique. Several analyses found that the shift in the bar crossing frequencies was directly proportional to and caused by fluctuations in the rotational speed of the refiner. However, local bar crossing frequencies were generally not as clear as expected. The measurements indicated that some specific radial positions determined by the tapered plate pattern, dominated the responses. Among the most dominating frequencies were periodicities associated with the rotational speed of the refiner, the number of bars in the breaker bar and coarse zones as well as the transition zone between the intermediate and fine bar zones. When the plates were new, a 25.2 kHz periodicity that arose from a bar-to-bar passage in the fine bar zone dominated the pressure readings. These pulses probably propagated in the disc so that the whole disc vibrated. It is not assumed that these vibrations generated large plate gap variations. However, these fluctuations were predominant compared with the local generated pressure pulses. The pressure variations probably propagated through the steam and pulp pad as pressure waves. Thus the pulp and steam flow through the whole refining zone was affected. The local bar crossing frequencies were suppressed and only visible to a minor extent. It is conceivable that the steam and pulp interacts through a two-phase flow. Thus the steam may have a repressive effect on the interaction between the pulp pad and the bar patterns on the discs. Recordings during different controlled operating conditions were used to study the reliability of the sensors. The relationship between the pressure signals and the process variables has been investigated as well. The most reliable relationship was obtained when changing the chip flow to the refining zone after 1000 hours of operating time. The motor load and acceleration variables were strongly correlated with the pressure readings. No strong correlations were found between the data from the pressure sensors and the process variables shortly after start-up using new plates. The pulp samples that were collected and analysed did not give a good relationship between the pressure signals and the quality data. However, during this period other process disturbances affected the conditions in the refining zone more than the randomized manipulation of the control variables. This was observed as time dependent variations in the pressure recordings as well as several process variables. The most probable disturbance was the amount of chips fed to the refiner and subsequent changes of the plate gap. Further indications imply that the pressure sensors were sensitive to variations in the incoming stream of chips. Besides the successful test when changing the chip flow to the refining zone, frequency analyses have showed that the sensors also were affected by periodicities in the pulp flow that were related to the inner part of the refining zone and the breaker bar section. This was shown both as sideband effects as well as plain peak frequencies. Sideband effects are interpreted as periodic waves of pulp flow propagated through the refining zone. This study indicates that the average pressure in the intermediate zone of the refining zone of the mill-scale TMP refiner was as high as 20-30 bar. This was somewhat higher than expected from the theoretical considerations. However, the theoretical calculations have some limiting factors, which there are some disagreement in the literature. The area where the energy is applied in the refining zone as well as the tangential friction coefficient is not clearly determined. Both are included in one of the theoretical approaches. Thus there are some uncertainties associated with the models. However, some uncertainties are related to the determination of the pressure levels for the measurements using the fibre-optic sensors as well. This is mainly due to the lack of well-established demodulation techniques for the sinusoidal relation between the sensor signal and the pressure. This is the greatest weakness in this part of the investigation. The pressure levels obtained using the piezoresistive sensors were to some extent agreeable with the fibre-optic measurements. However, there was greater uncertainty in the piezoresistive measurements due to probable disturbances related to electromagnetic noise or moisture having a detrimental influence on electric circuits in the sensors. The most interesting results obtained using the piezoresistive sensors in the mill-scale experiment have been observations of process relevant periodic signal patterns. Stable average pressures between 2 and 4.5 bar were found in the atmospheric pilot refiner during normal operating conditions. Since the local bar crossing frequency did not dominate the periodicities obtained from these recordings it is assumed that the steam affected the pressure conditions in the refining zone. This is assumed despite the fact that the refiner was atmospheric. Pressure peaks above 10 bar were observed frequently. When the plate gap decreased, the pressure pulses were considerably higher. Pressure peaks up to 60 bar were observed during operation with a small plate clearance. The local bar crossing frequency was clearly visible under such conditions, not otherwise.