Canadian Standard Freeness (CSF) and Schopper-Riegler (SR) are two well-known quality measurements in stock preparation. Both methods are ways to measure and control refining to ensure that the pulp is strong enough but also that the dewatering rate on the paper, board or tissue machine is in the right interval. To reduce the energy consumption of the refiners it is common to have a plan for how to control them, called refiner strategy.
CSF and SR can be a nuisance to measure manually, and it is common to skip a measurement or just copying the previous result. It is not due to laziness but a consequence of prioritizing, there is not enough time to do everything. The result may differ between operators, it also takes a long time to receive the correct compensated result. There are large savings to be made by moving from manual to automatic measurements, not only in operator resources but also in refining energy. On top of that an uniform furnish is an ultimate condition to the papermaker. Figure 1 shows a comparison between manual CSF measurements and automatic measurements in a mill in Asia. As can be seen in the graph there are large variations in between the manual measurements that would be missed without the automatic measurement.
Tangible savings in refiner energy
After the implementation of automatic measurement of CSF and SR when the values have been monitored for a while it is time to start to control and optimize the refining. As soon as stock preparation operators begin to rely on the automatic measurements they are starting to manually change the refining settings. It can be hard to distinguish which savings comes from what part of the optimization, as it is common to include the investment of automatic CSF or SR in an overall stock preparation optimization project. Tangible savings specifically from the automatic CSF or SR measurements are usually savings in refiner energy, these are easy to measure before and after implementation. Other ways to use the automatic CSF or SR measurements, is to monitor incoming pulp quality before refining or beating in stock preparation, to ensure that no variations occur. If variations are detected, actions can be made to ensure that the board or papermaker will receive an even furnish. If no measures can be taken in the stock preparation, the paper or board maker can hopefully, with the knowledge of the deviation, take necessary actions to reduce downgrading.
It is possible to have a refiner strategy in place before implementing an automatic CSF or SR measurement. The refiner control consist of several levels, and the first two levels, power control and specific energy control, are the most common. To implement a specific energy control, it is important to have a valid refiner flow signal and an accurate consistency transmitter installed. The next level in the strategy is freeness control mode which is based on the input from the automatic CSF measurement. Moving from power control mode to specific energy control can reduce the energy consumption of the refining process by 22%. Moving to the next level, freeness control, can reduce the energy consumption with an additional 19%. The highest level of control is advance process control which requires a soft sensor. Based on signals from the process, the soft sensor models and calculates the CSF value between the automatic measurements. The soft sensor then updates the freeness model as soon as an automatic measurement is finished.
Implementing an automatic CSF or SR measurement is not only beneficial as it reduces energy consumption but also as it liberates stock preparation operators to perform other tasks. It also opens up for implementing a higher level of refiner control.