Paper breaks are common on paper machines. More obvious causes of paper breaks are relatively easy to address – however, when all the easier issues have been resolved, it becomes more challenging to improve further.
This study aimed to identify potential process conditions that increase the probability of paper breaks. Results of this assessment motivated a modification to the broke return flow control philosophy.
Dry mass for the bottom layer was calculated considering long and short fibre flow as well as the broke return flow.
Figure 1 shows the time series graph representing the bottom layer dry mass (blending).
Data contained in Figure 1 was converted into a Time-in-State® map – Time-in-State® extracts data patterns from process data. These patterns provide insight into latent variable inter-relationships that are otherwise difficult to spot.
The red coloured matrix positions represent the highest values in the time series whereas blue coloured blocks represent lowest values. The following set of Time-in-State® maps provides further detail of the assessment.
Variability illustrated in Figure 4 impacts the Bottom-to-Top Layer Ratio. Figure 5 shows that the layer ratio is 1:1 top left while the ratio changes to 1:1.4 bottom right.
Bottom-to-Top Layer Ratio exceeds 1:1.2 for 34% of the time.
Probably more significant is the rate at which the ratio changes. Figure 6 illustrates a ratio change from 1.1 to 1.4 in a matter of 5 minutes. This variability has been identified by the project team as a potential source contributing toward paper breaks.
This assessment confirms that the highly variable broke return flow can cause instability in the bottom/top layer blending ratio. Data used in this study shows that broke return flow introduces disturbances into the blending section. A redesigning of the broke return flow control philosophy can contribute to higher levels of stability. By implication it will eliminate variability in the blending section that has been linked to paper breaks.