Monitoring Felt for Improved Paper Quality

Paper quality is significantly affected by the operating condition of components in the paper machine wet section. After the wet pulp leaves the front end wire section, the web still has high water content. The press section reduces the water content from 80% to 50%, when suction rolls apply pressure against the paper web, which is sandwiched between absorbent material or “felts”. A felt is a long fabric, which may be grained in a variety of ways, a diagonal seam in this case, making a continuous loop. The felt carries the paper through the press nips, or roll contact points.

Whenever a felt seam becomes disjointed or uneven, a force impulse is generated as the seam passes through a nip. The impulse is large and causes the roll to bounce imperceptibly, often resulting in
paper flaws. Further, bad rolls cause felt damage. In many instances, felts are continually replaced without correcting the underlying cause. This results in high maintenance costs, not to mention the
resulting lost production and paper recycling costs. In the past, these impulse forces were difficult and time consuming to detect, let alone to identify the faulty machine components causing them.

The phenomenon creates a difficult measurement problem because it is characterized by a low signal to noise ratio, which is further attenuated by the structural path of the impulse. Further complicating the measurement problem is the low sensitivity of these low frequency ranges.

Using a measurement technique known as synchronous time averaging enhances the coherent signal components associated with these forces, while non-coherent random noise is reduced to a zero average. Performing synchronous time averaging measurements requires the use of a reliable instrument trigger referenced to roll or felt rotational speed.

As detailed in this application note, SKF Multilog continuous monitoring systems from SKF allow operators to make multi-parameter measurements at each sensor location. Both synchronous time averaging and acceleration enveloping measurements can thus be assigned to each channel through software configuration control. And acceleration enveloping can even be performed with synchronous time averaging.