Signals Before Press Felt Become Clogged

How to Identify Press Felt Clogging Before It Becomes Severe?

When press felts are running continuously, they are prone to clogging due to the accumulation of fibers, fillers, fine particles, and chemical residues. Before severe clogging occurs, a series of warning signs often appear, but these are frequently overlooked in daily operation. Failure to address these signs in a timely manner leads to problems such as decreased press felt dehydration, increased energy consumption, and even paper defects. Most press felt unload that come off the paper machine are not worn out, but rather due to blockage.

If the clogging problem can be successfully resolved, the lifespan of the blanket can be extended by at least 30% to 50%, saving you a considerable amount of operating costs. Sun hong has compiled common press felt clogging warning signs to help you address problems before they become severe.

Vacuum level Continues to Rise, But the Dewatering Effect of the Press Felt Becomes Worse and Worse

This phenomenon is the most easily overlooked signal before the press felt becomes clogged in the early stage. Under normal circumstances, the vacuum level of the vacuum roll is relatively stable. If it is found that the vacuum level is continuously rising, but the dryness of the paper web in the pressing zone is decreasing, this is a typical manifestation of the paper making felt pores starting to become clogged. The principle is that the pores of the felts are gradually blocked by fillers such as fine fibers, calcium carbonate or talc, and sizing agents, and the porosity decreases, so the water in the paper web cannot be discharged smoothly. Therefore, the vacuum level naturally rises. At this time, the press felt can no longer absorb water, resulting in a poor dewatering effect.

Diagnosis and solutions: The vacuum level is found to be 15% to 20% higher than normal, and the dryness of the pressure outlet area is found to be 2% to 3%. The above two phenomena have been observed for several days and cannot be restored to normal. If this situation is detected, increase the spray nozzle water jet spray pressure and increase the frequency of chemical cleaning. If there is still no improvement after two consecutive cleanings, it indicates that the press felt is severely clogged, and it is time to estimate the usage cycle and consider replacing the press felt.

Paper Surface Streaks, Embossing, and Felt Marks

Generally, when paper surface problems occur, the first reaction is to adjust the paper machine parameters (increase pressure, decrease pressure, and adjust machine speed). However, if the problem recurs, the cause may not be the paper machine but the press felt itself.

After the press felt becomes partially clogged, the elasticity and permeability of the clogged area are significantly lower than those of the normal area. When the paper web passes through the pressure zone, the pressure at the clogged area is concentrated, resulting in poor drainage and crushing or embossing of the paper in that area. This manifests on the paper surface as regular streaks and felt marks.

There are several troubleshooting methods:

• Stop the paper machine and check by touching the press felt surface; the clogged area is darker in color and feels harder.
• Compare the spacing of the paper surface streaks with the periodicity of the felt structure to determine if it is felt marks.
• Check if the streaks on the paper surface show a periodic pattern; if so, it is basically determined that the problem is caused by the press felt.

It is recommended to strengthen fixed-point cleaning to alleviate local clogging. If the clogged area exceeds 30% of the felt width, it is recommended to replace press felt directly; otherwise, the paper quality will be downgraded, resulting in excessive losses.

Press Felt Hardening and Significant Decrease in Elasticity

A healthy press felt will feel noticeable springback when pressed by hand. However, with use, fine fibers and fillers will continuously penetrate into the blanket’s interior, filling the gaps and causing the felt to become increasingly hard. This phenomenon is quite serious; after the press felt loses its elasticity, it cannot effectively store and release moisture in the compressed area. The dewatering efficiency of the pressing zone decreases significantly, the dryness of the paper web exiting the pressing zone decreases, and the subsequent drying section will consume more steam, directly increasing the energy consumption per ton of paper.

The method to judge is to press the surface of the paper making felt with your hand. If the rebound is slow or the pressure is too strong after pressing, it indicates that the internal structure of the press felt has hardened significantly. It is recommended to record the thickness and feel of new press felt after they are installed on the machine, and check and compare them every time the machine is stopped, so that you have a clear understanding instead of relying on your feelings.

Paper Machine Forced to Slow Down in Order to Maintain Paper Quality

As a common problem, the paper quality of press felt becomes unstable after a period of use, and most people choose to “slow down” as a solution. Reducing the speed does alleviate the problem. With a lower speed, the paper web stays in the pressing zone for a longer time, allowing for more thorough dehydration and surface recovery. However, this comes at the cost of reduced output and sales revenue; increased energy and steam consumption per ton of paper; and increased cost of press felts per ton of paper, although the lifespan remains the same and output is reduced. Press felt need to be changed promptly.

Hardening Appears on Both Sides of the Press Felt

A distinct “hard band” will appear on both sides of the press felt in the width direction. The color is darker than the middle, the texture is hard, and the elasticity is almost imperceptible. The main causes are as follows:

First: The spray nozzle cleaning on the paper machine typically only covers the central area of the press felt, leaving the edges uncleaned, leading to the accumulation of debris.

Second: The two side edges are more likely to come into contact with the paper machine guide rollers, absorbing more impurities and oil.

Even if the hardened band is located at the edge of the press felt and seems not to affect the central papermaking area, it still causes uneven dehydration at the edge, resulting in uneven moisture distribution across the paper web. The hardened edge will gradually spread to the center, and once it blocks the papermaking area, paper quality will be affected.

It is necessary to check the spray nozzle spray range to ensure the entire blanket is cleaned. If necessary, add a dedicated edge cleaning device. If the edge hardening is severe, replace press felt promptly; do not wait until the hardening spreads to the center.