Five Signals Before The Press Felt Clogged

Some Signals to Watch Out for in Paper Machine Press Felt

Sun Hong has specialized in manufacturing the press felt for over thirty years, serving hundreds of paper mills. 

We have observed a common phenomenon: more than 90% of paper mills remove and replace press felts not because of wear, but due to severe clogging.

There is a fundamental fact underlying this phenomenon: if you can effectively solve the clogging issue, the lifespan of your felt will be extended by at least 30-50%. So you can save a significant amount of press felt consumables each year.

But this causes another problem, how can the problem be detected promptly when the press felt begins to clog?

In fact, the felt emits an "SOS signal" before an impending blockage causes a shutdown, yet these signals are often overlooked.

Today, Sun Hong has compiled a list of the five most common signs of felt clogging to help you address the issue before it escalates.

Five Press Felt Signals You Should Pay Attention To

Increasing Vacuum Demand with Reduced Dewatering Efficiency 

The Phenomenon

This is one of the earliest and most easily overlooked signs that a press felt is becoming clogged.

Under normal conditions, the vacuum level of the vacuum press roll remains relatively stable. So, if you observe a continuous, gradual rise in the vacuum level accompanied by a drop in sheet dryness at the press exit, this is a typical sign that the press felt pores are beginning to clog.

The Principle

This is because, as operating time increases, the pores within the press felt gradually become clogged with fine fibers, fillers (such as calcium carbonate and talc), sizing agents, and other impurities, resulting in a decrease in the felt's effective porosity.

When the water-permeability channels of the press felt become obstructed, moisture from the paper web cannot be effectively drained through the felt. Consequently, the vacuum system must provide a higher vacuum level to maintain the same dewatering performance.

But the result is obvious: the press felt has lost enough water absorption and wicking capacity, so even if the vacuum level continues to increase, the actual dewatering efficiency will still gradually decrease.

How To Determine

Paper Surface Defects Appear

This is a clear signal of a paper defect, so you can easier to find this phenomenon than others when the press felt is clogged.

Streaks, embossing marks, or felt marks appear on the paper surface

Localized clogging of the press felt will result in poor water permeability and reduced elasticity in the affected area. When the drainage is obstructed and localized pressure concentration in the pressing zone, the paper sheet is easily crushed, creating regular streaks or impressions of the felt's weave pattern on the paper surface.

Uneven paper moisture or paper holes

Localized severe clogging of the press felt will lead to uneven dewatering, while the dislodging of accumulated material causes holes or spots on the paper web.

In such cases, localized clogging of the press felt can be alleviated by intensifying targeted cleaning. However, if the clogged area exceeds 30% of the felt's width, direct replacement is recommended. Otherwise, the financial loss resulting from paper downgrading will far outweigh the cost of the felt.

Changes in Press Felt Physical Condition

This signal can be felt simply by touch.

The Felt Becomes Stiff

Under normal circumstances, a press felt in good condition should exhibit excellent elasticity and resilience. When pressing down on the felt with your hand, you can clearly feel its flexibility and bounce.

But as the press felt is used over time, fine fibers and filling materials gradually migrate into its interior, filling the gaps between fibers and causing the felt to mat and harden. Consequently, its resilience upon compression noticeably slows, or it becomes difficult to compress at all.

A "Hardened Zone" Appears at the Press Felt’s Edge

In addition to overall hardening, localized hardening along the edges of the press felt is also a signal that warrants attention.

The edges of the press felt (10-20 cm) are more prone to developing "hardened zones" that are darker in color, harder in feel, and less elastic due to inadequate cleaning or contact with impurities. These hardened zones may also spread towards the center.

When noticeable hardening is observed at the edges of the felt, it usually indicates that localized clogging has developed, necessitating a timely inspection of the cleaning system and the operating condition of the felt.

To facilitate early detection of this phenomenon, you can record the thickness and tactile feel of the new press felt as baseline data upon installation, and subsequently compare these against the baseline during scheduled maintenance shutdowns.

Equipment Operation and Energy Consumption Anomalies

Low Pressing Dryness

Once the press felt loses its resilience, it can no longer effectively store and release moisture from the paper web within the nip, resulting in a significant drop in nip dewatering efficiency. As a result, the dryness of the paper web will decrease when it leaves the pressing zone. This requires the subsequent drying section to consume more steam and affects the overall forming effect of the paper.

Reducing Machine Speed to Maintain Paper Quality

When the dewatering efficiency of the press felt decreases, it results in low dryness of the paper sheet leaving the press nip.

To avoid fluctuations in paper quality, some paper mills may choose to reduce the operating speed of the paper machine, thereby allowing the paper web to remain in the press nip for a longer period to achieve more stable dewatering.

However, reducing speed can not solve the problem, but will lead to decreased output, higher energy consumption per ton of paper, and increased hidden costs.

Declining Water and Air Permeability of Press Felt

In addition to assessing press felt blockage based on operating status, confirmation can also be obtained through performance test data.

If monitoring data indicates a significant downward trend in the press felt's water permeability or shows abnormal fluctuations, it suggests that the pores have become severely clogged with contaminants and that performance is unlikely to be restored after cleaning.

In cases of severe clogging, the water permeability of the felt is difficult to restore to its initial level, even after cleaning. Replacing the press felt is the best option in such situation