Solve Steam Spots Pulp Penetration and Rewetting in Press Felts

Press felt, as one of the three major paper making clothing and a crucial component in the paper machine press section, are subject to inevitable production problems, impacting overall production efficiency, quality, and costs. Sun Hong will now analyze, based on its own experience, abnormalities such as “steam spots,” “slurry penetration,” and “paper rewetting” in felts. These phenomena are not only surface defects but also symptoms of imbalance. If left unaddressed, they can lead to web breaks, quality issues, and other problems.

What is Press Felt Steam Spot?

Steam spot is typically caused by a dryness issue between the fabric and the dryer cylinder in the dryer section. It’s often caused by overheating on the dryer surface or wet streaks on the paper web. When the wet paper contacts the dryer cylinder, the heat converts the internal moisture into vapor. If the steam accumulated between the wet paper and the dryer cylinder doesn’t pass through the paper in time, it can force small pieces of paper out of close contact with the dryer cylinder surface. This phenomenon results in a pitted appearance on the paper surface, indicating poor gloss and excessive moisture in the point state. The customer encountered a pitted appearance due to the steam spot problem when producing glossy paper (the paper required to have high gloss on one side).

Causes of steam spots:

• The moisture content of the wet paper is too high and the moisture content is uneven across the width of the paper.
• The quantitative of the wet paper is uneven.
• The drying temperature of the Yankee dryer is too high.
• The water filtration performance of the paper making press felt is reduced and its own water content is high, resulting in a high wet paper sheet support pressure content.
• The drying cylinder hood exhaust system is weak and the temperature of its own auxiliary heating system is low.
• The coating of the dryer cylinder is uneven and there are problems with the lateral distribution.

What is slurry penetration in press felts?

Slurry penetration in press felts refers to the phenomenon in which fine fibers and fillers in the wet paper sheet, along with water, enter and penetrate the press felt during the vacuum dewatering process after the wet paper sheet is transferred from the wire to the press felt.
Slurry entanglement, which gradually increases, can be seen on the inner ring support roller surface and other roller surfaces. When the entangled slurry reaches a certain thickness, it flakes off due to speed. The lumps that fall off can easily land on the vacuum press rolls and vacuum transfer rolls, causing partial blockage of the needle holes. This can also lead to slower paper machine speeds, rough paper sheet surfaces, large differences between the two sides, and increased press felt clogging, shortening the press felt’s lifespan.

Analysis of the Causes of Press Felt Slurry Seepage

There are two main reasons for this problem: (1) Internal cause: The paper making press felt has too high air permeability, too low density, and large water filtration gaps, which makes it easy for fine fibers and fillers in the wet paper to enter the paper making felt along with the water during the pressing and dehydration process.

(2) External cause: The suction effect of the vacuum press roller absorbs the moisture of the wet paper and also absorbs fine fibers and fillers into the paper making press felt. In addition, the good air permeability of paper making press felts will also cause the vacuum degree of the vacuum pressing roller and the press felt washing vacuum box to be unable to reach a high level, resulting in too high moisture content in the wet paper and increased moisture content in the press felt.

Solutions to Press Felt Slurry Seepage:

First, consider the press felt itself. When designing a press felt for a paper making machine, it’s crucial to carefully control its air permeability. Excessive or insufficient air permeability can lead to poor press felt performance and reduced machine speeds. Another external factor contributing to bleed-through is vacuum suction. Once bleed-through occurs, reducing the vacuum level of the vacuum press rolls cannot alleviate the problem. However, if the vacuum level of the vacuum press rolls exceeds the paper machine’s standard requirements, reducing the vacuum level will inevitably reduce the dewatering capacity of the press section, making this counterproductive.

After the pulp seepage phenomenon occurs, we need to further tighten the press felt, soften the felt with alkaline water, and use a blowtorch to singe the press felt surface to increase the density and reduce the air permeability. Then we should strengthen the cleaning of the inner ring of the felt roller. Cleaning means using spray water to clean the roller surface to prevent the accumulation of fine fibers and fillers. Cleaning means using a scraper to clean the roller surface to discharge the spray water and pulp foam in a concentrated manner. Once again, the cleaning of the press felt must be strengthened, and low-pressure fan-shaped spray water, high-pressure mobile water needle and vacuum water suction box should be used reasonably to clean out the fine fibers and fillers that enter the press felt in time without accumulation.

What is Paper Rewetting?

This phenomenon occurs when moisture from the press felt returns to the wet paper after the wet paper leaves the press along with the press felt. According to Xinhang’s previous analysis, the paper making press felt uses fine-denier fibers on the surface layer, coarse-denier fibers in the middle layer, and the coarsest fibers on the bottom layer. The finer the surface fibers, the greater the layup and the denser the layer, the greater the resistance to moisture returning from the press felt to the paper, reducing the probability of rewetting. The smaller the paper making press felt structure ratio (the ratio of fiber layer to base wire layer), the more susceptible to rewetting. Therefore, it is important to ensure the layup on the front side of the press felt, and not simply focus on improving the press felt’s water filtration performance while ignoring other aspects of its performance.

Furthermore, the dryness of the wet paper sheet entering the nip zone and the moisture content of the press felt are also important factors that determine the dryness of the wet paper sheet leaving the nip zone. The greater the dryness of the wet paper sheet entering the nip zone and the smaller the moisture content of the
press felt, the greater the dryness of the wet paper sheet leaving the nip zone. When the speed of the paper machine is increased, in order to maintain the same amount of water removed from the wet paper sheet, the pressure of the press line must be increased. For press felts with high moisture content entering the nip, the fluid pressure in the nip increases, which reduces the press felt’s ability to absorb water from the nip and increases the tendency of wet paper embossing.

Reducing and Eliminating Rewetting:

Based on the above analysis, the structure of the press felt itself is crucial to reducing and eliminating rewetting. Therefore, during the design of the press felt, it is necessary to give it certain characteristics (such as an inverted funnel-shaped water filtration channel) through customization and structural design to improve its ability to withstand rewetting.

Promptly separating the paper from the press felt after exiting the press area is crucial for reducing and eliminating rewetting. Reducing the moisture content of the wet paper and press felt prior to pressing significantly improves press dryness.

To sum up, these problems with paper making press felts are not accidental. They are closely related to the press felt’s own structure, air permeability, etc. Only by establishing a press felt maintenance plan, conducting regular diagnosis and inspection, and thoroughly understanding and analyzing the causes of the problems can we effectively maintain production efficiency and quality.