Common Problems and Solutions in Paper Machine Press Felt Operation

The Importance of Press Felt

Press felt is essential consumable fabrics in the paper machine production process, playing a crucial role in paper transfer and dewatering in the press section. Different press structures produce different types of paper, resulting in significant variations in the technical parameters of the press felt, such as dewatering capacity, air permeability, and basis weight. Selecting the appropriate felt based on the paper machine’s performance is vital for stable production. Improper selection and use can directly lead to paper defects and breaks. In severe cases, it can cause the paper making felt to fold or break, resulting in substantial losses. Sun Hong has compiled typical problems commonly encountered with paper press felt during the paper making process and provides corresponding solutions.

Common Problems and Solutions for Press Felt

Press Felt Causes Paper to Fold

In paper making, the phenomenon of paper folding in the press section is analyzed and investigated. The causes of folding are as follows:

• The press felt has a high moisture content and the paper web also has a large moisture content, which causes the felt to stick to the paper.
• The difference in air permeability between the upper and bottom felt is less than 15 cfm, resulting in paper sticking to the upper felt and causing folds.
• Small separation angle at the press zone exit, unstable paper exit direction, and insufficient paper web dryness (less than 20%) at the press section, leading to paper crushing.
• Inadequate press felt base mesh structure design and insufficient flocking.

Improvement Measures:

• • Adjust the press felt usage cycle and alkaline washing cycle based on the test results. A strobe light can be used to detect the peeling of the paper web from the felt during high-speed operation.
  • Install a paper-stabilizing air pipe below the press felt at the suction roller to ensure good adhesion between the paper web and the paper making felt.
  • Increase the return angle of the No. 2 suction roller at the first pressure outlet to 120°, and add a transfer vacuum box at the first pressure blanket outlet to directly peel the paper web from the upper press felt to the bottom felt, solving the problem of sticking to the upper felt.
  • Adjust the design of the first pressure felt. The upper press felt adopts a surface layer of circular weave and a bottom layer of twill weave. The multi-layer composite base mesh structure has large openings, a stable structure, and is conducive to cleaning and uniform dewatering in the later stages. The upper felt is designed with a triple layer composite twill weave structure, which can withstand a linear pressure of 450kN/m, and the press felt thickness and elasticity remain stable and consistent. The bottom felt adopts a triple layer composite twill weave and a full monofilament structure, which can effectively prevent paper from sticking to the pressure zone outlet, making the bottom felt stronger than the upper felt in adsorption of wet paper web, preventing paper from sticking during separation and reducing moisture return.

Reasons for Press Felt Folding

The main reason for press felt folding is due to human manipulation, for example:

• The paper making felt tension is less than 2.0 kN/m.
• Foreign objects in the vacuum box plug partially lift up the felt, causing vacuum leakage.
• When the paper machine is running at high speed, the high pressure water is turned on but the vacuum system is not turned on, resulting in high moisture content of the press felt and folding.
• When washing paper making felt with alkali, the speed of the machine is greater than 100m/min, and the alkali solution accumulates, causing folds.

Adjustment Measures:

To address the issue of pleating caused by human error, standardized process procedures can be established to improve this situation.

• • Press felt tension standard: 2.5 to 35 kN/mm.
  • Clean the vacuum chamber promptly or switch to a spare vacuum chamber.
  • The press felt’s operating status is interlocked with the corresponding vacuum pump. If the vacuum level is less than -20 kPa, the operating adjustment is not met, effectively preventing misoperation.
  • Prepare a standard 3% to 5% NaOH solution and wash the press felt at a speed of 25 m/min.

Abnormal Press Felt Shedding

The press felt gradually shed to varying degrees during use. Several factors can cause this:

• Excessive vacuum (greater than -50 kPa) exacerbates paper making felt wear.
• High-pressure water pressure exceeding 3 MPa causes press felt shedding during washing.
• Wear on the felt guide roller or localized wear on the pressure roller causes felt shedding.
• A speed difference greater than 1% between the upper and lower rollers in the same press group causes sliding friction between them.
• Blockage of the press felt lubricating water leads to localized wear.
• Wear on the vacuum chamber panel or a surface roughness greater than 0.5 μm, etc.

Shedding Improvement Measures:

To address the causes of press felt shedding, the following improvement measures are implemented:

• • During the press felt’s adaptation period, the vacuum level should be controlled between -25 and -20 kPa. After 24 hours, adjust it to -50 to -35 kPa.
  • The panel material of the vacuum box is changed from polyester fiber to micro-nano ceramic panel with 99% alumina content (note that the panel flatness is 0.05mm/m, straightness is 0.05mm/m, porosity is less than 0.02%, hardness is 1530kg/mm², and surface roughness is 0.2 to 0.3pm).
  • The guide roller surface is equipped with full-width low-pressure lubricating water of 0.05 to 0.1MPa to reduce the wear between the doctor blade and the roller surface, and at the same time reduce the roller surface temperature.
  • The speed difference between the upper and lower rollers on the press roller surface should be controlled to be less than 0.5%, and the pressure of the press roller doctor blade should be adjusted to less than 0.05 MPa, with the doctor blade angle at 23 to 25 degrees.
  • Regularly check for blockages in the vacuum chamber’s suction gaps and clean them promptly.

Difficulty in Washing and Poor Dehydration of Press Felt

In the later stages of use, paper making felt may experience difficulties in washing and dehydration. Workers need to regularly inspect and analyze the press felt to determine their condition. The main reasons for poor dehydration and difficulty in washing felts are as follows:

• The press felt weaving method is not conducive to dehydration, and the dehydration channels are easily blocked.
• The vacuum level is below -25 kPa.
• The pressure on the pressing line is low and fluctuates greatly.
• The high-pressure water pressure is less than 15 MPa, and the spray nozzle is scattered or blocked.
• The press felt cleaning agent was added incorrectly, and the alkaline washing operation after shutdown was not standardized.
• Excessive contamination in the slurry, and failure to clean the roller surface contamination in a timely manner.

Adjustment Measures:

• • The base material of the press felt was changed to monofilament, increase air permeability to 70-85 cfm, refine the flocking thread diameter of the surface layer, adjust the upper felt to 22 dtex, and the bottom felt to 17 dtex.
  • Vacuum standard range: -50 to -30 kPa.
  • Disassemble and clean the hydraulic proportional valve, circulate and filter the hydraulic oil, and install an online vibration detector.
  • When cleaning the press felt, the high-pressure water pressure should be greater than 2.0 MPa, ensuring the spray nozzles are free of blockages, splatter, and blind spots; control the amount of cleaning agent used per press felt to 20-25 mL/min, and ensure the spray water covers the mesh surface without blind spots.
  • The machine can be shut down once every 15 days for cleaning with 3%-5% NaOH solvent for 30 minutes.
  • Improve the quality of slurry treatment; rinse the guide rollers every 8 hours to reduce slurry buildup and roller surface contamination.

Press Felt Deviation

In the paper production process, press felt deviation is mainly caused by equipment. The following are common causes:

• The corrector is incorrectly adjusted and is not within the effective stroke range; the reversing valve is not flexible; the air source pressure is low or there is water accumulation in the air source pipeline; the cylinder has poor sealing; air leakage in the cylinder; slow adjustment speed, etc.
• The tensioning distances on both sides of the tensioning roller are different, and the tensioning is not synchronized.
• Slurry entanglement on the guide roller causes uneven tension on both sides of the press felt.
• Uneven pressure on both sides of the pressure roller leads to a large difference in linear velocity between the two sides.

Adjustments Measures:

• • Adjust the calibrator base to the center position, ensuring the effective adjustment stroke is within 20cm; install a gas-liquid separator on the compressed air source, with a pressure greater than 0.4MPa.
  • Install scales on both sides of the tensioner track for visualization; install mechanical and instrument limits at the extreme positions for dual protection.
  • Install a doctor blade on the felt guide roller to prevent prolonged roller wrapping.
  • Regularly inspect and replace the pressure roller, and conduct pressure zone tests to check for bias.

Wear and Tear on The Edges of Press Felt

The main reason for this is that the high breathability of the outer felt of the paper sheet results in low moisture content in the press felt, leading to wear and tear.

Adjustment Methods:

• Insert the suction box sealing strip to the edge of the paper sheet.
• Add a spray pipe to the inside of the paper making felt.
• Control the vacuum degree of the water suction box.

Additionally, if the press felt has edges, reduce the spray pipe opening or remove the last sealing strip outside the paper sheet. Clean the paper making felt edges with a cleaning agent.

Press Felt Edges

In terms of breathability, the press felt is smaller than the bottom wire, and the suction transfer roll width being too large, affecting breathability. To address this, the suction transfer roll width should be reduced, press felt breathability improved, and paper making felt washing intensified.

The Edge of Press Felt is Falling off

This occurs when the edges of the paper are unstable under the suction felt. The width of the suction roller can be increased appropriately, and the press felt washing can be strengthened. At the same time, poor spray nozzle cutting will also affect these two phenomena, and the blockage of the suction roller at the edge of the paper sheet is relatively serious.