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Performance Optimization of Paper Machine Clothing With Preventive Maintenance

Different elements (such as the fabric design) play key roles in the life optimization and quality of operation of paper machine clothing; however, the focus of this article is the maintenance aspect.

Following a detailed checklist would lead to the optimization of the equipment during scheduled shutdowns. Much like a person with pains everywhere needs a complete check-up by a doctor, a detailed inspection would help prolong the useful life of the fabric since it would give an idea of the history of the performance of the previous fabrics. For machine clothing, this checklist would include a systematic analysis of the used fabrics, although experience proves that it is not easy for suppliers to get these fabrics.

Examples:

1. While on the machine, if the PresTuner analysis (measure of moisture profile of the press fabric transfers from the sheet of paper) shows inadequate press crowning, one can suggests doing nip impression analysis at the next scheduled shutdown. However, the used fabric analysis in which we find a caliper profile shows this problem of over- or under-crowning or cross section profile. A fabric that has a life of 45 days with a speed of 1000 m/minute represents thousands of nip impressions.

2. Forming fabrics often have a shortened life because of the wear on the edges of the fabric. Having assisted with the start-up of new paper machines over the past years, I have seen a paper company install two Twin-Wire units that were almost identical yet with a request to the machine builder to increase lubrication on the deckles of the forming shoe for the second unit installed. The result was that the edges of the fabric were less worn than the rest of the fabric on that unit, with a net improvement over the average life on this type of former.

A prioritized checklist in regard to a specific problem has a better chance of being completed than a general list. This prioritized list should be based on the used sample analysis or other inspection done during the fabric’s operation. For example, if the analysis of a fabric (whether forming, pressing or dryer) shows more wear in the back centre, the prioritized list should precisely pinpoint the worn place. It is important for the maintenance people to be informed, so that during inspection, they will concentrate their efforts at that place. This is not always the case when a general list is used.

Observations

during the operation:

In addition to the used fabric analysis, the clothing must be closely inspected and the results will then help establish the priorities when the shutdown occurs:

Look at the guideline of the press fabric (is it ahead or back in the centre or is it sideways?). In general, 20cm are tolerated, depending on the width of the machine, but the greater the distortion is, the more it can indicate a bad alignment or levelling of the machine or possibly deflexion of rolls that could be caused by high tension. This adds premature wear to the cushions. The follow-up done by suppliers usually links this parameter with the operating tension. A fabric with too much distortion will not only not give a uniform cross section profile but will also have a tendency not to wear uniformly.

Darker or lighter spots may indicate a cleaning efficiency problem or a wear problem. If it is cleaning efficiency problem, the high pressure-cleaning shower should be synchronized with the speed of the machine. The shower should move the equivalent of the shower nozzles at each evolution. Other parameters (like pressure, angle, shower distance, the stroke and quality of the water) must also be verified. There are many articles available on that subject. Used nozzles introduce air and provoke defibrillation of fabric. A used nozzle is easily distinguished by its jet profile.

A sheet producing a blowing effect in the press section, even with an anti-blow box, can indicate a problem of blocked or bad levelling of the vacuum box. It is also important to change the flexible hoses of this type of box once in a while because the hoses can destroy themselves from the inside (collapsing problem), causing bad vacuum control and breaks on the paper machine.

A sheet that bounces at a specific location in the dryer section can indicate a problem of plugged dryer fabric at that location or a failure of the anti-blow box system.

A sheet at the entrance of the dryer section that lifts at a certain frequency corresponding to the length of the dryer felt may indicate that a seam is too open. This is a frequent problem; certain actions to mask the problem have been taken by many units but they do not solve the problem at its source.

Add to that the remarks of the follow-up analysis of the suppliers.

In general, papermakers have verification lists for showers, rolls and doctor blades during shutdowns. It is obvious that these verifications must continue to be done. Articles focussing on these lists are still available. However, to optimize machine clothing and improve its life and its performance, it is absolutely necessary to know its weak points and establish a list of corrections to follow-up, according to the established parameter.

The follow-up of thickness of fabric by the regular supplier during the operation are valid, but they can only be taken on the edges of the fabric, so that in the case of over-crowning, these measures may indicate that the fabric is still in good shape despite crushing problems in the centre which can cause defects in the sheet. Towards the end of its life, this may cause vibration that, if tolerated too long on the machine, would leave permanent markings on the rolls leading to a need for resurfacing. So the fabric analysis in this case becomes necessary in order to determine the thickness and the density in the centre of the fabric. The Prestuner and Feltperm (porosity profile of the fabric) trials also give a good idea of the cross section compaction of the fabric.

Many theories on high pressure cleaning showers exist. However, the purpose is to clean the fabrics and my only remark on this subject is to verify its efficiency with a trial run of fibre carry back in the case of forming fabrics, of Feltperm or void volume for press fabrics. The laboratory analysis for all positions is very useful in specific cases. Operating pressure levels are suggested by all clothing suppliers; keeping the pressure as low as possible, will improve the life of the fabrics and there will be fewer mechanical problems for the pumps and piping.

Shutdown inspections:

When changing doctor blades, inspect them carefully (consult used doctor blades charts for comparison, since there are examples of the defect related to profiles). The profiles can be undulating which indicates a lack of even lubrication on the width and these undulations will, after a time, transmit to the rolls which, in turn, will transmit to the fabric, causing variations of basis weight or humidity profiles depending on the section.

On scheduled shutdowns, the tension of the forming fabric should be brought down slightly (from about 30 pli to 20 pli). The longer the fabric, the more necessary it is to lower tension, especially if the forming fabric contains nylon. Nylon is less abrasive but is also less stable to moisture than polyester. The tension of press fabrics should be brought down a lot, which is not the case for dryer fabrics since they are already dry.

If possible, the tension should be left low for forming fabrics at start-up. This would help prevent making holes in the fabric in case of a hard lump getting in the nip of a roll. It is better to advance by jerks, making sure to wash carefully at each roll nip. The larger the machine, the less certain that there is nothing in the centre or back (usually the location where these hard lumps are found since the washing is from front to back). In the case of changing rolls, mostly in the forming section, it is necessary to re-verify the tolerance or level the forming board. It is very critical in the case of the breast roll since this element must be returned properly in place following a fabric instal
lation or any other action that would have required removal.

The speed of paper machines increases constantly and uses more filling products on deinked pulp. Therefore, it becomes important to pay special attention to the holes in the suction rolls and to make sure that they are not plugged (with a bar) when changing the fabric, specially at the pickup roll.

During a shutdown of a paper machine, the forming fabric should be turned slowly and examined for defects, especially on the edges (loose yarns), creases, small holes, dirt or soft pockets. Doing an inspection on a shutdown allows for stopping the fabric at the right place to repair the defects. The same inspection should be done at start-up as well at slow speed to make sure that nothing was broken during shutdown, preventing lost time trying to re-thread the sheet.

When installing new fabric, inspect the condition of:

Forming foils, forming shoe, bedknife (wear, crack, cutting edge, etc.)

Rolls (wear, rough surface, defects)

Vacuum box covers

Suction rolls holes for clogging (once in a while check the blade of the couch roll)

Press grooves

Rust on the frame

Check also for deposits of pitch or stickies

During shutdown, it is good to inspect both sides of the forming fabric under microscope (30 X) for wear.

For the press section, special attention should be given to the suction box covers, especially if using deinked pulp or clay. Inspect the blades for wear since blades with round, protected bars (ceramic or other) can, over time, have abrasive particles lodged between the bars and the blade. These can cause the fabric to lose fibres. The same thing can happen with full ceramic blades (depending on the quality). When operating the press fabric, the interior walls of these blades must be inspected to make sure that they are not sharp since there is a tendency to enter through the opening of the uhle box.

When changing a press fabric, recalibration of the speed (speed-match) is necessary because the tiniest change in the thickness of the fabric can cause a difference of speed sufficient to be taken in consideration, especially in the case of tri-nip presses.

The dryer fabric must be verified to see if it is hydrolysed. Folding the sides of the fabric by hand should not break it. If the fabric is hydrolysed, discuss the problem with the supplier since there is informative material available to address this issue.

Check the tolerances of the anti-blow box seals for the dryer section. Many mills wash their dryer fabric with a caustic solution during the shutdowns, which helps clean the surface of the dryers as well. However, a good rinsing must be done after the washing. Otherwise, when the steam pressure is put back on, the water in the caustic solution will evaporate and the concentration (approximately 5% at the beginning) increases quite a bit, reducing the useful life of the dryer fabric and causing damages to the seams. Check the seams of the dryer and press fabrics as needed to repair them during the shutdown.

Gatan Houle is a paper-making specialist, working with Albany International Canada since 1993.