Solving guiding problems on forming fabrics

When a forming fabric doesn’t want to run straight it usually causes many problems and costs. In the worst case it actually runs into the machine frame and the fabric is completely destroyed. But also continuous strong guiding corrections lead to excessive fabric wear and hence high costs. This article explains the principles of forming fabric guiding and gives the papermaker tips on how to systematically solve guiding problems.

The principle of fabric guiding

In the loop of each fabric there is one roll equipped with the task to keep the fabric running straight: the guiding roll. One side of the guiding roll, usually at the tending side of the machine, can be moved in the running direction of the machine

The fabric will always travel toward the side of the guide roll that it contacted first. This is the right hand side or drive side (DS) of the machine. The left edge of the fabric reaches the guide roll first and moves the fabric to the left or tending side (TS) of the machine.

The importance of friction

The friction created between the guide roll and the forming fabric is the key of the guiding process. When one edge of the fabric encounters friction, it will be slowed down a little. This again will generate a cross direction force in the fabric, moving it toward the side with more friction. Please note that this applies for all items contacting the fabric; therefore a misaligned suction box can cause tremendous guiding problems too!

There are several ways to increase the friction on the guide roll:

  • Increasing the fabric tension;
  • Increasing the angle of wrap on the guide roll;
  • Increasing the diameter of the guide roll (thereby increasing the contact area);
  • Increasing roll softness.

Water reduces friction

Apart from the 4 points mentioned above, another critical consideration in maintaining friction is the amount of water on and around the guide roll. Water acts as a lubricant and when it becomes trapped between the roll and the fabric and it drastically reduces the guide system’s effectiveness.

Since fabric showering is critical to the total paper making operation, showering changes are normally used as a last resort to improve fabric guiding.

Wire tension

As mentioned before, guiding is directly controlled by surface friction between the fabric and the guide roll. Surface friction in its turn is strongly affected by the fabric tension; the higher the fabric tension, the more surface friction and hence a more responsive guiding system.

If experiencing guiding problems, always check for the correct operation tension on your fabric. Check the tension on both sides of the machine (TS and DS) and at the same position, for example before the guiding roll. Tension differences between the two sides are usually related to either misalignments or the forming fabric itself, but should always be resolved.

Be sure that your tension measuring device is giving correct values; see our related article on poorly maintained tension gauges for additional useful information.

Angle of wrap around the guide roll

The larger the angle of wrap around the guide roll, the higher the effectiveness of the guiding system. The minimum wrap angle varies a bit with the length of the fabric. For longer fabrics, like on Fourdrinier machines, a wrap angle of at least 25° (figure 1) is needed to adequately guide a fabric. For shorter fabrics, like top formers or gap formers, it can also be a little less to avoid a hyper-sensitive control system.

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Figure 1

Location of the guide roll in the machine

The location of the guide roll on some (older) machines can contribute to guiding problems. When the guide roll is adjacent to the stretch roll, either on the ingoing or outgoing side, the wrap angle will change when the stretch roll moves.
When a relatively short fabric is installed, as in the example in figure 2, the wrap angle will decrease and hence guiding problems may occur. Ideally the guide roll is positioned between two stationary rolls.

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Figure 2

Guiding roll cover

Roll cover material and its hardness are critical in maintaining adequate friction between fabric and guide roll. Rubber-covered rolls with a hardness of 20 P&J are recommended for guiding today’s fabrics.
However, because rubber rolls tend to harden over time, which reduces the friction between guide roll and fabric, periodic roll grinds are necessary to maintain the correct roll hardness. These rolls typically need to be reground every 2-3 years.

Polyamide yarns in the fabric

In a forming fabric, the yarns in cross machine direction are usually made of polyester and/or polyamide. Polyamide is more wear resistant and helps to increase the fabric life. However, the friction coefficient between polyamide and the guiding roll is about 30% less than with polyester yarns. This means that the previous remarks on tension, wrap angle and roll cover are even more important for forming fabrics with high polyamide content.

Drill and twill patterns

Sometimes a guiding situation occurs where the fabric guides one way with the stock off and in the other direction when the stock is on. This situation is normally caused by one of two possibilities.
Misalignment of a vacuum element
A misaligned vacuum element will cause the fabric to guide, with the stock off, towards its leading edge. When the stock is applied and the vacuum becomes effective, the fabric tends to guide in the other direction. This usually requires a severe and fast adjustment by the guiding system to keep the fabric running properly. Always check for proper alignment of especially the vacuum elements when this phenomenon occurs.
Drilled suction box covers
If there is no misalignment and the machine has suction boxes with drilled covers, then the latter in combination with the fabric design may cause the guiding problems.
There is a twill pattern in every woven cloth: it is actually the diagonal pattern caused by the cross-over points in the weaving process. When this fabric twill pattern and the suction box drill pattern match, the fabric will have a tendency, when vacuum is applied, to guide in that direction (figure 3).

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Figure 3

To eliminate the possibility, the best option is to replace the drilled suction box covers with slotted ones. This will not only reduce the guiding problem, but it will also reduce fabric wear and improve the dewatering capacity of the suction box.
If replacing the covers is not possible, the second best option is to alternate the drill pattern angle in successive suction boxes. The last suction box in running direction should have the drill angle opposite to the twill angle of the fabric.

Troubleshooting list for guiding problems

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  • Check the guide roll mechanism
  • Check the function (and position) of the guide plate
  • Increase the wire tension
  • Shut off the flooded nip shower and …
  • … increase pressure of the needle showers
  • Close or reduce lubrication showers near the guiding roll
  • Check the suction box deckles, couch roll deckles and pick up roll deckles
  • Eliminate pumping of the wet box/vacufoil
  • Reduce power consumption
  • Check the shaking mechanism
  • Check the drill and twill pattern (at the last suction box)
  • Check the water treatment of the lubrication showers
  • During a stop: check the roll surface of the guide roll (20 P&J)
  • During a stop: check the alignment of all the elements touching the fabric

This is one of a series of articles on Paper Machine Clothing related topics which can be found in knowledgebase of the Feltest website.

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Pulsation study with the FiberScan

Paper machines are complicated mechanical systems which are susceptible to a large number of vibration related problems, some of which can affect the quality of the paper which is being produced.

Perspective

Looked at from one perspective, a paper machine can be thought of as a gigantic multi-channel tape recorder. The effects of mechanical vibrations, pulsations, control loop faults, non-uniform consistency of the pulp, together with a number of smaller scale random variations all get written into the paper web. If not kept within bounds, they can affect the efficiency of the manufacturing process as well as the paper quality.

The FiberScan

The technology partners of Feltest Equipment invested many years in the development of the FiberScan measuring instrument that by now completely replaced the well-known ‘gamma gauge’. The FiberScan has two important advantages on old ‘gamma gauge’. First it uses harmless microwave technology, compared to the radioactive backscattering technology with all its down-sides on safety, transport and accuracy.

Second, the FiberScan can measure with a very high frequency and this countless number of measuring values is used for a mathematical algorithm (FFT) to create an “amplitude-frequency-histogram”. By means of this method it is possible to find periodical variations (pulsations) in the forming section. With this instrument it is possible to make pulsation studies very quick and uncomplicated.

Case study: streaks and MD variations in the paper web

To introduce the possibilities of this measuring instrument, we turn to a case study. At one of the board machines of an Austrian mill an unknown phenomenon regularly occurs. It comes up suddenly and is uncontrollable. It leads to un-sellable paper quality and many sheet breaks, hence much production loss.

As shown in the picture the problem is unstable streaks, meaning variations of mass, moisture and caliper in the paper web. These streaks are fluctuations of high and low basis weight in machine direction and are completely parallel over the full sheet width.

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At the darker parts there is a higher basis weight than at the lighter parts. This leads to different paper characteristics in relation to basis weight, thickness and moisture. The mass and moisture variations in machine direction point towards pulsations; the cause can be in periodically or continuously working machines (e.g., mixing pump: flapping of wings of the traversing wheel, pressure sorter, rotor frequency), raw turbulence, mechanical oscillations, aerial inclusions or foil and/or vacuum oscillations.

After several experiments like air in the approach flow system and in the de-foamer, attempts with the cleaner facility and its surroundings, the vertical sorter, head box, diffuser blocks and jet angles it was still unclear whether the variations were caused by the approach flow system or at the forming section. As there was no defined frequency which could be used to continue the search, the cause of the streaks remained unknown.

High speed measuring at the forming fabric

Now it was believed that the new high speed measuring technology of the FiberScan could shed some light on this problem. The task was to find out if the variations already existed in the approach flow system or if they are formed in the forming section, by means of undesirable foil angles and distances and /or the vacuum system. Also the question “do the stripes get amplified by the vacuum system” needed to be answered.

When the pulsations appeared, FiberScan measurements were immediately carried out. First a pulsation measurement (FFT) and immediately after that the standard drainage measurement, to assure that the conditions during the measurements are known for the pulsation study. It can also be investigated if the drainage behavior influences the pulsations.

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As shown in machine sketch it was possible to measure at the positions 1-5. To be as accurate as possible, white-water consistency probes were taken from elements A-C which were analyzed in the wet lab of the mill.

Result

Already at the first measuring point – immediately after the forming board – a dominant frequency of 26.27 Hz with a high amplitude appeared.

This reinforced itself due to the Varioline dewatering boxes up to an amplitude of more than 150 dB – please note the difference in the Y-axis scale.

After the divert roll the amplitude is substantially weaker but the peak is still clearly obvious.
With help of the FiberScan measurements it was possible to define the frequency and to limit the search area for the cause to the approach flow system. Clearly, the question if the vacuum system amplified the streaks could be answered with a yes.

Summary

In the past to find out pulsation frequencies required a large number of diagnostic and measurement tools, time and money. The FiberScan is a measuring instrument which allows quick and easy pulsation studies in the forming section. It gives an accurate frequency analysis (FFT) which helps to limit the search for these kinds of defects.

This is one of a series of articles on Paper Machine Clothing related topics which can be found in knowledgebase of the Feltest website.

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Correct felt permeability: how open do press felts need to be?

In order to remove water from the sheet in the press section, water must flow easily into the press felt. On the other hand, at the exit of the press nip, water must be kept from flowing back into the sheet. These conflicting demands ensure that felt permeability is always a compromise between good dewatering and re-wetting.

Introduction

This article gives some background information on felt permeability and describes hands-on tools to determine and optimize the permeability of felts which are running in the paper machine.

The CFM value on the datasheet

As part of the manufacturer’s quality control, the air permeability of every felt is tested and usually recorded on the datasheet. Although air permeability is an important parameter for the felt manufacturer, it is of limited value for the paper manufacturer.

Air permeability, in CFM or l/dm2.min, is determined by applying a very small pressure difference (of about 0.018-0.029 PSI or 1.27-2 mbar) between the two sides of a fabric and then measuring how much air flows through the fabric. In the case of a very fine, dense fabric less air will pass through and thus it will have a lower CFM value. This procedure works well for plain fabrics, such as forming fabrics and dryer screens.

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However, press felts are not plain woven fabrics, as they have multiple layers of very fine batt needled onto the base fabric. The needled batt is much denser than the base fabric and therefore only the batt determines the quantity of airflow and hence the outcome of a perm test.

In addition, the batt is usually made of polyamide which absorbs some water. This means that the tested CFM value may vary with local air humidity during the test – the higher the humidity, the thicker the batt fibers become, the lower the air permeability becomes.

One possible conclusion is that this test is hardly representative for the situation in which felts are used: large amounts of water, not air, passing through a compressed felt under high pressure. It is safe to say that the CFM value of a press felt is of no value for the papermaker.

Press concepts and permeability

When it comes to removing water from the sheet in the press section, there are two basic concepts: nip dewatering and Uhle box dewatering. The two concepts have slightly different properties when it comes to felt permeability.

Uhle dewatering

In the case of Uhle dewatering, the felt acts as temporary storage for water. It absorbs water in the nip and releases it in the Uhle box. This means there are some conflicting demands in respect of the felt permeability:

  • In the mid-nip the paper side of the felt must be open, so that water can freely move from the web into the felt.
  • To avoid re-wetting of the web at the nip exit, the paper side of the felt must be as dense as possible.
  • At the Uhle box, the felt must be open again, so that it can dispose of the water it contains within a relatively short time.

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Nip dewatering

For effective nip dewatering, the felt needs to carry water into the press nip. Since the sheet also carries water into the nip, the hydraulic pressure in the nip will rise and at a certain moment water will move in the direction of least resistance.

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From a felt permeability point-of-view, nip dewatering is a little easier than Uhle dewatering:

  • At the nip entrance, the felt must help to build up hydraulic pressure, so must be relatively dense.
  • In the mid-nip the water should be able to flow through the felt. To avoid the fines and fillers being washed out, the flow speed must not be too high: the felt may be a little dense.
  • Again, to avoid re-wetting of the web at the nip exit, the paper side of the felt must be as dense as possible.

Other runnability problems

When felts have become too open or too dense, the machine has its own way of letting you know: poor runnability. The table below lists some common phenomena, together with their relation to felt permeability.

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The ‘right’ felt permeability

It may come as no surprise that there is no general rule for correct felt permeability. From the above it may be surmised that machines running with nip dewatering require somewhat denser felts than those with Uhle dewatering, but that is about the only rule of thumb which applies. Optimal permeability depends on local circumstances, such as press and roll cover design, speed, stock quality, contamination etc., but also on the requirements of the paper manufacturer, such as break-in time, paper smoothness, minimum felt life etc.
As is very often the case, it is a matter of investigating what works best in a particular situation. Measuring felt permeability at regular intervals, such as every day at 9 AM, gives objective data for all machine conditions. The accumulated data gives the papermakers a point of reference, supporting them as they choose the most appropriate action to take.

Measuring felt permeability

To avoid the negative effects of incorrect felt permeability on the machine’s runnability, it is important to judge the condition of the press felts on a regular basis. Felt manufacturers often use instruments such as L&W Feltperm™ or Cristini PermFlow™. These instruments inject pressurized water into the felt and then measure the water flow. As these instruments are relatively expensive and not often used by the pulp & paper mills, they remain beyond the scope of this white paper.

Airflow through the felt

An easy and affordable way to get an indication of permeability is to measure how much air the Uhle box draws through the felt. Using an instrument such as the Feltest AirSpeed/2 this airflow can be measured accurately.

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Of course the measured airflow is closely related to the applied vacuum. Therefore it is good practice to also record the vacuum in the Uhle box when measuring the airflow. The Feltest RealVac is a handy portable manometer for measuring directly inside the Uhle box, rather than having to use defective manometers on the paper machine.

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Dynamic permeability indicator

In order to easily compare multiple measurements over time, it is best to have a single indicator which takes into account both airflow and the applied vacuum. When airflow is divided by the applied vacuum the result is such a ‘Dynamic Permeability Indicator’. The example in the table below shows that, although the measured airflow remains constant, the felt becomes denser as the vacuum increases.

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Compaction or contamination?

After the installation of a felt, (dynamic) permeability starts to fall; the felt is compacting. However contamination will also make it denser. To distinguish between these two possible causes for a denser felt, the felt caliper must be taken into account.

In order to measure the felt caliper, the same applies as in the case of measuring the airflow: only doing so on a regular/daily basis will provide a point of reference in order to assess today’s test result as ‘relatively thick’ or ‘relatively thin’. The Feltest Caliper Profiler has been specifically designed to measure fabric and felt thickness on the running paper machine and is therefore a very suitable instrument. The combination of the three tests (vacuum, airflow and caliper) provides very valuable information on the felt’s overall condition, as can be seen in the table below.

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Adjustments to running felts

There are only limited ways of adjusting unfavorable felt permeability. If a felt is dense and not too compacted, thorough felt washing makes sense. On the other hand, when the felt is compacted, installing a new felt may prove more (cost) effective.

For open felts, the options are also limited. When felts are open because they are worn, they need to be re- placed as soon as possible. When a felt is too open and still bulky, it is probably still in its break-in period. This situation can be improved by trying to compact the felt, for example by increasing the press load or introducing more water into the nip (by switching off one Uhle box).

Conclusions

Permeability is always a compromise between dewatering and re-wetting. The CFM value on the felt’s datasheet is of no value to the paper manufacturer. Typically felts for nip dewatering are somewhat denser than for Uhle dewatering, but that is about the only rule-of-thumb for felt permeability.
It is important that the paper manufacturer know the usual felt permeability, so that they can detect deviations and react in time. Combining the test results from airflow measurement, vacuum and felt caliper gives an excellent indication of the felt’s condition.

This is one of a series of articles on Paper Machine Clothing related topics which can be found in knowledgebase of the Feltest website.

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Poorly maintained tension gauges may cause severe damages

Almost every paper mill using forming fabrics has one: a mechanical tension gauge to measures the tension of Paper Machine Clothing, mostly forming fabrics. When these instruments are not in a perfect condition they can cause serious problems on the paper machine like torn fabrics, too narrow and/or too long fabrics and bearing failure.

Working principle

To understand the risks, it is important to understand how the mechanical tension gauges work. In the illustration underneath the working principle is shown.

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The sensor bar at the bottom is pushed into the fabric by a predefined spring load. The amount of travel of the bar is converted through spindle and clockwork into a value on the dial. The higher the fabric tension, the more the bar is pushed upwards, causing the needle to rotate clockwise, indicating a higher value on the dial.

What happens when the devices get older?

After a certain period of use, the instrument will start showing signs of wear and tear. Mechanical wear of the sensor bar and a leaking bellow seal are the most common problems with older tension gauges and exactly these two phenomena represent the highest risks.

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Worn sensor bar

In many older tension gauges the sensor bar is worn down for several millimeters, resulting in a flat contact surface of the bar compared to the original rounded bar.

Now remember the working principle of the gauge: in fact the travel of the sensor bar is converted into a tension value. In the illustration underneath a worn bar is compared to a new bar. The spring load is equal in both cases, and also the fabric deformation will be practically the same with an old or worn sensor bar. But the illustration shows that the worn sensor bar will travel further downwards than its new counterpart, resulting in different travel distances between the old and new sensor bar.

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Leaking bellow seal

The tension gauges are used in a very wet environment and the splash water can be quite corrosive. Aging effects of the rubber material and the moving spindle will wear out the bellow, making it no longer watertight. When process water can enter the instruments interior, soon the bearings of the spindle will start corroding. This internal corrosion will give the spindle extra mechanical resistance during the measurement. In other words: with a certain amount of fabric tension the sensor bar will not travel as easy upwards as it should.

Conclusion: worn tension gauges give too low test results

Summarized the two most common defects on mechanical tension gauges are a worn sensor bar and a leaking bellow, causing internal friction. Both defects have the same effect on the measurements: the gauge will show lower tension values than what they really are. A worn sensor bar will move the spindle downwards and hence the needle moves counter-clockwise towards lower values on the dial. More friction on the bearings makes the spindle ‘stick’ where it should be moving up; again resulting in lower values on the dial.

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What happens in daily practice

In many paper mills the tension gauges are used as long as the needle is still moving; the accuracy is too often not questioned. For every Paper Machine Clothing position the operators know the desired fabric tension; for example 6 kN/m. If the fabric tension is measured and the gauge shows 5 kN/m, the machine tension is increased until the gauge shows the desired 6 kN/m on its dial. However, due to the problems described above, the true fabric tension will then be clearly over the 6 kN/m mark.

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Consequences of machine clothing running at too high tension

When the Paper Machine Clothing is running at a higher than expected tension a number of things can happen:

  • the fabrics may stretch, finally running out of tension possibilities;
  • the fabrics can become narrow;
  • the dewatering behavior of forming fabrics can change;
  • the seam can be overloaded, resulting in torn fabrics;
  • when the fabric guiding rolls cannot handle the extra tension, roll-bending or bearing failure may occur.

Summary

The negative effects of poorly maintained tension gauges are often underestimated. The conclusion of this article is clear: a regular service or timely replacement of this precision measuring instrument can prevent many costly problems and damages like production loss. Bearing failure and increased clothing costs.
Feltest offers repair and re-calibration services for most common brands of tensometers. If you prefer a new instrument, the Feltest TensioMaster mechanical tensiometer combines high accuracy with a wide measuring range and durability.

This is one of a series of articles on Paper Machine Clothing related topics which can be found in knowledgebase of the Feltest website.

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LINKEDIN GROUP FOR PMC PROS

Join the Paper Machine Clothing Professionals group on LinkedIn. Anyone dealing with Paper Machine Clothing is invited to join this group. PMC Sales & Service Engineers, paper mill Production Managers, anyone with questions or knowledge on wires, fabrics, felts and canvas is welcome to join!

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