Managing forming fabrics during rebuilds.
Wayne Freeman, technical director-forming fabrics, Weavexx Corp., Youngsville, North Carolina, USA
Keith Kemp, Asten Johnson, Charleston, South Carolina
Cynthia Mathe, paper grade specialist, Albany International, Albany, New York, USA
John Miller, graphics applications specialist, Voith Paper Fabrics, Joliette, Quebec, Canada.
Ken Stager, product manager-graphic paper machines, Voith Paper, Appleton, Wisconsin, USA
Frank Swietlik, senior product manager-paper machines, Metso Paper, Norcross, Georgia
SOLUTIONS! Forming sections are one of the most frequently rebuilt sections on existing paper machines, which typically require changing machine clothing. How can mills work with machinery and fabric suppliers to manage this process during rebuilds?
SWIETLIK: There are several levels of involvement that a mill can pursue to ensure that fabrics are correctly selected for a rebuild. Very often, pilot trials are run with customer furnish to verify post-rebuild performance. Making fabric selection a part of the trial process can shorten the learning/startup curve after the rebuild. It is also essential to accurately set production and quality goals prior to fabric selection. The effect of fabric design on these variables is often overlooked until quality parameters become an issue. In addition, the machinery manufacturer knows the expected drainage characteristics for each element of the new former as well as the vacuum levels and can, along with the fabric supplier, provide a complete system to meet the requirements. The former designs of today tend to push the limits of fabric design as machine speeds approach 2000 meters/m and z direction stability is also challenged as widths increase beyond 11 meters.
FREEMAN: The forming fabric supplier should get involved with the mill several months ahead of a rebuild. An obvious reason is to plan the transition of forming fabric designs and dimensions from existing to new requirements. Close cooperation can minimize clothing costs and eliminate risk of supply problems.
This early involvement also enables the forming fabric supplier to fully understand the objectives of the rebuild and to create fabric designs that can enhance the probability of meeting these objectives. Today, most rebuilds are expected to positively impact selected sheet properties and output. This often demands clothing that both drains very well and provides excellent sheet support. This balance of properties can be designed into triple-layer products such as Huytexx, Synergie, and Vortexx.
Meetings with the mill and the machinery supplier to become very familiar with machine elements that affect drainage rates, sheet properties, fabric stability, and fabric life enable the fabric supplier to select the most appropriate fabric specifications to work with these elements. Often, the mill and the supplier will agree on two or three different fabric specifications to be supplied at start-up to deal with the range of possible impacts of the machinery rebuild. This can shorten time to meet rebuild objectives.
KEMP: The key is for the mill to begin early to work with both suppliers and realize that each brings a different skill set. Machinery suppliers bring a deeper level of experience with their specific equipment, an understanding of their design and controls logic, and start-up expertise. Their objective is to get the machine restarted and meet the warranty as soon as possible. The clothing suppliers often provide a broader level of experience, usually with other machinery suppliers. We look at clothing life and the longer term performance perspective. Our objectives are what affects our clothing performance in terms of sheet quality and life, how difficult installation will be, and what issues need to be addressed as the machine speeds up, changes weight, etc.
Getting both involved early also helps the mill better plan for timelines, resources, and "watch outs." Will a valve here speed up lock out time? Will this interlock protect unnecessary wear to the fabric? Also, involving machinery and fabric suppliers early can help make the pre-rebuild training much more valuable.
MATHE: Communication and understanding the mill's goals is paramount when rebuilding the forming section. The communication must start at the early stages of planning the rebuild and should include the mill, machine clothing supplier, and machine builder. The mention of clothing supplier, rather than just forming fabric supplier, is intentional because even though it may be a forming section rebuild, the entire machine process is affected. When the rebuild occurs, it often corresponds
with a complete change in grade structure. Regardless, the machine clothing supplier and the mill should work together to collect baseline data to be used as the benchmark. This data may include drainage surveys, collection of sheet samples for structural testing, furnish compilation, and machine run data. It can then be compared to the "after rebuild" data and offer defined direction for optimization of the former, papermaking process, and future clothing direction. Based on the machine rebuild objectives and grade structure, a change in the forming fabric design--along with possible changes to the press and dryer fabric designs--may be required to meet the mill's goals.
Considerations related to the rebuild of the forming section and clothing application include:
* Fabric designs that offer flexibility, including a large operating window.
* Maximum machine take-up (1-2% is desirable). Many older machines have minimal take-up, which is often overlooked when the wet end is rebuilt. Interferences with cross-bracing, pans, or showers can greatly reduce the machine's take-up capacity.
* Fabric size--installation lengths, minimum running length, and maximum running length are essential.
* Installation procedures may change greatly and should be well defined prior to the start-up.
* Packaging of the forming fabric needs to be optimized to accommodate new installation procedures and paper machine rigs, as well as forming fabric storage and handling.
* Forming fabrics inventory must be closely managed so as not to jeopardize machine up-time during the start-up phase. Damage can often occur from drive issues, guiding issues, metal shavings, and other contamination from the construction phases of the rebuild.
STAGER/MILLER: The optimum solution is to work with a supplier that offers both the machinery components as well as the fabrics themselves. This allows the customer and the supplier to run trials on the supplier's trial machine and have complete control over all variables. Such control ensures that future machine configuration, with all the associated settings, is optimized in conjunction with the engineered fabrics. A supplier with both machine and fabrics expertise is better able to provide the correct combination, minimizing the need to run unnecessary trials. Information sharing between fabric and machine supplier regarding grade structures, speeds, quality demand, and machine characteristics simplifies the optimization phase.
As an example, Voith recently rebuilt a competitor's high-speed gap former in France. The Voith TQv, with its loadable blade technology, required different machine settings, consistencies, and flows. Voith Paper and Voith Paper Fabrics delivered a near perfect design with the first set of fabrics. With separate suppliers, the customer might have had to go through several sets of clothing before finding the right design for the desired results.
SOLUTIONS! As consumption patterns change, many paper machines are being converted to higher value grades. How does this affect forming machinery and fabrics?
SWIETLIK: Higher value grades require improvements in both the fiber mix and the forming philosophy. As the fiber mix changes, physical properties--such as fiber length and freeness--change as well. The drainage properties of the stock change along with it. Both the former elements and the fabric design must match the new requirements as well as the new end product quality targets. The customer once again has the job of setting the new quality and production targets for the new grades. Also, with higher value grades the topography of the fabric has a greater influence on the end product quality and forming element selection.
FREEMAN: Higher value grades demand sheet properties that enhance the function of the paper in the end users' processes. This frequently means a smoother sheet with fiber characteristics and filler content on or near each surface that delivers high print quality. The best printing surface can only be achieved if the sheet is formed on a fabric with very high support quality. Density differences resulting from calendaring a sheet produced on an inferior fabric will result in inferior print quality.
Higher value grades require a forming fabric with excellent support for fibers aligned in the machine direction (MD). Shear in the delivery system results in this MD fiber alignment in the flow from the headbox, and the higher speeds often associated with rebuilds may increase this orientation--particularly in the portions of headbox flow that directly land on the fabric. To best support these MD-oriented fibers, forming fabrics must present higher levels of support that are oriented in the cross-machine direction. High performance products today present as many as 120 cross-machine direction support yarns, resulting in very high-quality support that makes the smoothest possible sheet on the former.
Modern former components as are used to produce higher value grades often use elements that enhance formation and water-removal rates. These elements can flush fines and filler from the surface of the forming sheet. The fabric design can effectively counter this flushing action by providing the high support quality discussed earlier.
KEMP: The change to "higher value grades" can have a huge impact on machinery and fabrics--particularly in certain grades. This change almost always includes significant variations in filler type and amount, which can increase the abrasion wear and will change drainage rates. Changes in fibers and chemicals--everything from microparticulates to optical brighteners--typically require fabric modifications. Once you add in forming section modifications, which are always more aggressive and usually have certain expectations concerning dewatering splits, it is almost guaranteed that the fabric design must change to address these issues.
MATHE: Conversion to higher value grades is becoming a common occurrence and challenge for paper mills. Sheet quality parameters vary greatly when, for example, a mill moves from newsprint to offset or lightweight coated (LWC) grades. Changing to higher value grades often involves change in basis weight, furnish, fillers, and dyes. It may also include changing coating application. In any case, production efficiency, sheet quality, retention, and fabric performance are top mill priorities. High sheet quality at low cost per ton becomes increasingly important as the mill competes against other mills producing similar grades.
High technology forming fabric designs improve retention and sheet smoothness. As a result, significant savings can be realized through reduced chemical and/or fiber usage resulting in lower cost per ton. Fabric life can be adversely affected due to increased machine speeds, vacuum levels, and filler usage--which can be addressed by a change in forming fabric design or adding wear resistant material to the current design. Again, communication is key to understanding the mill's priorities and challenges as they transition to higher value grades.
STAGER/MILLER: We continually see grade structures being converted to higher value grades. These grades usually have higher filler content and increased quality demands on sheet properties (such as fines/filers distribution [Z direction], two-sidedness, opacity, formation, tensile, etc.). When changing to higher value grades, a mill must know if its present former type can successfully produce these new grades. Conversion to a value-added grade requires an in-depth investigation of the required paper properties of the new grade. Issues such as formation, ash two-sidedness, or the final product's printing method may be critical on the new grade. Adding loadable blade technology and equalizing the top wire/bottom wire flows in the correct manner are instrumental in achieving these goals. With combined machinery and fabrics know-how, these properties can be optimized beforehand.
Communication and synergy between suppliers increases the odds for a successful conversion. For example, with an SC-A rotogravure paper machine, the ash distribution and surface structure requirements are incredibly important. The mechanical rebuild components require a fabric with a different weave pattern and number of layers. An assumption on the part of the machinery supplier, "naturally the mill will run a three-layer fabric," or an assumption by the fabric supplier, "naturally the retention of the rebuilt machine will be the same so we can stick with a two-layer fabric," can spell disaster for a startup. The one-supplier approach minimizes these risks.
EDITED BY ALAN ROOKS, EDITORIAL DIRECTOR
WHAT YOU WILL LEARN
* How mills can work with machinery and fabric suppliers to manage machine clothing changes during rebuilds
* How paper machines conversions to higher value grades affects forming machinery and fabrics
* "The basics: What you need to know about forming fabrics," by Richard Reese, Solutions!, August 2005. To access this article, type in the following product code in the search field on www.tappi.org: 05AUGSO33. Or call TAPPI Member Connection at 1 800 332-8686 (US); 1 800 446-9431 (Canada); +1 770 446 1400 (International).
* "The forming section: Beyond the fourdrinier," by Jim Atkins, Solutions!, March 2005. Product Code: 05MARSO28
* "Managing machine clothing: A Solutions! roundtable," Solutions!, November 2002. Product Code: 02NOVSO35
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|Title Annotation:||PAPER MACHINERY/CLOTHING|
|Publication:||Solutions - for People, Processes and Paper|
|Date:||Mar 1, 2006|
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