The value of on-line and off-line analysis.
The online monitoring tool most commonly found in mills is the gamma gauge. "This instrument utilizes a small nuclear source placed under the forming fabric at strategic points along the forming section to measure the amount of water removed by the forming fabric after the various drainage elements in the forming zone," explained Terry Lemerande, director, Voith Process Solutions, Wilson, North Carolina, USA. "Readings are taken at either regularly scheduled intervals during the life of the clothing or on an as-needed basis depending on paper machine overall performance. This method is useful in determining the efficiency of both drainage elements and fabric design. It is also used to help ascertain when a forming fabric has reached the end of profitable performance."
Technicians generally use the gamma gauge as a hand-held tool, and evaluate the fabric while the machine is running. Yet, as Lemerande reports, "some mills have installed a gamma gauge head (usually near the couch roll) to continuously monitor sheet solids into the press section."
According to Dr. Wolfgang Niessen, product manager, graphical paper for Andreas Kufferath GmbH & Co. KG, Duren, Germany, gamma gauge measurements include the following parameters: stock/water suspension [g/[m.sup.2]], consistency [%], absolute [l/m/min] and relative [%] water removal as well as solids loss [%]. "Values can be given for each dewatering element," Niessen said. "Fabric caliper and tension can also be measured during production. Observation of the stock/water suspension by means of a strobe lamp is used to visualize stock turbulences and to detect irregularities in the stock/water suspension such as air bubbles, stripes, or pulsations."
Tension is an important measurement for fabrics in every machine area, said our experts. "Tensioning devices are used to monitor and control tension on dryer fabrics and press fabrics; typically there are load cells that monitor tension. There may also be instances of such a device on forming fabrics," said Joel Cason, process analyst group manager, Albany International Inc., Albany, New York, USA. "Proper tension of dryer fabrics is critical to the ability of the fabric to aid in the efficient transfer of heat from the dryer shell to the sheet. In the case of a silent drive machine where the dryer fabric drives the dryer cans, it is the tension of the fabric that maintains a constant speed from can to can within the section. Proper tensioning of dryer, press and forming fabrics also help maintain the targeted fabric CFM.
In the press section, online measurements usually include fabric caliper, permeability and moisture, Lemerande explained. "Caliper is done during a paper machine shutdown and measurement is restricted to the fabric's edges. Permeability is most commonly measured by reading the change in vacuum level through the fabric at the uhle boxes. The higher the vacuum readings, the more filled, or compacted, the press fabric. Clothing suppliers can also use a hand-held, backpack-style permeability measurement tool to measure permeability changes in the cross direction on each press fabric where safe accessibility allows."
Papermakers can also measure moisture level in the press fabric to determine the amount of water removed from the sheet after each press nip, and to evaluate the amount of water left in the fabric after the uhle boxes. This helps determine the efficiency of the press nips and the water removal capabilities of each press fabric.
"There are also online devices used to monitor vibration, typically in the press section," said Albany International's Cason. These devices measure vibration levels and adjust the stretch roll cross machine levelness (cocking). The path of the fabric is changed slightly to keep vibration levels down to a minimum. These devices are being used effectively, and we expect to see more systems of this type as word spreads about their usefulness and reliability," he added.
"In the forming section, a properly calibrated tensometer micrometer, and a high intensity strobe light are the most cost effective tools," said Marc White, sales manager, packaging & board, U.S. and Canada for Kufferath. "For more detailed analysis, mills should consider investing in an NDC backscatter gauge, either fixed or portable, plus high speed video in key areas. The most effective and increasingly popular forming fabric tool for the papermaker during shutdowns is a digital camera with a super-macro lens."
Dryer fabric monitoring is usually done online while the paper machine is down, said Lemerande. Technicians can measure the fabric condition with a handheld permeability unit, or can use tape measures to monitor stretch or narrowing. "The effect of dryer fabrics on the sheet is measured when the machine is running by comparing pocket moisture levels from one dryer section to another and one fabric type to another in the same dryer section. On dryer positions where excessive fabric wear is a problem, a portable, hand-held microscope and/or digital camera is used to monitor wear to prevent unscheduled replacement of dryer clothing." he said.
ROOM FOR TRADITION
Though online, real time measurement and analysis tools have found their place throughout the mill, traditional laboratory analysis still plays an important role in analyzing machine clothing performance. It provides accurate, important data that cannot be obtained online, and offers the freedom to perform more direct, invasive testing that can yield critical information. In addition, breakthrough technologies are helping to improve lab analysis.
"Laboratory analyses offer far more capabilities to better understand the overall properties of machine clothing," said Steve Cole, director of technology, Weavexx Corp., Wake Forest, North Carolina, USA. "It is invaluable for assessing many different issues--photographic analysis in terms of filling (location), mechanical damage, hydraulic damage, chemical damage, fabric surface analysis to paper surface, shower damage." Lab tests are also required for contamination identification (using elemental analysis, for instance) and residual strength measurements to determine material and construction integrity, Cole said.
"Laboratory analysis plays an important role in analyzing used fabrics," added Kufferath's Niessen. "By means of a microscopic analysis, the wear condition of the fabric can be evaluated. Additionally, a caliper profile is provided, which helps to calculate the remaining life time. Mills can detect high wear areas, which may give hints regarding machine related problems."
Mills can also archive caliper profiles and photos, to track wear condition development from fabric to fabric. Such a record can be a real boon to those trying to determine the possible causes for damaged fabrics.
In fact, the ability to compare performance history on a particular machine position is "probably the most important role traditional laboratory analyses provide," according to John Hawes, vice president technology, pressing, for Albany International. "In this manner, machine and environmental conditions can be evaluated in the context of their impact on performance, particularly when several fabrics of the same design populate the database for that position. Wear rates, contaminant/filling levels, and a true CD 'signature' of the position can be compared to history to alert the papermaker and the clothing designer of changes in the system."
In addition, traditional laboratory analyses are useful for special cause investigations--such as contaminant or damage/failure analysis--valuable to both supplier and paper maker," Hawes continued. "Over the last several years, new profiling and measurement equipment have improved the precision and accuracy of caliper, mass, and air permeability in samples many meters wide or long. New high-resolution imaging technology also has improved the investigative quality of laboratory analyses. These breakthroughs, combined with the overall gains obtained in information technology (such as Internet posting of reports) allow today's laboratory analyses to be performed more accurately and delivered to the customer more quickly than ever."
Scott Makepeace, product and materials analysis team leader, Asten Johnson, Charleston, South Carolina, USA noted that increased machine speeds tend to obscure both the MD and CD measurements made online, even with high speed measurement.
"Increased detail can be gleaned from analyzing paper machine clothing off-line in the traditional laboratory, thereby, more precisely separating the MD from the CD component allowing the diagnosis of many machine related issues," he said. "The traditional laboratory also offers a cleaner, more stable environment in which to do measurements, compared to the paper machine setting where fluctuations in temperature, humidity and contamination can negatively affect test results."
Papermakers benefit from the wide range of tests possible in the lab, said Dan Aldrich, North American quality manager, Voith Fabrics, Wilson, North Carolina, USA. These include continuous cross-direction profiling of the dry returned fabric sample, as well as filling analysis performed at selected locations using solvent extraction and SEM analysis.
"As needed, additional specialized tests can be performed, such as intrinsic viscosity, compression properties, water permeability, IR, bending stiffness, loop strength, surface roughness, pore size distribution, and others," he said. "Some of these tests are proprietary to Voith Fabrics, while others are standard textile tests. The results of these tests aid in troubleshooting paper machines and improving clothing design and application," Aldrich concluded.
In the forming section, as the reliability and precision of online tools such as the NDC backscatter gauge, high intensity strobe lights, and high speed video have improved, so has the net value of troubleshooting results, said White. "At Kufferath, field analysis tools such as precision micrometers and super-macro lenses for digital cameras are used during downtime and complement the online measurements," he said.
As for most industries, the breakthroughs helping to improve lab analysis for the paper industry focus on the level of data acquisition and the ability of today's computers to handle the high volume of data collected, according to Asten Johnson's Makepeace.
"Automated Fourier Analysis (FFT) routines offer a level of diagnosis that removes a good deal of the guesswork as to the origin of machine related variations," he said. "Breakthroughs also exist in the analysis of the sheet with the enhancement of being able to better quantify the sheet quality/printability through the Print Quality Index (PQI) developed by AstenJohnson. This technique provides a non-subjective, calculated value to each of the sheet surfaces that allows for consistency when comparing different sheets or monitoring important paper machine improvements."
Felt surface and paper surface imaging are definitely growing in demand, added Cole. "Weavexx has developed a computer imaging system to identify patterns and surface relationships to help understand the cause-effect dynamic for clothing and paper.
"The company is also developing a unique system using three-dimensional, non-invasive testing that measures the topography of machine clothing and paper sheets at resolutions of less than 1 micron," he continued. "Using this system, our design/applications engineers will be able to understand the relationships between paper machine clothing and the sheet at unprecedented levels."
Cole also noted that Weavexx has recently expanded its lab capabilities with a proprietary Continuous Profile machine that automatically measures machine clothing for mass variation, residual vs. original porosity and caliper, and several other variables as well.
WHAT YOU WILL LEARN
* How mills are using new and improved online measurement and analysis tools.
* The value of traditional laboratory analysis and how it is being optimized by new information technology tools.
* "Machine clothing strategies keep wraps on cost," by Janice Bottiglieri, Solutions! August 2004. To access this article, type the following Product Code in the search field on www.tappi.org: 04AUGSO26.
* "Optimizing PMC operations, by Janice Bottiglieri, Solutions!, October 2004. Product Code: 04OCTSO25.
* "Machine clothing: How do the best mills operate?, by Janice Bottiglieri, Solutions!, January 2004. Product Code: 04JANSO70.
RELATED ARTICLE: "SQUEEZE TOOLKIT:" GET THE MOST FROM MACHINE CLOTHING
TAPPI Press offers the "Machine Clothing Squeeze Toolkit," a set of essential reference materials and the latest information on machine clothing.
The Toolkit includes:
* Paper Machine Wet Press Manual, Fourth Edition, Edited by Richard Reese This book provides an introduction to wet pressing technology in the paper industry and includes information on practical press section considerations on paper machines. (224 pages)
* Energy Cost Reduction in the Pulp and Paper Industry, Published by PAPRICAN. Covers the potentially large energy cost reductions to be had by improving the energy efficiency of existing pulp and paper operations. (234 pages)
* A custom mixed, searchable CD-ROM containing more than 40 documents related to machine clothing from Solutions!, TAPPI JOURNAL, Conference Proceedings, and Technical Information Papers (TIPs).
Purchase these three items as a kit, and save! Non-Member Price: $US 265.00, Member Price, $199.00. TAPPI members save more than $100! To purchase the Machine Clothing Squeeze Toolkit, go to www.tappi.org and enter "Squeeze Toolkit" in the search field, or call U.S. (800) 332-8686; Canada (800) 446-9431; International: +1 (770) 446-1400.
RELATED ARTICLE: HAPPY RETIREMENT TO BILL JEFFREY!
After a 42-year paper industry career (including 28 years with Voith Fabrics), William L. "Bill" Jeffrey retired in November 2004. Bill began his career in 1962 with Simpson Paper in Vicksburg, Michigan. His paper mill experience included technical service, customer service and sales functions for several companies.
In 1976, Bill joined Porritts & Spencer, part of the Scapa Group, where he sold press fabrics in the upper Midwest. He was promoted to sales manager in 1979 and moved to the Wilson, N.C. press fabric plant. He went on to become corporate accounts manager for Scapa, which was acquired by Voith in 1999. During his years of service to the company, Jeffrey built strong customer relationships through his sales management positions, and served as a member of Voith's Fabrics Marketing and Communications team. Newly retired, Jeffrey is enjoying time with his wife, Dorothy, his three children and six grandchildren.
JAN BOTTIGLIERI, SENIOR EDITOR
ABOUT THE AUTHOR
Janice Bottiglieri is senior editor for Solutions! and editor of TAPPI JOURNAL Jan can be reached at email@example.com or by phone at +1 847 466-3891.
|Printer friendly Cite/link Email Feedback|
|Title Annotation:||MACHINE CLOTHING|
|Publication:||Solutions - for People, Processes and Paper|
|Date:||Jan 1, 2005|
|Previous Article:||ABS Scanpump AB.|
|Next Article:||Paper machine survey tracks performance issues.|