Troubleshooting: turning problems into opportunities: when bad things happen to good clothing, call the troubleshooters!
Troubleshooting paper machine clothing, then, is a key part of effective machine operations. Solutions! asked several machine clothing experts to define troubleshooting and provide case studies that illustrate key aspects of the troubleshooting process.
Shorter than expected clothing life is a common problem in all types of machine clothing, according to Randy Kimpfbeck, papermaking services manager, for Albany International, Albany, New York, USA. "Clothing life can be categorized in several different ways," he said. "First is the actual time the clothing is on the paper machine, and second is the amount of time the clothing is operating at peak efficiency (steady state)."
According to Kimpfbeck, actual clothing running time on the machine is influenced by many factors. One is excessive fabric wear caused by poor roll conditions, ceramics in either poor condition or improper ceramics application, and inadequate conditioning.
Steady state machine clothing performance is also affected by poor conditioning, he says. Poor conditioning-such as inadequate lubrication-can lead not only to excessive wear but to fabric filling as well. This lends to poor performance and ultimately early removal from the machine. "Attention to details can have a dramatic impact on machine clothing," says Kimpfbeck. "Regular roll changes, regular checks of ceramics, and inspection and repair of conditioning equipment can all enhance clothing life." (See Case Study 2 on page 31.)
KEY IS WATER REMOVAL
Most paper machine clothing troubleshooting focuses on effective and controlled water removal from the paper stock, according to Terry Lemerande, manager, paper machine technology for Voith Fabrics, Raleigh, North Carolina. Water removal is monitored and measured at the wet end from the time the stock leaves the headbox to passing over the couch roll on the way to the press. The amount of water removed at each drainage point in the wet end affects the amount of water still trapped in the fiber slurry in the sheet.
The objective is to present a sheet to the press with a minimum amount of water and a maximum amount of fiber, said Lemerande. "Today, more attention is placed on the micro-formation of tint sheet as it is formed and pressed," he stated. "The forming fabric troubleshooter must account for the type of specialized furls being used to keep micro-turbulence in the moving slurry while evaluating the relative performance of the forming fabric."
The growing use of gap formers adds still another dimension to forming fabric designs that are exposed to higher water removal forces, Lemerande continued. He noted that distance of the headbox lip from the wire gap and angle of the stock out of the headbox into the gap must be carefully calculated to develop higher fiber support structures. These structures are needed to maintain the maximum amount of fiber in the sheet through these formers. Drainage measurements taken at the wet end determine the fabric designs for the machine that will effectively seal fiber in the sheet while removing water. "However, this should not seal the sheet so rapidly that the water is trapped within the interior of the sheet as it passes over the couch roll and enters the press," he added.
Lemerande noted that a press fabric troubleshooter must consider many options in press felt design when trying to improve the quality or production rate through the press section. "Since there are few fundamental changes that can be made to the mechanical parts of the press-such as roll diameters, draw lengths and maximum nip pressures-most press section troubleshooting centers on the water removal components and the press fabrics themselves," he said. "Uhle box covers and slot sizes can be optimized to take fullest advantage of vacuum on the paper machine. Felt tensions can be a factor affecting uniform water removal. Efficient felt showering programs, which use the minimum amount of water to keep the press Felt surfaces open, are extremely effective in maximizing felt life and performance over the life of the felt."
True Stories: Machine Clothing troubleshooting in action
Mill Case Study 1
TOO MUCH FIBER, NOT ENOUGH CONSISTENCY
A mill operating a coated board machine was having to use 130 + pounds of fiber/1,000 sq. ft. to achieve sheet caliper requirements for a 125-pound grade. Sheet consistency exiting the press section was low because the third press was only loaded to the weight of the roll, again to achieve caliper/bulk requirements. The press felt design was inadequate to deliver the required sheet smoothness, dryness, and bulk.
In this case, the goals of the troubleshooting study were to increase sheet caliper/bulk (thereby reducing fiber usage); increase sheet consistency; improve sheet smoothness; reduce sheet two-sidedness; and reduce operating costs.
Weavexx technical engineers and product design engineers did a complete press section analysis using Press-Manager computer simulation techniques. The simulation revealed two distinct but equally critical problems: the misapplication of press felts and roll covers.
The troubleshooting team focused on the earlier presses because the sheet there is wettest and most easily affected. First press nip width was increased by 86%, and nip peak pressure was reduced by 46%. In the second nip, nip width was increased by 28%, and nip peak pressure reduced by 22%. (See Figures 1 and 2.) With a dryer sheet entering the third press, loading was applied to the third press without loss of caliper. The Weavexx team relied upon dwell time in the nip vs. sheer peak pressure to dewater the sheet. This is the identical theory behind long nip and shoe type presses. Improved press felt designs and softer roll covers produced wider nips with reduced peak pressures.
[FIGURES 1-2 OMITTED]
After complete evaluations of the machine's vacuum and cleaning systems, the Weavexx team determined that the mill could use and benefit from more sophisticated press felt designs. These improved, laminated press felt designs-using patented needling processes called Huyperpunch-D-along with reduced nip peak pressures from the improved roll cover design produced a smoother sheet with reduced two-sidedness.
After the mill made the operating changes and converted to the new felt design, it achieved a 3.0+ pounds reduction in fiber usage, resulting in more than $1 million/yr, savings. The mill also reduced sheet two-sidedness by 20-30 points and realized a 1 to 2% increase in sheet consistency exiting the last press. Plus, the mill enjoyed improved press felt performance, since there was a reduced rate of compaction from the use of softer nips and reduced peak pressures.
Mill Case Study 2
THE SHORT LIFE MYSTERY
A large fine paper machine was experiencing extremely short forming fabric life on the backing position. The life on this position had originally been more than 100 days, but this steadily dropped until it reached a point where fabrics could be run for just one shutdown cycle. In this particular position no vacuum elements contacted the fabric so stationary elements were ruled out as a source of the problem. The condition of the roll surfaces-which had been monitored for some time-continued to deteriorate. The roll surface roughness on several rolls was too high to measure and there was visible scoring of some rolls caused by roll doctors not oscillating. While the Albany International service team made the mill aware of these problems, the mill could not make large-scale roll changes due to maintenance time constraints and spare equipment.
During a shutdown, the Albany team installed reflective tape on all former rolls to facilitate speed checks. After startup, these speed checks showed there was no significant speed difference between the rolls and fabric. During a subsequent maintenance shutdown with the top fabric removed, roll diameter checks determined there was a significant diameter difference between the edges and the center of one of the rolls. Since this roll was a relatively small nominal diameter roll, the diameter difference was causing a large speed difference from edge to center of the roll. The roll ends were larger in diameter than the center, so the ends were running taster, causing accelerated wear on the fabric edges.
The Albany team matched the roll's diameter profile with the wear pattern on returned fabric samples, showing that the roll profile matched the wear pattern of the fabric. The roll was changed and life immediately improved. Albany International also added wear resistant material to the fabrics for the position to further enhance life. With the changes made by the mill and by Albany International, the mill was able to run fabrics a minimum of two shutdown cycles, but has yet to match previous performance. To return to the 100-plus day fabric life, more attention must be paid to routine maintenance items, such as regular roll changes and doctor oscillator maintenance.
Mill Case Study 3
THE NEED FOR SPEED
Mill Case Study A paper machine with a gap former had run well at 3300 fpm (1000 meters/min) with few operating problems associated with the paper machine clothing. However, as the machine speed approached 3600 fpm (1097 meters/min.) there was an unexplained increase in the number of wet end breaks as well as reduced wire life. Since the machine was designed to operate at 4000 fpm (1219 meters/min.), the mill wanted to investigate alternate wet end clothing designs to increase speed without sacrificing paper machine efficiency.
The Voith Fabrics technical team assigned to this paper machine used gamma gauge readings to determine how water was being removed from the sheet at the higher speeds compared to lower speeds. The team used high-speed video equipment to determine the angle of stock impingement out of the headbox slice into the gap formed by the two converging forming wires.
By methodically changing toil angles and vacuum pressures and re-directing the headbox delivery, the gamma gauge reveled alterations in the drainage profile and water levels in vacuum chambers. The Voith Fabrics team correlated this data with moisture readings taken at the reel-in both the machine direction and cross-machine direction.
These troubleshooting steps proved that the wire designs used on the paper machine were not optimum for higher machine speeds. Not enough water was being removed from the sheet before it entered the press section. This produced a wetter, weaker sheet that broke easily in the press. The Voith Fabrics team also determined that it was important to use the proper headbox angle to deliver stock into the gap. This angle, it was discovered, is highly dependent on the type of forming wire being used.
Voith Fabrics' final recommendation for this machine was to change the single layer forming fabrics to modified double layer fabrics. The increased wire caliper created longer wire life and, with related changes in the former, increased sheet dryness at the couch. This produced the desired fabric life and reduced wet-end breaks.
About the author: Alan Rooks is editorial director Of Solutions Magazine. Contact him at +1847 172-9065, or by e-mail at email@example.com.
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|Title Annotation:||Machine Clothing|
|Publication:||Solutions - for People, Processes and Paper|
|Date:||Feb 1, 2002|
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