Press fabrics: a troubleshooting guide: when something goes awry in your press fabrics operation, isolating the problem is half the battle.
CONDITIONING IS KEY
By Randy Kimpfbeck, papermaking services manager, Albany International Corp.
Any one piece of paper machine clothing can cause issues, but press fabrics are particularly susceptible to poor performance if not properly conditioned. The time to make certain your fabrics will maintain peak lifetime performance is while the machine is down for maintenance.
When the machine is down and old fabrics have been removed from the machine, the saveall pans, showers, and framework should be completely cleaned of stock buildup. This procedure should include cleaning roll ends and bearing housings of any grease or oil buildup, as well as repairing any oil leaks. If the press section is not properly cleaned, the press fabric will become dirty during the installation.
During maintenance outages, the condition of all surfaces touching the press fabrics should be checked for uneven wear and overall surface condition. These surfaces should include press roll felt rolls and uhle box covers. Worn or rough surfaces can lead to non-uniform wear of the fabrics and can produce unusual moisture profiles. While checking the press roll surfaces, suction rolls must be checked for plugging, grooved rolls for filling, and press rolls for cover looseness. Press roll doctor blades are normally changed during outages, but all too frequently papermakers ignore changing the felt roll doctors--quite often leading to wear on felt roll covers.
The showering equipment must be properly maintained during these shutdowns. Two frequently ignored areas are nozzles on lube showers and high pressure showers. Papermakers frequently use welding tip cleaners to clean plugged nozzles, but the tip cleaners are harder than the steel used in making the nozzles. The cleaners will both enlarge and roughen the bore in the nozzle. This will produce an uneven flow pattern from the nozzle and can cause more frequent plugging of the damaged nozzle. Nozzles should always be cleaned with material softer than the nozzle. While a paper clip can be used to dean the nozzles, the best procedure is to change plugged nozzles. This is a small investment compared to what non-uniform wear can do to press fabric performance.
IMPROVING PRESS SECTION EFFICIENCY
By Daniel Hedou, product business leader--press, AstenJohnson
Sometimes we miss the little things that could make life so much easier. It is easy to lose focus working on the press section of a paper machines. We become concerned with peak pressure calculations, specific vacuum readings, roll crowns, and moisture contents. Then the little things go awry and undo all our good work.
Here are three pieces of bite-sized advice for your consideration:
1. Move high pressure showers within 3-4 inches of the press fabric surface. Having the nozzles further away increases the odds of losing the laminar flow critical for fabric cleaning. Turbulent flow will gradually wear out the fabric surface. Vacuum levels will stay low, but the resulting fabric degradation will affect profiles, leading to shedding and reduced fabric life.
2. Check and replace doctor blades. We often take doctoring press rolls for granted. In 17 years of field experience I have seen at least several dozen situations where simply replacing a worn doctor changes a machine from constant haying-out mode to smooth operation. Doctors are cheap! Replace them routinely.
3. Measure solids. Papermakers are doing a better job of measuring solids, but many press sections still lack good data that tracks press section water removal. Water removal is "Job One." Each percentage of consistency gained is worth countless dollars in drying costs or can be traded as efficiency from increased sheet integrity.
My favorite measurement is a well-maintained water collection and measurement system. If used daily, it will provide excellent feedback on the state of individual presses and fabrics. Until you have this device, refer to a steam reading from the dryer section. It won't provide the same level of detail, but will give you an overall performance indicator. Draws can also be used, but they can be affected by the adhesion properties of the solid roll and the precision of drive readings used.
STEADY STATE OPERATIONS
By Andy Lilburn, director, process systems, Voith Process Solutions (VPS is Voith Paper Technology' s papermaking consulting and troubleshooting team)
The performance of press fabrics is best measured by how little performance changes over the life of the fabric. Assuming an optimum design, the fabric must be maintained in a condition as close to new as possible. The fabric must maintain its ability to absorb water removed from the sheet as a result of hydraulic pressure in the press nip. The felt condition should not change the hydraulic pressure in a given nip over time.
The ability to absorb water is a function of void volume and the absence of hydrophobic material on the surface of batt fibers or base fabric. The most important variable affecting a fabric's ability to absorb water is conditioning equipment and chemicals. Improper, inefficient and variable conditioning is the single most common cause of variability in press fabric performance.
Online press fabric condition can be measured by regular moisture scans that identify variable water content in the fabric over time. Fabrics that no longer absorb water carry less water because of the reduced void volume of the fabric or because of hydrophobic material in the fabric. Moisture scans can help identify variations in the water content in the fabric's cross machine direction in the fabrics.
Uhle box vacuums are also used to identify conditioning problems. A contaminated fabric restricts airflow through the fabric and vacuum will increase up to the header pressure of the vacuum supply. The most effective way to measure press fabrics water handling is to install weirs to measure the flows leaving the press section, including press pans. These measurements can be used to calculate a material balance on a continuous basis. Individual fabrics and press nips can be monitored simultaneously, allowing problems to be quickly and accurately identified. The dewatering path for individual nips can be monitored to determine the proportion of water removed via press fabrics and uhle boxes versus nip dewatering through vented rolls or in press pans. Many times the runnability of the machine will change for the worse as the water removal path changes.
In the short term, mills can use these indicators to change fabrics, batch clean fabrics, or make other adjustments, such as reduced press loads. In the long term, they should be used to evaluate press fabric designs and to guide decisions about optimizing the press fabric-conditioning program.
The bottom line is that press fabrics need to perform consistently over time. The less variation, the more profitable the operation.
USING YOUR UHLE BOX CORRECTLY By Chuck Moore, press felt design engineer, Weavexx, Starkville, Mississippi, USA
Proper positioning of uhle boxes on a paper machine can help solve issues with premature felt wear, streaks and non-uniform profiles. To check your uhle box for misalignment, run a piece of string on the front and backside of the machine between the rolls, crossing over the top of the uhle box. With your string as a reference, check to make sure the uhle box is level from front to back and positioned properly relative to the machine direction run. The uhle box should be just touching the string if the uhle box and roll are on the same side of the felt. If the uhle box and roll are on opposite sides of the felt, compensation for the felt caliper should be taken into consideration (see Figure 1).
[FIGURE 1 OMITTED]
If the uhle box is too far into the position of the felt it can cause wear. If the uhle box is too far away from the felt it can cause leakage. If the uhle box is not level in the cross machine or positioned properly in the machine direction then leakage and/or wear will occur, causing streaks and a non-uniform profile.
Proper lubrication showering of press felts is another commonly overlooked, but critical part of paper machine operations. Water should be carried on the surface of the felt to allow for lubrication as the felt crosses the surface of the uhle box. To achieve this, position lubricating showers to allow the water jet to strike the felt as close to the uhle box as possible, without spraying the box, to protect against all the water being absorbed into the body of the felt. Double coverage of spray nozzles--and oscillation of the showers on a high-speed machine--is ideal. A regular check across the machine for plugged shower nozzles is important to protect against moisture streaks in the felt, which can cause profile problems. S!
Alan Rooks, editorial director of Solutions! magazine, edited this article using contributions of experts from Albany International, AstenJohnson, Voith Fabrics, and Weavexx.
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|Title Annotation:||Practical Solutions|
|Publication:||Solutions - for People, Processes and Paper|
|Date:||Jun 1, 2003|
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