Papermakers share ideas on improving mill performance.
1. Pulp testing -- What pulp tests provide the best indication of pulp quality coming to paper machines?
Some recommended tests include fiber length, surface tension (becoming more popular), water retention value (WRV) or modified WRV, conductivity, and charge (although conductivity can skew numbers). Pulp quality analyzers can be used with advanced multivariate analysis to help predict effects of furnish changes on machine performance ahead of time. Some fine paper mills found water retention value useful as they converted from acid to alkaline.
2. Use of hand-held computers -- Is anyone familiar with mills using hand-held computers to assist with operator and maintenance rounds?
One mill is using them to input infrared temperature measurements of the sheet and fabrics on a diagram of the machine. Others are using bar codes to ensure that measurements are taken in the right location.
Some mills in Scandinavia using supplier service programs have extended the program so that when observations are entered outside the normal range, a server will automatically send emails to supplier representatives. Success depends on operator buy-in and acceptance.
3. Stock approach hydraulic instability -- Has anyone had issues with hydraulic instabilities around a pressure screen? The instabilities show up as head variation with a 6-20 second period, and they occur whether there is any fiber in the system or not. These are conventional screens where height has been extended to add capacity.
This range could be caused by a standing wave in a tank or a very fast-acting control loop. One mill had to install in-line attenuation to address a similar problem. A mill in Wisconsin had a similar issue, so they bypassed the pressure screen, but it did not make any difference. There may be a manufacturing issue with this type of screen.
4. Machine Direction (MD) Instability -- A fine paper machine had MD instability that showed up in MD tensile strength orientation (TSO) strips as frequency and fiber angle variation. The frequency was not apparent in pulsation measurements. Piping changes reduced the variation, but it has not gone away. Frequency tends to be 6-7 Hz, with some random low-frequency variation. Any recommendations?
Chemistry variations can cause variations like this, especially with a top wire former. Headbox vibration would have to be very bad to cause this type of variation.
5. Pinholes -- What might be causing pinholes on 20-lb bag paper? The machine has no deaerator, and air entrainment at the wet end is 1.5%-2.0%. Defoamers did not help.
Suggestions included installing a deaerator and graduating vacuum on the table to change the rate of water removal in that area on the table. Entrained air should be less than 0.5%.
Wet strength agents can cause foam problems. Pump seals can allow air to leak into stock. Sand has also been known to cause pinholes through the calender stack.
6. Shoe Press Fabric Degradation -- In three recent cases, shoe press belts and fabrics have degraded prematurely on paper machines producing kraft grades. Belt life has been as short as 13 days. The failures are not solvent related. Are there unusual chemicals that could cause this degradation?
Participants noted the trend toward using ammonium bromide biocides, but it seems unlikely that this could be causing the fabric problems.
7. Wrinkle -- A pulp dryer with a fourdrinier had a wrinkle in the center of the sheet at the first press that started when the third press fabric was changed. Couch and first press roll alignment appeared to be okay. Changing the first press fabric made the problem worse, but going to an older "basket weave" fabric design improved the situation. What else can the mill try?
Participants suggested checking the water permeability of the press fabrics.
8. Bulk/Basis Weight Ratio -- How can a tissue mill improve tissue bulk/basis weight ratios with a crescent former? (They already over-dry the sheet to less than 2% moisture.)
Suggested areas to look at to improve ratios include crepe ratio, triple-layer forming fabrics, checking wire tension, doctor blade design, and Yankee coating packages.
9. Dryer Dusting -- How can we correct dryer dusting issues on a corrugating medium machine? The machine runs very low (10-15 psig) steam pressures in wet end dryers.
The problem might be corrected by using significantly higher (60 psig or greater) wet end pressures. Dryer can passivation was also mentioned as a way to eliminate picking. Look at the charge of the fines portion of whitewater by comparing filtered and unfiltered samples.
Some machines have picking issues when fines get close to isoelectric or even cationic. The relative split between upward and downward drainage in the press section was also mentioned as a possible contributor.
10. Whitewater Sampling -- What is the best way to ensure representative sampling for on-line whitewater solids control?
Each machine tends to have a different location to provide the most representative sample for effective control.
11. High Consistency Refining -- Is high-consistency refining needed to produce extensible bag grades?
A mill representative indicated that nearly all producers of extensible grades have high consistency refining (around 30%) systems to achieve cross directional (CD) stretch properties, and that they feel they could not make these grades without it.
12. Killing Fluorescence -- Are there any additives that effectively quench fluorescence?
Bleach and titanium dioxide are possible choices. Some quench products can distort dye colors, however.
13. Press Section Weirs -- Would it make sense to set up a small blower to blow off foam build-up in press section weirs?
Participants indicated that bubble tubes can provide simple, accurate weir box level measurements. Some mills are controlling vacuum based on these flows. If felt water is reused, tanks should be set up to allow foam to overflow.
14. Dryer Linting -- How can a mill producing lightweight coated paper reduce dryer linting? The mill has already valved off several wet end dryers to minimize linting.
One participant indicated that good vacuum capability is needed for wet end dryers. His rule of thumb is that the early dryers should be no more than 20[degrees]F hotter than the sheet Press solids, center roll condition, and the condition of hood air systems were mentioned as additional areas to check.
15. Oak Vessel Segments -- What is the best way to handle oak vessel segments on fine paper grades?
High consistency refining (30%-32%) followed by latency chests (about 30 minutes retention time) may be one of the best ways to minimize the effects of vessel segments. Additional ways to minimize effects include using high quality size press starch with good film-forming capability, low-intensity refiner plates, and internal starch.
16. Bottom Dryer Felting -- Why do some mills opt to run their board machines without bottom dryer fabrics?
Increased drying capacity is often offset by decreased machine efficiency from clearing breaks and wads, especially if a machine does not have a good break detection system. Machines with no inching or reverse drives are not able to clear wads easily.
The benefit from bottom felting is greatest at the wet end of the dryer section (where the potential for wrapping dryers is also greatest) and less at the dry end because sheet-to-dryer contact improves as the web gets stronger.
17. Larger Size Press Rods -- What are the effects of using larger rods in metering size presses?
Several machines are beginning to use larger rods in the size press to increase life and improve coating. Rod size is sometimes limited by mechanical limitations of the rod drives.
18. Press Fabric Design Trends -- What are the trends for fabrics in shoe presses?
The newest fabrics are being designed to have a smooth enough surface to obtain desired surface properties, while making sure water can get out from the fabric. Multi-axial and nonwoven products are becoming more common in Europe.
Opening up the base structure can increase fabric life. Some fabrics incorporate multiaxial base fabrics with nonwoven or membrane structures on top. Stratified webs can have open base materials as great as 40 denier inside, then graduating to 25, 15, and 3 denier closer to the fabric surface.
Some designers are talking about going even lower than 3 denier. Pure nonwoven designs have gone from 2% to 15% use in Europe. Shoe press fabric designs tend to lead the way, and roll presses follow. Roll presses are converting to more multiaxial designs.
Pure nonwovens are very compressible and can have shorter life spans. Pressing trends to eliminate the third nip and single-nip pressing are putting greater demands on the fabrics.
The fourth machine with a single-nip press section is reportedly starting up now. Essentially, some board machines are now running tissue fabrics.
WHAT YOU WILL LEARN
* How to eliminate pinholes.
* Methods to reduce dryer dusting.
* Trends in press fabric designs.
* 2004 TAPPI Paper Summit, Papermaking, PCE & I, and P & PQ & Environmental Conference Proceedings CD-ROM. To purchase this CD-ROM (US$ 55 for TAPPI members, US$ 80 for non-members), go to www.tappi.org and type "Paper Summit 2004" into the search field. Click on the first link.
Editor's Note: Dick Reese will present the results of the TAPPI Paper Machine Survey in a special feature in the January 2005 issue of Solutions! magazine.
ABOUT THE AUTHOR
Dick Reese has worked on paper machines for more than 40 years in various production and engineering/technical support roles. He has been a TAPPI member since 1973 and is an active member of the Papermaking and Water Removal committees. He was named a TAPPI Fellow in 1992, received the Manufacturing Division Technical Award in 1997, and the Engineering Division Technical Award in 1999. He is currently an independent papermaking consultant in Norcross, Georgia, USA. He can be reached by phone at +1 770 448-8002 or email at email@example.com.
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|Publication:||Solutions - for People, Processes and Paper|
|Article Type:||Cover Story|
|Date:||Dec 1, 2004|
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