Paper machine survey tracks performance issues.
* Provide feedback on machine information provided by paper company representatives by comparing performance data to TAPPI guidelines,
* Increase awareness of paper machine performance guidelines included in TAPPI technical information papers,
* Compile information for the next revisions of technical information papers,
* Obtain information for an article in Solutions! magazine.
Performance data were submitted from paper machines in the United States, Brazil, Canada, Scotland, and some unidentified countries. Grades included fine paper, newsprint, bleached board, virgin and recycled corrugating medium, virgin and recycled linerboard, kraft bag, old corrugated container (OCC) bag, tissue, towel, and some specialty grades. Table 1 summarizes the submitted information. Machine-specific comments were sent to all representatives who responded to the survey, except for those who did not supply contact information. The compiled information include the following characteristics:
1. The range of basis weights and reel speeds is wide due to inclusion of light and heavyweight grades.
2. Headbox consistencies range from 0.2% on bag and tissue grades to 1.26% on a twin-wire newsprint machine. TAPPI Technical Information Paper (TIP) 0404-47 includes headbox consistency guidelines for all of the grades listed.
3. The range of sheet consistencies to the press section is also very broad. The lowest reported consistency (32%) was on a bag machine; the highest reported consistency (49.5%) was on a corrugating medium machine with a shoe press. Sheet consistency targets by grade are included in TIP 0404-47 Sheet dryness leaving the press section is a critical number for maximizing production rates and minimizing energy consumption. TAPPI TIP 0404-19 (press section monitoring) and TIP 0404-52 (press section optimization) provide guidelines for evaluating and optimizing press section operation.
4. Calculated TAPPI drying rates showed a wide range--from fair to excellent--when compared to the TAPPI drying rate curves. The procedure for calculating TAPPI drying rate is given in TIP 0404-07. Drying rate curves for major grades are included in TIPs 0404-08 (corrugating medium), 0404-09 (linerboard), 0404-12 (woodfree printing and writing), 0404-14 (bleached board), 0404-15 (newsprint), 0404-16 (kraft papers), and 0404-56 (pulp-can dryers). The drying rate curves are updated regularly by the TAPPI Water Removal Committee. Machines with the highest drying rates typically have dryer bars, modern stationary siphons with optimum clearance settings, dryer fabrics with tensions of at least 8 pli, and good steam and condensate system design, maintenance, and operation. TIP 0404-33 (dryer section performance monitoring) also provides guidelines for evaluating dryer section performance.
5. The newsprint machines included in the survey were reported to have very low average steam pressures. One mill reported an average main section steam pressure of 6.6 psig. Better water removal in press sections, lighter newsprint basis weights, and use of stationary siphons and dryer bars have reduced drying requirements and improved drying efficiency. Some equipment suppliers are increasing syphon clearances and detuning dryer bars to better control steam pressures ranges.
6. The top performing tissue machine had a total drying rate of 22 lb of water evaporated per hour per square foot of Yankee dryer surface. Total drying rate includes drying by the Yankee cylinder and hood. Top total drying performance is more than 30 lb water evaporated/hr/ft2. Typical drying contributions are 7-12 lb/hr from the Yankee and 18-23 lb/hr on high efficiency hoods. High Yankee dryer contribution provides the best energy efficiency because steam usually has lower cost per Btu than alternative fuels used to heat hood air make-up. Key parameters in optimizing drying on machines with Yankee dryers include condensate removal and hood design (temperature, air velocities, clearance, etc.). TAPPI guidelines are needed on key parameters in tissue drying on Yankee dryers.
7. Sheet moisture content at the dry end ranged from 4% on a tissue machine to 9% on a corrugating medium machine. Raising moisture content entering a size press or at the reel permits operating at higher speeds or reducing energy consumption. Moisture profile variation typically increases when average moisture targets are raised, unless the machine has moisture profile correction capability.
8. Reported total machine downtime varied from 2.9% on a recycled liner machine to 22.6% on a kraft bag machine. The TAPPI guideline is 7% (TIP 0404-47). The difference in operating with 2.9% and 22.6% downtime represents a very large difference in potential production and profitability.
9. Reported lost time due to sheet breaks ranged from 0.5% on a towel machine to 5.5% on a newsprint machine. The lowest reported sheet-break lost time on a machine operating without a Yankee dryer was 0.8% on a recycled liner machine. One mill reported that lost time due to wet-end breaks had decreased significantly after a video camera system was installed. The TAPPI guideline is <3 % for machines without Yankee dryers. Tissue and towel machines normally have fewer breaks than other machines since the web is supported until it leaves the Yankee dryer.
10. Reported average lost time per break ranged from 6 to 39 minutes on machines not running a Yankee dryer. A good performance target is 15 minutes. Machines with low lost time per break have good crew teamwork with each crew member having assigned jobs, performing tasks quickly, and then helping to thread the machine. Sheet threading systems have to perform well to keep lost time per break low. Papermakers have improved sheet threading by aligning pulleys and sheaves, installing dryer groove fillers, and upgrading sheet threading devices.
11. Reported scheduled maintenance downtime ranged from 1.0% to 5.5%. There did not appear to be a correlation between amount of scheduled maintenance downtime and unscheduled maintenance downtime. The TAPPI guideline for scheduled downtime is less than 1.5%.
12. Unscheduled maintenance ranged from 0.4% on a recycled liner machine to 6.7% on another liner machine. The TAPPI guideline for unscheduled maintenance is less than 1%.
13. Process downtime (washups, unscheduled clothing and rope changes, etc.) ranged from 0.3% to 6.5%. The 6.5% was on a kraft bag machine. The TAPPI guideline is less than 1.0%.
14. Reported grade change lost time ranged from zero to 1.5%. The amount of grade change lost time is machine specific. It varies with the number of grades produced and basis weight range. Compact approach system designs and a specific focus on reducing the time required to change grades have significantly reduced lost time on some machines.
15. Reported lost time caused by other departments ranged from zero in some recycled fiber mills to 1.9%. Well-run mills have all departments operating efficiently with other departments causing less than 0.5% of downtime.
16. Reported first quality ranged from 85.8% to 99.6%. Percent first quality is defined as the percentage of paper on the reel shipped to customers or converting operations. Losses include slabs, off-quality, trim losses, etc. Trim losses are 2% to 3% of reel trim with good machine scheduling and properly sized machines. TAPPI guidelines for percent first quality range from 93% on fine paper to 97% on containerboard grades.
17. Overall machine efficiency is defined in TIP 0404-47 as percent uptime multiplied by percent first quality. The range on machines participating in the TAPPI survey was 66.4% on a kraft bag machine to 95.2% on a recycled liner machine. The 66.4% value indicates that that machine produces two-thirds of the saleable paper that could be produced if it ran good paper all of the available time, without trim losses.
Major operating concerns included drying rates, safety, drive performance, water quality, sufficient sheet tests to permit running lower basis weights, maintenance and preventive maintenance, press fabric life, cross machine profiles, carrier rope life, sheet handling, creping blade performance, and Yankee dryer doctoring. Mills also want to reduce breaks, downtime, wrinkles, former bearing failures, clothing damage, cull due to Cobb tests, furnish cost, dryer linting, slime buildup, and stickies problems.
Additional concerns included dryer head issues, tiger striping, higher mixed waste, and freeness control. Suggestions for resolving operating issues were included with feedback responses.
Jasper Mardon developed a maintenance evaluation obtained by dividing unscheduled maintenance by scheduled maintenance down time. Mardon suggested that a properly maintained paper machine would have a maintenance ratio index of less than 0.3. Maintenance ratio indices calculated from data from the TAPPI survey ranged from 0.4 to 3.0. The Mardon target of 0.3 appears very low, but a ratio index of 0.4 to 1.0 appears to be a practical target for modern mills.
Many mills have reduced maintenance spending in an effort to lower operating costs. This approach typically reduces short-term cost, but has an adverse long-term effect because equipment failures that occur are more significant and the repairs often require long machine outages. It is interesting that the recycled liner mill that had a 0.4 maintenance ratio also had the lowest total lost time of 2.9%. The newsprint machine that had a 3.0 maintenance ratio reported total lost time of 20.7%. Several articles published in recent years indicate that drastically reducing maintenance spending has an effect similar to "shooting yourself in the foot."
All of the recycled liner machines in the survey were U.S. "mini-mills" installed in the 1990s. The mini-mill concept was to install paper machines and secondary fiber systems at minimal capital cost with only equipment necessary to produce competitive containerboard. These recycled liner machines all reported total lost time of less than 7% and high overall machine efficiency. The recycled liner machines in the survey all appear to be performing better than most liner machines using a high percentage of kraft pulp.
One of the recycled liner machines had the highest overall performance of all the machines in the survey. This machine, with a reported downtime of only 2.9%, exceeded all TAPPI performance guidelines.
Some email communications developed in the course of analyzing survey information:
* A fine paper mill in Brazil wanted to develop a better understanding of TAPPI drying rate calculations.
* A mill in Scotland reported that it experienced a significant reduction in wet end breaks and increased yield by 2% after installing video cameras at sheet edges on a twin-wire machine.
* One respondent's machine had the only mild steel machine-glazed cylinder running in the world. He would like assistance in exploring further possibilities of using mild steel cylinders in place of cast iron. (Please contact the author if you can provide assistance.)
The TAPPI Paper Machine Performance Survey generated information that will be helpful in updating TIPs and should be helpful to paper companies trying to benchmark their paper machines. Similar surveys could be conducted on other topics such as paper machine energy consumption. Please contact the author with suggestions.
PARAMETER UNITS RANGE TAPPI GUIDELINE Basis Weight lb/3,000 sq 11.2-207 ft (gsm) (18.2-337) Speed fpm (mpm) 705-4,400 (215-1,341) Headbox Consistency % 0.2-1.26 0.2-1.2* Dryness off Couch % 17-28 20-27* Dryness Leaving Press Section % 32-49.5 40-48* Average Steam Pressure psig 6.6-150 Moisture at Dry End % 4.0-9.0 Total Downtime % 2.9-22.6 <7 Break Downtime % 0.5-5.1 <3 Scheduled Maintenance Downtime % 1.0-5.5 1.5 Unscheduled Maintenance Downtime % 0.4-6.7 <1 Process Downtime % 0.3-6.5 1 Grade Change Downtime % 0.0-1.5 * Downtime Caused by Other Departments % 0.06-1.9 0 Average Lost Time per Break min 2.5-39 Mardon Maintenance Ratio Index 0.4-3.0 Percent First Quality % 85.8-99.6 92-97* Overall Machine Efficiency % 66.4-95.2 88-90* * Varies by Grade Table 1: TAPPI PM Survey Summary.
IN THIS ARTICLE YOU WILL LEARN:
* The operating characteristics of the best machines surveyed.
* Major operating concerns for mill staff who responded to the survey.
* "Papermakers share ideas on improving mill performance," by Richard Reese, Solutions!, Dec. 2004.
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 firstname.lastname@example.org.
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|Author:||Reese, Richard A.|
|Publication:||Solutions - for People, Processes and Paper|
|Date:||Jan 1, 2005|
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