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In 1998, Inland Paperboard and Packaging Inc. implemented cost reduction efforts at its linerboard mill in Maysville, Ky., as part of the company's Mplus50 Project. This project's goal was to increase revenue by $50 million through process improvements. As part of the project, all papermaking processes were audited and a paper yield task team was formed at each mill to identify cost saving opportunities. Since the price of paper was low, the focus was on reducing costs rather than increasing production.
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Inland Paperboard and Packaging has installed stationary siphons on 45 of 55 dryer cans.
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Based on the recommendations of the paper yield task team at the Maysville mill, stationary siphons were installed on dryer cans in five of the No. 1 paper machine's six dryer sections. Conversion to stationary siphons made a significant contribution to lowering costs and increasing production by reducing steam consumption, dryer flooding, dryer felt wear, and steam joint maintenance, while increasing machine uptime. The installation of stationary siphons on the remaining dryer cans is planned for the near future.
DRYER SECTION DESCRIPTION. Inland's Maysville, Ky., mill produces 26-lb to 69-lb 100% recycled linerboard on its No. 1 paper machine, which has a trim width of 289 in. at the reel. The dryer system has 55 dryer cans divided into six steam sections. The first section has five dryer cans, two of which are not heated. The remaining sections have ten dryer cans each. With the exception of the unirun section, the dryer drives are "silent-drive" type, meaning the motor only drives the last four cans in each section and the remaining cans turn due to friction with the felts. Rotary siphons were installed on all dryers at startup in 1992. The third and fourth section dryer cans were also equipped with turbulence bars.
The mill uses a recirculating thermocompressor differential pressure control system to remove condensate from the dryer cans. With this system, condensate is drawn out of the dryer cans, since a lower pressure is maintained in the condensate flash tanks.
To maintain the differential pressure, the blow through steam is recirculated into a thermocompressor where it is recompressed and injected into the steam header. When the thermocompressor valve is wide open and differential pressure is still below set point, excess steam is discharged to the off-machine silos for heating. When the silo valve is wide open and the differential pressure is still below set point, the blow through steam is then vented to the atmosphere (Figure 1).
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Figure 1. The 55 dryer cans are divided into six steam sections.
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DRYER FLOODING CHALLENGES. As the Maysville mill's No. 1 paper machine was optimized and production increased, dryer flooding became increasingly problematic. The mill's paper yield task team identified several issues associated with this flooding:
Energy loss. Dryer can flooding caused extensive energy loss. Differential pressures as high as 18 psi in the early dryer sections were required to evacuate the dryer cans. This resulted in excessive blow through steam. Also, venting was a serious problem during production of linerboard weights of 33 lb and lower.
Machine speed limitations. Dryer flooding limited machine speed on the No. 1 paper machine. When the dryer drive motors became loaded due to dryer flooding, the machine speed was reduced to regain normal operation. This occurred when producing linerboard weights of 42 lb and heavier. Also, when dryer cans flooded during operations, it could take as long as ten hours to drain them.
Delayed threading. Dryer flooding during sheet breaks delayed threading and extended the time required to bring the machine back up to speed. Leaking steam valves caused dryer flooding in all the sections. The second section flooded in minutes after shutdown, because the silent-drive allowed the rotary siphons to stop at positions other than six o'clock. This section took 30 minutes to one hour to drain during a shutdown. During startup, dryer flooding delayed threading and caused repeated drive trips while the section was brought up to speed.
Felt wear. Dryer flooding caused excessive wear on the silent-drive felts of the second section. Flooded dryers in the second section required more force from the felts to turn them. By 1997, the felts on the second section that had an expected life of 200 days were splitting at the seams within 60 days.
STATIONARY SIPHONS. The use of rotary siphons in the silent-drive second section had been a concern since the 1992 startup of the No. 1 paper machine. The stationary siphons on the market at that time were considered unreliable due to siphon shoe erosion, vibration, and other mechanical problems. However, by 1998, stationary siphons were considered a reliable, efficient, and low-maintenance alternative to rotary siphons.
The paper yield task team investigated stationary siphons as a means to reduce the mill's dryer flooding problems. (Figure 2) Because condensate does not have to resist centrifugal forces to exit the dryer cans through stationary siphon systems, less differential pressure is required to evacuate condensate out of the cans. Consequently, there is less blow through steam and steam venting. The stationary siphons are particularly suitable for use on silent-drive sections, where rotary siphons may not stop in the six o'clock position during shutdowns. The paper yield task team estimated that significant savings could be achieved for second section felts by converting from rotary siphons to stationary siphons.
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Figure 2. The mill team saw stationary siphons as as way to reduce dryer flooding.
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The paper yield task team evaluated and received good recommendations on several stationary siphon vendors. Because of factors such as low-maintenance design, ease of installation, and level of service, the team recommended conversion of the rotary siphons to Deublin stationary siphons. The conversion was approved in two phases. Stationary siphons and turbulence bars were installed in 13 dryer cans in August 1999. The next 30 dryer cans were converted for Phase II in March 2000.
A primary consideration for recommending stationary siphons was the maintenance required on the steam joints. The existing dryer system required extensive maintenance and one or more dryer cans were constantly valved out for repair. The paper yield task team saw the Deublin joint as an attractive alternative, because it allows seal face measurement to assess joint wear and predict remaining seal life. This measurement permits seal replacement as required, not after failure or earlier than necessary to avoid failure. The joints were also designed to minimize loading and friction on the two seal faces to extend seal life and minimize maintenance.
RESULTS. During Phase I of the installation, an unexpected bolt pattern on 12 of the 13 dryer cans required that the pattern be measured so the steam joints could be re-cut. For Phase II, the supplier provided joints with both bolt patterns, so the installation went more smoothly. The task team was very pleased with the results of the dual bolt pattern. Since the first conversion in August 1999, there has been only one stock issue for Deublin parts, and it was a result of the original incorrect bolt pattern and not a typical repair. Along with a noticeable reduction in required maintenance, the following benefits from the conversion have been noted:
Energy savings. There was a substantial improvement in the lb steam/lb paper consumption after Phase I and further improvement after Phase II. There was a significant reduction in blow through steam and venting. Steam savings amounted to 200 lb steam/ton of paper produced.
Machine speed increase. After Phase II, there was a big reduction in steam required to dry the sheet. Before the conversion, the dryer can maximum allowable pressure limited the drying rate and machine speed. After the conversion, improved heat transfer from a minimized condensate layer resulted in the same drying rate at lower operating pressures. This enabled a significant machine speed increase.
No dryer flooding. There were no more instances of dryer flooding in the sections converted to stationary siphons. After Phase I, the maximum differential pressures required to evacuate cans was reduced from 18 psi to 10 psi. After Phase II, operators lowered differential pressures as low as 5 psi. The operators have been reluctant to reduce differential pressure further due to the risk of dryer flooding. This is shown by the gradual reduction of lb steam/lb paper since the Phase II conversion in Figure 3.
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Figure 3. Since the stationary siphons were installed, the Maysville mill has seen a gradual reduction in steam use.
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"Silent-drive" felt performance. Felt roll damage continued to limit the second section felt life until the damaged felt rolls were replaced in March and August 2000. Since that time, the second section felts are running until scheduled off after the expected life is reached.
Better seal life. Since the conversion of the steam joints, seal wear measurements have been taken regularly once every two months with a telescopic gage. Based on current measurements, the seal life of the balanced mechanical seal is extrapolated to five years service life—a significant improvement over the old design—that will further reduce maintenance man-hours and dollars. The Inland maintenance department also reports that no siphon-related problems have occurred since the conversion.
The conversion of the ten dryers in the last section to stationary siphons has been approved and will go ahead in the near future. The mill also conducts operator training on the stationary siphon components to acquaint operators with the performance and capacity of the new condensate removal system.
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