By Mark Williamson, Journalist/Engineer, Thornhill, ON, Canada
BRUSSELS,
July 16, 2009
(RISI) -
It's often the case that end use requirements drive the need for improved quality and productivity of paper and board production lines. When the end product converting process takes a step change, the performance of the entire production chain certainly must move up a step in concert. To keep up with new, more rigorous end-user demands, paper and board producers should update their existing processes regularly and invest wisely in new equipment and employee tools and skills required to meet their customers' expectations.
PABCO Paper in Vernon, CA, was confronted with this challenge when PABCO Gypsum, its main customer and sister company, increased the production of the gypsum wallboard plant near the company's Las Vegas, NV, gypsum quarry by expanding to a new, high speed wallboard line in 2005. PABCO Paper and PABCO Gypsum are part of the Pacific Coast Building Products group, one of USA's leading suppliers of building products and services to the construction industry.
The mill took a hard look at how to achieve the increased production and better quality needed to meet the Las Vegas plant's needs. Then it decided to invest in improved process and product quality measurements, process stabilizing controls, and vastly expanded operator information and decision-making tools supplied by Metso. Product quality variability is much improved. In fact, customers rate the quality as the best ever. Production levels are up significantly, and the costs of energy and raw materials have been reduced.
PABCO acquired the idled mill in 1984. Since the acquisition and restart, the mill's production facilities have been modernized in several stages to meet PABCO's wallboard production and quality demands. These updates included a new wet press, a new winder and seven 7 Horne pressure formers replacing the original dry vats.
The 110-in. wire width machine produces 225 to 250 tons/day of gypsum face and back linerboard and other specialty chip board grades (from 16 to 40 point caliper) that are used for a variety of commercial applications including popcorn boxes, brick pallet sheets, wine box separation sheets, tablet backing board, and specialty cake and donut boxes. This production flexibility allows the mill to adapt its product mix to market conditions and adds to the mill's viability in more difficult economic times. The furnish is 100% recycled materials, much of which is sourced in the Los Angeles area.
End users need lower variability
PABCO Paper was aiming at much more than the extra production required to satisfy the wallboard production requirements. As well as extra production the new gypsum wallboard process required more stringent and consistent quality. Mike Willoughby, vice president of product quality and paper manufacturing, states, "The new gypsum wallboard line drove the linerboard quality requirements. For the wallboard process we need to produce uniform appearance, sheet flatness, basis weight and moisture. These are critical elements for our customer. "
Bill Fraser, manufacturing manager-paper, adds his thoughts about the justification and expected returns from improving repeatability of the process and product quality: "We required the extra production, but to run faster we needed more stable control and repeatability on the machine. The new gypsum wallboard machine does not accept too much variability. Also, over the long term, we aimed to lower our costs of manufacturing by reducing chemical consumption, energy cost and by reducing basis weight."
The gypsum linerboard grades are sold by coverage area and the specialty chip board grades are specified by caliper, so reducing fiber consumption by minimizing basis weight is a desirable and legitimate goal. But first, stability is needed. Otherwise there could be trouble if quality specs dip too low.
Mark Wasson, technical director, interjects," When a customer makes a large change in the process their supplier must respond as well. Our customer does not want to chase a moving target, so we needed to understand our process and the critical quality parameters that our customers want and then operate the board plant in a consistent way." Establishing standard operating procedures was a key objective of the automation project, as well as the automated controls. More accurate and available information about the furnish preparation and the board making processes was needed.
Integrated solution
So the mill asked automation suppliers to bid on an integrated solution to raise the level and consistency of production. After the technical evaluation, the mill chose Metso for an integrated system, which combines a metsoDNA DCS, Paper IQ Plus QCS and DNAhistorian information management functions. The Vernon mill had a good level of confidence in Metso's technical credentials. Willoughby reports that Metso's "performance metrics" were rated as the best in the industry by the company he previously worked for.
But before the automation system was commissioned a considerable amount of preparation work and investment was required to bring the machine's instrumentation and control capabilities to the higher level required to meet their quality objectives. A Metso-supplied kajaaniMAP online sampling analyzer was installed in May 2007 to measure furnish freeness in the stock preparation system.
The analyzer gathers online data and process responses required to provide up to date operator information, implement standard operating procedures and eventually close the refining energy control loop. The analyzer has three sampling lines which measure the freeness before and after the filler ply refiner and after the liner ply refiner with an analysis turnaround time of four minutes per point. The measurement results are displayed on SPC charts on a video screen in the stock prep control room. The refiner plates are now adjusted manually to achieve a KW energy use target. The more consistent furnish freeness has a stabilizing effect on sheet strength properties and machine draws. Closed loop KWH/ton controls are planned for the future.
Optimizing formation
Much more information was needed to properly control the furnish preparation, chemical addition and forming processes, so the mill invested in process instrumentation and control actuators. For precise control over ply weight, formation and sheet strength properties each pressure former was equipped with thin stock flow meters and variable-speed fan pump drives. Jet to wire ratio is calculated and controlled using an input of slice opening checked manually at every planned shutdown. This control optimizes formation and directional sheet strength properties. The furnish preparation system is instrumented with flow measurement and control valves for controlling furnish sizing and retention aid chemicals on a pounds per ton basis. Metso supplied 25 Jamesbury valves with Metso's Neles ND9000 intelligent positioners.
Improved cross direction (CD) weight control was also an important target, so a new Kadant radial flow distributor with automatic CD control valves supplied by Metso was installed on the No. 4 filler ply former. The combined QCS and DCS system started up smoothly in July 2007. Wasson notes that the factory calibration of quality sensors was "pretty much on target right from the beginning." The dry end reel scanner is equipped with basis weight, moisture and caliper sensors.
Training of the operators was a challenge since many of them had limited computer knowledge. The PABCO management team coordinated their training efforts, along with Metso support, to provide 24-hour coverage to help operators adapt to the new system, which has so much more information. Fraser reports it is difficult to simulate every scenario at the beginning but, over time, standard operating procedures have been developed. "Now, the operators want to see even more," he says.
The operators now have much better control over furnish and forming condition uniformity and CD basis weight profiles. Regulation of the drying process is also much better and drying energy is used more efficiently. Rather than use the reel moisture measurement to control the drying section, new infrared temperature sensors at three positions across the sheet at the dryer exit are used for steam pressure control. Water boxes on the wet calender add moisture later on, but that addition depends somewhat on sheet furnish quality so reel moisture is not the best solution for drying control.
No blue smoke
The mill previously installed an infrared temperature sensor at one location. Now, with three sensors across the sheet, it has an indication of CD profile. The mill staff say that sheet temperature control gives more uniform sheet quality, provides tighter control of the drying section, avoids overdrying of the sheet and, therefore, saves drying energy. In this mill there is no telltale "blue smoke" that is common in many board mills where the sheet is over-dried.
The dryer control strategy uses an automatic pressure letdown function when there is a sheet break. Mill staff report that sheet break recovery time has improved significantly. With the improved measurements and controls, grade changeovers are much better, adding to the production flexibility of the machine.
More available and accurate information, standardized operating procedures, more precise process and quality controls have all added up to a significant return for the mill's investment in automation. Product variability in both the MD and CD has been cut significantly, Table 1.
More stable quality can be leveraged to give some impressive savings. Since gypsum linerboard provides surface coverage, furnish consumption can be cut if the quality is more uniform. The same leveraging factor applies to chipboards that are sold by caliper. The mill reports basis weights have been reduced by 14% with the improved automation.
Other cost savings stand out. Steam consumption has been cut by more than 3.5% and chemical consumption has been reduced by almost 2.9%. Wasson reports: "The reduction in chemical use is possible by targeting lower feed rates with the reduced MD variation of basis weight and ply weights and also having better control of these systems. Steam savings are possible by using lower target sheet temperature settings due to the improved CD and MD variation of both basis weight and moisture." He stresses that the new automation system has promoted better process understanding and increased their focus and attention on important process issues. That did not exist before the modern automation was installed.
| |
| Summary of results | Improvement |
| Basis weight reduction (fiber savings) | 14% |
| Line speed increase | 6.41% |
| Drying steam savings | 3.57% |
| Chemical cost savings | 2.87% |
| Reduced customer complaints | 25.96% |
Stability = productivity
A stable machine with more consistent quality is often a more productive machine, and that is exactly what happened at the Vernon mill. Line speed has increased by an average of 6.41%. Willoughby sums up the results: "We now have the ability to detect and control our process parameters and we make a consistent product with higher quality compliance, to meet our customer demands. Reduced basis weight and higher machine speeds have followed. But we could not speed up without the improvement in variability."
The learning and process improvements initiated by the mill staff are continuing. The original system, specified with 200 input and output (I/O) points, was expanded to 400 points when it was installed. As new measurement and control possibilities were discovered, the system point count was expanded by another 120 points since startup.
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