The new shoe press section boosts productivity, improves quality, and reduces energy consumption.
The growth of containerboard demand in recent years has spurred new production capacity and some grade conversions of former publication paper machines. However, there is a third and very viable option for containerboard producers to modernize key parts of existing production assets—thereby boosting productivity, gaining ground in quality and trimming production costs.
The Greif Packaging mill in Riverville, VA, is a case in point. As the focal point of the PM 1 rebuild, Greif replaced a 1975-vintage bi-nip press section with a Valmet shoe press section.
PM 1 produces corrugating medium based on hardwood semi-chemical pulp and OCC furnish.
The on-time and under-budget project, which concluded in June 2015 after a 26-day shutdown, was accomplished with no accidents in more than 100,000 man-hours. The project included replacing the DC drives with variable frequency AC drives, several wet end improvements, a reel rebuild to accomplish four set reels, and a winder rebuild with an auto-slitter section.
David Scott, vice-president of mill operations for Greif Inc., summarizes the exceptional results: “Prior to the rebuild, we were either drying- or drive speed-limited. This press section project, in concert with a couple of others, has enabled us to increase machine speed by 20 percent consistently while running the 23# grade.”
He continues, “The press section has been very reliable and threading through the press is virtually automatic. All of the product characteristic goals set by the team were met or exceeded. We now deliver a much drier, denser, and stronger sheet to the dryer section, which has improved the runnability of the machine and greatly reduced its energy intensity. The press is no longer a bottleneck. All the feedback from the market has been positive.”
MODERNIZING THE LARGEST ASSET
Scott explains the rationale behind the rebuild. “This project was based on meeting the needs of our customers, reducing the energy intensity of the operation, and modernizing the largest asset in our system,” he says. “Before the rebuild we were competing against sheets made with more modern press technology. We needed to produce a product competitive in the market-place while gaining operating efficiencies.”
Regarding the press supplier selection, Scott says, “The project team selected Valmet based on the team, the technology they brought to the project, and the recent successful projects that the company had executed on similar grades.” Valmet also supplied the initial machine clothing and continues to do so today. “The project team elected to give Valmet full responsibility, essentially one source for the success of the press, and that included clothing design,” adds Scott.
Project execution was another important ingredient. The project’s success at all stages was credited to meticulous attention to detail and teamwork between Greif staff, suppliers, and contractors. Mitch Heishman, technical manager and Greif project manager, describes some of the benefits of this attention to detail: “The assembly installation and start-up phases of the project went very well due to the pre-planning in all phases of design, contractor preparation, and Valmet project group’s support. The delivered equipment was very well organized, packaged, and documented. This allowed the project team and contractors to easily confirm that all equipment was on site prior to the start of the assembly.” He says the Valmet press section footprint was an advantage when fitting it into an existing machine. Only one dryer can was removed.
Scott adds, “Our project team was committed to the aspiration of a perfectly executed project and Valmet did not hesitate to commit the resources required to support that aspiration. Valmet has provided the resources needed at each step during the project, including field engineering services to study machine imbalance, and to collect field measurements for equipment design.”
FELTS MATCHED TO NIP DEWATERING
The new press section overcomes limitations of the old press by increasing the sheet dryness and saving drying energy, closing up the long open draw to the dryer section, and providing a stable center roll sheet release. The new press consists of a three-nip configuration with a shoe press in the third nip. This is Valmet’s OptiPress Center, which is a new name for SymPress B. More than 80 OptiPress Center and SymPress B deliveries have been made since 1996—more than 30 of those on board machines. It’s a well-proven workhorse.
The press section and matching SMO felt designs give maximum nip dewatering by expressing water from each nip, rather than the felts carrying water to the uhle boxes. The SeamMaster Open (SMO) felt designs have lower overall caliper and are lighter overall compared to conventional felt designs. The SMO felt designs also have much shorter startup time, to achieve quick and effective nip dewatering. Clint Lipscomb, PM 1 production manager, says they are very pleased with the felts’ running performance. He says the original SMO press clothing designs at startup have not been changed.
The project team selected Valmet’s new uhle boxes with perforated covers rather than the traditional slotted design, and a hard ceramic coating. They provide better felt dewatering at lower vacuum levels and less drag and wear on the felts, leading to longer felt life.
PRODUCTIVITY UP, ENERGY USE DOWN
Sheet dryness to the dryer section is much improved—from 41 percent to 48 percent—thereby saving a significant amount of drying energy. Specific energy consumption was reduced by approximately 10 percent over the whole mill and 25 percent on PM 1, based on the lower evaporation load. This lowers the cost of energy per ton of board produced.
Production speeds are up significantly from 2,350 ft/min (717 m/min) to 2,850 ft/min (870 m/min) on the 23# grade. Since the press startup, the mill has set new production records—more than 100 t/d better performance. Once the mill has corrected a dryer felt roll imbalance problem, the speed is expected to be 30 percent higher, with a target of 3,150 ft/min (960 m/min).
The mill also targeted quality test improvements, and that has been achieved. With the denser sheet, caliper has been lowered, and that contributes to better runnability on high speed corrugators. The target for higher tensile strength has been met. Lipscomb adds, “We are able to meet industry strength properties at higher production rates than previous.”
Both Scott and Heishman agree that the new press has given the mill extra capability for increased productivity—in fact, more than expected. The mill can leverage this capability by targeting improvements in other parts of the machine, to improve efficiency and meet customer needs. After the press installation, Valmet UnoRun blow boxes, twin run blow boxes, a new UnoRun section, and optimized section transfers were added to the dryer section to stabilize the sheet at the higher speeds.
Further plans include solving the dryer felt roll imbalance problems and adding a Valmet steam profiler in the press section. Valmet will supply a different felt yarn formulation to resist felt compaction at the higher operating temperatures.
Mark Williamson is a journalist engineer based in Ontario, Canada. Contact him at [email protected]
PM 1 At-A-Glance
• Fabric width: 282 inches (716 mm)
• Design speed: 3000 ft/min ( 914 m/min)
• Design basis weight: 26 lbs/1000 sq.ft. (127 g/m2)
• Basis weight range: 23 to 40 lbs (112 to 195 g/m2)
The new press section getting the finishing touches before startup.
Mitch Heishman (left) and David Scott.
The perforated uhle box covers provide better felt dewatering at lower vacuum levels, and less drag and wear on the felts.
Schematic of Greif Riverville’s PM 1