Pelletizing woody biomass is hard on equipment. This reality is widely accepted by pellet producers, pellet press manufacturers and lubricant providers. The friction and heat generated inside an operating pellet press begin wearing down dies, roller shells, roller bearings and roll pins almost immediately. While the average lifespan of these wear parts varies from plant to plant, their replacement and refurbishment represent a significant portion of producer operations and maintenance (O&M) budgets across the industry.
A roller assembly is made up of a number of parts, including a roller shell, a roll pin, a number of seals and O-rings and roller bearings. One roller assembly has two bearings, each with a price tag of nearly $500. With two, and sometimes three roller assemblies in each press, producers have thousands of dollars’ worth of bearings installed in each press. “Roll bearings are the single biggest issue in any pellet plant,” says Jase Locke, an aftermarket account manager at Andritz. “Everyone who has a pellet press is trying to figure out how to prolong that roll bearing life.”
Roller bearings, Locke explains, enable the roller shell to turn around the roll pin. The friction generated at the intersection of the pellet die and the roller shell is transferred to the bearing, protecting even more expensive components like the roll pin from early failure.
As the industry matures and grows, best practices for prolonging the lifetime of these wear parts have begun to emerge, and producers are putting a number of different approaches to work, calling upon press manufacturers and other players for more.
“What we are seeing today in the industry are these new, larger mills popping up, and you might have a plant manager or a maintenance manager with some experience, but the actual operators—the folks doing the work—are all green, they’re fresh, they’re new,” says Travis Fetzer, an aftermarket coordinator for Andritz. “They don’t recognize the sounds of a pellet press that isn’t running properly.”
While new facilities may not have the tenure that established facilities have, they do know the financial impact of premature failure of these expensive components, and they are increasingly looking to press manufacturers and lubricant providers for ideas.
Holger Streetz, international operations manager for Bathan AG, a Swiss manufacturer of lubricants, suggests that producers involve more than just their pellet press OEMs in efforts to drive down operating expenses of these assets. “I think everyone is beginning to cook their own meals,” Streetz says. “I do think that producers are trying to improve their own maintenance processes and bring more of their roll maintenance in house.”
The lubricants offered by Bathan AG include tiny ceramic particles, and initial results are incredibly promising. Streetz points to one of Bathan AG’s first pellet customers, a producer in Switzerland, as proof of the product’s potential. “Our Swiss customer is achieving 10,000 hours of use out of a set of bearings,” Streetz says. “That’s 10 times what you would normally expect from a bearing. This is our best case scenario.”
Whether or not producers are ready to invest in the type of highly specialized lubricants offered by Bathan AG, attention to lubrication programs is vitally important to their maintenance program. “You need to follow your OEM guidelines, whatever they prescribe in their manual, for the amount and types of grease to use,” Fetzer explains. “Giving a bearing too much grease is just as bad as giving a bearing, not enough grease. It is very important to follow the guidelines.”
Attention to lubricants is aimed at protecting components from early failure, most notably the roll pin. Fetzer explains that almost half of the cost of a new roller assembly is in the pin. The case-hardened steel and precision machining that are required to manufacture the pin make it an expensive item to replace. After the pin, producers should focus on bearings. Roller shell wear is an inevitability.
Producers should strive to protect those roll pins and achieve the upper range of expected bearing life. “If you are just replacing the shell and your bearings are all okay, you’re really only looking at about 20 percent of the cost of a new assembly,” Fetzer says. “It’s a significant cost savings.”
As the industry surges forward with new demand from European and Asian power producers, interest in reducing the operating expense of pellet presses is at an all-time high.
Fetzer identifies two innovations he sees gaining traction with producers—one of them already well-deployed, and the other is on the horizon. The first is auto-roll adjust. Auto-roll adjust takes the guess work and manual adjustment of the distance between the roller and the die out of the hands of the producer. This prevents the roll from being set too close to the die, unnecessarily increasing the load on roller assembly. Fetzer says that this technology is widely deployed across Europe, and he expects it to gain traction with North American producers soon.
“The next step is putting sensors into rolls,” Fetzer says. For now, pellet producers must rely on manual, walk-by inspections of pellet presses to gauge how the machine is running. In some ways, much of the diagnostic work is guesswork. Precise data about the temperature inside an operating press, or the speed at which a roller is turning, is currently unknown to producers. Roll assemblies with built-in sensors would be a game-changer. “If you can detect that a roll is beginning to slip or turn at an improper speed, or if you can sense that the bearings are starting to get overheated, a potential failure, you can stop the machine, pull it out and avoid that catastrophic failure,” Fetzer says.
The moment a pellet press is brought online, the components that make it operate begin to wear. Their replacement costs producers not only money, but perhaps more importantly, precious operational time. “When your pellet press is offline because you are performing maintenance on a roller bearing, you aren’t making pellets,” Streetz says. The point is easy enough to understand, but stakeholders agree there is tremendous opportunity across the industry to prolong wear-part life, drive replacement and refurbishment costs to the bottom line, and, most importantly, keep those presses up and making saleable product.
Author: Tim Portz
Executive Editor, Biomass Magazine