Case study on how a sealing expert provided a permanent solution for a paper mill that had leaking agitators. Jason Putnam reports
The Domtar Nekoosa pulp and paper mill has been operating along the banks of the Wisconsin River for more than 130 years. The facility is an integrated mill, producing pulp and publishing grade paper. Annual paper production capacity is 118,000 tons, and an annual pulp production is 155,000 tons.
The facilities management team continually seeks ways to improve the reliability and performance of key equipment at the mill. In an era of tough competition and closing paper mills, this approach has contributed to the mill’s continued success.
Six side entry agitators were used in critical applications in the pulp and paper making process as they continuously mixed product in the tanks to prevent solids from settling to the bottom. Shaft deflection and misalignment, a common problem in most side entry agitators, made effective shaft sealing impossible on these units. Unreliable shaft seals lead to product leakage and contribute to premature bearing failure, causing an increase in unscheduled downtime, expensive maintenance repairs and costly product loss.
Two of the tanks located outdoors contain white liquor, a hot, corrosive and hazardous by-product of paper pulp production. One agitator was sealed with a double mechanical seal with a conventional glycol seal pot support system. The other agitator was sealed with conventional packing. When white liquor leaked through the seal, it was not only a maintenance issue but also an environmental concern.
The other four agitators were located on tanks inside the mill. These tanks held abrasive ground Calcium Carbonate and Filler Clay slurries. Because the slurries are abrasive, wear to the shafts and packing was excessive. This led to frequent re-packs, unnecessary maintenance and significant product loss to the drain. Flushing the packing with clean water would delay the breakdown, but wasn’t acceptable due to dilution of the product in the tanks.
Annual costs for maintenance and lost product on these six agitators at the mill averaged over US$82,000. The maintenance team recognised it was time to investigate new sealing options to improve performance and reduce costs.
In late 2005, the team met with Chuck Kimball, president of MRO Tech, a local Inpro/Seal distributor, to discuss options for upgrading the sealing method used on these agitators. Kimball recognised the side-entry agitators on the large tanks were an ideal application for the Inpro/Seal Air Mizer. The maintenance team was eager to try the new sealing technology. “The team's early adopter culture makes it very gratifying to work with them and find solutions to troublesome applications, such as these agitators,” stated Kimball.
The Inpro/Seal Air Mizer product seal is a non-wearing, permanent shaft seal that uses a positive purge to create an effective seal, preventing product or harmful emissions from escaping into the atmosphere. It seals by directing small amounts of air, inert gas or water through a precision clearance between the throttle and shaft, pressurising the clearance. The sealing medium is evenly dispersed around the shaft to prevent dead spots and is directed toward the product and atmosphere evenly to safeguard against both product loss and contamination.
Unlike traditional seals, the Air Mizer fully articulates to accommodate shaft deflection, run-out and misalignment and does not require periodic adjustments. Its unique non-contacting design does not wear out or groove the shaft, therefore, providing a permanent sealing solution. All seals are custom engineered to fit the application, and provide an effective shaft seal for powders, liquids, slurries and bulk solids.
In December 2005, a split custom-engineered stainless steel Air Mizer was installed on one of the white liquor tank agitators. It proved to be the perfect solution for the leaking agitators. Other installations followed in rapid succession – the second white liquor tank agitator in Aug 2006, and the indoor slurry agitators in 2006, 2007 and 2008. With the exception of a frozen air line five years ago, the two white liquor seals have run trouble-free for eight years. The first two clay slurry seals installed in 2006 have operated leak-free continuously since initial installation.
By switching to Air Mizers, the Nekoosa mill has realised impressive savings. After initial and continuous air costs, the seals installed are saving the mill: US$23,000 per year on the two white liquor tank agitators; and
US$51,000 per year on the four calcium carbonate and filler clay tank agitators.
Because the cost of the new seals consisted mainly of up-front, one-time expenditures, the team expects the US$74,000 savings per year to continue. A significant portion of this saving is the elimination of product loss.