Thinking big in seals
Giant perfluoroelastomer O-rings seal double-arm kneaders

Hermann Linden Maschinenfabrik of Marienheide, Germany, has chosen 1.7-m O-rings of Kalrez Spectrum 7075 perfluoroelastomer from DuPont Dow Elastomers to seal its double-arm (Z-type) kneaders. The company reports extended service life, reduced maintenance costs, and improved productivity.

Hermann Linden supplies mixers and kneaders for high-viscosity product mixing at temperatures from 100 °C to 300 °C in the chemical, pharmaceutical, plastics and other industries. Originally, both sides of the mixing trough were sealed with sheet gaskets containing aramid fibers and asbestos substitutes. But these seals were prone to occasional leaks that could be difficult to repair. Flat seals with metal inserts failed to achieve more than a slight increase in service life, and a suggestion to caulk the joints with soft metal was rejected as being costly to replace and not very maintenance-friendly.

A better idea was an O-ring seal, which would be easy to fit and replace, and compared with the flat seal would require far lower contact pressure to seal reliably. “We looked for a sealing material that could withstand kneader operating conditions over several years”, explained Alexander Schneider, general manager of Linden. “The seal had to resist temperature cycles from 100 °C to 300 °C, and exposure to the chemicals used.”
Linden asked customers to test a silicone seal, a graphitized aramid-fiber cord and a perfluoroelastomer seal under typical operating conditions. Although the silicone seal stood up to high temperatures, it was severely attacked by the process chemicals.
The graphitized aramid fibers resisted the chemicals but charred at high temperatures. Only the perfluoroelastomer seal withstood both aggressive chemicals and high temperatures. The next step was to find a reliable supplier of perfluoroelastomer O-rings in diameters above 1.5 m. The result is a Kalrez O-ring with a cord thickness of 9.66 mm and a diameter of 1,705 mm. “We are enthusiastic about the performance of the Kalrez perfluoroelastomer O-rings, and calculate at least a six-fold increase in maintenance intervals compared with the flat
seals. We plan to use Kalrez parts for our other kneaders and mixers, operating similarly”, said Schneider.