The new seals had to meet high requirements: torque and leakage had to be kept at a minimum even at pressures of 120 bar and rotational speeds of up to 261 rpm. Furthermore, they had to be designed to work independent of the direction of rotation since the RAM machine frequently turns in different directions to open and close the reactor tube. Last but not least, the new seals had to work reliably as this was the weak point of the previous products. In case a mechanical seal fails, costs for the operator resulting from lost revenue and maintenance expenses can range somewhere between 500,000 up to one million dollars per day. Seal failure also exposes the service technicians working on the RAM machine to higher radiation doses.
Unexpected problems call for a scientific approach
When developing the new mechanical seals, EagleBurgmann’s engineers were confronted with an unexpected problem. “Under certain circumstances, the seal’s torque increased significantly after a few turning cycles. A behavior we had not anticipated”, explains Jens Hofmann, head of technical sales support pump seals power, mining, pulp & paper, water at EagleBurgmann. To get to the bottom of what caused the rapid increase in friction, the material specialists at EagleBurgmann had to take a closer look at the atomic binding forces between the sliding faces. In cooperation with colleagues from their parent company, the Freudenberg group, they carried out an atomic simulation which turned out to be the key to understanding the effect.
“During leakage-free operation without media between the sliding faces, we observed cold welding between the carbon atoms. The sliding faces were intermeshed on a molecular level”, explains Hofmann. On the grounds of these findings, the scientists used further simulations to gradually approach the perfect material pairing. Eventually, a combination of crystalline diamond coating and silicon-carbide slide surfaces turned out to be the ideal solution. “This combination reduces cold welding to a minimum and keeps frictional resistance permanently low. Wear on the faces is also reduced. This increases the ervice life of the seal to four years – almost five times longer than the previous one”, Hofmann elaborates.