Contact elimination in mechanical face seals using active control

Wear and failure of mechanical seals may be critical in certain applications and should be avoided. Large relative misalignment between the seal faces is the most likely cause for intermittent contact and the increased friction that eventually brings failure. Adjustment of the seal clearance is probably the most readily implemented method of reducing the relative misalignment and eliminating seal face contact during operation. This method is demonstrated with the aid of a noncontacting flexibly mounted rotor (FMR) mechanical face seal test rig employing a cascade control scheme. Eddy current proximity probes measure the seal clearance directly. The inner loop controls the clearance, maintaining a desired gap by adjusting the air pressure in the rotor chamber of the seal. When contact is detected the outer loop adjusts the clearance set point according to variance differences in the probes’ signals. These differences in variance have been found to be a reliable quantitative indication for such contacts. They are complimentary to other more qualitative phenomenological indications, and provide the controlled variable data for the outer loop. Experiments are conducted to test and verify this active control scheme and strategy. Analysis and results both show that contrary to intuition for the seal under investigation, reducing seal clearance can eliminate contact, and the outer cascade loop indeed drives the control toward this solution.