The Most common design of Mechanical Face Seals is the FO design, also called as O Ring 
design, where O Ring acts as the secondary sealing element. FO Type Mechanical Face Seal 
consist of 2 identical metallic sealing rings which seal against each other on a lapped seal face. 
The metallic Seal rings are precisely lapped to seal perfectly. The seal is designed so one of the 
two seal rings rotates while the other is stationary.