Shaft seals keep compressed gas from leaking out. The need for leak prevention may include process economy, safety and environmental protection. Moreover, the process gas will usually have to be kept away from the compressor bearings. In essence, seals are located between the compressed gas and bearings. A number of different sealing arrangements and configurations are found in industry. Fig. 1 is an overview of four principal seal types—wet-floating ring or bushing seals, trapped-wet-bushing seals, liquid-lubricated mechanical contact seals and dry-gas non-contacting face seals.¹ Within each of the four principal types, there are numerous variants: