Centrifugal blowers were historically used to transfer ambient air in various low pressure(15 PSI max) blower installations as well as some vacuum, or pneumatic conveying, applications. The internal shaft seals at either end of the blower only needed to reduce the leakage of compressed air at the outlet end, and prevent ingestion of unfiltered air at the inlet end of the blower, to maintain a design performance. The shaft seals were never meant to completely stop any leakage of air, or gas, from the blower.  

Lamson standardized on the labyrinth seal, which Wikipedia defines as a type of mechanical seal that provides a tortuous path to help prevent leakage. The numerous series of lands and grooves make it very difficult for the air, or gas, to escape. It is, however, a non-contacting seal. There is always a space between the rotating shaft, and the fixed seal housing.

Hoffman standardized on the carbon ring seal, which had been used for years in other applications where a little better sealing was required. The carbon rings are spring loaded to maintain contact with the shaft, thereby providing increased sealing ability, and enabling the blowers to be marketed to more gas applications. Most landfill gas blowers use the carbon ring seal. In some higher pressure(vacuum) situations, 2 sets of carbon rings are used-the double carbon seal, which almost reduces the leakage to zero. Adding a gas "purge" (usually nitrogen) to the double carbons all but eliminates any leakage.

Continental standardized on the graphite impregnated rope packing, which has been used successfully as a seal in many types of water pumps. The rope packing seals snugly around the shaft and seems to withstand the adverse elements associated with landfill gas blower applications. In my experience, it seems to be the better choice for seals in flare gas blowers.

A key point regarding any of these seals is they only work when the blower is operating-the shaft must be turning. When the blower is not being used, air or gas can leak out to the atmosphere. The purged gas seal may still prevent most leakage during machine downtime. The most effective way to seal the shaft has been with the addition of closed bearing housings. The shaft end of the blower would then need a small seal packing assembly mounted ahead of the bearing housing.

Shaft seals & bearings are the only wear items on centrifugal blowers. If the bearing is bad, it most likely failed due to a faulty seal. Be sure to at least check the seals when replacing bearings. A little extra preventative maintenance may save a premature bearing failure. Invest in PM, it will save you money!