Coaxial seals

Coaxial agitators with a central shaft running inside a tubular shaft call for special design measures. The trouble is that the gap between these two shafts has to be sealed as well. Though the use of stuffing boxes and dry-running seals - as already described - is very limited, the applicaion of a buffer-fluid-lubricated mechanical seal creates the additional need for a verticall housing linked with the buffer fluid reservoir. Two very different designs have been developed to meet the demands of this system, which involves three distinct relative velocities.

Coaxial seal at the end of the tubular shaft
In many applications, two separate sealing cartridges are used. The job of sealing the tubular shaft against the vessel flange is handled by a doulbe-acting standard cartridge, as in a normal agitator. For sealing the gap between the two shafts, a mechanical seal is installed between the gearbox and the centrall shaft drive. This seal consists of one seal ring for each shaft, enclosed within two stationary seats sealed in the housing. This yields three sealing gaps between the four seal rings. Fig. 10 shows how this seal is positioned and built.

This sealing concept, which requires a tubular drive shaft prepared to accept the seal, can be operated only with pressureless buffer fluid. Each of the two mechanical seal cartridges is supplied by its own buffer fluid reservoir.

Coaxial seal inside the tubular shaft
The seal cartridge consists of a twin mechanical seal. This arrangement shown in Fig. 11 eliminates the problems caused by the corotating housing and tubular shaft when it comes to sealing the gap between the wto shafts. It does so because the tubular shaft itself acts as the seal housing and the connections terminate in the buffer chamber of the outer seal, which closes the gap between tubular shaft and vessel flange.

The advantages of this design lie in the use of standard seal rings for double-acting mechanical seals and in the possibility of pressurization, which broadens the application limits. Added bonuses are a much shorter product-wetted gap, which is important where special materials are involved, and the need for just one buffer fluid system.