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Various types of seals, together with pumps and compressors, were exhibited at the Mos-cow International Exhibition "Pumps and Compressors --75." Special mechanical seals for agi-tator shafts (both top and bottom entry) were exhibited by Pacific (Federal German Republic,FGR), Burgmann (FGR), Crane Packing (UK), Bestobel! Seals (UK), etc. These firms specialize in producing sealing devices. Double balanced and unbalanced mechanical seals are used, pro-viding highly reliable operation.9834
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Top-entry mixing devices are simple to construct and operate and are widely used~ How-
ever, this construction also has certain disadvantages. Rotational speed of the shaft is
limited if there is no lower bearing to limit radial play of the lower end of the shaft~
Allowable radial vibration is reduced with larger diameter or more rigid shafts and higher
driver stands. Shaft replacement and seal and agitator repair require complete dismantling
of the upper half of the vessel. This is another disadvantage of this construction. This
design has the advantages of repairing the seal and agitator without draining the liquid from
the vessel and the low probability of fluid escaping from the vessel on seal failure~
Bottom-entering mixers are more effective, especially in fluids containing solids with
high settling rates. The short drive shaft can rotate at a higher speed with relatively
small radial vibration of the agitator. Seals installed in the upper section of the vessel
operate in the vapor-- gas phase. Seals located in the bottom head of the vessel are in di-
rect contact with the working fluid. This results in some difficulty in seal fabrication
and selection of material for the mating surfaces and presents special requirements for re-
liability and operational quality. A major disadvantage is the requirement for draining the
vessel to repair or replace the seal and agitator.
Pacific, Burgmann, Crane Packing and Bestobell have developed two types of double me-
chanical seal for top-entry agitators: axial (Figs. I and 2)and radial (Fig. 3). Table 1
presents the technical features of these seals.
Axial seals are assembled from standardized sealing elements that are also used to pro-