Term used for single or double mechanical seals for sealing shaft passages through the vessel walls of agitator vessels and other processing machinery and apparatus. Though it is possible in principle to employ the same mechanical seals in agitators as in pumps, a number of characteristic differences arise in practice: Agitator shafts run at lower speeds than pump shafts and have a larger range of diameters (from a few mm for laboratory agitators to shaft diameters of around 500 mm for large-scale agitators). As a rule, agitator seals are made as a cartridge unit, with or without a shaft bearing, shut-down bearing leakage removal and connections for a quick-change device integrated in the seal casing. A distinction is drawn between agitator seals for top, side and bottom drive, depending on the direction from which the shaft enters the agitation vessel.

Top drive:

The majority of all agitator drives are top-mounted drives. In these cases the agitator seal is usually accommodated in an agitator gear stool, which for heavy mechanical seals is often equipped with a quick-change device. In modern drives the shaft bearing is designed with a locating or axial thrust bearing integrated in the gear-box and a floating bearing (self-aligning roller bearing) in the mechanical seal casing. In high-pressure seals the axial thrust bearing is usually integrated together with a floating bearing jointly in the seal casing. The parts of the agitator seal facing the inside of the vessel are not usually in contact with the process stock itself but with its gas phase or a gas overlay. Selective discharge of the boiler-side leakage from the agitator seal is possible, therefore, via simple auxiliary equipment (leakage trap). Given boiler temperatures > 200° C, it is customary to install a cooling flange between the vessel and the agitator seal, resulting in a much lower temperature at the mechanical seal parts.

Side drive:

Large-diameter agitation vessels, e.g. storage tanks, washers and oxydizers, are usually fitted with side drives. In this type of design the parts of the agitator seal on the vessel side are enveloped completely or partially by the process stock. The latter usually contains solids, so it requires either a mechanical seal whose sealing gap on the product side lies directly in the process stock or – should a shut-down seal be needed – a flushing facility between the mechanical seal and the shut-down seal.

Bottom drive (floor drive):

Large reaction vessels are equipped with bottom drives for technical (mixing) reasons and in order to avoid long, floating shafts. Agitator seals for side and bottom drives are similar in design.

Agitator gear stool for side drive with built-in mechanical seal

Glass lined agitation vessel with top drive