Sealing assembly for a shaft passing through the wall of a vessel and subject to fluctuating pressure, esp. in agitator shaft in a sterile, sealed vessel, comprises two radial shaft seals, the second seal being on the side of the first seal which is subject to the alternating pressure; the intermediate space between these two seals is filled with a flowable pressurised fluid esp. a grease, and the axial distance (a) between the two seals can be varied. Specifically, the second seal can be axially displaced relative to the first. The second seal is pref. mounted inside a sleeve which has limited axial movement from its initial distance (a) from the first seal. The opposite sides of this sleeve pref. have different areas. In partic. for agitator shafts for sterile vessels in the chemical, pharmaceutical and cosmetic industries, where the internal pressure of the mixing vessel can range from high value to sub- atmospheric values. The arrangement prevents dry running when the pressure falls to vacuum and does not require special external pressurising devices.
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