Abstract: A fluid seal having differing fluid levels is provided on opposite sides of a downwardly opening cup-shaped container that is longitudinally lapped by an upwardly opening annular container that is sealed to a vessel such that the lip of the cup-shaped container is submerged. The fluid seal creates a hydrostatic pressure barrier between ambient and a vessel, for example at a rotating vertical shaft. Relatively high speeds are permitted without leakage due to a number of internal horizontal and vertical baffles on one or more walls of the annular container. Barrier fluid losses due to evaporation or the like can be replaced in an embodiment wherein condensed process fluid is used as the barrier fluid, and wherein excess barrier fluid is returned to the vessel while maintaining a differential pressure barrier via the seal.