Submersible pumps are vertical shaft pumps in which the pump unit is positioned inside the pumped medium and the drive motor outside the pumped medium. A distinction is drawn generally between two types: in the single-pipe submersible pump, the shaft guide pipe serves simultaneously as pressure line. The mechanical seal is usually situated in the pumped stream and is pressurized by the pump's full discharge pressure. The twin-pipe version either has no shaft seal or its mechanical seal is positioned above the level of liquid. If it is necessary to seal the gas phase above the liquid level, single-mechanical seals with a liquid quench are used for pressureless gas phases; for gas phases with a vapour pressure greater than 1 bar or for inert gas overlays, double mechanical seals with a buffer fluid circuit or single-acting mechanical seals with connection of pressure pipe joints and a throttle bushing between the mechanical seal and tank space (with or without a quench) are used.