Pumps are driven machines that transmit the input drive energy to a stream of liquid or gas in the form of pressure energy and flow energy. Pumps are named according to diverse criteria, e.g. method of operation and design features (cf. VDMA 24261, Part 1), purpose of application, material, etc. Pointers for the design of a pump's mechanical seal can often be drawn from the pump's designation or description. For example, mechanical seals in eccentric spiral pumps, twin-flow pumps, small screw pumps etc. must (usually) seal the inlet pressure. In single-tube submersible pumps it is the pump discharge pressure which acts on the mechanical seal; in designs with a separate lifting tube, on the other hand, only the gas phase above the pumping medium needs to be sealed.