Centrifugal pumps apply centrifugal force to generate velocity, use rotating impellers to increase velocity, and push fluids through an outlet valve. Positive displacement pumps use rollers, gears, or impellers to move fluid into a fixed cavity so that when liquid exists, the vacuum that is created draws in more fluid. Jet pumps force fluid through a nozzle, converting the fluid’s energy into velocity energy. Airlift pumps mix air or gas with the liquid media so that the air-liquid mixture, with a lower specific weight than the liquid, rises in the pipe and is discharged above the level of the liquid.