Positive Displacement pumps is a category of pumps that causes a fluid to move by trapping a fixed amount of it then forcing (displacing) that trapped volume into the discharge pipe.
A positive displacement pump can be further classified according to the mechanism used to move the fluid: Rotary-type, for example, the lobe, external gear, internal gear, screw, shuttle block, flexible vane or sliding vane, helically twisted roots or liquid ring vacuum pumps. The gears turn away from each other, creating a current that traps fluid between the teeth on the gears and the outer casing, eventually releasing the fluid on the discharge side of the pump as the teeth mesh and go around again. Many small teeth maintain a constant flow of fluid, while fewer, larger teeth create a tendency for the pump to discharge fluids in short, pulsing gushes.
Screw pumps are a more complicated type of rotary pumps, featuring two screws with opposing thread – that is, one screw turns clockwise, and the other counter-clockwise.