A screw pump is a type of rotary pump which is equipped with screws that mesh together and rotate within a cylindrical cavity or liner. The fluid enters from the suction side of the pump and moves linearly along these intermeshing screws to the discharge side of the pump. The clearances between the screws and the liner are very small hence the fluid gains pressure while moving through the pump.
A Screw Pump is a type of Positive Displacement Pump. This means that it moves fluid by continually displacing the area that the fluid occupies. The screws are encased inside of a liner, usually made of some sort of metal. The fluid fits into the screw cavities within this liner and is forced through the pump and out of the discharge as the screws rotate and inter-mesh.
Since there needs to be some clearance between the liner and the screws, it is possible for any fluid that is pumped to slip backwards into the pump to lower pressure zones. For high viscosity fluids like this volumetric slippage is usually a non-issue. As the viscosity decreases, however, this slippage becomes substantial thus; reducing the efficiency of the pump. This has to be taken into consideration when pumping water or similar fluid, and particularly in multi-phase applications where vapor slugs are mixed into the fluid stream. In these cases, all the clearances within the pump must be minimized to reduce slip.