A screw pump, also known as a water screw, is a positive-displacement (PD) pump that use one or several screws to move fluid solids or liquids along the screw(s) axis. In its simplest form (the Archimedes' screw pump), a single screw rotates in a cylindrical cavity, thereby moving the material along the screw's spindle. This ancient construction is still used in many low-tech applications, such as irrigation systems and in agricultural machinery for transporting grain and other solids.
Development of the screw pump has led to a variety of multiple-axis technologies where carefully crafted screws rotate in opposite directions or remain stationary within a cavity. The cavity can be profiled, thereby creating cavities where the pumped material is "trapped".[clarification needed]
In offshore and marine installations, a three-spindle screw pump is often used to pump high-pressure viscous fluids. Three screws drive the pumped liquid forth in a closed chamber. As the screws rotate in opposite directions, the pumped liquid moves along the screws' spindles.
Three-spindle screw pumps are used for transport of viscous fluids with lubricating properties. They are suited for a variety of applications such as fuel-injection, oil burners, boosting, hydraulics, fuel, lubrication, circulating, feed and so on.
Compared to centrifugal pumps, positive-displacement pumps have several advantages. The pumped fluid is moving axially without turbulence which eliminates foaming that would otherwise occur in viscous fluids. They are also able to pump fluids of higher viscosity without losing flow rate. Also, changes in the pressure difference have little impact on PD pumps compared to centrifugal pumps.
The term ‘screw pump' is often used generically. However, this generalization can be a pitfall as it fails to recognize the different product or ‘screw' configurations, as well as the uses, advantages and design considerations for each. The design differences of each screw configuration and pump type make each suitable for different applications and handling fluids with varying characteristics.
Each ‘screw pump' operates on the same basic principle of a screw turning to isolate a volume of fluid and convey it. However, the mechanical design of each is different. The primary difference is the number of screws: one, two, three or more.
The screw pump is the oldest positive displacement pump. The first records of a water screw, or screw pump, dates back to Ancient Egypt before the 3rd century BC. The Egyptian screw, used to lift water from the Nile, was composed of tubes wound round a cylinder; as the entire unit rotates, water is lifted within the spiral tube to the higher elevation. A later screw pump design from Egypt had a spiral groove cut on the outside of a solid wooden cylinder and then the cylinder was covered by boards or sheets of metal closely covering the surfaces between the grooves.
A cuneiform inscription of Assyrian king Sennacherib (704–681 BC) has been interpreted by Stephanie Dalley to describe casting water screws in bronze some 350 years earlier. This is consistent with classical author Strabo, who describes the Hanging Gardens as watered by screws.
The screw pump was later introduced from Egypt to Greece. It was described by Archimedes, on the occasion of his visit to Egypt, circa 234 BC. This suggests that the apparatus was unknown to the Greeks before Hellenistic times.