Because the fluid travels axially in closed cavities that move continuously from suction to discharge, rather than being displaced in discrete pushes. There is no moment where delivery stops and restarts, so the output is close to steady - which is what removes pressure surges and vibration.

It improves performance, which is the opposite of a centrifugal pump. Thicker fluid seals the running clearances more effectively and reduces slip back from the discharge side, so volumetric efficiency rises with viscosity. This is why screw pumps dominate heavy oil and polymer duty.

High ones, because the several cavities in series each carry a share of the total pressure rise rather than one element taking it all. Longer screws with more turns achieve higher pressure. Actual capability depends on the design, the fluid and the clearances specified.

Poorly. The fine running clearances between screws and liner are what create the seal, and abrasive particles open them up quickly, after which capacity and pressure both fall. Clean fluid, or effective upstream filtration, is a requirement rather than a recommendation for this family.

It must not. The fluid lubricates the screws against each other and against the liner, and dry running causes contact, galling and rapid destruction. Dry-run protection is standard where the supply can be lost, and it is far cheaper than replacing a screw set.

Marine engine rooms for fuel and lubricating oil, hydraulic power units, lubrication systems, heavy fuel oil transfer, asphalt, polymer and resin handling, and elevator and machine tool hydraulics. The common requirements are viscosity, high discharge pressure and quiet pulseless delivery, and it is usually the combination rather than any one of them that rules out a cheaper pump family.

Low noise and vibration, high pressure capability, and excellent handling of the viscous fuel and lubricating oils an engine room runs on. In a confined space where noise transmits through the structure, the pulseless characteristic is a genuine operational advantage rather than a refinement.

A progressive cavity pump uses a single helical rotor turning inside an elastomeric stator, so there is contact and it tolerates solids and abrasion well. A screw pump uses intermeshing metal screws with clearances, giving higher pressure, higher speed and smoother flow but far less tolerance of solids.

Generally yes once wetted, and they will draw a reasonable suction lift, which suits drawing from tanks and bunkers. Priming performance depends on the clearances, so a worn pump primes poorly - often the first noticeable symptom of clearance loss.

Two or three in the common industrial types, and the count changes the character. Twin-screw designs can be timed so the screws do not touch, allowing better solids tolerance and double-suction balance. Triple-screw designs use an untimed driven screw and two idlers, which is simpler and quieter.