J. R. BRENNAN
G. J. CZARNECKI
J. K. LIPPINCOTT
A. J. PRANG
Screw pumps are a special type of rotary positive displacement pump in which the flow
through the pumping elements is truly axial. The liquid is carried between screw threads
on one or more rotors and is displaced axially as the screws rotate and mesh (see Figure
1). In all other rotary pumps, the liquid is forced to travel circumferentially, thus giving the
screw pump with its unique axial flow pattern and low internal velocities a number of
advantages in many applications where liquid agitation or churning is objectionable.
The applications of screw pumps cover a diversified range of markets including navy,
marine, and utilities fuel oil services; marine cargo; industrial oil burners; lubricating oil
services; chemical processes; petroleum and crude oil industries; power hydraulics for
navy and machine tools; and many others. The screw pump can handle liquids in a range
of viscosities, from molasses to gasoline, as well as synthetic liquids in a pressure range
from 50 to 5000 lb/in2 (3.5 to 350 bar) and flows up to 8000 gal/min (1820 m3/h).
Because of the relatively low inertia of their rotating parts, screw pumps are capable
of operating at higher speeds than other rotary or reciprocating pumps of comparable dis-
placement. Some turbine-attached lubricating oil pumps operate at 10,000 rpm and even
higher. Screw pumps, like other rotary positive displacement pumps, are self-priming and
have a delivery flow characteristic, which is essentially independent of pressure, provided
there is sufficient viscosity in the liquid being pumped.
Screw pumps are generally classified into single- or multiple-rotor types. The latter is
further divided into timed and untimed categories.
The single-screw or progressive cavity pump (see Figure 2) has a rotor thread that is
eccentric to the axis of rotation and meshes with internal threads of the stator (rotor hous-
ing or body). Alternatively, the stator is made to wobble along the pump center