pumps are used aboard ship to pump fuel and lube
oil and to supply pressure to the hydraulic system.
In the double-screw pump, one rotor is driven by
the drive shaft and the other by a set of timing
gears. In the triple-screw pump, a central rotor
meshes with two idler rotors.
In the screw pump, liquid is trapped and
forced through the pump by the action of rotating
screws. As the rotor turns, the liquid flows in
between the threads at the outer end of each pair
of screws. The threads carry the liquid along
within the housing to the center of the pump
where it is discharged.
Most screw pumps are now equipped with
mechanical seals. If the mechanical seal fails, the
stuffing box has the capability of accepting two
rings of conventional packing for emergency use.
Figure 9-10.—Double-screw, low-pitch pump.
Figure 9-11.—Triple-screw, high-pitch pump.
SCREW PUMPS.— Several different types of
screw pumps exist. The differences between the
various types are the number of intermeshing
screws and the pitch of the screws. Figure 9-10
shows a double-screw, low-pitch pump; and figure
9-11 shows a triple-screw, high-pitch pump. Screw
SLIDING VANE PUMPS.— The sliding-vane
pump (fig. 9-12) has a cylindrically bored housing
with a suction inlet on one side and a discharge
outlet on the other side. A rotor (smaller in
diameter than the cylinder) is driven about an axis
that is so placed above the center line of the
cylinder as to provide minimum clearance between
the rotor and cylinder at the top and maximum
clearance at the bottom.
The rotor carries vanes (which move in and
out as the rotor rotates) to maintain sealed spaces
between the rotor and the cylinder wall. The vanes
trap liquid on the suction side and carry it to the
discharge side, where contraction of the space
expels liquid through the discharge line. The vanes
slide on slots in the rotor. Vane pumps are used
for lube oil service and transfer, tank stripping,
bilge, aircraft fueling and defueling and, in
general, for handling lighter viscous liquids.
Figure 9-12.—Sliding vane pump.
9-9
