pump; and figure 7-11 shows a triple-screw, high-
pitch pump. Screw 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 be-
tween 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
Figure 7-12.--Sliding vane pump.
mechanical seals. If the mechanical seal fails, the
stuffing box has the capability of accepting two
The rotor carries vanes (which move in and
rings of conventional packing for emergency use.
out as the rotor rotates) to maintain sealed spaces
between the rotor and the cylinder wall. The vanes
SLIDING VANE PUMPS.--The sliding-vane
trap liquid on the suction side and carry it to the
pump (fig. 7-12) has a cylindrically bored
discharge side, where contraction of the space
housing with a suction inlet on one side and a
expels liquid through the discharge line. The vanes
discharge outlet on the other side. A rotor (smaller
slide on slots in the rotor. Vane pumps are used
in diameter than the cylinder) is driven about an
for lube oil service and transfer, tank stripping,
axis that is so placed above the center line of the
bilge, aircraft fueling and defueling, and, in
cylinder as to provide minimum clearance between
general, for handling lighter viscous liquids.
the rotor and cylinder at the top and maximum
clearance at the bottom.
Jet Pumps
The pumps discussed so far in this chapter
have had a variety of moving parts. One type of
pump you find in the engine room is the jet pump,
usually called an eductor. Figure 7-13 shows an
eductor, which has no moving parts. These pumps
are used for pumping large quantities of water
overboard in such applications as pumping bilges
and dewatering compartments. Eductors are also
being used on distilling plants as air or brine
eductors.
Eductors use a high-velocity jet of seawater
to lower the pressure in the chamber around the
converging nozzle. Seawater is supplied to the
converging nozzle at a relatively low velocity and
exits the nozzle at a high velocity. As the seawater
leaves the nozzle and passes through the chamber,
air becomes entrained in the jet stream and is
pumped out of the chamber. Pressure in the
chamber decreases, allowing atmospheric pressure
to push the surrounding water into the chamber
and mix with the jet stream. The diverging
nozzle allows the velocity of the fluid to decrease
and the pressure to increase; the discharge
pressure is then established.
Figure 7-11.--Triple-screw, high-pitch pump.
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