ENGINEMAN 1 & C
the reed in the neutral position, the fluid pressure
is relieved to chamber C through the nozzles and
a pressure drop allows the centering springs to
return the spool valve to a central position; in this
position, the valve blocks the pressure and return
line, creating a hydraulic lock in chambers E and
D. By energizing the left-hand solenoid, the
magnetic reed will move to the left and the entire
process will be reversed.
The position of the spool valve can be adjusted
by using the centering screw. Fixed orifices are
used so that the pressure drop in the hydraulic
servovalve will not create a pressure drop in the
opposite nozzle which is closed. Note that the ser-
vovalve is basically a sliding spool valve. This type
of valve has many other applications in hydraulic
systems. For example, servovalves are used in the
guidance systems of missiles and in the control
systems of aircraft.
DISTILLING PLANTS
This section will deal with the operation,
troubleshooting, and repairing of the submerged
tube and the flash type distilling plants that are
used by the Navy. For additional and more de-
tailed information than is provided by this train-
ing manual, consult the manufacturers technical
manual for the type of distilling plant installed
on your ship.
Distilling plants in naval ships are of three
general types: (1) vapor compression, (2) low-
pressure steam, and (3) heat recovery. The
major differences between the three types are the
kinds of energy used to operate the units and the
pressure under which distillation takes place.
Vapor compression units use electrical energy (for
heaters and compressors). Low-pressure steam
distilling units use low-pressure steam from either
the auxiliary exhaust steam systems or the aux-
iliary steam system. Heat recovery distilling units
use diesel engine jacket water instead of steam as
the heat source. Vapor compression units boil the
feedwater at a pressure slightly above atmospheric
pressure. Low-pressure steam and heat recovery
units depend on a relatively high vacuum for
operation.
Vapor compression type distilling units are
used in submarines and small diesel-driven sur-
face craft where the daily requirements do not
exceed 4000 gallons per day (gpd). Since the vapor
compression type found on surface crafts is
being replaced with the heat recovery distilling
units, vapor compression distilling units will not
be covered in this manual. Chapter 531 (9580-II)
of Naval Ships Technical Manual contains infor-
mation on these plants.
The low-pressure steam distilling unit is used
in all steam-driven surface ships and nuclear sub-
marines. Enginemen usually share responsibility
with Machinists Mates for the maintenance and
operation of the low-pressure steam distilling
plants.
There are two reasons why low-pressure steam
distilling units are considered low pressure: (1)
they use low-pressure steam as the source of
energy, and (2) their operating shell pressure is
less than atmospheric pressure.
The three major types of low-pressure steam
distilling units are submerged tube, flash type, and
vertical basket.
In this section of the chapter we will be
discussing only two of these distilling unitsthe
submerged tube and the flash type.
SUBMERGED TUBE PLANTS
Low pressure, submerged tube plants differ
from ship to ship, but the operating conditions
and the maintenance procedures are basically the
same. In almost all instances, the personnel who
stand watches on distilling plants are also respon-
sible for the maintenance of the plants. This gives
them ample opportunity to detect abnormal
operating conditions before such conditions reach
advanced stages. When operating troubles do
occur, it is the responsibility of the EN1 or ENC
on duty to locate the trouble and to make the
necessary adjustments or repairs.
Steady operating conditions are essential for
satisfactory results. Except under emergency con-
ditions, no plant should be forced beyond its rated
capacity, because higher steam pressures will be
required and the resulting higher temperatures will
cause more rapid scaling of the evaporator tubes.
During operation, the various elements of any
plant are interdependent due to the heat and fluid
balances throughout the plant. Adjustment of any
one control can produce widespread effects on
these balances. For example, an increase in the
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