the bleed air pressure is then reduced by a pressure
the PMS. Except for the module bleed air cutout valves,
regulating valve before the air is sent to the masker air
you may in some cases use a type of planned
maintenance program. You will also maintain the valves
distribution manifold. The air then enters the
in the remaining subsystems of the bleed air system
distribution manifold where it can be directed manually
according to the PMS and possibly a planned
to any of the desired emitter belts.
maintenance program. The planned maintenance
DDG-51 Class Ships. The basic masker air system
programs are usually contractual agreements between
installed on the DDG-51 class ships is the same as the
the Navy and the manufacturers of the valves. In some
one installed on the CG-47, DD-963, and DDG-993
cases, valve manufacturers may keep a rotational pool
class ships. The only difference worth mentioning is the
of valves that will allow the ship to remove and replace
way in which the masker air (cooler) valve operates. The
all of the system valves with new or rebuilt ones during
type of masker air valve installed on the DDG-51 class
an SRA. Plans, such as these, will significantly reduce
ships is manually operated. The type installed on the
CG-47, DD-963, and DDG-993 class ships is an electric
Because all the bleed air system valves are basically
solenoid-controlled valve that is operated from the
maintained in the same manner, this is the last time we
PACC or the PLCC.
will discuss their maintenance requirements.
FFG-7 Class ships. The masker air system
installed on the FFG-7 class ships has only two
Masker and Prairie Air Systems
characteristics in common with the masker air systems
installed on the other ship classes: (1) where the masker
Both the masker and prairie air systems take the hot
air comes from and (2) the basic principles of operation.
bleed air from the bleed air collection and distribution
system. These two systems cool the air in independent
In the FFG-7 class ships, the air is piped from the
coolers before sending it to the masker emitter belts or
bleed air cooler through a normally open, manually
leading edge of the propeller blades. The purpose of
operated cutout valve and then to the masker air pressure
masker air is to muffle or modify the ship's machinery
regulating valve. The air pressure is reduced and the air
noises that are transmitted into the water through the
is then piped to the two masker air supply cutout valves.
ship's hull. Prairie air is then used to disguise the
These two solenoid-controlled valves are then operated
propeller noise signature. We will discuss the basic
from the auxiliary control console (ACC) in the CCS to
design and operational characteristics and maintenance
send air to one or both emitter belts.
requirements of both systems in the following
PRAIRIE AIR SYSTEM. Now let's discuss the
paragraphs.
prairie air system. The prairie air systems installed on
MASKER AIR SYSTEM. The number of valves
all gas turbine-powered ships have the same basic
used and the direction of airflow through the masker air
design. They all have a solenoid-controlled valve that is
system will depend on the ship's class. Because of the
operated from the control consoles, a cooler, and the
differences in masker air system designs, we will
associated piping that supplies air to the rotary air seal
discuss the system installed on the CG-47, DD-963, and
on the OD box for distribution to the propeller blades.
DDG-993 class ships. After we discuss this system, we
Because these valves are operator selected either open
will then point out the differences in the systems
or closed, the airflow and pressure are not reduced or
installed on other gas turbine-powered ships.
regulated by these valves before the air is required for
CC-47, DD-963, and DDG-993 Class Ships. For
use.
the masker air system in these ships, the air is first
TESTS AND INSPECTIONS. For both the
extracted from the bleed air header and then piped to the
masker and prairie air systems, all tests and inspections
masker air valve (masker cooler valve). The masker air
must be carried out according to the PMS.
valve is located between the header and the masker air
There are two primary tests that are normally
cooler. When the operator selects the ON position for
conducted on these systems. In the first test, you will use
this valve, the hot bleed air flows through the cooler to
a portable air flowmeter to check for the proper airflow.
an air/water cyclone-type separator and then to the
masker transfer valve. The masker transfer valve is a
For the other test, you will check the operational
two-way directional flow valve. The valve position
effectiveness of these two systems against an acoustical
range. In most cases, you can perform the two tests at
selected by the operator will determine the direction of
airflow, either to the start air system or to the masker air
the same time unless all flow rates arc set correctly
system. As the air is directed to the masker air system,
before you make the required passes over the range.