control consoles or, in an emergency, manual
cubic feet per minute (scfm) at an approximate
discharge pressure of 12 psig. The emitters are
override can be used to select the position of the
valve.
perforated with 3/64-inch holes through which
After the masker air passes through the
masker air discharges from the keel to the design
waterline. An orifice plate is fitted into the
masker/start transfer valve, it is piped to the
masker air pressure-regulator valves. The
emitter belt halves on the port side to balance the
regulating valve maintains a 30 psig regulated
air flow. A check valve is provided to prevent the
pressure in the masker emitter ring flow
backflow of seawater into the system.
manifolds. Masker air then passes into the
emitter flow distribution manifold. As the masker
PRAIRIE AIR SYSTEM
air leaves the manifold and enters each branch
line, it passes through a flowmeter orifice to a
The prairie air system modifies the thrashing
manual flow control valve. The flowmeter orifice
noise produced by the ship's propellers to disguise
creates the differential pressure required to
the sonar signature of the propellers. The system
operate its associated flowmeter. The manual flow
does this by taking hot bleed air from the bleed
control valve establishes the prescribed masker air
air header, cooling it, and distributing it to the
flow to its respective emitter ring. From the
leading edges of the propeller blades.
manual flow control valve, masker air continues
The prairie air system is basically the same on
on through a check valve and a shutoff valve. It
all classes of gas turbine ships. Bleed air is directed
then flows into the hollow emitter ring on the
from the header to a prairie air cooler by the
ship's outer hull. The emitter ring has a series of
prairie air valve. On the CG-, and DD-
1/16-inch diameter holes in its outer wall through
class ships, the prairie air valve is operated from
the control console. On the FFG-class ships, the
which the air escapes.
prairie air valve is a thermostatically operated
FFG Masker Air System
valve, which automatically closes when the
discharge air from the cooler exceeds 150F. The
In the FFG masker air system, the discharge
bleed air is cooled to 100F on the CG-, and DD-
header from the bleed air heat exchanger divides
class ships and to 125F on the FFG. These
into two branch headers, one for the prairie/
are the maximum prairie air temperatures. An
alarm indication is given when these temperatures
masker air and one for the starting air. Between
the bleed air heat exchanger and the two branch
are exceeded.
headers are a temperature switch and an RTD.
A manually operated flow valve is used to
The temperature switch shuts the bleed air
establish the prescribed prairie air flow to the
reducing valve when the bleed air discharge
propeller. From the prairie air flow control valve,
temperature reaches 425F. The RTD supplies a
prairie air is piped to the oil distribution (OD) box
temperature signal to the auxiliary control
on the front of the MRG. At the OD box, the
console (ACC) for display and alarm (400F)
prairie air enters the propeller shaft through a
indication.
rotoseal and travels through the shaft to the
The prairie/masker header divides into two
propeller blade. It then enters the water through
a series of holes on the leading edge of each blade.
branches, one for prairie air and one for masker
air. The prairie/masker branch header has a
prairie isolation valve. The masker air system
contains a pressure-reducing valve to reduce the
COMPRESSED AIR SYSTEMS
pressure from 75 psig to 28 psig. The masker air
Compressed air systems consist of the SSAS
system divides into two branches, one for the
or LP air system and the HP air system. The
forward emitter belts and one for the aft emitter
systems vary between ship classes and are
belts. The forward and aft masker flow control
described in the following sections.
valves are operated by push buttons from the
ACC. Venturi-type flowmeters are provided in
each masker system branch to measure airflow.
CG, AND DD
The emitter belts are located forward and aft
LOW-PRESSURE AIR SYSTEM
in port and starboard halves, each half belt
having a separate air connection. The masker air
The SSAS is the general-purpose air system.
system discharges bleed air through each of the
It provides LP air throughout the ship for
operation of most pneumatically operated
four emitter connections at a rate of 425 standard
4-21
