OPERATOR, UNIT, AND DIRECT SUPPORT MAINTENANCE
AIR COMPRESSOR FOR
INLAND AND COASTAL LARGE TUG (LT)
THEORY OF OPERATION
This work package describes the theory of operation for the Inland and Coastal Large Tug's (LT) compressed air
system. The theory of operation for the system as a whole is discussed first. Following the system theory of
operation, the theory of operation for the air compressor is discussed.
Two electric powered, two-stage air compressors (figure 1, item 1) compress air for use throughout the vessel.
See the Air Compressor paragraph below for theory of operation for the compressors.
After leaving the compressors (figure 1, item 1), the compressed air passes through air dryers (figure 1, item 2).
These air dryers remove moisture from the air before the air reaches the starting air receivers (figure 1, item 3).
Removal of moisture from the compressed air is necessary to prevent the accumulation of water in the receivers.
This accumulated water can corrode the interior of the receivers, and can also travel downstream in the com-
pressed air system damaging components further down the line. Each dryer is equipped with a bypass valve
(figure 1, item 4) to permit air to bypass the dryer in the event of a dryer failure.
All air discharged from the air compressors (figure 1, item 1) arrives first at the starting air receivers (figure 1, item
3). Here, the air is stored at 250 PSI (17.2 bar) to ensure that an adequate supply of compressed air is on hand
for use in starting the main engines, the bow thruster engine, the pump drive engine, and the engine for SSDG 2.
SSDG 1 is equipped with an electric starter, and does not require compressed air.
Compressed air for ship's service use (air tools, ship's whistle, sea chest blowdowns, etc.) is stored in the ship's
service air receiver (figure 1, item 5). This receiver is supplied from the starting air receivers (figure 1, item 3)
through a regulator (figure 1, item 6). This regulator reduces the starting air pressure to 125 PSI (8.6 bar) before
the air is permitted to enter the ship's service air receiver. A bypass valve (figure 1, item 7) permits manual filling
of the ship's service air receiver in the event of a regulator failure.
All three receivers (figure 1, items 3 and 5) are equipped with relief valves (figure 1, item 8), local pressure
gauges (figure 1, item 9), manual drain valves (figure 1, item 10), and automatic drain valves (figure 1, item 11).
The relief valves prevent overpressurization of the receivers, which could result in catastrophic failure. The relief
valve for the starting air system is set at 275 PSI (19.0 bar) and the relief for the ship's service system is set for
137.5 PSI (9.5 bar). The manual drains permit manual draining of water from the receivers. They also permit the
system to be drained when receiver or compressed air system maintenance is required. The automatic drains
open on a preset schedule, keeping the receivers drained under normal operating conditions.
Although the starting air is stored at 250 PSI (17.2 bar), this pressure is too high for the engines' starters to
accommodate without damage. Therefore, each engine's starting air piping is fitted with a regulator. The regu-
lator for the bow thruster engine (figure 1, item 12) and the pump drive engine (figure 1, item 13) each reduces the
starting air pressure to 125 PSI (8.6 bar). The regulators (figure 1, item 14) for the main engines each reduce the
engines' starting air pressure to 200 PSI (13.8 bar).
Additional regulators are provided for the forward (figure 1, item 15) aft (figure 1, item 16) and high level (figure 1,
item 17) sea chest blowouts. These reducing valves reduce the ship's service air to 25 PSI (1.7 bar) to prevent
damage to the sea chests.