pressed air to an appropriate pressure from the ship’s
low-pressure air system. In some systems, a hose is
used to pressurize the tank through a quick-disconnect
valve. In other systems, a permanent pipe installation
is connected to the expansion tank through a pressure-
relief valve and an air shutoff valve.
The ship’s low-pressure air system is used to
charge the pressure tank, and then it is secured to pre-
vent a possible flood back of coolant into the low-
pressure air system. The relief valve protects the tank
and the distilled-water system from being overpres-
surized. The sight glass and the low-level alarm
switch function the same as those on the gravity ex-
pansion tank.
In both types of expansion tanks, the bottom of the
tank is connected by piping to the return main of the
secondary cooling system. Changes in coolant volume
cause the coolant to flow into or out of the reservoir,
as necessary, to maintain a stable return-line pressure.
Makeup water (distilled water) is added to the
expansion tank through the funnel on the top of the
tank. A funnel cap is provided for the funnel to pre-
vent dirt from entering the system. When you fill the
pressurized expansion tank, you must first isolate the
tank from the cooling system and the air supply
before you vent the air pressure off through the vent
pipe at the top of the tank. The makeup water can be
obtained directly from the ship’s evaporators and
preferably when the ship is making boiler-feed water,
because the water is double distilled. At no time
should potable (drinking) water or treated boiler-feed
water be used in any electronic cooling systems.
After the water is drawn from the ship’s evapo-
rators, it should be transported only in a clean, capped
container. You should take a sample of the water from
the container and have it tested for chloride by the
ship’s water test facility before any of the water is
used in the cooling system. The maximum permis-
sible level of chloride is 0.065 ppm (equivalent parts
per million). The supply system provides an alternate
source of makeup water.
The expansion tank sight glass is your best indi-
cation of a coolant leak in the secondary cooling
system. When the system uses excessive makeup
water, you should inspect the whole secondary sys-
tem, including the telltale drains on the heat ex-
changer, to locate the source of the leak. A small drip
can amount to several gallons of water a day. On the
pressurized expansion tank, a very small air leak (in-
dicated by a pressure drop on a tank gage) can be
located by brushing on a leak detector (a thick, clear,
soapy liquid, such as concentrated liquid dishwashing
soap) over the suspected area of the leak. The escap-
ing air causes bubbles to form in the leak detector.
SEAWATER STRAINERS
Strainers are used in the seawater cooling system
to remove debris and sea life, which could clog the
pressure and flow-control device (orifice) and/or the
tubes of the heat exchanger. The two types of in-line
seawater strainers most commonly used in weapons
cooling systems are the simplex (single) and duplex
(double) basket strainers.
The simplex basket strainer, shown in figure 2-8,
has a Y-pattern body. (Some simplex strainers have a
small drain on the cover to allow you to drain the
water off before removing the cover.) The basket is
removed periodically for cleaning and inspecting for
deterioration. This type of strainer requires that the
seawater be secured before you clean the basket.
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