The duplex pressure gage monitors the differential
pressure between the inlet and outlet ports of the
duplex strainer. The duplex gage provides a visual in-
dication of a clogged strainer basket. To correctly use
the gage, you should mark it when the basket is clean.
When the basket is clogged, the pressure reading is
usually 5 to 10 psi above the clean-basket reading. If
the pressure drop is less than the clean-basket reading,
you should check for a damaged or missing basket.
The basket handle (spring handle) acts as a spring-
load to seat and hold the basket in the housing. A
damaged spring handle will permit debris to bypass
the strainer basket and clog the heat exchanger tubes.
In some cases, the basket may spin inside the duplex
strainer and physically wear away the basket seat and/
or the side of the duplex strainer. The duplex strainer
would then have to be removed for extensive repairs,
possibly off the ship. New or replacement baskets
should always be checked for proper spring-handle
pressure against the top of the basket cover.
You should use only the correct gasket material
for the basket covers, as specified in the Coordinated
Shipboard Allowance List (COSAL). Inferior material
can stretch and can be forced out from under the
cover, permitting seawater to spray out and possibly
flood the space.
TEMPERATURE-REGULATING VALVES
The temperature-regulating valves regulate the
amount of cooling water flowing through or by-
passing a heat exchanger to maintain a desired tem-
perature of distilled water going to the electronic
equipment. Temperature regulating is usually pro-
vided by a three-way or two-way temperature-regu-
lating valve or a combination of both valves. The
three-way valve is used where seawater is the primary
cooling medium in the heat exchanger, whereas the
two-way valve is used where chilled water is the pri-
mary cooling medium.
Three-Way Temperature-Regulating Valves
Three-way temperature-regulating valves are in-
stalled in liquid-cooling systems so that the incoming
distilled water to the valve can be directed to the heat
exchanger or caused to bypass the heat exchanger.
More accurately, the distilled water is proportioned
between these two paths.
The valve senses the temperature of the distilled
water downstream of the junction between the heat
exchanger outlet and the bypass and then proportions
the two flows to obtain the desired temperature. The
operating range of the three-way temperature-regu-
lating valve is within ±5° of the setting on the valve.
The bulb contains a volatile liquid that vaporizes
and expands when heated. The pressure generated in
the bulb is a function of the temperature around it and
is transmitted through the capillary tubing to the
flexible bellows, which are loaded by the spring. Both
the bellows and the spring rest on the end of the valve
stem. Expansion or contraction of the bellows causes
movement of the stem and the piston in the valve
body. Movement of the bellows is opposed by the
spring, which can adjust the operating temperature by
the spring-tension adjustment wheel.
A drop in the temperature at the thermostatic bulb
reduces the pressure in the thermostatic assembly,
causing it to exert less force and resulting in an up-
ward movement of the stem because of the force of
the spring. As the stem is connected to the piston, the
piston also moves upward, enabling more liquid to
pass from the bottom inlet through the right outlet
(bypass) side and, at the same time, restricting flow
through the left outlet (heat exchanger) side. A rise in
temperature at the thermostatic bulb results in a re-
versed effect.
Figure 2-10 shows a three-way temperature-regu-
lating valve.
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