The duplex strainer (fig. 1-14) consists of two
removable baskets located in parallel at the seawater
inlet. Seawater flows into the top of one basket and out
through the perforated sides to the outlet. This
arrangement allows maintenance to be performed on
one basket while the system is in operation, A selector
valve is arranged so that with the handle in one position,
seawater flows through one of the baskets, leaving the
other basket accessible for removal and cleaning. By
switching the valve handle to the alternate position, flow
is shifted over to the other basket.
A duplex pressure gauge monitors the differential
pressure between the inlet and outlet ports of the duplex
strainer. The purpose of the duplex gauge is to provide
a visual indication of a clogged strainer basket. To
correctly use the gauge, it should be marked when the
basket is clean. When the basket is clogged, the pressure
reading is usually 5 to 10 psi above the clean-basket
If the pressure drop is less than the
clean-basket reading, a check should be made for a
damaged basket 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 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 COSAL. Inferior material can stretch
and be forced out from under the cover, and then
seawater could spray out and possibly flood the space.
The temperature regulating valve regulates the
amount of cooling water flowing through or bypassing
a heat exchanger to maintain a desired temperature of
distilled water going to the electronic equipment.
Temperature regulating is usually provided by either a
three-way or a two-way temperature regulating valve or
a combination of both valves, as shown in figure 1-7.
The three-way valve is used where seawater is the
primary cooling medium in the heat exchanger, and the
two-way valve is used where chilled water is the primary
Three-Way Temperature Regulating
The three-way regulating valve (fig. 1-7) is installed
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
Figure 1-14.-Seawater duplex strainer.