the third deck level. The height and size of the
overflow loop act as a relief device. It limits the
pressure that can be exerted on the tanks (within the
maximum allowable limits) when maximum pump
capacity is discharged overboard. This would be the
condition when the delivery of gasoline is stopped and
the seawater pump continues to operate. However,
the height of the loop and the expansion tank also
maintains an adequate back pressure on the tanks to
force gasoline to the suction side of the gasoline
pumps. This ensures a positive pressure (0.5 to 1 psi)
is maintained when maximum delivery of gasoline is
being made. A one-way check valve and a shutoff
valve are installed near the end of the overflow line.
The shutoff valve is normally LOCKED OPEN.
Steam-heating coils are installed around the overflow
line at the shell connection to keep the line clear
during icing conditions.
A vent line extends from the top of the loop to
the atmosphere at the 02 level. The vent line is
provided to break the syphoning effect of the overflow
loop to prevent lowering the pressure at the gasoline
pump suction header. This line also may be equipped
with steam-heating coils.
OPERATION OF THE
SEAWATER SYSTEM
The seawater system serves to force the gasoline
through the tank and up to the gasoline pump
suction. A pressure of about 0.5 to 1 psi is required at
the gasoline pump suction to prevent the gasoline
pumps from becoming vapor locked.
The seawater pump should be put into operation
before starting up the gasoline pump. The seawater
pump will discharge to the outer tank. As gasoline is
drawn from the tank, it is automatically replaced
with seawater, thus maintaining a positive pressure
on the gasoline pump suctions. Excess seawater will
automatically be discharged overboard through the
over-flow line.
1. To line up the system to take suction from the
sea and to discharge to the outer tank supply riser,
take the following actions:
a. Open the shutoff valve between the sea
chest supply riser and the pump suction header.
b. Open the shutoff valve between the pump
discharge header and the outer tank supply riser.
2. To align the seawater pump, take the
following actions:
a. Open the shutoff valve in the pump
suction line.
b. Vent the pump through a petcock at the
top of the pump casing. When seawater appears, close
the petcock.
c. Start the pump with the discharge valve
closed. When the pump discharge pressure builds up,
open the discharge valve SLOWLY.
DOUBLE-WALLED
GASOLINE PIPING
When MOGAS passes through spaces, it is
carried in double-walled piping that consists of two
concentric pipes (fig. 6-11). The inner pipe is copper
Figure 6-11.—Double-walled piping.
6-11
