prevent progressive flooding through the CHT
The collection element consists of separate soil
system drains. This eliminates the need for
and waste drains that are equipped with diverter
bulkhead stop valves.
valves. Depending on the position of these diverter
valves, the soil and waste drains can be diverted or
Where CHT system valves are designated as
directed overboard or into the CHT tank.
damage control closures, the valve bonnet and
handwheel are labeled, The labels are X-BAY,
The holding element is a holding tank. The
YOKE, or ZEBRA, with the direction to be turned
tank is designed large enough to hold all of the
marked by an arrow. Similar marking or labeling is
ship's sewage that would normally be generated over
shown on the remote valve operators, located at the
a 12-hour period.
damage control deck.
The transfer element includes sewage pumps,
The piping primarily used in CHT systems is
overboard and deck discharge piping, and deck
made of copper-nickel (90 to 10). The main drain
headers are normally 4-inch pipe. However, some
larger ships will have 6- or even 8-inch pipe.
For identification purposes, the piping will be
The CHT system plan requires that waste drains
stencilled as either a soil drain or waste drain. An
be kept separate from soil drains wherever practical
arrow will also be stencilled on the piping to
until they reach the overboard diverter valve. From
indicate the direction of flow. It is not necessary to
this point, they may be combined into a single drain
color-code the piping. However, the valves and
line. All drains above the waterline may be diverted
valve handles should be painted gold according to
overboard by gravity. Drains located below the
the color coding system.
waterline cannot be diverted directly overboard and
must use the CHT system as an ejection system. In
this case, t h e CHT system must operate
We said earlier that the CHT tank is normally
designed large enough for a 12-hour holding period,
NOTE: Waste drains are plumbing drains that
but the ship's size, shape, and mission will affect the
service sinks, showers, galleys, and other similar
volume of the tank. The number of tanks will vary
drains that transport their waste water, which is
from ship to ship. Each tank has inside surfaces
known as gray water. Soil drains are plumbing
that are generally free of structural members such as
drains that service commodes, urinals, and other
stiffeners, headers, and brackets. However, very
drains that transport human wastes, also known as
large tanks may have swash bulkheads to dampen
the movement of the contents of the tank. The tank
bottom slopes about 1.5 inches per foot toward the
All drain piping is pitched to ensure rapid and
pump suction. All internal surfaces of the tank are
complete drainage of the waste and soil drain
coated to prevent corrosion. (See NSTM, chapter
systems. The piping pitch is l/4 inch per foot
631 (9190).) Each CHT tank has a vent to the
whenever possible, but not less than l/8 inch per
atmosphere, an overflow to the sea, and a manhole
foot relative to the ship's trim.
for internal maintenance.
Garbage grinder drains connected to the waste
The firemain connection is used to flush and
drains have a minimum slope of 3 inches per foot.
clean the CHT tank. Seawater is sent into the top
These drains also have a check valve to prevent
of the tank through washdown nozzles, which spray
backflow from the waste drain system.
the inside of the tank. The tank should be flushed
each week for 30 minutes whether in port or at sea.
Plumbing drains may pass through watertight
The firemain is used also to flush the discharge
bulkheads. Normally, each bulkhead penetration
piping and the transfer hose when the ship is
has a bulkhead stop valve. Each bulkhead stop
preparing to leave port but while it is still connected
valve is a full-port, plug-, or ball-type valve. The
to the shore sewage fittings.
valve should be operable both at the valve and from
the damage control deck. In some installations,
diverter valves (three-way valves) are used to