The collection element consists of separate soil
prevent progressive flooding through the CHT
and waste drains that are equipped with diverter
system drains. This eliminates the need for
valves. Depending on the position of these diverter bulkhead stop valves.
valves, the soil and waste drains can be diverted or
directed overboard or into the CHT tank.
Where CHT system valves are designated as
damage control closures, the valve bonnet and
The holding element is a holding tank. The
handwheel are labeled, The labels are X-BAY,
tank is designed large enough to hold all of the YOKE, or ZEBRA, with the direction to be turned
ship’s sewage that would normally be generated over
marked by an arrow. Similar marking or labeling is
a 12-hour period.
shown on the remote valve operators, located at the
damage control deck.
The transfer element includes sewage pumps,
overboard and deck discharge piping, and deck The piping primarily used in CHT systems is
discharge fittings. made of copper-nickel (90 to 10). The main drain
headers are normally 4-inch pipe. However, some
Collection Element
larger ships will have 6- or even 8-inch pipe.
The CHT system plan requires that waste drains For identification purposes, the piping will be
be kept separate from soil drains wherever practical
stencilled as either a soil drain or waste drain. An
until they reach the overboard diverter valve. From arrow will also be stencilled on the piping to
this point, they may be combined into a single drain indicate the direction of flow. It is not necessary to
line. All drains above the waterline may be diverted color-code the piping. However, the valves and
overboard by gravity. Drains located below the valve handles should be painted gold according to
waterline cannot be diverted directly overboard and the color coding system.
must use the CHT system as an ejection system. In
this case, the CHT system must operate Holding Element
continuously.
We said earlier that the CHT tank is normally
NOTE: Waste drains are plumbing drains that designed large enough for a 12-hour holding period,
service sinks, showers, galleys, and other similar but the ship’s size, shape, and mission will affect the
drains that transport their waste water, which is volume of the tank. The number of tanks will vary
known as gray water. Soil drains are plumbing from ship to ship. Each tank has inside surfaces
drains that service commodes, urinals, and other that are generally free of structural members such as
drains that transport human wastes, also known as stiffeners, headers, and brackets. However, very
black water. large tanks may have swash bulkheads to dampen
the movement of the contents of the tank. The tank
All drain piping is pitched to ensure rapid and bottom slopes about 1.5 inches per foot toward the
complete drainage of the waste and soil drain pump suction. All internal surfaces of the tank are
systems. The piping pitch is l/4 inch per foot coated to prevent corrosion. (See NSTM, chapter
whenever possible, but not less than l/8 inch per 631 (9190).) Each CHT tank has a vent to the
foot relative to the ship’s trim. atmosphere, an overflow to the sea, and a manhole
for internal maintenance.
Garbage grinder drains connected to the waste
drains have a minimum slope of 3 inches per foot. The firemain connection is used to flush and
These drains also have a check valve to prevent clean the CHT tank. Seawater is sent into the top
backflow from the waste drain system. of the tank through washdown nozzles, which spray
the inside of the tank. The tank should be flushed
Plumbing drains may pass through watertight each week for 30 minutes whether in port or at sea.
bulkheads. Normally, each bulkhead penetration The firemain is used also to flush the discharge
has a bulkhead stop valve. Each bulkhead stop piping and the transfer hose when the ship is
valve is a full-port, plug-, or ball-type valve. The preparing to leave port but while it is still connected
valve should be operable both at the valve and from to the shore sewage fittings.
the damage control deck. In some installations,
diverter valves (three-way valves) are used to
17-6
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