ENGINEMAN 1 & C
can be diverted overboard or into the CHT tank.
The basic CHT system concept requires that waste
drains be kept separate from soil drains wherever
practical until they reach their respective over-
board diverter valves. Downstream of their over-
board diverter valves both waste drains and soil
drains may be combined into a single drain line.
All drains above the waterline may be diverted
overboard by gravity. Drains located below the
waterline cannot be diverted directly overboard
and must use the CHT system as an ejection
system. In this case, the CHT system must operate
continuously in all modes.
All drain piping is pitched to insure rapid and
complete drainage. Pitch is 1/4-inch/ft whenever
possible, but not less than 1/8-inch/ft relative to
the operating trim.
Very large tanks may require swash bulkheads to
dampen movement of the tank contents. The tank
bottom slopes approximately 1.5 inches/ft toward
the pump section. All internal surfaces of the tank
are coated in accordance with procedures given
in the Naval Ships Technical Manual chapter
63(9190). Preservation of Ships in Service, for
protecting sanitary tanks, and to prevent corro-
sion. Each CHT tank is fitted with a vent to the
atmosphere and an overflow to the sea. In addi-
tion, a manhole is provided for internal
maintenance. Vents should be positioned to avoid
intake of CHT gases into the air compressor or
Garbage grinder drains connected to the waste
drains are installed with a minimum slope of 3
inches/ft. Garbage grinder drains are also pro-
vided with a check valve to preclude back-flow
from the waste drain and a diverter valve to per-
mit drainage to either the CHT tank or overboard.
When the garbage grinder employs seawater for
flushing, the waste piping downstream of the gar-
bage grinder is of copper-nickel alloy.
Plumbing drains may penetrate watertight
bulkheads. Usually, each bulkhead penetration
below flooding water level (FWL-1) is provided
with a bulkhead stop valve. The stop valve is a
round, full-port plug or ball valve. The stop valve
is operable at the valve and the damage control
deck. In some installations, diverter valves (3-way
valves) are used to prevent progressive flooding
throughout the CHT system drains, eliminating
the need for a bulkhead stop valve.
Each tank is equipped with two nonclog
marine sewage pumps connected in parallel. The
pumps may discharge sewage to a tender, barge,
shore facility, or directly overboard, depending
on the position of the discharge diverter valve.
Each pump is equipped with full-port plug or ball
suction and discharge valves, and a discharge
swing check valve with a hold-open device. An
explanation of pump characteristics curves is given
in Naval Ships Technical Manual, Chapter
Where CHT system valves are designated as
damaged control closures, the valve bonnet and
hand wheel is labeled SET X-RAY, SET YOKE,
or SET ZEBRA, with the direction to be turned
marked with an arrow. Similar labeling is required
at the damage control deck box. The damage con-
trol labeling is in addition to the CHT classifica-
tion and label plate.
Two types of CHT systems are installed. The
type selected for a particular ship depends on the
holding tank capacity. Systems with tanks with
a capacity of more than 2000 gallons use a com-
minutor and aeration system. Smaller systems
with tanks having a capacity of less than 2000
gallons use strainers.
The CHT tank is usually sized for a 12-hour
holding period. Individual ship constraints may
affect this design objective. Each tank has inside
surfaces which are usually free of structural
members such as stiffeners, headers, and brackets.
In a comminutor-type system the comminutor
located in the soil drain or the combined soil and
waste drain serves to macerate solids passing in-
to the CHT tank. A bypass is fitted upstream of
the comminutor. If the comminutor jams or
plugs, the bypass provides drainage around the
comminutor and into the tank. If a valve is fit-
ted in the bypass, it should always remain open.
Isolation valves are fitted directly before and after
the comminutor to allow for maintenance. Most
installations include an access port, or cleanout,