CHT system should not be lagged. This will allow
The pump suction is located one pipe diameter
ready detection of leaks. A drip pan will be
off the bottom of the tank. This maintains a
installed under any valve or flange located in food
flooded suction. The pump's impeller is a nonclog
preparation or food storage areas, medical spaces,
marine type. The semi-open impeller allows solids
or spaces where leakage can reach any bilges that
up to 2 l/2 inch in diameter to pass through.
Double mechanical seals are used to prevent
will help detect leakage and prevent the spread of
leakage. The oil reservoir of the seal is filled with
2190 TEP oil.
A firemain washdown system is used to flush the
Drip pans or a coaming are installed to contain
tank, discharge piping, and hoses. The pump room
any leaks at the pump or pipe connections. The
can also be washed down. The water left in the
drip pans and coaming areas should be inspected
pump room can be disposed of through the
regularly; if they have been contaminated, they must
pump-room sump using the installed eductor.
be cleaned and disinfected.
A relief valve set at about 125 psi is installed in
Full-ported suction and discharge valves are
the discharge piping system. This valve prevents
installed in the piping on the respective sides of the
excessive pressure in the hose during the flushing
pump. These valves will either be ball or plug
Controller, Alarms, and Level Sensors
A spool piece consisting of a short flanged pipe
is installed in the suction line near the pump. The
The CHT system provides for both manual
spool is a convenience item to allow maintenance
(MAN) and automatic (AUTO) operation. In the
personnel to inspect the pump. If the pump is
MAN 1 mode, the operator may start either or both
clogged or jammed, you can remove the spool and
pumps independently of the tank liquid level
gain access to correct the situation.
sensors. In the MAN 2 mode, the pumps stop at
the low-liquid level (10 percent). In the AUTO
Most of the new installations are fitted with
mode, the pump controller automatically performs
oil-filled diaphragms to keep the sewage out of the
the following functions as a result of signals from
sensitive parts of the pressure gauges. The pump
the level sensors in each tank:
has a vacuum/pressure gauge on the suction side,
and a discharge/pressure gauge.
The controller alternates the duty or
A special swing-check valve is installed in each
pump discharge line to prevent one pump from
pumping sewage back to the tank through the
The low-liquid level sensor stops the duty
adjacent pump. The swing-checkvalve also prevents
pump when the liquid level reaches approximately
sewage from another nested ship from being
10 percent of the tank volume to maintain flooded
pumped into the tank. These swing-check valves
have a manual hold-open device to permit drainage
of the discharge piping.
The 30 percent liquid level sensor signals the
controller to start the duty pump.
The 60 percent liquid level sensor signals the
The piping assembly for the CHT system should
controller to start the standby pump in the event of
be straight with as few elbows as possible. A slight
failure or inadequacy of the duty pump.
downward slope is needed to allow for gravity flow.
The piping transfers soil and waste water from the
The 85 percent liquid level sensor provides
source to the overboard discharge or to the CHT
a visual (sight) and audible (sound) high-level alarm
tank, depending on the operational mode.
signal. The high-liquid level alarm operates both
audibly and visually in the CHT pump area, in
The piping in the pump room should not be
damage control central, and on the quarterdeck.
lagged. However, the piping outside the pump
The sound alarm in the pump room can be silenced
room should be lagged. Valves or flanges within the