2. CHT pump set: There is one pump set per
AERATION SUBSYSTEM.--In
the
tank.
A pump set consists of the following
comminutor-type CHT system, air is supplied to the
components:
tank to prevent the contents from generating
hydrogen sulfide gas (odor of rotten eggs) and also
a. Two motor-driven (mixed flow)
to keep solids in suspension. (See fig. 17-2,
centrifugal pumps
at the end of this chapter.) Air enters the tank at
or near the top and is piped to nonclog air diffusers
b. Two plug- or ball-type suction valves
located at the bottom of the tank. These diffusers
are spaced 2 feet apart. Air pressure of 5 to 12 psi
c. Two plug- or ball-type discharge valves
from motor-driven blowers is supplied at the
diffusers to overcome the hydrostatic head of the
d. Two pump discharge check valves with a
overlying liquid. In some systems, ship's service air
hold-open device
is provided as a secondary source of the aeration.
CHT systems in some aircraft carriers do not use
e. A pump controller
blowers for aeration. Instead, they use ship's service
air as the primary source of air for tank aeration.
f. A high-level alarm
The aeration subsystem is operated when the CHT
system is in any operational mode except the at-sea
g. One liquid-level sensor for the high-level
mode.
alarm
Strainer-Type System
3. Inflow strainers with plug- or ball-type stop
valves and swing-check valves
The strainer-type system is basically the same as
the comminutor system except that the strainer
4. Overflow strainer inside the CHT sewage
system does not have a comminutor or an aeration
tank
subsystem, and the holding tank is less than 2,000
gallons in capacity.
5. Firemain flushing connections and washdown
nozzle for spray cleaning the interior of the tank
The strainer-type system has an overflow
strainer within the CHT tank and an inflow strainer
CHT COMMON COMPONENTS
mounted on the discharge side of each pump. The
drain collection pipe directs sewage flow through the
There are definite differences between the
overflow strainer where liquids may overflow into
comminutor system and the strainer system, but the
the CHT tank in the event the inflow strainer or the
two systems are similar in several ways. The
pumps become clogged. Solid and liquid wastes
operational modes, hoses, pumps, alarms, level
flow through the ball or plug valve and check valves
sensors, and controls are basically the same. The
until they reach the pump discharge piping. At this
manufacturer and the size may differ from one
junction, the sewage flow passes through the inflow
system to the other, or even from one ship to the
strainer where large solids are collected. Then they
next. However, the operation of the equipment will
pass through the pump and back into the CHT tank.
vary only slightly.
The inflow strainer limits the flow of solids, but
liquids are allowed to pass through the pump into
Sewage Pumps
the tank. Each time the pump operates, its inflow
strainer is cleaned by the reverse flow of liquid
Each CHT system has a set of two sewage
being pumped from the tank.
pumps. The pumps may be operated both at the
same time or by alternating with each other. The
The major components of the strainer-type
pump transfers the sewage from the tank to the pier
system are shown in figure 17-3 (at the end
connection, or directly overboard. The destination
of this chapter) and are discussed in the following
of the sewage depends on the lineup of the piping
paragraphs.
and the diverter valves.
1. CHT tank: The capacity of each strainer
system tank is less than 2,000 gallons.
17-8