begin to take over. Anaerobic bacteria require no
Therefore, the tank is to be flushed and pumped out
oxygen to survive. The anaerobic bacteria produce
at least weekly whether in the in-port mode or the
hydrogen sulfide, methane, and other toxic and
at-sea mode.
flammable gases. H2S is not limited to the CHT
system. If drainage water from refrigeration boxes
Hydrogen sulfide (H2S) is the most common gas
gets into a confined space and is overlooked for
you will find in the CHT system, and it can be
long periods of time, it too can produce H2S.
produced within 6 to 12 hours. It can also exist as
a liquid at low temperatures and high pressures.
Other gases of concern are as follows:
H2S is a colorless gas that has a strong odor of
rotten eggs. It can affect an individual's sense of
smell and cause an irritation to the eyes, nose, and
--Methane (CH4) is a colorless, odorless,
throat. A headache, dizziness, and an upset
nontoxic gas that is highly flammable. Flammable
stomach are caused by inhaling low concentrations
vapors may spread from a spill. The heat of a fire
of H2S. At higher concentrations of 1,000 to 2,000
can cause a container of CH4 to explode. Its
ppm, you can go into a coma or meet sudden death
flashpoint is -188C and ignition temperature is
after just one single breath. It is also possible that
538C. The lower flammable limit by volume is 5.53
you may be completely unaware of the presence of
percent.
H2S.
CH4 itself has no effect on humans. However,
H2S is also a flammable gas. It has a lower
it does reduce the oxygen concentration of the air.
flammable limit by volume of 4.3 percent and an
If the oxygen concentration is dropped below 20
upper limit of 46 percent. The au toignition
percent, an individual without an OBA, or similar
temperature is 260C (500F). When H2S burns, it
device, will suffocate.
has a pale blue flame. This type of fire may be
extinguished by using either CO, or water in the fog
--Carbon dioxide (CO,) is primarily a colorless
form. The gas can travel a considerable distance to
and odorless gas. However, CO, is also available as
a source of ignition. Once ignited, it will flash back
a liquid and as a solid. The maximum allowable
to the source.
dose of CO, authorized by OSHA is 5,000 ppm
averaged during an 8-hour period. NIOSH has
The toxicity level for H2S is currently set at 20
recommended that the amount be changed to 10,000
ppm by the Occupational Safety and Health
ppm averaged over a period of up to 10 hours per
Administration (OSHA). The National Institute for
day. For our purposes, and keeping these figures in
Occupational Safety and Health (NIOSH) has
mind, an acceptable limit of less than 0.5 percent is
recommended that the permissible exposure limit be
authorized.
reduced to IO ppm averaged over a lo-minute
period. At the time of this printing, the acceptable
CO2 is nonflammable, but it is an asphyxiant and
safe limits are set at 10 ppm averaged over an
a potent respiratory stimulant. It is also a stimulant
8-hour period.
and a depressant to the central nervous system.
CO2 displaces the oxygen in the compartment into
An aeration system must be provided for each
which it is discharged. When you enter a space
CHT collection tank. The aeration system is to be
where carbon dioxide has been discharged, you must
operated continuously while in the in-port mode.
wear an approved respirator or similar breathing
The tank is to be pumped down each 6- to 12-hour
device.
period when in the in-port mode to reduce the
amount of H2S that may be produced.
--Hydrogen (H) is a colorless and odorless gas.
It is flammable with an ignition point of 585C and
Fresh sewage contains a mixture of bacteria,
has a wide range of explosive limits. H is harmless
normally aerobic bacteria. The aerobic bacteria
in itself, but it can lower the amount of oxygen in
survive and grow in the presence of oxygen. If the
the air if a leak occurs. You will suffocate if the
sewage is not provided with additional oxygen (use
percentage of oxygen is reduced below 20 percent.
of the aeration system), and the sewage is allowed
to remain in the tank in excess of 6 to 12 hours, the
--Ammonia (NH3) is a colorless gas with a sharp
bacteria will consume all the oxygen and die off. As
and intensely irritating odor. It liquifies readily
the aerobic bacteria die off, the anaerobic bacteria
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