Page 53 - International safety guide for oil tankers and terminals

HAZARDS OF PETROLEUM

2.3.9.3 Nitrogen Oxides
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Fresh flue gases typically contain about 200 ppm by volume of mixed
nitrogen oxides. The majority is nitric oxide (NO), which is not removed by
water scrubbing. Nitric oxide reacts slowly with oxygen, forming nitrogen
dioxide (NO 2 ). As the gas stands in tanks, the total concentration of
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nitrogen oxide falls over a period of 1-2 days to a level of 10-20 ppm as
the more soluble nitrogen dioxide goes into solution in free water, or by
condensation, to give nitrous and nitric acids. Further decrease below this
level is very slow.

Nitric oxide is a colourless gas with little smell at its TLV-TWA of 25 ppm.
Nitrogen dioxide is more toxic with a TLV-TWA of 3 ppm.

2.3.9.4 Sulphur Dioxide
Flue gas produced by the combustion of a fuel oil that has a high sulphur
content typically contains about 2,000 ppm of sulphur dioxide (SO 2 ). Inert
gas system water scrubbers remove this gas with an efficiency that
depends upon the design and operation of the scrubber, giving inert gas
with a sulphur dioxide content of typically between 2 and 50 ppm.

Sulphur dioxide causes irritation of the eyes, nose and throat and may
also cause breathing difficulties in sensitive people. It has a distinctive
smell at its TLV-TWA of 2 ppm.
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2.3.9.5 Carbon Monoxide
Carbon Monoxide (CO) is normally present in flue gas at a level of only a
few parts per million. Abnormal combustion conditions and slow running
can however give rise to levels in excess of 200 ppm. Carbon monoxide is
an odourless gas with a TLV-TWA of 25 ppm. It is insidious in its attack,
which is to restrict oxygen uptake by the blood, causing a chemically
induced form of asphyxiation.

2.3.10 Oxygen Deficiency

The oxygen content of the atmosphere in enclosed spaces may be low for
several reasons. The most obvious one is if the space is in an inert condition,
and the oxygen has been displaced by the inert gas. Oxygen may also be
removed from an atmosphere by chemical reactions, such as rusting or the
hardening of paints or coatings.

As the amount of available oxygen decreases below the normal 21% by
volume, breathing tends to become faster and deeper. Symptoms indicating
that an atmosphere is deficient in oxygen may give inadequate notice of
danger. Most people would fail to recognise the danger until they were too
weak to be able to escape without help. This is especially so when escape
involves the exertion of climbing.
While individuals vary in susceptibility, all will suffer impairment if the oxygen
level falls to 16% by volume.

Exposure to an atmosphere containing less than 10% oxygen content by
volume inevitably causes unconsciousness. The rapidity of onset of
unconsciousness increases as the availability of oxygen diminishes, and death
will result unless the victim is removed to the open air and resuscitated.

© ICS/OCIMF/IAPH 2006 19