Page 178 - International safety guide for oil tankers and terminals

INTERNATIONAL SAFETY GUIDE FOR OIL TANKERS AND TERMINALS

10.2.5 Oxygen Deficiency
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Before initial entry is allowed into any enclosed space, the atmosphere
should be tested with an oxygen analyser to check that the air contains
21% oxygen. This is of particular importance when considering entry into
any space, tank or compartment that has been previously inerted. Lack of
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oxygen should always be suspected in all enclosed spaces, particularly if
they have contained water, have been subjected to damp or humid
conditions, have contained inert gas or are adjacent to, or connected with,
other inerted tanks.

10.2.6 Products of Inert Gas

By-products of combustion when inert gas is produced from boiler flue
gas or from an inert gas generator include carbon monoxide and carbon
dioxide.

Carbon monoxide is a toxic gas that may be present in cargo tank
atmospheres following gas freeing and in spaces containing components
of the inert gas plant.

Carbon dioxide is not toxic, but presents a smothering hazard. Adequate
ventilation is required to maintain a normal oxygen level in air of 21% by
volume in the space and to eliminate any hazard.
Atmosphere Tests Prior to Entry ึ

10.3

No decision to enter an enclosed space should be taken until the atmosphere within
the space has been comprehensively tested from outside the space with test
equipment that is of an approved type and that has recently been calibrated and
checked for correct operation (see Section 8.2).
The appropriate atmosphere checks are:

Oxygen content is 21% by volume.
Hydrocarbon vapour concentration is less than 1% LFL.
No toxic or other contaminants are present.

Care should be taken to obtain measurements from a representative cross-section of
the compartment by sampling at various depths and through as many deck openings
as practicable. When tests are being carried out from deck level, ventilation should
be stopped and a minimum period of about ten minutes should be allowed to elapse
before readings are taken.

Even when tests have shown a tank or compartment to be safe for entry, pockets of
gas should always be suspected.

If extensive work is to be carried out within a large space, such as a cargo tank, it is
recommended that a full assessment of the tank atmosphere is undertaken after the
initial tests have been satisfactorily carried out and recorded. The person undertaking
the full assessment should enter the tank carrying an emergency escape breathing
device and a personal gas monitor, in addition to the gas testing instrument. The tank
atmosphere should be checked frequently during this entry, with particular attention
being placed on testing the work location(s) and places that are inaccessible for
testing from the deck. On satisfactory completion of this additional atmosphere test,
the results should be recorded as required by the appropriate safety procedure in the
Safety Management System.

144 © ICS/OCIMF/IAPH 2006