Page 126 - International safety guide for oil tankers and terminals

INTERNATIONAL SAFETY GUIDE FOR OIL TANKERS AND TERMINALS

7.1.5.2 Inert Gas System Maintenance
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There should be close co-operation between the deck and engine
departments to ensure proper maintenance and operation of the inert gas
system. It is particularly important to ensure that non-return barriers
function correctly, especially the deck water seal or block and bleed
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valves, so that there is no possibility of petroleum gas or liquid petroleum
passing back to the machinery spaces.

To demonstrate that the inert gas plant is fully operational and in good
working order, a record of inspection of the inert gas plant, including
defects and their rectification, should be maintained on board.

7.1.5.3 Degradation of Inert Gas Quality

Tanker personnel should be alert to the possible degradation of inert gas
quality within tanks as a result of air being drawn into the tanks due to
inappropriate operation of the inert gas or cargo systems. For instance:
Not topping up the inert gas promptly if the pressure in the system
falls, due to temperature changes at night.
Prolonged opening of tank apertures for tank gauging, sampling and
dipping.

When water is drained from a non-inerted tank, air will be entrained into
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the drainings delivered to the slop tank and may ultimately enter into
inerted tank atmospheres. The volume of air entrained in this manner can
be particularly high if an eductor is used on recirculation to the slop tank.
Therefore, when liquid is to be drained to the slop tank, the inert gas
quality in all tanks should be closely monitored.

7.1.6 Application to Cargo Tank Operations

Before the inert gas system is put into service, the tests required by the
operations manual or manufacturer’s instructions should be carried out. The
fixed oxygen analyser and recorder should be tested and proved to be in good
order. Portable oxygen and hydrocarbon meters should also be prepared and
tested.

7.1.6.1 Inerting of Empty Tanks

When inerting empty tanks that are gas free, for example following a dry
docking or tank entry, inert gas should be introduced through the
distribution system while venting the air in the tank to the atmosphere.
This operation should continue until the oxygen content throughout the
tank is not more than 8% by volume. Thereafter, the oxygen level will not
increase if a positive pressure is maintained by using the inert gas
system to introduce additional inert gas when necessary.

If the tank is not gas free, the precautions against static electricity given in
Section 7.1.6.8 should be taken until the oxygen content of the tank has
been reduced to 8% by volume.

When all tanks have been inerted, they should be kept common with the
inert gas main and the system pressurised with a minimum positive
pressure of at least 100 mm water gauge. If individual tanks have to be
segregated from a common line (e.g. for product integrity), the segregated
tanks should be provided with an alternative means of maintaining an
inert gas blanket.

92 © ICS/OCIMF/IAPH 2006