Page 129 - International safety guide for oil tankers and terminals

SHIPBOARD SYSTEMS

action to be taken in the event of failure of the inert gas plant during
discharge from inerted tanks, see Section 7.1.12.
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If hand dipping of a tank is necessary, pressure may be reduced while
dipping ports are open, but care must be taken not to allow a vacuum to
prevent ingress of air. Whenever pressure falls to the low pressure alarm์นาวี
develop since this would pull air into the tank. To prevent this, it may be
necessary to reduce the cargo pumping rate, and discharge should be
stopped immediately if there is a danger of the tanks coming under
vacuum.

7.1.6.7 Ballast Passage
งานห้องสมุด ศูนย์ฝกพาณิชย
During a ballast passage, cargo tanks other than those required to be gas
free should remain in the inert condition and under positive pressure to

level, the inert gas plant should be restarted to restore the pressure, with
due attention being paid to the oxygen content of the inert gas delivered.

7.1.6.8 Static Electricity Precautions

In normal operations, the presence of inert gas prevents the existence of
flammable gas mixtures inside cargo tanks. Hazards due to static
electricity may arise however, mainly in the case of a failure of the inert
gas system. To avoid these hazards, the following procedures are
ึ
recommended:
If the inert gas plant breaks down during discharge, operations should
be suspended (see Section 7.1.12). If air has entered the tank, no
dipping, ullaging, sampling or other equipment should be introduced
into the tank until at least 30 minutes have elapsed since the injection
of inert gas ceased. After this period, equipment may be introduced
provided that all metallic components are securely earthed. This
requirement for earthing should be applied until a period of five hours
has elapsed since the injection of inert gas ceased.
During any necessary re-inerting of a tank following a failure and
repair of the inert gas system, or during initial inerting of a non-gas
free tank, no dipping, ullaging, sampling or other equipment should be
inserted until the tank is in an inert condition, as established by
monitoring the gas vented from the tank being inerted. However,
should it be necessary to introduce a gas sampling system into the
tank to establish its condition, at least 30 minutes should elapse after
stopping the injection of inert gas before inserting the sampling
system. Metallic components of the sampling system should be
electrically continuous and securely earthed. (See also Chapter 3 and
Section 11.8.)

7.1.6.9 Tank Washing, Including Crude Oil Washing

Before each tank is washed, the oxygen content must be determined,
both at a point 1 metre below the deck and at the middle level of the
ullage space. At neither of these locations should the oxygen content
exceed 8% by volume. Where tanks have a complete or partial swash
bulkhead, the measurement should be taken from similar levels in each
section of the tank. The oxygen content and pressure of the inert gas
being delivered during the washing process should be continuously
recorded.

If, during washing, the oxygen content in the tank exceeds 8% by volume
or the pressure of the atmosphere in the tanks is no longer positive,

© ICS/OCIMF/IAPH 2006 95