Page 233 - International safety guide for oil tankers and terminals

SHIPBOARD OPERATIONS

11.4.6 Portable Fans
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Portable fans or blowers should only be used if they are water,
hydraulically, pneumatically or steam driven. Their construction materials
should be such that no hazard of incendiary sparking arises if, for any
reason, the impeller touches the inside of the casing. If steam driven fans
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are utilised, care should be taken to ensure that the exhaust does not
vent into the cargo tank, in order to prevent possible build-up of static
electricity.

The capacity and penetration of portable fans should be such that the
entire atmosphere of the tank on which the fan is employed can be made
non-flammable in the shortest possible time.

To assist in gas freeing deep cargo tanks and tanks with deep structural
members in the tank bottom, the use of extension tubes for the fans is
known to be effective. Where these extension tubes incorporate synthetic
materials, care should be taken to ensure that they are effectively bonded
to the ship’s structure.

Portable fans should be placed in such positions, and the ventilation
openings so arranged, that all parts of the tank being ventilated are
equally and effectively gas freed.

Ventilation outlets should generally be as remote as possible from the
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fans.

Portable fans should be so connected to the deck that an effective
electrical bond exists between the fan and the deck.

11.4.7 Ventilating Double Hull Ballast Tanks

The complexity of the structure in double hull and double bottom tanks
makes them more difficult to gas free than conventional ballast tanks. It is
strongly recommended that the Company develops guidelines and
procedures relating to the ventilation of each tank. An efficient method is
to fill each tank with ballast water and to then empty it. Account must be
taken of the stress, trim and loadline factors. However, it must be borne in
mind that any hydrocarbon leaks into the tank will mean that the ballast
will be dirty ballast and must be handled in accordance with MARPOL
regulations. When ballasting the tank, it should not be allowed to overflow
onto the deck.

Whenever possible, these guidelines and procedures should be
developed in conjunction with the shipbuilder and should be based on
actual tests and experiments, as well as on calculation. They should give
details of the configuration of each tank, the method of ventilation and
the equipment to be used. Details should also include the time required
for each method of ventilation to gas free the tank for entry. This should
be the time to remove all contaminants, rather than a simple volume/rate
calculation.

Where portable fans are used for ventilation purposes, the above
information should be provided for a range of drive pressures and
different numbers of fans.

Where tanks are identical in structure and size, and where the method of
venting is identical, the data can be obtained from tests on a

© ICS/OCIMF/IAPH 2006 199