Page 225 - International safety guide for oil tankers and terminals

SHIPBOARD OPERATIONS

To Control the ‘Fuel’ in the Tank Atmosphere
(See Figure 11.4 Non-inert tank washing flow chart.)
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Before Washing:

The tank bottom should be flushed with water, so that all parts
งานห้องสมุด ศูนย์ฝกพาณิชย์นาวี
are covered, and then stripped. This flush should be
undertaken using the main cargo pumps and lines.
Alternatively, permanent pipework extending the full depth of
the tank should be used. This flush should not be undertaken
using the tank washing machines.

The piping system, including cargo pumps, crossovers and
discharge lines, should also be flushed with water. The flushing
water should be drained to the tank designed or designated to
receive slops.
The tank should be ventilated to reduce the gas concentration
of the atmosphere to 10% or less of the Lower Flammable
Limit (LFL). Gas tests must be made at various levels and due
consideration should be given to the possible existence of
pockets of flammable gas, in particular in the vicinity of
potential sources of ignition such as mechanical equipment
that might generate hot spots, e.g. moving parts such as found
in in-tank (submerged) cargo pump impellors.
During Washing: ึ
Tank washing may only commence once the tank atmosphere

reaches 10% or less of the LFL.

Atmosphere testing should be frequent and taken at various
levels inside the tank during washing to monitor the change in
LFL percentage.
Consideration should be given to the possible effect of water
on the efficiency of the gas measuring equipment and
therefore to suspension of washing to take readings.
Mechanical ventilation should, whenever possible, be
continued during washing and to provide a free flow of air from
one end of the tank to the other.

The ability to mechanically ventilate concurrent with tank
washing is recommended but, where mechanical ventilation is
not possible, the monitoring of the tank atmosphere should be
more frequent as the likelihood of rapid gas build-up is
increased.

The tank atmosphere should be maintained at a level not
exceeding 35% LFL. Should the gas level reach 35% LFL at
any measured location within a tank, tank washing operations
in that individual tank MUST immediately cease.
Washing may be resumed when continued ventilation has
reduced and is able to maintain the gas concentration at 10%
or less of the LFL.

If the tank has a venting system that is common to other tanks,
the tank must be isolated to prevent ingress of gas from other
tanks.

© ICS/OCIMF/IAPH 2006 191