Page 201 - International safety guide for oil tankers and terminals

SHIPBOARD OPERATIONS
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11.1.7.4
Table 11.1 shows approximate volumetric flow rates that
correspond to a linear velocity of 1 metre/second in piping of
various diameters.
The reasons for such a low linear velocity as 1 metre/second are
threefold:
1. At the beginning of filling a tank, there is the greatest
likelihood of water being mixed with the oil entering the tank.
Mixtures of oil and water constitute a most potent source of
static electricity.
2. A low product velocity at the tank inlet minimises turbulence
and splashing as oil enters the tank. This helps reduce the
generation of static electricity and also reduces the dispersal
of any water present, so that it quickly settles out to the
bottom of the tank where it can lie relatively undisturbed when
the loading rate is subsequently increased.
3. A low product velocity at the tank inlet minimises the formation
of mists that may accumulate a charge, even if the oil is not
considered to be a static accumulator. This is because the
mist droplets are separated by air, which is an insulator. A mist
can result in a flammable atmosphere even if the liquid has a
high flashpoint and is not normally capable of producing a
flammable atmosphere.
ึ
Figure 11.1 provides a flow chart to assist in deciding the
precautions that need to be taken when loading static accumulator
cargoes.
Minimising Hazards From Water
Because mixtures of oil and water constitute a potent source of
static electricity, care should be taken to prevent excess water
from operations such as water washing, ballasting or line flushing
entering a tank that contains or will contain a static accumulator
oil. For example, cargo tanks and lines that have been flushed
with water should be drained before loading and water should not
be permitted to accumulate in tanks. Lines should not be
displaced with water back into a tank containing a static
accumulator cargo. (For an explanation of line displacement, see
Section 11.1.15.2.)
Any water remaining within the shore or ship pipeline system after
the initial filling period might be flushed into the cargo tank when
loading at the maximum rate. (The minimum product velocity for
flushing water out of pipelines effectively is 1 metre per second.)
The resulting mixing and agitating of the oil and water in the tank
will increase the generation of static charge to a level that is
unsafe in a flammable atmosphere. Before increasing to the bulk
loading rate, it is therefore necessary to ensure that, so far as
practicable, all excess water that may have been lying in low spots
in the pipelines has been flushed out of the system either before
loading commenced or during the initial filling of the tank (see
Section 11.1.7.3 for advice on the process).
Under normal circumstances, and provided that the
aforementioned precautions to prevent excess water have been
taken, the amount of water still present in the system after the
initial filling period will be insufficient to increase static separation
© ICS/OCIMF/IAPH 2006 167