Page 206 - International safety guide for oil tankers and terminals

INTERNATIONAL SAFETY GUIDE FOR OIL TANKERS AND TERMINALS

Operators should be aware that the maximum velocity might not
occur at the minimum diameter of the pipeline when the pipeline
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feeds multiple branch lines. Such configurations would be where a
pipeline feeds multiple loading arms or hoses or, on a ship, where
a main cargo line feeds multiple drop lines or tank inlets. For
งานห้องสมุด ศูนย์ฝกพาณิชย์นาวี
example, where a 150 mm diameter pipeline feeds three 100 mm
branch lines, the highest velocity will be in the 150 mm pipeline,
not in the branch lines.

Figure 11.2 also shows that the smallest diameter section of
piping in the system is the cargo hose, which has a diameter of
250 mm. If a loading velocity of 7 metres/second is acceptable to
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the ship and shore, a maximum loading rate of 1,281 m /hour
should be requested.

11.1.7.9 Antistatic Additives
If the oil contains an effective antistatic additive, it is no longer a
static accumulator. Although in theory this means that the
precautions applicable to a static accumulator can be relaxed, it is
still advisable to adhere to them in practice. The effectiveness of
antistatic additives is dependent upon the length of time since the
additive was introduced to the product, satisfactory product
mixing, other contamination and the ambient temperature. It can
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never be certain that the product’s conductivity is above 50 pS/m,
unless it is continuously measured. (See Section 12.5.2.2.)

11.1.7.10 Loading of Different Grades of Product into
Unclean Tanks (Switch Loading)
Switch loading is the practice of loading a low volatility liquid into
a tank that previously contained a high volatility liquid. The
residues of the volatile liquid can produce a flammable
atmosphere even when the atmosphere produced by the low
volatility liquid alone is non-flammable.

In this circumstance, it is important to reduce charge generation
by avoiding splash loading and other charge generating
mechanisms such as filters in the pipeline. The flow rate should
be restricted as per Sections 11.1.7.3 and 11.1.7.8 during the
initial and bulk loading periods respectively.

Product specification and quality requirements normally mean that
switch loading does not arise on tankers handling finished
products. This situation however may be encountered when
handling cargo slops or off-grade product for which no tank
preparation may be required as the grades can be mixed without
a risk of product contamination. In this situation, the precautions
outlined for switch loading described above should be
implemented.

11.1.8 Loading Very High Vapour Pressure Cargoes

Cargoes with high vapour pressure (see Section 2.5.6.2) introduce
problems of cargo loss due to excessive vapour release and can also
cause discharge difficulties due to gassing-up of cargo pumps. Special
precautions may therefore be necessary. These include:

Permitting only closed loading methods (see Section 11.1.6.6).

172 © ICS/OCIMF/IAPH 2006