Page 86 - International safety guide for oil tankers and terminals

INTERNATIONAL SAFETY GUIDE FOR OIL TANKERS AND TERMINALS

3.1.2 Charge Separation
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Whenever two dissimilar materials come into contact, charge separation occurs
at the interface.

The interface may be between two solids, between a solid and a liquid or
Flow through fine filters (less than 150 microns) that have the ability toิชย์นาวี
between two immiscible liquids. At the interface, a charge of one sign (say
positive) moves from material A to material B so that materials A and B
become respectively negatively and positively charged.

While the materials stay in contact and immobile relative to one another, the
charges are extremely close together. The voltage difference between the
charges of opposite sign is then very small, and no hazard exists. However,
when the materials move relative to one another, the charges can be separated
and the voltage difference increased.
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The charges can be separated by many processes. For example:

The flow of liquid petroleum through pipes.

charge fuels to a very high level, as a result of all the fuel being brought
into intimate contact with the filter surface where charge separation occurs.
Contaminants, such as water droplets, rust or other particles, moving
relative to oil as a result of turbulence in the oil as it flows through pipes.
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The settling of a solid or an immiscible liquid through a liquid (e.g. water,
rust or other particles through petroleum). This process may continue for up
to 30 minutes after completion of loading into a tank.
Gas bubbles rising up through a liquid (e.g. air, inert gas introduced into a
tank by the blowing of cargo lines or vapour from the liquid itself, released
when pressure is dropped). This process may also continue for up to
30 minutes after completion of loading.
Turbulence and splashing in the early stages of loading oil into an empty
tank. This is a problem in the liquid and in the mist that can form above the
liquid.
The ejection of particles or droplets from a nozzle (e.g. during steaming
operations or injection of inert gas).
The splashing or agitation of a liquid against a solid surface (e.g. water
washing operations or the initial stages of filling a tank with oil).

The vigorous rubbing together and subsequent separation of certain
synthetic polymers (e.g. the sliding of a polypropylene rope through gloved
hands).

When the charges are separated, a large voltage difference can develop
between them. A voltage distribution is also set up throughout the neighbouring
space and this is known as an electrostatic field. Examples of this are:
The charge on a charged petroleum liquid in a tank produces an
electrostatic field throughout the tank, both in the liquid and in the ullage
space.
The charge on a water mist formed by tank washing produces an
electrostatic field throughout the tank.

If an uncharged conductor is present in an electrostatic field, it has
approximately the same voltage as the region it occupies. Furthermore, the
field causes a movement of charge within the conductor; a charge of one sign
is attracted by the field to one end of the conductor and an equal charge of the

52 © ICS/OCIMF/IAPH 2006