Page 88 - International safety guide for oil tankers and terminals

INTERNATIONAL SAFETY GUIDE FOR OIL TANKERS AND TERMINALS

3.1.4.1 Types of Discharge
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Electrostatic discharge can take the form of a ‘corona’,a ‘brush
discharge’,a ‘spark’ or a ‘propagating brush discharge’, as described
below:
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Corona is a diffuse discharge from a single sharp conductor that slowly
releases some of the available energy. Generally, corona on its own is
incapable of igniting a gas, such as propane, or a vapour such as
gasoline.

Brush Discharge is a diffuse discharge from a highly charged
non-conductive object to a single blunt conductor that is more rapid than
corona and releases more energy. It is possible for a brush discharge to
ignite gases and vapours. Examples of a brush discharge are:

Between a conductive sampling apparatus lowered into a tank and the
surface of a charged petroleum liquid.

Between a conductive protrusion (e.g. fixed tank washing machine) or
structural member and a charged petroleum liquid being loaded at a
high rate.

Spark is an almost instantaneous discharge between two conductors
where almost all of the energy in the electrostatic field is converted into
heat that is available to ignite a flammable atmosphere. Examples of
charged liquid and the adjacent tank structure. ึ
sparks are:
Between an unearthed conductive object floating on the surface of a

Between unearthed conductive equipment suspended in a tank and
the adjacent tank structure.

Between conductive tools or materials left behind after maintenance
when insulated by a rag or piece of lagging.

Sparks can be incendive if various requirements are met. These include:
A discharge gap short enough to allow the discharge to take place
with the voltage difference present, but not so short that any resulting
flame is quenched.

Sufficient electrical energy to supply the minimum amount of energy to
initiate combustion.

Propagating Brush Discharge is a rapid, high energy discharge from a
sheet of material of high resistivity and high dielectric strength with the
two surfaces highly charged but of opposite polarity. The discharge is
initiated by an electrical connection (short circuit) between the two
surfaces. The bipolar sheet can be in ‘free space’ or, as is more normal,
have one surface in intimate contact with a conducting material (normally
earthed).

The short circuit can be achieved:
By piercing the surface (mechanically or by an electrical
break-through).
By approaching both surfaces simultaneously with two electrodes
electrically connected.
When one of the surfaces is earthed, by touching the other surface
with an earthed conductor.

54 © ICS/OCIMF/IAPH 2006