Page 85 - International safety guide for oil tankers and terminals

Chapter 3
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STATIC

งานห้องสมุด ศูนย์ฝกพาณิชย์นาวี
ELECTRICITY

This Chapter describes hazards associated with the generation of static electricity
during the loading and discharging of cargo and during tank cleaning, dipping, ullaging
and sampling. Section 3.1 introduces some basic principles of electrostatics in
order to explain how objects become charged and to describe the effect of those
charges on other objects in close surroundings.

The risks presented by static electricity discharges occur where a flammable
atmosphere is likely to be present. The main precaution for tankers against
electrostatic risks is to conduct operations with the cargo tanks protected by inert
gas. Section 3.2 describes, in general terms, precautions against electrostatic
hazards in tanks that are not protected by inert gas; these are discussed in more
detail in Chapter 11 (Shipboard Operations). Section 3.3 considers other likely
sources of electrostatic hazards in tanker and terminal operations.
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3.1 Principles of Electrostatics

3.1.1 Summary
Static electricity presents fire and explosion hazards during the handling of
petroleum and during other tanker operations such as tank cleaning, dipping,
ullaging and sampling. Certain operations can give rise to accumulations of
electric charge that may be released suddenly in electrostatic discharges with
sufficient energy to ignite flammable hydrocarbon gas/air mixtures. There is, of
course, no risk of ignition unless a flammable mixture is present. There are
three basic stages leading up to a potential electrostatic hazard:
Charge separation.
Charge accumulation.
Electrostatic discharge.

All three of these stages are necessary for an electrostatic ignition of a
flammable atmosphere.

Electrostatic discharges can occur as a result of accumulations of charge on:
Liquid or solid non-conductors, for example a static accumulator oil (such
as kerosene) pumped into a tank, or a polypropylene rope.
Electrically insulated liquid or solid conductors, for example mists, sprays or
particulate suspensions in air, or an unbonded metal rod hanging on the
end of a rope.

The principles of electrostatic hazards and the precautions to be taken to
manage the risks are described fully below.

© ICS/OCIMF/IAPH 2006 51