Page 89 - International safety guide for oil tankers and terminals

STATIC ELECTRICITY

A propagating brush discharge can be highly energetic (1 joule or more)
and so will readily ignite a flammable mixture.
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Scientific studies have shown that epoxy coatings greater than 2 mm
thick on tanks, filling pipes and fittings may give rise to conditions
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whereby there is a possibility of a propagating brush discharge. In these
cases, there would be a need to seek expert advice on requirements to
explicitly earth the cargo. However, on most ships, the thickness of epoxy
coatings is not generally greater than 2 mm.

3.1.4.2 Conductivity

Materials and liquid products that are handled by tankers and terminals
are classified as being non-conductive, semi-conductive (in most
electrostatic standards the term ‘dissipative’ is now preferred to
‘semi-conductive’) or conductive.

Non-Conductive Materials (or Non-Conductors)
These materials have such low conductivities that once they have
received a charge they retain it for a very long period. Non-conductors
can prevent the loss of charge from conductors by acting as insulators.
Charged non-conductors are of concern because they can generate
incendive brush discharges to nearby earthed conductors and because
they can transfer a charge to, or induce a charge on, neighbouring
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insulated conductors that may then give rise to sparks.
Liquids are considered to be non-conductors when they have
conductivities less than 50 pS/m (pico Siemens/metre). Such liquids
are often referred to as static accumulators.

Petroleum products, such as clean oils (distillates), frequently fall into this
category with a conductivity typically below 10 pS/m. Chemical solvents
and highly refined fuels can have conductivities of less than 1 pS/m. The
solid non-conductors include plastics, such as polypropylene, PVC, nylon
and many types of rubber. They can become more conductive if their
surfaces are contaminated with dirt or moisture. (Precautions to be taken
when loading static accumulator oils are addressed in Section 11.1.7.)

Semi-Conductive Materials (or Dissipative Materials or
Intermediate Conductors)
The liquids in this intermediate category have conductivities exceeding
50 pS/m and, along with conductive liquids, are often known as static
non-accumulators. Examples of semi-conductive liquids are black oils
(containing residual materials) and crude oils, which typically have
conductivities in the range of 10,000-100,000 pS/m. The solids in this
intermediate category generally include such materials as wood, cork,
sisal and naturally occurring organic substances. They owe their
conductivity to their ready absorption of water and they become more
conductive as their surfaces are contaminated by moisture and dirt.
However, when new or thoroughly cleaned and dried, their conductivities
can be sufficiently low to bring them into the non-conductive range.

If materials in the intermediate conductivity group are not insulated from
earth, their conductivities are high enough to prevent accumulation of an
electrostatic charge. However, their conductivities are normally low
enough to inhibit production of energetic sparks.

For materials with intermediate conductivities, the risk of electrostatic
discharge is small, particularly if practices in this Guide are adhered to,

© ICS/OCIMF/IAPH 2006 55