Page 87 - International safety guide for oil tankers and terminals

STATIC ELECTRICITY

opposite sign is left at the opposite end. Charges separated in this way are
known as ‘induced charges’ and, as long as they are kept separate by the
---ใช้เพื่อการศึกษาเท่านั้น---
presence of the field, they are capable of contributing to an electrostatic
discharge.

งานห้องสมุด ศูนย์ฝกพาณิชย์นาวี
3.1.3 Charge Accumulation
Charges that have been separated attempt to recombine and to neutralise
each other. This process is known as ‘charge relaxation’. If one or both of the
separated materials carrying charge is a very poor electrical conductor,
recombination is impeded and the material retains or accumulates the charge
upon it. The period of time for which the charge is retained is characterised by
the relaxation time of the material, which is related to its conductivity; the lower
the conductivity, the greater the relaxation time.
If a material has a comparatively high conductivity, the recombination of
charges is very rapid and can counteract the separation process, and
consequently little or no static electricity accumulates on the material. Such a
highly conductive material can only retain or accumulate charge if it is
insulated by means of a poor conductor, and the rate of loss of charge is then
dependent upon the relaxation time of this lesser conducting material.

The important factors governing relaxation are therefore the electrical
conductivities of the separated materials, of other conductors nearby, such as
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ship’s structure, and of any additional materials that may be interposed
between them after their separation.
Refined clean products tend to have very low conductivity, such that the
relaxation time is about half a minute. This is not to be confused with the
‘settling time’ referred to in Section 11.8.2.3.

3.1.4 Electrostatic Discharge

Electrostatic discharge occurs when the electrostatic field becomes too strong
and the electrical resistance of an insulating material suddenly breaks down.
When breakdown occurs, the gradual flow and charge recombination
associated with relaxation is replaced by sudden flow recombination that
generates intense local heating (e.g. a spark) that can be a source of ignition if
it occurs in a flammable atmosphere. Although all insulating media can be
affected by breakdowns and electrostatic discharges, the main concern for
tanker operations is the prevention of discharges in air or vapour, so as to
avoid sources of ignition.

Electrostatic fields in tanks or compartments are not uniform because of tank
shape and the presence of conductive internal protrusions, such as probes
and structure. The field strength is enhanced around these protrusions and,
consequently, that is where discharges generally occur. A discharge may occur
between a protrusion and an insulated conductor or solely between a
conductive protrusion and the space in its vicinity, without reaching another
object.

© ICS/OCIMF/IAPH 2006 53