Page 91 - International safety guide for oil tankers and terminals

STATIC ELECTRICITY

3.1.5 Electrostatic Properties of Gases and Mists
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Under normal conditions, gases are highly insulating and this has important
implications with respect to mists and particulate suspensions in air and other
gases. Charged mists are formed during the ejection of liquid from a nozzle,
for example:
งานห้องสมุด ศูนย์ฝกพาณิชย์นาวี
Petroleum products entering an empty tank at high velocity.
Wet steam condensing.

Water from tank washing machines.
Crude oil during crude oil washing.

Although the liquid, for example water, may have a very high conductivity, the
relaxation of the charge on the droplets is hindered by the insulating properties
of the surrounding gas. Fine particles present in inert flue gas, or created
during discharge of pressurised liquid carbon dioxide, are frequently charged.
The gradual charge relaxation, which does occur, is the result of the settling of
the particles or droplets and, if the field strength is high, of corona discharge
at sharp protrusions. Under certain circumstances, discharges with sufficient
energy to ignite hydrocarbon gas/air mixtures can occur. See also Section
3.3.4.

ึ
3.2 General Precautions Against Electrostatic Hazards

3.2.1 Overview

Whenever a flammable atmosphere could potentially be present, the following
measures must be taken to prevent electrostatic hazards:
The bonding of metal objects to the metal structure of the ship to eliminate
the risk of spark discharges between metal objects that might be electrically
insulated. This includes metallic components of any equipment used for
dipping, ullaging and sampling.
The removal from tanks or other hazardous areas of any loose conductive
objects that cannot be bonded.
Restricting the linear velocity of the cargo to a maximum of 1 metre per
second at the individual tank inlets during the initial stages of loading,
i.e. until:
a) the filling pipe and any other structure on the base of the tank has been
submerged to twice the filling pipe diameter in order that all splashing
and surface turbulence has ceased and
b) any water collected in the pipeline has been cleared. It is necessary to
load at this restricted rate for a period of 30 minutes or until two pipeline
volumes (i.e. from shore tank to ship’s tank) have been loaded into the
tank, whichever is the lesser.

Continuing to restrict the product flow to a maximum of 1 m/s at the tank
inlet for the whole operation unless the product is ‘clean’.A ‘clean’ product,
within this context, is defined as one which contains less than 0.5% by
volume of free water or other immiscible liquid and less than 10 mg/l of
1
suspended solids .
Avoiding splash filling by employing bottom entry using a fill pipe
terminating close to the bottom of the tank.

1
CENELEC Technical Report CLC/TR 50404, “Electrostatics – Code of Practice for the Avoidance of Hazards Due to
Static Electricity”, June 2003.

© ICS/OCIMF/IAPH 2006 57