Page 94 - International safety guide for oil tankers and terminals

INTERNATIONAL SAFETY GUIDE FOR OIL TANKERS AND TERMINALS

measures are not necessary if the ship is limited to crude or black oil service
or if the tanks are inerted.
---ใช้เพื่อการศึกษาเท่านั้น---

3.3.3 Free Fall in Tanks
งานห้องสมุด ศูนย์ฝกพาณิชย์นาวี
Loading or ballasting over the top (overall) delivers charged liquid to a tank in
such a manner that it can break up into small droplets and splash into the
tank. This may produce a charged mist as well as an increase in the petroleum
gas concentration in the tank. Restrictions upon loading or ballasting overall
are given in Section 11.1.12.

3.3.4 Water Mists

The spraying of water into tanks, for instance during water washing, gives rise
to electrostatically charged mist. This mist is uniformly spread throughout the
tank being washed.

The electrostatic levels vary widely from tank to tank, both in magnitude and in
sign.

When washing is started in a dirty tank, the charge in the mist is initially
negative, reaches a maximum negative value, then goes back through zero
and finally rises towards a positive equilibrium value. It has been found that,
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among the many variables affecting the level and polarity of charging, the
characteristics of the wash water and the degree of cleanliness of the tank
have the most significant influence. The electrostatic charging characteristics of
the water are altered by re-circulation or by the addition of tank cleaning
chemicals, either of which may cause very high electrostatic potentials in the
mist. Potentials are higher in large tanks than in small ones. The size and
number of washing machines in a tank affect the rate of change of charge, but
they have little effect on the final equilibrium value.

The charged mist droplets created in the tank during washing give rise to an
electrostatic field, which is characterised by a distribution of potential (voltage)
throughout the tank space. The bulkheads and structure are at earth (zero)
potential and the space potential increases with distance from these surfaces
and is highest at points furthest from them. The field strength, or voltage
gradient, in the space is greatest near the tank bulkheads and structure, more
especially where there are protrusions into the tank. If the field strength is high
enough, electric breakdown occurs into the space, giving rise to a corona.
Because protrusions cause concentrations of field strength, a corona occurs
preferentially from such points. A corona injects a charge of the opposite sign
into the mist and is believed to be one of the main processes limiting the
amount of charge in the mist to an equilibrium value. The corona discharges
produced during tank washing are not strong enough to ignite the hydrocarbon
gas/air mixtures that may be present.

Under certain circumstances, discharges with sufficient energy to ignite
hydrocarbon gas/air mixtures can occur from unearthed conducting objects
already within, or introduced into, a tank filled with charged mist. Examples of
such unearthed conductors are a metal sounding rod suspended on a rope or
a piece of metal falling through the tank space.

An unearthed conductor within a tank can acquire a high potential, primarily by
induction, when it comes near an earthed object or structure, particularly if the
latter is in the form of a protrusion. The unearthed conductor may then
discharge to earth giving rise to a spark capable of igniting a flammable
hydrocarbon gas/air mixture.

60 © ICS/OCIMF/IAPH 2006