Page 95 - International safety guide for oil tankers and terminals

STATIC ELECTRICITY

The processes by which unearthed conductors give rise to ignitions in a mist
are fairly complex, and a number of conditions must be satisfied
---ใช้เพื่อการศึกษาเท่านั้น---
simultaneously before an ignition can occur.

These conditions include the size of the object, its trajectory, the electrostatic
capable of producing incendive discharges being produced by portable types์นาวี
level in the tank and the geometrical configuration where the discharge takes
place.

As well as solid unearthed conducting objects, an isolated slug of water
produced by the washing process may similarly act as a spark promoter and
cause an ignition. Experiments have shown that high capacity, single nozzle,
fixed washing machines can produce water slugs which, owing to their size,
งานห้องสมุด ศูนย์ฝกพาณิชย
trajectory and duration before breaking up, may satisfy the criteria for
producing incendive discharges. However, there is no evidence of water slugs

of washing machine. This can be explained by the fact that, if the jet is initially
fine, the length of slugs that are produced are relatively small so that they have
a small capacitance and do not readily produce incendive discharges.

Following extensive experimental investigations and using the results of
long-term experience, the tanker industry has drawn up the tank washing
guidelines set out in Section 11.3. These guidelines are aimed at preventing
excessive charge generation in mists and at controlling the introduction of
unearthed conducting objects when there is charged mist in the tank.
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Charged mists, very similar to those produced during tank washing, occur from
time to time in partly ballasted holds of OBOs. Due to the design of these
ships, there may be violent mist-generating impacts of the ballast against the
sides of the hold when the ship rolls in even a moderate sea. The impacts also
give rise to free flying slugs of water in the tank so that if the atmosphere of
the tank is flammable all the elements for an ignition are present. The most
effective countermeasure is to have tanks either empty or fully pressed up so
that violent wave motion in the tank cannot take place.

3.3.5 Inert Gas
Small particulate matter carried in inert gas can be electrostatically charged.
The charge separation originates in the combustion process and the charged
particles are capable of being carried through the scrubber, fan and
distribution pipes into the cargo tanks. The electrostatic charge carried by the
inert gas is usually small, but levels of charge have been observed well above
those encountered with the water mists formed during washing. Because the
tanks are normally in an inert condition, the possibility of an electrostatic
ignition has to be considered only if it is necessary to inert a tank which
already contains a flammable atmosphere or if a tank already inerted is likely
to become flammable because the oxygen content rises as a result of ingress
of air. Precautions are then required during dipping, ullaging and sampling.
(See Section 11.8.3.)

3.3.6 Discharge of Carbon Dioxide

During the discharge of pressurised liquid carbon dioxide, the rapid cooling
which takes place can result in the formation of particles of solid carbon
dioxide that become charged on impact and contact with the nozzle. The
charge can be significant with the potential for incendive sparks. Liquefied
carbon dioxide should not be used for inerting, or injected for any other reason
into cargo tanks or pump rooms that may contain flammable gas mixtures.

© ICS/OCIMF/IAPH 2006 61