Page 71 - International safety guide for oil tankers and terminals

HAZARDS OF PETROLEUM

An adjacent jetty which, although it is usually regarded as free of sources of
ignition, is a work area and thoroughfare.
การศึกษาเท่านั้น---
Adjacent vessels such as lightering ships, bunker and stores craft, pilot and
crew transfer boats.
opposed by a tendency to sink because its density is greater than that of์นาวี

2.5.4 Variables Affecting Dispersion

2.5.4.1 The Dispersion Process

A mixture of hydrocarbon gas and air (or inert gas), issuing vertically from
งานห้องสมุด ศูนย์ฝกพาณิชย
an outlet, rises under its own momentum as a plume above the outlet. If
there is no wind, the plume remains vertical, but otherwise it is bent over
in the downwind direction. The rise of the plume due to its momentum is

the surrounding air.

The flow velocity of the issuing gas is at its maximum as it passes
through the outlet, and decreases as air is drawn into the plume. This air
decreases the hydrocarbon gas concentration and hence the gas density
in the plume. The progressive decreases in velocity, hydrocarbon
concentration and density, together with the wind speed and other
meteorological factors, determine the final shape of the plume and hence
of the flammable zone.
ึ
The type of vent being used affects the dispersion of the gas plume.
During normal loading operations, the venting will be either via:
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A high velocity vent installed at a minimum height of 2 m above the
deck, which causes the vapour to be vented at a speed of
30 m/second irrespective of the loading rate of the cargo, or
A vent riser with a minimum height of 6 m above the deck.

These high velocity vents and risers may not be placed closer than 10 m
to any accommodation house vent, to ensure that cargo vapours will be
safely dispersed before they reach these locations.

2.5.4.2 Wind Speed
For many years, it has been recognised that the dispersion of
hydrocarbon gas/air mixtures is inhibited by low wind speeds. This
recognition is based upon experience on tankers and little experimental
work has been done to obtain quantitative information on the effect of
wind speed. Much depends upon the quantity of gas being vented and
how it is vented, but experience at terminals seems to suggest that, at
wind speeds above about 5 metres/sec (10 knots), dispersion is sufficient
to avoid any flammability risk.

2.5.4.3 Rate of Flow of Gas

As the rate of flow of a hydrocarbon gas/air mixture of fixed composition
is increased through a given opening, several effects come into play. In
the first place, the rate of emission of the hydrocarbon constituent
increases in proportion to the total gas flow rate and therefore the
distance the plume travels before it is diluted to the LFL should be
greater. On the other hand, the higher the velocity, the more efficient is
the mixing of the initially hydrocarbon-rich gas with the air and this tends
to counterbalance the first effect.

© ICS/OCIMF/IAPH 2006 37