Page 70 - International safety guide for oil tankers and terminals

INTERNATIONAL SAFETY GUIDE FOR OIL TANKERS AND TERMINALS

to the atmosphere. This is achieved by containing the vapour in empty
cargo tanks, by simultaneous ballasting and cargo discharge, or by other
---ใช้เพื่อการศึกษาเท่านั้น---
approved means.

2.5.2.4 Inert Gas Purging
งานห้องสมุด ศูนย์ฝกพาณิชย์นาวี
If inert gas purging is being carried out by the displacement method (see
Section 7.1.4) any dense concentrated hydrocarbon layer at the bottom of
the tank is expelled in the early stages, followed by the remainder of the
tank atmosphere as it is pressed downwards by the inert gas. If there is a
uniformly high concentration throughout the tank, for example after crude
oil washing, the hydrocarbon concentration of the vented gas remains
high throughout the purging process until the inert gas reaches the
bottom of the tank.

If inert gas purging is being carried out by the dilution method (see
Section 7.1.4), the gas concentration at the outlet is highest at the
beginning of the operation and falls continuously as it proceeds.

2.5.2.5 Gas Freeing
In a gas freeing operation, air is delivered into the tank where it mixes
with the existing tank atmosphere and where it also tends to mix together
any layers that may be present. The resultant mixture is expelled to the
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outside atmosphere. Because the process is one of continuous dilution
with the air, the highest hydrocarbon concentration is vented at the
beginning of gas freeing and decreases thereafter. For example, on a
non-inerted ship, gas freeing of a motor gasoline tank that has been
battened down can give initial concentrations as high as 40% by volume,
but in most circumstances the concentration in the vented gas is much
lower, even at the start of the operations.

On inerted ships, after purging to remove hydrocarbon vapour before gas
freeing, the initial concentration will be low, 2% by volume or less.

2.5.3 Gas Dispersion

Whether the hydrocarbon gas at the outlet is mixed with air or with inert gas
will have no bearing on the dispersion of the gas after it has left the outlet.

As the hydrocarbon gas displaced during loading, ballasting, gas freeing or
purging issues from the vent or vents on the tanker, it immediately starts to mix
with the atmosphere.

The hydrocarbon concentration is progressively reduced until, at some
distance from the vent, it passes below the LFL. At any point below the LFL, it
ceases to be of concern as a flammability hazard because it cannot be ignited.
However, there exists in the vicinity of any vent a flammable zone within which
the gas concentration is above the LFL.

There is a potential danger of fire and explosion if this flammable zone
reaches any location where there may be sources of ignition, such as:

Superstructures and accommodation blocks into which the gas can enter
through doors, ports or ventilation intakes.

The cargo deck which, although it is usually regarded as free of sources of
ignition, is a work area and thoroughfare.

36 © ICS/OCIMF/IAPH 2006