Page 68 - International safety guide for oil tankers and terminals

INTERNATIONAL SAFETY GUIDE FOR OIL TANKERS AND TERMINALS

2.5.2 Gas Evolution and Venting
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2.5.2.1 Evolution During Loading

As a high vapour pressure petroleum cargo enters an empty gas free
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tank, there is a rapid evolution of gas. Because of its high density, the gas
forms a layer at the bottom of the tank that rises with the oil surface as
the tank is filled. Once it has been formed, the depth of the layer
increases only slowly over the period of time normally required to fill a
tank, although ultimately an equilibrium gas mixture is established
throughout the ullage space.

The amount and concentration of gas forming this layer at the beginning
of loading depend upon many factors, including:

True Vapour Pressure (TVP) of the cargo.
Amount of splashing as the oil enters the tank.
Time required to load the tank.
Occurrence of a partial vacuum in the loading line.

The hydrocarbon gas concentration in the layer varies with distance
above the liquid surface. Very close to the surface, it has a value close to
that corresponding to the TVP of the adjoining liquid. For example, if the
ึ
TVP is 0.75 bar, the hydrocarbon gas concentration just above the surface
is about 75% by volume. Well above the surface, the hydrocarbon gas
concentration is very small, assuming that the tank was originally gas
free. In order to consider further the influence of gas layer depth, it is
necessary to define this depth in some way.

When considering dispersion of gases outside cargo tanks, only high gas
concentrations in the vented gas are relevant. For this purpose therefore,
the gas layer depth will be taken as the distance from the liquid surface to
the level above it where the gas concentration is 50% by volume. It should
be remembered that hydrocarbon gas will be detectable at heights above
the liquid surface several times the layer depth defined in this way.

Most high vapour pressure cargoes give rise to a gas layer with a depth in
these terms of less than 1 metre. Its precise depth depends upon the
factors listed above and most of the advice with respect to vented gas
given in this Guide is intended for such cargoes. However, gas layers
greater than 1 metre in depth may be encountered if the cargo TVP is
great enough. Cargoes giving rise to these deeper gas layers may require
special precautions (see Sections 2.5.6.2 and 11.1.8).

2.5.2.2 Venting During the Loading of Cargo
Once the dense hydrocarbon gas layer has formed above the surface of
the liquid, its depth, as defined in Section 2.5.2.1, increases only very
slowly. As the liquid rises in the tank, the hydrocarbon gas layer rises with
it. Above this layer, the atmosphere originally present in the tank persists
almost unchanged and it is this gas that enters the venting system in the
early stages of loading. In an initially gas free tank, the gas vented at first
is therefore mainly air (or inert gas) with a hydrocarbon concentration
below the LFL. As loading proceeds, the hydrocarbon content of the
vented gas increases.

Concentrations in the range 30%-50% by volume of hydrocarbons are
quite usual in the vented gas towards the end of loading, although the

34 © ICS/OCIMF/IAPH 2006