Page 69 - International safety guide for oil tankers and terminals

HAZARDS OF PETROLEUM

very high concentration immediately above the liquid surface remains in
the final ullage space on completion of loading.
as fresh cargo enters the tank.เท่านั้น---
Subsequently, evaporation continues until an equilibrium hydrocarbon gas
concentration is established throughout the ullage space. This may be
งานห้องสมุด ศูนย์ฝกพาณิชย์นาวี
very high indeed, depending upon the cargo composition and
temperature, and values as high as 90%-95% by volume have been
observed with crude oils. However, this gas is only vented by breathing of
the tank, and thus only intermittently. When the oil is discharged, this very
dense gas mixture travels to the bottom of the tank with the descending
liquid surface and may contribute to the gas vented during the next
operation in the tank. If the tank is not initially gas free, the hydrocarbon
gas concentration in the vented gas during loading depends upon the
previous history of the tank. For example:

In an unwashed crude oil tank that is to be loaded soon after
discharge of a previous cargo, there is a layer of highly concentrated
---ใช้เพื่อการศึกษา
gas at the bottom of the tank, with hardly any hydrocarbon gas above
it. This gas is expelled immediately ahead of the layer that is formed

In an unwashed crude oil tank after a long ballast voyage, there is a
homogeneous hydrocarbon gas concentration of up to 10% by volume
throughout the tank. When the tank is next loaded, this is the gas that
is expelled until the concentrated gas layer immediately above the
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liquid surface begins to exert its influence. Thereafter, this
concentrated layer dominates the composition of the vented gas.
In a crude oil tank that has been crude oil washed but not
subsequently purged with inert gas or gas freed, a uniform gas
concentration exists throughout the tank. Depending on the crude oil
used and its temperature, this concentration is usually well above the
flammable range and may be as high as 40% by volume. This mixture
is displaced from the tank throughout the subsequent loading until the
possibly even richer gas adjacent to the liquid surface approaches the
top of the tank.

Shortly after the discharge of a motor or aviation gasoline cargo, there
is a layer at the bottom of the tank where concentrations of 30%-40%
by volume of hydrocarbons have been measured. If loaded at this
stage, the gas enters the venting system immediately ahead of the
concentrated layer formed by the next cargo.

In motor or aviation gasoline tanks that have been battened down after
discharge and not gas freed, uniform hydrocarbon gas concentrations
as high as 40% by volume have been measured throughout the tanks.
This concentration is expelled to the vent system throughout the next
loading until the concentrated layer above the liquid surface
approaches the top of the tank.

Note that in all loading operations, whether the tank is initially gas free or
not, very high gas concentrations enter the venting system towards
completion of loading.

2.5.2.3 Ballasting into a Cargo Tank
The atmosphere in cargo tanks before ballasting will be similar to that
before the loading of oil cargo, given a similar tank history. The gas
concentration expected to enter the venting system during ballasting will
therefore be comparable to that in the examples given above. If it is
necessary for ships using crude oil washing to load ballast into cargo
tanks before departure, some ports require controls on vapour emissions

© ICS/OCIMF/IAPH 2006 35