Page 37 - International safety guide for oil tankers and terminals

Chapter 1
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BASIC PROPERTIES

งานห้องสมุด ศูนย์ฝกพาณิชย์นาวี
OF PETROLEUM

This Chapter describes the physical and chemical properties that have the greatest
bearing on the hazards arising from handling petroleum liquids. These properties
are vapour pressure, the flammability of the gases evolved from the liquids and the
density of these gases.

1.1 Vapour Pressure

1.1.1 True Vapour Pressure
All crude oils and the usual petroleum products are essentially mixtures of a
wide range of hydrocarbon compounds (i.e. chemical compounds of hydrogen
and carbon). The boiling points of these compounds range from -162ºC
ึ
(methane) to well in excess of +400ºC, and the volatility of any particular
mixture of compounds depends primarily on the quantities of the more volatile
constituents (i.e. those with a lower boiling point).

The volatility (i.e. the tendency of a crude oil or petroleum product to produce
gas) is characterised by the vapour pressure. When a petroleum mixture is
transferred to a gas free tank or container, it starts to vaporise, that is it
liberates gas into the space above it.
There is also a tendency for this gas to re-dissolve in the liquid, and
equilibrium is ultimately reached with a certain amount of gas evenly
distributed throughout the space. The pressure exerted by this gas is called the
equilibrium vapour pressure of the liquid, usually referred to simply as the
vapour pressure.

The vapour pressure of a pure compound depends only upon its temperature.
The vapour pressure of a mixture depends on its temperature, constituents
and the volume of the gas space in which vaporisation occurs; that is, it
depends upon the ratio of gas to liquid by volume.

The True Vapour Pressure (TVP), or bubble point vapour pressure, is the
pressure exerted by the gas produced from a mixture when the gas and liquid
are in equilibrium at the prevailing temperature. It is the highest vapour
pressure that is possible at any specified temperature.
As the temperature of a petroleum mixture increases, its TVP also increases. If
the TVP exceeds atmospheric pressure, the liquid starts to boil.

The TVP of a petroleum mixture provides a good indication of its ability to give
rise to gas. Unfortunately, this is a property that is extremely difficult to
measure, although it can be calculated from a detailed knowledge of the
composition of the liquid. For crude oils, it can also be estimated from the
stabilisation conditions, making allowance for any subsequent changes of
temperature or composition. In the case of products, reliable correlations exist
for deriving TVP from the more readily measured Reid Vapour Pressure and
temperature.

© ICS/OCIMF/IAPH 2006 3