Page 38 - International safety guide for oil tankers and terminals

INTERNATIONAL SAFETY GUIDE FOR OIL TANKERS AND TERMINALS

1.1.2 Reid Vapour Pressure
---ใช้เพื่อการศึกษาเท่านั้น---
The Reid Vapour Pressure (RVP) test is a simple and generally used method
for measuring the volatility of petroleum liquids. It is conducted in a standard
apparatus and in a closely defined way. A sample of the liquid is introduced
into the test container at atmospheric pressure, so that the volume of the liquid
งานห้องสมุด ศูนย์ฝกพาณิชย์นาวี
is one fifth of the total internal volume of the container. The container is sealed
and immersed in a water bath where it is heated to 37.8ºC. After the container
has been shaken to bring about equilibrium conditions rapidly, the rise in
pressure due to vaporisation is read on an attached pressure gauge. This
pressure gauge reading gives a close approximation, in bars, to the vapour
pressure of the liquid at 37.8ºC.

RVP is useful for comparing the volatilities of a wide range of petroleum liquids
in a general way. It is, however, of little value in itself as a means of estimating
the likely gas evolution in specific situations, mainly because the measurement
is made at the standard temperature of 37.8ºC and at a fixed gas/liquid ratio.
For this purpose, TVP is much more useful and, as already mentioned, in
some cases correlations exist between TVP, RVP and temperature.

1.2 Flammability

1.2.1 General
ึ
In the process of burning, hydrocarbon gases react with the oxygen in the air
to produce carbon dioxide and water. The reaction gives sufficient heat to form
a flame, which travels through the mixture of hydrocarbon gas and air. When
the gas above a liquid hydrocarbon is ignited, the heat produced is usually
enough to evaporate sufficient fresh gas to maintain the flame, and the liquid is
said to burn. In fact, it is the gas that is burning and is being continuously
replenished from the liquid.

1.2.2 Flammable Limits
A mixture of hydrocarbon gas and air cannot be ignited and burned unless its
composition lies within a range of gas in air concentrations known as the
flammable range. The lower limit of this range, known as the Lower Flammable
Limit (LFL), is that hydrocarbon concentration below which there is insufficient
hydrocarbon gas to support and propagate combustion. The upper limit of the
range, known as the Upper Flammable Limit (UFL), is that hydrocarbon
concentration above which there is insufficient air to support and propagate
combustion.

The flammable limits vary somewhat for different pure hydrocarbon gases and
for the gas mixtures derived from different petroleum liquids. Very roughly, the
gas mixtures from crude oils, motor or aviation gasolines and natural gasoline
type products can be represented respectively by the pure hydrocarbon gases
propane, butane and pentane. Table 1.1 gives the flammable limits for these
three gases. It also shows the amount of dilution with air needed to bring a
mixture of 50% by volume of each of these gases in air down to its LFL. This
type of information is very relevant to the ease with which vapours disperse to
a non-flammable concentration in the atmosphere.

In practice, the Lower and Upper Flammable Limits of oil cargoes carried in
tankers can, for general purposes, be taken as 1% and 10% by volume
respectively.

4 © ICS/OCIMF/IAPH 2006