Flammable liquids generally have a low flash point. This is the temperature above which vapour is given off at sufficient rate to form an explosive mixture with air. Liquids with flash points below normal ambient temperatures automatically release vapour in sufficient volume to provide an explosive mixture; thus leakage of such liquids is potentially as dangerous as a flammable gas leak.
Some fuels such as Diesel or Jet Fuels have relatively high flash points (>30oC) and therefore vapour accumulations can only be detected when ambient temperatures exceed this level.
When an explosive mixture of gas or vapour and air has developed, it can be ignited either by a spark of sufficient energy or simply by being at a sufficiently high temperature. The lowest temperature which will cause a mixture to burn or explode is called the ignition temperature (sometimes called auto-ignition temperature).
In practice, the molecular weight of a compound is the sum of the atomic weights of the species as given in the molecular formula. (In practice the terms, molecular mass, molecular weight, formula mass and formula weight are used interchangeably by chemists).
Knowing the molecular weight of a substance enables judgement to be made as to whether a gas or vapour will accumulate at high or low level upon release (ie whether it’s lighter or heavier than air), and also enables conversion from the mass concentration (mg/m3) to volumetric measures (ppm)
A: To calculate the Molecular Weight of the compound carbon monoxide (CO).
Carbon monoxide is composed of one atom of carbon (C) and one atom of oxygen (O). Its formula is CO.
Atomic weight of carbon = 12.01 (taken from the Periodic Table)
Atomic weight of oxygen = 16.00 (taken from the Periodic Table)
Molecular Weight for carbon monoxide = atomic weight carbon + atomic weight oxygen
Therefore Molecular Weight = 12.01 + 16.00 = 28.01
B: To calculate the Molecular Weight of the compound carbon dioxide, CO2
Carbon dioxide is composed of one atom of carbon (C) and
two atoms of oxygen (O).
Atomic weight of carbon = 12.01
Atomic weight of oxygen = 16.00
Molecular Weight for carbon dioxide = atomic weight carbon +
2 x atomic weight oxygen
Therefore Molecular Weight = 12.01 + (2 x 16.00) = 12.01 + 32.00 = 44.01
Density and Vapour Pressure
VAPOUR DENSITY or Relative Density is a measure of the density of a gas or vapour relative to air. It is calculated by dividing the Molecular Weight of the gas by that of air (28.80). Gases or vapours with a vapour density of less than one are lighter than air and they tend to rise from the point of escape and may therefore be readily dispersed (or they may be trapped at a higher level). Gases or vapours with a vapour density of greater than one are heavier than air and tend to sink to lower levels. Such heavy gases can remain trapped for a long time in ducts, inspection pits, etc, ready to explode as soon as a source of ignition is introduced. It should be noted that the dispersion behaviour of gases and vapours is also influenced by ambient temperature, storage temperature, ambient pressure, storage pressure, ventilation or wind patterns etc.
VAPOUR PRESSURE – In assessing the flammable hazard for a specific substance, it is extremely useful to know its vapour pressure. Any material which is liquid or solid at atmospheric temperature will have a vapour phase, and the proportion of vapour in the surrounding air will depend on the temperature. As the atmospheric temperature increases the proportion of vapour it can hold increases, and when the vapour pressure of a substance reaches atmospheric pressure, the substance is at its boiling point. Vapour pressure is usually expressed in millimetres of mercury (mmHG), atmospheres (atm), or kiloPascals (kPa). Normal atmospheric pressure at sea level is 760mmHG, 1 atm or 101.325kPa.
The maximum concentration of a substance in air at a given temperature can be calculated from its vapour pressure at that temperature. This means that we can assess whether a substance can give rise to a flammable concentration at that temperature. For most substances the relevant Material Safety Data Sheet will quote its vapour pressure (usually defined at 25oC).
Knowing the Vapour Pressure of a substance enables us to calculate whether sufficient vapour may be emitted to create an explosion hazard in the environment in which the substance is used.
To calculate the concentration of vapour in air, divide the substance vapour pressure by ambient pressure (normally 760mmHg) and multiply by 100 to obtain a % volume reading (ensure first that the pressures are expressed using the same units).
Vapour pressure at 25oC =
Therefore amount of vapour in air at 25oC =
0.0013% volume is equal to 13ppm. This concentration is far below the lower explosive level (LEL) of this substance.
This calculation shows that in normal ambient conditions a quantity of Caprolactone Monomer would emit levels of vapour insufficient to create a flammable risk.
A chart of vapour pressures for specific substances can be found in Section 3. Much of the information described in this section can be found for any substance on the Material Safety Data Sheet which can be obtained from the supplier of the substance.