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Talking gas #4: Gas ignition and evaporation properties

Flash point

Flammable liquids generally have a low flash. This is the temperature on which slightly is born in a ratio sufficient to form an explosive mixture with air. Liquids with flash points smaller than normal ambient temperatures can automatically evaporate enough volume to form an explosive mixture; therefore leaks of flammable liquids are also potentially as dangerous as a flammable gas leak.

Some fuels such as Diesel or jet fuel have a relatively high flash point (> 30oC) and therefore vapor accumulation can only be detected when ambient temperatures exceed this level.

Ignition temperature

When an explosive mixture of gas or vapor and air has been born, it can catch fire, either due to a sufficient spark of energy or simply at a sufficiently high temperature. The lowest temperature that can cause this mixture to burn or explode is called the ignition temperature (sometimes called automatic ignition temperature).

Molecular mass

In fact, the molecular mass of a compound is the total atomic mass of types when given in the molecular formula. (In fact the terms, molecular mass, molecules, mass formulas and weight formulas are used in place of each other).

Knowing the molecular mass of a substance that allows to judge whether or not the gas or vapor will accumulate at a high or low level when released (i.e. whether it is lighter or heavier than air), and also allows conversion from volume concentration (mg/m3) volume concentration (ppm)

Example A: To calculate the molecular mass of the carbon monoxide (CO) compound.

  • Carbon monoxide is consisting of a carbon atom (C) and an oxygen atom (O).Its formula is CO.
  • Atomic mass of carbon = 12.01 (taken from HTTH table)
  • Oxygen atomic mass = 16.00 (taken from HTTH table)
  • Molecular mass of carbon monoxide = oxygen atomic mass + carbon atom mass
  • So Molecular Mass = 12.01 + 16.00 = 28.01

Example B: To calculate the molecular mass of carbon dioxide, CO2 compounds

  • Carbon dioxide is composed of one carbon atom (C) and two oxygen atoms (O).
  • Atomic mass of carbon = 12.01
  • Oxygen atomic mass = 16.00
  • Molecular mass of carbon dioxide = carbon atomic mass + 2 x oxygen atom mass
  • So molecular mass = 12.01 + (2 x 16.00) = 12.01 + 32.00 = 44.01

Vapor weight and pressure

  • VAPOR RATIO – or relative proportion is a measure of the proportion of a gas or vapor to air. It is calculated by dividing the molecular mass of that gas by air (28.80). Gases or vapors with a lower density of air will tend to soar from the exit point and therefore can be easily dispersed (or may be trapped in an area of higher altitude). Gases or vapors with a ratio of slightly greater than one will be heavier than air and tend to sink to lower levels. Such heavy gas can remain trapped for a long time in ducs, test pits, etc., ready to explode as soon as a source fires. It should be noted that the properties of dispersing gases and vapors are also affected by ambient temperature, storage temperature, ambient pressure, storage pressure, ventilation or air ducs etc.
  • VAPOR PRESSURE – In assessing the fire risk for a particular substance, it is useful to know its vapor pressure. Any material that is liquid or solid at air temperature will have a stage of evaporation, and this percentage of vapor in the surrounding air will depend on the temperature. When the air temperature increases its density can carry slightly increased vapors, and when the vapor pressure of a substance reaches atmospheric pressure, it is at the time of its boiling. Vapor pressure is usually expressive in millimeters of mercury (mmHg), atmosphere (atm), or kilopascals (kPa). Normal atmospheric pressure at sea level is 760mmHG, 1 atm or 101.325kPa.

The maximum concentration of a substance in the air at a certain temperature can be calculated from the evaporation pressure at that temperature. This means that we can assess whether a physical substance give birth to flammable concentrations at that temperature. For most of the substances involved MSDS will announce its vapor pressure (usually determined at a temperature of 25oC).

Knowing the vapor pressure of a substance allows us to calculate whether the vapor can be sufficiently generated to create an explosive risk in the environment in which the substances are used.

To calculate the concentration of vapors in the air, divide the vapor pressure by the ambient pressure (usually 760mmHg) and cause it by 100 to get a volume in % (first ensuring the pressure is used in the same unit of calculation).


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