2.1 Properties of flammable and explosive materials and the mechanisms by which they ignite FIRE

Question 1

Flammability category and criteria Highest

Answer 1

CHIP Extremely Flammable: Flash point < 0°C and initial boiling point ≤ 35°C CLP Category 1: Flash point < 23°C and initial boiling point ≤ 35°C

Question 2

Flammability category and criteria Medium

Answer 2

CHIP Highly Flammable: Flash point < 21°C and not classified as Extremely Flammable CLP Category 2: Flash point < 23°C and initial boiling point > 35°C

Question 3

Flammability category and criteria Lowest

Answer 3

CHIP Flammable: Flash point ≥ 21°C and ≤ 55°C CLP Flash point ≥ 23°C and ≤ 60°C

Question 4

Flash point (FP): is defined as:

Answer 4

… the lowest temperature, corrected to a pressure of 101.325 kPa (atmospheric pressure at sea level), at which a liquid evolves vapours, under the conditions defined in the test method, in such an amount that a flammable vapour/air mixture is produced in the test vessel.

Question 5

The key points to note about the flash point are: 6

Answer 5

 It relates to volatile liquids.  It determines the flammability hazard classification.  It is established through various approved test methods.  It is the lowest temperature (at standard pressure) at which a liquid gives off enough vapour to form an ignitable mixture with air.  The presence of an ignition source is necessary to cause a flash.  The vapour will only burn briefly. For continuous combustion to be sustained a higher temperature (the fire point) is required.

Question 6

Fire point:

Answer 6

The fire point is the lowest temperature at which a vapour above a liquid will continue to burn once ignited; the fire point temperature is higher than the flash point.

Question 7

Vapour density:

Answer 7

The density of a gas or vapour is the ratio of the vapour’s mass divided by its volume, expressed in mg/m3

Question 8

Relative density:

Answer 8

The ratio of the density of a substance to the density of a standard substance under specified conditions. For liquids the comparator is water at 4oC, and for gases and vapours the comparator is air. Relative density can therefore be considered to be the 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 (approximately 28.90). Note: Air is approximately 21% Oxygen, 78% Nitrogen, 1% other gases – the molecular weight of Oxygen (O2) is 32 and Nitrogen (N 2 ) is 28.

Question 9

Flammability limits:

Answer 9

Vapour-air mixtures will ignite and burn only over a well-specified range of compositions. The mixture will not burn when the composition is lower than the lower flammable limit (LFL); the mixture is too lean for combustion. The mixture is also not combustible when the composition is too rich, that is, when it is above the upper flammable limit (UFL). A mixture is flammable only when the composition is between the LFL and the UFL. Lower explosive limit (LEL) and upper explosive limit (UEL) are used interchangeably with LFL and UFL. For example, the lower and upper flammability limits of toluene are approximately 1.3% and 7% respectively.  If the concentration of toluene vapour in air is less than 1.3% then combustion will not occur as the mixture of flammable vapour in air is too lean.  if the concentration of toluene vapour in air is greater than about 7% then combustion again will not occur; the mixture is to too rich.

Question 10

Stoichiometry:

Answer 10

The branch of chemistry and chemical engineering that deals with the quantities of substances that enter into, and are produced by, chemical reactions. Stoichiometry provides the quantitative relationship between reactants and products in a chemical reaction.

Question 11

The combustion equation was explained earlier. The process of combustion can be broken down into the following stages of growth and decay:

Answer 11

(1) Induction (2) Ignition (3) Growth (4) Steady state (5) Decay