It is important to remember that a hazardous situation can be created with chemicals but it does not necessarily follow that a hazardous event will occur. Most chemical reactions require an energy source to initiate the reaction or a particular concentration for the creation of a toxic event. This is why a knowledge/analysis of the chemistry is critical to any risk assessment of chemical hazards. SDS data is often insufficient! Also, the amount of a chemical is important because the risk is very dependant on this factor.
An understanding of fire and explosion requires you to be familiar with the concepts of energy, reaction kinetics, oxidation and reduction, and the states of matter (solids, liquids and gases). The ideas of vapour pressure and how temperature and pressure changes affect gases are also important.
Dust explosions can occur in “galleries” (e.g. coal mines) and in equipment and buildings. Probably the earliest examples recognised were in coalmines where an initial explosion of methane (fire-damp) created sufficient turbulence to form a cloud of coal dust. When this ignites (tiny particles of fuel in the presence of air), the shock waves create more dust, and ultimately a devastating explosion occurs.
The most severe “non-coal” dust explosions have occurred in grain elevators or in flourmills. Movement of grain in silos or on conveyer belts generates dust by attrition, and any spark can ignite the dust. Generally speaking, any combustible dust (e.g. flour, coal, sulfur, metal powder) can be ignited provided a stoichiometically equivalent mass of oxygen is present. The energy released by organic dust clouds is roughly the same as that of an organic vapour cloud. The more finely divided the dust is, the more rapidly it will be consumed.
No fire is spontaneous. It has to be started by something - a match, the sun’s rays concentrated by a piece of glass, lightning strike, heat, friction, electricity etc. A source of ignition is essential. Once the ignition (heat; activation energy) is provided, the fire (redox reaction) starts, generating its own heat to increase its rate until either all the fuel (reducing agent) or the available oxygen has been consumed. Note that heat is evolved in an exothermic reaction, and most reactions speed up by a factor of 2-3 times for every 10°C rise in temperature.
A concept used by the Fire Service in the theoretical treatment of fire is the “Fire Triangle”. In terms of the Fire Triangle all three, fuel, oxidiser and ignition are necessary. If any one side of the fire triangle is removed, fire will not be able to occur or continue burning.
To cause a fire, the molecules of a liquid must have reactivity with oxygen, and the liquid must have an appreciable vapour pressure at the temperature under study. That vapour pressure depends on (among other factors such as molecular size) the strengths of the intermolecular forces in the liquid. Water, which has extensive hydrogen bonding in the liquid, has a lower vapour pressure than for example ethanol which is less extensively hydrogen bonded.
|Temperature °C||Vapour pressure KPa||Vapour pressure KPa||Vapour pressure KPa|
The flash point is the lowest temperature at which a liquid has a sufficient vapour pressure to form an ignitable mixture with air near the surface of the liquid. Many common organic liquids have a flash point below room temperature e.g. acetone ( -18°C), diethyl ether ( -45°C). It is important to note that some flammable liquids will maintain their flammability even at concentrations as low as 10% by weight in water. Methanol and isopropanol have flash points below 38°C at concentrations as low as 30% by weight in water; acetonitrile/water mixtures of 15% to 30% acetonitrile are flammable.
The ignition temperature is the lowest temperature at which gaseous material will
spontaneously catch fire in air (with no external source of ignition other than the heat
necessary). In this instance merely warming to the ignition temperature serves as the
ignition source (activation energy provided by the warming).
A bleve (boiling liquid expanding vapour explosion) is one of the worst types of fire. It produces a fireball.
The fire triangle and fire rectangle concepts are not only useful for the understanding of fires, but they also point to ways in which fires can be prevented or extinguished.
In order to extinguish a fire:
• remove the fuel
• remove the oxidizer
• remove free radicals (with hydrocarbon fires)
• remove the source of ignition (or decrease the energy to the extent that sufficient
activation energy is no longer available).
Free radicals (hydrocarbon) can be removed using halon extinguishers. In the past
carbon tetrachloride was used in some fire extinguishers, but both it and its combustion products are toxic and harmful to the environment. Some fires are unable to be extinguished by water or carbon dioxide e.g. sodium fires. Such fires involve very strong reducing agents, capable of removing oxygen from the water or CO2.
An explosion is merely a reaction which has got out of control. Remember that reactions increase in rate by about two to three times per 10°C rise in temperature. An exothermic reaction gets hotter and hotter and faster and faster. This can happen very rapidly (virtually instantaneously) if the reagents are in close contact.
Like fires, explosives need a fuel, an oxidizer and an initiator. Usually, explosives have the oxygen already incorporated in the mixture or molecule. They don’t need to take it from the air. Both oxidizer and reducer are present in the formulation. Only a source of ignition is needed. Some other explosive materials have very weak bonds (unstable) and have free radicals on site. e.g. KClO3 (potassium chlorate), KClO4 (potassium perchlorate), H2O2 (peroxide).
As an explosive reaction rockets out of control, the gases formed in the reaction(s) heat up very rapidly and expand. The increase in volume and pressure is very rapid giving the blast effects. In a confined space this can be devastating and the container may shatter.
There two types of explosives:
Low explosives: need to be ignited; e.g, blasting powder
High explosives: shock sensitive; e.g, nitroglycerine, gelignite,
These limits define the range of concentrations in mixtures with air (or oxygen depending on definition) that will propagate a flame and cause an explosion. The lower values of these limits are normally well above the levels legally allowed as ambient in laboratories and workplaces but can be easily exceeded following a spillage. The upper limits of the flammability range offer little margin of safety because, when a solvent is spilled in the presence of a source of ignition, the lower level will be reached quickly and fire or explosion will occur before the upper limit is attained.
|Chemical||Flash point||Boiling point||Ignition temp||Flammable limit
(% by volume in air)
|Carbon disulfide||-30.0||46.1||80.0||1.3 50.0|
|Diethyl ether||-45.0||35.0||160.0||1.9 36.0|
|Methyl ethyl ketone||-6.1||80.0||515.6||1.8 10.0|
From the tables above:
• name two flammable liquids
• are there any combustible liquids?
• identify one liquid which would have a very low vapour pressure
• identify a liquid which could go on fire merely in contact with a hot water pipe
• what is the rationale for including the columns headed “flammable limit”?
• in these columns, why does the ratio of air to fuel matter?