A laboratory houses several solvents which play diverse roles in analysis and synthesis activities. The quantities of such solvents in laboratories are often much lower than those found in warehouses. Fortunately the fire hazards posed by flammable solvents are less in laboratories in comparison to places of bulk storage. The risk in laboratories is further reduced due to provision of adequate ventilation, fume hoods and air- conditioning. However, fire hazards in laboratories cannot be completely ruled out and tips on prevention of laboratory fire accidents provides some suggestions on minimizing such unfortunate incidents.
It becomes all the more important for a laboratory worker to have a clear understanding of the flammability characteristics that define combustible behaviour of such solvents so that required precautions are taken during their handling and storage.
Flammable and combustible characteristics of liquids
First of all it is important to understand the dividing line between a flammable and combustible liquid. Flammable liquids have the potential to ignite and catch fire at normal laboratory temperatures whereas combustible liquids burn at higher temperatures. In other words flammable liquids have flashpoints below \(37.8^0C\) (\(100^0\) F) whereas flashpoints of combustible liquids are above \(37.8^0C\) (\(100^0\)F) but below \(93.3^0C (200^0\)F).
It now becomes essential to know what is meant by flashpoint. It is the lowest temperature at which a liquid produces sufficient vapour to ignite in presence of a source of ignition. The entire liquid does not burn but the solvent vapour above the liquid surface starts burning. The lower the flashpoint the higher is the fire risk. Common laboratory solvents such as acetone, diethyl ether,toluene,etc exhibit below ambient temperature flash points. An example is diethyl ether which has high fire potential as its flashpoint is – \(45^0\)C which is far below ambient temperature of a laboratory.
Auto ignition point
A liquid can auto ignite by itself when it is heated. The liquid catches fire on reaching the auto ignition temperature even in the absence of a source of ignition. Fortunately auto ignition temperatures of common solvents are high (400 to \(500^0\)C) than laboratory ambient temperatures but it is advisable to keep such solvents away from hot plates and other heat generating equipments.
Flammable and explosive limits
Flammable limits define the concentration limits of solvent vapour in air that can lead to burning or explosion in presence of a flame or any other source of ignition.
Lower explosive limit (LEL) is the lowest vapour concentration that will burn or explode if ignited. In other words the air- fuel mixture is lean on fuel for ignition
Upper explosive limit (UEL) is the highest vapour concentration that will burn or explode if ignited. Above this concentration the mixture is too fuel rich to support combustion
It can be inferred that the solvent – air mixture can be ignited only between the lower and upper explosive limits
On the basis of Flash point the National Fire Protection Association (NFPA) has classified solvents into different classes based on their fire hazard potential. NFPA 30 code classifies such solvents according to their fire hazard potential.
You should keep in mind that fires resulting from flammable and combustible liquids spread at incredible speeds besides releasing thick, black and toxic fumes.
Sprays and aerosols of such liquids can also result in hazards if there are sources of high temperature in close vicinity. Any spill of such liquids also poses a potential fire hazard as it flows easily and can spread across work benches and laboratory floors. Such spills need to be cleaned up on priority to prevent fire hazards.