Fire And Fire Extinguishing

Even though chemists handle flammable substances frequently, fire in a chemical lab is rather the exception, not the rule.

Fire is the result of a chemical reaction between a (oxidizable) fuel (gas or fume) and oxygen. Liquids in itself do not burn, only the fumes above the liquid do.
 

What is a fire?

Flames and fires are two aspects of the same chemical phenomenon.

Both result from the oxidation of a fuel by oxygen (deflagration), where both the supply of fuel and of oxygen are diffusion controlled.

When the fuel mass is large and larger surfaces are involved, the reaction is called a fire.

In general, the oxidation reaction is not spontaneous and must be ignited. The lab contains several sources of ignition: Bunsen torches, electrical heaters, electrical switches,Ö Pulling an electrical cord form a plug can create a spark, that can constitute the source of ignition.

Flammable substances are characterized by their flash point and their auto ignition temperature.

The following table indicates that volatile solvents like acetone or ether have a flash point of lower than 0°C. These solvents can cause fire even in a refrigerator when ignited.
 

Flash point and auto ignition

Flammable gases and fumes can ignite and burn when the mixing ratio with oxygen is between the so-called lower and upper ignition limits.

Under constant pressure, the effect of temperature on the ignition limits is important. The lower limit will decrease and the upper limit increase with increase in temperature, widening the danger window. This may even result in an non-flammable mixture becoming a flammable one.

Liquids cannot burn provided the temperature is low enough to deplete the atmosphere above the liquid from flammable fumes and to render the oxygen-fuel mixture to below the lower ignition limit. Every liquid is characterized by a temperature below which it cannot ignite.

This temperature is the flash point: the temperature measured at 1 bar pressure above which a mixture of oxygen and the equilibrated fumes of a liquid can ignite by means of a small energy source.

The auto ignition temperature is the temperature at which a mixture of fuel and oxygen ignites spontaneously.

The auto ignition temperature is higher than the flash point.

Table: Comparison between flash point and auto ignition temperature.

Solvent Flash point in °C Auto ignition temperature in °C

Acetone

 -19 538

Acetonitrile

 2 524

Ethanol

 12 365

Ethyl acetate

 4 426

Di-ethyl ether

 -45 160

Fire extinguishing

Once ignited, fires become self sustaining as the increase in temperature heats the fuel above its flash point. Fires must be extinguished by eliminating at least one of the constituents in the chemical reaction: fuel, oxygen or heat energy. Taking away the fuel, cutting the oxygen supply and lowering the temperature of the burning mass and surroundings are effective methods.

Water has a double effect: it cools down the burning mass and the steam formed displaces the oxygen.

A moist towel is very efficient in fighting the initial phase of a fire.

The following pictures contain some tips to fight larger fires

WRONG

TIPS

RIGHT
Approach fire under the lee.
Work from the front end on.
Use extinguishers simultaneously, not sequentially.
Stay alert to avoid re-ignition.
Have used extinguishers replenished immediately.

Fire extinguishers

  • At or close to  each lab (mostly gray) CO2 fire extinguishers are located. These apparatuses contain 2 or 5 kg of pressurized CO2 gas. When operated, the gas is decompressed and turns into a solid substance, CO2 snow, that covers the burning mass. The heat evaporates the CO2 again and this in turn displaces oxygen. CO2 extinguishers are ideal to fight most starting fires, but are useless for larger fires, as their capacity is too limited. Every extinguisher can operate some 20 seconds only.

    Care must be taken when handling fires involving liquid fuels not to point the CO2 into the liquid, as this might result in splattering and spreading the fire rather than eliminating it. CO2 extinguishers are the primary choice to fight fires involving electrical devices, as it leaves no traces.

    Avoid hitting people with the CO2 jet, as it causes freeze wounds.

  • The corridors and staircase halls are equipped with red powder extinguishers. They contain an inert powder such as NaHCO3 that is expelled by a driver gas. The finely divided powder covers the burning mass, cutting the oxygen supply. Their capacity is larger than that of CO2 extinguishers, but still limited. They suffer from a major drawback: the powder is a nasty substance to clean after  the fire.





  • Every building containing labs at K.U.Leuven is equipped with fire blankets. Their location is marked with the following symbol:

    People that caught fire can be wrapped while laying on the floor. These blankets can also be useful to cover fires and cut oxygen supply. Verify the location of the nearest fire blanket.
     
  • All buildings are equipped with fire hose reels to be used by the Fire Department and intervention teams. A fire hose is a small diameter rubber hose of approximately 30 meters length. It is connected to the water mains and its location is marked by the symbol:

Last revision: May 20, 2008

 
 
[K.U.Leuven - CWIS]