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Flammability - The Combustion Triangle


There is always a temptation to use solvents in cleaning applications.  Although it is a "no-brainer" that there is a risk of fire or explosion when using a flammable solvent, it won't hurt to review what flammability is all about. You may be familiar with the "Combustion Triangle." In simple terms, there are three things needed for fire.
  • Fuel, which may be a solid, liquid or vapor
  • Oxygen (which is present in the air) or some other oxidizer
  • Heat to initiate the oxidation process which we call fire.  This heat is sometimes called a source of ignition.
If any of the above three are missing, there can be no fire.  But in some cases, there are other conditions that must be met.  Most flammable liquids and solids will not burn unless they are first vaporized.  Using a candle as an example, we all know that you can hold a match so that the flame touches the side of the candle and it will not start on fire.  The wax may melt, but even the liquid wax will not burn.  When we light the candle at the wick, the flame of the match first heats the wax in the wick (or near the wick)  to a temperature at which it first liquefies and then becomes a gas.  Once the wax has become a gas and mixes with the oxygen in the air, the heat from the match ignites the mixture of gaseous wax and oxygen from the air.  Once the candle is burning, the flame of the candle melts more wax which wicks up through the wick where it is further heated to become gas which mixes with the unlimited supply of oxygen in the air and is ignited by the existing flame.  The candle flame, once lit, is self-sustaining.  It creates the fuel, it mixes with oxygen and is ignited. The candle also provides us some good examples of how the combustion triangle can be broken.
  • Fuel - Once all of the wax in the candle has burned away, there is no longer any fuel available and the candle goes out.  The combustion triangle is broken by the lack of fuel.
  • Oxygen - If a burning candle is placed in a glass jar and the lid is screwed on, the candle will eventually use up all of the available oxygen in the jar and the candle will stop burning.  The combustion triangle is broken by the lack of oxygen.
  • Heat or Ignition - This one is a little tricky!  We are all familiar with "blowing" a candle out.  In fact, the rapid movement of air moves the flame (where the oxidation is taking place) far enough away from the fuel and oxygen mixture to prevent it from being ignited.  The white "smoke" that appears immediately after a flame is blown out is actually a very flammable mixture of wax vapor and oxygen but, with no flame, it is not ignited.  You may have noticed easy it is to re-ignite a candle that has just been extinguished vs. one that has never been lit.  "Pinching" a candle to put it out temporarily removes oxygen and essentially removes the heat of the burning flame which is required to sustain combustion.
Interesting Note - Those birthday candles that you can't blow out?  Well, the trick is that the wick is impregnated with a material similar to that coating the wires of "sparklers" which we burn at various celebrations.  This material contains both the fuel and the oxidizer (similar to gun powder) and is mixed in such a way that the temporary removal oxygen will not cause the oxidation process to stop.  The resulting sparks re-ignite the candle flame even though the flame itself has been temporarily removed. The above provides some basics.  Upcoming blogs will address additional practical details about combustion and the risk of using solvents in cleaning.  They will also explore ways to use solvents (even flammable solvents) safely for cleaning.

-  FJF  -