Process Safety Across the Chemical Engineering Curriculum

Initiative Led by Professor H. Scott Fogler

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Fire Triangle

Introduction:

The purpose of this tutorial is to illustrate the components necessary for the ignition of a fire and to identify ways to prevent fires. The fire triangle, also referred to as the combustion triangle, is a visual representation of the three essential components needed for the ignition of a fire. Each side of the triangle represents a different component.



Oxygen:

An oxidizing agent is present in any combustion reaction and reacts with the fuel in an exothermic reaction. The most common oxidizing agent is oxygen because of its abundance in air.



Fuel:

The fuel of the fire is any material that can burn. This can be any combustible material, like wood, paper, flammable liquids or gases, rubber, or fabrics. The more fuel, the longer the fire can burn and spread.

A fuel usually needs to be “primed”, or at a minimum temperature, before it can ignite. Flammable vapor mixtures occur at, or above, the flash point temperature of the liquid. The flash point is the temperature at which a vapor-air mixture above a liquid is capable of sustaining combustion after ignition from an energy source. A fine droplet mist can be flammable below the flash point of the liquid. Flammable gas mixtures can form at any temperature.



Ignition Source:

An ignition source is needed to preheat the fuel and keep the fire alight. About 98% of all ignitions come from open flames, hot surfaces, hot gases, sparks from mechanical or electrical equipment, static electricity, and chemical reactions. Other ignitions sources can include lightning strike, electromagnetic radiation, or stray electrical currents.



Flammability Limit:

Vapor in mixtures will only ignite and burn over a well-specified range of compositions.



Lower Flammability Limit (LFL):

Below the LFL the mixture will not burn as it is below the lower flammability limit, i.e., the mixture is too lean (i.e., insufficient fuel) for combustion.



Flash Point:

Temperature at which a vapor-air mixture above a liquid is capable of sustaining combustion after ignition from an energy source.



Upper Flammability Limit (UFL):

Above the UFL the mixture will not be combustible as the composition is too rich (e.g., insufficient oxygen).



Ways to Prevent Fires:

To stop a fire only one of the three components needs to be removed. There are many ways to reduce the risk of a fire, and a few are listed below:

Steps to take to protect people and facilities from fire damage:



References:



Prepared in collaboration with Kara Steshetz and Marina Miletic