Flammable and combustible liquids are liquids that can burn. They are classified, or grouped, as either flammable or combustible by their flash points. Generally speaking, flammable liquids will ignite (catch on fire) and burn easily at normal working temperatures. Combustible liquids have the ability to burn at temperatures that are usually above working temperatures.
There are several specific technical criteria and test methods for identifying flammable and combustible liquids. Under the Workplace Hazardous Materials Information System (WHMIS) 1988, flammable liquids have a flash point below 37.8°C (100°F). Combustible liquids have a flash point at or above 37.8°C (100°F) and below 93.3°C (200°F).
Flammable and combustible liquids are present in almost every workplace. Fuels and many common products like solvents, thinners, cleaners, adhesives, paints, waxes and polishes may be flammable or combustible liquids. Everyone who works with these liquids must be aware of their hazards and how to work safely with them.
the minimum temperature at which a liquid gives off vapor within a test vessel in sufficient concentration to form an ignitable mixture with air near the surface of the liquid. The flash point is normally an indication of susceptibility to ignition.
The flash point is determined by heating the liquid in test equipment and measuring the temperature at which a flash will be obtained when a small flame is introduced in the vapor zone above the surface of the liquid.
Combustible liquid: any liquid having a flash point at or above 100ºF (37.8ºC).
Combustible liquids shall be divided into two classes as follows:
- CLASS II LIQUIDS SHALL INCLUDE THOSE WITH FLASH POINTS AT OR ABOVE 100ºF (37.8ºC) AND BELOW 140ºF (60ºC), EXCEPT ANY MIXTURE HAVING COMPONENTS WITH FLASH POINTS OF 200ºF (93.3ºC) OR HIGHER, THE VOLUME OF WHICH MAKE UP 99 PERCENT OR MORE OF THE TOTAL VOLUME OF THE MIXTURE.
- CLASS III LIQUIDS SHALL INCLUDE THOSE WITH FLASH POINTS AT OR ABOVE 140ºF (60ºC). CLASS III LIQUIDS ARE SUBDIVIDED INTO TWO SUB CLASSES:
- Class IIIA liquids shall include those with flash points at or above 140ºF (60ºC) and below 200ºF (93.3ºC), except any mixture having components with flash points of 200ºF (93.3ºC), or higher, the total volume of which make up 99 percent or more of the total volume of the mixture.
Class IIIB liquids shall include those with flash points at or above 200ºF (93.3ºC). This section does not regulate Class IIIB liquids. Where the term “Class III liquids” is used in this section, it shall mean only Class IIIA liquids.
- When a combustible liquid is heated to within 30ºF (16.7ºC) of its flash point, it shall be handled in accordance with the requirements for the next lower class of liquids.
Flammable liquid: any liquid having a flash point below 100ºF (37.8ºC), except any mixture having components with flash points of 100ºF (37.8ºC) or higher, the total of which make up 99 percent or more of the total volume of the mixture. Flammable liquids shall be known as Class I liquids. Class I liquids are divided into three classes as follows:
- CLASS IA SHALL INCLUDE LIQUIDS HAVING FLASH POINTS BELOW 73ºF (22.8ºC) AND HAVING A BOILING POINT BELOW 100ºF (37.8ºC).
- CLASS IB SHALL INCLUDE LIQUIDS HAVING FLASH POINTS BELOW 73ºF (22.8ºC) AND HAVING A BOILING POINT AT OR ABOVE 100ºF (37.8ºC).
- CLASS IC SHALL INCLUDE LIQUIDS HAVING FLASH POINTS AT OR ABOVE 73ºF (22.8ºC) AND BELOW 100ºF (37.8ºC).
It should be mentioned that flash point was selected as the basis for classification of flammable and combustible liquids because it is directly related to a liquid’s ability to generate vapor, i.e., its volatility. Since it is the vapor of the liquid, not the liquid itself that burns, vapor generation becomes the primary factor in determining the fire hazard. The expression “low flash – high hazard” applies. Liquids having flash points below ambient storage temperatures generally display a rapid rate of flame spread over the surface of the liquid, since it is not necessary for the heat of the fire to expend its energy in heating the liquid to generate more vapor.
The above definitions for classification of flammable and combustible liquids are quite complex. The diagram below should aid in their understanding.
Liquid itself burn:
Flammable and combustible liquids themselves do not burn. It is the mixture of their vapours and air that burns. Gasoline, with a flash point of -40°C (-40°F), is a flammable liquid. Even at temperatures as low as -40°C (-40°F), it gives off enough vapour to form a burnable mixture in air. Phenol is a combustible liquid. It has a flash point of 79°C (175°F), so it must be heated above that temperature before it can be ignited in air.
Flammable or explosive limits:
A material’s flammable or explosive limits also relate to its fire and explosion hazards. These limits give the range between the lowest and highest concentrations of vapour in air that will burn or explode.
The lower flammable limit or lower explosive limit (LFL or LEL) of gasoline is 1.4 percent; the upper flammable limit or upper explosive limit (UFL or UEL) is 7.6 percent. This means that gasoline can be ignited when it is in the air at levels between 1.4 and 7.6 percent. A concentration of gasoline vapor in air below 1.4 percent is too “lean” to burn. Gasoline vapor levels above 7.6 percent are too “rich” to burn. Flammable limits, like flash points however, are intended as guides not as fine lines between safe and unsafe.
Auto ignition Temperature:
A material’s auto ignition or ignition temperature is the temperature at which a material self-ignites without any obvious sources of ignition, such as a spark or flame.
Most common flammable and combustible liquids have auto ignition temperatures in the range of 300°C (572°F) to 550°C (1022°F). Some have very low auto ignition temperatures. For example, ethyl ether has an auto ignition temperature of 160°C (356°F) and its vaporous have been ignited by hot steam pipes. Serious accidents have resulted when solvent-evaporating ovens were heated to temperatures above the auto ignition temperature of the solvents used. Auto ignition temperatures, however, are intended as guides, not as fine lines between safe and unsafe. Use all precautions necessary.