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Chemical Safety Hazard Alert - Ammonia Has Flammable Properties
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Subject: Ammonia has flammable properties Source: Liaison, A
Newsletter for the Users of the CD-ROM CCINFO
Summer 1998, Volume 13, No. 3
Liaison is a publication of the Canadian Centre for Occupational Health and
Safety (CCOHS)
250 Mains Street East, Hamilton, Ontario L8N 1H6, 1-800-668-4284
Ammonia is a colorless, corrosive gas with a sharp, pungent odor which can
be detected by smell at low concentrations.
During preparation of the CHEMINFO review for ammonia (CHEMINFO record
number 48), an important and not adequately recognized fire hazard of
ammonia came to light. There is a widespread belief in North America that
ammonia does not burn. This perception probably arises from North American
transport regulations which classify anhydrous ammonia as “a non-flammable”
gas. In Europe, however, it is classified as flammable.
Upon examining the literature, Canadian Center for Occupational Health and
Safety (CCOHS) scientific staff learned that there is a history of violent
ammonia-air explosions, particularly with equipment in refrigeration plants.
Much of the equipment was poorly maintained and located in uninhabited,
inadequately ventilated confined spaces, such as a basement or cold storage
room. In these explosions, an ammonia leak occurred and the lower explosive
limit of 15% was reached in the confined space. The air-gas mixture was then
ignited explosively by an ignition source such as an unprotected tungsten
filament lamp, a spark from a motor or the heat of a welding or cutting
torch.
In one case, a leak occurred in the anhydrous ammonia refrigeration system
in a cold storage warehouse. The fire fighters believed that they were
dealing with stabilized conditions and that anhydrous ammonia gas was
non-flammable based on the U.S. DOT and Bureau of Explosives
classifications. They decided to use an electric forklift to replace the
leaking valve. Unfortunately, the truck crashed into a wall and a large
explosion followed, resulting in the death of one fire fighter and extensive
damage to the building. Investigators determined that the cause of the
explosion was the ignition of a hazardous accumulation of ammonia gas. The
ignition source was either an electric arc from the forklift truck of a
spark produced when the steel base of the truck hit the concrete wall.
Factors contributing to the explosion were the formation of a flammable
mixture of ammonia and air and the fire fighters’ lack of awareness that an
explosion hazard existed.
What do explosive or flammable limits mean?
The flammable or explosive concentration of ammonia in air is 15-28%. When
the concentration of ammonia in air is below the lower explosive limit (LEL)
of 15%, the mixture is too lean to burn. Similarly, when the concentration
of ammonia is above the upper explosive limit (UEL), the mixture is too rich
to burn. If the ammonia concentration is in the flammable range, a large and
intense energy source can cause ignition and explosion. The explosive range
is widened by mixing ammonia with other flammable gases such as oxygen. The
presence of oil or other combustible materials also increases the fire
hazard.
For additional information on this topic:
See The Environmental Protection Agency
Chemical Emergency Preparedness and Prevention Office's Chemical Safety
Alert
Last Modified: February 1, 2008
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