Ammonia is a colourless, corrosive
gas with a sharp, pungent odour 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,
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 or 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.
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