Group Issues Reminder about Flammability of Ammonia

The Canadian Centre for Occupational Health and Safety reports that during a chemical review for ammonia, an important and "not adequately recognized fire hazard" came to light.

The Canadian Centre for Occupational Health and Safety (CCOHS) reports that during preparation of the CHEMINFO review for ammonia (CHEMINFO record number 48), an important and, according to the center, "not adequately recognized fire hazard" came to light.

"There is a widespread belief in North America that ammonia does not burn," says the warning from the center. "This perception probably arises from North American transport regulations which classify anhydrous ammonia as ''a non-flammable'' gas." In Europe, the center notes, ammonia is classified as flammable.

Ammonia is a colorless, corrosive gas with a sharp, pungent odor that can be detected by smell at low concentrations.

During an examination of the available 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 basements or cold storage rooms. In these explosions, an ammonia leak occurred and the lower explosive limit of a 15 percent concentration of ammonia 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, CCOHS notes, 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. Department of Transportation 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.

by Sandy Smith ([email protected])

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