Characteristics of Anhydrous Ammonia
Physical Properties Anhydrous Ammonia is a chemical compound containing the elements nitrogen and hydrogen. The formula is NH3, indicating that each volume of nitrogen is combined with 3 volumes of hydrogen. Since the atomic weights of these elements differ, the weight ratio is 82.5% nitrogen to 15.7% hydrogen. At atmospheric temperature and pressure, ammonia is a colorless gas with a very sharp, characteristic odor. This gas can become a liquid by cooling or by applying pressure. In appearance, liquid anhydrous ammonia resembles water. The boiling point at atmospheric pressure is -28 F. Above 900 F ammonia begins to decompose, reverting to hydrogen and nitrogen. At -107.9 F anhydrous ammonia freezes to form white crystals.
Effects on the Human Body Anhydrous ammonia is not a poison. It has no cumulative toxic effects on the human body. It is, however, very corrosive when it comes in contact with human flesh.
Anhydrous ammonia primarily affects three areas of the body - the lungs, eyes and skin tissue lining in the lungs, throat, mouth and nose are particularly sensitive to anhydrous ammonia. The effect on eyes may range from mild irritation to blindness.
When liquid ammonia comes in contact with skin, a severe burn is the result. This is caused because anhydrous ammonia is an alkali. Alkalis turn flesh into a sticky goo and mix with the affected tissue causing even more damage. As a result anhydrous ammonia burns keep spreading until the ammonia is diluted. This is why water is so important in first aid treatment of anhydrous ammonia injuries. In addition to being a strong corrosive, anhydrous ammonia causes a freeze dry effect similar to frostbite on the skin.
Flammability Anhydrous ammonia will burn, but only if ammonia gas is mixed with the proper proportion of air. The flammable limits of a mixture are reported as 16-25% of ammonia vapor in air. In addition, ammonia-air mixtures are difficult to ignite. It requires an intense source of ignition and a relatively high concentration of gas. The auto-ignition temperature is in excess of 1562 F using a quartz test container and 1204° F in the presence of an iron catalyst.
Caution should be exercised. It is impossible to test mixture percentage inside a storage tank, nurse tank, applicator, or piping. Welding or cutting should not be done on ammonia equipment until the equipment has been completely decontaminated and thoroughly cleaned of all salt and ammonia. A certified welder must do welding on ammonia storage tanks.
Never apply heat to lines plugged at both ends, or use an open flame on tanks, lines or valves containing solution. Tapers are not recommended in testing for ammonia leaks; they may ignite escaping vapors.
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Compatibility All anhydrous ammonia tanks, piping, tubing and fittings must be made of steel. NH3 is not compatible with zinc (galvanized steel), brass, copper, silver and any alloys of these metals. Piping shall be at least ASTM A53 Grade B seamless or ERW pipe. Pipe joints shall be threaded, welded or flanged. Pipe shall be at least Schedule 40 when joints are welded or welded and flanged. Pipe shall be at least Schedule 80 when joints are threaded. Welding shall be done by a welder certified in accordance with ASME Code, Section IX, “Welder Qualifications.” All piping shall conform to ASME B31.3.
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