June 23, 2014
The health risks associated with anhydrous ammonia are well documented. They range from caustic effects capable of burning the respiratory system to environmental fallout on a highly toxic scale. And, thanks to a clear gaseous composition, the substance can spread far-and-wide, only detected when a concentrate reaches the odor threshold. Stored as one of industries most commonly employed chemicals, a key component of fertilizers, the compound is typically held cooled in its liquid form.
Communities go about their daily lives close to a storage facility containing anhydrous ammonia vessels. Thousands of facilities store substantial amounts of the compound, with many of these facilities located close to farming areas where fertilizers are used to grow bumper crops. These storage areas obviously institute regulated risk management plans, but the impetus lays upon engineering reliable vessels to safely store the substance. The vessels are designed from materials with a chemical certitude that guarantees no reaction with the caustic compound. That means no copper or zinc can be used in the construction of the tank or the fittings. Brass, a mix of copper and zinc, can find no place in the project. Brass and copper are soft metals, and it's unlikely they'd be employed in the construction of a large process vessel in any eventuality, but there's always the possibility of brass being used in fixtures, on gauge pipes and as seating, a scenario that must be avoided.
Using precision-fabricated steel and iron in the manufacturing process is the recommended approach. This construction also accounts for the pressurization constraints of the hazardous substance, since anhydrous ammonia is typically stored under pressure to increase the volume of the substance before it's used to manufacture a fertilizer product. This is simple physics and chemistry in action. Let's illustrate the point by stating that a single cubic foot of the compound in its liquid state equals 855 cubic feet of ammonia gas. The rated pressure vessel must then withstand an internal pressure of at least 250 pounds per square inch, although this figure may differ slightly depending on the regulatory authority or country where the vessel is bound for use. The total capacity of the tank must account for pressure factors and for the chemical characteristics of the substance. For example, there's an expansion factor to consider, a function of chemical inflation that classifies 85% of the vessel as having a usable capacity while the remaining 15% is defined as vapor space. Also, note that the compound has an affinity for water, and make provisions to keep the tank platform dry.
A number of safety factors and relevant codes shape the design of the tank, most of which we've already covered. Refer to data sheets for further details on safety procedures, on the chemical properties of the compound and the hazards to organic tissue. The vessel must also be installed at a recommended distance from traffic and built on a suitable foundation. Finally, all gauges must be free of copper, zinc and alloys employing the two elements.
Fusion - Weld Engineering Pty Ltd
ABN 98 068 987619
1865 Frankston Flinders Road,
Hastings, VIC 3915
Ph: (03) 5909 8218
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