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Ammonia Refrigeration

Properties of Ammonia

Ammonia
Ammonia

Ammonia is a chemical consisting of one atom of nitrogen and three atoms of hydrogen. It is designated in chemical notation as NH3. Ammonia is extremely soluble in water and is frequently used as a water solution called aqua ammonia. Ammonia chemically combines with water to form ammonium hydroxide. Household ammonia is a diluted water solution containing 5 to 10 percent ammonia. On the other hand, anhydrous ammonia is essentially pure (over 99 percent) ammonia. "Anhydrous" is a Greek word meaning "without water;" therefore, anhydrous ammonia in ammonia without water.

Refrigerant grade anhydrous ammonia is a clear, colorless liquid or gas, free from visible impurities. It is at least 99.95 percent pure ammonia. Water cannot have a content above 33 parts per million (ppm) and oil cannot have a content above 2 ppm. Preserving the purity of the ammonia is essential to ensure proper function of the refrigeration system.



Keeping Ammonia Pure

Impurities can enter the ammonia system:

  • While charging the system.

  • Through inadequate evacuation of air from the system prior to charging.

  • From valve stem packing.

  • From piping repairs.

  • From piping leaks.

  • From the normal breakdown of ammonia.

Potential Hazards:

  • Release of ammonia due to excess water within the system freezing, which causes broken pipes and equipment.

  • Ineffective refrigeration due to excess oil within the system, causing the system to work harder than necessary, thus stressing the system components.

  • Oxygen levels of more than a few ppm in liquid ammonia or a few thousand ppm in gaseous ammonia can promote stress corrosion cracking in steels.

    • This may result in a catastrophic failure of bulk storage vessels.

    • This may result in ammonia weeping from a crack within the refrigeration system.

    • Stress corrosion cracking proceeds more rapidly at high temperatures.

Possible Controls:

  • Provide employees information pertaining to the hazards of ammonia. [29 CFR 1910.119(d)(1)]

    • Ensure ammonia being introduced to the system is refrigerant grade.

    • See Physical and Chemical properties of ammonia for more information.
  • Conduct a process hazard analysis. [29 CFR 1910.119(e)]

  • Provide operating procedures [29 CFR 1910.119(f)] to ensure proper operation of the system.

    • Follow operating procedures for charging the system to prevent impurities from being introduced.
       
  • Provide training specific to the handling of ammonia. [29 CFR 1910.119(g)]

  • Conduct a pre-startup safety review prior to introducing ammonia when the equipment is new or modified significantly enough to require a change in the process safety information. [29 CFR 1910.119(i)]

    • Ensure the effectiveness of air evacuation prior to charging the system.
       
  • Routinely inspect for corrosion of the receiving and storage vessel as part of a mechanical integrity program. [29 CFR 1910.119(j)]

    • Install a non-condensable gas (oil and water) separator (purge units) into the system.

      • Maintain the purge unit according to manufactures specifications.
         
    • Test and analyze the ammonia periodically for oxygen and water content.

    • Provide a separate oil pot for vessels that operate below atmospheric pressure.
  • Establish and implement written procedures to manage changes to equipment, procedures, and to facilities. [29 CFR 1910.119(l)]

  • Investigate incidents. [29 CFR 1910.119(m)]

    • Investigate accidents and near misses that could have resulted in a release of ammonia.
  • Establish and implement an emergency action plan in case of release. [29 CFR 1910.119(n)]

Additional References:

  • Bulletin No. 108. Guidelines for Water Contamination in Ammonia Refrigeration Systems. International Institute for Ammonia Refrigeration (IIAR), (1986). Offers insights on where the water can come from and how to minimize continued infiltration. Provides an analytical approach to quantifying water concentrations, and recommends apparatus to remove the water.. See the Table of Contents.

  • Bulletin No. 110. Guidelines for Start-Up, Inspection, and Maintenance of Ammonia Mechanical Refrigerating Systems. International Institute for Ammonia Refrigeration (IIAR), (1993, March). Covers ammonia characteristics and hazards, inspection and maintenance of equipment, start-up issues, reference standards, safety equipment, and log book record-keeping. See the Table of Contents.
 


Physical Properties

Anhydrous ammonia is a clear liquid that boils at a temperature of -28F. In refrigeration systems, the liquid is stored in closed containers under pressure. When the pressure is released, the liquid evaporates rapidly, generally forming an invisible vapor or gas. The rapid evaporation causes the temperature of the liquid to drop until it reaches the normal boiling point of -28F, a similar effect occurs when water evaporates off the skin, thus cooling it. This is why ammonia is used in refrigeration systems.

Liquid anhydrous ammonia weighs less than water. About eight gallons of ammonia weighs the same as five gallons of water.

Liquid and gas ammonia expand and contract with changes in pressure and temperature. For example, if liquid anhydrous ammonia is in a partially filled, closed container it is heated from 0F to 68F, the volume of the liquid will increase by about 10 percent. If the tank is 90 percent full at 0F, it will become 99 percent full at 68F. At the same time, the pressure in the container will increase from 16 pounds per square inch (psi) to 110 psi.

Liquid ammonia will expand by 850 times when evaporating:

Anhydrous ammonia gas is considerably lighter than air and will rise in dry air. However, because of ammonias tremendous affinity for water, it reacts immediately with the humidity in the air and may remain close to the ground.

The odor threshold for ammonia is between 5 - 50 parts per million (ppm) of air. The permissible exposure limit (PEL) is 50 ppm averaged over an 8 hour shift. It is recommended that if an employee can smell it they ought to back off and determine if they need to be using respiratory protection.

Summary of properties:

Boiling Point

-28F

Weight per gallon of liquid at -28F

5.69 pounds

Weight per gallon of liquid at 60F

5.15 pounds

Specific gravity of the liquid (water=1)

0.619

Specific gravity of the gas (air=1)

0.588

Flammable limits in air

16-25%

Ignition temperature

1204F

Vapor pressure at 0F

16 psi

Vapor pressure at 68F

110 psi

Vapor pressure at 100F

198 psi

One cubic foot of liquid at 60F expands to

850 cubic foot of gas



 


Chemical Properties

Anhydrous ammonia is easily absorbed by water. At 68F, about 700 volumes of vapor can be dissolved in one volume of water to make a solution containing 34 percent ammonia by weight. Ammonia in water solution is called aqua ammonia or ammonium hydroxide.

Ammonia, especially in the presence of moisture, reacts with and corrodes copper, zinc, and many alloys. Only iron, steel, certain rubbers and plastics, and specific nonferrous alloys resistant to ammonia should be used for fabrications of anhydrous ammonia containers, fittings, and piping.

Ammonia will combine with mercury to form a fulminate which is an unstable explosive compound.

Anhydrous ammonia is classified by the Department of Transportation as nonflammable. However, ammonia vapor in high concentrations (16 to 25 percent by weight in air) will burn. It is unlikely that such concentrations will occur except in confined spaces or in the proximity of large spills. The fire hazard from ammonia is increased by the presence of oil or other combustible materials.

Anhydrous ammonia is an alkali.

 


Health Effects

Ammonia is not, strictly speaking, a poison and repeated exposure to it produces no additive (chronic) effects on the human body. However, even in small concentrations in the air it can be extremely irritating to the eyes, throat, and breathing passages.

Anhydrous ammonia primarily affects three areas of the body:

  • Eyes
  • Lungs
  • Skin

Eyes

Everything from mild irritation to destruction of the eye can occur depending on whether a spray or gas is involved. Ammonia penetrates the eye more rapidly than other alkalis.

Lungs

In the lungs, liquid anhydrous ammonia causes destruction of delicate respiratory tissue.
Exposure to ammonia vapor may cause:

  • Convulsive coughing.
  • Difficult or painful breathing.
  • Pulmonary congestion.
  • Death.

Skin

Skin damage depends upon the length and concentration of exposure and can range from mild irritation, to a darkened freeze-dry burn, to tissue destruction. Because liquid ammonia boils at -28F, the expanding gas has the potential to freeze anything in its path of release, including human flesh and organs.

Because water can absorb ammonia so readily, it is a factor that contributes to human toxicity. Ammonia will keep spreading across contacted skin until the chemical is diluted by skin moisture.

Alkalis effect tissue differently than acids, which tend to burn and seal off a wound. Alkalis, such as ammonia cause liquidization of tissue and turn tissue into a sticky "goo" and mix with this tissue, causing further damage. As a result, anhydrous ammonia burns keep spreading until the chemical is diluted.

In addition to liquidization, super-cooled anhydrous ammonia spray causes a freeze dry effect like frost bite when it hits the skin. The spray is also capable of freezing clothing to skin so that if the clothing is removed incorrectly whole sections of skin can be torn off.

High concentrations in the air can also dissolve in the moisture of the skin or perspiration and result in a corrosive action on the skin and mucous membranes.

First Aid

Decontaminate the victim as quickly as possible.

  • First, flush the eyes with clean water. Then flush the whole body or the exposed area with generous amounts of water; includes the hair, ears, under chin, and armpits. Any water source is acceptable; such as showers, hoses, or stock tanks.

  • Remove contaminated clothing, but only after careful flushing and warming to prevent the previously mentioned problem of skin sticking to the clothing.

  • See First Aid for more information.
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