Safety operation of anhydrous ammonia equipment

Treating straw with anhydrous ammonia has many advantages, such as high efficiency, low cost, simplicity of operation and elimination of parasite eggs and weed seeds. However, anhydrous ammonia is toxic and explosive, so pressure containers are required for transport and storage. It is very important to follow relevant national regulations for safe operation. The basis of these regulations should be well understood. Explosions of ammonia containers happen occasionally and have resulted in persons injured or even killed. All persons working with ammonia must learn from these accidents.

Physical and chemical characteristics of ammonia, and safe operation

Ammonia is a colourless gas with suffocating and penetrating odour. Under pressure or at low temperature (-33 °C) it condenses to a clear liquid. The physical and chemical characteristics of ammonia are:

In the presence of water or steam, copper, zinc and aluminium, and alloys containing these metals, will be corroded by ammonia. These metals are therefore not to be used in ammonia equipment.

The pressure change in an ammonia container according to temperature is illustrated in Figure 6-33.

Figure 6-33. Ammonia pressure change with temperature

The chemical and physical characteristics of ammonia are the basis of the selection of ammoniation equipment and for safe operation:

• From Figure 6-33 it can be seen that the ammonia pressure is 2 x 105, 5.3 x 105 and 14.8 x 105 Pa for temperatures of -10, 10 and 40°C, which correspond to normal winter, spring and autumn temperature in Beijing, respectively. According to regulations, the ammonia should not be used at temperature over 50°C, equivalent to a pressure of 2.07 x 106 Pa. Other factors are taken into account, and the design pressure for a truck tank is 2.2 x 106 Pa and 3 x 106 Pa for a bottle. Pressurized containers should not be exposed to the sun for a long period, nor be brought near fire.

• Ammonia easily dissolves in water, a very important consideration in safety protection and accidents. On handling sites, enough water should be available, preferably tap water. A sign indicating "safety water" must be clearly displayed. If ammonia gets into the eyes or on the skin, it should be rinsed immediately with abundant water. If ammonia leaks into the air, water spray should be used to "knock down" the vapour. Spraying polluted areas with water reduces gas emission. Covering the nose with a wet towel is an efficient method of protection.

• Ammonia's density is lower than air density. If a large amount of ammonia is released to the air, people should rapidly leave the polluted area, moving into the wind. When driving out air from an ammonia container, the gas outlet valve should be in a low position.

• The boiling point of anhydrous ammonia is -33.34°C. When anhydrous ammonia vaporizes, a great amount of heat is absorbed. If the heat source is not sufficient, the process of volatilization is slowed down; and ammonia will remain liquid for longer. If anhydrous ammonia comes into contact with the skin, temperature will decrease and water will be absorbed, causing injury. If a great amount of ammonia liquid escapes from a damaged valve, if possible a soil or sand dam or a dig pit should be made around the discharge (because the ammonia boiling off will cool surfaces to -33.34°C) to reduce gas emission. Heat absorption from the air can be further reduced by spraying foam on the liquid ammonia, or simply covering it with a plastic sheet.

• If it is impossible to dig a hole or to make a dam, the container should be rapidly turned so that the damaged valve is uppermost (from liquid to gas position), thereby reducing the speed of ammonia release, since 1 litre of anhydrous ammonia will vaporize into 1 000 litre of ammonia gas.

• When the ammonia content in the air gets to 15-28 percent, it has reached the explosion limit. Below 15 percent, there is not enough ammonia for the explosion, and above 28 percent there is not enough oxygen. In practice, ammonia in the air or in containers should be kept below the explosion limit. For new ammonia containers or for those that have not been used for a long time, it is necessary to drive air out before using them. Adding a little ammonia into the container and opening the lower valve, the gas is pushed out into water through the hose. Repeating the process several times until there are no more bubbles in the water means that all discharged gas is ammonia. Since ammonia dissolves rapidly and thoroughly in water, in order to prevent air from going into ammonia containers, a little ammonia should be left in the bottle: that is to say, the pressure meter should remain at 0.05 MPa (0.5 atmospheres pressure). In addition, avoid fire on the handling sites to prevent explosions and self-ignition at 630°C. Attention should be paid during welding or repairing ammonia containers: ammonia should be purged beforehand.

• Containers should not be filled to excess. Figure 6-33 clearly shows that the vaporizing pressure increases with temperature. If the container is completely full of ammonia, with no empty space for liquid expansion with increasing temperature, the pressure will increases at the rate of 1.5 x 106 Pa/°C. If the temperature increases by 5°C, an explosion will occur. The amount of ammonia to be put in a container (G) should be calculated using the formula: G = 0.53V, where V represents the real volume of the container (in litres) and 0.53 is the maximum filling factor (kg/litre).

Safety supervision of ammonia containers

In order to emphasize the management of pressure containers with explosion risk, Safety supervision regulations for gas bottles, Supervision regulations for pressure containers, and Management regulations for liquefied gas tank trucks have been promulgated by the National Council, the Ministry of Work and Personnel, and the Ministry of Chemical Industry. Clear regulations have been established for the design, manufacture, installation, usage and inspection or repair of pressure containers, as well as for reporting accidents.

Ammonia tank trucks and bottles are classed as medium-pressure container transport. As ammonia is very toxic, a container of more than 440 litre volume is a class II container, which can implies serious consequences in a explosion.

According to national regulations, the following points should be observed when using pressure containers:

• Inspection and acceptance. The design and manufacturing company must have a Certificate of Approval from the appropriate authorities, and the products must have a quality certificate.

• Keeping technical records. Detailed records should be kept, including dates, inspections, repairs, measures taken to resolve problems, and so on.

• Personnel training. Operators must be trained, and receive the required qualification after passing an examination.

• Care in handling. Valves should be opened slowly to prevent pressure from rising or falling too quickly.

• Avoid overfilling. Do not overfill, to avoid high pressure and temperature.

• Regular inspection. According to the specifications of the pressure containers, they should be inspected externally once a year and internally once every two years by the relevant authorities. After inspection, a visible and permanent label of the authority, with the year of inspection, must be affixed to the container.

• Safe transport. Ammonia containers should be protected from impacts or overturning. Valves should be closed and caps well screwed on during transport. Loading and unloading must be done slowly. Ammonia containers should never be placed together with other containers, especially oxygen bottles. Fire extinguishers must be available.

• Regular maintenance. Pressure containers must be repaired and maintained regularly to prevent leaking. They should be kept in dry areas when not used.

• Qualified drivers and operators. The drivers and operators of tank trucks must be trained and qualify by passing an examination. Moving and stopping ammonia tank trucks should respect not only normal traffic regulations but also special conditions. The relevant regulations should be written and displayed in the appropriate position in or on the vehicle.

• Accident reporting. Any accident should be reported immediately to the relevant administration, police and labour department.

Regulations for safety in straw ammoniation

Ammonia is toxic and may explode under certain conditions. The ammonia containers are classed as "pressure containers with risk of explosion." Treating straw with anhydrous ammonia can endanger life and health if not done correctly. It is therefore necessary to follow the precautions and regulations made by the Bureau of Animal Production and Health (MOA, 1990):

Safety operation requlations for straw ammoniation

· Operators must have the necessary training, pass the examination and obtain their qualification.

· Safety equipment available at the working site should include:

- an abundant supply of fresh water;

- filter masks with ammonia filters;

- gloves and boots made of rubber or another suitable material; and

- fire extinguishers.

· Each new ammonia container must have an accompanying quality certificate that is properly filed.

· Never overfill whenever adding ammonia to a container.

· In summer, do not put ammonia containers in the sun. In winter, if ammonia flow is low, move containers to a warm place. Never heat them with fire.

· During ammonia transport, storage and straw ammoniation, check containers and hoses for leakage or swollen parts. Stop operation to make appropriate repairs to damage if found.

· Ammonia bottles must be firmly secured to the vehicle during transport to prevent them from falling due to movement.

· Be sure not to hit or to roll ammonia bottles during loading and unloading, which should be done slowly.

· Screw and tighten valves during bottle transportation and storage.

· Transport of anhydrous ammonia and oxygen in same wagon and storage in the same shed is strictly prohibited to avoid danger of explosion.

· If there is still pressure in a container, do not repair or fasten any part under pressure.

· When ammoniating straw, first place the spear into straw, then open the valve.

· Open and close ammonia-injecting valves slowly. During ammonia injection the gas pressure must be below 0.8 Pa. Do not hit the ammonia bottle with any tool.

· Ammonia flow should stop when pressure drops to 0.05 Pa.

· The ammonia containers can not be used to store any other material without the written consent of the working department.

· Ammonia containers must be checked regularly.

· The operators must wear protective equipment, including rubber gloves and protective suit, and stand upwind from the ammonia source.

· If ammonia leaks to the air, people should lay down on the ground, cover their noses with a wet towel and move upwind.

· Fire is forbidden in the vicinity of the ammonia station and the site of straw ammoniation.

· If an accident occurs, it should be immediately reported to administration and labour departments.