Engineering Data Sheet 4-04: Anhydrous Ammonia-MOL

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Source:  Ontario Ministry of Labour

Introduction

Anhydrous ammonia (NH3) exists naturally in a gaseous state under atmospheric pressure and temperature. Under moderate pressure it changes easily to a liquid, becoming a gas again when the pressure is reduced. Industries take advantage of this characteristic by shipping and storing liquefied ammonia in pressurized railway cars, tank trucks and cylinders of various sizes. It is primarily used as fertilizer. It is also widely used as a refrigerant and in manufacturing nitric acid, explosives and plastics.

Properties

Anhydrous ammonia is one of the most water-soluble of all gases, but it should not be confused with aqueous ammonia, which is a 15 to 30 per cent solution of ammonia in water.

As a gas it is colourless and has a specific gravity of 0.597 (air = 1.0) at 25°C under standard atmospheric pressure. Generally, ammonia has good warning properties because of its characteristic pungent and irritating odour.

With rising temperature ammonia expands rapidly, increasing the internal pressure in vessels and pipes, etc. This must be considered in the design and operation of ammonia systems.

Hazards

Fire

Ammonia gas has an explosive range of 16 to 25 per cent by volume in air. Although it is classified as non-flammable under the Workplace Hazardous Materials Information System (WHMIS) and the Transportation of Dangerous Goods Regulations, its flammability potential is only slightly less than that of some gases that do meet the official flammability criteria.

Explosion

Contact of ammonia with certain chemicals—including fluorine, chlorine, bromine, iodine, mercury and silver oxide—can create explosive compounds. Moist ammonia will vigorously attack silver, copper, zinc and many of their alloys. Aluminum is attacked to a lesser extent. Iron and steel are inert to ammonia.

Health

Anhydrous ammonia, either as liquid or gas, is a strong irritant to skin, eyes and the respiratory tract. Direct exposure by contact can cause severe burns.

For ammonia the time-weighted-average exposure value is 17 mg/m3 (25 ppm). The short-term exposure value is 24 mg/m3 (35 ppm).

Storage And Handling

Tank Car Loading and Unloading

A dead-end siding must be provided.

Tracks must be level, and cars must be adequately chocked and braked.

The siding must be protected by a locked switch or a locked derail.

Caution signs must be placed on the track or car so as to give necessary warning to persons approaching the car from the open end of the siding. The signs must remain in place until the car is loaded and disconnected from the discharge connections.

The siding must not be closer than 90 m (300 ft.) to a school, hospital or place of public assembly.

An ammonia hose that is rated for a bursting strength of 12 MPa (1750 psig) and a minimum operating pressure of 2.4 MPa (350 psig) must be used to connect the car with permanent plant piping.

The ammonia hose connection line must be water-tested at least once every three months and be capable of withstanding a test pressure of 3.4 MPa (500 psig).

Excess-flow valves must be incorporated in the eduction lines.

Pumphouse Buildings

The building that houses the system for transferring loads to tank cars or tank trucks must:

1.be located

•at least 7.5 m (25 ft.) from the nearest property line, and

•at least 3 m (10 ft.) from any other structure (e.g., a building, storage tank or loading or unloading point) that contains electrical equipment not suitable for hazardous atmospheres;

2.be constructed of non-combustible material; and

3.have:

•two or more egress doors if the distance of travel to an egress exceeds 4.5 m (15 ft.) or if the room area exceeds 15 m2 (160 ft2). Egress doors must be located so that the distance of travel to an egress does not exceed 23 m (75 ft.). Doors must be equipped with panic hardware and be vertically hinged to open outwardly.

•screened openings to the outdoors near ceiling level. The total area of the openings should be at least 0.2 per cent of the area of the floor. Similar openings must be provided at the opposite wall near floor level to provide maximum air circulation.

•explosion venting to the outdoors, designed in conformity with good engineering practice, such as described in NFPA 68, “Venting of Deflagrations” or Factory Mutual 1-44, “Damage Limiting Construction”. Pressure-resisting walls and roofs should be designed to withstand explosion pressures of at least five times the relieving pressure that the explosion vents are designed for without collapse.

•heating systems (if the building is heated) that comply with Engineering Data Sheet No. 5-03, “Heating For Areas Containing Flammable or Explosive Gases, Vapours, Dust or Fumes.”

Storage Tanks

Storage tanks inside buildings must be:

1.located in a well-ventilated area; and

2.protected by automatic sprinklers.

The building or room must be used exclusively for the storage of anhydrous ammonia and must be kept free of combustible materials.

Wiring and equipment may have to meet the requirements of the Ontario Electrical Safety Code for Class I, Group D hazardous locations in accordance with the requirements for machinery rooms found in CSA B52, the Mechanical Refrigeration Code.

Outdoor storage tanks over 9 m3 (2000 imp. gal.) capacity must be located:

1.15 m (50 ft.) from the nearest building, property line or source of drinking water (e.g., wells and rivers); and

2.90 m (300 ft.) from schools, hospitals or other places of assembly.

Storage tanks must be protected from all sources of heat above 54°C (130°F).

Tanks must be equipped with one or more safety valves (spring- loaded or equivalent) and with loose-fitting rain caps on the discharge pipe. Provision should be made to drain any moisture that accumulates in the vent lines.

Safety valves should be tested and the entire system should be inspected periodically; this inspection should include the condition of paint, static grounding, etc.

Safety valves must be vented vertically to the outside atmosphere at least 2.1 m (7 ft.) above the tank. For tanks with capacity greater than or equal to 9 m3 (2000 Imp. gal.) and with no access to the top of the tank, the vent must be at least 2.1 m (7 ft.) above the ground.

Unrefrigerated tanks must have a minimum design pressure of 1.7 MPa (250 psig).

Refrigerated Storage

1.Refrigerated tanks must have a minimum design pressure of 415 kPa (60 psig).

2.A stand-by compressor of sufficient capacity to handle the holding load must be provided for each refrigerated system.

3.An emergency power supply must be provided unless the ammonia gas can be safely disposed of while the refrigeration system is not in operation.

4.An automatic alarm system must be provided for unattended systems.

Cylinders

Cylinders must not be filled to more than 85 per cent of capacity by tare weight as marked on the cylinder by the manufacturer. Cylinders must never be full of liquid ammonia at temperatures higher than 54°C (130°F).

Cylinders must be protected from excessive heat. Storage areas must be kept free from all combustible material.

Cylinders may be manifolded to discharge either liquid or gaseous ammonia. However, when liquid is discharged, a check valve must be installed on the discharge line of each cylinder. Wherever practical, arrange manifolded cylinders to discharge gaseous ammonia.

Heating to increase the discharge rate must be by hot water with a maximum temperature of 43°C (110°F).

The amount of anhydrous ammonia in main plant areas must be limited to two 10-cylinder banks of 72 kg (160 lbs.) per cylinder or the minimum required for a 24-hour supply, whichever is smaller. Where larger quantities are needed, the cylinders must be located outdoors, in a detached building or in a cut-off room, i.e. a room with all openings on the exterior of the building.

Process Equipment Design

Totally enclosed systems are preferable for processes using anhydrous ammonia.

The Boilers and Pressure Vessels Regulation (O. Reg. 220/01) sets safety standards for the design, fabrication and inspection of pressure vessels. Ammonia system installations must be approved by and registered with the Technical Standards and Safety Authority.

Tanks, piping and electrical equipment must be bonded and grounded (refer to Engineering Data Sheet 10-0, “Static Electricity”).

Wiring and equipment may have to meet the requirements of the Ontario Electrical Safety Code for Class I Group D hazardous locations.

Only wrought iron or steel may be used for ammonia containers, piping, fitting and equipment.

Piping must be schedule 40 with welded joints and welded fittings or schedule 80 with either threaded or welded joints and welded fittings.

High-pressure welding may be performed only by qualified welders.

Industrial Processes Using Ammonia

Machinery Rooms

All machinery rooms must meet the requirements of the Mechanical Refrigeration Code (CSA Standard B52-95). Note the different requirements for Class T Machinery Rooms, which are mainly used in skating and curling rinks and various industrial refrigeration processes.

Dissociation Rooms

Dissociation rooms must comply with section 7-4 of National Fire Protection Association (NFPA) Standard 86C, “Industrial Furnaces Using a Special Processing Atmosphere”. (“Dissociation” is the decomposition of ammonia into its two elements, nitrogen and hydrogen, using heat and a catalyst.)

Employee Safety

Education and Training

Safety in the handling of ammonia depends to a large degree upon effective employee education. The Workplace Hazardous Materials Information System (WHMIS) legislation includes requirements for employers to instruct employees on such things as:

1.hazards that may result from improperly handling ammonia;

2.emergency response procedures, including education, emergency repairs, the locations of safety showers, eye wash stations and the closest source of water;

3.and correct use of respiratory protective devices and other protective equipment; and

4.recognizing equipment failures and reporting them without delay.

Ammonia Leaks

Repairs must be made by trained personnel using approved protective equipment and clothing. All other personnel must be sent upwind of a leak.

Water or water spray must be used to control and absorb serious ammonia leaks. Since it may contain high concentrations of absorbed ammonia, this water should not be discharged into sewage systems.

Minor leaks may be located by using:

1.Ammonia test paper. Phenolphthalein or litmus paper will change colour in the presence of ammonia.

2.Dilute hydrochloric acid. The fumes will produce a dense white fog if ammonia is present.

3.Sulphur dioxide gas. Available in aerosol containers, it produces a dense white fog if ammonia is present.

Protective Equipment

Where appropriate, the following equipment must be kept on hand, readily accessible and properly maintained:

Eye Protection

A pair of tight-fitting, indirect-ventilation splash goggles must be worn at all times when handling ammonia. Contact lenses must not be worn when handling ammonia.

Respiratory Equipment

1.For concentrations up to 250 ppm: A half-facepiece chemical- cartridge respirator, with cartridges providing protection against ammonia, may be used as a minimum if eye protection is also supplied.

2.For concentrations between 250 and 300 ppm: A full-facepiece chemical cartridge respirator is required, with cartridges that provide protection against ammonia.

3.For emergency or planned entry into unknown concentrations or IDLH (immediately dangerous to life or health) concentrations, i.e. over 300 ppm: A self-contained breathing apparatus (SCBA) with a full facepiece and operated in a pressure-demand, or other positive-pressure, mode.

Protective Clothing

1.Rubber gloves must always be worn when handling ammonia.

2.In spill situations, both the person entering the spill area and a back-up/rescue person must wear a gas-tight suit in addition to the full-facepiece positive-pressure SCBA that is required for emergency or planned entry into unknown concentrations or IDLH (immediately dangerous to life or health) concentrations.

A Safety Shower and Eye Wash Station

This must have a means of heating the water to about 21°C. See ANSI standard Z358.1-1990, “Emergency Eyewash and Shower Equipment”.

Note: Protective equipment should never be used as a substitute for safe work practices.

Fire Protection

Water is the best extinguishing medium because anhydrous ammonia is highly soluble in water. The spread of escaping gas can be readily controlled by water spray.

Portable fire extinguishers must be available for extinguishing fires in electrical motors.

For bulk storage exceeding 9 m3 (2000 imp. gal), at least one fire hydrant fitted with a 6 cm (2 1/2″) hose must be provided.

References

Regulations and Codes

  1. Anhydrous Ammonia Bulk Storage Regulations, under the National Transportation Act and the Railway Act, Statutes of Canada 1978, Ch 1146
  2. Boiler and Pressure Vessels Regulation, O. Reg. 220/01
  3. Regulations for Industrial Establishments, R.R.O. 1990, Reg. 851
  4. Mechanical Refrigeration Code, CSA B52-92
  5. Ontario Electrical Safety Code, 21st ed.
  6. Ontario Fire Code, R.R.O. 1990, Reg. 454

Standards, Guides, etc.

  1. “Damage Limiting Construction”, Factory Mutual Loss Prevention Data 1-44
  2. “Emergency Eyewash and Shower Equipment”, ANSI Z358.1-1990
  3. “Guide for Venting of Deflagrations”, NFPA 68, 1994
  4. “Standard for Industrial Furnaces Using a Special Processing Atmosphere”, NFPA 86C, 1995
  5. Mossman, A.L. and Braker, W., Matheson Gas Data Book, 6th ed., 1980
  6. NIOSH Pocket Guide to Chemical Hazards, National Institute for Occupational Safety and Health, 1994

Ministry of Labour Engineering Data Sheets

  1. #5-03 Heating For Areas Containing Flammable or Explosive Gases, Vapours, Dusts or Fumes
  2. #10-0 Static Electricity