ammonia refrigeration hazards & controls
Because of the recent push to “go green” and conserve energy, many companies whom require refrigerated environments (e.g. food processors, cold storage facilities, etc.) are changing out their old Freon-based equipment for “mechanical refrigeration” systems that utilize ammonia (NH3). Ammonia systems have a significant cost advantage over Freon, however, the hazards are also significantly higher.
- Ammonia is a toxic gas under ambient conditions. Personnel safety is of course of utmost importance.
- Ammonia contaminates food. Food that is exposed to a release of ammonia- ever so slightly- will need to be discarded under government food safety regulations (e.g. USDA, FDA)
- Ammonia is liquefied under pressure. Because of this pressure, it will rapidly release into the air. Typically, the ammonia will rise (specific gravity is lighter than air), however, in the presence of moisture (such as high relative humidity), the liquefied anhydrous ammonia gas forms vapors that are heavier than air. These vapors may spread along the ground or into low-lying areas with poor airflow where people may become exposed.
- Explosions have been attributed to releases of ammonia contaminated with lubricating oil (a study conducted to determine the influence of oil on the flammability limits of ammonia found that oil reduced the lower flammability limit as low as 8 percent, depending on the type and/or concentration of oil).
- Ammonia can accumulate without appropriate ventilation. Leaks in rotating seals and pipes can occur. Forklifts or aerial equipment can hit pipes, valves, and evaporators. And of course, ammonia can escape during delivery (e.g. hose leaks).
- Ammonia is not flammable by the strict definition of flammability, however, when mixed with air in the 16% to 25% range, it is potentially explosive.
Key Risk Management Controls
- Regulatory standards include EPA, which requires a Process Safety Management Program when over 10,000 lbs of ammonia is used. OSHA’s Hazard Communication standard applies and requires employees to be furnished and trained with regard to the hazards, proper handling, personal protective equipment, etc. OSHA’s standard also sets standards for parts per million (PPM) for employee exposure. Best practice consensus standards include The American Conference of Governmental Industrial Hygienist (ACGIH), which sets exposure limits (both short-term and time-weighted average for an 8 hour period.
- From a risk management (including fire protection) standpoint, the industry consensus best-practice standard (“bible”) that professional practitioners adhere to is published by the American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE). The particular standard is #15- Safety Standard for Refrigeration Systems (2007 is the most recent revision). This standard provides guidelines on the following:
Ammonia Machine Room
- Location of ammonia refrigeration machinery is a detached building. A “cut-off room” (having one or more exterior walls with doors leading directly outside) is acceptable. Construction should be non-combustible (nor should combustible materials be stored in this room).
- Deflagration (“blow-out”) panels should be installed in the exterior wall (without any hazards on the other side of this wall). The National Fire Protection Association (NFPA) Standard 68, Guide for Venting of Deflagrations, provides details on the design requirements. Interior walls should be vapor tight with a one-hour fire resistance rating and designed to be pressure resistive so the deflagration panel does its job. NFPA 68 specifies further details on the hardware.
- Doors should be self-closing, tight fitting and open into the machine room. To prevent leaks, doorjambs should have a minimum 5/8 inch overlap and all penetrations through interior walls and floors/ceilings should be sealed with a listed fire resistant vapor-tight caulking (UL Listed/Factory Mutual Approved).
- Separate ventilation intake louvers and exhaust devices (fan or roof openings) should be located to promote mixing and airflow. All exhausts should vent directly to the outdoors. The exhaust should be at the ceiling level and the fresh air intakes should be at floor level (again- ammonia is lighter than air).
- Ammonia machine rooms are classified as hazardous locations per NFPA 70- National Electric Code (NEC). This standard requires that all electrical equipment in machine rooms be listed for Class I, Division 2 Group D atmospheres per NEC, Article 500. If this is in place, natural ventilation at the ceiling is acceptable
- If class I, Div 2 electrical is not in place, the following must be provided:
- A two-stage ammonia detection system with detectors installed according to the manufacturer’s listing and arranged to sound the alarm in a constantly attended location. The detection system must sound continuously and automatically activate the emergency ventilation at 25% lower explosive limit (LEL). The detection system must then shut down all equipment and close valves at 50% LEL.
- Continuous ceiling-level exhaust ventilation for the machine room, sized at a minimum of 1.0 cfm/ft2.
- Emergency ventilation providing 10 cfm/ft2, with a minimum rate of 20,000 cfm regardless of room size.
- A power source for ventilation systems (continuous or emergency) that is separate from the machine room power, so that shutting down power to the machine room does not affect the ventilation systems.
- A means to activate emergency ventilation fans that is separate from the machine room electrical system.
- Ventilation and explosion venting is not required in the refrigerated areas if equipment is limited to only the piping & all valves are located outside the refrigerated area (e.g. outside the building on the roof is common).
- Machine rooms of combustible construction particularly should be sprinkled (Ordinary Hazard Group 1 occupancy is sufficient per NFPA 13). It is optimal that the entire building be sprinklered, but that is a topic for another day.
- Not the least- preventative maintenance is critical- both short-term (daily inspections) and long-term (rebuilding/replacement of compressors and other critical equipment). ASHRAE provides inspection checklists (as well as EPA, equipment manufacturers, etc.).
- Optimally, ammonia detection systems should be installed in refrigerated areas containing materials and products subject to ammonia contamination. Ammonia detection and alarm systems should be tested according to the manufacturer's recommended testing frequency, but no less than annually.
- Ammonia is easily diluted with air or water, but because of the hazards noted above, it needs to be detected as early as possible. Key personnel should be trained/educated where the valves are located and how to shut them (again- these valves should be located outside confined areas- outside the building is preferred). Access to valves should be as easy as possible.
- Piping/equipment should be protected against mechanical damage, pressure-relief devices should be in place and arranged to discharge to a safe outdoor location, and automatic shutoff valves should be provided for liquid-level gauges.
- An emergency response team should be organized and trained. The following should be included:
- Training should include the entire system such as ventilation equipment/systems, dispersion/diluting procedures, equipment, preventative maintenance safety equipment (e.g. personal protective equipment such as SCBA gear), containment/compartementation through the use of closing building sectional doors, etc.
- Diagrams should be available with the location of all components. Copies should be kept in multiple places (e.g. off-site) but readily available.
- The fire department should be involved in pre-fire planning.