AMMONIA COMPRESSORS AND REFRIGERATION PLANT
INTRODUCTION 1 This circular gives advice on the precautions to be taken against the toxic, fire and explosion hazards presented by refrigeration systems containing ammonia. These are most likely to be found by LA enforcement officers at cold stores and food distribution warehouses. It applies to the entire system not simply the compressor house. It provides interim advice on matters of concern to enforcement officers pending revision of BS 4434:1980. 2 Appendix 1 outlines the general principles of refrigeration, Appendix 2 gives information on the results of the programme of special visits carried out in 1983 by Factory Inspectorate (F1) to examine present standards in the food industry and Appendix 3 gives detailed guidance on electrical standards. Enforcement officers should not overemphasise the hazards of ammonia compared with other refrigerants.
HAZARDS Toxicity 3 Ammonia is a chemically reactive gas that is very soluble in water and is much lighter than air (vapour density 0.59 of that of air). Cold vapour (e. from leaks) may however be denser than air.
Although there have been incidents of exposure to harmful concentrations of ammonia in the UK there have been few fatal accidents. Ammonia is characterised by a typical pungent odour and is detectable by most people at levels of about 50 ppm in the atmosphere. Although workers become tolerant to this effect and in the past have been able to work without distress at levels up to 70 ppm, currently the recommended exposure limit for ammonia is 25 ppm, 8 hour TWA (0.0025%) and the short term exposure limit is 35 ppm, 10 minute TWA. At 400 ppm, most people experience immediate nose and throat irritation, but suffer no permanent ill-effects after 30-60 minute exposure. A level of 700 ppm causes immediate irritation to the eyes, and a level of 1,700 ppm (0.17%) will give rise to repeated coughing and can be fatal after about 30 minutes exposure. Exposure to concentrations exceeding 5,000 ppm (0.5%) for quite short periods can result in death. Response to the effects of ammonia varies widely between individuals, and the dose-response effects described above are likely to be those experienced by the more susceptible members of the population.
Fire and explosion 4 Ammonia forms a flammable mixture with air at concentrations between 16 and 25% v/v. There have however been very few incentive explosions involving ammonia compressor houses in the UK and all of the reported incidents involved ammonia leakage from plant under maintenance. Existing guidance 5 Current guidance on the precautions which should be taken with ammonia refrigeration plant may be found in: British Standard 4434: 1980 "Requirements for Refrigeration Safety: Part 1, General". The requirements (particularly from the f ire and explosion standpoint) are similar to those in the earlier (1 969) version. However a' full revision of BS 4434 is taking place. Precautions 6 Under normal circumstances people will not be able to bear ammonia concentrations at even a fraction of the flammable limit. The appropriate precautions are mainly those applicable against toxic effects in occupied areas and to work where sudden exposures are foreseeable, such as maintenance and repair work, including in particular filling and oil draining. Precautions against fire and explosion will be appropriate however, in unoccupied areas such as compressor houses and unattended plant such as cold stores where accumulations of vapour may go unnoticed. PRECAUTIONS AGAINST TOXIC RISK Respiratory protective equipment 7 Any person entering an area in which ammonia vapour is likely to be present at a significant level (eg for rescue or fault-finding purposes) must wear self-contained or airline breathing apparatus. This does not include routine visits to plant rooms etc.
A suitable and properly maintained set should be conveniently sited close to, but outside, any area in which high levels of .ammonia vapour might arise. In no circumstances should anyone enter an area where a flammable concentration of gas may be present. Details of suitable apparatus are contained in Form 2501 "Certificate of Approval (Breathing Apparatus)," published annually by HSE. See also Guidance Note GS 5 regarding entry into confined spaces. 8 Suitable respiratory protective equipment must be worn by every person carrying out engineering maintenance work on any system where there is a risk of release of ammonia. Full face canister respirators with type A (blue) canisters give good protection in atmospheres up to 2% concentration or 20,000 ppm, for one hour. Work in such a concentration is likely to lead to discomfort quickly due to skin irritation as ammonia dissolves in perspiration. A list of suitable equipment is given in form 2502 "Certificate of Approval (Canister Gas Respirators)". For substantial jobs impervious suits may be necessary if the gas cannot be cleared.
9 Everyone who is likely to need to use respiratory protective equipment must be properly trained in its use and must be fully aware of its limitations. The equipment must be maintained, kept clean and examined at least once a month. Appropriate records should be kept. If canister respirators are used there must be an effective system for deciding when the canisters should be renewed. Evacuation and emergency procedures 10 lt is essential that a clear emergency procedure is drawn up which details the precise duties of all staff and the arrangements for evacuation, rescue, first aid, plant isolation etc. It is particularly important that evacuation procedures are clearly set out and regularly practised where refrigeration systems are in working areas. A common method which may be suitable is to use the fire alarm provided that actuating points are immediately available at working areas. Personnel should be warned not to approach any vapour clouds. (Clouds may often look like steam because of the cooling of the released gas).
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