| INTRODUCTION | | | | the plant are found there is often |
| 1 This circular gives advice on the | | | | inadequate attention to evacuation and |
| precautions to be taken against the | | | | emergency action; and |
| toxic, fire and explosion hazards | | | | (5) where enforcement officers encounter |
| presented by refrigeration systems | | | | maintenance contractors they should make |
| containing ammonia. These are most | | | | enquiries about their working practices |
| likely to be found by LA enforcement | | | | and training. |
| officers at cold stores and food | | | | Further advice |
| distribution warehouses. It applies to | | | | 30 This is a complicated technical |
| the entire system not simply the | | | | subject and there are strong trade |
| compressor house. It provides interim | | | | pressure groups. Enforcement officers |
| advice on matters of concern to | | | | are recommended to seek the advice of |
| enforcement officers pending revision of | | | | HSEs Field Consultant Group (FCG) via |
| BS 4434:1980. | | | | the Local Enforcement Liaison Officer |
| 2 Appendix 1 outlines the general | | | | (ELO), before considering enforcement |
| principles of refrigeration, Appendix 2 | | | | action. |
| gives information on the results of the | | | | ________________________________________ |
| programme of special visits carried out | | | | Ammonia is used as a refrigerant because |
| in 1983 by Factory Inspectorate (F1) to | | | | of particular thermodynamic properties |
| examine present standards in the food | | | | which enable it to move heat far more |
| industry and Appendix 3 gives detailed | | | | efficiently than other refrigerant gases |
| guidance on electrical standards. | | | | such as halogenated hydrocarbons. It is |
| Enforcement officers should not | | | | particularly suited to working in the |
| overemphasise the hazards of ammonia | | | | range approximately OoC to -30oC and |
| compared with other refrigerants. | | | | hence is widely used for food |
| HAZARDS | | | | preservation, the chilling of liquids |
| Toxicity | | | | such as milk, beer and soft drinks, and |
| 3 Ammonia is a chemically reactive gas | | | | in the chemical industry. New systems |
| that is very soluble in water and is | | | | continue to be installed. |
| much lighter than air (vapour density | | | | 2. A simple system theoretically needs 4 |
| 0.59 of that of air). Cold vapour (e.g. | | | | components: |
| from leaks) may however be denser than | | | | (1) evaporator; |
| air. Although there have been incidents | | | | (2) compressor; |
| of exposure to harmful concentrations of | | | | (3) condenser; and |
| ammonia in the UK there have been few | | | | (4) reducing valve |
| fatal accidents. | | | | In practice other components such as oil |
| Ammonia is characterised by a typical | | | | separator, intercooler, liquid receiver, |
| pungent odour and is detectable by most | | | | surge drum and liquid pumps are often |
| people at levels of about 50 ppm in the | | | | found. |
| atmosphere. Although workers become | | | | 3 The useful refrigeration is produced |
| tolerant to this effect and in the past | | | | at the evaporator. Liquid ammonia at low |
| have been able to work without distress | | | | pressure, and hence low temperature, |
| at levels up to 70 ppm, currently the | | | | takes in heat by vaporising. This vapour |
| recommended exposure limit for ammonia | | | | is removed by the compressor which, in |
| is 25 ppm, 8 hour TWA (0.0025%) and the | | | | compressing it, raises the temperature |
| short term exposure limit is 35 ppm, 10 | | | | from below to above ambient. The hot |
| minute TWA. At 400 ppm, most people | | | | compressed gas gives up the heat by |
| experience immediate nose and throat | | | | condensing to a liquid in the condenser. |
| irritation, but suffer no permanent | | | | The high pressure liquid then passes |
| ill-effects after 30-60 minute exposure. | | | | through the pressure reducing valve to |
| A level of 700 ppm causes immediate | | | | the evaporator. At the valve the liquid |
| irritation to the eyes, and a level of | | | | is cooled as some vapour flashes off. |
| 1,700 ppm (0.17%) will give rise to | | | | The remaining liquid is available for |
| repeated coughing and can be fatal after | | | | use in the evaporator. |
| about 30 minutes exposure. Exposure to | | | | 4 In a practical system it is likely |
| concentrations exceeding 5,000 ppm | | | | there will be other items of plant. An |
| (0.5%) for quite short periods can | | | | oil separator removes suspended oil |
| result in death. Response to the effects | | | | carried over from the compressor and |
| of ammonia varies widely between | | | | either returns it to the (pressurised) |
| individuals, and the dose-response | | | | crank-case or holds it for draining in |
| effects described above are likely to be | | | | some way. There may be a multi-stage |
| those experienced by the more | | | | compressor with an intercooler. This is |
| susceptible members of the population. | | | | cooled by bleeding high pressure liquid |
| Fire and explosion | | | | into the low pressure side. Downstream |
| 4 Ammonia forms a flammable mixture with | | | | of the condenser is generally a liquid |
| air at concentrations between 16 and 25% | | | | receiver. Downstream of the reducing |
| v/v. There have however been very few | | | | valve is often found a surge drum which |
| incentive explosions involving ammonia | | | | acts as a reservoir of cold liquid and |
| compressor houses in the UK and all of | | | | evens out demand on the compressor and |
| the reported incidents involved ammonia | | | | condenser. The liquid ammonia is drawn |
| leakage from plant under maintenance. | | | | from the surge drum by a pump. Oil |
| Existing guidance | | | | drains may be found on surge drums, |
| 5 Current guidance on the precautions | | | | liquid receivers, and elsewhere on large |
| which should be taken with ammonia | | | | plants. There is also likely to be an |
| refrigeration plant may be found in: | | | | automatic control system on all but the |
| British Standard 4434: 1980 | | | | oldest and smallest plants. |
| "Requirements for Refrigeration Safety: | | | | A simple practical refrigeration system |
| Part 1, General". The requirements | | | | 1 The aim was to collect information |
| (particularly from the f ire and | | | | about a cross section of installations. |
| explosion standpoint) are similar to | | | | One hundred and forty eight returns were |
| those in the earlier (1 969) version. | | | | used in the analysis which used the |
| However a' full revision of BS 4434 is | | | | Edinburgh FCG microcomputer. |
| taking place. | | | | 2 Returns covered a wide range of |
| Precautions | | | | processes in the food and drinks |
| 6 Under normal circumstances people will | | | | industries. The largest single sector |
| not be able to bear ammonia | | | | was dairying (chilled water supply) with |
| concentrations at even a fraction of the | | | | substantial returns also from frozen |
| flammable limit. The appropriate | | | | food producers and cold stores. In the |
| precautions are mainly those applicable | | | | drinks sector cooling and soft drinks |
| against toxic effects in occupied areas | | | | carbonators were the principal uses. |
| and to work where sudden exposures are | | | | There were a wide range of other uses |
| foreseeable, such as maintenance and | | | | reported; most parts of the food |
| repair work, including in particular | | | | industry require controlled temperatures |
| filling and oil draining. Precautions | | | | below ambient at some part of their |
| against fire and explosion will be | | | | process. A wide range of sizes of |
| appropriate however, in unoccupied areas | | | | installations from 45 kg to 45 tonne |
| such as compressor houses and unattended | | | | chargeweight were reported, 13% were |
| plant such as cold stores where | | | | over 5 tonnes, 40% between 1 and 5 |
| accumulations of vapour may go | | | | tonnes, 35% between 100kg and 1 tonne |
| unnoticed. | | | | and 12% 1 00 kg or below. |
| PRECAUTIONS AGAINST TOXIC RISK | | | | The oldest component reported was |
| Respiratory protective equipment | | | | pre-war and there was a fairly even |
| 7 Any person entering an area in which | | | | spread of age from 1960 to the present. |
| ammonia vapour is likely to be present | | | | 3 Eighty-nine per cent of installations |
| at a significant level (eg for rescue or | | | | had a separate compressor room. |
| fault-finding purposes) must wear | | | | Forty-nine percent had the system |
| self-contained or airline breathing | | | | charging point in the compressor room |
| apparatus. This does not include routine | | | | and 38% had it outdoors. Twenty-seven |
| visits to plant rooms etc. A suitable | | | | per cent of the sample could positively |
| and properly maintained set should be | | | | be identified as having doors to the |
| conveniently sited close to, but | | | | outside of the building only. Thirty-six |
| outside, any area in which high levels | | | | per cent of the other compressor rooms |
| of .ammonia vapour might arise. In no | | | | did not have self-closing doors and 17% |
| circumstances should anyone enter an | | | | did not have well-fitting doors. With |
| area where a flammable concentration of | | | | compressors in a separate room this is a |
| gas may be present. Details of suitable | | | | surprisingly large number where even the |
| apparatus are contained in Form 2501 | | | | most rudimentary precautions to prevent |
| "Certificate of Approval (Breathing | | | | the spread of escaping gas has not been |
| Apparatus)," published annually by HSE. | | | | taken. Fifty-five per cent had |
| See also Guidance Note GS 5 regarding | | | | condensers mounted above ground level |
| entry into confined spaces. | | | | outside - typically on the roof. This |
| 8 Suitable respiratory protective | | | | raises questions of safe access and also |
| equipment must be worn by every person | | | | escape in the event of an emergency. |
| carrying out engineering maintenance | | | | 4 Thirty-six percent, had the evaporator |
| work on any system where there is a risk | | | | in the workroom. (These were usually |
| of release of ammonia. Full face | | | | product freezers in the frozen food |
| canister respirators with type A (blue) | | | | sector and carbonators at the soft |
| canisters give good protection in | | | | drinks plants). This points to the need |
| atmospheres up to 2% concentration or | | | | for effective emergency procedures in |
| 20,000 ppm, for one hour. Work in such a | | | | the event of leakage, particularly if it |
| concentration is likely to lead to | | | | is in the workroom. |
| discomfort quickly due to skin | | | | 5 Only 3% of installations were |
| irritation as ammonia dissolves in | | | | identified as having pipework or plant |
| perspiration. A list of suitable | | | | capable of being damaged by, for |
| equipment is given in form 2502 | | | | example, fork lift trucks. Half of the |
| "Certificate of Approval (Canister Gas | | | | entire survey however had unmarked |
| Respirators)". For substantial jobs | | | | pipework. (Notes of many proformas |
| impervious suits may be necessary if the | | | | suggested that this would receive early |
| gas cannot be cleared. | | | | attention). |
| 9 Everyone who is likely to need to use | | | | 6 lt proved impossible to carryout |
| respiratory protective equipment must be | | | | meaningful analysis of the ventilation |
| properly trained in its use and must be | | | | provided in compressor rooms. A common |
| fully aware of its limitations. The | | | | installation seemed to rely largely on |
| equipment must be maintained, kept clean | | | | natural ventilation (perhaps assisted by |
| and examined at least once a month. | | | | a small fan) for normal ventilation. |
| Appropriate records should be kept. If | | | | Where there was provision of ventilation |
| canister respirators are used there must | | | | specifically for emergencies, it tended |
| be an effective system for deciding when | | | | to be a separate system rather than a 2 |
| the canisters should be renewed. | | | | speed fan on the normal ventilating |
| Evacuation and emergency procedures | | | | system. Only 23% of the installations |
| 10 lt is essential that a clear | | | | had 2 ventilation rates available and |
| emergency procedure is drawn up which | | | | only half the ventilation systems of any |
| details the precise duties of all staff | | | | kind could be controlled from outside |
| and the arrangements for evacuation, | | | | the compressor room. Only half of these |
| rescue, first aid, plant isolation etc. | | | | ventilation systems were automatically |
| It is particularly important that | | | | controlled. |
| evacuation procedures are clearly set | | | | 7 Only 16% of all system charging was |
| out and regularly practised where | | | | done by a person on his own; the usual |
| refrigeration systems are in working | | | | arrangement was 2 men. Oil draining was |
| areas. A common method which may be | | | | done by one man on his own at 30% of all |
| suitable is to use the fire alarm | | | | Installations. At 51 % of all |
| provided that actuating points are | | | | installations it was carried out more |
| immediately available at working areas. | | | | than once a month. Only 26% of |
| Personnel should be warned not to | | | | installations had spring-loaded valves |
| approach any vapour clouds. (Clouds may | | | | or a catchpot system at oil drains. Most |
| often look like steam because of the | | | | of the rest had simply a short stub of |
| cooling of the released gas). | | | | pipe from a vessel containing liquid |
| 11 Adequate exits should be maintained | | | | ammonia closed by a single valve. In 71 |
| from plant rooms at, all times. | | | | % of cases where the oil drain was |
| Personnel seriously affected by an | | | | unsatisfactory inspectors considered |
| ammonia escape suffer streaming eyes and | | | | that the reasonably practicable |
| violent coughing and rapidly become | | | | improvement was the fitting of |
| disorientated. They therefore require | | | | self-closing valves. The 30% of |
| clear prior knowledge of a safe exit | | | | installations where one man did the oil |
| route. | | | | draining on his own included 6 which had |
| Training in plant operation and | | | | no respirator of any kind. |
| maintenance | | | | 8 Forty-two per cent of compressor |
| 12 All personnel involved in the | | | | houses had no gas detectors. Sieger was |
| operation and maintenance of the plant | | | | by far the most common supplier (60%) of |
| must be adequately trained. The training | | | | all detection systems. The most common |
| should cover not only general principles | | | | service period of twice per year |
| of refrigeration but also specific | | | | reflects that company's normal service |
| points related to the particular plant. | | | | contract. Nineteen per cent of detector |
| This applies as much to maintenance | | | | systems were never checked. |
| contractors as to an employer's own | | | | Approximately half of the detector |
| staff. | | | | installations only had one operating |
| PLANT LOCATION | | | | level. Twenty-seven per cent of systems |
| Plant not designed for outdoor location | | | | did not shut down the plant but merely |
| 13 In the case of standard refrigeration | | | | raised the alarm. Ten per cent of the |
| plant (ie plant not specifically | | | | systems had no separate alarm. |
| designed for outdoor location) exposure | | | | 9 Sixty-six percent of compressor room |
| to excessively low air temperatures may | | | | electrical installations were not fully |
| cause liquefaction of ammonia within the | | | | equipped to Zone 2 standard even where |
| compressor leading to compressor damage, | | | | much of the plant was under the control |
| which could be hazardous. This type of | | | | of detectors. Seventy-five per cent of |
| plant should therefore be sited in a | | | | all compressor installations could be |
| compressor house using the precautions | | | | switched off elsewhere outside the |
| described in BS 4434:1980 and outlined | | | | compressor room (even if only at the |
| below. Compressor-houses should, where | | | | main supply). Of the remainder, the main |
| reasonably practicable, be fitted with | | | | switchgear was either in the compressor |
| explosion relief (eg by using | | | | room or access to it was through the |
| lightweight fragile roof). Where loosely | | | | compressor room. |
| held panels are used as explosion | | | | 10 Eighty-eight percent of all sites had |
| relief, they should be suitably | | | | 2 or more sets of respiratory protection |
| restrained (eg by chains) to prevent | | | | of some kind. Six installations (4%) had |
| them becoming dangerous missiles in the | | | | none at all. At 83% of sites there was |
| event of an explosion. | | | | said to be some sort of training in the |
| 14 ln order to facilitate the provision | | | | use of respiratory protection but only |
| of ventilation and explosion relief, | | | | 43% had some sort of systematic |
| compressor-houses should incorporate at | | | | examination. At only 5 installations |
| least one external wall. The siting of | | | | (3.4%) were there possible limitations |
| compressors in confined areas, | | | | of space which conflicted with provision |
| basements, etc should be avoided | | | | of respiratory protection and the main |
| wherever practicable. Doors between | | | | problem seemed to be access up ladders |
| plant rooms or compressor-houses and | | | | or around congested items of plant. |
| other parts of the building should be | | | | 11 Forty-seven per cent of sites had |
| self-closing and well-fitting. | | | | reviving apparatus available usually for |
| Plant designed for outdoor location | | | | general first aid rather than |
| 15 Only plant specifically designed for | | | | specifically because of the ammonia. |
| the conditions should be installed | | | | 12 Twenty-seven percent of sites had |
| outdoors. Such installations should be | | | | Draeger (or similar) detector tubes for |
| sited in a safe position in the open air | | | | measuring low concentrations of ammonia. |
| with, if necessary, weather protection | | | | Many others had sulphur sticks or |
| using a Dutch barn type structure which | | | | hydrochloric acid for detecting small |
| has an evenly distributed minimum open | | | | leaks. |
| area equivalent to at least 50% of the | | | | 13 Fifty-nine percent of installations |
| total wall area. | | | | were maintained at least partly by |
| Plant in workrooms | | | | contractors. Apart from a few major |
| 16 As a general principle the amount of | | | | suppliers and installers of equipment |
| plant containing ammonia situated in | | | | there were many local refrigeration |
| workrooms and other populated areas | | | | engineers who only appeared once or |
| should be minimised. Ancillary plant | | | | twice in the survey. No information is |
| such as surge drums and liquid pumps | | | | available about the standard of training |
| should wherever possible be sited away | | | | or workmanship of these contractors. |
| from working areas. Compressors are | | | | 14 Fifty-five percent of all sites |
| often noisy and this is another reason | | | | appeared to have emergency evacuation |
| for not having them in working areas. | | | | procedures (43% used the fire alarm) but |
| Ventilation | | | | slightly fewer (50%) appeared to give |
| 17 Compress or houses should be provided | | | | any training in these procedures. Only |
| with adequate and suitable ventilation | | | | 24% appeared to have detailed rescue |
| to meet the following requirements: | | | | arrangements. Twenty-two per cent had |
| (1) Normal Ventilation Sufficient | | | | written systems of work which appeared |
| permanent ventilation should be provided | | | | comprehensive and only 34% had what |
| to prevent build up of toxic | | | | appeared to be effective plant operator |
| concentrations of ammonia from | | | | training. |
| operational leakage (eg from seals, | | | | ________________________________________ |
| glands etc). It is probable that the | | | | 1 Compressors and refrigeration plant |
| redrafted British Standard will insist | | | | sited in out door locations in |
| on mechanical car ventilation rather | | | | accordance with para 14 of this Circular |
| than rely on rather uncertain natural | | | | in otherwise non-hazardous areas will |
| ventilation. | | | | not normally require specially protected |
| (2) Emergency ventilation Provision | | | | electrical equipment. |
| should be made for sufficient mechanical | | | | INTERNALLY SITED PLANT |
| ventilation to prevent flammable ammonia | | | | 2 A flow chart of the basic requirements |
| air mixtures accumulating in the event | | | | relating to the electrical apparatus for |
| of reasonably foreseeable plant or | | | | internally sited plant is given in the |
| operational failure (eg valve failure). | | | | supplement to this appendix. The |
| In such circumstances the aim should be | | | | operational approaches are given below. |
| to keep concentrations below 25% of the | | | | Option 1 - Use of explosion protected |
| lower explosive limit (ie 4%). | | | | electrical apparatus |
| 18 The ventilation requirements for a | | | | 3 Hazardous area classification should |
| particular installation will depend on | | | | be carried out by a competent person. |
| the type, capacity, operating conditions | | | | Electrical apparatus should then be |
| and location of the plant and may | | | | selected in accordance with BS 5345: |
| require individual assessment by a | | | | Part 1: 1976 Section 2. The majority of |
| ventilation engineer with appropriate | | | | compressor-houses should be regarded as |
| expertise. However, the following | | | | Zone 2 areas. Type "N" explosion |
| general points apply: | | | | protected equipment (including any |
| (1) permanent natural or mechanical | | | | emergency ventilation fans) will be |
| ventilation, or a combination of both, | | | | suitable for these locations. |
| may be used for normal or emergency | | | | Option 2 - Detection of leaks by |
| ventilation. Mechanical ventilation | | | | personnel or gas detectors |
| initiated by gas detectors or manually | | | | 4 ln this approach, non-explosion |
| (in the case of continuously manned | | | | protected electrical apparatus, with |
| plants) may also be used for emergency | | | | qualifications, may be used in |
| ventilation (see para 26); and Appendix | | | | combination with a readily available |
| 3 for electrical safety of the system; | | | | means of isolating the electricity |
| (2) the ventilation should discharge to | | | | supply. The method of achieving the |
| a safe place in the open air; | | | | latter can be accomplished either |
| (3) in considering the ventilation to be | | | | automatically after detection of a |
| provided, the potential effects of cold | | | | leakage by a gas detector system, or |
| on plant should be taken into account | | | | manually after a leakage has been |
| (see para 12); | | | | detected by personnel. The use of these |
| (4) flow of air through cracks around | | | | techniques as a first line of defence is |
| windows, doors etc, or the opening of | | | | limited to applications solely involving |
| windows or doors should not be relied on | | | | ammonia in refrigeration plants. This |
| for ventilation; | | | | approach is considered acceptable |
| (5) the formulae in BS4434 for | | | | provided that the general principles |
| quantifying ventilation requirements are | | | | outlined in paras 10-17 are followed and |
| rules of thumb based on unstated | | | | that sufficient account is taken of |
| assumptions (eg they take no account of | | | | paras 5-9. |
| room size or leak rates). Inspectors | | | | Gas detectors |
| should advise that the formulae may be | | | | 5 The detectors should be suitably |
| used as a basic guide but discretion in | | | | positioned taking into account the |
| their detailed application to a | | | | physical characteristics of the plant |
| particular plant should be stressed. | | | | room, the pattern of airflow movement in |
| This is particularly important with very | | | | it and the most likely sources of |
| large systems when the ventilation | | | | potential leakage. Due regard should be |
| required by the formulae becomes | | | | paid to any dead pockets or recesses. |
| impracticable; and | | | | Experience has shown that, in certain |
| (6) it should be noted that the standard | | | | circumstances, it is possible for cold |
| of ventilation given by the formulae in | | | | ammonia vapour to stratify initially at |
| BS 4434: 1980 is not intended to deal | | | | low levels. Unless the occupier has |
| with prolonged releases from major plant | | | | adequate expertise within his own |
| failure. However, the latter is very | | | | organisation, it would be advisable for |
| unlikely to occur in properly designed, | | | | him to consult a firm which specialises |
| constructed and maintained plant. | | | | in the design and installation of gas |
| Control of sources of ignition and plant | | | | detection systems. |
| shutdown (see paras 22-26) should also | | | | 6 As a rough guide only, one might |
| provide protection in such | | | | expect to see detectors in the vicinity |
| circumstances. Manually operated | | | | of the compressors and other non-static |
| controls for emergency ventilation | | | | items of plant and at ceiling level |
| should be located in a safe, easily | | | | where one detector per 36M2 of ceiling |
| accessible place along with the control | | | | area would probably be sufficient, |
| or switch for turning off the | | | | although more may be necessary if there |
| compressor. | | | | are deep beams creating recesses. The |
| Plant integrity | | | | objective is to ensure that the ammonia |
| 19 There can be serious corrosion of the | | | | is detected and the apparatus rendered |
| low pressure. parts of pipework and | | | | safe before flammable concentrations |
| plant due to condensation. It can | | | | reach a source of ignition. (This |
| progress unnoticed under lagging which | | | | objective, which is also applicable to |
| is not effectively vapour sealed and is | | | | "detection" of a leak by personnel, is |
| particularly rapid on plants which run | | | | particularly critical with regard to |
| intermittently and pass-through OoC. The | | | | electrical apparatus which is not |
| general principles relating to the | | | | specially designed to be non-sparking, |
| safety of pressure systems are | | | | non explosion-protected electrical |
| appropriate. The system should be | | | | apparatus and electrical apparatus with |
| thoroughly examined by a competent | | | | temperatures above 630'C). |
| person at regular intervals in | | | | 7 The detectors should be suitably |
| accordance with a written scheme. There | | | | explosion protected. |
| should be an effective maintenance | | | | 8 The detectors used are of the |
| scheme. | | | | "pellistor" type and may be subject to |
| Pipework | | | | poisoning by airborne contaminants. They |
| 20 All parts of refrigerating systems | | | | should therefore be properly installed |
| and in particular pipework should be | | | | and maintained and regularly checked. |
| positioned or protected to minimise the | | | | The operation of the detectors should be |
| risk of impact damage, for example by | | | | checked using standard ammonia gas |
| fork lift trucks. Pipework and valves | | | | mixtures. Certain V-belt dressings |
| should be clearly marked to indicate | | | | containing antimony have been shown to |
| their contents and function. | | | | poison detectors and gradually reduce |
| Oil drain system | | | | their response. |
| 21 Many of the reported incidents | | | | 9 The detectors should be capable of |
| involving ammonia refrigeration systems | | | | detecting concentrations of ammonia at 1 |
| have been the result of a malfunction of | | | | % v/v or less. |
| the oil drain system (designed to catch | | | | Associated electrical apparatus |
| the "carry-over" of oil from the | | | | 10 Account should be taken of the |
| compressors). In most cases oil is | | | | electrical control system circuitry and |
| drained from below liquid ammonia and is | | | | the maximum possible degree of failure |
| saturated with it. In addition the oil | | | | to safety should be achieved, so far as |
| is viscous because it is cold. In order | | | | is reasonably practicable. General |
| to minimise the risk of escape from this | | | | guidance is contained in BS 5304: |
| cause the following measures should be | | | | 1975 "Safeguarding of Machinery" Section |
| advised: | | | | 6. |
| (1) where short distances are involved | | | | 11 The isolating device(s), whether |
| and adequate observation of the drain is | | | | manually or automatically operated, |
| possible oil drain pipes should | | | | which cuts off the electricity supply to |
| terminate in a safe location in the open | | | | the ammonia plant room, should be |
| air. Valves on any pipe extension should | | | | located in a non-hazardous area. It can |
| not introduce the possibility of liquid | | | | be either a contractor or circuit |
| ammonia being trapped; a bleed valve or | | | | breaker. If the criteria in paras 5-9 |
| hydrostatic relief valve venting to a | | | | above have been satisfied, the following |
| safe place should be provided in the | | | | recommendations in paras 13-17 should be |
| sections between valves, as appropriate; | | | | adopted. (Although certain specific |
| (2) a double valve arrangement should be | | | | details have been taken from BS |
| provided at oil drains. In addition to | | | | 4434:1980, by way of example, they are |
| the operational manual valve, there | | | | intended to indicate the general |
| should be an automatic closing spring or | | | | principles of this approach and not |
| weight-loaded valve; and | | | | specific requirements -which will have, |
| (3) The use of oil drain catchpots. | | | | to be determined in each particular |
| These are a useful feature on new plant, | | | | case). |
| but existing plant cannot normally be | | | | 12 Attention will need to be paid to the |
| easily modified. Before the oil is | | | | control of other circuits which enter |
| drained, the catchpot is isolated from | | | | the plant room and are not directly |
| the liquid ammonia/oil feedline and the | | | | associated with the plant, eg socket |
| catchpot is electrically heated to boil | | | | outlets for portable tools. |
| off any ammonia which flows as a vapour | | | | Continuously manned rooms |
| to the low pressure side of the system. | | | | 13 Isolation of all electrical circuits |
| When the catchpot is warm, it is also | | | | should be effected by isolating devices |
| isolated on the vapour side and the oil | | | | located in a non-hazardous area. These |
| is then drained from it. | | | | devices should be controlled by push |
| Ammonia filling point | | | | buttons immediately outside the plant |
| 22 Ammonia filling points should be | | | | room, or controlled by a gas detection |
| located in safe, well ventilated | | | | system as described pare 14, and |
| positions and, where reasonably | | | | arranged to give visual and audible |
| practicable, in the open air. Filling | | | | alarms to switch on equipment for |
| points should be sited away from sources | | | | emergency ventilation and/or emergency |
| of ignition. | | | | lighting (if installed). Any electrical |
| PRECAUTIONS AGAINST FIRE AND EXPLOSION | | | | apparatus that is required to operate in |
| RISK | | | | the room after a leakage has been |
| Sources of ignition | | | | detected, such as ventilation equipment |
| 23 All likely sources of ignition (naked | | | | and emergency lighting, should be |
| flames etc) should be eliminated from | | | | suitably protected for the hazardous |
| compressor houses and from the immediate | | | | area in which it is sited, ie Zone 2. |
| vicinity of externally located plant. | | | | Few compressor rooms are continuously |
| Electrical equipment | | | | manned. Detection of leakage by |
| 24 Guidance on electrical apparatus for | | | | operators is only reliable if they are |
| use in potentially explosive atmospheres | | | | continuously present in the room. If for |
| is given in RS 5345: Part 1: 1976 "Code | | | | example they have other duties, or leave |
| of Practice for the Selection, | | | | the area for meals etc, or use an |
| Installation and Maintenance of | | | | isolated noise refuge then the speed of |
| Electrical Apparatus for Use in | | | | response is likely to be substantially |
| Potentially Explosive Atmospheres, Part | | | | slower than that of automatic detectors. |
| 1, Basic Requirements for all Parts of | | | | Unmanned plant rooms |
| the Code"; BS 4434: 1980, Clause 13 | | | | 14 lsolation of all electrical circuits |
| "Electrical Installations". The | | | | should be effected by isolating devices |
| approaches followed by the above | | | | located in a non-hazardous area and |
| documents differ. | | | | controlled by one or more suitable |
| 25 As a general principle, electrical | | | | ammonia gas detectors which should also |
| equipment should be sited outside the | | | | be arranged to give a visual and audible |
| compressor room in a safe location. | | | | alarm and to switch on equipment for |
| However, when it is necessarily sited in | | | | ventilation and/or emergency lighting, |
| the room, it should be in accordance | | | | if installed. The ventilation air should |
| with the guidance given in para 27. | | | | be discharged to the outside of the |
| 26 Where the ammonia compressors and | | | | building in such a manner as not to |
| refrigeration plant are located in the | | | | cause distress or danger to persons in |
| same room as the supply switch gear for | | | | the vicinity of the building. Circuit |
| the-premises relocation would probably | | | | isolation should be effected at ammonia |
| be inconvenient and costly. In such | | | | concentrations below 25% LEL and an |
| cases, Field Consultant Group (FCG) | | | | alarm setting of 1.5% v/v followed by |
| advise on the most suitable safety | | | | circuit isolation at 3% v/v is |
| precautions in the particular case | | | | suggested. |
| should be sought. | | | | 15 Maintenance personnel are required to |
| Electrical apparatus selection criteria | | | | enter unmanned plant rooms and adequate |
| 27 The use of electrical apparatus in | | | | means of escape should be provided. |
| refrigeration plants using ammonia has | | | | 16 Personal protection including |
| been considered a special case because | | | | breathing apparatus, and possibly |
| of the flammability characteristics of | | | | impervious suits, may be needed in any |
| the gas (high LEL and narrow explosive | | | | room or space if maintenance men are |
| range) and the fact that it can be | | | | likely to dismantle pipework or do any |
| detected at very low levels by smell. | | | | other operation liable to release liquid |
| This has resulted in a number of options | | | | ammonia or substantial quantities of |
| which may be considered when selecting | | | | gas. |
| electrical apparatus for ammonia plants | | | | Unmanned plant rooms linked to a |
| and these are considered in Appendix 3. | | | | continuously manned control room |
| OTHER RISKS | | | | 17 ln certain applications, (eg chemical |
| 28 Refrigeration systems often have | | | | plant), sudden loss of cooling |
| associated risks which may require | | | | facilities caused by automatic shut-down |
| attention, These include the risk of | | | | of a refrigeration plant might possibly |
| trapping in cold stores and chills, the | | | | create a hazard. It is unlikely that |
| handling of very cold products and | | | | this situation will ever arise in any |
| microbiological problems associated with | | | | premises in the JA sector of |
| cooling towers used for the condenser. | | | | enforcement. In these circumstances |
| ENFORCEMENT APPROACH | | | | isolation of the electrical equipment by |
| 29 Enforcement officers should advise | | | | manual intervention may be acceptable, |
| that ammonia refrigeration plant should | | | | provided that the detector/alarm system |
| comply with the guidance in BS 4434: | | | | is directly linked to a continuously |
| 1980 as amended and augmented by the | | | | manned control room. Other actions |
| information in this circular. They | | | | initiated by the detector such as the |
| should however bear in mind: | | | | operation of emergency ventilation may |
| (1) ammonia presents a toxic risk at | | | | still be feasible. Acceptance of this |
| concentrations far below those at which | | | | procedure, when automatic plant |
| it presents any fire or explosion risk. | | | | shut-down has been shown to be not |
| There have been 2 gassing fatalities | | | | reasonably practicable, will also |
| between 1977 and 1983 in the UK but only | | | | require that: |
| 3 incentive ammonia/air explosions in | | | | (1) the alarm arrangement and monitoring |
| the last 20 years; | | | | of the alarms (ie the manning of the |
| (2) the potential consequences of an | | | | control rooms) is satisfactory; |
| incident in terms of injury to | | | | (2) suitable isolation facilities for |
| personnel, and the general public should | | | | the compressor and unprotected |
| be assessed; | | | | electrical equipment are available in a |
| (3) BS4434 was first published in 1969 | | | | safe place; and |
| and was not intended to be | | | | (3) as a safe system of work is provided |
| retrospective, although improvements in | | | | for entry into the compressor room and |
| installations which pre-date the | | | | for the overall assessment of the |
| standard should be recommended, where | | | | potential hazard and any other necessary |
| reasonably practicable; | | | | action (eg plant isolation). (see paras |
| (4) analysis of the l983 visits strongly | | | | 6-8 of this circular for personal |
| suggest that where poor conditions of | | | | protection). |