Protect your workers with the right equipment


Ammonia Compressors and Refrigeration Plant

INTRODUCTIONthe plant are found there is often
1 This circular gives advice on theinadequate attention to evacuation and
precautions to be taken against theemergency action; and
toxic, fire and explosion hazards(5) where enforcement officers encounter
presented by refrigeration systemsmaintenance contractors they should make
containing ammonia. These are mostenquiries about their working practices
likely to be found by LA enforcementand training.
officers at cold stores and foodFurther advice
distribution warehouses. It applies to30 This is a complicated technical
the entire system not simply thesubject and there are strong trade
compressor house. It provides interimpressure groups. Enforcement officers
advice on matters of concern toare recommended to seek the advice of
enforcement officers pending revision ofHSEs 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 2action.
gives information on the results of the________________________________________
programme of special visits carried outAmmonia is used as a refrigerant because
in 1983 by Factory Inspectorate (F1) toof particular thermodynamic properties
examine present standards in the foodwhich enable it to move heat far more
industry and Appendix 3 gives detailedefficiently than other refrigerant gases
guidance on electrical standards.such as halogenated hydrocarbons. It is
Enforcement officers should notparticularly suited to working in the
overemphasise the hazards of ammoniarange approximately OoC to -30oC and
compared with other refrigerants.hence is widely used for food
HAZARDSpreservation, the chilling of liquids
Toxicitysuch as milk, beer and soft drinks, and
3 Ammonia is a chemically reactive gasin the chemical industry. New systems
that is very soluble in water and iscontinue to be installed.
much lighter than air (vapour density2. 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 typicalseparator, intercooler, liquid receiver,
pungent odour and is detectable by mostsurge drum and liquid pumps are often
people at levels of about 50 ppm in thefound.
atmosphere. Although workers become3 The useful refrigeration is produced
tolerant to this effect and in the pastat the evaporator. Liquid ammonia at low
have been able to work without distresspressure, and hence low temperature,
at levels up to 70 ppm, currently thetakes in heat by vaporising. This vapour
recommended exposure limit for ammoniais removed by the compressor which, in
is 25 ppm, 8 hour TWA (0.0025%) and thecompressing it, raises the temperature
short term exposure limit is 35 ppm, 10from below to above ambient. The hot
minute TWA. At 400 ppm, most peoplecompressed gas gives up the heat by
experience immediate nose and throatcondensing to a liquid in the condenser.
irritation, but suffer no permanentThe high pressure liquid then passes
ill-effects after 30-60 minute exposure.through the pressure reducing valve to
A level of 700 ppm causes immediatethe evaporator. At the valve the liquid
irritation to the eyes, and a level ofis cooled as some vapour flashes off.
1,700 ppm (0.17%) will give rise toThe remaining liquid is available for
repeated coughing and can be fatal afteruse in the evaporator.
about 30 minutes exposure. Exposure to4 In a practical system it is likely
concentrations exceeding 5,000 ppmthere will be other items of plant. An
(0.5%) for quite short periods canoil separator removes suspended oil
result in death. Response to the effectscarried over from the compressor and
of ammonia varies widely betweeneither returns it to the (pressurised)
individuals, and the dose-responsecrank-case or holds it for draining in
effects described above are likely to besome way. There may be a multi-stage
those experienced by the morecompressor with an intercooler. This is
susceptible members of the population.cooled by bleeding high pressure liquid
Fire and explosioninto the low pressure side. Downstream
4 Ammonia forms a flammable mixture withof 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 fewvalve is often found a surge drum which
incentive explosions involving ammoniaacts as a reservoir of cold liquid and
compressor houses in the UK and all ofevens out demand on the compressor and
the reported incidents involved ammoniacondenser. The liquid ammonia is drawn
leakage from plant under maintenance.from the surge drum by a pump. Oil
Existing guidancedrains may be found on surge drums,
5 Current guidance on the precautionsliquid receivers, and elsewhere on large
which should be taken with ammoniaplants. There is also likely to be an
refrigeration plant may be found in:automatic control system on all but the
British Standard 4434: 1980oldest and smallest plants.
"Requirements for Refrigeration Safety:A simple practical refrigeration system
Part 1, General". The requirements1 The aim was to collect information
(particularly from the f ire andabout a cross section of installations.
explosion standpoint) are similar toOne 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 isEdinburgh FCG microcomputer.
taking place.2 Returns covered a wide range of
Precautionsprocesses in the food and drinks
6 Under normal circumstances people willindustries. The largest single sector
not be able to bear ammoniawas dairying (chilled water supply) with
concentrations at even a fraction of thesubstantial returns also from frozen
flammable limit. The appropriatefood producers and cold stores. In the
precautions are mainly those applicabledrinks sector cooling and soft drinks
against toxic effects in occupied areascarbonators were the principal uses.
and to work where sudden exposures areThere were a wide range of other uses
foreseeable, such as maintenance andreported; most parts of the food
repair work, including in particularindustry require controlled temperatures
filling and oil draining. Precautionsbelow ambient at some part of their
against fire and explosion will beprocess. A wide range of sizes of
appropriate however, in unoccupied areasinstallations from 45 kg to 45 tonne
such as compressor houses and unattendedchargeweight were reported, 13% were
plant such as cold stores whereover 5 tonnes, 40% between 1 and 5
accumulations of vapour may gotonnes, 35% between 100kg and 1 tonne
unnoticed.and 12% 1 00 kg or below.
PRECAUTIONS AGAINST TOXIC RISKThe oldest component reported was
Respiratory protective equipmentpre-war and there was a fairly even
7 Any person entering an area in whichspread of age from 1960 to the present.
ammonia vapour is likely to be present3 Eighty-nine per cent of installations
at a significant level (eg for rescue orhad a separate compressor room.
fault-finding purposes) must wearForty-nine percent had the system
self-contained or airline breathingcharging point in the compressor room
apparatus. This does not include routineand 38% had it outdoors. Twenty-seven
visits to plant rooms etc. A suitableper cent of the sample could positively
and properly maintained set should bebe identified as having doors to the
conveniently sited close to, butoutside of the building only. Thirty-six
outside, any area in which high levelsper cent of the other compressor rooms
of .ammonia vapour might arise. In nodid not have self-closing doors and 17%
circumstances should anyone enter andid not have well-fitting doors. With
area where a flammable concentration ofcompressors in a separate room this is a
gas may be present. Details of suitablesurprisingly large number where even the
apparatus are contained in Form 2501most rudimentary precautions to prevent
"Certificate of Approval (Breathingthe spread of escaping gas has not been
Apparatus)," published annually by HSE.taken. Fifty-five per cent had
See also Guidance Note GS 5 regardingcondensers mounted above ground level
entry into confined spaces.outside - typically on the roof. This
8 Suitable respiratory protectiveraises questions of safe access and also
equipment must be worn by every personescape in the event of an emergency.
carrying out engineering maintenance4 Thirty-six percent, had the evaporator
work on any system where there is a riskin the workroom. (These were usually
of release of ammonia. Full faceproduct freezers in the frozen food
canister respirators with type A (blue)sector and carbonators at the soft
canisters give good protection indrinks plants). This points to the need
atmospheres up to 2% concentration orfor effective emergency procedures in
20,000 ppm, for one hour. Work in such athe event of leakage, particularly if it
concentration is likely to lead tois in the workroom.
discomfort quickly due to skin5 Only 3% of installations were
irritation as ammonia dissolves inidentified as having pipework or plant
perspiration. A list of suitablecapable of being damaged by, for
equipment is given in form 2502example, fork lift trucks. Half of the
"Certificate of Approval (Canister Gasentire survey however had unmarked
Respirators)". For substantial jobspipework. (Notes of many proformas
impervious suits may be necessary if thesuggested that this would receive early
gas cannot be cleared.attention).
9 Everyone who is likely to need to use6 lt proved impossible to carryout
respiratory protective equipment must bemeaningful analysis of the ventilation
properly trained in its use and must beprovided in compressor rooms. A common
fully aware of its limitations. Theinstallation seemed to rely largely on
equipment must be maintained, kept cleannatural ventilation (perhaps assisted by
and examined at least once a month.a small fan) for normal ventilation.
Appropriate records should be kept. IfWhere there was provision of ventilation
canister respirators are used there mustspecifically for emergencies, it tended
be an effective system for deciding whento be a separate system rather than a 2
the canisters should be renewed.speed fan on the normal ventilating
Evacuation and emergency proceduressystem. Only 23% of the installations
10 lt is essential that a clearhad 2 ventilation rates available and
emergency procedure is drawn up whichonly half the ventilation systems of any
details the precise duties of all staffkind 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 thatcontrolled.
evacuation procedures are clearly set7 Only 16% of all system charging was
out and regularly practised wheredone by a person on his own; the usual
refrigeration systems are in workingarrangement was 2 men. Oil draining was
areas. A common method which may bedone by one man on his own at 30% of all
suitable is to use the fire alarmInstallations. At 51 % of all
provided that actuating points areinstallations it was carried out more
immediately available at working areas.than once a month. Only 26% of
Personnel should be warned not toinstallations had spring-loaded valves
approach any vapour clouds. (Clouds mayor a catchpot system at oil drains. Most
often look like steam because of theof the rest had simply a short stub of
cooling of the released gas).pipe from a vessel containing liquid
11 Adequate exits should be maintainedammonia closed by a single valve. In 71
from plant rooms at, all times.% of cases where the oil drain was
Personnel seriously affected by anunsatisfactory inspectors considered
ammonia escape suffer streaming eyes andthat the reasonably practicable
violent coughing and rapidly becomeimprovement was the fitting of
disorientated. They therefore requireself-closing valves. The 30% of
clear prior knowledge of a safe exitinstallations where one man did the oil
route.draining on his own included 6 which had
Training in plant operation andno respirator of any kind.
maintenance8 Forty-two per cent of compressor
12 All personnel involved in thehouses had no gas detectors. Sieger was
operation and maintenance of the plantby far the most common supplier (60%) of
must be adequately trained. The trainingall detection systems. The most common
should cover not only general principlesservice period of twice per year
of refrigeration but also specificreflects that company's normal service
points related to the particular plant.contract. Nineteen per cent of detector
This applies as much to maintenancesystems were never checked.
contractors as to an employer's ownApproximately half of the detector
staff.installations only had one operating
PLANT LOCATIONlevel. Twenty-seven per cent of systems
Plant not designed for outdoor locationdid not shut down the plant but merely
13 In the case of standard refrigerationraised the alarm. Ten per cent of the
plant (ie plant not specificallysystems had no separate alarm.
designed for outdoor location) exposure9 Sixty-six percent of compressor room
to excessively low air temperatures mayelectrical installations were not fully
cause liquefaction of ammonia within theequipped 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 ofof detectors. Seventy-five per cent of
plant should therefore be sited in aall compressor installations could be
compressor house using the precautionsswitched off elsewhere outside the
described in BS 4434:1980 and outlinedcompressor room (even if only at the
below. Compressor-houses should, wheremain supply). Of the remainder, the main
reasonably practicable, be fitted withswitchgear was either in the compressor
explosion relief (eg by usingroom or access to it was through the
lightweight fragile roof). Where looselycompressor room.
held panels are used as explosion10 Eighty-eight percent of all sites had
relief, they should be suitably2 or more sets of respiratory protection
restrained (eg by chains) to preventof some kind. Six installations (4%) had
them becoming dangerous missiles in thenone 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 provisionuse of respiratory protection but only
of ventilation and explosion relief,43% had some sort of systematic
compressor-houses should incorporate atexamination. 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 avoidedof respiratory protection and the main
wherever practicable. Doors betweenproblem seemed to be access up ladders
plant rooms or compressor-houses andor around congested items of plant.
other parts of the building should be11 Forty-seven per cent of sites had
self-closing and well-fitting.reviving apparatus available usually for
Plant designed for outdoor locationgeneral first aid rather than
15 Only plant specifically designed forspecifically because of the ammonia.
the conditions should be installed12 Twenty-seven percent of sites had
outdoors. Such installations should beDraeger (or similar) detector tubes for
sited in a safe position in the open airmeasuring low concentrations of ammonia.
with, if necessary, weather protectionMany others had sulphur sticks or
using a Dutch barn type structure whichhydrochloric acid for detecting small
has an evenly distributed minimum openleaks.
area equivalent to at least 50% of the13 Fifty-nine percent of installations
total wall area.were maintained at least partly by
Plant in workroomscontractors. Apart from a few major
16 As a general principle the amount ofsuppliers and installers of equipment
plant containing ammonia situated inthere were many local refrigeration
workrooms and other populated areasengineers who only appeared once or
should be minimised. Ancillary planttwice in the survey. No information is
such as surge drums and liquid pumpsavailable about the standard of training
should wherever possible be sited awayor workmanship of these contractors.
from working areas. Compressors are14 Fifty-five percent of all sites
often noisy and this is another reasonappeared to have emergency evacuation
for not having them in working areas.procedures (43% used the fire alarm) but
Ventilationslightly fewer (50%) appeared to give
17 Compress or houses should be providedany training in these procedures. Only
with adequate and suitable ventilation24% appeared to have detailed rescue
to meet the following requirements:arrangements. Twenty-two per cent had
(1) Normal Ventilation Sufficientwritten systems of work which appeared
permanent ventilation should be providedcomprehensive and only 34% had what
to prevent build up of toxicappeared to be effective plant operator
concentrations of ammonia fromtraining.
operational leakage (eg from seals,________________________________________
glands etc). It is probable that the1 Compressors and refrigeration plant
redrafted British Standard will insistsited in out door locations in
on mechanical car ventilation ratheraccordance with para 14 of this Circular
than rely on rather uncertain naturalin otherwise non-hazardous areas will
ventilation.not normally require specially protected
(2) Emergency ventilation Provisionelectrical equipment.
should be made for sufficient mechanicalINTERNALLY SITED PLANT
ventilation to prevent flammable ammonia2 A flow chart of the basic requirements
air mixtures accumulating in the eventrelating to the electrical apparatus for
of reasonably foreseeable plant orinternally sited plant is given in the
operational failure (eg valve failure).supplement to this appendix. The
In such circumstances the aim should beoperational approaches are given below.
to keep concentrations below 25% of theOption 1 - Use of explosion protected
lower explosive limit (ie 4%).electrical apparatus
18 The ventilation requirements for a3 Hazardous area classification should
particular installation will depend onbe carried out by a competent person.
the type, capacity, operating conditionsElectrical apparatus should then be
and location of the plant and mayselected in accordance with BS 5345:
require individual assessment by aPart 1: 1976 Section 2. The majority of
ventilation engineer with appropriatecompressor-houses should be regarded as
expertise. However, the followingZone 2 areas. Type "N" explosion
general points apply:protected equipment (including any
(1) permanent natural or mechanicalemergency ventilation fans) will be
ventilation, or a combination of both,suitable for these locations.
may be used for normal or emergencyOption 2 - Detection of leaks by
ventilation. Mechanical ventilationpersonnel or gas detectors
initiated by gas detectors or manually4 ln this approach, non-explosion
(in the case of continuously mannedprotected electrical apparatus, with
plants) may also be used for emergencyqualifications, may be used in
ventilation (see para 26); and Appendixcombination with a readily available
3 for electrical safety of the system;means of isolating the electricity
(2) the ventilation should discharge tosupply. The method of achieving the
a safe place in the open air;latter can be accomplished either
(3) in considering the ventilation to beautomatically after detection of a
provided, the potential effects of coldleakage by a gas detector system, or
on plant should be taken into accountmanually after a leakage has been
(see para 12);detected by personnel. The use of these
(4) flow of air through cracks aroundtechniques as a first line of defence is
windows, doors etc, or the opening oflimited to applications solely involving
windows or doors should not be relied onammonia in refrigeration plants. This
for ventilation;approach is considered acceptable
(5) the formulae in BS4434 forprovided that the general principles
quantifying ventilation requirements areoutlined in paras 10-17 are followed and
rules of thumb based on unstatedthat sufficient account is taken of
assumptions (eg they take no account ofparas 5-9.
room size or leak rates). InspectorsGas detectors
should advise that the formulae may be5 The detectors should be suitably
used as a basic guide but discretion inpositioned taking into account the
their detailed application to aphysical characteristics of the plant
particular plant should be stressed.room, the pattern of airflow movement in
This is particularly important with veryit and the most likely sources of
large systems when the ventilationpotential leakage. Due regard should be
required by the formulae becomespaid to any dead pockets or recesses.
impracticable; andExperience has shown that, in certain
(6) it should be noted that the standardcircumstances, it is possible for cold
of ventilation given by the formulae inammonia vapour to stratify initially at
BS 4434: 1980 is not intended to deallow levels. Unless the occupier has
with prolonged releases from major plantadequate expertise within his own
failure. However, the latter is veryorganisation, 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 plantdetection systems.
shutdown (see paras 22-26) should also6 As a rough guide only, one might
provide protection in suchexpect to see detectors in the vicinity
circumstances. Manually operatedof the compressors and other non-static
controls for emergency ventilationitems of plant and at ceiling level
should be located in a safe, easilywhere one detector per 36M2 of ceiling
accessible place along with the controlarea would probably be sufficient,
or switch for turning off thealthough more may be necessary if there
compressor.are deep beams creating recesses. The
Plant integrityobjective is to ensure that the ammonia
19 There can be serious corrosion of theis detected and the apparatus rendered
low pressure. parts of pipework andsafe before flammable concentrations
plant due to condensation. It canreach a source of ignition. (This
progress unnoticed under lagging whichobjective, which is also applicable to
is not effectively vapour sealed and is"detection" of a leak by personnel, is
particularly rapid on plants which runparticularly critical with regard to
intermittently and pass-through OoC. Theelectrical apparatus which is not
general principles relating to thespecially designed to be non-sparking,
safety of pressure systems arenon explosion-protected electrical
appropriate. The system should beapparatus and electrical apparatus with
thoroughly examined by a competenttemperatures above 630'C).
person at regular intervals in7 The detectors should be suitably
accordance with a written scheme. Thereexplosion protected.
should be an effective maintenance8 The detectors used are of the
scheme."pellistor" type and may be subject to
Pipeworkpoisoning by airborne contaminants. They
20 All parts of refrigerating systemsshould therefore be properly installed
and in particular pipework should beand maintained and regularly checked.
positioned or protected to minimise theThe operation of the detectors should be
risk of impact damage, for example bychecked using standard ammonia gas
fork lift trucks. Pipework and valvesmixtures. Certain V-belt dressings
should be clearly marked to indicatecontaining antimony have been shown to
their contents and function.poison detectors and gradually reduce
Oil drain systemtheir response.
21 Many of the reported incidents9 The detectors should be capable of
involving ammonia refrigeration systemsdetecting concentrations of ammonia at 1
have been the result of a malfunction of% v/v or less.
the oil drain system (designed to catchAssociated electrical apparatus
the "carry-over" of oil from the10 Account should be taken of the
compressors). In most cases oil iselectrical control system circuitry and
drained from below liquid ammonia and isthe maximum possible degree of failure
saturated with it. In addition the oilto safety should be achieved, so far as
is viscous because it is cold. In orderis reasonably practicable. General
to minimise the risk of escape from thisguidance is contained in BS 5304:
cause the following measures should be1975 "Safeguarding of Machinery" Section
advised:6.
(1) where short distances are involved11 The isolating device(s), whether
and adequate observation of the drain ismanually or automatically operated,
possible oil drain pipes shouldwhich cuts off the electricity supply to
terminate in a safe location in the openthe ammonia plant room, should be
air. Valves on any pipe extension shouldlocated in a non-hazardous area. It can
not introduce the possibility of liquidbe either a contractor or circuit
ammonia being trapped; a bleed valve orbreaker. If the criteria in paras 5-9
hydrostatic relief valve venting to aabove have been satisfied, the following
safe place should be provided in therecommendations in paras 13-17 should be
sections between valves, as appropriate;adopted. (Although certain specific
(2) a double valve arrangement should bedetails have been taken from BS
provided at oil drains. In addition to4434:1980, by way of example, they are
the operational manual valve, thereintended to indicate the general
should be an automatic closing spring orprinciples of this approach and not
weight-loaded valve; andspecific 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 be12 Attention will need to be paid to the
easily modified. Before the oil iscontrol of other circuits which enter
drained, the catchpot is isolated fromthe plant room and are not directly
the liquid ammonia/oil feedline and theassociated with the plant, eg socket
catchpot is electrically heated to boiloutlets for portable tools.
off any ammonia which flows as a vapourContinuously manned rooms
to the low pressure side of the system.13 Isolation of all electrical circuits
When the catchpot is warm, it is alsoshould be effected by isolating devices
isolated on the vapour side and the oillocated in a non-hazardous area. These
is then drained from it.devices should be controlled by push
Ammonia filling pointbuttons immediately outside the plant
22 Ammonia filling points should beroom, or controlled by a gas detection
located in safe, well ventilatedsystem as described pare 14, and
positions and, where reasonablyarranged to give visual and audible
practicable, in the open air. Fillingalarms to switch on equipment for
points should be sited away from sourcesemergency ventilation and/or emergency
of ignition.lighting (if installed). Any electrical
PRECAUTIONS AGAINST FIRE AND EXPLOSIONapparatus that is required to operate in
RISKthe room after a leakage has been
Sources of ignitiondetected, such as ventilation equipment
23 All likely sources of ignition (nakedand emergency lighting, should be
flames etc) should be eliminated fromsuitably protected for the hazardous
compressor houses and from the immediatearea in which it is sited, ie Zone 2.
vicinity of externally located plant.Few compressor rooms are continuously
Electrical equipmentmanned. Detection of leakage by
24 Guidance on electrical apparatus foroperators is only reliable if they are
use in potentially explosive atmospherescontinuously present in the room. If for
is given in RS 5345: Part 1: 1976 "Codeexample they have other duties, or leave
of Practice for the Selection,the area for meals etc, or use an
Installation and Maintenance ofisolated noise refuge then the speed of
Electrical Apparatus for Use inresponse is likely to be substantially
Potentially Explosive Atmospheres, Partslower than that of automatic detectors.
1, Basic Requirements for all Parts ofUnmanned plant rooms
the Code"; BS 4434: 1980, Clause 1314 lsolation of all electrical circuits
"Electrical Installations". Theshould be effected by isolating devices
approaches followed by the abovelocated in a non-hazardous area and
documents differ.controlled by one or more suitable
25 As a general principle, electricalammonia gas detectors which should also
equipment should be sited outside thebe 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 inventilation and/or emergency lighting,
the room, it should be in accordanceif installed. The ventilation air should
with the guidance given in para 27.be discharged to the outside of the
26 Where the ammonia compressors andbuilding in such a manner as not to
refrigeration plant are located in thecause distress or danger to persons in
same room as the supply switch gear forthe vicinity of the building. Circuit
the-premises relocation would probablyisolation should be effected at ammonia
be inconvenient and costly. In suchconcentrations below 25% LEL and an
cases, Field Consultant Group (FCG)alarm setting of 1.5% v/v followed by
advise on the most suitable safetycircuit isolation at 3% v/v is
precautions in the particular casesuggested.
should be sought.15 Maintenance personnel are required to
Electrical apparatus selection criteriaenter unmanned plant rooms and adequate
27 The use of electrical apparatus inmeans of escape should be provided.
refrigeration plants using ammonia has16 Personal protection including
been considered a special case becausebreathing apparatus, and possibly
of the flammability characteristics ofimpervious suits, may be needed in any
the gas (high LEL and narrow explosiveroom or space if maintenance men are
range) and the fact that it can belikely 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 optionsammonia or substantial quantities of
which may be considered when selectinggas.
electrical apparatus for ammonia plantsUnmanned plant rooms linked to a
and these are considered in Appendix 3.continuously manned control room
OTHER RISKS17 ln certain applications, (eg chemical
28 Refrigeration systems often haveplant), sudden loss of cooling
associated risks which may requirefacilities caused by automatic shut-down
attention, These include the risk ofof a refrigeration plant might possibly
trapping in cold stores and chills, thecreate a hazard. It is unlikely that
handling of very cold products andthis situation will ever arise in any
microbiological problems associated withpremises in the JA sector of
cooling towers used for the condenser.enforcement. In these circumstances
ENFORCEMENT APPROACHisolation of the electrical equipment by
29 Enforcement officers should advisemanual intervention may be acceptable,
that ammonia refrigeration plant shouldprovided that the detector/alarm system
comply with the guidance in BS 4434:is directly linked to a continuously
1980 as amended and augmented by themanned control room. Other actions
information in this circular. Theyinitiated by the detector such as the
should however bear in mind:operation of emergency ventilation may
(1) ammonia presents a toxic risk atstill be feasible. Acceptance of this
concentrations far below those at whichprocedure, when automatic plant
it presents any fire or explosion risk.shut-down has been shown to be not
There have been 2 gassing fatalitiesreasonably practicable, will also
between 1977 and 1983 in the UK but onlyrequire 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 ancontrol rooms) is satisfactory;
incident in terms of injury to(2) suitable isolation facilities for
personnel, and the general public shouldthe compressor and unprotected
be assessed;electrical equipment are available in a
(3) BS4434 was first published in 1969safe place; and
and was not intended to be(3) as a safe system of work is provided
retrospective, although improvements infor entry into the compressor room and
installations which pre-date thefor the overall assessment of the
standard should be recommended, wherepotential hazard and any other necessary
reasonably practicable;action (eg plant isolation). (see paras
(4) analysis of the l983 visits strongly6-8 of this circular for personal
suggest that where poor conditions ofprotection).



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