The mother of all hazardous areas
01 September 2008
The munitions industry is perhaps the archetypal hazardous area industry. Graham Park provides an insight into the explosive industry’s approach to classification and zoning
Primary high explosive detonation with debris cloud
The explosive industry was one of the first industrial processes to be regulated by government. The 1772 Act was an attempt in improving safety within the explosives industry and marked the beginning of state regulation of industry as a whole. This Act provided standard methods and materials for the construction of gunpowder works along with the regulation of working practices [1].
Since then a number of other regulations and legislation have come and gone. The present main governing legislation for the explosives industry are the Manufacture and storage of explosives regulations 2005 (MSER). These regulations cover a number of subjects associated with the explosives industry including: the prevention of fires and explosions, measures to limit the extent of fire or explosions, protecting people from fire and explosions, licensing and regulation requirements and prohibitions concerning certain explosives and miscellaneous provisions. The explosive industry generally also comes under the requirements of The Control of Major Accident Hazards Regulations (Comah) [2].
The explosive industries, while presenting a number of hazards that are common with processes covered by DSEAR and ATEX, differ in a number of significant respects. The DSEAR and ATEX regulations specifically exclude explosives – dusts of explosives that do not require atmospheric oxygen for combustion, or to pyrophoric substances – as do a number of the British standards associated with hazardous areas. One reason for this is that explosives produce there own oxygen and as such some common methods of control would not work. It is well known that in The Fire Triangle if you remove one corner, you remove the hazard. This is not so with explosives.
Explosives also pose a problem that is not encountered with most other dusts; a thin dust layer still presents a significant explosion hazard. For this reason it is common to blanket-zone an explosive areas to a single zone.
The industry has for some time worked within a number of Ministry of Defence standards, Health and safety executive guidance documents along with specific company based standards and recommendations that have been developed over a good many years. With the advent of MSER the HSE published an approved code of practice [3] that gives guidance and quote a number of recommended ways of managing an explosive site.
The method used to classify and zone explosive hazardous areas takes parts from the ATEX user directive along with the older classifying methods from the HSE guidance PM82 [4] and Ministry of Defence standards. Buildings and areas that are likely to contain explosives are divided into categories according to the nature of the explosives they are intended to hold. Table 1 shows the current definitions of the classification and zoning in current use.
A few explosive materials present both a vapour and a dust hazard, in these cases the precautions taken along with the classification and zoning of the area in which they are handled is derived from the outcome of a detailed risk assessment.
Explosive material, as with other potentially hazardous items have hazard data sheets produced to indicated there potential hazards. These explosive hazard datasheets in addition to the normal information given on toxicity etc. also includes the results of a number of standard UN EMTAP tests. These EMTAP tests cover a range of test parameters, some of the more common tests include mallet friction tests with results give for steel against steel, and maple against Yorkstone, plastic, hardwood and softwood. Some of the other tests undertaken include a rotter impact test; a rotary friction test; ease of ignition; temperature of ignition; and ignition by flash. Other tests include behaviour of inflammation test where the materials property for supporting a burn train is assessed; temperature and chemical stability; compatibility tests; and ignition by electric spark – plus any special precautions required to be taken. See the Cranfield website for further details [5].
The results from the sensitivity to electric spark are broken down in to three sections that determine the level of static dissipative precaution needed to be taken. Some materials are not sensitive to electric spark but some are very sensitive with values of less than 100 micro joules producing ignitions.
Table 1: Categories and zoning of explosive buildings and areas
Category A:
Buildings containing, or liable to contain, explosives which produce flammable vapours, but not explosives dust
n Category A zone 0:
An area in a category A building in which a flammable gas or vapour and air mixture is continuously present, or is present for long periods
n Category A zone 1:
An area in a category A building in which a flammable gas or vapour and air mixture is likely to occur during normal working.
n Category A zone 2:
An area in a category A building in which a flammable gas or vapour and air mixture is not likely to occur in normal operation and if it occurs it will only occur for a short time.
Category B:
Buildings containing or liable to contain exposed explosives or explosives which may give rise to an atmosphere of explosives dust, but not flammable vapour.
n Category B zone 0:
A place in which explosive atmospheres in the form of a cloud of explosives dust is present continuously, or for long periods or frequently.
n Category B zone 1:
A place in which explosive atmospheres in the form of a cloud of explosives dust is likely to occur in normal operation occasionally.
n Category B zone 2:
A place in which explosive atmospheres in the form of a cloud of explosives dust is not likely to occur in normal operation but, if it does occur, will persist for a short period only.
Category C:
Buildings containing or likely to contain explosives that do not give rise to flammable vapours or explosive dusts.
Category D:
Buildings containing or likely to contain explosives that is contained and unexposed.
References
[1] Cocroft, W. D. (2000) Dangerous energy, the archaeology of gunpowder and military explosives manufacture, English Heritage
[2] HSE, (2006) A guide to the control of major accident hazards regulations 1999 as amended (L111), 2nd edition, London, HMSO
[3] HSE, (2005) Manufacture and storage of explosives approved code of practice and guidance (L139), London, HMSO
[4] HSE, (1997) The selection, installation and maintenance of electrical equipment for use in and around buildings containing explosives (PM82), London, HMSO
[5] http://www.cranfield.ac.uk/dcmt/cotec/index.jsp
The author
Graham Park MIET is with BAE Systems, Land Systems, Munitions at Glascoed, Usk Monmouthshire
mailto: graham.park@baesystems.com