Where toxic dusts are handled, in most cases occupiers will need to carry out testing of the product for its explosion properties. Companies able to undertaken such testing are listed in the IChemE's book on the prevention of dust explosions. There is no legally defined test for an explosible dust. However, for many years we have used a small-scale screening test, the vertical tube test, described in HSG 2. The issues about representative samples of dust, and other factors that might cause the results to vary are also discussed in this guidance.
For most chemical products it is preferable to test dust taken from the process, but if the particle size distribution varies, it is common to test material that passes a micron sieve, and take this as the worst case. Ignition due to a hot surface is possible, but the temperature needed to ignite a dust layer depends on layer thickness and contact time. For COMAH sites with toxic dusts, the most likely hazard would arise in drying processes, if substantial quantities were held for extended periods hot enough to start self heating or smouldering combustion.
Existing codes of practice provide information with respect to good practice for hazardous area classification. The standards detailing selection of appropriate electrical apparatus have been updated to take into consideration ventilation effects. European equipment standards may become 'harmonised' when a reference to them is published in the Official Journal of the European Community.
Equipment built to such a harmonised standard may assume automatic conformity with those essential safety requirements of relevant directives that are covered by the standard. The EPS regulations describe the conformity assessment procedures that apply to different types of equipment. The most recent general source of advice was drafted by a European standards working group, but was published in the UK as BS PD R and not as a full standard.
It contains much useful advice about limiting pumping speeds, electrostatic risks from clothing, and many detailed operations. The two parts of the older BS The two parts are: This was a study led by a consortium of the chemical, electrical and mechanical engineering institutes, and showed how the subject spanned the traditional divides.
Electrical equipment in hazardous areas - Wikipedia
It was important in the development of ideas, but provides no new methodology for users. Dust Explosion Prevention and Protection: A practitioner's handbook - Electrical installation and maintenance in potentially explosive atmospheres , Publication No. Skip to content Skip to navigation. Health and Safety Executive. A - switch to normal size A - switch to large size A - switch to larger size. Hazardous Area Classification and Control of Ignition Sources This Technical Measures Document refers to the classification of plant into hazardous areas, and the systematic identification and control of ignition sources The relevant Level 2 Criteria are 5.
General Principles Hazardous Area Classification for Flammable Gases and Vapours Area classification may be carried out by direct analogy with typical installations described in established codes, or by more quantitative methods that require a more detailed knowledge of the plant. Zoning Hazardous areas are defined in DSEAR as "any place in which an explosive atmosphere may occur in quantities such as to require special precautions to protect the safety of workers".
Hazardous areas are classified into zones based on an assessment of the frequency of the occurrence and duration of an explosive gas atmosphere, as follows: An area in which an explosive gas atmosphere is present continuously or for long periods; Zone 1: An area in which an explosive gas atmosphere is likely to occur in normal operation; Zone 2: An area in which an explosive gas atmosphere is not likely to occur in normal operation and, if it occurs, will only exist for a short time. The most common values used are: A hazardous area extent and classification study involves due consideration and documentation of the following: The flammable materials that may be present; The physical properties and characteristics of each of the flammable materials; The source of potential releases and how they can form explosive atmospheres; Prevailing operating temperatures and pressures; Presence, degree and availability of ventilation forced and natural ; Dispersion of released vapours to below flammable limits; The probability of each release scenario.
Classification of the hazardous area as in zones shown in the table above ; Temperature class or ignition temperature of the gas or vapour involved according to the table below: Ignition Sources - Identification and Control Ignition sources may be: Electromagnetic radiation of different wavelengths Vehicles, unless specially designed or modified are likely to contain a range of potential ignition sources Sources of ignition should be effectively controlled in all hazardous areas by a combination of design measures, and systems of work: Using electrical equipment and instrumentation classified for the zone in which it is located.
New mechanical equipment will need to be selected in the same way. Lightning Protection Protection against lightning involves installation of a surge protection device between each non-earth bonded core of the cable and the local structure. Vehicles Most normal vehicles contain a wide range of ignition sources. Factors for Assessor of a Safety Case to Consider Is a full set of plans identifying hazardous areas available? For a large site they need not all be provided in the report, but those examples relevant to the representative set of major accidents upon which the ALARP demonstration is based must be included.
Have all flammable substances present have been considered during area classification, including raw materials, intermediates and by products, final product and effluents? Commonly these will be grouped for the purposes of any area classification study. Locations where a large release is possible and the extent of hazardous areas has been minimised by the use of mechanical ventilation should be identified, e.
Some reference to design codes, and commissioning checks to ensure the ventilation achieves the design aim, should be provided. The consequences of a loss of power to the system should be included in any section looking at other consequences of power loss. Have appropriate standards been used for selection of equipment in hazardous areas?
Electrical Installations in Hazardous Areas
Existing plant will not meet the formula in DSEAR, but older standards distinguished between electrical equipment suitable for zones 0, 1 and 2. Does the report identify old electrical equipment still in service in a hazardous area, and what assessment has been made to ensure it remains safe for use? Explosion prevention and protection. Basic concepts and methodology. Factors that could be considered during an on site inspection If there are any large areas of zone 1 on the drawings, is there evidence that by design and operation controls, the sources of release and consequently the location and extent of hazardous areas have been minimised?
Do any zone 2 areas extend to places where the occupier has inadequate control over activities that could create an ignition source, or is there any suggestion that the zone boundaries have been arbitrarily adjusted to avoid this? Has ignition protected electrical equipment been installed and maintained by suitably trained staff. Are the risks from static discharges controlled properly? Earthing of plant, drums and tankers is the most basic requirement; other precautions are described in the references What control measures over ignition sources are adopted in hazardous areas during maintenance; where ignition sources must be introduced, typical precautions include the use of supplementary ventilation, portable gas detectors, and inerting of sections of plant.
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The explosibility of dusts is dependent upon a number of factors: Status of Guidance Existing codes of practice provide information with respect to good practice for hazardous area classification. Appendix 3 describes the requirements for hazardous area classification. The use of BS EN Model Code of practice in the Petroleum Industry' Part 15 are recommended. It suggests all drum stores should be zone 2, to a height 1m above the stack. Discussions with industry on the relaxation of this in particular circumstances are ongoing. HS G 71 2 Chemical warehousing: This contains very limited information on hazardous area classification or control of ignition sources HS G 2 Safe handling of combustible dusts: This is aimed mainly at small scale handling, with containers of litres or less.
HS G 2 Formula for health and safety: The guidance describes the requirements for hazardous area classification, and gives some typical examples. These should now be seen as rather conservative. This is basic level guidance, and COMAH reports should normally reference more specific publications, such as the other HSG series books listed, and other items in this list.
Paragraphs 35 to 39 describe the requirements for hazardous area classification. This cross references BS EN Institute of Petroleum Model Code of Safe Practice, part 15, area classification for installations handling flammable fluids, 2nd edition Appendix 2 describes the requirements for hazardous area classification. Contains useful information about electrostatic hazards during unloading.
Design, installation and operation of vessels located above ground, LP Gas Association, LPGA codes have not previously drawn a clear distinction between hazardous areas, and separation distances required for other reasons. These are currently under revision, and will specify hazardous areas, that in most cases will be smaller than the separation distance.
Model Code of practice in the Petroleum Industry' Part 15 is recommended. The guidance also recommends that zones be recorded in a plan to prevent sources of ignition being brought in. Equipment built to older standards, including purely national standards may remain in service, provided it is properly maintained. The IEC range of standards also includes documents on selection, installation and maintenance of equipment for use in explosive atmospheres.
Non-electrical equipment The first standard for explosion protected non-electrical equipment is BS EN part 1 1. It describes requirements for "Category 3" equipment. Further parts of this standard are well advanced and will appear during Basic concepts and methodology, British Standards Institution. This gives additional general advice on the many of the issues covered in this TMD.
Electrostatic ignition risks The most recent general source of advice was drafted by a European standards working group, but was published in the UK as BS PD R and not as a full standard. The different parts of this standard set out requirements for construction of equipment for use in atmospheres containing explosive dusts; information about selection and maintenance; and BS EN Code of practice for protection of structures against lightning, British Standards Institution.
Section 9 provides guidance on lightning protection of structures with inherent explosive risks. The inductance of the signal bell coils, combined with breaking of contacts by exposed metal surfaces, resulted in sparks which could ignite methane, causing an explosion. In an industrial plant such as a refinery or chemical plant , handling of large quantities of flammable liquids and gases creates a risk of leaks.
In some cases the gas, ignitable vapor or dust is present all the time or for long periods.
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Other areas would have a dangerous concentration of flammable substances only during process upsets, equipment deterioration between maintenance periods, or during an incident. Refineries and chemical plants are then divided into areas of risk of release of gas, vapor or dust known as divisions or zones.
The process of determining the type and size of these hazardous areas is called area classification. Guidance on assessing the extent of the hazard is given in the NFPA or NFPA standards published by the National Fire Protection Association for explosive gas or dust atmospheres respectively, or RP and RP standards published by the American Petroleum Institute for explosive gas or dust atmospheres respectively, and IEC or IEC standards published by the International Electrotechnical Commission for explosive gas or dust atmospheres respectively.
The principles of the NEC Division and Zone classification systems are applied in countries around the globe, such as in the United States.
The NEC Zone classification system was created to provide multinational companies with a system that could be harmonized with IEC classification system and therefore reduce the complexity of management. Canada has a similar system with the Canadian Electrical Code defining area classification and installation principles.
Typical gas hazards are from hydrocarbon compounds, but hydrogen and ammonia are common industrial gases that are flammable. Flammable dusts when suspended in air can explode. An old system of area classification to a British standard used a system of letters to designate the zones. The boundaries and extent of these hazardous locations should be decided by a competent person. There must be a site plan drawn up of the factory with the divisions or zones marked on. Explosive atmospheres have different chemical properties that affect the likelihood and severity of an explosion. Such properties include flame temperature, minimum ignition energy, upper and lower explosive limits, and molecular weight.
Empirical testing is done to determine parameters such as the maximum experimental safe gap MESG , minimum igniting current MIC ratio, explosion pressure and time to peak pressure, spontaneous ignition temperature, and maximum rate of pressure rise. Every substance has a differing combination of properties but it is found that they can be ranked into similar ranges, simplifying the selection of equipment for hazardous areas. Flammability of combustible liquids are defined by their flash-point. The flash-point is the temperature at which the material will generate sufficient quantity of vapor to form an ignitable mixture.
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The flash point determines if an area needs to be classified. A material may have a relatively low autoignition temperature yet if its flash-point is above the ambient temperature, then the area may not need to be classified. Conversely if the same material is heated and handled above its flash-point, the area must be classified for proper electrical system design, as it will then form an ignitable mixture. Group IIC is the most severe Zone system gas group.
Hazards in this group gas can be ignited very easily indeed.
The above groups are formed in order of how explosive the material would be if it was ignited, with IIC being the most explosive Zone system gas group and IIA being the least. The groups also indicate how much energy is required to ignite the material by energy or thermal effects, with IIA requiring the most energy and IIC the least for Zone system gas groups. Another important consideration is the temperature classification of the electrical equipment.
The temperature classification on the electrical equipment label will be one of the following in degree Celsius:. The auto-ignition temperature of a liquid, gas or vapor is the lowest temperature at atmospheric pressure at which the substance will ignite without any external heat source, such as a spark or flame. This is used for classification of temperature class for industry and technology applications. Such temperatures for common substances are:. Examples for common materials are:.
To ensure safety in a given situation, equipment is placed into protection level categories according to manufacture method and suitability for different situations. Category 1 is the highest safety level and Category 3 the lowest. Although there are many types of protection, a few are detailed. Equipment has flameproof gaps max 0. A 'Zener Barrier', opto-isolator or galvanic unit may be used to assist with certification. The types of protection are subdivided into several sub classes, linked to EPL: The a subdivisions have the most stringent safety requirements, taking into account more the one independent component faults simultaneously.
Many items of EEx rated equipment will employ more than one method of protection in different components of the apparatus. These would be then labeled with each of the individual methods. The required Protection level is linked to the intended use in the zones described below:. All equipment certified for use in hazardous areas must be labelled to show the type and level of protection applied.
The CE marking is complemented with the Ex mark, followed by the indication of the Group, Category and, if group II equipment, the indication relating to gases G or dust D. Industrial electrical equipment for hazardous area has to conform to appropriate parts of standard IEC for gas hazards, and IEC for dust hazards, and in some cases, be certified as meeting that standard.
Independent test houses known as Notified Bodies are established in most European countries, and a certificate from any of these will be accepted across the EU. In North America the suitability of equipment for the specific hazardous area must be tested by a Nationally Recognized Testing Laboratory. The label will always list the Class es , Division s and may list the Group s and temperature Code. Directly adjacent on the label one will find the mark of the listing agency. All equipment in Division 1 areas must have an approval label, but certain materials, such as rigid metallic conduit, does not have a specific label indicating the Cl.
Some equipment in Division 2 areas do not require a specific label, such as standard 3 phase induction motors that do not contain normally arcing components. Also included in the marking are the manufacturers name or trademark and address, the apparatus type, name and serial number, year of manufacture and any special conditions of use.
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