Europe and most countries from Asia and Africa have been influenced by the system of approximation to three Zones from the International Electrical Engineering Commission (IEC), these standards separate explosive atmospheres based on probability of occurrence and on the time in which the potentially explosive mix is present; the equipment to be used in these zones are examined and approved in the testing facilities approved by the European Committee of Electrical Engineering Standardization (CENELEC in Spanish), using the European Standards (EN in Spanish). North America, Canada, and South American countries have used the class and Division system, where the class identifies the risk of the presence of Gas, vapor or fog and flammable powder or fibers; the Division defines the normal or abnormal condition in which the risky material may be present, this is defined by the National Electric Code (NEC 501).
The IEC defines the explosive atmosphere zones for Gas, vapor or fog as:
For powder or fiber in:
An explosive atmosphere is a mix with air, in atmospheric conditions, of flammable substances in the form of gas, vapor, fog, or powder, where after an ignition combustion may propagate towards the unburnt mix.
The term is not applicable when the risk of explosion comes from unstable substances, such as explosives and pyrotechnic substances, or when the explosive mix is out of what is understood as normal atmospheric conditions, reason why it excludes processes in hyperbaric conditions. This means that for processes in conditions exceeding the atmospheric ones, complying the requirements of guidelines on explosive atmospheres known as ATEX, is not a guarantee of being in safe conditions.
We understand atmospheric conditions as those where temperature is between -20 º C and 60 º C, and pressure is in the rank 0,8 bar to about 1800 meters.
For an explosion to take place the explosive atmosphere and an ignition focal point must coincide. This requires the existence of a flammable substance (gas, vapor, fog or powder), of an oxidant (air) in a certain concentration range, and at the same time the presence of an energetic source capable of initiating the reaction.
The interval of concentration determined between combustible substance and oxidant is limited by the Explosiveness lower Limit (LEL) and the Explosiveness upper Limit (UEL), which is given in a percentage form and depends of each flammable substance when we work with gas, vapor or fog, and in the case of powder the limits are given by the Minimum Explosible Concentration (MEC) and the Explosiveness Maximum Concentration.
Minimum energy for combustion: it is the energy we must supply to an explosive atmosphere for ignition to occur. The basic parameters on explosive atmospheres due to the presence of flammable powder or fiber are:
- Explosiveness minimum concentration: this equals the limit of explosiveness of gases.
- Explosiveness maximum concentration: this equals the limit of explosiveness of gases.
- Minimum temperature of ignition to cloud (AIT): this equals the combustion temperature.
- Minimum Temperature of ignition in layer (TIC in Spanish):This equals the maximum superficial temperature.
- Minimum Energy of ignition: This equals the minimum energy for combustion
- Maximum concentration of oxygen allowed preventing ignition: this is the maximum concentration of oxygen where no explosion of flammable powder occurs.
- Maximum pressure of explosion: Maximum pressure reached during the explosion.
- Maximum gradient of pressure. Speed of growth of the pressure. It gives us an idea of the force of the explosion.
Examples of flammable substances and relevant data:
Regulations for the classification of zones procedures that may be taken as general; the first step is to get to know and analyze the characteristics of the material. In gases: density, flash point, explosiveness limits; for powders: size of the particle, powder humidity, minimum temperature of ignition in cloud and layer and its resistance. Sorting of zones is based on the determination of the presence of zones of escape and accumulation of powder and on the probability for explosive mixes of gas/air or powder/air to form. Therefore, for determining the zones it is necessary to:
- Identify the locations: Identify the locations and their different characteristics.
- Identify the flammable substances: All present flammable substances and their main characteristics should be identified.
- Identify the sources of escape: sources of escape should be identified, and the possibility of eliminating them or of limiting the volume of escape as much as possible should be verified.
- Degree of escape: for each source of escape the frequency and duration (degree of escape) of emission of flammable substances should be established, and the possibility of eliminating or limiting as much as possible the escapes in a continuous degree and primary degree should be corroborated, or at least it should be corroborated whether or not it is possible to reduce the amounts.
- Analyze the influences of all the escapes: the influences of all the escapes should be analyzed over the sorting of the place, considering their degree and bearing in mind especially the parts of the assembly with high concentration of escape sources, which could give room to simultaneous emissions and influence each other reciprocally.
- Calculate the escape rate: for each source of escape a rate of escape should be calculated or estimated, always taking security precautions.
- Define the degree and availability: for each place reference values of the environment temperature and the characteristics of the ventilation (degree and availability) are defined.
- Determine the type of dangerous zone: for each source of escape the type of dangerous zone should be determined.
- Calculate the extension of the zone: for each source of escape the extension of the zone should be calculated
Sorting of the dangerous place: the sorting of the dangerous place is obtained from the group of individual dangerous zones.
The basic parameters on explosive atmosphere due to the presence of gases, vapor or fog for liquid fuels or gas are:
- Explosiveness range: in order for the atmosphere to turn explosive, the concentration of the aforementioned elements should be within a range. Under or over this range it cannot be considered as such. The range is determined by the limits of explosiveness.
- Explosiveness Inferior Limit (LIE in Spanish): it is the minimum concentration of flammable gas, vapor or fog in the air under which the mix is not explosive.
- Explosiveness Superior Limit (LSE in Spanish): it is the maximum concentration of flammable gas, vapor or fog in the air over which the mix is not explosive.
- Combustion temperature or flash point: it is the temperature at which the release of vapors is enough for combustion to occur due to the supply of energy to an external focal point.
- Ignition or self-ignition temperature: at this temperature the mix makes a spontaneous explosion. It does not need a source of external energy for the ignition to occur.
- Maximum superficial temperature: the maximum temperature a material may reach without becoming a focal point of ignition for the explosive atmosphere around it.
Explosion risks may appear in any company where flammable substances are handled. Among these are numerous raw materials, intermediate products, finished products, and residual material from Industriel processes.
The risk of forming an explosive atmosphere exists in processes and procedures of more diverse operations, therefore it affects almost all branches of Industriel activities.
1.1 Industries in which can be generated Explosive Atmospheres.
1.1.1 Chemical Industrie: In the chemical Industrie, flammable gases, liquids and solids are transformed and used in several of the processes. Plants for production and manipulation of brimstone. Operation, handling and storage areas. Places where flammable volatile liquids are transferred from one recipient to another. Places with storages of flammable liquids that are or may be opened. Rooms for flammable gas or liquid pumps or compressors.
1.1.2- Generation of energy: Biomasses and other solid fuels are explosive. Refrigeration of alternators with H2 implies a risk of explosion.
1.1.3- In the treatment of waste water: Digestion gases generated in the treatment of waste water in depurators may form explosive gas/air mixes. Dry mud is also explosive.
1.1.4- In the supply of gas: In case of escapes of natural gas as a result of a leak or similar event, explosive gas/air mixes may be formed.
1.1.5- Industrie of work with wood: When working with wood pieces, wood dust is generated, which may form explosive powder/air mixes in filters or silos. Industries of wood processing, such as carpentries, saw mills, etc.
1.1.6- Industrie of paint and enamel: the fog resulting from the pulverization formed during the enameling of surfaces with paint pistols in lacquering cabins, as well as the vapors of released solvents, may provoke a very explosive atmosphere in contact with air. Powdery pigments may be very explosive. Areas inside cabins for painting with pulverization pistols and the nearby area.
1.1.7- Manufacture of pieces made of light metals and metallic carpentry: In the manufacture of light molding metal pieces, its surface treatment (grinding) may generate explosive metallic powders, especially in the case of light metals (aluminum, titanium, magnesium, etc.). These explosive powders may provoke explosion risks in separators and other operations. Areas where metallic powders made of light material (aluminum, magnesium, etc.) are produced, processed, handled, or packaged.
1.1.8- Agricultural facilities: In some agricultural exploitation facilities biogas generation areas are used. In case of leaks, explosive biogas/air mixes may be formed. Fodder dehydrators, almond or peanut peelers and other similar facilities generate explosive atmospheres. Storage and use of fertilizers (ammonium nitrate).
1.1.9- Vehicle repair: Normally, the amount of flammable products is reduced, and confinement and ventilation make this sorting unnecessary. However, as a general note, the possibility of formation of explosive atmospheres may be analyzed if there are significant amounts of flammable materials in garages and vehicle repair workshops.
.1.10- Impowderrial car wash and laundry places, where flammable liquids are stored and used.
1.1.11- Food inpowderria: transport and storage of flours, grains and derivatives may generate explosive powders. If these are vacuumed and separated in filters, an explosive atmosphere may appear in the filter. Places where fats and oils are extruded, where flammable solvents are used. Drying places for material with flammable solvents. Work, handling, and storage areas. Among flammable powders are flour and derivatives, starch, sugar, cocoa, powdered milk and powdered egg, spices, etc. Bread flour plants. Manufacture of bread and pastry products.
1.1.12- Pharmaceutical inpowderria: In pharmaceutical production, alcohols are often used as solvents. Active and ancillary explosive substances may also be used, such as lactose, vitamins, paracetamol, etc. Work, handling, and storage areas.
1.1.13- Refineries: Hydrocarbons handled in refineries are all flammable, and, according to their ignition point, they may provoke explosive atmospheres even at room temperature. The area of petrol transformation equipment is almost always considered an explosion risk zone.
1.1.14- Waste recycling inpowderrias: recyclable waste treatment may entail explosion risks resulting from containers not fully emptied from their flammable gas or liquid content, or for paper or plastic material powder.
1.1.15- Textile and similar inpowderria: Storages and delivery docks (bags or containers). Textile treatment areas, such as for cotton. Fiber Manufacture and processing plants. Cotton mills. Linen processing plants. Clothing factories.
1.1.16- Farming inpowderrias: Manufacture of animal feed and its composing elements for livestock. Manufacture of vitamin/mineral correctors. Silos for storage of cereals. Among flammable powders are cereals, grains and derivatives, starch, hay, etc. drying places and dehydrators for cereals. Manufacture of paper and cellulose.
1.2 – Ignition focal points
Explosive atmospheres should always be evaluated and checked for; as well as for possible focal points for ignition, or the reactivation of these focal points. This is defined as anything that is capable of generating energy that will raise the temperature of a gas, vapor or fog, to the temperature of combustion, and in the case of powders to the minimum ignition temperature.
2- Lightning as ignition sources.
In the lightning formation process, the most important five instants of the discharge are when we may conclude whether its values put it into the ignition focal point category and more risk, if we are in the presence of explosive atmospheres in Zone 0 as well as in Zones 1 and 2 in the case of gases, and of Zone 20, and Zones 21 and 22 in the case of powder.
2.1- First instant: electrostatic charges during the formation of the leader. At the moment of the presence of electric shadow on ground, generated by the cloud, the electric field present in the structures goes from low tension values to very high tension values; at that moment the point effect may appear anywhere, especially in the highest points of the area. This effect is visually transformed into sparks coming from materials exposed to the electric shadow. In the case of a lightning rod, the electrostatic charges generate interferences and noises that may connect to the data lines or TV and radio signals. During this electrical phenomenon, currents with a minimum of 150 Amp circulate through the ground wire of the lightning rod. Why? Because the point effect sparks appear after the ionization of the air, and a minimum of 1,500 volts are needed at the point of an electrode (depending on the quality of the air) in order to ionize the air; if we apply Ohm’s Law and take the 1,500 volts as tension reference (V) and the 10 Ohms of the resistance as a maximum of the ground connection (R), we will have a current intensity (I) that will circulate through the wire as in: I = E / R, 150 A = 1.500 V / 10 Ohm.
2.2- Second instant: Electrostatic pulses (ESP). Electrostatic pulses are atmospheric transitory, and they appear as a result of the sudden variation of the electrostatic field present in the zone during the storm; the cause of this phenomenon is generated by the difference of potential between the cloud and the ground. Lightning Strikes near an installation generates induced currents in the earthing system, overloading the equipment electrically since they are connected to ground It is also worth mentioning that anything that is suspended in the air in reference to the ground within the electric shadow, will be electrically charged with a tension proportional to its height and the electrostatic field present, as though it was a condenser. As a reference, at 10 meters high, the data lines or telecommunications isolated from the ground, may receive tensions from 100 to 300,000 volts in reference to the ground, within a medium electrostatic field, and tensions or electric arcs may appear in the ground mesh that make a shadow for the wires in reference to the ground
2.3- Third instant: Electromagnetic pulses (EMP). At the very instant of the lightning impact on a lightning rod or in any element, the physical contact of the lightning’s energy at the point of contact generates a spark that transforms into an electromagnetic pulse that travels through the air; at the same instant the flow of the current circulating through the electric conductor to earth to the ground connection, generates a magnetic field proportional to the intensity of the lightning discharge current and inversely proportional to the distance where one wants to measure such magnetic field in units A/meters; the formula to use for this calculation is I / 2 x Pi x d. The energy radiated by the electromagnetic pulse in the air travels at the speed of light, inducing by contact everything in its way in reference to the ground, in a 1,500 meter radius. The intensity of the electromagnetic pulse varies according to the intensity of the lightning discharge and of the point of physical contact with the impacted element, the time of transfer of the current to the ground and the level of absorption of the physical ground; this will determine the electric contact values.
2.4- Fourth instant: Over-tension and passage tensions during the lightning’s impact. Its direct impact on conductive elements generates a current wave that propagates upstream and downstream, creating an over-tension of high energy. These conductive elements (service lines, data, telecommunications, energy, gas, water, etc.) suffer consequences such as material destruction, premature aging, breaking of equipment and/or conductive elements connected to such service lines, with a danger of fire. In the case of mid-intensity lightning (40,000 A), the coupling in equipment that is not connected to the same ground connection, or floating, will have the risk of electric arcs appearing that will jump among masses of different potential during the instant of lightning discharge nearby; the tension values that may appear are over 400,000 Volts.
2.5- Fifth instant: Earth currents. According to the intensity of the lightning’s discharge on the lightning rod, the ground connections do not get to absorb the whole potential energy discharged in less than 1 second, thus generating electric returns through the ground connections into the interior of the electrical installations. This phenomenon may generate dangerous passage tensions. Another phenomenon that impacts ground tensions is the difference in the potential among masses or ground electrodes nearby the lightning’s impact; when it strikes all the aforementioned phenomena interact among themselves and tend to discharge on the ground, according to the distance between electrodes a resistance will be generated that is typical of the semi-conductive (the chemical composition of the physical ground), and dangerous passage tensions will appear among electrodes
The operative life of ground electrodes and wires is shortened after each lightning discharge in a lightning rod, and they suffer a loss of material because of the sudden ion exchange in each energy transfer process. Each lightning impact on a lightning rod generates a very high current leak that goes through the copper wire to the ground electrode in order to dissipate on physical ground; at that moment a ion exchange or natural electrolysis is generated between the electrode’s material and the physical ground; this high and instant ionic exchange reacts with the surrounding, creating a crystallization of the physical ground, and a degradation and oxidation of the metal electrodes. Each lightning discharge evaporates the water that the ground has around, modifying the resistance of the ground connection and enhancing the risk of appearance of tensions in the next discharge. In time, the electrodes that are used as earthing disappear; already in their first year of life they lose physical contact with the ground and their transfer ability diminishes because of the oxidation. We need to bear in mind that all materials or points of contact with the ground have different electric behavior values, their own resistance as electrical conductive elements may considerably vary according to the conditions surrounding them (humidity, temperature, chemical contamination, etc.). Maintenance and yearly revision of the electric ground connections are mandatory in order to guarantee a good dissipation of the current leaks, and the electric safety of workers and equipment.
Orientating values related with the lightning phenomenon
3- Explosive Atmosphere zones, some examples
3.1- Loading of a tanker truck
Zone 0: Inside the tank.
Zone 1: 1.5 mts around and above the loading openings, down to the ground level. The same distance will be taken for the flexible coupling of the loading arm. Inside the draining channel (if this channel or pit was in a zone already sorted as zone 1, a superior level should be considered because of the lack of ventilation, that is, zone 0).
Zone 2: 1.5 meters around zone 1. 1.5 meters around and over the possible leak, that will be located under the tank and around the draining channel.
Remark 1: When vapor recovery systems are used, the extensions of the zones may be reduced.
Remark 2: The inside of the deposits is considered zone 0 because of the liquid-air interface. If there is a procedure, interlock or control system that will guarantee a minimum level or liquid, below that level may be considered a non-sorted zone, as there will always be just liquid, never air, and consequently an explosive atmosphere may not be formed.
3.2- Unloading of a tanker truck
Zone 0: Inside the tank. Inside the underground deposit.
Zone 1: 1.5 meters around and above the unloading openings, down to the ground level. The same distance will be taken for the coupling of the hose to the deposit’s entry opening. Although superior loading openings are not opened during the unloading, the surroundings should be sorted as zone 1. If there was a draining channel it would be considered zone 1, except if it is under the zone already sorted as 1, in which case the channel or pit would be zone 0.
Zone 2:1.5 meters around zone 1. 1.5 meters around and above the possible leak, that will be located under the tank and around the loading opening going to the deposit.
Remark 1: When vapor recovery systems are used, the extensions of the zones may be reduced.
Remark 2: The inside of the deposit is considered zone 0 because of the liquid-air interface. If there is a procedure, interlock or control system that will guarantee a minimum level or liquid, below that level may be considered a non-sorted zone, as there will always be just liquid, never air, and consequently an explosive atmosphere may not be formed.
3.3 Flour Silos
Zone 20: Inside the transportation pipes, of the silos and the arms.
Zone 21: 1m around and above the depressurizing arms.
Zone 22: 1m around and above the pipe of vent exit.
3.4. Vent channels of underground deposits for liquid fuel
Zone 0: Inside the vent channel.
Zone 1: A 1m sphere with a center at the top part of the vent is taken.
Zone 2: A 2m sphere with a center at the top part is taken
Remark 1: The amount of fuel inside the storage should be considered. Note the load status and the existence of interlocks or controls of the minimum liquid amount, for enhancing zones 1 and 2.
3.5. Pumps in Liquid and Gas service stations
Zone 0: Inside the conduits.
Zone 1: In the pump itself.
Zone 2: 1m around zone 1 and up to the pump’s height and the height of its hoses column.
Remark 1: The previous definitions and loading statuses should be considered.
4- Accidents resulting from explosive atmospheres; ignition focal point: lightning
This information was obtained and translated from the internet page about ARIA Accidents “Accidentologie foudre les accidents à l’étranger“ and pertain to a selection of two per year, in years 2000, 2001, and 2002.
03/01/2000 – JORDAN. Oil refinery. The lightning impacted on 2 deposits containing 33,000 lts. of petrol in a refinery and initiates a fire. Damages are minor and the refinery operates normally: the fuel-oil treatment units were not affected, as a result of the fire being speedily controlled. There is no calculation of damages.
11/05/2000 – United States. Chemical base inpowderria. The lightning strikes an electric transformer in a chemical product factory. Fire develops in a storage area containing methanol, solvents, and other dangerous matter. Witnesses tell about explosions after the initial impact. The fire provokes a black smoke visible several kilometers away. Inhabitants from the riverside are evacuated in an 800 meter radius; approximately some 200 people. Three fire brigades work for 3 hours in order to control the fire. The deposit is completely destroyed. A brigade specialized in dangerous material intervenes to help the firefighters. There is no calculation of losses.
06/67/2001 – United States. Oil Refining. In a refinery, a storage with a capacity of nearly 43,000
m3 and containing 24,000 m3 of gas-oil was struck by lightning, which started a fire in the storage.
07/19/2001 – RUSSIA – Chemical products manufacture. A fire starts in a storage as a result of lightning; three people are dead, economic damage was not evaluated.
03/28/2002 – MOROCCO – Manufacture of diverse mineral products. Lightning impacts on a storage containing flammable products in a cobalt production factory (1,500 T per year). There is no calculation of losses.
05/05/2002 – POLAND – Oil refinery. In a refinery, lightning provokes fire in a container with 2,000 T of oil. There is no risk of fire propagating to other storages. As a precaution, neighbors are evacuated. Damages account for a wounded firefighter.
1- Page www.int-sl.ad , Articles written by Mr. Angel Rodríguez Montes
2- Articles written for the Ministry of Work and Immigration, National Institute of Work Safety and Hygiene, Spanish Government.
3- Royal Decree 681/2003 and BOE nº 145.
4- Guidelines on ATEX
5- Norms EN60079-10
6- Norms IEC 60079-10
7- Norms NEC 501
8- Web page from the company Emerson Inpowderrial Automation