Sources of ignition
Sources of Ignition
Explosion hazard only exists if, in addition to the presence of an explosive mixture such as gas with air, an ignition source is present. In fact, there are only two ways to ignite an explosive mixture:
- By a spark (a chemical reaction also reacting with the surrounding explosive mixture);
- By a hot surface (molecules of the explosive mixture absorb so much energy they disintegrate and start to react with each other).
Most common sources of ignition
The ways in which these sparks or hot surfaces are generated are very different and can be classified into thirteen categories according to EN 1127-1:
- Hot surfaces;
- Flames and hot gases;
- Mechanically generated sparks;
- Electrical equipment;
- Stray currents and cathodic corrosion protection;
- Static electricity;
- Lightning, thunderstorms;
- Electromagnetic (radio) waves;
- Electromagnetic (optical) waves;
- Ionizing radiation;
- Ultrasonic noise;
- Adiabatic compression and shock waves;
- Exothermic chemical reactions).
The first seven ignition sources are the most common and should always be taken into account. In addition, for dust and gas environments, optical waves and exothermic reactions must be taken into account. The other ignition sources (radio waves, ionizing radiation, ultrasonic noise and adiabatic compression) are rarely present in installations.
Hot surfaces as potential source of ignition
If an explosive gas mixture comes into contact with a hot surface, the gas mixture can heat up and ignite locally. Combustion then spreads throughout the whole mixture.
In an explosive dust environment, a hot surface will heat up the flammable dust lying on top of it. As soon as anywhere in the dust layer the so-called smoldering or glowing temperature is reached, the dust layer will ignite there. The rest of the dust, present as either solid dust or dust cloud, will then ignite too.
Whether a hot surface can ignite an explosive atmosphere easily or not depends on:
- The type of gas or dust;
- The concentration of gas or dust;
- The temperature of the surface;
- The size of the hot surface;
- The shape of the surface (smooth or rough);
- The type of surface material;
- The rate of movement of the explosive atmosphere;
- For dust: the thickness of the dust layer.
A thicker layer of the dust will have two effects: the minimum ignition temperature decreases and the insulating effect increases. A lot of equipment has been tested dust-free and a maximum dust layer thickness of 5 mm must be assumed (the thickness of the dust deposit is not indicated). If the layer thickness of the dust could be more than 5 mm then the maximum permissible surface temperature should be reduced according to the graph below.
Graph maximum surface temperature versus layer thickness
Surface temperature ignition source
If the smoldering temperature at a layer thickness of 5 mm (T5) is less than 250 °C additional laboratory tests should be carried out to determine the exact smoldering temperature at the indicated layer thickness. The same applies to layer thickness over 50 mm.
Due to the effects mentioned above, values for minimum ignition temperature should be used with caution. The minimum ignition temperature can be found in professional literature and in chemistry charts.
| Zone | Surface temperatures |
| 0 |
Surface temperature must not exceed 80% of the auto-ignition temperature of the explosive atmosphere present, even in the case of rare malfunctions.
|
| 1 | Surface temperature must not exceed the self-ignition temperature of the explosive atmosphere present, even in case of expected malfunctions. If heating up to surface temperature of the gas or vapor cannot be avoided, the temperature of the hot surface must not exceed 80% of the auto-ignition temperature of the explosive atmosphere present. This temperature may only be exceeded in case of rare malfunctions. |
| 2 |
During normal operation, the surface temperature must not exceed the auto-ignition temperature of the explosive atmosphere present.
|
| 20 |
Surface temperature, even in case of rare malfunctions, must not exceed:
|
| 21 |
Surface temperature, even in the case of foreseeable malfunctions that normally have to be taken into account, must not exceed:
|
| 22 | Surface temperature during normal operation must not exceed:
|
Table 7.1 Zone and permissible surface temperatures
The previous graph clearly shows that the permissible surface temperature strongly depends on the thickness of the dust layer.
Fire ignition zones (Flames and hot gases)
Flames and hot gases ATEX zones
Flames occur during combustion reactions and the temperature may vary between 300 °C and 1100 °C. These reactions produce both hot gases and glowing solid particles. Flames, even very small ones, are among the most active ignition sources and are very capable of igniting an explosive atmosphere.
| Zone | Measures against flames |
| 0 / 20 |
In general, all flames should be avoided in zone 0 and 20. Combustion gases or other hot gases are not permitted unless adequate measures have been taken, e.g., the gases must be cooled to below the ignition temperature of the explosive atmosphere.
|
|
1 / 2 21 / 22 |
Enclosed fire and flames are permitted provided the enclosure temperature meets the requirements for hot surfaces. The casing must be fire resistant.
|
| 1 / 2 | In a zone 1 or zone 2 air may only be supplied to the fire provided that adequate measures have been taken to prevent e.g., a flashback to the zone. The temperature of hot gases must be below the ignition temperature of the explosive atmosphere. Combustion gases must be filtered and must not contain any hot particles which could ignite the explosive atmosphere. |
Table 7.2 Zone and measures with regard to flames
Open flames caused by welding or smoking must be prevented by organizational measures.
Mechanically generated sparks
Mechanical work (hammering, chopping, drilling, grinding and sanding) can produce loose particles that may be hot. If these particles consist of an oxidizable substance such as aluminum, iron or steel, the temperature can be increased even more by oxidation. Such hot particles may ignite an explosive atmosphere. Especially rust and light metals such as co, magnesium, titanium and zirconium are very reactive and can become very hot.
Grinding
When foreign bodies penetrate devices or installations they may cause sparks. Think of stones or pieces of metal in machines or filters. Flames during sanding orimpact sparks can be limited by choosing a suitable combination of materials (e.g., for fans).
Equipment generating mechanical sparks can never be used in an environment where the following highly reactive gases may be present, unless it has been established there is absolutely no risk of explosion:
- Acetylene;
- Carbon disulphide;
- Ethylene oxide;
- Carbon monoxide;
- Hydrogen;
- Hydrogen sulphide.
| Zone | Measures with regard to sparks |
| 0 |
Equipment generating mechanical sparks may not be used in zone 0. Friction must be avoided between aluminum or magnesium on one side and iron or steel on the other. Stainless steel is allowed because no rust particles will occur. Friction and impact between titanium or zirconium and any other hard material must be avoided.
|
|
1 |
If possible, the zone 0 requirement must be met. In any case sparks must be avoided during normal operation and when malfunctions occur.
|
| 2 |
Sparks must be prevented during normal operation.
|
|
20 / 21 / 22 |
Sparks should be avoided if possible, considering the risk of a dust cloud or dust layer being present. Since dust clouds require much more spark energy than gas clouds to ignite (by a factor 100 or more), the risk is much lower. |
Table 7.3 Spark zone and measures
Usage of Hand tools in hazard explosion atmosphers
Hand tools for use in hazardous areas are divided into two groups:
- Tools which, when used, produce only a single spark, e.g. a screwdriver;
- Tools which, when used, produce a shower of sparks, e.g. a grinding wheel.
Non-sparking screwdriver
| Zone | Measures |
| 0 |
Tools used in zone 0 should not be able to produce sparks at all.
|
|
1 / 2 |
Tools used in zone 1 or 2 must be made of steel and, when used, can only produce a single spark. Other tools may only be used when it has been established no explosive atmosphere is present.
|
| 1 |
In a zone 1 where one or more of the following gases may be present, steel tools cannot be used unless there is no atmosphere present at the workplace concerned during the work:
|
|
20 / 21 / 22 |
Sparks should be avoided if possible, considering the risk of the presence of a dust cloud or dust layer. Since dust clouds require much more spark energy than gas clouds to ignite (by a factor 100 or more), the risk is much lower. |
Table 7.4 Zone and use of hand tools
Electrical equipment as potential Source of Ignition
Electrical equipment can form all possible sources of ignition and this group is therefore seen as a separate ignition source. Consider for instance:
- Electrically and mechanically generated sparks;
- Hot surfaces;
- Stray currents.
Where electrical equipment is concerned, both the ignition sources present in the equipment and those generated by the equipment should be examined. Appropriate measures must then be taken to prevent each source from actually igniting an explosive atmosphere. See chapter 8 for various methods of protection.
For this reason, only existing electrical equipment complying with the requirements of Directive 1999/92/EC, Annex II, may be used in potentially explosive atmospheres. New equipment should be chosen based on the categories described in Directive 2014/34/EU (ATEX 114).
Stray currents and cathodic corrosion protection
Stray currents are currents unintentionally flowing through metal structures. Where there isa poor connection, this can lead to an increase in temperature. Sparks can occur when disconnection or connecting parts of installations where stray currents are flowing.
Stray currents can flow in electrically conductive systems as a result of:
- Return current in power generation or transport systems;
- Ground fault in electrical installations;
- Magnetic induction;
- Lightning;
- Cathodic protection;
- Electric welding when the ground terminal is not located close to the welding spot.
Cathodic protection using impressed current produces the same hazards (sparks, increased temperature). In case of passive protection by sacrificial electrodes, ignition by sparks is unlikely, unless aluminum or magnesium electrodes are used.
| Measures |
| All conductive parts must be protected in accordance with the applicable installation standard. |
| In the case of cathodic protection, insulators must be fitted to contain stray currents. |
Table 7.5 Measures with regard to stray currents
Static electricity as potential source of ignition
When an object, eitherin solid, liquid or gaseous form, moves against another object, it will acquire an electrostatic charge relative to the other object. This is due to the friction between the objects, where electrons show a preference for either one of the objects. Thus, an object may be positively or negatively charged.
Static electricity
If the charge becomes high enough, it will discharge by means of a spark. When the spark contains sufficient energy, it can ignite an explosive atmosphere. The following forms of discharge may occur under normal operating conditions:
- Spark discharges are caused by charging of non-earthed conductive parts;
- Brush discharges are caused by charged non-conductive parts like most plastics;
- Propagating brush discharges occur during fast separation processes, such as unrolling of film over rollers, pneumatic transport in metal pipes or containers with an insulating coating, driving belts, etc.;
- Corona discharges particularly occur in during pneumatic filling of silos.
Most gases, vapors, mists and dust/air mixtures can be ignited by any of these forms of discharge. Propagating brush discharges only need to be considered as a possible active source of ignition in case of dusts.
Static electricity is everywhere. It is impossible to prevent it completely, but it can be controlled. The most important measure is good grounding of all conductive parts of an installation. Charging of non-conductive parts, materials and persons present must be either prevented or reduced to a safe level.
Various techniques are available for this purpose, such as:
- Avoiding materials with low electrical conductivity;
- Reduction of non-conductive surfaces;
- Use of conductive coatings;
- Ionizing the air;
- Use of conductive footwear on a suitable floor (maximal mutual resistance of 108Ω).
Lightning and thunder
Lightning is nothing more than static electricity on a large scale. In the Netherlands, lightning strikes around 300,000 times each year, often enough to justify measures against the potentially harmful consequences. Discharges can lead to ignition of an explosive atmosphere in more than one way:
- Direct ignition by lightning;
- Lightning conductors and other metal parts of an installation will act as an ignition source due to very high currents running through them for short periods of time, causing high temperatures;
- Spark discharges between insulated metal parts close to a lightning strike due to high voltages generated by induction.
Lightning strike
The most important measures to prevent damage caused by lightning strikes are:
- Grounding of metal parts; pay attention the cathodic protection of the installation remains intact;
- Installation of overvoltage protection, especially when a lightning strike can affect a zone 0 or zone 20 environment.
Electromagnetic (radio) waves
Radio waves (10 kHz to 1 THz) can be received by the conductive parts of an installation. These parts will act as antennas. So much energy can be received it will cause thin conductors to heat up. Also, potential differences may become large enough to cause a spark between two conductors.
The amount of energy received depends on:
- The distance to the transmitter;
- The power transmitted;
- The size and effectiveness of the antenna.
Radio Wave Spectrum
Radio waves do not care about zones. Check the environment to see whether there are any radio transmitters or mobile phone antennas nearby. If the distance to the transmitter is sufficient, additional measures will not be necessary. Certified transmitting equipment such as mobile phones and walkie-talkies can be used safely in a hazardous area. Usually, no measures are necessary for a dust environment, unless the specific dust has a very low ignition limit.
Electromagnetic (optical) waves
These are waves in the frequency range from 1 x 109 to 3 x 1015 Hz, for example sunlight and laser light. By absorption of energy these waves, especially when focused, can cause dust to heat up. The heated dust particles can then act as a source ofignition.
Equipment that produces these waves is only allowed if:
- The energy or power emitted is so low that the explosive atmosphere cannot be ignited;
- The waves are limited to the housing of the device. In that case the housing must be eithergastight or capable of withstanding an explosion. The temperature of the housing must be insufficient to ignite an explosive atmosphere.
Ionizing radiation
Ionizing radiation, from X-ray tubes, for example, and radiation from a radioactive source in a smoke detector, can be sufficiently energy-rich to ignite, in particular, dust particles present in an explosive atmosphere. Such a dust particle then absorbs so much radiant energy that the temperature quickly rises above the ignition temperature of the explosive mixture.
Ionizing radiation, from for instance X-ray tubes or radiation from a radioactive source inside a smoke detector, can contain enough energy to ignite especiallydust particles in an explosive atmosphere. The dust particles absorbso much radiation energy that the temperature quickly rises above the ignition temperature of the explosive atmosphere.
In addition, the radioactive source can become very hot by internal absorption. Radiation can also cause a decomposition reaction releasing highly reactive radicals that in their turn may cause ignition.
Equipment emitting ionizing radiation is only permitted in a gas environment if:
- The radiation energy or power emitted is so low that ignition of the explosive atmosphere is not possible;
- The radiation is confined to the housing of the equipment. In that case the housing must be eithergastight or capable of withstanding an explosion. The temperature of the housing must be insufficient to ignite an explosive atmosphere.
Equipment designed for zone 0 or 1 must comply with the above even in case of rarely occurring or foreseeable malfunctions.
Usually, no measures are necessary for a dust environment, unless the specific dust has a very low ignition limit.
Ultrasonic sound as potential source of ignition
Ultrasonic sound has a frequency above the audible range (approx. 20 kHz). It is used, for example, when measuring levels in storage tanks or in flow measurement inside
process pipes. When ultrasonic sound is used, a large part of its energy is absorbed bysolids and liquids. In extreme situations this can lead to high temperatures, sufficient to ignite an explosive mixture.
In a gas environment, the following applies:
- For sound with frequencies up to 10 Mhz, power density should not exceed 1 mW/mm2 unless it is established that a higher density will not constitute a source of ignition;
- Ultrasonic noise at frequencies above 10 Mhz should be avoided completely. Sound at such high frequencies can cause molecular resonance, leading to a sharp increase in temperature.
Conventional level and flow measurement equipment generally uses a much lower power density so there is no risk of ignition. Only when sound waves are bundled a power density above the limit may occur. In those cases, constructive measures are necessary to preventthis bundling.
In a dust environment, usually no action is necessary unless the specific dust has a very low ignition limit.
Adiabatic compression and shock waves
With (pseudo) adiabatic compression, a rapid increase in pressure takes place combined with little or no heat exchange with the surrounding environment. The result is a locally high temperature.
An everyday example is a bicycle pump, where the temperature at the bottom of the pump has risen considerably after inflating a bicycle tire. The diesel engine is based on this principle. In industrial installationspressures can be much higher, with even higher temperatures. The temperature reached depends on the ratio between pressure before and after compression.
Shock waves may occur when gas under high pressure is released into a low-pressure pipeline. The propagation rate within the pipeline will be above the speed of sound, causing shock waves and locally high temperatures.
Adiabatic compression
In general, shock waves should be avoided. This can be done, for example, by ensuring that valves between sections with high pressure differences can only be opened slowly. Depending on the zone, the following operating conditions should be observed:
Zone 0: no compression and shock waves allowed, even in the case of rare malfunctions;
Zone 1: no compression and shock waves allowed, even in case of regularly occurring or foreseeable malfunctions;
Zone 2: no compression and shock waves allowed under normal operating conditions.
For a dust environment, usually no measures are necessary unless the specific dust has a very low ignition limit.
Exothermic chemical reactions
An exothermic chemical reaction is a reaction that releases heat. A well-known everyday example is drain unblocker that reacts violently when in contact with water, producing a lot of heat. Many chemical reactions are exothermic.
The height of the final temperature during reaction depends on:
- Ratio between the volume and surface of the reaction vessel;
- Ambient temperature;
- Thermal insulation from the surrounding environment;
- Amount of the reactants;
- Amount of time that the reaction takes
Many of these reactions can produce flammable gases, vapors or mists that may form an explosive mixture. The danger is therefore doubled; on the one hand the reaction can form an explosive mixture, on the other hand it can produce enough heat to ignite one.
All (unwanted) exothermic reactions should be prevented. The best way to do this must be must be assessed separately for each situation. In general, the following measures can be taken:
- Inertization;
- Atabilization;
- Improving heat dissipation;
- Limiting temperature and pressure;
- Storage at a reduced temperature.
