Explosion, fire and technical parameters of dusts, gases and flammable liquids
Every explosive atmosphere is characteristic by its fire or explosion parameters and qualities. These characteristics are typical for each matter, but may change due to the real conditions, in which the explosive atmosphere exists. Explosion and fire characteristics are the subject of Chapter 9 of the Material Safety Data Sheet (MSDS) - if they're known or has any relevant meaning to the specific matter. These characteristics are set in lab and they are always meant to be interpreted during normal conditions (surrounding temperature 20°C, atmospheric pressure 1013 hPa, zero humidity, extremely low particle size (dust), purity over 99% (gases), and so on). In real practical conditions these characteristics may vary in both ways according to the real conditions in specific industrial installation (see below).
If the explosive atmosphere contains more various dusts or if it is a mixture of gases or vapours, always the matter with the worst characteristic is examinated and the mixture is handled like there is 100% of the matter with the worst characteristics. Sometimes there can be a situation with two dusts where dust A has worse half of the parameters and the dust B is worse in another characteristics. In these case the characterics are combined according to for what purposes are they used (for exmaple for setting the maximum temperature of electric devices the ignition and glowing temperatures of one dust is used, but parameter KST or pmax for setting the necessary venting are of the other dust is used).
List of explosion or fire-technical parameters of dusts
Explosive dusts are typical in these characteristics:
Lower explosion limit - LEL
Lower explosion limit (or the lowest explosion concentration) is the mass of the dust in grams per cubic meter. When reached, the dust-air mixture is able to be ignited if there is strong enough ignition source. If the concentration is lower, it is not possible to set the explosion because there are not enough dust particles in the mixture to spread the explosive fire.Typical values of LEL are in tens or (rarely) hundreds of grams per cubic meter. In the areas outside the devices this concentration rarely exists because it is unbreathable and it looks like very dense fog. On the other side, usually it is not a problem to reach the explosion concentration inside the industrial devices.
Besides Lower Explosion Limit there is also Upper Explosion Limit - theoretical value defined as maximal mass of the dust in the mixture with air (also in grams per cubic meter) where the mixture is not explosive because there is not enough air for the combustion. Between the values of lower and upper explosion limit there is so called the zone of explosibility. Approximatelly in the middle of this zone, there is a point of optimal concentration where minimum energy is needed to ignite explosion which reaches the maximum explosion pressure (ignition energy and explosion pressure depend on the concentration).
In technical praction the value of upper explosion limit has not very big meaning, because to be reached, the matter must go through the zone of explosibility.
Minimum ignition energy - EMIN
Minimum ignition energy is the lowest possible energy which is able to ignite the dust in optimal concentration. If the energy is lower than this value, it is not sufficient to ignite the dust. With rising energy also the mixtures of non-optimal parameters (concentration, particle size, humidity,...) can be ignited.The values of minimum ignition energy of dusts are very diverse (starting at several mJ - milijoules up to tens of joules). It means that some kinds of dust (for example fine and dry sugar or plastics) can be ignited also by a strong electrostatic spark and by all kinds of mechanical sparks, on the other side some other kinds of dusts (like carbon black) are hard to ignite even by very strong ignition sources. Minimum ignition energy depends on concentration of the mixture (see the graph below) and it should be understood as approximate value because in the technology there can be some sources strong enough to ignite any kind of explosive atmosphere (for example flames, electric sparks, hot surfaces) so it is not good rely on relatively high minimum ignition energy of dusts.
Maximum explosion pressure - pMAX
Maximum explosion pressure is the highest value of pressure which can be reached in closed vessel of one cubic meter after ignition of dust-air mixture of optimal concentration by strong ignition source.The most common values of maximum explosion pressure are 3 to 10 bar, some dist can reach up to 12 bar. The explosion pressure depends also on the shape and volume of the vessel where the explosion occured and on the concentration of the dust (and some other parameters). The highest value of pMAX is reach when the explosion is ignited in optimal concentration (when you also need the lowest energy).
Maximum explosion pressure has strong influence on the contruction parameters of devices because if the machine is constructed to be explosion pressure resistant (like coal mills in power plant), usually it is not necessary to protect them against explosion (of course, the risk of explosion spreading must be always considered!)
Reduced pressure (pred, or pred max), which is the maximum pressure that can occur in the device protected by some kind of explosion protective system is much lower (usually 0.3 to 1 bar) and its calculation is cruicial for setting the parameters of the protective system.
Graph showing the dependence of explosion pressure (pmax) and minimum ignition energy (MIE) on the concentration of the dust.
EXPLANATION:
red - maximum explosion pressure (pmax)
yellow - minimum ignition energy (MIE)
As you see, the graph doesn't start at zero, but on minimum explosion concetration (LEL) - point A. With rising concentration, the pmax rises too and the ignition energy descents. In the point of optimal concentration - B - the dust air mixture has the potential to reach the highest explosion pressure with the lowest MIE. With concentration rising on, the explosion pressure descents as well as the MIE rises (there becomes insufficient amount of oxygen int he mixture for optimal explosion). After reaching point C - maximum explosion concentration, there is not enough of air/oxygen even to ignite the misture and the dust cloud is not explosible any more.
Constant of speed of increasing pressure in the volume of 1 m3 - KST
Because the maximum explosion pressure is the value depending also on the volume of the dust cloud, volume of the vessel and the concentration, there is a constant describing how fast the pressure rises in the closed volume of 1 m3 which is fully filled by the dust cloud with optimal concentration. This value (KST) is specific for each dust and it is something similar (but not exactly) to brisance of the explosives.The value of KST is extremely important to know when considering the explosibility of the dust and usually it is more important value than pmax, because it tells very much about expected running of the explosion, which is the most critical value for the construction and design of all dust explosion protective systems. In other words: it is not so important how high the explosion pressure can be, but how fast the explosion runs.
According to KST the dusts are devided into three groups of explosibility - St 1, St 2, St 3 (comming from German word for dust - "staub"). Here are the values for dividing the dusts into these groups:
St 1 - KST 0 - 199 bar*m/s
St 2 - KST 200 - 299 bar*m/s
St 3 - KST more than 300 bar*m/s
The class of the dust (or it value of KST) for which the explosion protection system designed and certified for is the most important to know when evaluating suitability of specific system. The reason is that not each system is suitable for all classes and this must be always considered. Since the St 3 class has no upper level (and there are dust which can have values of KST even exceeding 800 bar*m/s), there is no protective system certified for the whole class St 3, because it would have to have unlimited efficiency. If there are protective systems suitable for values of KST over 300 bar*m/s, it must be stated in the certificate with specific value of KST.
Ignition temperature of a dust cloud - MIT
It is a temperature of a hot surface which is high enough to ignite a dust cloud of optimal concentration. This value is important for setting the maximum surface temperature of electric devices whic are placed in the areas with dust explosion risk (zone). Because dust-air mixtures are not homogenic like gas-air mixtures, safety coeficient of 1/3 of the value is applied. It means that maximum allowed temperature of a electric device or any hot surface in the are with the risk of explosion os 2/3 of the MIT of the dust which is present. If there are or could be more kinds of dust in the same area, the one with the lowest MIT is used for consideration.Minimum ignition temperature of dust clouds are usually in the range of 350°C to 750°C, but there are some dusts with MIT below 300°C.
Glowing temperature of the layer of the dust - MGT
Glowing temperature is the lowest temperature of a hot surface which is able to start independent glowing of the dust in uncompressed layer with thickness of 5 mm. After reaching this temperature, the dust continue glowing itself with need of any other source of external energy and its temperature will continue rising independently. This value is most important especially for electric devices to be used outside the technology handling with explosive dust, where the layers and pills are expected (like drives, lighting, switchboards,...). Because the temperature of environment can be different from the temperature during the testing of MGT (20°C), safety coefficient 75 K (75°C) is used. It means that the maximum allowed value of surface temperature of the device to be used in the area where the layers of dust can be expected is MGT-75°C.The range of MGT of majority of dust is 150°C to 600°C.
Additional explosion or fire-technical parameters dusts
Besides described parameters some other values can be important for practical use.
Lower oxygen concentration - LOC
This values is usually tested for the fossil fuels (coal, biomass, turf) and describes how the other explosion characteristic change with lowering the ammount of oxygen in the atmoshpere. The reason is that in many power plant burnign coal the fuel is dried by hot exhaust gases, which has the temperature above ignition temperature, but since there is only low ammount of oxygen in these gases (usually approx. 3-5%), the atmoshere is not explosive even the ignition source is still present. The problem is that the LOC changes also with temperature, so checking this value is very important for these installations.Lower oxygen concentration sets the minimum concentration of oxygen which allows the dust cloud of optimal parameters to explode. Normal concentration of oxygen in the air is approx. 21%. Majority of organic dusts has LOC around 10%, but it can be even lower, in some cases even around 7%. With rising temperature, this value can even descent to 6-5%.
Self-ignition (better self-glowing) temperature
Setting this value has sense for the dusts which can have tendences to self ignite, which means that they can rise their temperature even without presence of any external source of energy or ignition source. This effect is usually caused by exotermical chemical reactions or by microorganisms producing heat. This is a domain of organic dusts, especially coal and grain. Some types of coal can have the self-ignition temperature (during special circumstances like humidity, particle size, presence of oxygen,...) even as low as 50°C. It means that the way of their storage or cooling is extremely important.List of explosion or fire-technical parameters of gases, vapours and mists
Explosive gases and vapours of flammable liquids are characterized by these parameters:
Lower explosion limit - LEL, or minimum concentration Cmin
It is a minimum concentration (described in percentage of gas in the total volume of the atmosphere) when the mixture becomes explosive. Below this concentration the mixture is not explosive (in normal conditions) even with extra strong ignition source.Lower explosion concentrations are usually between 2% to 20% of the volume.
Upper explosion limit - UEL, or maximum concentration Cmax
It is a maximal concentration (described in percentage of gas in the total volume of the atmosphere) when the mixture becomes non-explosive because of low percentage of oxygen. This values is not very important for the dusts, but for gases it is very important value with big practical meaning since lot of gas technologies works in non-oxygen atmospheres with overpressure to normal atmosphere (like natural gas transportation and handling technologies). In these cases the descending of the concentration under UEL can be critical. In comparasion with dusts, where upper explosion limit is usually 100 times higher than lower explosion limit, gases has smaller zone of explosibility and it is usually only several percent of volume (for example the zone of explosibility of natural gas is 5% to 15%).Flash point
Flash point is the temperature of the matter (flammable liquid) which is high enough for evaporating the amount of vapours which makes
explosive atmoshere above the liquid surface. This temperature has nothing common with ignition source!
When the temperature of the liquid is below flash point, the concentration of vapours is not explosive (the vapours are spread in the
air and the concentration is below LEL) and no ignition source can flame the atmoshere.
According to flash point, the flammable liquids are sometimes devided into groups of hazards:
| Group | Flash point |
| I | < 21°C |
| II | 21°C - 55°C |
| III | 55°C - 100°C |
| IV | >100°C |
Ignition point (temperature)
Ignition point pr ignition temperature is the lowest temperature of the ignition source which flashes the vapours or gases of optimal concentration. According to this temperature the gases and flammable liquids are divided into temperature classes. This temperature (or class) is very important for the design of devices which have to be used in the gas explosion risk areas.The dividing into the groups:
| temperature class | ignition point |
| T1 | > 450°C |
| T2 | 300°C - 450°C |
| T3 | 200°C - 300°C |
| T4 | 135°C - 200°C |
| T5 | 100°C - 135°C |
| T6 | 85°C - 100°C |
Some other parameters wich can be important to know about the explosive gases or flammable liquids can be for example melting point, vapours pressure at 20°C, relative density of vapours (or if the vapour is heavier than air or not) solubility in water and some others.