Conduct of the explosion
Dust explosion
Although explosion can occur like extremely fast and single action, its running can be divided into several parts, which always take place and which has their rules. Also, different matters have different explosion conducts because of their different nature and explosion characteristics.
On this graph, there are 2 different dust explosions pictured. These two dusts have various KST. Yellow graph is the conduct of explosion which is more aggressive, "faster" (the dust has higher KST) than red dust. Notice that in the same period of time, the pressure rise was much higher in the first case. Also you can notice that dusts with higher KST usually (not always) reach also higher maximum explosion pressure (pmax).
On the yellow graph we can also demonstrate the basic phases of explosion:
A - ignition
Point A is the point of ignition - it means that this is the time in which the ignition source occured in dust-air atmosphere and the energy of this ignition point is higher than lower explosion limit (LEL) of the dust in the atmosphere. From this point relatively slow combustion of small ammount of particles very close (or touching) from ignition source are burning, heating neighbour particles and compressing the atmosphere very close to the ignition source. The pressure in the vessel starts rising, but relatively slowly because volume of burning particles (producing exhaust gases of bigger volume than original atmosphere - reason of explosion overpressure) is small comparing to the whole volume of the vessel.A-B - compression
Between the points A and B the process of burning starts spreading in chain reaction (every burning particle emblaze several others in the same time). The ammount of exhaust gases is bigger and it starts effecting the whole vessel. This phase is about 3-10 miliseconds long and this the most critical part for the functionality of the explosion protective system - in this phase the explosion shuld be detected and the explosion protection system should start acting against the explosion.B - combustion of the whole mixture starts
In the point B the explosive mixture is so hot and so comprimed, that the ignition and burning spread through the whole volume by extremely high speed. This is the main phase of the explosion. Immediatelly after this point the prssure rises to the point which is above the static opening pressure of explosion venting systems (panels, membranes, rupture discs, special valves). In this point also the extinguish agent from suppression systems starts to flow inside.B-C - the main phase of the explosion
This is the phase described as explosion. The speed of spreading of the explosion depends on KST parameter of the dust and also on original pressure (if there is higher pressure before the explosion, the mixture needs only shorter time to compress) and the original temperature (higher temperature means that the mixture reaches temperature necessary for spreading of the explosion much faster).This period is approximately 10-500 milisecond long, depending on the volume of explosive atmosphere and its parameters. If there is no protective system acting during the explosion or if the vessel is not explosion pressure resistant, this is the time when it collapses or dispart and the explosion starts to spread to connected devices and to the surroundings of the device.
C - reaching of the maximum explosion pressure
After all particles of dust are burnt down, the pressure stops rising, reaching its maximum. After this point the mixture starts cooling down and releasing of the pressure by the breaches and holes. This is the point when interesting efect can occur: if the exhaust gases will cool down fast, damaged device is not able to withstand underpressure caused by cooling and shrinkage of the exhaust gases inside and the device collapses inside itself (implode). This happens usually to silos projected to withstand big overpressure caused by the weight of the material inside, but where the designer never expected underpressure inside the device.
Picture: tank collapsed due internal underpressure. Although this time the explosion wasn't the reason, this can happen even to big industrial and agricultural silos
Gas explosion
Explosion of a explosive gas or vapours of flammable liquids in closed vessel has the same running as the dust explosion, but with much faster rising pressure, reaching higher values. It is true that the gas or vapour explosion almost always ends up with the destruction of the vessel where the explosion occured with subsequent fire. Because there is no effective protective system for gas explosion in closed vessel (but some special membranes for slow running mixtures), it has no big sense to study the running of the explosion of the gases and vapours of flammable liquids.
In this case much more important is to check how the explosion spreads through the pipelines - the pressure acts mostly in axial direction, round pipeline is also usualy strong enough not to collapse during the explosion. It means that it is crucial to stop the explosion, sparks or ignition source going throught the pipelines, which could ignite the explosive atmosphere on the places where the pipeline goes to. The explosion or flame going throught the pipeline can be easily stopped, so it is good to know how it goes.
In contradistinction to the dust explosin it is not the time what is important but the distance from the point of ignition or from the beginning of the ducting. The nature of the exlosion changes with different distance from the point of ignition, as could be seen on the graph above.
A-B - deflagration phase (yellow)
In the phase of deflagration, which starts in the ignition point and ends approx. at the distance of 10 * diameter of the pipeline the speed and pressure are rising almost linearly. The mixture warms up and compresses, particles start to burn from each other. In this place, the explosion is realtively easy to stop due slow speed and small pressure, that's why the protective system (flame arrester) is usually placed here - deflagration type of the flame arrester is needed. Detonation version can be used too, but it is not necessary.B-C - unstable detonation phase (red)
In the zone of unstable detonation the speed and pressure rising change rapidly due the big turbulences in the mixture. There can be several peaks of pressure in this zone and the values of pressure are extreme. It is the most complicated place to stop the explosion and there is only few types of flame arresters abel to stop even ustable detonation. To be true: it is usually useless because the flame arresters can be usually installed closer or farrer from the ignition source, to the zone of deflagration or stable detonation. Also it is possible to add some extra pipeline just to get far enough and it is cheaper than to invest in unstable detonation flame arrester.In this zone the speed of explosion passes the speed of sound (explosion spreading becomes supersonic).