An explosion is characterized by the sudden release of energy, producing a shock wave, or blast wave, that may be capable of causing remote damage. There are two distinct types of sources, namely, the high explosive and the pressure burst. The high explosive is typified by compounds such as trinitrotoluene (TNT) and cyclotrimethylenetrinitramine (RDX). These compounds are highly exothermic species, decomposing to release substantial quantities of energy. Although thermally stable (although some are less so and require desensitization to make them safe to handle), they can be induced to detonate, with decomposition, propagating at the velocity of sound through the solid. If the amount of energy released is high enough, a blast wave will propagate from the source with the potential to do significant damage at a distance. By assessing remote damage, one can estimate the size of the explosion in terms of “TNT equivalent” (normally in metric tons). This technique relies on the large amount of data that has been gathered on the damage potential of TNT (much of it during wartime), and uses empirical scaling laws which have been developed from studies of the damage caused by known quantities of TNT.
In peacetime, high explosives are used in a variety of activities, including mining, quarrying and major civil engineering works. Their presence on a site represents a particular hazard that requires specific management. However, the other source of “explosions” can be equally devastating, particularly if the hazard has not been recognized. Overpressures leading to pressure bursts can be the result of chemical processes within plants or from purely physical effects, as will occur if a vessel is heated externally, leading to overpressurization. The term BLEVE (boiling liquid expanding vapour explosion) has its origins here, referring originally to the failure of steam boilers. It is now also commonly used to describe the event in which a pressure vessel containing a liquefied gas such as LPG (liquified petroleum gas) fails in a fire, releasing the flammable contents, which then ignite to produce a “fireball”.
On the other hand, the overpressure may be caused internally by a chemical process. In the process industries, self-heating can lead to a runaway reaction, generating high temperatures and pressures capable of causing a pressure burst. However, the most common type of explosion is caused by the ignition of a flammable gas/air mixture which is confined within an item of a plant or indeed within any confining structure or enclosure. The prerequisite is the formation of a flammable mixture, an occurrence which should be avoided by good design and management. In the event of an accidental release, a flammable atmosphere will exist wherever the concentration of the gas (or vapour) lies between the lower and upper flammability limits (table 41.1). If an ignition source is introduced to one of these regions, a premixed flame will propagate rapidly from the source, converting the fuel/air mixture into combustion products at an elevated temperature. This can be as high as 2,100 K, indicating that in a completely closed system initially at 300 K, an overpressure as high as 7 bars is possible. Only specially designed pressure vessels are capable of containing such overpressures. Ordinary buildings will fall unless protected by pressure relief panels or bursting discs or by an explosion suppression system. Should a flammable mixture form within a building, the subsequent explosion can cause significant structural damage—perhaps total destruction—unless the explosion can vent to the outside through openings (e.g., the failure of windows) created during the early stages of the explosion.
Explosions of this type are also associated with the ignition of dust suspensions in air (Palmer 1973). These are encountered when there is a substantial accumulation of “explosible” dust which is dislodged from shelves, rafters and ledges within a building to form a cloud, which is then exposed to an ignition source (e.g., in flour mills, grain elevators, etc.). The dust must (obviously) be combustible, but not all combustible dusts are explosible at ambient temperatures. Standard tests have been designed to determine whether a dust is explosible. These can also be used to illustrate that explosible dusts exhibit “explosibility limits”, similar in concept to the “flammability limits” of gases and vapours. In general, a dust explosion has the potential to do a great deal of damage because the initial event may cause more dust to be dislodged, forming an even greater dust cloud which will inevitably ignite, to produce an even greater explosion.
Explosion venting, or explosion relief, will only operate successfully if the rate of development of the explosion is relatively slow, such as associated with the propagation of a premixed flame through a stationary flammable mixture or an explosible dust cloud. Explosion venting is of no use if detonation is involved. The reason for this is that the pressure relief openings have to be created at an early stage of the event when the pressure is still relatively low. If a detonation occurs, the pressure rises too rapidly for relief to be effective, and the enclosing vessel or item of a plant experiences very high internal pressures which will lead to massive destruction. Detonation of a flammable gas mixture can occur if the mixture is contained within a long pipe or duct. Under certain conditions, propagation of the premixed flame will push the unburnt gas ahead of the flame front at a rate that will increase turbulence, which in turn will increase the rate of propagation. This provides a feedback loop which will cause the flame to accelerate until a shock wave is formed. This, combined with the combustion process, is a detonation wave which can propagate at velocities well in excess of 1,000 m/s. This may be compared with the fundamental burning velocity of a stoichiometric propane/air mixture of 0.45 m/s. (This is the rate at which a flame will propagate through a quiescent (i.e., non-turbulent) propane/air mixture.) The importance of turbulence on the development of this type of explosion cannot be underestimated. The successful operation of an explosion protection system relies on early venting or early suppression. If the rate of development of the explosion is too fast, then the protection system will not be effective, and unacceptable overpressures can be produced.