Pronunciation key ( iks-plō′zhən ) |
ex•plo•sion
n.
[L. explosio, driving off stage by clapping < pp. of explodere see EXPLODE].
- Blowing up, an exploding. Act of bursting with a loud noise; detonation.
- The sharp, loud noise made by exploding.
- Loud outburst or breaking forth; such as an emotional explosion of rage.
- Sudden and great increase. Such as the explosion of social media..
- Phonetics. Plosion. Sudden release of breath in the articulation of a plosive.
The sudden and instantaneous liberation of energy or violent expansion of a substance accompanied by a sudden release of energy. Explosions are created by rapid chemical reactions in various substances and mixtures, a nuclear reaction or bursting out of gases or vapors under pressure. Other phenomena, including the failure of pressure vessels such as steam boilers, compressed-air tanks and pressure cookers may also be called explosions. The cause of such is different and though the end result may be similar to an explosion they are more appropriately called pressure ruptures. The burst of energy associated with the Atom bomb, produced by rapid atomic fission and fusion are also called explosions.
The loud noise that is associated with an explosion is created when vibrations set up in the surrounding air by the sudden burst of energy. An explosion which is contained in a closed vessel strong enough to withstand the pressure may produce very limited noise.
Substances and mixtures which undergo chemical reactions typically produce gaseous products by those reactions. One exception is copper acetylide. It produces hot particles of copper and carbon but practically no gas. Gaseous reaction products and surrounding air are expanded quickly by the heat that is involved in an explosive chemical reaction. In confined conditions this expansion exerts force. A stimulus such as a spark, flame, friction or shock typically are needed to initiate an explosive chemical reaction. The stimulus material required to ignite such a reaction varies depending on the material or mixture. Generally, there are two types of explosive chemical reactions:
- Rapid combustion. This is usually called deflagration and produces flash fires is not contained and explosive pressure when contained. Ordinarily it results from the chemical union of oxygen with other chemical substances (for further information see Combustion). Rapid combustion only occurs if conditions are right, such as in,
- Highly flammable solids or liquids containing enough oxygen for near complete combustion
- Particles of a combustible gas, vapor or dust are suspended in the air in close vicinity to each other permitting propagation of flame from one to the other yet separated far enough apart, leaving space for enough oxygen required for combustion./li>
With rare exception, combustible gases, vapors and dust are not explosive alone. However, their mixture in certain proportions can make them explosive. This fact is especially dangerous in regard to human safety factors. The minimal concentrations of combustible substances in the air that will produce an explosive reaction is known as the lower limit of flammability or lower explosive limit. There is also a maximum proportion of air to chemical material, above which the material is deemed non-explosive. This is known as the upper limit of flammability or upper explosive limit. Between these two limits is a concentration that provides the most powerful explosion of which the mixture has potential. Relationships between concentrations measured by volume of gasoline vapor in the air and the pressure which develops from such an explosion is demonstrated in the following graph.
Relationship between percent of gasoline vapor in the air and associated pressure by the explosion of the mixture.The graph illustrates that a concentration of about 2.25% is the optimal concentration to produce the most powerful explosion. Graphs of other combustible substances in air also reveal that an increase in concentration over the lower limit result in an increase in pressure until reaching the maximum, after which pressure begins to decline until the upper limit is reached and the mixture will cease to be explosive.
The lower explosive limit for most flammable vapors is very low. With gasoline, about 1.4% at normal temperature and less at increased temperatures. Its upper limit is about 6%. Explosive limits of flammable vapors vary, for example,
- Benzene 1.5-8%
- Carbon disulfide 1-50%
- Ether 1.9-36.6%
Most flammable vapors produce a detectable odor in the air at their lower limit which explains why a spark does not always produce an explosive reaction. Needless to add, a sense of smell does not provide a reliable hazard meter.
Lower explosive limits in many common flammable gases are quite low. The typical limits are
- Methane 5.3-14
- Butane 1.6-8.5
- Propane 2.3-9.5
- Hydrogen 4-74
- Carbon monoxide and ammonia are exceptions, with comparably higher limits in the low range at 12.5 and 16.
Explosive limits for dust cannot be determined as effectively as those for vapors and gas, however, any mixtures permitting rapid combustion of particles are said to be within the explosive limit range. Dust explosions occur when combustible dust is lingering in the air. The degree of its flammability depends on moisture content and upon size of particles. Increased flammability occurs when the particles are drier and finer. Substances that cause such explosions are from products such as powdered starch, sugar, flour, wood, coal, certain plastics, powdered milk, cocoa, sulfur, aluminum, magnesium and similar substances.
The only exception to the rule, is acetylene (when it comes to necessitating a mixture of air with explosive materials to produce an explosion). If this substance is under the pressure of 2 or more atmospheres it tends to undergo explosive decomposition if exposed to a spark or some other source that may ignite it. For this reaction to occur, air is not necessary.
Solids that undergo rapid combustion and cause explosion when confined are known as explosives. They are classified as low explosives to distinguish them from explosives that are detonated. Some typical examples are black powder and smokeless powders which will rapidly burn when compared to the speed of other substances, but are relatively slow when compared to detonated explosive reactions. Due to the slower reaction the explosions created by them exert a push rather than the shattering shock produced by detonation. This characteristic makes them especially useful for propellant charges in firearms and blasting in mines and quarries, as it is likely more desirable to break material vs shattering it.
The chemical transformation of low explosives occur at the surface of particles, proceeding inward to their center. Deflagration is accelerated by the hot gases that develop, especially when the substance in confined. The rate of deflagration depends on the size and shape of the explosive particles and the pressure and temperature, including chemical composition.
- Detonation. This reaction is almost instantaneous in chemical change of a substance to the liberation of heat energy and production of gases. It is a shock wave maintained by a chemical reaction and advances through the explosive substance at a constant speed. The part of any detonating substance that has not yet been hit by the shock wave, or the detonation front, remains at its initial pressure. However, when hit by the front its pressure will rise nearly instantaneously to its highest value, possibly 10-100x the original pressure in gases and possibly as much as 200,000 atmospheres in dense solid explosives. The detonation front travels about 3500 meters per second in gases and 8000 meters per second in solids and liquids which is relatively rapid. Such detonations are by far more abrupt and violent than deflagration explosions.
Detonations are created by heat or impact. Perhaps both. Heat may be produced by a spark, flame, friction or other source. Usually, it first produces deflagration, which is changed to detonation. The transformation from deflagration to detonation is assisted by the confinement of the substance. Many high explosives burn without detonation if ignited in the open. Impact may require a mechanical blow or the detonation of an alternative explosive. The effect is the production of an immediate detonation in the explosive substance which is subjected to the impact.
Some chemical mixtures producing deflagration also detonate under certain conditions. Such as a mixture of gasoline vapor and air will ordinarily burn in the cylinders of an automobile engine, producing a relatively slower reaction that delivers smooth power to the pistons. However, under different circumstances, the mixture can detonate and produce an extreme blow to the pistons and cause the engine to lose power, followed by producing a noise called a knock or a ping.
In many military or civilian jobs, there are useful purposes for the violent force associated with such explosions as they are used in bursting charges in shells, bombs, mines, depth charges and similar weapons. Also, they are used commercially for demolition projects and blasting when the purpose is to shatter rather than breaking material.
Explosions in aircraft are often caused by the vapors of gasoline and other liquid fuel. Generally an explosion tends to be followed by fire and if in flight, a crash landing. Breaks and leaks in the fuel system are attributed to vibration or causes that permit formation of an explosive mixture of vapor and air in the fuselage and this may be ignited by the electrical, heating system or static electricity. Another cause on military aircraft can be ammunition and transport-planes which carried explosives as cargo. Other explosions in aircraft, were formerly caused by hydrogen used as a lifting gas. By changing over to Helium (non-explosive) has eliminated this problem.
Explosions in ships are due to their structure. Ships are generally airtight in their construction, therefore increasing the hazard of explosive gas or vapor spreading from oil-fired boilers, internal combustion engines and fuel tanks. The ventilation system on ships may create an especially dangerous setup due to an explosion traveling through the ducts from one deck to the next.
Tankers which transport fuel oil, gasoline or explosives in general have special explosion hazards.
Explosion crater. A large crater that is formed when meteorites having a velocity of greater than 3-4 km per second strike the surface of the Earth. The meteorite and adjacent soil are broken up by the shock wave that spreads from the point of impact. The wave overcomes the molecular cohesion of the solid meteorite, transforming it into a highly compressed gas that expands and explodes. Typically, only tiny fragments will remain of the meteorite's original material.
Explosion craters range in size, anywhere from 100 meters to several kilometers in diameter. Rock flour and impactites (fragments of bedrock that fuse, forming bubble-like, glassy masses) are generally discovered within. The largest craters formed by meteorites are produced by explosion, the largest of which is the Ungava-Quebec Crater of Canada. The diameter is more than three kilometers or about two miles. See meteorite impacts and crater design and meteorite impact stimulation.
Explosion seismology. The use of man-made explosions which aide in the determination of structures of the earth, generally on a larger scale. Large chemical explosive detonations and underground nuclear blasts generate seismic waves which can then be interpreted to learn about the interior of the Earth. This method provides an advantage over earthquake sources being controllable in size, location and timing of the source impulses. After correlating the different acoustical refracted and reflected recorded vibrations, it is possible to ascertain estimations of depth and character of subsurface geologic structures. Explosion seismology is often used to locate petroleum and mineral deposits. Major limitations of the explosion seismology method are receiver sensitivity and the inhomogeneity of the earth structures. For more information see marine sediment composition and structure.
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