Introduction :-
The injuries and damage are in the first place caused by the shock-wave of the explosion itself. People are blown over or knocked down and buried under collapsed buildings or injured by flying glass. Although the effects of over-pressure can directly result in deaths, this would be likely to involve only those working in the direct vicinity of the explosion.The history of industrial explosions shows that the indirect effects of collapsing buildings, flying glass and debris cause far more loss of life and severe injuries.
The effects of the shock-wave vary depending on the characteristics of the material, the quantity involved and the degree of confinement of the vapour cloud. The peak pressure in an explosion therefore vary between a slight over-pressure and a few hundred kilopascals (kPa). Direct injury to people occurs at pressure of 5-10 kPa (with loss of life generally occurring at a greater over-pressure), whereas dwellings are demolished and windows and doors broken at pressure of as low as 3-10 kPa.
The pressure of the shock-wave decreases rapidly with the increase in distance from the source of the explosion. As an example, the explosion of a tank containing 50 tonnes of propane results in a pressure of 14 kPa at 250 metres and a pressure of 5 kPa at 500 metres from the tank.
•When an explosive was first used?
It may never be known with certainty who invented the first explosive, black powder, which is a mixture of saltpetre (potassium nitrate), sulfur, and charcoal (carbon). The consensus is that it originated in China in the 10th century, but that its use there was almost exclusively in fireworks and signals.
•How Explosions Happen?
An explosion occurs when a large amount of energy is released into a small volume of area in a very short time. The energy released comes in many forms, including chemical (artificial explosives), nuclear and hydrothermal ( volcano eruptions ).
Burning very rapidly, explosive material releases concentrated gas that expands quickly to fill the surrounding air space and apply pressure to everything in it. That is, it explodes in a blast powerful enough to blow away nearby trees, cars, buildings and anything else in its path.
On the set of an action flick, blasts are often cultivated using nifty camera angles, CGI and miniature explosive devices, with notable exceptions like 2009 Oscar winner "The Hurt Locker" which used actual full-sized explosives. In real life, movie-type blasts require an explosive like C-4, TNT or gasoline, which cause explosions when they burn and mix with oxygen. C-4, for example, combines combustible material with plastic, often in a block form. The explosive is ignited using a detonator, which burns and causes the block to release a number of gases, including nitrogen and carbon oxides at a very fast rate and with a whole lot of force (the explosion).
And those fiery automobile explosions that are the centerpiece of any good car chase scene? More likely caused by strategically placed C-4 than an explosion in the gas tank. These days, cars on the street are equipped with highly durable gas tanks precisely to prevent them from detonating in the event of a crash. It is also important to remember that gas burns in vapor, but not liquid form. That means that, even if a car's tank were ruptured, the liquid gas in it would have to convert to vapor, mix with the air in the proper proportion, and be ignited in order to the car to blow up.
•Causes of Explosion.
~Natural
Explosions can occur in nature due to a large influx of energy. Most natural explosions arise from volcanic or stellar processes of various sorts. Explosive volcanic eruptions occur when magma rising from below has much dissolved gas in it; the reduction of pressure as the magma rises causes the gas to bubble out of solution, resulting in a rapid increase in volume. Explosions also occur as a result of impact events and in phenomena such as
hydrothermal explosions (also due to volcanic processes). Explosions can also occur outside of Earth in the universe in events such as supernova . Explosions frequently occur during bushfires in
eucalyptus forests where the volatile oils in the tree tops suddenly combust.
~Astronomical
Among the largest known explosions in the universe are supernovae , which results when a star explodes from the sudden starting or stopping of nuclear fusion gamma-ray bursts, whose nature is still in some dispute. Solar flares are an example of a common explosion on the Sun, and presumably on most other stars as well. The energy source for solar flare activity comes from the tangling of magnetic field lines resulting from the rotation of the Sun's conductive plasma. Another type of large astronomical explosion occurs when a very large meteoroid or an asteroid impacts the surface of another object, such as a planet.
~Chemical
The most common artificial explosives are chemical explosives, usually involving a rapid and violent oxidation reaction that produces large amounts of hot gas. Gunpowder was the first explosive to be discovered and put to use. Other notable early developments in chemical explosive technology were Frederick Augustus Abel 's development of nitrocellulose in 1865 and Alfred Nobel's invention of dynamite in 1866. Chemical explosions (both intentional and accidental) are often initiated by an electric spark or flame in the presence of Oxygen. Accidental explosions may occur in fuel tanks, rocket engines, etc.
~Electrical and magnetic
A high current electrical fault can create an 'electrical explosion' by forming a high energy electrical arc which rapidly vaporizes metal and insulation material. This arc flash hazard is a danger to persons working on energized switchgear . Also, excessive magnetic pressure within an ultra-strong electromagnet can cause a
magnetic explosion .
~Mechanical and vapor
Strictly a physical process, as opposed to chemical or nuclear, e.g., the bursting of a sealed or partially sealed container under internal pressure is often referred to as an explosion. Examples include an overheated boiler or a simple tin can of beans tossed into a fire.
Boiling liquid expanding vapor explosions are one type of mechanical explosion that can occur when a vessel containing a pressurized liquid is ruptured, causing a rapid increase in volume as the liquid evaporates. Note that the contents of the container may cause a subsequent chemical explosion, the effects of which can be dramatically more serious, such as a
propane tank in the midst of a fire. In such a case, to the effects of the mechanical explosion when the tank fails are added the effects from the explosion resulting from the released (initially liquid and then almost instantaneously gaseous) propane in the presence of an ignition source. For this reason, emergency workers often differentiate between the two events.
~Nuclear
Nuclear explosion and Effects of nuclear explosions, In addition to stellar nuclear explosions, a man-made nuclear weapon is a type of explosive weapon that derives its destructive force from nuclear fission or from a combination of fission and fusion. As a result, even a nuclear weapon with a small yield is significantly more powerful than the largest conventional explosives available, with a single weapon capable of completely destroying an entire city.
•Deflagration and detonation
An explosive is classified as a low or high explosive according to its rate of combustion: low explosives burn rapidly (or deflagrate), while high explosives detonate. While these definitions are distinct, the problem of precisely measuring rapid decomposition makes practical classification of explosives difficult.
Explosions can occur either in the form of a deflagration or a detonation, depending on the burning velocity during the explosion. Deflagration occurs when the burning velocity or the flame speed is relatively slow, of the order of 1m/sec.In a detonation the flame speed is extremely high. The flame front travels as a shock-wave, with a typical velocity of 2,000-3,000 m/sec. A detonation generates greater pressures and is far more destructive than deflagration. The peak pressure caused by a deflagration in a closed atmospheric vessel reaches about 70-80 kPa, whereas a detonation can easily reach a pressure of 200 kPa. Whether a deflagration or a detonation takes place depends on the material involved as well as the conditions under which the explosion occurs. It is generally accepted that a vapour phase explosion requires some degree of confinement for a detonation to take place.
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