Explosion

What is explosion?

The explosion is the result of rapid combustion with a sudden, violent change of pressure involving the liberation and expansion of a large volume of gas. Thus the release of energy in a rapid and uncontrolled manner gives rise to an eruption. The released energy may appear as heat, light, sound or mechanical shock. Combustible dust or powders, flammable vapours and flammable gases can explode. A combustible material, air and source of ignition (or temperature) are the prerequisites for an explosion.

Thus explosion is a sudden and violent release of energy. Its effect depends on the rate at which the energy is released. The difference between fire and explosion is the rate of release of energy. In a fire, it is at a slow rate and in the eruption, it is at a high rate and sudden release.

Fire and explosion inflammable substance are possible only when it leaks and forms vapour in explosive range. Therefore the first step necessary is to regularly check the tank, container, piping equipment, etc for leakage and to stop it.

Three types of energy can be released: physical, chemical or nuclear. Bursting of tyre, vessel, pipe etc. due to overpressure or brittle fracture and flushing of superheated liquid (thermal energy) are examples of physical energy. Chemical energy is released due to chemical reactions. It may be uniformed as in the case of a vessel, or it may be propagating as in the case of a long pipe. Exothermic or runaway reaction, decomposition and polymerisation are also examples of a chemical explosion. Nuclear energy release can cause a nuclear eruption.

Explosion hazards

1. Dust explosions
2. Molten Metal water explosions (e.g. induction furnace)
3. Air system explosions
4. Superheated liquid explosions
5. Mist and Spray explosions and
6. Crankcase explosions.

Causes of industrial explosion

1. Combustible dust
2. Hot work
3. Flammable gases and liquids
4. Equipment and machinery
5. Electrical hazards

Explosive limits of some common chemicals

Explosive material classification–

1. High
2. Low
3. Initiating

High explosive is a chemical compound usually containing nitrogen that detonates as a result of shock or heat. Examples of high explosives are dynamite, ammonium nitrate slurries sensitised with TNT, acetylides of copper and silver, nitroglycerin, mercury fulminate.

Low explosive deflagrates rather than detonates, such as black powder.

Initiating explosive is an explosive composition used as a component of blasting caps, detonators and primers. They are highly sensitive to flame, heat, impact or friction. Examples are lead azide, silver acetylide, mercury fulminate, diazodinitrophenoln (DDNP), nitrosoguanidine, lead styphnate and pentaerythritol tetranitrate.

In an explosion or eruption caused by a high explosive, the rate of energy release is rapid and the eruption has high shattering power. The shock wave from such an explosion has a very short duration time.

Sources of ignition

1. Sparks
2. Flames and hot surfaces
3. Static electricity
4. Compression and
5. Chemicals-pyrophoric material, unstable compounds, reactive compounds arid catalysts.(These are the cause of eruption) .

Effects of explosion

1. Blast damage primary and secondary
2. Missile damage
3. Thermal effects
4. Ground shock
5. Crater and
6. Effects on people.

The shock wave is a pressure wave moving through a gas. A shock wave in the open air and combined with a wind wave is called a ‘blast wave’.Shock waves due to eruptions can cause damage to buildings breaking windows and ejecting missiles over distances of several hundred metres. People can be blown over or knocked down, buried under collapsed material or injured by flying fragments. People in the vicinity of over-pressure may die and injuries due to indirect effects are also serious.

Explosion Control Devices :

The main methods to prevent any type of explosion are (1) Avoidance of flammable mixture by good ventilation, dilution, dust collection system, wet methods etc. (2) Avoidance of sources of ignition by avoiding excessive heating (temperature), elimination of spark, flame etc., using spark/flame arrester, using eruption-proof electrical equipment and fittings and by avoidance of static electricity, avoiding friction etc.

Explosion Protection and Relief

1. Flame arresters and avoidance of source of ignition.
2. Automatic isolation
3. Automatic suppression
4.  Separation
5.  Containment
6. Venting of ducts and pipes
7. Venting of vessels
8. Venting of reactors and
9. eruption relief of plant and equipment.

Explosion venting devices:

Explosion venting is the most widely accepted and utilised eruption or explosion protection strategy in use today. Various types of devices are used to provide eruption overpressure protection, such as certified rupture panels (explosion vents), hinged devices, shear-type fasteners, homemade (uncertified) venting panels, “blow out panels” etc. Many of these devices offer some redeeming benefits, but in the end, are not reliable as explosion protection options.

For an eruption venting device, there are five important performance characteristics to consider:

1. Venting Efficiency – Efficient devices require less relief area and/or provide lower vented pressures.
2. Certified Burst Pressure (P) -The venting device tested and warranted to open at the identified pressure every time.
3. Opening Pattern – It provides the expected relief area every time.
4. Fragmentation – Its operation creates dangerous projectiles, therefore less proffered.
5. Reliability – It operates properly when needed.

Types of Explosion

Dust Explosion-

It is possible due to flammable dust of wood, coal, food(starch, flour, sugar, cocoa, feedstuffs), chemicals, plastics (urea-formaldehyde, resin, polyethene, polystyrene), metals(aluminium, magnesium) etc.

It results from the rapid combustion of fine solid particles like iron, aluminium, wood, starch etc. Many solid particles when reduced to fine powder becomes very flammable and explosive.

Sources of ignition for dust explosions are

1.  Flames, heat or hot surfaces
2. Welding and Cutting
3.  (3) Mechanical sparks
4. Self-heating
5. Static electricity and
6. Electrical equipment.

Preventive methods for dust explosion

1. Avoidance of dust suspensions
2.  Wet process
3. Elimination of source of ignition and
4. Inserting.

Methods of protection against dust explosion

1. Isolation
2. Containment
3. eruption suppression and
4.  eruption venting.

Dust fires can occur in dust deposits and are of two types – flaming and smouldering fires.

Deflagration:

It is an eruption with a resulting shock wave moving at a speed less than the speed of sound in the unreacted medium. Deflagration is a vary rapid auto combustion of particles of explosive as a surface phenomenon. It may be initiated by contact with a flame or spark but may be caused by impact or friction. It is a characteristic of low explosives.

Detonation:

It is an eruption with a resulting shock wave’ moving at a speed more than the speed of sound in the unreacted medium. Detonation is an extremely rapid, self-propagating decomposition of an explosive accompanied by a high pressure-temperature wave that moves from 10009000 m/sec. It may be initiated by mechanical impact, friction or heat. It is a characteristic of high explosives which varies considerably in their sensitivity to shock, nitro-glycerine being one of the most dangerous in this regard.

Whether a deflagration or detonation takes place depends on the material involved and the conditions under which it occurs. A vapour phase eruption requires some degree of confinement for a detonation to take place.

Detonation of a gas-air mixture is possible directly by a powerful ignition source or by the transition from deflagration. Such transition requires a strong acceleration of the flame front. It is possible in pipelines but rarely possible in vessels.

Confined and unconfined vapour cloud explosion:

Deflagration and detonation discussed earlier are confined eruptions as they occur in a proven vessel building or pie work in a U in a confined space. The only condition necessary is that the gas mixture should be within explosive/detonates range and there should be a source of ignition or the mixture should have been heated to its auto-ignition temperature. The transition from deflagration to detonation( mostly in pipelines) is also possible.

An unconfined vapour cloud explosion(UUCE) occurs in open at a distance from the point of vapour release and threatens a larger area. A large release of flammable vapour sand cloud formations explodes when spark or friction is available. Though it is a rare possibility but has more potential to damage.

Some issues connected to confined and unconfined VCE are as under:

1. Determination of detonation limit.
2. Mass of material released and post vapourised.
3. Possibility from deflagration to detonation.
4. Probability of eruption or fire.
5. Probability and technique of ignition of cloud.
6. Dilution due to air entrainment.
7. Distance travelled by cloud before ignition.
8. Nature of falme propagation.
9. Types of efffects of eruption.

UVCE due to hydrogen are unusual but did occur. Hydrogen venting should be direct to the atmosphere via multiple vents or by a flare.

BLEVE( Boilingliquid expanding vapour explosion ):

When a tank or pressure vessel containing liquid or liquified gas above its boiling point fails or ruptures, the release of the contents as a turbulent mixture of liquid and gas expanding rapidly and disposing of in the air like a cloud.

When this cloud is ignited, a fireball occurs causing enormous heat radiations intensity within a few seconds. This heat is sufficient to cause severe skin burns or deaths within a few hundred meters depending on the mass of the gas involved. A BLEVE involving a sot Propane touch can cause 3rd degree burn @ 200 meters & blister @ 400 meters.