2.1 Properties of flammable and explosive materials and the mechanisms by which they ignite EXPLOSION

Question 1

An explosion is

Answer 1

a rapid expansion of gases resulting in a rapidly moving pressure or shock wave. The expansion can be mechanical (by means of a sudden rupture of a pressurised vessel), or it can be the result of a rapid chemical reaction (such as combustion). Explosion damage is caused by the pressure or shock wave.

Question 2

Mechanical explosion:

Answer 2

An explosion resulting from the sudden failure of a vessel containing high-pressure, non-reactive gas.

Question 3

Deflagration:

Answer 3

An explosion in which the reaction front moves at a speed less than the speed of sound (sub-sonic) in the unreacted medium.

Question 4

Detonation:

Answer 4

An explosion in which the reaction front moves at a speed greater than the speed of sound (supersonic) in the unreacted medium.

Question 5

Confined explosion:

Answer 5

An explosion which occurs within a vessel or a building. These are most common and usually result in injury to the building inhabitants and extensive damage.

Question 6

Unconfined explosion:

Answer 6

Unconfined explosions occur in the open. This type of explosion is usually the result of a flammable gas release. The gas is dispersed and mixed with air until it comes in contact with an ignition source. Unconfined explosions are rarer than confined explosions because the explosive material is generally diluted below the lower flammable limit (LFL) by wind dispersion. These explosions are destructive because large quantities of gas and large areas are frequently involved.

Question 7

Boiling-liquid expanding-vapour cloud explosion (BLEVE):

Answer 7

A BLEVE occurs if a vessel that contains a liquid at a temperature above its atmospheric pressure boiling point ruptures. The subsequent BLEVE is the explosive vapourisation of a large fraction of the vessel contents, possibly followed by combustion or explosion of the vapourised cloud if it is combustible. BLEVE’s occur when an external fire heats the contents of a tank of volatile material. As the tank contents heat, the vapour pressure of the liquid within the tank increases and the tank’s structural integrity is reduced because of the heating. If the tank ruptures, the hot liquid volatilises explosively.

Question 8

Dust explosion:

Answer 8

This explosion results from the rapid combustion of fine solid particles. Many solid materials (including common metals such as iron and aluminium) become flammable when reduced to a fine powder.

Question 9

Shock wave:

Answer 9

An abrupt pressure wave moving through a gas. A shock wave in open air is followed by a strong wind; the combination of shock wave and wind is called a blast wave. The pressure increase in the shock wave is so rapid that the process is mostly adiabatic. (Adiabatic processes occur without input or release of heat within a system).

Question 10

Overpressure:

Answer 10

The pressure on an object as a result of an impacting shock wave.

Question 11

Critical temperature:

Answer 11

The temperature above which a gas cannot be liquefied, regardless of the pressure applied.

Question 12

Maximum explosion pressure (MEP):

Answer 12

The explosion pressure is the peak value of the time dependent pressure measured in a closed container (European standard test container) upon deflagration of an explosive mixture of defined composition. The maximum explosion pressure is the maximum value of the explosion pressure determined by varying the composition of the mixture.

Question 13

Rate of pressure rise:

Answer 13

The increase in pressure divided by the time interval necessary for that increase to occur. It is determined under the same test conditions as the maximum explosion pressure.

Question 14

A vapour cloud explosion may be confined (for example, in a tank or vessel) or unconfined (in the open air). The key requirements for a VCE to occur are: 2

Answer 14

 the presence of flammable vapour at a concentration between the upper and lower explosive limits  an ignition source that exceeds the minimum ignition energy.

Question 15

Confined vapour cloud explosions (CVCE)

Answer 15

An explosion can occur when a gas or vapour cloud is ignited within a confined volume such as a building or vessel. As the flame propagates through the vapour cloud it produces hot combustion products. The confinement prevents expansion of these combustion products and, as a consequence, the pressure increases. In general, this continues until the confining structure fails, in some cases catastrophically.

Question 16

Unconfined vapour cloud explosion (UVCE) UVCE’s occur in the following sequence:

Answer 16

 A large quantity of flammable vapour is suddenly released, usually as a consequence of a vessel rupture. (This is likely to be a minimum of 5 tonnes – for a flammable mix to occur in the open air).  The vapour disperses around the site and mixes with air.  The vapour cloud is ignited.

Question 17

There are two known mechanisms for generating an explosion in a relatively unconfined vapour cloud:

Answer 17

 Deflagration, where the flame accelerates to high speed, which requires a mechanism for generating the flame acceleration (turbulence).  Detonation which, if sustained, can be much more damaging. Detonation may arise from the coalescence of a strong shock wave and a fast-moving chemically reacting front, which together can propagate faster than the speed of sound and produce over-pressures at the wave front in excess of 10 bar.

Question 18

A BLEVE (ble-vee) occurs when

Answer 18

a tank containing a liquid, held above its atmospheric pressure boiling point ruptures, resulting in the explosive vapourisation of a large portion of the tank contents.

Question 19

Boiling-liquid expanding-vapour explosion (BLEVE) The process is: 10

Answer 19

 Fire impinges upon the walls of the LPG tank or cylinder.  Fire heats the walls of the tank.  The liquid within the tank helps to cool the tank metal.  As the temperature increases the tank contents boil and vapourise causing an increase in pressure.  The pressure relief valve operates letting off excess pressure (this vapour may be ignited by the fire and flare off).  Once the pressure is relieved the valve closes.  This cycle will be repeated.  The liquid levels within the tank fall.  The tank metal above the liquid level is not cooled, as it heats it expands and weakens until it eventually ruptures.  Upon rupture the tank contents are explosively vapourised.

Question 20

Precautions for controlling vapour phase explosions include: 7

Answer 20

 Segregation / separation of plant – reduces likelihood of ignition and minimises consequences of an explosion.  Hazardous area classification (HAC) and specification of appropriate electrical equipment (removal of potential ignition sources).  Designing plant to withstand pressure, or release it in a controlled manner via pressure relief valves, bursting discs, etc.  Regular inspection, testing and maintenance of plant to reduce the likelihood of loss of containment.  Reducing the inventory – UCVE’s in particular require a significant volume of vapour to create an explosive mix in the open air.  Use of inerting (replacing / reducing quantity of oxygen in air by replacing it with an inert gas) in fully enclosed systems to prevent the creation of an explosive atmosphere.  Monitoring of flammable vapour levels, linked to automatic ventilation or inerting systems.

Question 21

the dust explosion pentagon

Answer 21

For a dust explosion to occur the three sides of the fire triangle must come together: (1) Combustible dust (fuel) (2) Ignition source (heat) (3) Oxygen in air (oxidizer) The following additional conditions will also need to be met: (4) Dispersion of dust particles in sufficient quantity and concentration (5) Confinement of the dust cloud.

Question 22

For many organic materials the lower explosive limit is in the range

Answer 22

10 - 50g/m3dust cloud of this concentration resembles a very dense fog. Upper explosive limits are difficult to measure accurately, and have little practical importance, but are usually considered to be approximately 10 times the lower limit.

Question 23

The most violent explosions usually result from dust/air mixtures that are fuel rich. Other factors which can strongly affect the force of the explosion and the explosive limits include the shape and size of the dust particles, and the moisture content. Only weak explosions are likely where

Answer 23

the mean particle size of the dust exceeds 200 microns, or the moisture content exceeds 16%.