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Explosive hazard
 

2Definition and characteristics2 There are two types of explosion:

The first one is an explosion resulting from a high speed combustion (see “Fire hazard”), which can occur within an explosive product or device itself or within a gas or flammable dust particles mixture with air. In this case, a flame propagates through the mixture at a speed of 1 to 10 metres per second depending on the reactivity of the combustible and the mixture proportions. This flame projects ions which propagate the combustion reaction in the mixture. The gases generated by the combustion (notably CO2 and H2O) are disseminated at the back of the flame front on which they exert pressure.

Another type of explosion is pneumatic bursting, for example the bursting of a closed tank (typically a reactor) due to the excessive increase in the pressure of the gases it contains or the localised weakening of its wall strength.

There are also explosions resulting from the combination of chemicals referred to as “incompatible” and reacting violently. 2How explosions occur2 For gas or dust explosions, the causes are the same as those of fire. Possible sources of ignition are varied:

  • Naked flame or incandescent solid
  • Hot spot caused by Joule effect or friction
  • Lightning
  • Electric or electrostatic spark
  • Adiabatic compression of a gas (which subsequently heats up)
  • etc.

Ignition causes can be natural (lightning), human (carelessness, malice) or technological.

Air mixtures with flammable or combustible substances are more or less prone to ignition. The following characteristics are commonly used (established based on standards):

  • Lower explosive limit or LEL: minimum concentration of a combustible gas or dust particle in the air to form a mixture capable of exploding in the presence of a specified source of ignition (methane: 5% in volume, wheat dust: 50 g/m3); below LEL, the mixture is deemed “too weak” to explode
  • Upper explosive limit or UEL: maximum concentration of a gas in the air to form a mixture capable of exploding in the presence of a specified source of ignition (methane: 15% in volume); beyond UEL, the mixture is deemed “too rich” to explode.

The origin of the tank bursting accident is often the so-called “surcharge” of a liquid or liquefied gas. A sufficient part of the tank should always be reserved for the gaseous phase to allow the evaporation of the liquid during external temperature variations and the moderate increase in internal pressure. The surcharge and subsequent reduction in the volume dedicated to the gases can cause the bursting of the tank. 2Explosion effects2 The explosion mostly produces excess pressure and thermal effects as well as projection effects.

The effects of the excess pressure generated by an explosion due to the production of combustion gases are relatively limited in the open. According to the level of containment and congestion of the place where the explosion occurs, excess pressure effects can become significant. In the case of extreme containment, it can reach ten bar. Almost all gas or dust explosions have a flame velocity of under 100 metres per second and excess pressure of less than 10 bar: these are blasts. Under certain conditions (notably when products are confined), the transition from blast to detonation is possible.

The effects of a tank bursting are excess pressure effects, due to the sudden expansion of compressed gases and the instantaneous vaporisation of part of the liquid phase, as well as the projection of tank fragments.

Regulatory threshold values to evaluate the effects of excess pressure on man or structures are as follows:

  • 20 mbar corresponding with the effects of glass breakage
  • 50 mbar corresponding with irreversible effects and minimum damage to structures
  • 140 mbar corresponding with first lethal effects and substantial damage to structures
  • 200 mbar corresponding with significant lethal effects and serious damage to structures
  • 300 mbar corresponding with very serious damage to structures

The thermal effects of an explosion are due to the radiation of the flame and hot combustion gases. Their range and seriousness vary depending on the extent of the explosion’s propagation and speed. The more confined or congested the explosion, the greater the flame velocity and excess pressure; thermal effects are therefore less significant, as the flame “passes too fast” and the effects of excess pressure are largely predominant. In addition, the closer to the conditions for explosion the mixture is over a vast expanse, the more affected distant targets are by the cumulative effects of the radiation.

Description of the damage caused by heat radiation on man or structures: “Fire hazard”. 2How to prevent explosions or limit their effects?2 By making sure that the combustible (gas or dust), combustive (air) and the source of ignition never combine to create conditions conducive to an explosion, for example:

  • by removing the combustive: remove the air using floating covers in hydrocarbon tanks or replace it with an inert gas ceiling for highly flammable liquids or heated flammable liquids,
  • by removing the combustible: sufficient ventilation of premises likely to contain flammable gases (maintain the gas-air mixture below LEL), detection of gas leaks or flammable liquids, suction of combustible dust at source, regular cleaning of dust deposits (in cereal storage silos),
  • by removing the source of ignition: remove or eliminate any heat source, use insulated electrical equipment, adapted to potentially explosive atmospheres (ATEX), burning and hot work permit instructions.

Containment should also be limited to avoid significant excess pressure effects, for example by creating special surface areas (vents) which open under the effect of pre-calculated excess pressure, thereby evacuating the explosive energy before its transmission to other explosive atmosphere areas (in the case of silos).

In the case of dust explosions, the idea is to prevent the explosion transmitting from one volume to the next by creating resistant separations (isolation) between volumes. As the blast of an explosion is capable of suspending potential dust deposits accumulated on surface areas, it can use volumes originally without an explosive atmosphere to create new conditions conducive to the explosion, with possible accelerated chain growth effects (scenario of the accident which occurred in Blaye, Gironde, in 1997 - silo).