The University of Newcastle, Australia

Concerns with employing VAM abatement technologies

A critical area of concern for the operation of VAM abatement plant on a coal mine site is the explosion hazard and how to effectively counteract a potential explosion event should this mishap eventuate.

Upset conditions within the mine such as outburst or strata failure could lead to a sudden spike in methane levels within the VAM above the lower explosive limit or LEL of 5%.

The VAM abatement units are a likely ignition source if preventable controls do not divert or dilute the flammable mixture before it enters one or more of the abatement units.

An explosion with a resultant flame and pressure wave may be directed back towards the mine and claims lives and property damage.

Deflagration and Detonation

Deflagration and detonation are two types of explosions reported widely in extractive industries.

A deflagration is the most common type of fire and explosion characterised by a combustion front moving at a sub-sonic flame speed, a separate pressure wave of up to 10 Bar influenced by a heat transfer propagation mechanism.

A transition from deflagration-to-detonation or DDT can occur when the combustion zone couples with the shock wave to form a more violent detonation type explosion. A detonation is the most violent form of explosion with greatest potential for severe damage with pressures in the range of 20 to 80 Bar.

The flame can increase to supersonic speeds under a compression propagation mechanism by the influence of confinement and/or obstructions interaction.

Countermeasures

Countermeasures are defined as “an action taken to counteract a danger or threat”.

Such extrinsic or add-on safety measures typically are applied as separate layers of protection and are seen as a last line of defence should earlier actions directed to defeat the accident sequence (e.g. inherent safety, dilution etc.) fail to achieve the required level of tolerable risk.

Choosing an optimum countermeasure system involving one or more categories is recognised as being a complex and difficult task, particularly for a highly variable hybrid gas/dust mixture such as VAM.

Also one must consider the potential for a deflagration low intensity type of explosion to transition to a more intense detonation type explosion under suitable conditions.