June 29, 2007

scientists react to discovery of explosives in a car in central London

Experts on engineering and explosives commented on the explosives which were discovered in a car in central London.

Keith Plumb, a consultant chemical engineer, said:

“If you think of this in terms of Propane as a fuel, and a car engine as a controlled explosion you can imagine that a small amount of Propane could contain enough energy to move a car many miles. But Propane itself is not an explosive and only with the right mixture of Propane and air it will explode (like adjusting the fuel/air mix in your car engine). Propane needs a source of oxygen, unlike Nitro Glycerine, TNT and fertiliser, which have their own source. In the Flixborough disaster it was the mixing of Cyclohexane with oxygen in air in a huge vapour cloud that made the power of the explosion so dramatic.

“Furthermore, Propane in the cylinders will not explode – you would have to get it out of the cylinders. We might expect therefore that there would be a primary explosion to release the Propane followed by mixing of the gas with oxygen in the air and then ignition of the gas cloud which would create a large secondary explosion. However, to get the maximum potential explosive power is very difficult, and timing of ignition is absolutely crucial. That said, if the power of the explosion is constrained within a car with the addition of projectiles such as nails, this could cause a tremendous amount of damage to nearby people and buildings.”

Dr Clifford Jones, Reader in the Department of Engineering, University of Aberdeen, said:

“The explosive device in London this morning is believed to have comprised containers of flammable gas, most probably propane. These act differently from high explosives such as TNT and dynamite. A propane/air mixture on ignition will create an overpressure if there is significant turbulence: confinement, such as would occur if pre-ignition mixing of air and gas was in an enclosed vehicle, will promote turbulence. Overpressures from gas explosions are smaller than those from high explosives although they can be lethal to persons and capable of destroying a building. A fundamental distinction is that with a high explosive propagation is by detonation and is supersonic, hence the high overpressures and destructive potential. Flammable gas combustion such as would occur with propane/air having been admitted to the inside of a car propagates by deflagration which is always subsonic. That does not preclude overpressure effects if there is turbulence as noted but they will be milder than those from a substance which detonates.

“Initiation of a propane/air mixture requires only a simple ignition source such as a spark or a small flame. The minimum ignition energy of such a mixture is small and such amounts can arise adventitiously from the environment.”

Brian Baker, director of the Association for Petroleum and Explosives Administration, said:

“Propane is liquefied petroleum gas and patio heater gas is usually 97% propane. Cylinders of this type of gas are readily available to the public and can be bought in places such as petrol stations and iron monger in particular. Propane is heavier than air when released, highly flammable and easy to ignite. When released in to the atmosphere and only a small amount is a required to cause an explosive condition and this is worse in a confined space when subject to an ignition source. Its explosive properties mean that 10 litres of the gas is equivalent to 2770 litres of flammable gas and air mix. There are controls on using this type of gas and this is why the industry gives lots of safety advice about use.”