Confined flame theory

The amount of explosion overpressure is determined by the flame speed of the explosion. Flame speed is a function of the turbulence created within the vapor cloud that is released and the level of fuel mixture within the combustible limits. Maximum flame velocities in test conditions are usually obtained in mixtures that contain slightly more fuel than is required for stoichiometric combustion. Turbulence is created by the confinement and congestion within the particular area. Modem open air explosion theories suggest that all onshore hydrocarbon process plants have enough congestion and confinement to produce vapor cloud explosions. Certainly confinement and congestion are available on most offshore production platforms to some degree. 

Spalding, D. B. 1959. Theory of rate of spread of confined turbulent premixed flames. 7th Symposium (International) on Combustion Proceedings. Baltimore The Wilhams and Wilkins Co. 595-603. 

Deflagration-to-detonation transition is probably the least understood aspect of detonation theory at this time. Measurements have been made of the distance required to have transition to detonation in smooth tubes. Distances many times the tube diameter have been required. If obstacles are inserted into the tube, the required distance to detonation is greatly reduced. The motion of the expanding gases around the obstacles leads to greatly increased flame front area, rapid flame acceleration and rapid transition to detonation. Confinement greatly promotes transition, but one cannot rule out transition to detonation in a containment if a detonable mixture of sufficient size is present. The second question, "If a detonation is possible, what is the likelihood that it will occur " therefore cannot be answered with certainty at present. 

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