Overpressure Generation in Explosions

A Technical Introduction to the CPD course on Explosion Hazards 12-16 March 2018, Leeds UK.

Explosion Severity

The explosion severity and potential damage resulting from an accidental gas, vapour or dust explosion is evaluated on the basis of the maximum pressure generated and the duration of the pressure pulse.

In the above CCTV footage the fast flame (seen from two angles) is what causes the overpressure and damage. But what makes the flame go hundreds of times faster in this environment compared to on open field? - a brief explanation of the main driving mechanism is given towards the end of this article.

Most pressure plant and equipment will generally withstand overpressures (pressure above atmospheric) up to 0.5 bar (1 bar = 100,000 Pascals, 1 atm). Total destruction of plant and buildings occur at overpressures of 0.7 bar or higher. Accidental explosions are capable of generating overpressures from several bar to 15 to 30 bar in the most energetic events where transition to detonation occurs. Designing structures to withstand the full potential of such events is cost (and weight) prohibitive.

Understanding the characteristics and dynamics of explosion development and pressure generation in different scenarios is key in developing appropriate prevention and mitigation strategies.

In general there are two main scenarios: explosions within an enclosure (a process equipment or a building) or explosions out in open but generally congested areas (with equipment or structures) generally known as vapour cloud explosions.

Explosion Pressure Generation and Mitigation in Enclosures

In the extreme case, where the gas or dust cloud is totally enclosed in a vessel that can take high pressures, then typically overpressures of 7 to 8 bar are generated. The combustion process in this scenario may be thought of as changing the temperature of the gas/air volume in the vessel (typically within a fraction of a second) from near atmospheric temperature of 300 K to near adiabatic flame temperature of 2000-2500 K. This increase in temperature would result in 7 to 8 times increase in the volume of the combustion gases if allowed to expand but since the enclosure prevents such expansion the pressure goes up instead by the same factor.

As discussed earlier, unless specifically designed most process plant and buildings can only withstand a small fraction of this potential pressure rise, so mitigation measures are needed to prevent the structures experiencing such pressures. These measures include: (a) Explosion Suppression, where the explosion is detected very early (in first few milliseconds of its life and a calculated amount of suppressant agent is fast flooded into the enclosure extinguishing the explosion before it generates damaging overpressures. (b) Explosion Venting (see Fig.1) where a designed or fortuitous weak part of the structure breaks or opens up relieving the pressure to the outside and limiting the maximum pressure experienced by the structure to safe levels.

Fig. 1. Internal (left) and external (right) view of a vented gas explosion - Source: British Gas Research (Now DNV-GL)

Vapour Cloud Explosion Pressure Generation and Mitigation

Most "enclosure" explosions, discussed above typically involve a single process equipment, reactor, building etc. However, the most destructive explosions in the process industry are vapour cloud explosions (VCE).

VCEs involve the release of a large amount of flammable gas or vapour into an area of the plant and whilst the main event may be precipitated by ignition and explosion within an enclosure, it is the involvement of the larger cloud that is particularly destructive. In this scenario the mechanism of pressure generation is quite different, in that the expansion of the burnt gases is not restricted by any confinement. What causes the pressure rise in a VCE is the inertia of the surrounding atmosphere to the propagation of a a fast moving flame front. For this effect to be significant the flame has to be moving fast, typically from 100 to several hundred m/s. This is not possible in an explosion in an open space where such flames can only move at about 10 m/s. The congestion presented by the plant (or in some of major incidents by vegetation) is an essential component of the mechanism of pressure generation.

In a VCE as the hot combustion products behind the reaction front expand they generate flow ahead of the flame front. This flow interacts with rigid structures and generates flow turbulence downstream of the abstraction. When the flame front arrives in the turbulent region it distorts generating a much larger reaction front area and it burns locally faster, both of which effects generate a much faster burning and production of hot products which generate faster flow and stronger turbulence downstream of the next set of obstructions to the path flame. This generates a accelerating dynamic feedback loop which which can give rise to very fast flame and hence high overpressures of the order of 1 to 3 bar, and associated destruction. This mechanism is illustrated in Fig.2.

Fig. 2 Flame acceleration mechanism in Vapour Cloud Explosions.

In the CCTV video of the PEMEX refinery explosion, at the top of this article, the process plant piping and equipment are acting as the turbulence generating obstacles in the path of the propagating flame - this causes the flame to be accelerating through the plant generating destructive pressures.

Through this and other related, more complex mechanisms, the flame can be accelerated to flame speeds that can lead to transition to detonation which is associated with spikes of tens of bars and with devastating consequences..

Presenting the current understanding and knowledge on this area is one of the key objectives of the well established Explosion Hazards CPD Course at Leeds, 12- 16 March. The first two days of the course deal with enclosure explosions (gas, dust and vapour) while the second part of the week deals with Vapour Cloud Explosions an d modelling of explosions. A number of a leading industrial speakers will help with the demonstration of the application of the theoretical understanding in the design of effective mitigation measures and strategies while detailed analysis of major incidents (including Buncefield) will bring to focus some of the important lessons learnt.

Dr Phylaktou and Prof Andrews are directors of this high level CPD Course. Dr Phylaktou is a member of the Committee and Trustees of the UK Explosion Liaison Group (UKELG).

We hope to see some of you at the course, please click here for more details.