Gary Horgan (CMSE Consultancy Manager at the Chris Mee Group) and his team are outlining the path for companies to ensure they are compliant with Part 8 “Explosive Atmospheres at Places of Work” of the Health, Safety & Welfare at Work (General Applications) Regulation 2007 in a series of focussed blogs.
This is Blog number 4 in the series.
In this latest blog of our series looking at explosion safety, we will be discussing some of the terminology and science behind explosions. These are important to understand when putting together an Explosion Protection Document for your site.
First of all, it is important to clarify the similarities and differences between fires and explosions. Fire is caused is when a substance rapidly consumes oxygen to produce heat and light. An explosion is an event where energy rapidly expands outward from the source, often creating a damaging shock wave. A fire may not cause an explosion, and explosions can happen without fire. In an explosion there is a rapid expansion in volume associated with an extremely vigorous outward release of energy, usually associated with the generation of high temperatures and release of high-pressure gases (e.g. a blast).
Explosions can be due to a variety of causes, for example natural (e.g. due to volcanic activity), nuclear (e.g. from nuclear fission or from a combination of fission and fusion), electrical (e.g. an arc blast) or mechanical (e.g. the bursting of a sealed or partially sealed container under internal pressure).
However, the type of explosion which most often springs to mind is chemical explosions which involve combustion processes. e.g. artificial chemical explosives such as dynamite, usually involve a rapid and violent oxidation reaction that produces large amounts of hot gas. To understand how a chemical (combustion) explosion occurs we need an understanding of the combustion process.
When a flammable substance (fuel) is available with the presence of an adequate amount of oxygen, air having ~21%, and an ignition source is introduced, then the chemical oxidation reaction i.e. the combustion process, can be initiated.
To achieve combustion the amount fuel available must be within its flammable range i.e. the fuel must be in a concentration that will burn in air. Where the concentration is too low or similarly, where the concentration is too high, combustion will not occur. Any concentration in between, i.e. within the flammable range, will combust. The upper and lower combustible concentrations are known as the flammable limits. i.e. Lower Flammable Limit (LFL) and the Upper Flammable Limit (LFL) also known as the lower explosive limit and upper explosive limit respectively. This is true for gases and vapours and for dusts.
In normal combustion reactions (burning) the thermal expansion of gases will be easily dissipated and result in no large differential in pressure and therefore no explosion. For example, the burning of coal in an open fire will result in the evolution of heat and the formation of gases, but the liberation is not enough to build up a sudden pressure differential and result in an explosion.
Higher burning rates (rate at which the unburned gases move into the flame front) create higher flame speeds (the rate at the flame travels through a gas cloud) which result in a raid of energy and larger differential pressures known as explosions. Explosions are normally described as either deflagrations or detonations.
The speed of the reaction is what distinguishes an explosive reaction from an ordinary combustion reaction. Most “fires” from flames to explosions are deflagrations and involve subsonic combustion and propagate through heat transfer; i.e. hot burning material heats the next layer of cold material and ignites it.
The flame speed in deflagrations, can vary. A low-speed deflagration in the open air may result in “flash fire” and release of heat. However, large-scale, short-duration deflagrations can release a large amount of energy and result in damage to buildings, equipment and people. The amount of damage is dependant on the total amount of fuel burned (total energy available), the maximum flame velocity achieved, and whether the combustion gases are contained.
Detonations, however, involve supersonic flame speeds and an exothermic flame front accelerates through a flammable material. The accelerating flame front causes pressure waves which are pushed ahead of it and can eventually result in a shock front (shock waves) propagating directly ahead of the flame front. These can result in very large explosions, which can be very devastating to structures.
Now we have an understanding of the science behind explosions and the combustion process, we will explore some different types of explosions that can occur in industrial settings.
Where there is fire impingement on a vessel, depending on the nature of the vessel, this can cause an explosion known as Boiling-Liquid-Expanding-Vapor Explosion (BLEVE).
This is where the radiated heat causes the temperature in an exposed pressure vessel to rise, increasing the internal pressure. Where the heat input is large enough, the pressure within the vessel may continue build, and despite the activation of pressure relief valves, may cause the vessel to fail. This allows the liquid in the vessel which is above its atmospheric boiling point to rapidly depressurize. The instantaneous transition from liquid to vapor is accompanied by a large energy release.
A BLEVE involving flammable material is usually accompanied by a large aerosol fireball. The amount of radiated heat flux from such as fireball is very large and can have disastrous effects on the surroundings for a significant distance. However, it is not necessary for the liquid to be flammable to have a BLEVE occur. i.e. there would be blast effects from the vessel failure but no fireball.
The following YouTube video gives a good visual demonstration of BLEVEs:
Where a fire (low-speed deflagration) occurs within a closed vessel or structure, the heat release causes the combustion gases and excess air to expand thermally causing pressure rise within the vessel. The vessel must be strong enough to withstand the additional internal pressure, or it will fail, allowing the gases to escape. The failure may impact on other vessels or structures in the area. This is a growing concern within waste storage facilities.
This can be compared to the combustion of gases being contained inside a long tube or vessel. As the flame front propagates along the tube, the unburned gases ahead of the flame front are compressed, and thus heated. The amount of compression varies depending on the configuration but can be in the range of 2 to 8 times the initial pressure. Where multiple vessels are connected, this pressure piling may result in a transition from a deflagration to a detonation and very large explosion pressures.
Where there is a release of gas, vapour, or mist into the atmosphere, forming a cloud within the fuel’s flammable limits followed by its subsequent ignition, this is known as vapor cloud explosion.
The explosion resulting from the ignition of such vapour clouds can create flame speeds which accelerate to sufficiently high velocities to produce significant overpressure and transition from to deflagration or detonation with potentially damaging pressures within and well beyond the boundary of the cloud.
A typical example of a VCE with very large pressure effects took place in a fuel storage facility in Buncefield in UK in 2005. The initial explosion shock wave measured 2.4 on the Richter scale and was heard up to 150 km away.
The YouTube videos below provide further information on the Buncefield explosion, and vapour cloud explosions in general:
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