Industrial Accidents 2.1 Accidents
2.1.4 Incidental Scenarios
difficult to be found experimentally. Thus it remains undetermined.
Moreover, it is almost impossible to obtain a homogeneous system dust air with a uniform composition, because segregation phenomena usually happen. This results in hard measures. The reader should keep in mind that the data listed in Table 2.12 should be regarded to as order of magnitude rather than definitive values. In any investigation the forensic engineer should collect samples and make new
measurements whenever it is possible.
VCE and BLEVE, being significant in the industrial context, are discussed in the next Paragraph, dedicated to the most frequent incidental scenarios in the process industry.
outside. The context gives its contribution in defining the produced effects. Toxic, flammable or energy releases may be the consequence of the incident or the cause that generated them.
Having in mind the information provided by Figure 2.14, it is soundly accepted the following distinction for fires:
Flash Fire. It is a sudden blaze with a limited duration of few seconds. It is caused by the ignition of solids, vapors or gases. A rapid and subsonic flame front is its main feature. Figure 2.15 shows an example.
Jet Fire. It occurs when the mixture of oxidiser and gaseous fuel is ignited in one or more directions. The Jet Fire has a significant diffusion and power. The release of a gaseous substance, from a tank in pressure or piping, is its main cause. It may have different shapes (horizontal, vertical or inclined jet) depending on the local conditions, such as the presence of wind and the geometrical configuration.
There are several parameters to evaluate the radiated power.
Some of them are:
Quantity of fuel taking part in the reaction;
distance covered by the jet; and
the distance of the defeat (i.e. the crack) from the source of ignition.
Those parameters are then correlated to other conditions like the dimension of the crack and the inner pressure of the tank. Several parameters contribute to model the Jet Fire in a very complicated way. The interested reader can find further information on [23].
Figure 2.16 shows: on the left, the modelled jet fire for a fire
investigation [29], further discussed as case study at Chapter ; on the right, an example of jet fire.
Pool Fire. It is typical of cylindrical tanks. Generally, it is due to the spill of a flammable or combustible liquid. The horizontal dimension of the flames is comparable with the spill dimension while the height is almost double. It may be confined or not,
depending on whether the spill occurs in a tank or on the
unconfined ground. When the fuel is spilt on water, the pool fire may produce, under specific conditions, the so called “boil over”. It is the boiling of the underlying water with the indirect involvement of a significant quantity of fuel. This sudden phenomenon
produces an increase of one order of magnitude in the combustion rate. To evaluate the radiant power emitted by the pool fire, it is necessary to know:
The geometry of the flame;
the features of the flame;
the combustion rate; and
the radiation and the geometrical related factors.
Typically, the flame is modelled as a vertical cylinder, even if a more accurate model, taking care about its inclination, provides better results, especially in windy conditions. For more details, see [23]. Figure 2.17 shows an example.
Fireballs. They can be the consequence of two events:
The collapse of an LPG tank, with the consequent vaporisation and ignition of the containment; and
the ignition of a cloud of gaseous fuel (rarer. Generally it generates a flash fire).
In both the two cases, the cloud of fuel burns with a diffusive flame, as shown in Figure 2.18.
Figure 2.15 An example of Flash Fire.
Source: Frame from [28].
Figure 2.16 On the left, a modelled jet fire for a fire investigation
Source: [29]. On the right, an example of a jet fire Source: [30].
Figure 2.17 Example of Pool Fire.
Figure 2.18 Schematic representation of a fireball in the stationary stage.
Figure 2.19 A Vapor Cloud Explosion test.
Source: Reprinted with permission from [31].
Modelling a fireball allows an estimation of the thermal radiation produced. To do so, it is necessary to evaluate:
Involved mass of fuel;
diameter and duration; and
radiation and geometrical related factors.
Three of the models widely adopted to calculate the emitted power per unitary surface of the fireball (thus an estimation of the
radiation on the surroundings) are the following [23]:
Point source model. It considers that all the energy is produced from one single point, i.e. the centre of the fireball;
solid flame model. It estimates the power radiated by assuming that the flame is equivalent to a grey body; and
flame emissivity. It evaluates the emitted power starting from some peculiar parameters like temperature, dimension, and composition of the flame.
Regarding explosions, the following incidental scenarios can be observed:
Unconfined Vapor Cloud Explosion (UVCE). It is an
unconfined explosion of vapor in the atmosphere, even if partial confinement is usually the real context because of natural
obstacles, the presence of buildings or simple openings that deeply
influence its dynamics. This kind of explosion is worldly
considered among the most dangerous in the chemical industry sector. It is generated after the release of a significant amount of vapors in the atmosphere. The ignition and the subsequent
combustion create a flame front and an expansion of the
combustion products, causing a pressure wave that sometimes may evolve in a shock wave. The flame front may accelerate significantly in case of congestion (i.e. in presence of many small obstacles like pipings, trees) to supersonic value such that locally a detonation may occur. One of the most sadly famous examples is the one in Flixborough, which occurred in 1974, where 40 tonnes of
cyclohexane were released from a reactor, generating a cloud of half a million of cubic meters whose ignition caused 28 victims, the destruction of the plant and an economic loss of 150 billion US$.
Confined or partially confined explosion. In this kind of explosion, energy is released inside a containment structure, like a tank, a reactor, a room or building. If the explosion, involving piping or a vessel, is generated by a flammable gaseous mixture, it is possible to have deflagration or detonation. Instead, it is not clear whether detonation may be generated from a dust explosion, in the context of an industrial plant. Confined explosions produce pressure waves that may cause, in an interconnected system, the so called “pressure piling”. The phenomenon is caused by an increase in both temperature and pressure inside a tank, determining a similar growth in the connected system, thus
generating further increases. To avoid this complex behavior and isolate the various systems, it may be useful to install rapid
depressurization valves.
Boiling Liquid Expanding Vapor Explosion (BLEVE). It occurs when a tank, containing a liquid under pressure, collapses suddenly causing the rapid depressurization and the subsequent evaporation of the fluid, resulting in an extremely dangerous explosion (Figure 2.20 and 2.21). The most frequent cause is the engulfment in flames of a tank. Flames warm the upper part of a tank (the one above the liquid level), while the lower part remains at a lower temperature, because heat is transferred to the liquid
inside that changes its phase. This may result in a decrease of the mechanical resistance of the metal above the liquid level and a parallel increase of the inner pressure because of the increase of the vapor pressure. Therefore, from a structural point of view, actions increase and strength decreases. When the crack appears, the subsequent prompt evaporation, due to the sudden
depressurization, causes a catastrophic rupture. A famous example is the incident of Mexico City, which occurred on 19 November 1984, which caused about 500 victims and more than 7000 injured people. On that occasion, the rupture of a piping containing LPG was the reason for a release of a flammable cloud that was ignited by the torch of the plant. The Vapor Cloud Explosion (VCE) and the jet fire that followed were the two contributors of the first
BLEVE of an LPG spherical tank. Fifteen further BLEVEs followed, causing the complete destruction of the plant.
Figure 2.20 Sequence events to BLEVE.
Figure 2.21 Example of BLEVE.
Source: Frame from [32].