Industrial Accidents 2.1 Accidents
2.1.3 Explosions
Canisters of wasted polyethylene, stacked, height 4.5 m 53 Polyethylene bottles packed in cardboard boxes, height 4.5
m
82 Pallets made of polyethylene, stacked, height 1 m 145 Pallets made of polyethylene, stacked, height 2 m 31 ÷ 55 Single mattress in polyurethane, horizontal 115 Polystyrene tubs stacked in cardboard boxes, stacked, height 4.5 m
115 Polyethylene and polypropylene films in rolls, stacked,
height 4 m
38 Insulating panels in rigid foam in polystyrene, stacked,
height 4 m
6
Source: Data taken from [21].
home is destroyed. The difference between fire and explosion here is the combustion rate. In the first case gas burns slowly throughout the night. In the second case the gas burns in about 2 to 3 seconds. Same amount of gas, hence same energy delivered, but different duration hence different power.
The reader should also consider that not all combustion phenomena may result in an explosion. Combustion may occur in three ways:
Diffusive flame;
premixed flame; or smoldering (no flame).
When fuel and oxidant move to come into contact to make a reaction, you get a diffusive flame. Examples are a candle, the burning of a pool of liquid or a pile of wood. In general, all fires, even of a very huge size such the fire of a huge building, occur by diffusive flame. This is a rather slow phenomenon as the limiting phenomenon is mass transfer which drives the motion of fuel and oxidant that must come in contact.
Diffusive flames cannot produce explosions.
When fuel and oxidant are mixed together before the combustion starts, after ignition you will get a premixed flame. Examples are an oxyacetylene torch, a gasoline engine, or the flame you get after an unwanted release of flammable gas or vapor in air. Flame speed is not more limited by mass transfer and the flame front may reach very high speed. Premixed flames may cause an explosion.
Smoldering is the burning of a solid without flame. A typical example is the burning of a cigarette. Smoldering itself cannot produce
explosions but may produce a huge quantity of smoke containing unburnt flammable substances. It may, sometimes, cause the
accumulation of a flammable atmosphere in a closed environment. An example is the smoldering combustion of wood chip in a silo which often ends with the silo explosion.
An explosion is the ignition of a mixture of one or more flammable substances in air, with a consequent rapid volume expansion or a pressure increase, depending on the space, confined or not, in which the event takes place. The ignition starts the chemical reaction,
producing heat that is then transferred to the adjacent mixture, thus generating a reaction (or flame) front that moves from the combusted gases to the fresh mixture. The propagation rate of the reaction front depends on the velocity of the heat conduction. Explosions can be distinguished, according to their conformation and the reached propagation rate value, in:
Deflagration. It is an explosion characterised by a flame front that proceeds with subsonic speed. Usually, explosions of gases and airborne powders are part of this category, even if a blast inside a long duct, like a tunnel, may transform it into a detonation. A deflagration is a reaction propagated by heat transfer [22]; and detonation. It is an explosion whose flame front has a marked turbulent structure, proceeding at supersonic speed. Compression waves are generated, and they precede the reaction front,
propagating in the fuel mixture like a shock wave. Therefore, a detonation can be defined as a reaction propagated by a shock wave [20]. This phenomenon is typical of those real explosive substances that detonate.
The difference between detonation and deflagration are shown in Figure 2.12.
Figure 2.12 Shock front and pressure front in detonations and deflagrations.
Source: Adapted from [23].
An explosion is defined:
Mechanical, if it is due to the rupture of a recipient in pressure not containing a reactive gas;
chemical, if it is generated by the rapidly expanding gases produced by a chemical reaction;
physical, if it is due to the expansion of a liquefied gas, even if stored at room pressure;
confined, if it occurs inside a container, a vessel, a building. The confinement is responsible for a significant increase in pressure; or not confined, if it occurs outdoors. It may occur because of a leak of flammable gases or vapors.
Depending on the type of the explosion, mechanical or chemical
energy is produced. The energy released by the explosion is dissipated
by various phenomena, like a shock wave, radiation, acoustic energy, throw of fragments. Its estimation can be performed by calculating the Gibbs's free energy. The free energy of a compressed gas may be
estimated by different methods available in the literature [23].
Process plants may be affected by a number of different types of explosions [23]:
Condensed phase explosion;
physical explosion (hydraulic or pneumatic);
confined gas explosion;
vapor Cloud Explosion (VCE);
boiling Liquid Expanding Vapor Explosion (BLEVE); and dust explosion.
In the context of this book, only a few of them are discussed, since the other ones, like the condensed phase explosions (e.g. TNT) are far from being of interest for the process plant context, even if they stimulated the creation of several well established investigation methods. The interested reader can find additional information in [22]. The physical explosions are typically the result of overpressure during a fire. If the vessel is full of liquid, then a hydraulic explosion occurs, otherwise it is a pneumatic explosion, which is more violent.
The failure modality depends on the weakest feature of the
containment. A confined gas explosion may result in a deflagration or detonation, depending on the nature of the confined space. With a single vessel the explosion is likely to be a deflagration, with a uniform stress over the volume. But with a multi compartment volume the flame speed can accelerate, such as with piping, resulting in a
detonation. In a slow deflagration, bursting occurs at a pressure equal to the structural strength of the item. Instead, with a detonation, the bursting occurs at a higher pressure. In simple words, this happens because the slow deflagration is likely to cause tears, thus a brittle failure will affect the structural response of the item. Conversely, a detonation does not generally cause brittle fracture and the item can use its ductile reserve. Distinguishing the structural response (brittle or ductile) according to the type of detonation is one way that
investigators have to determine the type of explosion and the initial point. With this regard, the pattern of the bursting of a single vessel may not reveal too much; alternatively, in a multi component
configuration, the maximum damage is usually observed far from the source of ignition. However, it is harder to find the ignition source for a vapor cloud explosion rather than for solid materials [23]. Also, injury to humans is a potential source of information in the
investigation of an explosion. This information is discussed in [24]. A useful guide for explosion and bombing scene investigation is in [25].
Different measurable physical dimensions can be adopted to represent the effects of an explosion. They depend on the specific material and are experimentally determinable by using specific equipment. The maximum pressure of explosion and the deflagration index (a measure of the explosibility) are two examples of these physical dimensions.
Some peculiar values, available by using specific test apparatus for acquiring vapor explosion data [23], are in Table 2.11 and Table 2.12.
Table 2.11 is referred to in [23], where data have been selected from the three referenced sources.
Table 2.11 Characteristic explosion indexes for gasses and vapors.
Maximum pressure Pmax [barg]
Deflagration index K
· m/s]
Chemical NFPA 68 (1997)
Bartknecht (1993)
Senecal and
Beaulieu (1998)
NFPA 68 (1997)
Bartknecht (1993)
Senecal and
Beaulieu (1998)
Acetylene 10.6 109
Ammonia 5.4 10
Butane 8.0 8.0 92 92
Carbon disulphide
6.4 105
Diethyl ether 8.1 115
Ethane 7.8 7.8 7.4 106 106 78
Ethyl alcohol 7.0 78
Ethylbenzene .6.6 7.4 94 96
Ethylene 8.0 171
Hydrogen 6.9 6.8 6.5 659 550 638
Hydrogen sulphide
7.4 45
Isobutane 7.4 67
Methane 7.05 7.1 6.7 64 55 46
Methyl alcohol
7.5 7.2 75 94
Methylene chloride
5.0 5
Pentane 7.65 7.8 104 104
Propane 7.9 7.9 7.2 96 100 76
Toluene 7.8 94
Source: Data taken from [23].
Table 2.12 Characteristic explosion indexes for powders.
Deflagration index KSt [bar·m/s]
St class
0 St 0
1–200 St 1
200–300 St 2
>300 St.3
Dust Median
particle size [μm]
Minimum explosive dust
concentration [g/m3]
Pmax [barg]
KSt
[bar·m/s]
Minimum Ignition Energy [mJ]
Cotton, wood, peat
Cotton 44 100 7.2 24 –
Cellulose 51 60 9.3 66 250
Wood dust 33 – – – 100
Wood dust 80 – – – 7
Paper Dust <10 – 5.7 18 –
Feed, food
Dextrose 80 60 4.3 18 –
Fructose 200 125 6.4 27 180
Fructose 400 – – – >4000
Wheat grain dust
80 60 9.3 112 –
Milk powder 165 60 8.1 90 75
Rice flour – 60 7.4 57 >100
Wheat flour 50 – – – 540
Milk sugar 10 60 8.3 75 14
Coal, coal products Activated carbon
18 60 8.8 44 –
Bituminous coal
<10 – 9.0 55 –
Plastics, resins, rubber
Polyacrylamide 10 250 5.9 12 –
Polyester <10 – 9.0 55 –
Polyethylene 72 – 7.5 67 –
Polyethylene 280 – 6.2 20 –
Polypropylene 25 30 8.4 101 –
Polypropylene 162 200 7.7 38 –
Polystyrene (copolymer)
155 30 8.4 110 –
Polystyrene (hard foam)
760 – 8.4 23 –
Polyurethane 3 <30 7.8 156 –
Intermediate products, auxiliary materials
Adipinic acid <10 60 8.0 97 –
Naphthalene 95 15 8.5 178 <1
Salicylic acid – 30 – – –
Source: Data taken from [23].
Dust explosions deserve a separate discussion because of their
features. With the term “dust”, we mean solid combustible materials with a diameter lower than 500 μm. They are dispersed in air, forming a cloud that can rapidly burn if ignited and generate an explosion like a gas cloud.
The dynamic of a dust explosion is very peculiar, since the following two distinct phenomena develop:
A primary explosion. It involves those portions of dust that cause direct structural damages and, by expanding and generating convective motions, lift the dust eventually dispersed in ducts or generally present nearby; and
a secondary explosion. The dust lifted by the first explosion participates, enlarging significantly the destroying effects of the event.
The primary and secondary explosions are shown in Figure 2.13.
Figure 2.13 Primary and secondary dust explosion.
Source: Adapted from [26].
To determine the dangerousness of the explosive mixture dust air, some tests can be carried out, following the international technical standards. In order to have ignition and explosion of a dust cloud, it is necessary that the concentration of the fuel is between the
flammability limits already discussed: the LEL and the UEL. These boundaries of minimum and maximum concentration depend on several variables, including:
Condition of the superficial layer of the dust particle;
dimensions of the particle;
temperature and pressure;
presence of inert gases; and presence of inert dust.
Differentially from gases, UEL does not represent a solid reference for dust: this is not only because of the marked influence numerous
variables listed above but also because the upper bound is quite
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.