Industrial Accidents 2.1 Accidents
2.1.1 Principles of Combustion
2.1.1.1 Flammable Gases and Vapors
The flammability of gases and vapors is connected with the value of their composition in the mixture with the oxidising agent. Their quantity must be within two flammability limits that identify a range of values.
The Lower Flammability Limit (LFL) is the lowest concentration of a gas or vapor in the air that can sustain combustion at a given
temperature and pressure. In the context of this book, the LFL is
synonymous with Lower Explosive Limit (LEL). If the concentration is lower than the LFL, then combustion does not occur even if a source of ignition is present. In simple words, this happens because the quantity of fuel is not sufficient.
The Upper Flammability Limit (UFL), also referred to as Upper Explosive Limit (UEL), is the maximum acceptable concentration to have a flammable gas/vapor cloud that can sustain combustion. Over this limit, there is a lack of the oxidiser, hampering the combustion.
These basic concepts are shown in Figure 2.7.
Figure 2.7 Graphical representation of the concepts of LFL and UFL.
The LFL and the UFL vary with temperature and pressure: generally speaking, a higher temperature produces an increase in the UFL and a decrease in the LFL: this means that the gas/vapor mixture has a
greater range of possible concentration values falling inside the flammable region. Instead, the effect of pressure is usually hard to predict, since it may vary depending on the specific mixture. In Table 2.2, the flammability limits of some substances are listed.
Table 2.2 Flammability limits of some gas and vapors.
Flammability limits Substance LFL
[% in volume]
UFL
[% in volume]
Hydrogen 4 75
Methane 5 15
Butane 1.8 8.4
n Hexane 1.1 7.5
Gasoline 1.7 7.6
Toluene 1.2 7.1
Methanol 6.7 36
Ethanol 3.3 19
Source: Data taken from [11].
2.1 When the fuel is a mix of components, the flammability limits are
evaluated through additivity criteria, starting from the values of the single elements, using the empiric relation of Le Châtelier, known as
“the mixing rule”.
In equation 2.1, LFLmix is the LFL of the mixture, LFLi is the LFL of the generic single component, and xi is the molar fraction of the single generic component of the mixture of flammable species. The results based upon the additivity criterion have to be considered prudently, especially if there are components of different chemical structures that tend to react differentially and to influence each other. In order to calculate the effect of temperature and pressure on the limits, other empirical relations may be used to estimate them starting from the stoichiometric concentration in air. The temperature influences the characteristics of flammability of both gases and vapors significantly, acting on the vapor pressure, the reaction rate, the flammability limits, and the speed of flames propagations. Generally an increasing of the temperature produces an enlargement of the flammability range, with a particular focus on the upper limit. This growth may require the introduction of a larger quantity of inert substance to transform the mixture into a non flammable one. Pressure is also capable to vary the flammability range, but these modifications are less evident and their effect is not easy to identify because a single trend does not exist, but it depends on the specific mixture. Generally, a significant decrease of pressure causes a reduction of the flammability range.
Another important parameter governing the flammability of gas and vapors is the Minimum Oxygen Concentration (MOC), also known as Limiting Oxygen Concentration (LOC). It identifies a limiting value of the oxygen concentration below which combustion cannot occur. This value is expressed in units of volume percent of oxygen. It depends on pressure, temperature, and the type of inert (non flammable) gas. Some values of this parameter are in Table 2.3.
The knowledge of this value is particularly important in fire safety
engineering, since severe risks of explosion can be eliminated by adding the inert gas (like nitrogen or carbon dioxide) in the
compartment, forcing the O2 concentration to drop below the MOC.
Table 2.3 MOC values (volume percent oxygen concentration above which combustion can occur).
Gas or vapor N2/Air CO2/Air Gas or vapor N2/Air CO2/Air
Methane 12 14.5 Kerosene 10
(150°C) 13
(150°C)
Ethane 11 13.5 JP 1 fuel 10.5
(150°C) 14
(150°C)
Propane 11.5 14.5 JP 3 fuel 12 14.5
nButane 12 14.5 JP 4 fuel 11.5 14.5
Isobutane 12 15 Natural gas 12 14.5
nPentane 12 14.5 n Butyl chloride 14 –
Isopentane 12 14.5 12
(100°C) –
nHexane 12 14.5 Methylene
chloride
19 (30°C)
–
nHeptane 11.5 14.5 17
(100°C) –
Ethylene 10 11.5 Ethylene
dichloride
13 –
Propylene 11.5 14 11.5
(100°C) –
1 Butene 11.5 14 Methyl
chloroform
14 –
Isobutylene 12 15 Trichloroethylene 9 (100s
°C)
–
Butadiene 10.5 13 Acetone 11.5 14
3 Methyl butene
11.5 14 t butanol NA 16.5
(150°C)
Benzene 11.4 14 Carbon disulphide
5 7.5
Toluene 9.5 – Carbon monoxide 5.5 5.5
Styrene 9 – Ethanol 10.5 13
Ethylbenzene 9 – 2 Ethyl butanol 9.5
(150°C) –
Vinyltoluene 9 – Ethyl ether 10.5 13
Diethylbenzene 8.5 – Hydrogen 5 5.2
Cyclopropane 11.5 14 Hydrogen
sulphide
7.5 11.5
Gasoline Isobutyl formate 12.5 15
(73/100) 12 15 Methanol 10 12
(100/130) 12 15 Methyl acetate 11 13.5
(115/145) 12 14.5
Source: Data from [11].
Among the flammability properties of gases and vapors, there is the Auto Ignition Temperature (AIT). It is defined as the lowest temperature at which a substance starts to burn spontaneously when an oxidiser is present, without a direct source of ignition. In this case, the temperature is itself an efficient source of ignition to start the combustion. It is important to note that autoignition is not
synonymous with instantaneous ignition. Indeed, a period, named
“induction period” or “ignition delay”, exists and it varies according to the specific mixture and temperature. Typically, this period decreases as the temperature is much higher than the AIT and it increases as the temperature is close to the AIT. For instance, it is possible to expose a combination methane air, whose AIT is 580°C, to a jet of gas at a higher temperature, but only for a very short time. Autoignition provoked by contact of a flammable atmosphere with a hot surface often triggers an explosion, as described in [15, 16]. The relations among the flammability limits and the properties of gases and vapors are shown in Figure 2.8. Table 2.4 lists the values of AIT for some substances.
Figure 2.8 Relations among the flammability properties of gas and vapors.
Source: Adapted from [11].
Table 2.4 Approximate values of the Auto Ignition Temperature for some substances.
Substance Auto Ignition Temperature (AIT) [°C]
Methane 537
Gasoline 246
Hydrogen 570
Hexane 220
Paper 230
Wood 220–250
Synthetic rubber 300
Wool 205
The AIT is not an intrinsic parameter of the material since it depends on the same factors that influence the reaction rate in the gaseous
phase and on the peculiar system used for its measurement. They include:
Volume and geometry of the container, in particular the surface/volume ratio;
presence of inert (N2, CO2, water steam, and so on);
pressure;
presence of additives (inhibitors or promoters);
physical state of the fuel (fog, vapor);
cold flames;
ignition delay; and
superficial effects (correlated to the material of the container).
It is possible to classify the flammable gases depending on the way they are stored. According to this classification, it is possible to distinguish among:
Compressed gasses. They are stored at the gaseous state at a higher pressure than the atmospheric one. They are usually stored in
cylinders at the pressure listed in Table 2.5;
liquefied gases. They are liquefied by compression, at the room temperature. Hydrocarbons and their mixtures are typically stored in this way. The main advantage is the space saving, generally in a ratio of 1/800, meaning that from 1 litre of liquefied gas, 800 litres at the gaseous state are obtained (propane, ammonia, chlorine, LPG are some examples);
dissolved gases. They are stored at the gaseous state, dissolved in a liquid at a certain pressure (acetylene is a case); and
refrigerated gases. They are liquefied gases, by compression and low temperatures. Using low temperature allows storing them at a lower pressure than the compressed gas (liquid nitrogen is an example whose pressure is comparable to the atmospheric one).
Table 2.5 Storage pressure of some compressed gasses.
Gas Storage pressure [bar]
Hydrogen 250 Oxygen 250
Air 250
Methane 300
CO2 20