Industrial Accidents 2.1 Accidents
2.1.2 Fires
Fire is an uncontrolled combustion that develops without limits in time and space. The most visible part of fire is the flame, whose color depends on the temperature and the chemistry of the fire, and the smoke, which is an unwanted by product of fire. The distinct
characteristics of fires are:
Decomposition. The combustion reaction “consumes” the reactants, burning them into other products;
heat. Fire is an exothermic reaction; and
transfer of heat. The produced heat is transferred through conduction, convection and radiation, as already discussed.
When the extinguishment of the fire does not occur spontaneously, because all the fuel was burnt, then the usage of particular substances, named “extinguishers”, is required. They adopt different actions,
summarised in Table 2.8.
Table 2.8 Extinguishers and their actions.
Extinguisher Action Features Water Temperature reduction
Suffocation
Dilution of flammable substances
Inhibition of solid fuels
Good availability Cheap
To not use on electric parts It does not work on light hydrocarbons
Foam Separation between fuel and oxidiser
Suffocation
Classified in high, medium or low expansion
To not use on electric parts Powder Separation
Chemical inhibition Cooling
Inert gases Temperature reduction Reduction of oxidising concentration
Suitable on electric fire
Fires may be classified in different ways, according to the purpose of the classification. A typical distinction, also promoted by several
national technical regulations, concerns the material of the fuel.
Taking inspiration from [19], the identified classes are:
Class A: Fires of solid materials, typically of organic nature (they leave ashes when they combust). Wood, paper, rubber, trash, and cloth are examples. Class A fires are faced removing the “heat” side in the fire triangle, e.g. using water;
Class B: Fires of liquid substances or liquefiable solids, like petroleum, oils, greases, paints. They do not leave ashes when burning. The most used firefighting strategy to face Class B fires is to eliminate the oxidiser or, at least, its possibility to react with the fuel, using foam or carbon dioxide extinguishers;
Class C: Fires in live electrical materials. Circuit breakers, wirings, electrical outlets are examples of Class C combustible materials. If Class A extinguishers are used for a Class C fire, then severe
hazards of electrocution can occur. Indeed, Class C fires are
extinguished through dry chemical or powder. If the fire involves electronics, such as TVs and PCs, the residual damages likely caused by the powder may be avoided using carbon dioxide or halon extinguishers; and
Class D: Fires of combustible metallic substances. They include titanium, magnesium, aluminium, sodium, and so on. The best way to face these fires is to use a special powder, handled by trained personnel, to cover the combusting material, excluding the contact between the oxygen and the combusting metal. Typically, Class D fire extinguishers contain sodium chloride, or sodium carbonate.
Depending on the national context, other classes can also be
identified. The Class F fires, involving cooking oil or fat, are common in the UK, even if in the US these materials are listed as fuels in Class B fires.
Fires are also classified according to the spatial attribute in which they originate. Indeed, it is possible to distinguish between confined and not confined fires (a third category is the semi confined fire, being in the middle between the two extreme cases). This classification points out if fires occur outdoors or inside a closed space, like a room or
industrial building. On the one hand, confinement may limit the availability of oxygen, hampering the fire propagation; on the other hand, a confined fire takes into account the radiation from hot
elements (smokes, walls, ceiling) towards other combustible materials, propagating the fire. The initial evolution is similar for both confined and not confined fires, but when the power increases in time and the fire propagates in space, the confinement effect starts to become relevant and may generate two different conditions. The two subsequent regimes of combustion have been already discussed previously in this book. They are:
Oxygen controlled fire. It is typical of confined fire, and the
combustion rate depends on the availability of oxygen. An oxygen controlled fire is the one occurred at deck 3 of the Norman
Atlantic. The significant presence of black soot is a key indicator of the lack of oxygen (Figure 2.10); and
fuel controlled fire. It is typical of typical of not confined fire. Here, the combustion rate depends mainly on the fuel availability. A fuel controlled fire is the one occurred at deck 4 of the Norman
Atlantic, where the lateral openings supplied a continuous flow of fresh air, allowing the fuel to fully burn. In Figure 2.10 it is possible to see the different colors on the walls of the garage ramp of the ferryboat: on the right, the access to deck 3 is covered by black soot, while these signs are not present on the left, where there is the access to the windowed deck 4.
Figure 2.10 Different colors at the access of deck 3 and 4 of the Norman Atlantic, suggesting two different typologies of fire. The oxygen controlled fire at deck 3 (on the right) and fuel controlled fire at deck 4 (on the left).
The main parameters governing a fire are:
The maximum temperature and the temperature rate of the combustion products;
the quantity of heat being generated and the rate of its development;
the duration of the fire;
the required time to reach the maximum temperature;
the fire load, which is defined as the ratio between the heat
developed by a complete combustion of the fuels and the surface in the plan view of the considered space. It is expressed in kJ/m2. Conventionally, kilograms of equivalent wood (with a predefined value of 4400 kcal/kg) is used as an alternative measure of heat:
and
the availability of oxygen, which affects semiconfined fires and mainly depends upon the size of the openings.
The evolution in time of fire is usually represented through the Heat Release Rate (HRR) curve, shown in Figure 2.11. It shows the typical trend of a fire, focusing on the variation of the generated power
(energy per seconds) in time.
Figure 2.11 Evolution of a fire.
Three main stages may be found:
Development stage (or pre flashover). The average temperature of gases is low, and the fire is localised in its origin point. The
temperature slowly varies in time because the heat is mainly used to increase the temperature of the fuel materials above their AIT, and to warm the surrounding air and combustible materials next to the origin of the fire;
complete development stage (or flashover). The temperature rapidly increases because the number of materials involved in the fire grows and the increasing temperature causes a higher
combustion rate. The fire propagates, and the heat release rate
reaches the maximum values; and
decay stage: the fuel is almost all burned and combustion rate lowers. The temperature decreases because of the heat dispersion through the smokes and the irradiation toward the coolest zones.
In Table 2.9, the growth rate of fire has been classified into categories, depending on the time t1 required to reach the power threshold of 1 MW. The following Table 2.10 collects the values of t1 for some commonly used materials. These values have to be intended as approximated, thus purely as a guidance, because of the several variables that affect them.
Table 2.9 Categories of growth velocity of fire.
Category t1 [s]
Slow 600
Medium 300
Fast 150
Very fast 75
Source: Data taken from [20].
Table 2.10 Values of t1 for some materials commonly used.
Material t1 [s]
Wooden pallets, stacked, height 45 cm 155 ÷
310
Wooden pallets, stacked, height 1.5 m 92 ÷
187
Wooden pallets, stacked, height 3 m 77 ÷ 115
Wooden pallets, stacked, height 4.8 m 72 ÷ 115 Rolls of paper, vertical, stacked, height 6 m 16 ÷ 26 Clothing, cotton and polyester, shelves, height 4 m 21 ÷ 42 Paper, densely packed in cardboard boxes, stocked in shelf,
height 6 m
461
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].