Basics
For a fire or an explosion to occur, three conditions must be met:
- A combustable substance must be present
- An oxidizing agent must be present; usually this is atmospheric oxygen
- A source of ignition of sufficient energy must be active.
If and only if these three conditions prevail at the same time, fire or explosion are possible.
Fire protection may tackle any of the corners of this triangle.
Solid materials
The flammability of solids is characterized by the minimum temperature at which it can burst into flames without a further ignition source.
Examples are
| Phosphorus (white) | 60 °C |
| Peat | 230 °C |
| Sulphur | 250 °C |
| Spruce wood | 280 °C |
| Brown coal | 250 ... 280 °C |
| Hard coal | 330 ... 440 °C |
| Sugar | 410 °C |
| Tar | 500 °C |
| Rye flour | 500 °C |
These figures can only give a semi-quantitative description of the flammability. It is worthwhile noting that it is not usually the solid itself that starts burning but gases evolved from it.
Besides, a six-step scale exists to describe the flammability of solid substances, depending on whether and if, how quickly, fire spreads after the substance has been lighted.
Liquids
To characterize the flammability of liquids, the flash point is used.
The flash point is the minimum temperature at which a liquid evolves so much vapour that the gas phase above the liquid can be ignited with an external ignition source.
Examples are
| Carbon disulphide | - 30 °C |
| Diethyl ether | - 20 °C |
| Toluene | 6 °C |
| Ethanol | 12 °C |
| Fuel oil | > 55 °C |
The instruments and the testing procedures to determine the flash point have been standardized in several countries.
| Apparatus for determining the flash point
|
Without an external ignition source, gases and vapours burst into flames at the minimum ignition temperature, MIT, or ignition point.
Examples are
| Carbon disulphide | 102 °C |
| Diethyl ether | 170 °C |
| Petrol |  260 °C |
| Toluene | 535 °C |
| Natural gas | 595 °C |
Pieces of electrical equipment are grouped into six temperature classes according to the maximum allowable temperature of any of their surfaces.
| T 6 | 85 °C |
| T 5 | 100 °C |
| T 4 | 135 °C |
| T 3 | 200 °C |
| T 2 | 300 °C |
| T 1 | 450 °C |
Apparatus for determining the ignition point
|
(a) Observation mirror (b) Thermocouple (c) 200-ml Erlenmeyer flask (d) Heating (e) Resistance thermometer (f) Thermal insulation (g) Test gas (h) Pipette for liquids |
(Source: Fonds der Chemischen Industrie, Germany; imageseries "Sicherheit in der Chemischen Industrie")
Extinguishing
There are three ways to extinguish a fire:
- cool it down. This is why usually water is poured onto a fire, but this is not generally advisable.
- deprive it of oxygen. This is a popular way e.g. with gravure printing machines, where after some alarm time carbon dioxide gas is released into the production room. The alarm time is necessary because the room has to be evacuated from people. Note that some fires, e.g. burning light metal dusts (oh yes, they can !), will react with carbon dioxide, making the fire even worse.
- interfere with flame chemistry. Flame chemistry is essentially radical chemistry. Radicals produced by thermal decay of extinguishing agents will interfere with the flame reaction and stop it. The most popular compounds of this kind were the "halons", but these now have been banned because of their detrimental effects to the earth's ozone layer.
Types of reaction
Flames
are exothermic chemical oxidation reactions in the gaseous state. The
oxidant usually is atmospheric oxygen, which is supplied by diffusion from
the outside of the flame. Hence, the oxidation proceeds relatively slow.
Explosions
occur if the flammable substance has been premixed with the oxidant.
It is possible in the gaseous, liquid, or solid state. The chemical reaction
proceeds much more violently, as temperature rises more quickly. Propagation
speeds are typically around 100 m/s and may reach the velocity of sound
(333 m/s). The initial pressure may increase tenfold.
Time dependence of pressure in a gas explosion |
pin, initial pressure; , explosion
overpressure;pex, explosion pressure; dp/dt, rate of pressure rise |
| pmax [bar] | dp/dt [bar/s] | |
| Methane | 7,4 | 55 |
| Flour | 8,5 | 60 |
| Polyethylene dust | 9,0 | 200 |
| Hydrogen | 7,1 | 550 |
| Aluminum dust | 11,5 | 1,500 |
Detonations
are even faster and more violent than explosions. Their speed is above
the velocity of sound and typically reaches several thousand meters per
second; military explosives are as fast as 12,000 m/s. Energy tranfer is
not by conductance of heat, but by shock. The initial pressure may increase
thousandfold or more.
Explosion limits
In the gas phase above a liquid there is always some vapour of the
liquid. The pressure exercised by this vapour onto the walls of a closed
vessel is called the vapour pressure. When the vapour pressure is equal
to or higher than the ambient pressure, the liquid boils.
The vapour pressure depends on the temperature of the liquid.
The Clausius-Clapeyron equation says how:
ln p = -
/ RT + C
p, vapour pressure;
, heat of evaporation;
R, universal gas constant; T, temperature [K]; C, integration constant
So, the vapour pressure increases exponentially as temperature rises.
Dalton's law says that the partial pressures of gases can be added to
give the overall pressure in a gas mixture.
Hence, the concentration of a certain (flammable) gas in the air can be
estimated from its vapour pressure. It can only be estimated and not calculated
exactly, as there are no ideal gases.
Apparatus for determining the explosion limits of gases
(Source: Fonds der Chemischen Industrie, Germany; imageseries
"Sicherheit in der Chemischen Industrie")
Examples
| LEL [% by vol] |
UEL [% by vol] |
|
| Petrol | 0,6 | 8 |
| Toluene | 1,2 | 7,1 |
| Diethyl ether | 1,7 | 36 |
| Ethanol | 3,5 | 15 |
| Hydrogen | 4 | 76 |
| Methane | 5 | 15 |
Note that explosion limits, or flammable limits, are temperature-dependent, especially when given in [g/m³].
Example (from a German regulation, substance not given):
LEL (20 °C  290
K) |
40 g/m³ |
| LEL (310 °C 580
K) |
24 g/m³ |
This difference is easily understandable with regard to the universal gas
law,
pV = nRT,
which says 2 T ==>n/2;
the deviation from this result is mainly due to the dilution of the reactive
gases.
Dust explosions
Even combustible solids, when finely dispersed and raised, may explode. Such events are called dust explosions. Dust explosions usually cause severe damage.
Examples
(from the German "Handbook of Disasters")
In 1967 at Hawthorne, New Jersey, USA, starch was roasted in a chemical plant. Several dust explosions, caused by a steam-heated roaster, killed eleven people und destroyed several buildings. Estimated cost: $ 3,650,000.
In 1979 in Bremen, Germany, there was a fire near to a flour mill, causing a dust explosion there. 14 were killed and 17 injured. Estimated cost: more than 10,000,000 DM. As compared to military explosions, this disaster was calculated to be equivalent to the detonation of 20 tons of TNT, the standard military explosive.
| Test of a dust explosion | Destruction caused by dust explosion (1979) |
 (Source: Fonds der Chemischen Industrie, Germany; |
 imageseries "Farbstoffe und Pigmente") |
Paper dust, finely dispersed, may cause dust explosions. Here are the results of two tests, performed with paper dust from dust filters.
| (I) | (II) | |
| Size distribution [% by weight]  20 µm32 µm71 µm125 µm500 µmmedian |
S S  66909710010016 |
S S ???3365230 |
| LEL [g/m³] | 125 | - |
| [bar] |
8,7 | - |
| dp/dt [bar/s] | 60 | - |
| ignition point [°C] | 570 | - |
LEL - lower explosion limit;
-
explosion overpressure;
dp/dt - rate of pressure rise
Ten litres of explosive atmosphere are regarded dangerous in a medium-sized room.
How much toluene (standard solvent for gravure inks,
)
does it take to make up this amount of explosive mixture ?
The lower flammable limit is 1.2 % by vol (see above); so 120 ml of toluene vapour are enough.
1 mol of toluene is 7 · 12 g + 8 · 1 g = 92 g
1 mol of an ideal gas comprises 22.4 liters,
so 120 ml of toluene vapour require 0.5 g of toluene.
The density of toluene is about 0.7,
so about 0.7 ml of toluene are sufficient to create a dangerous explosion.
An "empty" 10-litre can contains by far more than that. If
you are lucky, you are even above the upper expolsion limit.
If not, ...
Explosion protection
There are three levels of protection against explosions:
| (1) Prevent explosive atmosphere, e.g. by use of less dangerous substances,
lower temperature, better ventilation, inerting,
(2) Prevent ignition sources, e.g. by safe electrical equipment, earthing of metal containers, use of antistatic materials, (3) Minimize the consequences, e.g. by injection of extinguishing agents, pressure-relief devices, lightweight construction, Basically, (1) should be preferred to (2), (2) is to be preferred to (3). |
It is common, however, to combine measures of all three types, so that they together bring about the desired level of safety.
Example from a regulation of the Berufsgenossenschaften, the German industrial accidents insurance.
 |
1 air in 2 air out 3 mixing chamber 4 heating chamber 5 intermediate drier 6 final drier 7 measuring device |
Flexo printing press |
Inertization means to reduce the oxygen concentration to a safe level.
The relevant parameter is the limiting oxygen concentration, LOC,
. It has
to be noted, however, thatdepends
on the other gases present.
Examples
|
[% by moles] | |
| inert gas |  |
 |
| Hydrogen | 5,0 | 5,0 |
| Petrol | 11.8 |
14.5 |
| Methane | 12,1 | 14,6 |
| Benzene | 11,2 | 13,9 |
These figures depend on pressure and temperature.
The concentrations in a system may be such that any addition of fuel cannot result in an explosive mixture (partial inertization) or such that any addition of air cannot make up a dangerous mixture (total inertization).
Diagrams like the triangular plot below sum up the explosive behaviour of flammable substances. The explosive region is shaded.
The arrows at point T explains how the concentrations are plotted.
Point D: LOC,
An ignition source must exceed some minimum energy to set off an explosion.
The relevant figure is the minimum ignition energy, MIE. This is the smallest amount of energy, which is sufficient to ignite the most readily ignitable fuel-air mixture, when stored in a capacitor and released in a spark discharge.
| Examples for MIEs | [mJ] |
| Hydrogen | 0,011 |
| Ethylene | 0,07 |
| Propane | 0,25 |
| i-Propanol | 0,65 |
| Aluminum dust | 2 |
| Methyl cellulose | 1000 |
| Examples for frequent ignition energies |
[mJ] |
| Electrostatic discharge | 4 |
| Grinding machine | 100 |
| Welding spark | 10000 |
Electrical equipment is considered safe, if none of the following quantities can be exceeded:
1.2 V, 0.1 A, 20 µJ, 25 mW.
Where possible, it is an engineer's task to design containers, reactors, etc., in such a way that they can withstand the pressures built up if the content goes off. This is what the chemical industry mainly relies on. Safety valves or rupture disks combined with pressure relief pipes leading outward or - preferably - to safety tanks will direct the gases in a preset way and thus protect the rest of the installation or even the building. It must be made sure that the hot gases that might escape do not cause further harm.
In the construction of buildings where explosions might occur, it is
important to create sufficient pressure relief areas, such as pressure
relief windows or walls. These are lightly built structures that will collapse
if under small pressure, thus saving people in the building and possibly
the building itself. Be careful with pressure relief roofs: these tend
to eventually fall down again.
Feedback Updated
on: 20. January 1998