6.0 Fires and Explosions

Chemicals present a very substantial hazard due to their potential to generate fires and

explosions. The combustion of one gallon of toluene can destroy an ordinary

chemistry laboratory in minutes; persons present may be killed. The potential

consequences of fires and explosions in pilot plants and process plant environments

are even greater.

The three most common chemical plant accidents are fires, explosions and toxic

releases, in that order. Organic solvents are the most common source of fires and

explosions in the chemical industry.

Yearly losses due to fires and explosions are substantial. Property losses for

explosions in the United States are estimated at over 200million. Additional losses

due to business interruptions are estimated to exceed 250million annually. To prevent

accidents due to fires and explosions, engineers must be familiar with:

• the fire and explosion properties of materials,

• the nature of the fire and explosion process, and

• procedures to reduce fire and explosion hazards.

6-1 THE FIRE TRIANGLE

The essential elements for combustion are fuel, oxidizer, and an ignition source.

These elements are illustrated by the fire triangle shown in Figure 6-1.

Fire, or burning, is the rapid, exothermic oxidation of an ignited fuel. The fuel can

be in solid, liquid or vapor form, but vapor and liquid fuels are generally easier to

ignite. The combustion always occurs in the vapor phase; liquids are volatized and

solids are decomposed into vapor prior to combustion.

When fuel, oxidizer, and an ignition source are present at the necessary levels,

burning will occur. The fire triangle tells us a fire will not occur if (1) fuel is not

present or is not present in sufficient quantities, (2) an oxidizer is not present or is not

present in sufficient quantities, and (3) the ignition source is not energetic enough to

initiate the fire.

Two common examples of the three components of the fire triangle are wood, air,

and a match; or gasoline, air, and a spark. However, other, less obvious combinations

of chemicals can lead to fires and explosions. Various fuels, oxidizers and ignition

sources common in the chemical industry are:

FUELS

Liquids Gasoline, acetone, ether, pentane.

Solids Plastics, wood dust, fibers, metal particles.

Gases Acetylene, propane, carbon monoxide, hydrogen.

OXIDIZERS

Gases Oxygen, fluorine, chlorine.

Liquids Hydrogen peroxide, nitric acid, perchloric acid.

Solids Metal peroxides, ammonium nitrate.

IGNITION SOURCES

Sparks, flames, static electricity, heat

6-2 DISTINCTION BETWEEN FIRES AND EXPLOSIONS

The major distinction between fires and explosions is the rate of energy release.

Fires release energy slowly, while explosions release energy very rapidly, typically on

the order of microseconds. Fires can also result from explosions and explosions can

result from fires.

6-3 DISTINCTION BETWEEN FIRES AND EXPLOSIONS

Some common definitions related to fires and explosions are given.

Combustion or Fire: Combustion or fire is a chemical reaction in which a substance

combines with an oxidant and releases energy. Part of the energy released is used to

sustain the reaction.

Autoignition temperature (AfT): A fixed temperature above which a flammable

mixture is capable of extracting enough energy from the environment to self-ignite.

Flash Point (FP): The flash point of a liquid is the lowest temperature at which it

gives off enough vapor to form an ignitable mixture with air. At the flash point, the

vapor will burn, but only briefly; inadequate vapor is produced to maintain

combustion. The flash point generally increases with increasing pressure.

There are several different experimental methods used to determine flash points.

Each method produces a somewhat different value. The two most commonly used

methods are open cup and closed cup, depending on the physical configuration of the

experimental equipment. The open cup flash point is a few degrees higher then the

closed cup.

Flammability Limits (LFL and UFL): Vapor-air mixtures will only ignite and burn

over a well-specified range of compositions. The mixture will not burn when the

composition is lower than the lower flammable limit (LFL); the mixture is too lean

rich; that is, when it is above the upper flammable limit (UFL). A mixture is

flammable only when the composition is between the LFL and the UFL. Commonly

used units are volume percent fuel (percent of fuel plus air).

Lower explosive limit (LEL) and upper explosive limit (UEL) are used

interchangeably with LFL and UFL.

Explosion: An explosion is a rapid expansion of gases resulting in a rapidly moving

pressure or shock wave. The expansion can be mechanical (via the sudden rupture of a

pressurized vessel) or it can be the result of a rapid chemical reaction. Explosion

damage is caused by the pressure or shock wave.

Deflagration: An explosion with a resulting shock wave moving at a speed less then'

the speed of sound in the unreacted medium.

Detonation: An explosion with a resulting shock wave moving at a speed greater than

the speed of sound in the unreacted medium.

Confined explosion: An explosion occurring within a vessel or a building. These are

most common and usually result in injury to the building inhabitants and extensive

damage.

Unconfined explosion: Unconfined explosions occur in the open. This type of

explosion is usually the result of a flammable gas spill. The gas is dispersed and

mixed with air until it comes in contact with an ignition source.

Boiling liquid expanding vapor explosion (BLEVE): A BLEVE occurs if a vessel

ruptures which contains a liquid at a temperature above its atmospheric-pressure

boiling point. This type of explosion occurs when an external fire heats the contents of

a tank of volatile material. As the tank contents heat, the vapor pressure of the liquid

within the tank increases and the tank's structural integrity is reduced due to the

heating. If the tank ruptures the hot liquid volatilizes explosively.

Dust explosion: This explosion results from the rapid combustion of fine solid

particles. Many solid materials (including common metals such as iron and aluminum)

become very flammable when reduced to a fine powder.

6-4 FLAMMABILITY CHARACTERISTICS OF LIQUIDS AND VAPORS

Flammability characteristics of some important organic chemicals (liquids and gases)

are shown in Table 6.1.

Liquids

The flash point (FP) is one of the major physical properties used to determine the fire

and explosion hazards of liquids. Flash points for pure components are easily

determined experimentally. Table 6-1 lists flash points for a number of substances.

Flash points can be estimated for multicomponent mixtures if only one component is

flammable and if the flash point of the flammable is known. In this case the flash

point temperature is estimated by determining the temperature at which the vapor

pressure of the flammable in the mixture is equal to the pure component vapor

pressure at its flash point. Experimentally determined flash points are recommended

for multicomponent mixtures with more than one flammable component.

Example 6-1

Methanol has a flash point of 11℃ and its vapor pressure at this temperature is 53 mm

Hg. What is the flash point of a solution containing 75% methanol and 25% water by

weight?

Solution The mole fractions of each component are needed to apply Raoult's Law.

Assuming a basis of 100 pounds of solution

Raoult's law is used to compute the vapor pressure (psat) of pure methanol, based on the

partial pressure required to flash.

p = xpsat

psat= p / x = 53/0.63 = 84.1 mm Hg

Using a graph of the vapor pressure versus temperature, shown on Figure 6-2, the flash

point of the solution is 17°C

Vapors

Flammable limits for vapors are determined experimentally. Vapor-air mixtures of

known concentration are added to a closed vessel and then ignited. The maximum

explosion pressure is measured.

This test is repeated with different concentrations to establish the range of

flammability for the specific gas. Figure 6-3 shows the results of one such

experimental run; this particular substance has an LFL of 2.2 per cent and a UFL of

7.8 per cent.

Vapor Mixtures

Frequently LFLs and UFLs for mixtures are needed. These mixture limits are

computed using the Le Chatelier equation'

Where

LFL; is the lower flammable limit for component i in volume % of component

i in fuel and air,

yi is the mole fraction of component i on a combustible basis, and

n is the number of combustible species.

Similarly,

Where

UFL; is the upper flammable limit for component i in volume % of component i in

fuel and air.

The Le Chatelier's equation is an empirically derived equation which is not

universally applicable. The limitations are covered in the literature.

Example 6-2

What is the LFL and UFL of a gas mixture composed of 0.8% hexane, 2.0% methane, and

0.5% ethylene by volume?

Solution The mole fractions on a fuel only basis are calculated below. The LFL and

UFL data are obtained from Table 6-1.

Equation 6-1 is used to determine the LFL of the mixture.

Equation 6-2 is used to determine the UFL of the mixtures,

Since the above mixture contains 3.3% total combustibles, it is flammable.

6-5 MINIMUM OXYGEN CONCENTRATION (MOC) AND INERTING

The LFL is based on fuel in air. However, oxygen is the key ingredient and there is a

minimum oxygen concentration required to propagate a flame. This is an especially

useful result, because explosions and fires are preventable by reducing the oxygen

concentration regardless of the concentration of the fuel. This concept is the basis for

a common procedure called inerting

Below the MOC, the reaction cannot generate enough energy to heat the entire

mixture of gases (including the inerts) to the extent required for the self propagation

of the flame.

The MOC has units of per cent oxygen in air plus fuel. If experimental data are

not available, the MOC is estimated using the stoichiometry of the combustion

reaction and the LFL. This procedure works for many hydrocarbons.

Example 6-3

Estimate the MOC for butane (C4HIO).

Solution The stoichiometry for this reaction is

C4H10 + 6.5O2 4CO2 + 5H2O

The LFL for butane (from Table 6-1) is 1.6% by volume. From the stoichiometry,

By substitution

The combustion of butane is preventable by adding nitrogen, carbon dioxide or even water

vapor until the oxygen concentration is below 10.4 %. The addition of water, however, is not

recommended because any condition which condenses water would move the oxygen

concentration back into the flammable region.

6-6 IGNITION ENERGY

The minimum ignition energy (MIE) is the minimum energy input required to initiate

combustion. All flammables (including dusts) have minimum ignition energies.

The MIE depends on the specific chemical or mixture, the concentration, pressure,

and temperature. A few MIEs are given in Table 6-2.

Experimental data indicates that

• The MIE decreases with an increase in pressure,

• The MIE of dusts are, in general, at energy levels comparable to combustible

gases, and

• An increase in the nitrogen concentration increases the MIEs.

Many hydrocarbons have MIEs of about 0.25 mJ. This is low when compared to

sources of ignition. For example, a static discharge of 22 mJ is initiated by walking

across a rug, and an ordinary spark plug has a discharge energy of 25 mJ. Electrostatic

discharges, as a result of fluid flow, also have energy levels exceeding the MIEs of

flammables and can provide an ignition source, contributing to plant explosions

6-7 AUTOOXIDATION

Autooxidation is the process of slow oxidation with accompanying evolution of heat,

sometimes leading to autoignition if the energy is not removed from the system.

Liquids with relatively low volatility are particularly susceptible to this oroblem.

Liquids with high volatility are less susceptible to autoignition because they self cool

as a result of evaporation.

Many fires are initiated as a result of autooxidation, referred to as spontaneous

combustion. Some examples of autooxidation with a potential for spontaneous

combustion include:

• Oils on a rag in a warm storage area,

• Insulation on a steam pipe saturated with certain polymers, and

• Filter aid saturated with certain polymers. Cases have been recorded where ten

year old filter aid residues were ignited when the land-filled material was

bulldozed, allowing autooxidation and eventual autoignition.

These examples illustrate why special precautions must be taken to prevent fires

due to autooxidation and autoignition.

6-8 ADIABATIC COMPRESSION

An additional means of ignition is adiabatic compression. For example, gasoline and

air in an automobile cylinder will ignite if the vapors are compressed to an adiabatic

temperature which exceeds the autoignition temperature. This is the cause of

preignition knock in engines which are running too hot and too lean. It is also the

reason why some over-heated engines continue to run after the ignition is turned off.

Several large accidents were caused by flammable vapors being sucked into the

intake of air compressors; subsequent compression resulted in autoignition. A

compressor is particularly susceptible to autoignition if it has a fouled after-cooler.

Safeguards must be included in the process design to prevent undesirable fires due to

adiabatic compression.

6-9 IGNITION SOURCES

As illustrated by the fire triangle, fires and explosions are preventable by eliminating

ignition sources. Various ignition sources were tabulated for over 25,000 fires by the

Factory Mutual Engineering Corporation and are summarized in Table 6-3. The

sources of ignition are numerous; consequently it is impossible to identify and

eliminate them all. The main reason for rendering a flammable liquid inert, for

example, is to prevent a fire or explosion by ignition from an unidentified source.

Although all sources of ignition are not likely to be identified, engineers must still

continue to identify and eliminate them.

Some very special situations might occur in a process facility where it is

impossible to avoid flammable mixtures. In these cases a very thorough safety

analysis is required to eliminate all possible ignition sources in each of the units

where flammable gases are present.

6-10 SPRAYS AND MISTS

Static electricity is generated when mists of sprays pass through orifices. A charge

may accumulate and discharge in a spark. If flammable vapors are present, a fire or

explosion will occur.

Mists and sprays also affect flammability limits. For suspensions with drop

diameters less than 0.01 mm, 'the lower flammability limit is virtually the same as the

substance in vapor form. This is true even at low temperatures where the liquid is

nonvolatile and no vapor is present. Mists of this type are formed by condensation.

6-11 EXPLOSIONS

Explosion behavior depends on a large number of parameters. A summary of the more

important parameters is shown in Table 6-4.

Detonation and Deflagration

Explosions are either detonations or deflagrations; the difference depends on the

speed of the shock wave emanating from the explosion.

Suppose a combustible mixture is placed within a long pipe. A small spark, flame,

or other ignition source initiates the reaction at one end of the pipe. After ignition, a

flame or reaction front moves down the pipe.

In front of the flame front is a pressure or shock wave. If the pressure wave moves

faster than the speed of sound in the unreacted medium the explosion is a detonation;

if it moves at a speed less than the speed of sound it is a deflagration.

Confined Explosions

A confined explosion occurs in a confined space, such as, a' vessel or a building. The

two most common confined explosion scenarios involve explosive vapors and

explosive dusts. Empirical studies have shown that the nature of the explosion is a

function of several experimentally determined characterisitics. These characteristics

are dependent on the explosive material used and include flammability or explosive

limits, the rate of pressure rise after the flammable mixture is ignited, and the

maximum pressure after ignition.

Explosion characteristics. The explosion characteristics determined using the vapor

and dust explosion apparatus are used in the following way.

1. The limits of flammability or explosivity are used to determine the safe

concentrations for operation or the quantity of inert required to control the

concentration within safe' regions.

2. The maximum rate of pressure rise is indicative of the robustness of an

explosion. Thus, the explosive behavior of different materials can be

compared on a relative basis. It is also used to design a vent for relieving a

vessel during an explosion before the pressure ruptures the vessel, or to

establish the time interval for adding an explosion suppressant (water, carbon

dioxide, or Halon) to stop the combustion process.

A plot of the log of the maximum pressure slope versus the log of the vessel

volume frequently produces a straight line of slope -1/3

This relationship is called the "Cubic Law."

1/3

max( / ) gdp dt V constant K (6-3)

1/3

max( / ) Stdp dt V K (6-4)

where Kg and Kst are called the deflagration indices for gas and dust respectively. As

the robustness of an explosion increases, the deflagration indices Kg and Kst increase.

The cubic law states that the pressure front takes longer to propagate through a larger

vessel. A few values for Kg and Kst are given in Tables 6-5 and 6-6. Dusts are further

classified into four classes, depending on the value of the deflagration index. These St

classes are shown in Table 6-6.

Dust explosions demonstrate --unique behavior. These explosions occur if finely

divided particles of solid material are dispersed in air and ignited. The dust particles

can be either an unwanted by-product or the product itself.

Explosions involving dusts are most common in the flour milling, grain storage,

and coal mining industries. Accidents involving dust explosions can be quite

substantial; a series of grain silo explosions in Westwego near New Orleans in 1977

killed thirty-five people.

An initial dust explosion can cause secondary explosions. The primary explosion

sends a shock wave through the plant, stirring up additional dust which may result in a

secondary explosion. In this fashion the explosion "leapfrogs" its way through a plant.

Many times the secondary explosions are more damaging than the primary.

To be explosive, a dust mixture must have the following characteristics.

• The particles must be below a certain minimum size:

• The particle loading must be between certain limits.

• The dust loading must be reasonably uniform.

For most dusts, the lower explosion limit is between 20 and 60 gm/m3 and the upper

explosion limit between 2 and 6 kg/m3.

Vapor Cloud Explosions (VCE)

The most dangerous and destructive explosions in the chemical process industries are

vapor cloud explosions (VCE). These explosions occur by a sequence of steps:

1. Sudden release of a large quantity of flammable vapor. Typically this occurs

when a vessel, containing a superheated and pressurized liquid, ruptures.

2. Dispersion of the vapor throughout the plant site while mixing with air.

3. Ignition of the resulting vapor cloud.

The accident at Flixborough, England is a classic example of a vapor cloud

explosion. A sudden failure of a 20-inch cyclohexane line between reactors led to

vaporization of an estimated 30-tons of cyclohexane. The vapor cloud dispersed

throughout the plant site and was ignited by an unknown source 45-seconds after the

release. The entire plant site was leveled and 28 people were killed.

A summary of twenty-nine vapor cloud explosions over the period 1974 through

1986 shows property losses for each event of between $5,000,000 to $100,000,000

and 140 fatalities (an average of almost 13 per year).

Some of the parameters that affect VCE behavior· are

Quantity of material released,

Fraction of material vaporized,

Probability of ignition of the cloud,

Distance travelled by the cloud prior to ignition,

Time delay before ignition of cloud,

Probability of explosion rather than fire,

Existence of a threshold quantity of material,

Efficiency of explosion, and

Location of ignition source with respect to release.

Methods which are used for preventing VCEs include keeping low inventories of

volatile, flammable materials; using process conditions which minimize flashing if a

vessel or pipeline is ruptured; using analyzers to detect leaks at very low

concentrations; and installing automated block valves to shut systems down while the

spill is in the incipient stage of development.

Boiling Liquid Expanding Vapor Explosions (BLEVE)·

A boiling liquid expanding vapor explosion (BLEVE, pronounced ble'.-vee) is a

special type of accident that can release large quantities of materials. If the materials

are flammable, a VCE might result; if toxic, a large area might be subjected to toxic

materials. For either situation, the energy released by the BLEVE process itself can

result in considerable damage.

A BLEVE occurs when a tank containing a liquid held above its atmospheric

pressure boiling point ruptures; resulting in the explosive vaporization of a large

fraction of the tank contents.

BLEVEs are caused by the sudden failure of the container due to any cause.

The most common type of BLEVE is caused by fire. The steps are as follows.

1. A fire develops adjacent to a tank containing a liquid.

2. The fire heats the walls of the tank.

3. The tank walls below liquid level are cooled by the liquid, increasing the liquid

temperature and the pressure in the tank.

4. If the flames reach the tank walls or roof where there is only vapor and no liquid

to remove the heat, the tank metal temperature rises until it loses it structural

strength.

5. The tank ruptures, explosively vaporizing its contents.

If the liquid is flammable and a fire is the cause of the BLEVE, it may ignite as

the tank ruptures. Often, the boiling and burning liquid behaves as a rocket fuel,

propelling vessel parts for great distances. If the BLEVE is not caused by a fire, a

vapor cloud might form, resulting in a VCE. The vapors might also be hazardous to

personnel via skin burns or toxic effects.

When a BLEVE occurs in a vessel, only a fraction of the liquid vaporizes; the

amount depends on the physical and thermodynamic conditions of the vessel contents.

The fraction vaporized is estimated using Raoult’s Equation and the Antoine

Equation.