ExplosionAn explosion is a rapid expansion of gases

resulting in a rapid moving pressure or shock wave. The expansion can be mechanical or it can be the result of a rapid chemical reaction. Explosion damage is caused by the pressure or shock wave.

A mechanical explosion is due to the sudden failure of a vessel containing high-pressure non-reactive gas.

Shock Wave

A pressure wave moving through a gas. A shock wave in open air is followed by a strong wind; the combined shock wave and wind is called a blast wave. The pressure increase in the shock wave is so rapid that the process is almost adiabatic. (Figure Next)

If the shock wave is caused by reaction, it is due to stoichiometric effects or thermal-expansion effects.

The Distinction Between Fires and Explosions

The major distinction 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 from fires. Example: the release of energy from a tire.

Explosion Types

Deflagration

An explosion with a resulting shock wave moving at a speed less than the speed of sound in the unreacted medium, e.g. 1/300 second in internal combustion engine. The pressure increase is typically several atmospheres.

Detonation

An explosion with a resulting shock wave moving at a speed greater than the speed of sound in the unreacted medium, e.g. 1/10000 second. The pressure rise is ten times or more higher th

at that of deflagration.

Confined Explosion

An explosion occurring within a vessel or building. These are most common and usually result in injury to the building inhabitants and extensive damage.

There are two types involving explosive vapors and explosive dusts.

Experimentally Determined Explosion Characteristics

The apparatus in Figures 3-10 and 3-13 can be used to determine

• Flammability or Explosive Limits

• The Rate of Pressure Rise

• The Maximum Pressure

Explosion Apparatus for Vapor

• Figure 3-10• A typical pressure versus time plot is shown in

Figure 3-11. Deflagration!• The pressure rate and maximum pressure for

different runs are plotted versus concentration in Figure 3-12.

• The flammability limits are 2% and 8% respectively; the max pressure is 7.4 bar; the max rate of pressure rise is 360 bar/s.

Explosion Apparatus for Dust

• Figure 3-13

• The device is similar to the vapor explosion apparatus, with the exception of a larger volume and the addition of a sample container and a dust distribution ring.

• The data are collected and analyzed in the same fashion as for the vapor explosion apparatus.

The Cubic Law

The results in Figure 3-14 can be found experimentally

Where and are constants. They are referred to as the deflagration indices for gas and dust respectively. (Table 3-8)

st

g

KVdt

dP

KVdt

dP

3

1

max

3

1

max

gKstK

Application of Cubic Law

This equation allows the experimental results to be applied to determining the explosive behaviors in buildings and process vessels.

ALEXPERIMENT

3

1

maxVESSELIN

3

1

max

Vdt

dPV

dt

dP

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.

Unconfined explosions are rarer than confined explosions since the explosive material is frequently diluted below the LFL by wind. These explosions are very destructive since large quantities of gas and large areas are often involved.

Vapor Cloud Explosion (VCE)

VCEs are unconfined explosions. They 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.

Hazardous Conditions

VCEs have increased in number in recent years due to an increase in inventories of flammable materials in process plants and operations at more severe conditions.

Any process containing large quantities of liquefied gases, volatile superheated liquid, or high pressure gases is considered a good candidate for a VCE.

VCE Characteristics1. The ignition probability increases as the size of

the vapor cloud increases.

2. Vapor cloud fires are more common than explosions.

3. The explosion efficiency is usually small; approximately 2% of the combustion energy is converted to a blast wave.

4. Turbulent mixing of vapor and air, and ignition of the cloud at a point remote from the release, increases the impact of explosion.

VCE Prevention Methods

• Keep low inventories of volatile, flammable materials;

• Use process conditions which minimize flashing of a vessel or pipeline is ruptured;

• Use analyzers to detect leaks at very low concentrations;

• Install automated block valves to shut the system down while the spill is initialized.

Boiling Liquid Expanding Vapor Explosion (BLEVE)

A BLEVE occurs if a vessel ruptures which contains a liquid at a temperature above its normal boiling point.

If the material is flammable, a VCE might result; if toxic, a large area might be subject to toxic material.

BLEVE Accident SequenceBLEVEs are caused by the sudden failure of the container du

e to any cause. The most common one is fire. A typical sequence of events is

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

2. The fire heats the tank wall.

3. The tank wall below liquid level is cooled by the liquid, increasing the liquid temperature and the pressure in the tank.

4. If the flames reach the tank wall or roof where there is 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.