Explosion
An 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 thermalexpansion 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 that 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
1
dP



 V 3  Kg
 dt  max
 dP 

 V  K st
 dt  max
Where K g and K st are constants. They are referred
to as the deflagration indices for gas and dust
respectively. (Table 3-8)
1
3
Application of Cubic Law
1
 dP 

3
V




 dt  max  IN
VESSEL
1
 dP 

   V 3 
 dt  max  EXPERIMENTAL
This equation allows the experimental results
to be applied to determining the explosive
behaviors in buildings and process vessels.
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 Characteristics
1. 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 Sequence
BLEVEs are caused by the sudden failure of the container
due 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.