Chemical Process Safety
Runaway Reactions
1
Two CSB Videos: Review
1. Reactive Hazards (31 July 2007)
2. Runaway: Explosion at T2 Laboratories (19 Dec 2007;
video: 22 Sep 2009)
“167 serious uncontrolled
reactions with 108 deaths
from 1980 – 2001”
2
Two CSB Videos: Review
1. Reactive Hazards:
a) What do you remember about the video?
b) Lessons “learned”
3
Two CSB Videos: Review
1. Reactive Hazards:
a)
1984 Bhopal
•
CSB formed & established chemical process safety
b) Synthron: butyl acrylate (solvents: toluene, cyclohexane)
•
1500 gal reactor
•
HE was used to condense solvent vapors & cool
exothermic reaction
•
Batch size increased
•
HE couldn’t remove enough heat
c)
BP Amoco: HP nylon
•
Polymerization reactor bypass to 750 gal waste tank
•
Overfilled waste tank; no PI or vent
•
Secondary decomposition reaction
d) MFG Chemical: allyl alcohol vapor release
•
30 gal test reactor (3rd test significant heat generation)
•
Production in 4000 gal reactor (SA/vol ratio: HE
inadequate)
e) 1st Chemical Corporation: mono-nitro toluene (MNT)
•
145’ distillation tower; MNT left in reboiler
•
Leaking steam valve
•
Heated to 450 oF – decomposition reaction
4
Two CSB Videos: Review
1. Reactive Hazards:
a) What do you remember about the video?
b) Lessons “learned”
5
Two CSB Videos: Review
2. Runaway: Explosion at T2 Laboratories:
a) What do you remember about the video?
b) Lessons “learned”
Producing a gasoline additive:
methylcyclopentadienyl manganese tricarbonyl (MCMT)
Reactor
6
Two CSB Videos: T2 Laboratories
Brief overview of process steps
• Added to reactor
– sodium metal in mineral oil
– methylcyclopentadiene dimer
– diethylene glycol dimethyl ether (diglyme)
• close the vessel
• set pressure to 3.45 bar and heating oil temp to 182.2 C
• heating melted sodium that reacted with
methylcyclopentadiene forming sodium methylcyclopentadiene,
hydrogen, and heat
•
•
•
•
Hydrogen gas was generated
when mix reached 100°C, agitation was shut off
at 150°C hot oil flow stopped
at 180°C cooling was initiated with water admitted to the
reactor jacket.
• maintain temperature from the exothermic reaction via water
evaporation.
7
Two CSB Videos: T2 Laboratories
175th batch exploded
Former Reactor Site
8
Figure 2. Control room.*
* From CSB final
report; Sep 2009.
9
Figure 4. Injury and business locations.*
* From CSB final
report; Sep 2009.
10
Figure 5. Portion of the 3-inch-thick
reactor.*
* From CSB final
report; Sep 2009.
11
Figure 4. Injury and business locations.*
* From CSB final
report; Sep 2009.
12
Two CSB Videos: T2 Laboratories
CSB Investigation
Runaway exothermic reaction
• Occurred during the first metalation step of the process
• An uncontrollable rise in temperature and resultant pressure
lead to the burst of the reactor
• Upon bursting, contents ignited in air
• Creating an explosion equivalent of 635 kg (1420 lb) of TNT
exploding from a single point
13
Two CSB Videos: T2 Laboratories
CSB Investigation
Possible causes for the explosion
Investigation considered:
– cross-contamination of the reactor
– contamination of raw materials
– wrong concentration of raw materials
– local concentration of chemical within the reactor
– application of excessive heat
– insufficient cooling
14
“The CSB determined insufficient cooling to be the only credible cause
for this incident, which is consistent with witness statements that the
process operator reported a cooling problem shortly before the
explosion. The T2 cooling water system lacked design redundancy,
making it susceptible to single-point failures including
• water supply valve failing closed or partially closed.
• water drain valve failing open or partially open.
• failure of the pneumatic system used to open and close the water
valves.
• blockage or partial blockage in the water supply piping.
• faulty temperature indication.
• mineral scale buildup in the cooling system.
Interviews with employees indicated that T2 ran cooling system
components to failure and did not perform preventive maintenance.
* From CSB final
report; Sep 2009. 15
Two CSB Videos: Review
2. Runaway: Explosion at T2 Laboratories:
a) What do you remember about the video?
b) Lessons “learned”
• “T2 did not recognize the runaway reaction hazard
associated with the MCMT it was producing.”
Contributing causes:
1. “The cooling system employed by T2 was susceptible to
single point failures due to a lack of design redundancy.
2. The MCMT reactor relief system was incapable of
relieving the pressure from a runaway reaction.”
16
Two CSB Videos: T2 Observations
•
Scaled up from 1 liter to 9300 liter directly
•
Batch 42 the recipe was increased by 1/3 (testing?)
•
Periodically experienced problems with cooling
•
No “backup” cooling system
•
Used city water supply (minerals?)
•
Did not recognize and control reactive hazards
•
No evidence found by CSB that T2 performed a
recommended HAZOP.
•
There was a need for reactive chemistry testing.
17
CSB Testing on T2 Recipe
CSB testing completed with a
VSP2 (Vent Sizing Package 2)
Adiabatic Calorimeter (116 ml
test cell)
diglyme decomposition
reaction 1
exotherm
* From CSB final
report; Sep 2009.
18
* From CSB final
report; Sep 2009.
19
Follow-up Topics
•
Key Findings of CSB investigation:
•
Cooling discussion
•
Overpressure
•
Runaway reactors
•
Hazard analysis
20
• A second exothermic reaction occurred
• This reaction became uncontrollable
around 200°C
• The reaction was the uncontrolled
decomposition of diglyme (the solvent
used)
• Probably catalyzed by the presence of
sodium.
• By the time the rupture disk opened
(28.6 bar)
• It was too late
• If the rupture disk had opened at 6.2
bar, then no explosion would have
occurred
* From CSB final
report; Sep 2009.
21
Over pressure Wave Profile, 1 Psi=0.07 bar
0.017
0.017 Bar
Bar
psi
1.7 Bar
0.14 Bar
* From CSB & SACHE
module by R. Willey, 2012.
22
Combustion Behavior – Most Hydrocarbons
Smoke and fire are very visible!
Slide courtesy of Reed Welker.
23
Combustion Behavior – Carbon Disulfide
No smoke and fire, but heat release rate just as high.
Slide courtesy of Reed Welker.
24
Combustion Behavior – Methane
Methane burns mostly within vessel, flame shoots
out of vessel.
25
Combustion Behavior – Dusts
Much of the dust burns outside of the chamber.
26
Definitions - 1
LFL: Lower Flammability Limit
Below LFL, mixture will not burn, it is too lean.
UFL: Upper Flammability Limit
Above UFL, mixture will not burn, it is too rich.
Defined only for gas mixtures in air.
UNITS:
27
Definitions - 2
Flash Point: Temperature above which a liquid
produces enough vapor to form an ignitable mixture
with air.
Defined only for liquids at 1 atm. pressure.
Auto-Ignition Temperature (AIT): Temperature above
which adequate energy is available in the
environment to provide an ignition source.
28
Definitions - 3
Limiting Oxygen Concentration (LOC): Oxygen
concentration below which combustion is not
possible, with any fuel mixture.
Expressed as volume % oxygen.
Also called: Minimum Oxygen Concentration
Max. Safe Oxygen Conc.
Others
29
Definitions - 4
Explosion: A very sudden release of energy
resulting in a shock or pressure wave.
Shock, Blast or pressure wave: Pressure wave that
causes damage.
Deflagration: Reaction wave speed < speed of
sound.
Detonation: Reaction wave speed > speed of sound.
Speed of sound in air: 344 m/s, 1129 ft/s at ambient
T, P.
Deflagrations are the
case with explosions
involving flammable materials.
30
Definitions - 5
Minimum Ignition Energy (MIE): Smallest energy to
initiate combustion.
•Higher for dusts & aerosols than for gases
•Many HC gases have MIE ~ 0.25 mJ
Auto-oxidation: slow oxidation and evolution of
heat can raise T and lead to combustion. i.e. liquids
with low volatility.
Adiabatic compression: of a gas generates heat,
increases temperature, and can lead to autoignition.
Ignition sources: usually numerous and difficult to
eliminate. Objective is to identify and eliminate, but
not to solely rely on this step to eliminate
combustion risk. (Table 6-5; Crowl)
31
Typical Values - 1
LFL
UFL
Methane:
5.3%
15%
Propane:
2.2%
9.5%
Butane:
1.9%
8.5%
Hydrogen:
4.0%
75%
See Appendix B
Flash Point Temp. (deg C)
Methanol:
12.2
Benzene:
-11.1
Gasoline:
-43
32
Typical Values - 2
AIT (deg. C)
Methane:
Appendix B
632
Methanol:
574
Toluene:
810
Great variability in
reported AIT values!
Use lowest value.
LOC (Vol. % Oxygen)
Methane:
12%
Ethane:
11%
Hydrogen:
5%
Table 6-2
33
Flammability Relationships
Figure 6-2
34
Aerosol Flammability
Too rich
Too lean
M. Sam Mannan, Texas A&M, Mary Kay O’Conner Process Safety Center
35
Minimum Ignition Energies
What: Energy required to ignite a flammable mixture.
Typical Values: (wide variation expected)
Vapors:
Dusts:
Dependent on test device --> not a reliable design
parameter.
Static spark that you can feel: about
mJ
Lightning: about 500 megajoules
Or ~ 500,000,000,000 mJ
Table 6-4
36
Minimum Ignition Energies
37
Ignition Sources of Major Fires
38
Experimental Determination - Flashpoint
Figure 6-3
Cleveland Open Cup
Method.
Closed cup
produces a better
result - reduces
drafts across cup.
39
Experimental Determination - Flashpoint
40
Setaflash Flashpoint Device
41
Setaflash Flashpoint Device – Close-up
42
Setaflash Flashpoint Device – Close-up
Window
43
Setaflash Flashpoint Device – Close-up
44
Auto-Ignition Temperature (AIT) Device
45
Auto-Ignition Temperature (AIT) Device
46
Experimental Determination - LFL, UFL
Maximum Explosion
Pressure (barg)
Run experiment at different fuel compositions with
air:
10
8
Need a criteria to
define limit - use 1
psia pressure
increase. Other
criteria are used with different
results!
6
4
2
LFL
0
0
2
UFL
4
6
8
10
See Figure 6-5
Fuel Concentration in air
(vol%)
Flammability limits are an empirical artifact of
experiment!
47
Experimental Determination: P versus t
TI
PI
Ignitor
Pressure (bar-abs)
10
Pmax
8
6
(dP/dt)max
4
2
0
0
50
100
150
200
250
Time (ms)
Final experimental result:
48
Experimental Apparatus
49
Experimental Determination - LFL, UFL
50
Flammability Limit Behavior -1
As temperature increases:
UFL increases, LFL decreases
--> Flammability range increases
100CP
0.75
LFLT  LFL25 
T  25  LFL25 
T  25
H c
H c
0.75
Equations 6-4, 6-5
T  25
UFLT  UFL25 
H c
T :o C
H c : kcal/mole, heat of combustion
Approx. for many
hydrocarbons
51
Flammability Limit Behavior -2
As pressure increases:
UFL increases
LFL mostly unaffected
UFLP  UFL  20.6 * (logP  1)
P is pressure in mega-Pascals, absolute
No theoretical basis for this yet!
Pressure and temperature effects on flammability
limits is poorly understood – estimation methods
are poor.
52
Flammability Limits of Mixtures
Le Chatelier Rule (1891)
LFLmix 
1
n
yi

i 1 LFLi
UFLmix 
1
n
yi

i 1 UFLi
yi on a combustible basis only
n is the number of combustible species
Assumptions:
1) Product heat capacities constant
2) No. of moles of gas constant
3) Combustion kinetics of pure species unchanged
4) Adiabatic temperature rise the same for all species
Details provided in Process Safety Progress, Summer 2000.
53
Flammability Limits - Le Chatelier
LeChatelier’s rule shows that the LFL can be approximated by:
C p T *
 LFL 


 100   hc 
Where Cp is the product heat capacity, T is the adiabatic
temperature rise, and hc is the heat of combustion.
*
1200 K is frequently used as the adiabatic temperature rise at
the flammability limit.
A similar expressions is written for the UFL.
54
Flammability Limits of Mixtures
C p T *
 LFL 


 100   hc 
From this equation, a plot of the flammability limit
vs. 1/(Heat of Combustion) should yield a straight
line if Le Chatelier’s rule is valid.
If this is done, one finds that:
Le Chatelier’s rule works better at the lower
flammability limit than the upper flammability
limit.
Assumptions are more valid at LFL.
55
Lower Flammability Limit and Heat of Combustion
LFL [Vol% Fuel in air]
8.0
7.0
LFLN Comp. = 5327.4[1/ hc ]
R2 = 0.9478
6.0
LFLHC Comp. = 4569.1[1/ hc ]
R2 = 0.8849
5.0
LFLOxy. Comp. = 5030.7[1/ hc ]
R2 = 0.9338
4.0
Hydrocarbons
3.0
Oxygen Compounds
2.0
Nitrogen Compounds
1.0
0.0
0.0000
Sulfur Compounds
0.0002
0.0004
0.0006
0.0008
1/hc [kJ/mole]-1
Linear (Nitrogen
0.0010
0.0012
0.0014
Compounds)
Linear (Hydrocarbons)
Linear (Oxygen
Compounds)
56
UFL [vol. % fuel in air]
Upper Flammability Limit and Heat of
Combustion
100
80
60
40
Hydrocarbons
Oxygen Compounds
Nitrogen Compounds
20
Sulfur Compounds
0
0.000
0.001
0.001
0.002
0.002
1/(- hc) [kJ/mol]-1
57
Estimating Flammability
Jones equation where
LFL = 0.55Cst
UFL = 3.50Cst
the stoichiometric concentration, Cst, is vol% fuel in fuel plus air.
From the general combustion equation,
CmHxOy + zO2 = mCO2 + x/2 H2O
It follows that z = m + x/4 – y/2, where z has the units of moles O2/mole fuel
Therefore,
Cst 
moles fuel
X 100 
moles fuel  molesair
100
100
100


 molesair 
 1  molesO2  1   z 
1  
 1  




0
.
21


 0.21 moles fuel 
 moles fuel 
The Jones equation can now be converted to
LFL 
0.55(100)
4.76m  1.19x  2.38y  1
3.50(100)
UFL 
4.76m  1.19 x  2.38 y  1
58
Estimating Flammability
Suzuki and Koide correlation
 3.42
LFL 
 0.569H c  0.0538H c2  1.80
H c
UFL  6.30H c  0.567H c2  23.5
where:
LFL and UFL are the lower and upper flammability limits (vol% fuel in air),
respectively, and
∆Hcis the heat of combustion for the fuel (in 103 kJ/mol)
NOTE that the accuracy of this and Jones methods are modest.
59
Estimating LOC
LOC limiting oxygen conc.
(1)Fuel + (z) Oxygen --> Products
[vol% O2]
Typically 8 - 10%
• Concentration required to generate enough energy to propagate flame
• Reduce O2 concentration below LOC to prevent the fire/explosion
• If data for LOC is not available, estimate using the stoichiometry of the
combustion process and the LFL
For example, the stoichiometry for butane:
C4 H10  6.5O2  4CO2  5H 2O
The LFL for butane is 1.9% by volume, therefore from stoichiometry
 m oles fuel  m olesO2 
 m olesO2 




LOC 

LFL
 total m oles m oles fuel 
 m oles fuel 






By substitution, we obtain,
 m oles fuel  6.5 m olesO2 

  12.4 vol% O2
LOC 1.9
total
m
oles
1
.
0
m
oles
fuel



60
LOC’s for Various Substances
61
Flammability Diagram
Upper limit in
pure oxygen
100
Air Line
20
Flammability
Zone
80
A
40
60
40
60
Lower limit in
pure oxygen
80
UFL
20
MOC
100
0
LFL
20
40
60
80
0
100
Nitrogen
62
Flammability Diagram
Useful for:
•
Determining if a mixture is flammable.
•
Required for control and prevention of flammable
mixtures
Problems:
•
Only limited experimental data available.
•
Depends on chemical species.
•
Function of temperature and pressure.
Flammability diagram can be approximated.
63
Flammability Diagram
(1) Fuel + (z) Oxygen ---> Products
0 100
CH4 + 2 O2 --> Products
z=2
 z 

 *100 
1

z


UFL
 z 

 *100
1 z 
100
0
Flammable
LFL
100
0
Nitrogen
64
Drawing an Approximate Diagram
1. Draw LFL and UFL on air line (%Fuel in air).
2. Draw stoichiometric line from combustion
equation.
3. Plot intersection of LOC with stoichiometric line.
4. Draw LFL and UFL in pure oxygen, if known (%
fuel in pure oxygen).
5. Connect the dots to get approximate diagram.
65
Example
Methane:
LFL: 5.3% fuel in air
Pure Oxygen:
UFL: 15% fuel in air
LFL: 5.1% fuel in oxygen
LOC: 12% oxygen
UFL: 61% fuel in oxygen
CH4 + 2 O2 --> CO2 + 2 H2O
--> z = 2
 z 
2

 *100    *100  66.7
1 z 
 3
% oxygen
66
Flammability Diagram - Example
0 100
LOC = 12% oxygen
61% Methane
66.7% O2
UFL = 15% fuel
100
0
LFL = 5.3% fuel
Nitrogen
100
0
5.1% Methane
67
Flammability Zone
0 100
Transition Boundary
Flammable
Non-Flammable
20
80
40
60
60
40
80
20
100
0
0
20
40
60
80
100
Nitrogen
68
Flammability Zone
0 100
Transition Boundary
Flammable
Non-Flammable
20
80
40
60
60
40
80
20
100
0
0
20
40
60
80
100
Nitrogen
69
Removal of Vessel from Service
70
Explosions - Definitions
Explosion: A very sudden release of energy
resulting in a shock or pressure wave.
Shock, Blast or pressure wave: Pressure wave that
causes damage.
Deflagration: Reaction wave speed < speed of
sound.
Detonation: Reaction wave speed > speed of sound.
Speed of sound in air: 344 m/s, 1129 ft/s at ambient
T, P.
Deflagrations are the
case with explosions
involving flammable materials.
71
Explosions
• Rapid release of energy
• Damage due to dissipation of energy in the form of
pressure wave, projectiles, sound, radiation, etc
• Reaction front moves out from ignition source
preceded by shock wave or pressure front. Once
combustible material consumed, reaction front
terminates, but pressure wave continues.
• Shock wave (results from abrupt pressure change)
and is associated with highly explosive materials
• Most damage due to blast wave (shock / pressure
wave followed by wind)
72
Detonations
• Energy releases short, < 1 ms, associated with
abrupt rise in P
• Shock and reaction front > speed of sound
• Reaction front provides energy to shock wave and
drives it at sonic or greater speeds
• P of shock wave: ~ 10 - 100 atm.
73
Deflagrations
• Energy release longer than detonation ~ 0.3 s,
• Pressure front = speed of sound; reaction front
behind at < speed of sound
• Mechanism: turbulent diffusion, mass transfer
limited
• P of wave: ~ a few atmospheres
• Can evolve, especially in pipes but not open spaces,
to a detonation due to adiabatic compression and
heating leading to pressure rise
74
Comparison of Behavior
Deflagration:
Reaction front moves at
less than speed of sound.
Ignition X
Pressure wave moves
away from reaction front at
speed of sound.
Detonation:
Ignition X
Reacted gases
Reaction / Flame Front
Pressure Wave
Unreacted gases
Reaction front moves
greater than speed of
sound.
Pressure wave is slightly
ahead of reaction front
moving at same speed.
75
Comparison of Behavior
Deflagration:
Ignition
P
Distance
Detonation:
Ignition
Reacted gases
P
Shock
Front
Reaction / Flame Front
Pressure Wave
Unreacted gases
Distance
76
Comparison of Behavior
Detonation
Localized Damage
No wall thinning
Lots of pieces
Deflagration
Damage all over
Wall thinning
A few pieces
77

Confined Explosions
Occurs in process or
building. Almost all of the
thermodynamic energy ends
up in the pressure wave.
Cubic Law:
dP 
1/3
  V  KG
 dt max
Ki
dP 
1/3
  V  K St
 dt max
Deflagration index (bar-m/s)
G gas
 (Staub)
St dust
Deflagration index:
Measure of explosion
robustness, higher value
means more robust.
Depends on experimental
conditions.
Not a fundamental property.
78
Deflagration Indexes
79
Deflagration
Indexes
80
Data: Max. P and KG
Stable Combustion Pressure
12
K G = V1/3 [dP/dt]max
K G = (0.02m3 )1/3 (316.7
bar/sec)
140
120
8
100
6
80
P = 7.6 bar
60
4
40
2
t = 24 ms
Pressure (psia) .
Pressure (bar) .
10
160
20
0
0
0
20
40
60
80
100
120
140
160
180
Time (ms)
81
Damage
Estimates from
Overpressure
Table 6-9; Crowl
82
Dust Explosions
• Finely divided combustible solids dispersed in air
encounter an ignition source
• Examples: flour milling, grain storage, coal mining,
etc
• Initial dust explosion produces secondary explosions
• Conditions for explosion:
a) particles < certain size for ignition & propagation
b) particle loading between certain limits
c) dispersion in air fairly uniform for propagation
83
Unconfined Explosions
Occur in the open. Only 2 to 10% of
thermodynamic energy ends up in pressure wave.
Use for this class:
VCE:
Vapor Cloud Explosion
- sudden release flammable vapor
- dispersion and mixing with air
- ignition vapor cloud
Flixborough
Prevention
- smaller inventories
- milder process conditions
- incipient leak detection
- automated block valves
84
BLEVE
BLEVE: Boiling Liquid Expanding Vapor Explosion
- Release large amount of superheated liquid after vessel
rupture (e.g. fire)
•
•
•
•
BLEVE: Explosive vaporization of a liquid at a
temperature above its normal boiling point caused by
container rupture. Ex: from external fire
If liquid is flammable, a VCE can result
Boiling liquid can behave as rocket fuel, propelling vessel
fragments
Fraction of liquid vaporized from Chapter 4, To > Tb
85
BLEVE
Effects: Blast + thermal
Vapor
Liquid
Vessel with liquid
stored below its
normal boiling point
Below liquid level –
Above liquid level –
86
BLEVE Consequences
87
Mechanical Explosions
Rupture of vessel
containing an inert gas at
high pressure.
Eqn. 6-31
 P
We  RgT ln
  PE
  PE 
  1  
P 
 
Max.
Mechanical
Energy
Where: We is the energy of explosion, P is abs. gas
pressure in vessel, PE is abs. ambient pressure, T is
abs. temperature.
88
Batch Reactor Explosion Consequences
89
Overpressures
Direct-on Overpressure
PI
Blast
Origin
PI
Blast wave
Side-on Overpressure
90
Peak Side-on Overpressures
o
Pressure
P
Peak overpressure
Explosion
origin
Direction of movement
Shock front
Ambient pressure
Pa
Distance from explosion origin
91
Peak Side-on Overpressures
Overpressure
Explosion Origin
t1
Direction of movement
t2
t3
t4
t5
t6
Distance
92
Consequences of Explosions: Table 6-9
Peak Side-on Overpressure
(psig)
Consequence
0.03
Large glass panes shatter
0.15
Typical glass failure
0.7
Minor house damage
1.0
Partial house demolition
P
3
Steel frame building distorted
> 15
100% fatalities
Distance
3 psig: Hazard zone for fatalities due to
structure collapse.
93
P
Distance
94
TNT Equivalency Method
po
ps 
pa
Scaled distance ze 
r
1/ 3
mTNT
95
P
Distance
96
TNT Equivalency for VCEs
mTNT
Where:
m Ec
Total Energy in Fuel


Energy/ m ass of TNT
ETNT
mTNT is the equivalent mass of TNT

is the explosion efficiency
m is the total mass of fuel
Ec is the heat of combustion
ETNT is the heat of combustion for TNT
(1120 cal/gm = 4686 kJ/kg = 2016 BTU/lb)
97
TNT Equiv. - Explosion Efficiency
mTNT 
m Ec
ETNT
  1 for confined explosion
  0.02 to 0.10 for unconfined explosion
Use a default value of
information is available.
unless other
98
Other Methods
Other methods are based on degree of congestion
or confinement. Basis is that confinement leads to
turbulence which increases the burning velocity.
•
TNO Multi-Energy Model (see pages 271-274)
•
Baker - Strehlow Model
Both produce essentially the same answer.
Need much more information, i.e. confinement info.
99
TNT Equivalency Procedure
Problem: Determine consequences at a specified
location from an explosion.
1. Determine total mass of fuel involved.
2. Estimate explosion efficiency.
3. Look up energy of explosion (See Appendix B in text).
4. Apply Equation 6-24 to determine mTNT.
5. Determine scaled distance.
z
r
1/ 3
mTNT
6. Use Figure 6-23 or Equation 6-23 to determine overpressure.
7. Use Table 6-9 to estimate damage.
100
TNT Equivalency Procedure
The problem with the application of this approach to
exploding vapor is that:
Overpressure curve developed from detonation data,
i.e. TNT, and flammable vapor explodes as a
deflagration.
The TNT method applied to vapor explosions tends
to underpredict overpressures at some distance
from the explosion, and over-predicts the
overpressures near the explosion.
Detonation
P
Deflagration
Distance
P
Shock
Front
Distance
101
Example
Determine the energy of explosion for 1 lb of n-butane? What is
the TNT equivalent? Use an explosion efficiency of 2%.
C4 H10 
G 
13
O2  4CO2  5H 2O
2
 G of 
Pr oducts
 G of  4(94.26)CO2  5(54.636) H 2O  (4.1)C4 H10   646.21
Re ac tants
or G   646.21
kcal
gm ole
kcal
cal
gm ole
cal
X 1000
X 58
  11,142
gm ole
kcal
g
g
But the explosion has an efficiency,   0.02
102
Example
cal
g
Gavailable  11,142
* 454 *1lb * 0.02  101,169cal
g
lb
mTNT
 101,169cal

 90.33 g TNT  0.093kg TNT
cal
 1120
g TNT
103
Example
1 lb n-butane overpressure vs distance
Po (overpressure) [psi]
100
10
1
0
0
5
10
15
20
25
30
35
r (distance from explosion) [m]
104
TWA - 800: July 17, 1996
105
TWA - 800: July 17, 1996
106
Example
Determine equivalent TNT mass for TWA 800
explosion.
Assume: 18,000 gallon fuel tank, P = 12.9 psia, T =
120 F, Concentration of fuel = 1%, Energy of
explosion for jet fuel = 18,850 BTU/lb, M = 160.
Mass of fuel in vapor:
ntotal
PV (12.9 psia)(18,000 gal)(0.1337 ft 3 / gal)


RgT
(10.731 psia ft 3 / lb-mole o R)(580o R)
= 4.99 lb-moles total
107
Example
Moles of fuel = (0.01)(4.99 lb-moles)
= 0.0499 lb-moles = 7.98 lb of fuel
Assume 100% efficiency (confined explosion).
mTNT 
 mEc
ETNT
(1)(7.98 lb)(18,850 BTU/lb)

2076 BTU/lb TNT
= 74 lb of TNT
108
Questions?
109
Flammability Diagram - 3
Air line always extends
FROM: Fuel:
TO:
0%, Oxygen: 21% Nitrogen: 79%
Fuel: 100%, Oxygen:
0%, Nitrogen: 0%
Equation for this line:
Fuel = -(100/79) Nitrogen + 100
110
Fuel/Air Explosive
CBU-72 / BLU-73/B Fuel/Air Explosive (FAE)
The the 550-pound CBU-72 cluster bomb contains three submunitions
known as fuel/air explosive (FAE). The submunitions weigh
approximately 100 pounds and contain 75 pounds of ethylene oxide with
air-burst fuzing set for 30 feet. An aerosol cloud approximately 60 feet in
diameter and 8 feet thick is created and ignited by an embedded
detonator to produce an explosion. This cluster munition is effective
against minefields, armored vehicles, aircraft parked in the open, and
bunkers.
During Desert Storm the Marine Corps dropped all 254 CBU-72s,
primarily from A-6Es, against mine fields and personnel in trenches.
Some secondary explosions were noted when it was used as a mine
clearer; however, FAE was primarily useful as a psychological weapon.
Second-generation FAE weapons were developed from the FAE I type
devices (CBU-55/72) used in Vietnam.
111