Introduction to Design and
Process Safety
Identify
Chemical &
Process
Hazards
Lecture 2:
Introduction to Flammability
1
Lecture 2 Objectives
Class Objectives
• Understand the causes of the 2005 Texas City explosion
• Understand the fire triangle
• Understand the language of fires and explosions, and flammability properties
• Calculate the effect of pressure on UEL
• Estimate the LEL and UEL of mixtures
• Understand types of fires and explosions
Microlearning objectives
• Understand fire & explosion types
2
Risk Based Process Safety
What is
Process
Safety?
What are
the
Hazards?
What could
go Wrong?
How Bad
could it be?
Prevent
Catastrophic
Incidents
Identify
Chemical &
Process
Hazards
Develop
Scenarios
Analyze
Consequences
How likely is
it to
happen?
Estimate
Frequency
How can we
sustain
Performance?
Is risk tolerable?
Assess Risk
Implement
Safeguards
Effective
Management
Systems
PSV
01
Lecture 1
• Introduction
• Risk Based
Process Safety
• PSI
• Regulations
Chapter 1, 2, 3
Lecture 2
• Flammability
Lecture 3
• Reactivity
Lecture 4
• Toxicity
Chapter 4,5,6,7
Lecture 5
• Incident
Classification
Chapter 9, 11, App E
Lecture 6
• Source models
Lecture 7
• Consequence
models
Chapter 13
Risk Based Process Safety
Lecture 8
• HAZOP
Lecture 9
• LOPA
Chapter 12
Not Covered in 3171
• Operational Readiness
• Management of Change
• SOP, Safe Work Practices,
Conduct of Operations &
Operational Discipline
• Emergency Management
• Management Systems
Chapter 17, 18, 19, 20, 21, 22,23
3
BP Texas City 2005
This incident has shaped the process safety regulatory environment since 2005
BP Texas City 2005
What are the Hazards of the process? Flammable gases and liquids, vapor cloud explosion pool fires
What is the Loss Event?
52,000 gallons of flammable raffinate at the boiling point and above the flash
point, was relieved through pressure relief devices to a blowdown drum. The drum
overflowed and created a flammable vapor cloud.
What are the Consequences?
Loss of primary containment, vapor cloud explosion resulting in 15 fatalities; 180
injured; severe damage and $5B in damages. BP stock has not recovered
What are the main Causes?
•
•
•
•
•
•
•
•
•
•
•
Faulty level transmitter in column
Lack of supervisory oversight during startup
Poor communication, poor shift relief practices
Operator fatigue having worked 12hour continuous shifts for 29 days
Training inadequate
Operating procedures inadequate
Inadequate design on relief blowdown drum
Previous near miss ignored
Facility Siting of trailers
Inadequate pre-startup reviews
Culture
The Fire Triangle
Fire
• A chemical reaction
• Rapid exothermic oxidation of an ignited fuel source
Fuels
• Liquids: gasoline, acetone, ether, pentane
• Solids: plastics, wood dust, fibers, metal particles
• Gases: acetylene, propane, CO, hydrogen, ethylene
Oxidizers
• Gases: oxygen, fluorine, chlorine
• Liquids: hydrogen peroxide, nitric acid, perchloric acid
• Solids: metal peroxides, ammonium nitrite
Ignition Sources
• Sparks, flames, static electricity, heat
From Chemical Process Safety, Third Edition,
By Daniel A. Crowl and Joseph F. Louvar (ISBN: 0131382268)
A fire needs fuel, oxidizer, and ignition
Relationship Between Flammability Properties
From Chemical Process Safety, Third Edition,
By Daniel A. Crowl and Joseph F. Louvar (ISBN: 0131382268)
Flammability Definitions
Term
Definition
Flash Point
The lowest temperature at which a liquid gives off enough vapor to form an ignitable mixture with air. At the flash point the vapor will
burn only briefly; inadequate vapor is produced to maintain combustion. The flash point determines the flammability of a substance.
FP < 100⁰F = Flammable
FP > 100⁰F = Combustible
Flammability Limits
LFL/UFL, LEL/UEL
Lower flammability limit/ upper flammability limit, lower explosive limit/ upper explosive limit (used interchangeably)
The region of a flammable vapor and air in which it will burn. Mixture will not burn below the LFL (too lean) or above the UFL (too rich)
Usually expressed as volume % in air
Autoignition
temperature
A temperature above which a flammable mixture is capable of extracting enough energy from the environment to self-ignite
Auto- oxidation
Process of slow oxidation with the evolution of heat, sometimes leading to autoignition. Liquids with low volatility might be subject
Pool Fire
Spill of flammable liquid, collects on ground, the vapors are ignited
Jet Fire
Occurs when pressurized vapor release occurs and is ignited – similar to a blow torch
BLEVE
Boiling liquid, expanding vapor explosion. Liquified gas vessel fails catastrophically, the sudden flash to vapor ignites
Minimum oxygen
concentration
Below this concentration of oxygen in a mixture, the reaction can’t generate enough energy to propagate a flame
Inerting
Use of an inert material such as nitrogen or helium to drop the vapor space below the minimum oxygen concentration
Minimum ignition
energy (MIE)
The minimum energy require to initiate combustion
Students should know and understand each term
Flammability Definitions
Term
Definition
Explosion
A rapid expansion of gases resulting in a rapidly moving pressure or shock wave
Detonation
An explosion with a shock wave that moves greater than the speed of sound
Deflagration
An explosion with a shock wave that moves less than the speed of sound
DDT
Deflagration to detonation transition
Confined Explosion
An explosion occurring within a vessel or building. These are the most common and usually result in injury and extensive
damage
Unconfined Explosion
An explosion that occurs in the open, usually from a flammable gas release. The gas is dispersed and mixed with air. As
the vapor disperses into the flammability limits, it may ignite if an ignition source is present.
Vapor Cloud Explosion
(VCE)
A confined or unconfined ignition of vapor
Overpressure
The pressure on an object as a result of an impacting shock wave, above ambient pressure
Peak side-on
overpressure
Maximum overpressure achieved on an object as a result of an impacting shock wave. Same as peak overpressure, or
maximum overpressure
Dust Explosion
The rapid combustion of solid particles in an oxidizer
Students should know and understand each term
Where to Find Flammability Data
•
•
•
•
•
Safety Data Sheets (MSDS/ SDS)
CAMEO Chemical Database
NFPA Standards
Internet search
CCPS Chemical Reactivity Worksheet (See reactivity lecture)
Chemical
Boiling Point
⁰C
Flash Point
⁰C
LEL
Vol. %
UEL
Vol. %
Autoignition
Temperature
⁰C
Acetone
56.1
-17.8
2.6
13
465
Ethyl Chloride
12.3
-50
3.5
15.8
519
Ethylene Oxide
10.5
-50
3
100
429
n-Hexane
68.7
-22
1.1
7.5
226
Methane
-161.5
-187.2
5
15
537
Styrene
110.6
4
1.08
7.06
480
Flammability Characteristics
Vapor Mixtures
Flammable limit dependence on
temperature & pressure
• In general, flammability range increases
with temperature
• Pressure has little effect on LFL
• UFL increases significantly as pressure
is increases
UFLp = UFL + 20.6(log (P) +1)
P = pressure (megapascals absolute)
UFL = upper flammable limit vol% of fuel
+ air at 1 atm
Use LeChatelier’s Equation to estimate LFL/UFL
𝐿𝐹𝐿𝑚𝑖𝑥 =
1
𝑛
𝑖=1
𝑦𝑖
𝐿𝐹𝐿𝑖
𝑈𝐹𝐿𝑚𝑖𝑥 =
1
𝑛
𝑖=1
𝑦𝑖
𝑈𝐹𝐿𝑖
LFLi UFLi = lower and upper flammable limits of component i in volume%
n = total number of combustible species
yi = mole fraction of species calculated on a combustibles only basis
Examples
Pressure effect on UFL
Vapor Mixtures
If the UFL for a substance is 11.0% by
volume at 0.0 MPa gauge, what is the
UFL at 6.2 MPa gauge?
What are the LFL and UFL of a gas mixture composed of 0.8% hexane,
2.0% methane, and 0.5% ethylene by volume? LFL/UFL given in table
𝐿𝐹𝐿𝑚𝑖𝑥 =
UFLp = UFL + 20.6(log P +1)
P = 6.2MPg + 0.101 = 6.301 MPa
UFLp = 11.0 + 20.6(log (6.301 ) + 1)
= 48 vol. % in air
NOTE: the formula is log(P) + 1, NOT
log(P+1)
1
𝑛
𝑖=1
𝑈𝐹𝐿𝑚𝑖𝑥 =
𝑦𝑖
𝐿𝐹𝐿𝑖
Gas
1
𝑛
𝑖=1
𝑦𝑖
𝑈𝐹𝐿𝑖
Vol. %
Mole Fraction
combustible basis
LFL
Vol. %
UFL
Vol%
Hexane
0.8
0.8/3.3 = 0.24
1.2
7.5
Methane
2.0
2.0/3.3 = 0.61
5.0
15.0
Ethylene
0.5
0.5/3.3 = 0.15
2.7
36.0
Total Combustible
3.3
Air
96.7
LFL = 0.24
1
0.61 0.15
+
+
1.2 5.0 2.7
= 2.65 Vol%
UFL = 0.24
1
0.61 0.15
+
+
7.5 15.0 36.0
= 13.0 Vol%
Ignition
MIE is measured in mili Joules (mJ)
Ignition Source
Electrical
23%
Smoking
18%
Friction
10%
Overheated materials
8%
Hot surfaces
7%
Burner flames
5%
Sparks
4%
Spontaneous ignition
4%
Cutting & welding
4%
Acetylene
0.020
Static
1%
Ethylene oxide
0.062
Chemical reaction
1%
Ethylene
0.080
Propane
0.250
Methane
0.280
We can’t eliminate ignition sources,
only minimize them
•
•
•
•
•
2 mJ is barely perceptible
10 mJ is “distinctly” perceptible
Spark discharge from walking across the room ~22 mJ
250 mJ results in severe shock
10,000 mJ is potentially lethal
Chemical
“Ignition source is always free” – Trevor Kletz
MIE (mJ)
Types of Fires & Explosion
Pool Fire
Jet Fire
BLEVE (Boiling Liquid Expanding Vapor Explosion)
Fires & Main Hazard
• Flash
• Pool
• Jet
Thermal Radiation
• Fireball
Vapor Cloud Explosion
Dust Explosion
• Explosion
Click HERE for CSB Video on Dust Explosions
Pressure Front
Types of Fires & Explosion
Vapor Cloud Explosion
• Vapor diffuses in air
• As it mixes, it may enter a
flammable region
• If an ignition source is present, the
cloud may ignite
• Main hazard is the pressure wave
produced by the expanding
reaction front
• Pressure wave can cause damage
to people and equipment
Pool Fire
• Vapor above the liquid burning
producing radiant heat
• Material in equipment could expand
and/or vaporize causing a physical
explosion
Pool Fire
Jet Fire
• Fuel rapidly feeding the burning
front produces intense radiant
heat
• Flame impingement on nearby
equipment can cause physical
explosion and/or metal failure
Types of Explosions
Thermal Explosions
Physical
Explosion
Thermal
Explosion
No Reaction
Front Present
Explosion
Chemical
Explosion
• Self-heating
• Chemically driven
•
•
Inhibitor depletion (runaway)
Melting with decomposition
•
Autocatalytic decomposition
Deflagration
Deflagration
• Chemical reaction propagation due
to diffusion of heat & chemical from
the reaction zone – flame
propagates by molecular diffusivity
less than sonic velocity
Detonation
Reaction Front
Present
• Propagation due to adiabatic shock
compression of unburned material
greater than sonic velocity
Detonation
Explosions
Typical pressure versus time data obtained from gas explosion apparatus
The maximum rate of pressure increase when a substance
is ignited indicates the robustness of an explosion and is
described by the Deflagration Index & Cubic Law
Gas Deflagration Index: KG = (dP/dt)max × V1/3
Dust Deflagration Index: KSt = (dP/dt)max × V1/3
From Chemical Process Safety, Third Edition,
By Daniel A. Crowl and Joseph F. Louvar (ISBN: 0131382268)
Vapor Cloud Explosions (VCE)
Vapor Cloud Explosions
• VCE overpressure determined by
combustion rate
• Burning velocity is speed of flame with
respect to unburned gas
• Laminar burning velocity is burning velocity
measured in a controlled apparatus
• Speed of the flame front, & therefore the
shape of blast cure dependent on the mass
& reactivity of fuel, confinement,
congestion, and ignition source
• Overpressure prediction (See consequence
assessment lecture)
• TNT equivalence
• Empirically defined Blast Curve
methods
Congestion
Low congestion <10%
Medium congestion >10%, <40%
High congestion >40%
Combustion Rate determines overpressure
Overpressure = f (mass of fuel, reactivity, confinement, congestion, ignition)
Confinement
• Eliminates area for vapor expansion, accelerates the reaction front
Congestion
• Generates turbulence increasing flame surface area and combustion rate
Reactivity
• Determined by laminar burning velocity
Ignition Source
Vapor Cloud Explosions (VCE)
Confinement
• Eliminates area for vapor expansion,
accelerates the reaction front
• Confined VCEs and much more
damaging the unconfined VCE
Unconfined
3-D flame
expansion
1-D flame
expansion
2-D flame
expansion
Dusts & Dust Explosions
Dust Terms
• Dusts are classified int St numbers: St-0 to St-3
• St-0 is non combustible
• St-3 is the most combustible
• The St class is determined by Kst
St
Kst
St 0
St 1
0 < Kst < 200
Dust
Particle
Size μm
Kst
(bar m/s)
Wood dust
33
-
Activated
Carbon
18
44
Characteristics of dust explosions
• As particle size or moisture content goes
down
• Kst ↑
• Pmax ↑
• Minimum explosive dust concentration
decreases
• Minimum explosive energy decreases
• To be explosive, a dust mixture must:
• Particles below a certain minimum size
• Particle density between certain limits
• Particle density reasonably uniform
Fuel
St 2
200 < Kst <
300
Cellulose
33
229
St 3
Kst > 300
Al powder
<10
515
Ignition
Dispersion
Oxygen
Confinement
Lecture 2 Summary
What’s Important
Typical Exam Problem
• The causes of the 2005 Texas City explosion
1.
The fire triangle is best described as:
a. Oxidizer, fuel, and ignition
b. Fire, oxygen, and fuel
c. Air, spark, and wood
d. UEL, LEL, and Flash point
2.
A material with a flash point = 150⁰F is best described as:
a. A flammable
b. Autooxidzer
c. A combustible
d. A TIH zone A material
• The fire triangle
• Know the definitions & how to apply the concepts
• Flammability characteristics and LeChatelier’s
Equation
• Dust explosion characteristics
• Concepts to prevent fires & explosions
• Inerting
• Grounding & bonding
22
Lecture 2 Homework
Required Reading
Micro Learning Lectures
• Chapter 4 in Process Safety for Engineers
2.1 Fires & Explosions
• OSHA Combustible Dust
Suggested Reading
• Chapters 5 & 6 ALOHA Technical Manual
• CEP Hotwork
• CSB Seven Key Lessons to Prevent Worker Death During Hotwork
Required Research
• Learn the definitions from this lecture and be able to apply the concepts
Material in required reading & research may be on exam
23
Microlecture
24
Introduction to Design and
Process Safety
Micro Learning 2.1
Flammability- Fires & Explosions
What are the
Hazards?
25
2.1 Objectives
Gain a visual understanding of various types of fires and explosions
• Flammable liquid
• Combustible liquid
• Unconfined dust
• Semi-confined dust
• Unconfined VCE
26
Flammable Liquid
Combustible Liquid
Aerosols
Dust
Dust, semi-confined
Unconfined VCE
Introduction to Design and
Process Safety
Micro Learning 2.2
FlammabilityPreventing Fires & Explosions
What are the
Hazards?
33
2.2 Objectives
Review concepts to prevent fires & explosions
• Grounding / Bonding
• Inerting
34
Ethylene Oxide Example
Ethylene oxide is a flammable liquid having a normal boiling point below
room temperature. Describe a system for transferring ethylene oxide from
a tank car through a piping system to storage. Include grounding, bonding,
and inerting.
Nitrogen
Vent
Check valve
Bonding
Grounding
Double block &
bleed
35
Vacuum Purge Example
Use a vacuum purge to purge oxygen from a 150 ft3 tank containing air. Reduce the oxygen
concentration to 1% using pure nitrogen as the inert gas at 80°F. Assume the vacuum purge
goes from atmospheric pressure to 20 mmHg absolute. Determine the total number of
purge cycles and amount of nitrogen used.
𝑛
𝑃
𝑦𝑗 = 𝑦0 (𝑛 𝐿 )𝑗 = 𝑦0 (𝑃 𝐿 )𝑗
𝐻
∆𝑛𝑁2 = 𝑗(𝑃𝐻 − 𝑃𝐿 )
𝐻
𝑉
𝑅𝑔 𝑇
𝑦𝑗 = concentration of oxidant after j cycles
𝑃𝐿 = Pressure at vacuum
𝑃𝐻 = Initial high pressure
∆𝑛𝑁2 =Total moles of nitrogen added for each cycle
𝑉 = Volume of container
𝑇= Temperature of container, consistent units
Solution
𝑦0 = 0.21 lbmol O2/total mol (concentration of oxygen in air)
𝑦𝑓 = .01
𝑃𝐻 = 1 atm = 760 mmHg, 𝑃𝐿 = 20 mmHg
𝑗=
∆𝑛𝑁2
ln(.01 .21)
ln(20 𝑚𝑚𝐻𝑔 760 𝑚𝑚𝐻𝑔)
= .84 = 1 purges
150𝑓𝑡3
= 1 1 𝑎𝑡𝑚 − 20/760
(0.7302 𝑎𝑡𝑚 − 𝑓𝑡 3 /𝑚𝑜𝑙𝑒 0 𝑅 540 ⁰𝑅
∆𝑛𝑁2 = 0.37 𝑙𝑏𝑚𝑜𝑙 = 10.4 𝑙𝑏 𝑁2
36
Sweep Through Purge Example
Use the sweep-through purging to reduce the concentration of toluene from an initial 20% to
1% in a room with a volume of 25,000 ft3. Assume that the room is purged with air at a rate
of 6 room volumes per hour. How long will it take to complete this purge process?
𝑄𝑣𝑡 = 𝑉 ln
𝐶1 − 𝐶0
𝐶2 − 𝐶0
𝑡=
Given:
V = volume vessel = 25,000
ft3
𝑡=
25000 𝑓𝑡3
150,000 𝑓𝑡3/ℎ𝑟
𝑉
𝐶1 − 𝐶0
ln
𝑄𝑣 𝐶2 − 𝐶0
0.2−0
ln 0.01−0 = .499 hr = 30 min
Qv = volumetric flowrate
= 6(25,000 ft3/hr) = 150,000 ft3/hr
Co = inlet contaminant concentration = 0
C1 = 0.2
C2 = 0.01
37