CFSE Exam Preparation: Section I-6
Section 6:
Consequence Analysis
• Statistical Consequence Analysis
• Toxicology / Probit Analysis
• Toxicity Benchmarks
• Chemical Release Modeling
– Vapor Cloud Explosions
– Physical Explosions
– Pool Fires
– BLEVE
– Toxic Dispersion
• Consequence vs. Impact
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CFSE Exam Preparation: Section I-6
Definition: Consequence
• A measure of the expected effects of an
outcome case (e.g., an ammonia cloud from a
10 lb/s leak under Stability Class D weather
conditions and a 1.4-mph wind traveling in a
northerly direction will cause 50 injuries)
– CCPS, Guidelines for CPQRA
2
CFSE Exam Preparation: Section I-6
Statistical Analysis of Consequences
• Consequence estimates can be the mean
(average) amount of loss from previous loss
events
– Mean Loss = Sum of Losses / Number of Incidents
• Statistical estimates have narrow applicability
– Extensive historical data
– Extensive loss base
– Very similar incident circumstances
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CFSE Exam Preparation: Section I-6
Statistical Analysis Example
• In the United States in 1999, there were 1,058 Boiler
Explosions (National Board of Boiler and Pressure Vessel Inspectors).
As a result of these boiler explosions there were 12
fatalities and 73 injuries
– Estimate the consequence of a boiler explosion in terms of
fatalities and injuries based on 1999 Board Data
Using statistical analysis, the average consequence, is the total
consequence divided by the total number of instances.
C(Fatality) = 12 Fatalities / 1,058 Accidents = 1.13 x 10–2 PLL
C(Injury) = 73 Injuries / 1,058 Accidents = 6.90 x 10-2 PI
4
CFSE Exam Preparation: Section I-6
Release Phenomena Modeling
• Consequences estimated by modeling
physical parameters of release
• Hazard stems from uncontrolled release of
energy potential
– Mechanical
– Thermal
– Chemical
– Electrical
5
CFSE Exam Preparation: Section I-6
Chemical release event tree
Incident –
Loss of Control
No Release –
Release BLEVE Physical Explosion
No Impact
Liquid and/or
Gas
Liquefied Gas
Liquid Flashes
Gas Vents
to Vapor Pool Slowly
Flame Jet Forms
(if ignited) Evaporates
Vapor Cloud Travels Pool Fire Occurs
Downwind
Vapor Cloud Ignites
(if not ignited)
-Explosion
Liquid Vapor Plume
Rainout Travels Downwind
Vapor Cloud Ignites
-Flash Fire Plume Ignites, Explosion
and/or Flash Fire Occurs No Ignition – Toxic
No Ignition – Toxic
Vapor Exposure
Vapor Exposure
Pool Fire
6 Occurs
CFSE Exam Preparation: Section I-6
Definition: Incident Outcome
• The physical manifestation of the incident; for
toxic materials, the incident outcome is a
toxic release, while for flammable materials
the incident outcome could be a Boiling Liquid
Expanding Vapor Explosion (BLEVE), flash
fire, unconfined vapor cloud explosion, toxic
release, etc. (e.g., for a 10 lb/s leak of
ammonia, the incident outcome is a toxic
release)
• CCPS, Guidelines for CPQRA
7
CFSE Exam Preparation: Section I-6
Definition:
Incident Outcome Case
• The quantitative definition of a single result of
an incident outcome through specification of
sufficient parameters to allow distinction of
this case from all others for the same incident
outcomes. For example, a release of 10 lb/s
of ammonia with D stability, 1.4 mph wind
speed gives a particular downwind
concentration profile, for example, 3,000 ppm
at a distance of 2,000 feet.
– CCPS, Guidelines for CPQRA
8
CFSE Exam Preparation: Section I-6
Definition: Effect Zone
• For an incident that produces an incident outcome of
toxic release, the area over which the airborne
concentration equals or exceeds some level of
concern… For a flammable vapor release, the area
over which a particular incident outcome case
produces an effect based on a specified
overpressure criterion… for thermal radiation effects,
the area over which a particular incident outcome
case produces an effect based on thermal damage
criteria.
– CCPS, Guidelines for CPQRA
9
CFSE Exam Preparation: Section I-6
Estimating Effect Zones
EPA Worst Case Scenario
• Effect zone determination can be very
complex
• EPA developed simplified protocols for Risk
Management Planning
• Scenarios reflect a “worst case scenario”,
yielding conservative results
• EPA Guidelines published in “Risk
Management Program Guidance for Offsite
Consequence Analysis”
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CFSE Exam Preparation: Section I-6
Effect Zone Calculation – EPA WCS
Vapor Cloud Explosion
• Procedure
– Determine the flammable mass
– Determine heat of combustion for flammable
material (use la Chatelier’s Principle for Mixtures)
– Calculate the Equivalent Weight of TNT using
Where, mTNT is the equivalent weight of TNT, lbs
mFlammable is the flammable mass, lbs
H c Hc is the heat of combustion, kcal/kg
mTNT m flammable Yf
1155 1155 is the heat of combustion of TNT
Yf is the explosive yield factor
11
CFSE Exam Preparation: Section I-6
Effect Zone Calculation – EPA WCS
Vapor Cloud Explosion (Cont’d)
– Use a 10% (0.1) Yield Factor (per EPA)
– Calculate distances according to equation below
– Typical Endpoints – Peak Overpressure in psi
• 5 psi – Equipment Damage
• 3 psi – Fatality
• 1 psi – Injury
– Impact zone is circular
1 / 3 3.5031 0.7241ln(O p ) 0.0398 (ln O p ) 2
d ( ft ) mTNT e
Where, d is the distance to given overpressure, ft
Op is the peak overpressure, psi
12
CFSE Exam Preparation: Section I-6
Example
VCE Effect Zone
Failure of a burner management system will result in the
release of 9,412 lbs of propane, it is expected to ignite
almost immediately. What is the magnitude of the effect
zone for a 3 psi endpoint.
Step 1 – Flammable Mass is 9,412 lbs
Step 2 – Heat of Combustion of Propane is 11,074 kcal/kg
Step 3 – Equivalent Weight of TNT is
mTNT = 9,412 * (11,074/1,155) * 0.1 = 9,024 lbs
13
CFSE Exam Preparation: Section I-6
Example
VCE Effect Zone – Cont’d
Step 4 – Calculate the overpressure distance
X = 9,0241/3 * exp(3.5031 – 0.7241 ln(3) + 0.0398 (ln(3)2)
X = 328 ft
Step 5 – Calculate effect area. Area is circular, with a
radius of 328 feet.
A = 3282 = 337,985 ft2
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CFSE Exam Preparation: Section I-6
Effect Zone Calculation – EPA WCS
Physical Explosion
• Procedure
– Convert pressure in vessel to equivalent weight of
TNT using equation below
– Proceed as for Vapor Cloud Explosion
5 P
mTNT 9.24 10 P V ln
14.7
Where, mTNT is the equivalent weight of TNT, lbs
P is the bursting pressure of the vessel, psia
V is the volume of the vessel, ft3
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CFSE Exam Preparation: Section I-6
Example
Physical Explosion Effect Zone
Low steam drum level will result in overpressure and
rupture of the vessel. A steam drum is expected to
rupture at 450 psig, and has a volume of 10,000 ft3.
What energy is released, in terms of equivalent weight
of TNT, by a rupture of the steam drum.
Step 1 – Convert pressure to psia, 450 + 14.7 = 464.7
Step 2 – Equivalent Weight of TNT is
mTNT = 9.24x10-5 * 464.7 * 10,000 * ln(464.7 / 14.7)
mTNT = 1,483 lbs
16
CFSE Exam Preparation: Section I-6
Effect Zone Calculation – EPA WCS
Pool Fire
• Procedure
– Determine the area of the flammable pool
• If confined, use confinement area
• If unconfined, calculate area for 1-cm pool depth
– Look-up chemical properties, for mixtures,
calculate with La Chatelier’s Principle
• Pool Fire Factor
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CFSE Exam Preparation: Section I-6
Example
Pool Fire Effect Zone
An isoprene tank is surrounded by a circular dike with a
radius of 50 feet. If the tank were to burst, the pool
could be ignited by the wiring of the transfer pump,
which is inside the dike. What is the effect zone of this
fire to a 5 kW/m2 endpoint?
Step 1 – Calculate pool area, since the pool is diked,
the area of the diking is the area of the pool.
A = 502 = 7,854 ft2
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CFSE Exam Preparation: Section I-6
Effect Zone Calculation – EPA WCS
Pool Fire
– Assume all property and persons in the pool are
destroyed
– Typical Endpoints
• Equipment Damage – 37.5 kW/m2
• Fatality – 12.5 kW/m2
• Injury – 5.0 kW/m2
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CFSE Exam Preparation: Section I-6
Effect Zone Calculation – EPA WCS
Pool Fire
– Calculate distance to endpoint (equation below)
1
d 70.71 PFF A
R 1000
Where, d is the distance to endpoint, ft
R is the radiation intensity endpoint, kW/m2
PFF is the Pool Fire Factor
A is the area of the pool, ft2
20
CFSE Exam Preparation: Section I-6
Example
Pool Fire Effect Zone
Step 2 – Look-up Physical Properties of Flammable
PFF = 5.5
Step 3 – Calculate distance to endpoint
d = 70.71 * sqrt(1/(5*1000)) * 5.5 sqrt (7854)
d = 487 ft
Step 4 – Calculate Effect Area
Area = 4872 = 745,000 ft2
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CFSE Exam Preparation: Section I-6
Effect Zone Calculation – EPA WCS
Toxic Gas Release
• Procedure
– Determine the quantity of material release (QS)
– Determine if release is mitigated by passive
mitigation
– Calculate release rate
• If no mitigation, QR = QS/10
• With passive mitigation, QR = (QS/10)*0.55
– Determine topography of release area, either
urban or rural
22
CFSE Exam Preparation: Section I-6
Effect Zone Calculation – EPA WCS
Toxic Gas Release
– Determine whether the gas is buoyant or dense
– Select an appropriate toxic endpoint for the injury
level being studied
– Select the appropriate reference table from the
OCA guidance document
– Determine the distance to endpoint based on the
selected table
– Assume that the effect area is circular, with a
diameter, of the distance to endpoint
23
CFSE Exam Preparation: Section I-6
Effect Zone Calculation – EPA WCS
Toxic Gas Release
• Assumptions
– Entire quantity of gas is released at a continuous rate over a
10-minute period
– Wind Speed is 1.5 meters/second – wind speed reference is
at 10-meters
– Atmospheric Stability is F
– Modeling uses 10-minute averaging time
– Buoyant gases can be modeled with the ALOHA protocol
– Dense gases can be modeled with the SLAB protocol, and
HCL is a representative surrogate for all dense gases
24
CFSE Exam Preparation: Section I-6
Example
Toxic Gas Release Effect Zone
A scenario has been identified where a phosgene tank
could relieve to the atmosphere, releasing 700 lbs.
What is the effect area up to a concentration of 0.00081
mg/L. The release will occur in a process plant with a
large amount of obstructions. The phosgene tank is not
enclosed.
Step 1 – Determine the release quantity. From the
problem statement, the release quantity QS = 700 lbs.
Step 2 – Determine the release rate.
QR = QS/10 = 700 lbs/10min = 70 lbs/min
25
CFSE Exam Preparation: Section I-6
Example
Toxic Gas Release Effect Zone
Step 3 – Select topography, since the release occurs in
the middle of a process plant with many obstructions,
select ‘urban’
Step 4 – Select dispersion type. Phosgene is a dense
gas, according to OCA Exhibit B-1
Step 5 – Select toxic endpoint. The problem statement
calls for an endpoint of 0.00081 mg/L
Step 6 – Select the appropriate OCA table. For a dense
gas release, 10-minutes, urban topography. Choose
reference table 7.
26
CFSE Exam Preparation: Section I-6
Example
Toxic Gas Release Effect Zone
Step 7 – Determine endpoint from table. The endpoint
of 0.00081 is closest to 0.0007. The release rate of 70
lbs/min is closest to 50. Thus, the table yields 8.1 miles.
Step 8 – The effect zone is estimated to be.
A = (8.12/4) = 51.53 miles2
27
CFSE Exam Preparation: Section I-6
Definition: Toxicity
• Toxicologists define toxicity as
– “the ability of a substance to produce an unwanted
effect when the chemical has reached a sufficient
concentration at a certain site in the body”
• NSC 1971, Fundamentals of Industrial Hygiene
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CFSE Exam Preparation: Section I-6
Probit Analysis
• Probit (Probability Unit) Method provides a
generalized time-dependent relationship
between probability of fatality (injury) and
amount of exposure
• Probit equations solved for the probit variable
Y are based on the causative variable V,
which represents the dose, and at least two
constants
Y = k1 + k2 lnV
29
CFSE Exam Preparation: Section I-6
Probit Analysis (Cont’d)
• Probit equations are available for a variety of
exposures – the causative variables are:
– Fire
• Product of Thermal Radiation Intensity and Exposure
Duration
– Explosion
• Peak Overpressure
– Toxic Gas
• Product of Toxin Concentration and Exposure Duration
30
CFSE Exam Preparation: Section I-6
Probit and Percentage
% 0 1 2 3 4 5 6 7 8 9
• Probit and 0 ~ 2.67 2.95 3.12 3.25 3.36 3.45 3.52 3.59 3.66
10 3.72 3.77 3.82 3.87 3.92 3.96 4.01 4.05 4.08 4.12
Percentage are 20 4.16 4.19 4.23 4.26 4.29 4.33 4.36 4.39 4.42 4.45
related and 30
40
4.48
4.75
4.50
4.77
4.53
4.80
4.56
4.82
4.59
4.85
4.61
4.87
4.64
4.90
4.67
4.92
4.69
4.95
4.72
4.97
converted either 50 5.00 5.03 5.05 5.08 5.10 5.13 5.15 5.18 5.20 5.23
60 5.25 5.28 5.31 5.33 5.36 5.39 5.41 5.44 5.47 5.50
by table or 70 5.52 5.55 5.58 5.61 5.64 5.67 5.71 5.74 5.77 5.81
80 5.84 5.88 5.92 5.95 5.99 6.04 6.08 6.13 6.18 6.23
equation 90 6.28 6.34 6.41 6.48 6.55 6..64 6.75 6.88 7.05 7.33
% 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
99 7.33 7.37 7.41 7.46 7.51 7.58 7.65 7.75 7.88 8.09
Y 5 Y 5
P 501 erf
Y 5 2
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CFSE Exam Preparation: Section I-6
Probit Equations –Toxic Gas
(World Bank – 1988)
Y = a + b ln(Cntc)
Y is the Probit
a,b,n are constants
C is the concentration in ppm by volume
tc is the exposure time in minutes
Substance a b n
Acrolein -9.93 2.05 1
Ammonia -9.82 0.71 2
Chlorine -5.3 0.5 2.75
Methyl Bromide -19.92 5.16 1
Phosgene -19.27 3.69 1
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CFSE Exam Preparation: Section I-6
Example
Probit Analysis
• What percentage of humans are expected to
survive an exposure to 45 ppm of Phosgene
for 15 minutes?
Step 1 – Solve the World Bank Equations for
Probit, using the constants for Phosgene
Y = -19.27 + 3.69 ln(45 * 15)
Y = 4.77
Step 2 – Convert Probit to Percentage (Table)
Y = 4.77 41%
33
CFSE Exam Preparation: Section I-6
Toxic Effect Benchmarks
• Numerical limits established by a number of
organizations for planning
• Limited human toxic response data available,
many benchmarks estimated
• Large amount of uncertainty, normally
requires expert judgment
• Little distinction between acute and chronic
exposure
34
CFSE Exam Preparation: Section I-6
Toxic Effect Benchmarks - ERPG
Emergency Response Planning Guidelines
• Prepared by AIHA (American Ind. Hygiene Ass.)
– Based on Exposure Time – 1 Hour
– ERPG-1
• Effect – No significant effects
– ERPG-2
• Effect – No irreversible health effects
– ERPG-3
• Effect – No life-threatening effects, more represents death
• Good fatality benchmark, e.g. 303ppm for H2S
• Becoming Industry/Government Norm
35
CFSE Exam Preparation: Section I-6
Toxic Effect Benchmarks
IDLH
• Immediately Dangerous to Health and Life
– Developed by NIOSH
– Effect – “exposure is likely to cause death or
immediate or delayed permanent health effects, or
prevent escape from such an environment”
– Exposure Time – 30 Minutes
36
CFSE Exam Preparation: Section I-6
Toxic Effect Benchmarks
EEGL
• Emergency Exposure Guidance Level
– Developed by the US National Research Council
– Based on exposure of Health Military Personnel
– Effect – “Exposure to concentrations at the EEGL
may produce transient irritation or central nervous
system effects but should not produce effects that
are lasting or would impair performance of a task”
– Exposure – 1 to 24 hours
37
CFSE Exam Preparation: Section I-6
Toxic Effect Benchmarks
SPEGL
• Short-term Public Exposure Guidance Level
– Developed by the US National Research Council
– General 10-50% of EEGL, for sensitive population
– Effect – “Acceptable concentrations for exposures
of members of the general public”
– Exposure – 1 to 24 hours
38
CFSE Exam Preparation: Section I-6
Toxic Effect Benchmarks
TLV-STEL
• Threshold Limit Value – Short Term Exposure
– Developed by ACGIH
– Effect – “Maximum concentration to which workers can be
exposed for a period of up to 15 minutes without suffering
(1) intolerable irritation (2) chronic or irreversible tissue
change (3) Narcosis sufficient to increase accident prone-
ness, impair self rescue”
– Exposure – 15 Minutes
• TLV-C provides a Ceiling Value
• TLV – TWA provides an 8-hour time weighted
average
39
CFSE Exam Preparation: Section I-6
Toxic Effect Benchmarks
PEL
• Permissible Exposure Limit
– Developed by OSHA and have force of law
– Exposure – 8-hour weighted average
– Effect – Similar to TLV
40
CFSE Exam Preparation: Section I-6
Toxic Effect Benchmarks
LOC
• Level of Concern
– Developed by EPA with the EPCRA Act
– Effect – “the maximum concentration of an
extremely hazardous substance in air that will not
cause serious irreversible health effects in the
general population”
– Exposure – 30 minutes (based on relation to
IDLH)
41
CFSE Exam Preparation: Section I-6
Incident Outcome vs. Consequence
(Impact Analysis)
• Incident outcome gives a magnitude of the
incident in terms of area where a certain
effect is expected
• Consequence is a measure of impact in terms
of expected fatalities, injuries, environmental
damage, and/or property damage
• Consequence is determined by estimating the
number of receptors in the effect area
42
CFSE Exam Preparation: Section I-6
Impact Analysis Calculation
• Determining consequence from effect area is
referred to as “Impact Analysis” or “Risk
Integration”
• Procedure
– Determine the personnel density of the plant
• Number of “outside operators” and maintenance
personnel working in the field divided by plant area
• May be different “zones” with different densities
43
CFSE Exam Preparation: Section I-6
Impact Analysis Calculation
– Determine the capital density of the plant
• Replacement cost of equipment divided by process area
– Determine vulnerability to the effect of the receptor
under study
• For quick conservative study, select effect endpoints
such that vulnerability is 100%
– Calculate consequence by multiplying effect area
by receptor density by vulnerability
44
CFSE Exam Preparation: Section I-6
Impact Analysis Calculation Review
• The quick conservative method presented
here may not provide an adequate estimate
• To ensure accuracy
– Overlay the effect zone on a plot plan
– Determine if any “normally occupied buildings” are
impacted
– Determine if any high capital cost equipment items
are impacted
• If the shortcut method is unsatisfactory, the
receptors can be counted per the plot plan
45
CFSE Exam Preparation: Section I-6
Annual Expected Value Calculation
• Expected values of loss can be annualized to
assist in risk decision making, especially cost-
benefit analysis
• Procedure
– Determine the consequence expected value for an
incident (e.g., PLL, PI, or $$)
– Determine the likelihood of the event as a
frequency. (Covered later in this course)
– Multiply consequence times frequency
46
CFSE Exam Preparation: Section I-6
Summary:
Consequence Analysis
• Statistical Consequence Analysis
• Toxicology / Probit Analysis
• Toxicity Benchmarks
• Chemical Release Modeling
– Vapor Cloud Explosions
– Physical Explosions
– Pool Fires
– BLEVE
– Toxic Dispersion
• Consequence vs. Impact
47
CFSE Exam Preparation: Section I-6
Exercise 1
Consequence Analysis
• Which toxic effect benchmark represents the
smallest consequence?
a. PEL
b. IDLH
c. ERPG-3
d. ERPG-2
48
CFSE Exam Preparation: Section I-6
Exercise 2
Consequence Analysis
An explosion scenario was modeled where the effect
zone for fatality (100% vulnerability) was determined to
be 280,000 ft2. The plant has an area of 15,000 ft by
15,000 ft. There are 10 outside operators, and 40
maintenance technicians, 75% of which are expected to
be in the process area. What is the expected
consequence in terms of probable loss of life?
49
CFSE Exam Preparation: Section I-6
Exercise 3
Consequence Analysis
An fire scenario is expected to result in $15 million of
damage (replacement cost). The scenario is expected
to occur once every 2,000 years, with no SIS installed.
If an SIS can remove 100% of the risk, what is the
highest annualized cost that should be paid for the
system?
50
CFSE Exam Preparation: Section I-6
Exercise 1
Consequence Analysis
• Which toxic effect benchmark represents the
smallest consequence?
a. PEL
b. IDLH
c. ERPG-3
d. ERPG-2
The correct answer is “a”. PEL is an 8-hour time weighted
average below which no effects are expected. IDLH is immediately
dangerous at 30 minutes. ERPG-2 and ERPG-3 are expected to
cause injury and fatality, respectively, at 1-hour of exposure.
51
CFSE Exam Preparation: Section I-6
Exercise 2
Consequence Analysis
An explosion scenario was modeled where the effect
zone for fatality (100% vulnerability) was determined to
be 280,000 ft2. The plant has an area of 15,000 ft by
15,000 ft. There are 10 outside operators, and 40
maintenance technicians, 75% of which are expected to
be in the process area. What is the expected
consequence in terms of probable loss of life?
Step 1 – Determine the personnel density. There are
10 + 0.75*(40) people in an area of 15,000 ft x 15,000 ft.
Yielding a personnel density of 1.7x10-7 person/ft2
52
CFSE Exam Preparation: Section I-6
Exercise 2
Consequence Analysis
Step 2 – The impact is the effect area times the
personnel density times the vulnerability, or:
PLL = 280,000 ft2 x 1.7x10-7 persons/ft2 x 100%
PLL = 0.0476 persons
53
CFSE Exam Preparation: Section I-6
Exercise 3
Consequence Analysis
An fire scenario is expected to result in $15 million of
damage (replacement cost). The scenario is expected
to occur once every 2,000 years, with no SIS installed.
If an SIS can remove 100% of the risk, what is the
highest annualized cost that should be paid for the
system?
The most that should be paid for a SIS, assuming that
100% of the risk is removed, is the annual expected
value of loss. In this case the AEV is - $15MM *
1/2,000 years, which is $7,500 per year.
54
