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Identification and Screening Identification and Screening of Scenarios for LOPAof Scenarios for LOPA
Ken FirstKen FirstDow Chemical CompanyDow Chemical Company
Midland, MIMidland, MI
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Layers of Protection Analysis (LOPA)
“LOPA is a semi-quantitative tool for analyzing and assessing risk. The primary purpose is to determine if there are sufficient layers of protection against an accident scenario (can the risk be tolerated?).”
Layers of Protection Analysis, Center for Chemical Process Safety, American Institute
of Chemical Engineers, New York (2001)
33
Key Concerns with Key Concerns with Implementation of LOPAImplementation of LOPA
Variability in identification of credible scenarios to enter into LOPA.
Consistent evaluation of scenario consequences in “understandable” terms of damage severity or human harm.
Overall effort for implementation and re-validation of LOPA.
44
Work Process Steps for Simplified Risk Analysis
Select equipment items to include in hazard evaluation
Compile chemical, process and plant information needed
Identify event sequences that could lead to an incident
Estimate the release quantity, rate and hazard distance
Quantify the Consequence in terms of potential damage or injury
Estimate the frequency or likelihood of event sequence
Estimate risk from consequence and frequency
Identify and Assess Independent Protective Layers
Is Risk Tolerable?
Yes
NoCan Risk be Reduced?
Manage Residual Risk
Full Risk Assessment
and/or Discontinue
ActivitySelect potential incident outcome
cases for review
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Select an Equipment Item to Include in Hazard Evaluation
Most chemical process facilities utilize the same basic process equipment – vessels, pumps, heat exchangers, columns, etc.
Select an equipment item based on the potential for a chemical process hazard similar to selection of a HAZOP “node” to begin the process.
66
Types of Chemical Process Hazards
Process Risk typically addresses acute hazards including:
Flammability
Toxicity (Inhalation)
Reactivity (Chemical Energy)
Pressure-Volume Energy
77
The potential to harm people, damage property or the environment depends upon: Chemical Properties (flash point, etc.)
Process Conditions (operating temp., etc.)
Equipment Parameters (volume, etc.)
Site and Plant Layout (distance to public, etc.)
Recognition of Chemical Process Hazards
88
Relationship between Chemical Properties and
Process ConditionsChem. PropertyChem. Property
Flash PointFlash Point
LFLLFL
MIEMIE
ERPG ERPG ConcentrationsConcentrations
Heat of ReactionHeat of Reaction
Gas GenerationGas Generation
Detected Onset Detected Onset TemperatureTemperature
Process ConditionProcess Condition
Temperature > FPTemperature > FP
Concentration > LFLConcentration > LFL
Ignition Source > MIEIgnition Source > MIE
Vapor Concentration > Vapor Concentration > ERPGERPG
Maximum Reaction Maximum Reaction Temp. and PressureTemp. and Pressure
Temperature > TTemperature > TNRNR
Pressure > Design Pressure > Design PressurePressure
Hazard
Flammability
Toxicity
Reactivity
P-V Energy
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Pump Deadhead (Blocked in while running)
Blocked in with Thermal Expansion
Overfill
Excessive Heat Input
Uncontrolled Reaction
Physical Damage
Etc.
Through operational experience, incident and hazard evaluation history; common process upsets may be categorized and related to specific types of equipment.
Common Categories of Process Upsets
1010
A Scenario Case represents an unplanned sequence of events leading to an incident
with undesired consequence.
Initiating Event
+ Enabling Conditions
Inci
dent Outcome with
Undesired Consequence
Failure of Independent
Protective Layers
Scenario IdentificationScenario Case
1111
Initiating Event starts an event sequence and is typically categorized as:
Control system failure
Human error
Mechanical failure
Incident is an unintended release of hazardous material or energy.
Consequence is a measure of the potential Outcome in terms of injury, damage, or economic loss.
Scenario IdentificationSequence of Events
1212
Categories of Chemical Process Incidents
Hole Size release rate.Standardized hole sizes simplify the screening analysis, for
example:- 5 to 10 mm to represent gasket failure.- 100 mm to full bore diameter to represent pipe or equipment
nozzle failure.
Overflow rate estimated from feed or fill rate. Excessive Heat vapor release rate estimated
from rate of heat input divided by heat of vaporization.
Catastrophic Failure or Rupture as a sudden release of entire equipment contents and reaction or pressure-volume energy.
1313
A Scenario Case also represents a relationship between Process Upset,
Initiating Event, Incident Category, and Outcome for a specific Equipment Type.
Scenario IdentificationScenario Case
These relationships may be used to pre-develop a list of scenario
cases to consider.
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Scenario IdentificationCommon Relationships
Scenario TypeScenario Type Parameter/ Parameter/ DeviationDeviation
Equipment Equipment TypesTypes
Initiating Initiating EventsEvents IncidentsIncidents ConditionsConditions
DeadheadDeadhead FlowFlow--NoneNone TempTemp--HighHigh
PumpPump CompressorCompressor
Control FailureControl Failure Human ErrorHuman Error RuptureRupture Max. Pressure > Burst PressureMax. Pressure > Burst Pressure
Overflow, Overfill, Overflow, Overfill, or Backflowor Backflow
LevelLevel--HighHighFlowFlow--BackflowBackflow
VesselVessel ColumnColumn
Control FailureControl Failure Human ErrorHuman Error
OverflowOverflow(thru Vent)(thru Vent)
Inventory > Equip Volume Inventory > Equip Volume andandFeed Pressure > Op PressureFeed Pressure > Op Pressure
Overflow Overflow (thru Relief)(thru Relief)
Inventory > Equip Volume Inventory > Equip Volume andand Max Pressure > Relief PressureMax Pressure > Relief Pressure
Excessive Excessive PressurePressure PressurePressure--HighHigh VesselVessel
ColumnColumnControl FailureControl Failure Human ErrorHuman Error
OverflowOverflow(thru Relief)(thru Relief)
Max Pressure > Relief Set Max Pressure > Relief Set PressurePressure
RuptureRupture Max Pressure > Burst PressureMax Pressure > Burst Pressure
Excessive HeatingExcessive Heating TempTemp--HighHigh Heat InputHeat Input--HighHigh
VesselVessel ColumnColumnExchangerExchanger
Control FailureControl Failure Human ErrorHuman Error
Vapor ReleaseVapor Release (Relief)(Relief)
Max Pressure > ReliefMax Pressure > ReliefSet PressureSet Pressure
RuptureRupture Max Pressure > Burst PressureMax Pressure > Burst Pressure
Loss of Loss of ContainmentContainment
FlowFlow--Loss ofLoss ofContainmentContainment AllAll MechanicalMechanical
IntegrityIntegritySmall HoleSmall HoleMedium HoleMedium Hole Large HoleLarge Hole
Frequency depends upon internal or Frequency depends upon internal or external corrosion, screwed versus external corrosion, screwed versus welded construction, etc.welded construction, etc.
Uncontrolled Uncontrolled ReactionReaction
TempTemp--HighHigh CompositionComposition-- WrongWrongFlowFlow--BackflowBackflow
VesselVessel ExchangerExchanger PumpPump
Control FailureControl Failure Human ErrorHuman Error Utility FailureUtility Failure
Vapor ReleaseVapor Release (Relief)(Relief) Max Pressure > Relief PressureMax Pressure > Relief Pressure
RuptureRupture Max Pressure > Burst PressureMax Pressure > Burst Pressure
Example Predetermined Scenario List
1515
Scenario IdentificationAnalysis Team
Use of a predetermined list of feasible scenarios may help the Analysis Team to quickly identify other cases to consider.
The Team may find additional relationships between scenario type, initiating event, incident category, and outcome to extend the predetermined list.
Elucidation of the initiating event (“How could this happen in my plant?”) may also help the Team identify scenario cases to consider.
1616
Overfill of T-127 acrylonitrile storage tank leading to a release at a rate equal to the fill rate caused by process control failure
resulting in . . .
Scenario IdentificationScenario Description
Process Upset Equipment Type
Incident Category Initiating Event
Chemical Involved
An Outcome must be selected based on the chemical process hazard and potential Consequence
to complete the Scenario Description.
1717
Simplified Analysis for Selection of Incident Outcome
Simplified source models are used to estimate release rates, airborne quantities, and hazard distances as part of determining feasible incident Outcomes.
For simplicity, selection of a single wind speed, stability, and surface roughness may
be appropriate for LOPA analysis.x
y
z
H
(x,0,0)
(x,-y,0)
(x,-y, z)
1818
Example Outcome Selection Criteria
Flash (or Jet) FirePersonnel exposure to 0.1 to 0.5 times LFL
Vapor Cloud Explosion1000 Kg flammable release (100 Kg for high flame speed)
Building ExplosionIndoor concentration exceeds LFL
Physical Explosion (and BLEVE)Exposure to 1 psi overpressure (0.3 psi for fragmentation)
Toxic Vapor Release (Indoor, Outdoor)Off-site exposure to > ERPG-2 concentration (60 min)On-site exposure to > ERPG-3 concentration
A single incident may have several potential outcomes.
1919
Simple Analysis of Outcome Consequence
A simple Consequence Analysis may be based on Hazards originating from a point source such that the effect zone is estimated in
terms of radial distance from the source.
Personnel within the effect zone are assumed severely impacted while
those outside of this area are assumed not affected.
Wind
Cloud Plume
Release Point
Effect Zone (Probability of
Severe Impact = 1)
Probability of Severe Impact = 0
2020
Quantification of Consequence Severity
The number of personnel severely impacted may be estimated as impact area times population density.On-site population density should account for
maintenance and other personnel in the process area and “worst case” wind direction.
For scenario cases where personnel are anticipated to be in close proximity to the release point, the number of personnel at risk assumed as the number in attendance.
2121
Quantification of Consequence Severity
In some cases, the number of personnel severely impacted is significantly less than one.May indicate a relatively low probability of
serious injury of fatality.May indicate that a minor injury is a more
likely consequence than a serious injury or fatality.
2222
Example Consequence Severity Categories
The estimated number of people impacted is not
precise such that a consequence
category representing an
“order of magnitude”
range may be more
appropriate.
Low Minor on-site injury, no lost timeMinor (non-reportable) environmental eventMinimal equipment damage or production loss
MediumPublic annoyance (odor, alert, etc.)Recordable on-site injury, not severeOffsite environmental impact or permit violationEquipment damage with some lost production
HighOne or more injuries to the publicOne or more severe on-site injuries or fatalitySignificant release/severe environmental impactMajor damage to process equipment (>$1 MM)
Very HighOne or more serious injuries or fatality to the publicMultiple severe on-site injuries or fatalitiesLong-term contamination and/or large kill of wildlifeMajor destruction and business loss (>$10 MM)
Catastrophic Significant off-site disruption with multiple injuries/fatalities resulting in public enquiry and prosecutions
Consequence Severity Description
No impact on the public
2323
Likelihood EvaluationProbability and Frequency
Estimates of frequency and probability are inherent in risk analysis as many scenario cases represent rare, but catastrophic, event sequences.
Initiating Events are represented as frequency (events per year).
Enabling Events or Conditions are represented by probability (between zero and one).
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Likelihood EvaluationEnabling Events
Enabling Events must generally be present or met for the event sequence to proceed from initiating event to incident outcome.
Probability of Ignition
Fraction of Time at Risk based on mode of operation (start-up, specific operational step or procedure, etc.)
Probability of Successful Evasive Action
Others
2525
Risk EstimationConsequence and Frequency
Initiating Event Frequency
XEnabling Condition
Probability
IPL Probability of
Failure on Demand
Tolerable Consequence Frequency to
Meet Risk Target
X <
Initiating Event
+ Enabling Conditions
Inci
dent Outcome with
Undesired Consequence
Failure of Independent
Protective Layers
2626
Risk EstimationExample Risk Matrix
10-2 / Year
10-3 / Year
10-4 / Year
10-5 / Year
10-6 to 10-8 / Year
FrequencyConsequence Severity
Low HighMedium CatastrophicVery High
The target frequency for a scenario case should be set conservatively compared with corporate
or regulatory risk criteria
* “As low as reasonably practicable” may apply.
Tolerable
Tolerable
Tolerable
Tolerable
Tolerable
TolerableTolerable
Tolerable
Tolerable
Tolerable
IntolerableTolerable
Tolerable*
Tolerable
Tolerable*
Tolerable*
Intolerable
Intolerable Intolerable
Intolerable
Intolerable
Intolerable
Intolerable
Intolerable
Intolerable
2727
A simple semi-quantitative risk analysis involving process upset (scenario type), incident category, and outcome is a promising means to identify and evaluate hazard scenario cases for LOPA.
Estimates of release rate, hazard distance, and people impacted provide a means for reducing variability in quantifying consequence and setting target frequencies.
Results may be validated against conventional quantitative risk analysis techniques and periodically updated to ensure they are appropriately conservative in meeting corporate or regulatory guidance.
LOPA Scenario IdentificationSummary and Conclusions
2828
LOPA Scenario Identification Key References
Layers of Protection Analysis, Center for Chemical Process Safety, American Institute of Chemical Engineers, New York (2001).
Guidelines for Hazard Evaluation Procedures, 2nd Edition, Center for Chemical Process Safety, American Institute of Chemical Engineers, New York (1992)
Guidelines for Chemical Process Quantitative Risk Analysis, 2nd Edition, Center for Chemical Process Safety, American Institute of Chemical Engineers, New York (2000)
Guidance on “as low as reasonably practicable” (ALARP) decisions in control of major accident hazards (COMAH), Health and Safety Executive, UK (2002), available at: http://www.hse.gov.uk.
Risk Management Program Guidance for Offsite Consequence Analysis, United States Environmental Protection Agency, available at: http://www.epa.gov/ceppo.
Freeman, R., Using Layer of Protection Analysis to Define Safety Integrity Level Requirements, Process Safety Progress, 26 (2007)
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