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Chemical Process Quantitative Risk Analysis
2009_2nd semester
En Sup Yoon
Introduction-1
CPQRA(Chemical Process Quantitative Risk Analysis)– A methodology designed to provide management with
a tool to help evaluate overall process safety
– Provide a quantitative method to evaluate risk and to identify areas for cost-effective risk reduction
Introduction-2
Definition (by CPQRA)– Evaluate the risk by defining the probability of failure,
the probability of various consequences and the potential impact of those consequences
– Risk = f(S,C,F) S = hypothetical scenario
C = estimated consequence
F = estimated frequency
Typical goal of CPQRA
To screen or bracket the range of risk present for further study
To evaluate a range of risk reduction measures
To prioritize safety investments
To estimate financial risk
To estimate employee risk
To estimate public risk
To meet legal or regulatory requirements
To assist with emergency planning
CPQRA Steps
Define the potentialAccident scenarios
Evaluate the eventconsequences
Estimate the potentialAccident frequencies
Estimate theEvent impacts
Estimate the risk
Evaluate the risk
Identify and prioritizePotential risk reduction
measures
CPQRA Definition-1
Risk– Is a combination of uncertainty and damage
– Is a ratio of hazards to safeguards
– Is a triplet combination of event, probability and consequences
Frequency– Number of occurrences of an event per unit of time
Hazard– A chemical or physical condition that has the potential
for causing damage to people, property or the environment
CPQRA Definition-2
Consequence– A measure of the expected effects of an incident
outcome case
Likelihood– A measure of the expected probability or frequency of
occurrence of an event– Expressed as a frequency (e.q. events/year)
Probability– The expression for the likelihood of occurrence of an
event or an event sequence during an interval of time or the likelihood of occurrence of the success or failure of an event on test or demand
– Expressed as a ranging from 0 to 1
Component Technique of CPQRA-1
Component technique covering in CPQRA ( Figure 1.3)– CPQRA definition– System description– Hazard identification– Incident enumeration– Selection incident– CPQRA model construction– Consequence estimation– Likelihood estimation– Risk estimation– Utilization of risk estimates
Component Technique of CPQRA-2
Prioritized CPQRA Procedure (Figure 1.4)– Step 1 : Define CPQRA
– Step2 : Describe the system
– Step 3 : Identify hazards
– Step 4 : enumerate incident
– Step 5 : select incidents, incident outcomes and incident outcome cases
– Step 6 : estimate consequences
– Step 7 : modify system to reduce consequences
– Step 8 : estimate frequencies
– Step 9 : modify system to reduce frequencies
– Step 10 : combine frequency and consequences to estimate risk
– Step 11 : modify system to reduce risk
Management of Incident Lists
Enumeration and selection of incident and tracking for effective management for CPQRA
Enumeration– Ensure that no significant incidents are overlooked
Selection– Reduce the incident outcome cases studied to
manageable number
Tracking– Ensure that no incident, incident outcome or incident
outcome case is lost in the calculation procedure
Enumeration
Objective– Identify and tabulate all members of the incident
classes
– Incident class Localized incident
– Localized effect zone, limited to single plant area
Major incident– Medium effect zone, limited to site boundaries
Catastrophic incident– Large effect zone, off site effects on the surrounding community
Selection-1
Goal– To limit the total number of incident outcome cases to
be studied to a manageable size
Incident– To construct an appropriate set of incident
– Type of incident list Reality list (all incidents)
Initial list (all incidents identified by enumeration)
Revised list (initial list less those handled subjectively)
Condensed list (revised list without redundancies)
Expansive list (list from which incidents for study are selected)
Representative set
Selection-1
Incident outcomes– The physical manifestation of the incident
– Develop a set of incident outcomes that must be studied for each incident included in the finalized incident study list
Incident outcome cases– 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
SMALL MEDIUM LARGE
ELEMENTARY SIMPLE SIMPLE/
INTERMEDIATE
INTERMEDIATE
ADVANCED SIMPLE/
INTERMEDIATE
INTERMEDIATE INTERMEDIATE/
COMPLEX
SOPHISTICATED INTERMEDIATE INTERMEDIATE/
COMPLEX
COMPLEX
NUMBER OF INCIDENT OUTCOME CASES
Safety assessment Technique
Qualitative methods– Safety Review
– Checklist Analysis
– Relative Ranking
– What-If Analysis
– HAZOP Analysis
– FMEA Analysis
Quantitative methods– FTA, ETA
– Cause-Consequence Analysis
– Human Reliability Analysis
– Dispersion Modeling
Safety Review
Purpose– Keeps operating personnel alert to the process hazards– Review operating procedures for necessary revisions– Seek to identify equipment or process changes that
could have introduces new hazard– Evaluate the design basis of control and safety system
Types of result– Qualitative descriptions of potential safety problem
and suggested corrective actions
Resource requirements– P&ID, flowcharts, plant procedures for start-up,
shutdown, maintenance and emergencies, hazardous incident reports, process material characteristics
Checklist Analysis
Purpose– Ensure that organizations are complying with standard
practices
Type of results– List of questions based on deficiencies or difference– Completed checklist contains “yes”, “no”, “not
applicable” or “need more information” answer to the question
Resource requirement– Engineering design procedure, operating practices
manual– Experiences manager or engineer with knowledge of
process
Relative Ranking
Purpose– Determine the process areas or operation that are the
most significant with respect to the hazard of concern in a given study
Types of result– An ordered list of process equipment, operation or
activities
Resource requirements– Basic physical and chemical data on the substance used
in the process or activity
What-If Analysis
Purpose– Identify hazards, hazardous situations or specific
accident events that could produce an undesirable consequence
Types of results– A list of questions and answers about the process– A tabular listing of hazardous situations, their
consequence, safeguards and possible options for risk reduction
Resource requirements– Experiences manager or engineer with knowledge of
process
HAZOP(Hazard and Operability Analysis)
Purpose– Review a process or operation in a systematic fashion
to determine whether process deviations can be lead to undesirable consequence
Types of results– Identification of hazards and operating problem and
recommendation
Resource requirements– P&ID, equivalent drawing other detailed process
information
FMEA(Failure Mode and Effect Analysis)
Purpose– Identify single equipment and system failure mode and
each failure mode’s potential effect on the system or plant
Types of results– Generates a qualitative, systematic reference list of
equipment, failure modes and effects
Resource requirements– A system or plant equipment list or P&ID, knowledge
of equipment function and failure modes, knowledge of system or plant function and response to equipment failures
Fault Tree Analysis
Purpose– Identify of equipment failure and human errors that
can result in an accident
Type of Results– System failure logic model that use Boolean logic gate
(AND, OR) to describe how equipment failure and human errors can combine to cause a main system failure
Resource requirements– Detailed understanding of how the plant or system
function, detailed process drawing and procedure, knowledge of component failure modes and their effects
Event Tree Analysis
Purpose– Identify the various accident that can occur in a
complex process
Types of results– Event tree models and the safety system successes or
failure that lead to each defined outcome
Resource requirements– Knowledge of potential initiating events and
knowledge of safety system function or emergency procedures that potential mitigate the effect of each initiating event
Cause-Consequence Analysis
Purpose– Identify the basic cause and consequence of potential
accident
Types of results– Generating diagrams portraying accident sequence and
qualitative description of potential accident outcomes
Resource requirements– Knowledge of component failure or process
– Knowledge of safety systems or emergency procedures
– Knowledge of the potential impacts of all these failure
Human Reliability Analysis
Purpose– Identify potential human errors and their effects or to
identify the underlying cause of human error
Types of results– Systematically lists the errors likely to be encountered
during normal or emergency operation, factors contributing to such error
Resource requirements– Plant procedure– Information from interviews of plant personnel– Knowledge of plant layout, function or task allocation– Control panel layout, alarm system layout
Overview of Consequence Analysis
Component of Consequence assessment
GIS
Information Collection, Parameter InputDischarge ModelingVapor phase Fugitive emission (emission factor DB)Vapor phase leak through a hole or pipeLiquid phase leak through a hole or pipeTwo phase leak through a hole or pipeVapor phase discharge by ruptureLiquid phase discharge by ruptureIn-Building dischargeInteractive flash calculationVaporization
Dispersion ModelingRichardson Number calculationLight gas Dispersion (Gaussian)Dense gas Dispersion (Pasquill-Gifford, Slab)Surface Roughness/Curvature effectBuilding effectRain/Snow effect
Effect ModelingPool fire Physical explosionBLEVE VCE & UVCEToxicity
RA Calculation, Reporting
Information System
Chemical Prop. DBEquip. Maintenance DBMeteorological DBOperation Schedule Info.Population DB
Case Storage DB Accident Scenario KB
Equipment info.Chemical info.
Meteorological info.Operating condition
Population
•Normal Operation
•Abnormal Operation
•Real-time Operation
Discharge
Dispersion
Effect
Statistical Report Graphical analysis
Risk Assessment Report
Related personOperator, Director, Manager...
Process AnalysisProcess Implementation
feedback
Calculation Flowchart
Discharge modeling-1
Aim– Prediction of the final state of the release as the material
emerges into the atmosphere
Input– Temperature, pressure, phase, liquid fraction etc.
Output– Mass flow rate, duration, pseudo-velocity, discharge
velocity, temperature, liquid fraction, droplet trajectories and size
Discharge modeling-2
Typical source term modeling– Estimate the release rate and the release duration for
vessel or pipe leak/rupture liquid release
vapor release
two-phase release (aerosol)
Air Dispersion Model
Use– Emergency Planning Mode
to make decision regarding mitigation measure
Consist solely of software
– Emergency Responding Mode consist of combination of software and hardware
real-time gathering the tank and meteorological data
– Complexity, Costs very greatly
Two Kinds of Dispersion Modeling
Modeling routine Emission e.g., SO2 gas from plant stack
Source strength well-defined, continuous and not time-Varying
Simple Gaussian model
Modeling accident release e.g., Leaking valve on a chlorine cylinder
More difficult to model than routine modeling
– users often guess important inputs such as source term
– pressurized releases not well understood
Gaussian model too simple
Two Stage of Analysis for Modeling Accidental Release
Source Strength(May have several subpart e.g., release from
containment evaporation if the pool is formed)
Dispersion Mechanism(May have several subparts)
Dispersion Mechanism
Neutrally buoyant
Dense gas
Ground
Wind
Slumping Stratified Passive
Initial rapid expansion of vapor on release
Dense turbulent plume release
Wind DirectionMixing due to initial momentum
Slumping dense plume phase
Gas slumps or spreads under gravity
Passive dispersion phase
Mixing due to atmospheric turbulence
Stages of a Continuous Release
Stage of an Instantaneous Release
Initial rapid expansion of vapor on release
Dense turbulent cloud phase
Slumping dense cloud phase
Passive dispersion phase
Wind Direction
Mixing due to initial energy
Cloud slumps or spreads under gravity
Mixing due to atmospheric turbulence
Meteorology and Local Condition
Wind Direction
Wind Speed
Atmospheric stability (A through F)
Ground roughness
Inversion
Wind Profile
Ele
vat
ion
Wind Speed
Atmospheric StabilityH
eig
ht
ab
ov
e g
rou
nd
Hei
gh
t a
bo
ve
gro
un
d
Warm air
Cool Air Warm air
Cool Air
Day-Unstable Night-Stable
Effect of Stability on Dispersion
Unstable Weather
Mixing
Stable Weather
Ground Roughness
Depend on the size and number of the surface feature on the terrain– When surface feature are smaller, so is the ground
roughness
– The smaller the roughness, the faster the cloud is dispersed
Existing Models
Models : Dispersion Model
-K-theory and other three-dimensional Model
-Modified Conventional Models : Pasquill-Gifford Model, Bureau of
Mines Model, Clancey Model
-British Gas/Cremer and Warner Model : Cox and Roe Model, Cox and
Carpenter Model
-Van Ulden Model : Van Ulden Model, Van Ulden Model 2
-Box and Slab Model : SLAB, FEM3
-Workbook Model : Britter and McQuaid Model
-Instantaneous Release Model : DENZ, CRUNCH
Selected Models
–Richardson Number calculation
–Light gas : Gaussian
–Dense gas : SLAB
–Surface Roughness/Curvature effect
–Building effect
–Rain/Snow effect
Existing Models
Models : Effect Model
Fire ModelRadiation Heat
Transfer ModelIgnition ModelUnsteady-State Model
Explosion ModelDetonation ModelDeflagration ModelTNT Explosion ModelMulti-Energy Model
Toxic ModelProbit AnalysisThreshold Limit ValueED, TD, LD, LC
Selected Models
Fire ModelRadiation Heat
Transfer Model
Explosion Model
TNT Explosion Model
Toxic ModelProbit Analysis