Chemical Process Quantitative Risk Analysis
2009_2
ndsemester
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 potential Accident scenarios Evaluate the event
consequences
Estimate the potential Accident frequencies Estimate the
Event impacts Estimate the risk
Evaluate the risk Identify and prioritize Potential 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 Input Discharge Modeling
Vapor phase Fugitive emission (emission factor DB) Vapor phase leak through a hole or pipe
Liquid phase leak through a hole or pipe Two phase leak through a hole or pipe Vapor phase discharge by rupture Liquid phase discharge by rupture In-Building discharge
Interactive flash calculation Vaporization
Dispersion Modeling
Richardson Number calculation Light gas Dispersion (Gaussian)
Dense gas Dispersion (Pasquill-Gifford, Slab) Surface Roughness/Curvature effect
Building effect Rain/Snow effect
Effect Modeling
Pool fire Physical explosion
BLEVE VCE & UVCE
Toxicity
RA Calculation, Reporting Information System
Chemical Prop. DB
Equip. Maintenance DB Meteorological DB
Operation 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 person
Operator, Director, Manager...
Process Analysis Process 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 Direction Mixing 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
Elev atio n
Wind Speed
Atmospheric Stability
Height above ground Height above ground
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 Model
Radiation Heat Transfer Model Ignition Model
Unsteady-State Model Explosion Model
Detonation Model Deflagration Model TNT Explosion Model Multi-Energy Model Toxic Model
Probit Analysis
Threshold Limit Value ED, TD, LD, LC
Selected Models
Fire Model
Radiation Heat Transfer Model
Explosion Model
TNT Explosion Model Toxic Model
Probit Analysis
