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

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Wind Speed

Atmospheric StabilityH

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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