1.1 Introduction(1)

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Techniques for AssessingIndustrial Hazards:

An Introduction

Dr. Samir I Abu-Eishah UAE University

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1- Techniques for Assessing Industrial

Hazards

1.1 Introduction

1.2 Definitions

1.3 Tasks of Hazard Assessment

1.4 Risk Reduction1.5 Computation of Risk

1.6 Acceptable and Maximum Risks

4- Consequence Calculations

4.1 Outflow Calculations

4.2 Behaviour Immediately after Release

4.3 Dispersion in the Atmosphere

4.4 Fires

4.5 Explosions4.6 Effects of Toxic Releases

2- The Structure of Hazard Analysis

2.1 Background

2.2 Description of the Hazard Analysis

Steps

5- Summary of Consequences

5.1 Results for Different Weather

Conditions

5.2 Ordering and Presenting The Results

3- Failure and Release Cases

3.1 Failure Cases

3.2 Release Cases

3.3 Properties of Released Materials

3.4 Consequences of a Release

6- Hazard Reduction

6.1 Reduction of Consequences

6.2 Reduction of Risk

6.3 Reduction of Impacts

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Course Contents:


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1.1 Introduction

The chemical and energy industries use a wide variety of manufacturing,storage and control processes. These processes involve many different

types of material that can be potentially harmful if released into the

environment, because of their toxic, flammable or explosive properties.

These materials are not usually kept at atmospheric pressure and

temperature: modern chemical and energy industries involve highpressures and temperatures.

Gases are frequently liquefied by refrigeration to facilitate storage in bulk

quantities.

It is essential to achieve and maintain high standards of plant integritythrough good design, management and operational control.

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Introduction ...

Controls and safeguards developed by industry are supposed to beentirely effective.

Accidents do occur and can cause serious injury to employees or the

public, and damage to property. Therefore, it is essential to identify

potential hazards.

The necessary steps can then be taken to reduce the hazard (by design) or

the risk (by high operating standards, safety devices, etc.)

To conduct a hazard analysis it is important to follow a structured

approach.

In addition, the calculation methods used should be straightforward and

reliable.

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Introduction ...

In the initial stages of a hazard analysis it is appropriate toapply simplified techniques in order to identify the most

serious potential hazards. More sophisticated techniques can

then be used to assess methods of reducing the hazards.

The techniques for hazard analysis should be used inconjunction with other methods of safety assessment, as

appropriate for the plant being analyzed.

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Introduction ...

Some methods of hazard analysis are Hazard and Operability Studies (HAZOP),

Fault Tree Analysis,

Event Tree Analysis,

Failure Modes and Criticality Effect Analysis (FMECA).

The analyst might also use other methods of

identifying and ranking potential hazards, such as the

Dow Index or the ICI Mond Index.

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1.2 Definitions

Risk:

The probability of injury, disease or death underspecific circumstance (EPA)

The likelihood of a specified undesirable eventoccurring within a specified period or in specifiedcircumstances. (IChem E)

A measure of potential economic loss or humaninjury in terms ofthe probability of the loss orinjury occurring and the magnitude of the loss orinjury if it occurs. (AIChE)


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Definitions

Risk perception what people believe poses a risk or

hazard

Risk assessment quantifying the risk associatedwith a hazard

Risk management evaluating whether real or

perceived risks are acceptable, and if not, addressingthese risks.


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Definitions

Accident a specific unplanned event or sequence ofevents that has a specific undesirable consequence. Theelements of accidents are shown in Table 1.

Consequence the results of an accident event sequence.

Here, it is considered to be the fire, explosion, and releaseof toxic material that results from the accident, but not thehealth effects, economic loss, etc., which is the ultimateresult.

Health a state of complete physical, mental and social

well-being, not merely the absence of disease or infirmity(WHO)

Source: Battelle, Guidelines for Hazard Evaluation Procedures, AIChE, NewYork(1985).

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Hazard Initiating

Event/Upsets

Intermediate Events

(System or Operator Responses to Upsets)

Propagating Ameliorative

Accident

ConsequencesSignificant Inventories

of

a) Flammable

Materials

b) Combustible

Materials

c) Unstable Materials

d) Toxic Materials

e) Extremely Hot or

Cold Materials

f) Inerting Gases

(CH4, CO)

Machinery and

Equipment

Malfunctions

a) Pumps, Valves

b) Instruments,

Sensors

Process Parameter

Deviations

a) Pressure

b) Temperature

c) Flow Rate

d) Concentration

e) Phase/State Change

Safety System

Responses

a) Relief Valves

b) Back-up Utilities

c) Back-up

Components

d) Back-up Systems

Fires

Explosions

Impacts

Highly Reactive Containment Failures Containment Failures Mitigation System

Responses

a) Reagents

b) Products

c) Intermediate

Products

d) By-products

a) Pipes

b) Vessels

c) Storage Tanks

d) Gaskets

a) Pipes

b) Vessels

c) Storage Tanks

d) Gaskets, Bellows, etc.

e) Input/output or

venting

a) Vents

b) Dikes

c) Flares

d) Sprinklers

Dispersion of

Toxic Materials

Dispersion of

Highly Reactive

Materials

Table 1. Elements of Accidents

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Hazard Initiating Event/Upsets

Intermediate Events

(System or Operator Responses to Upsets)

Propagating Ameliorative

Accident

Consequences

Reaction Rates Especially

Sensitive toa) Impurities

b) Process Parameters

Human Errors

a) Operationsb) Maintenance

c) Testing

Material Releases

a) Combustiblesb) Explosive Materials

c) Toxic Materials

d) Reactive Materials

Control Responses

Operator Responsesa) Planned

b) Ad Hoc

Loss of Utilities

a) Electricity

b) Water

c) Aird) Steam

Ignition/Explosion

Operator Errors

a) Omission

b) Commissionc) Diagnosis/Decision

-Making

Contingency

Operations

a) Alarms

b) EmergencyProcedures

c) Personnel Safety

d) Evacuations

e) Security

External Events

a) Floods

b) Earthquakesc) Electrical Storms

d) High Winds

e) High Velocity Impacts

f) Vandalism

External Events

a) Delayed Warning

b) Unwarned

External Events

a) Early Detection

b) Early Warning

Method/Information

Errors

a) As Designed

b) As Communicated

Method/Information

Failure

a) Amount

b) Usefulness

c Timeliness

Information Flow

a) Routing

b) Methods

c) Timing

Table 1. Elements ofAccidents


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Definitions

Hazard:

The agent or means by which an adverse effect canoccur in a particular situation

A physical situation with a potential for human injury,damage to property, damage to environment or somecombination of these. (IChem E)

A characteristic of the system/plant/process thatrepresents a potential for an accident. (AIChE)

Typical Hazards are shown in Table 2.

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Table 2: Typical Hazards

Significant Inventories of: Extreme Physical Conditions: Flammable materials Combustible materials

Unstable materials

Corrosive materials

Asphyxiants

Shock sensitive materials

Highly reactive materials

Toxic materials

Inerting gases

Combustible dusts Pyrophoric materials

High temperatures Cryogenic temperatures

High pressures

Vacuum

Pressure cycling

Temperature cycling

Vibration/liquid

hammering

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Definitions

Hazard Identification:

The techniques for finding out what hazards are

present in a plant or process.

Hazard Assessment:

The techniques for deciding how far we ought to go

in removing the hazards or protecting people from

the hazards.

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Definitions

Incident the loss of containment of material or energy(e. g., a leak of 10 lb/sec of ammonia from a connectingpipeline to the ammonia tank, producing a toxic vaporcloud).

Incident Outcome - the physical manifestation of theincident;

for toxic materials: a toxic release (e. g., a 10 lb/sec leak of

ammonia).

for flammable materials: could be a BLEVE (Boiling Liquid

Expanding Vapor Explosion), flash fire, unconfined vapor

could explosion, etc.

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Definitions

Incident Outcome Case - the quantitative definition of a singleresult of an incident outcome through specification of sufficient

parameters to allow distinction of this case from all others for the

same incident outcome [e. g., a concentration of 3333 ppm (v) of

ammonia 2000 ft downwind from a 10 lb/sec ammonia leak is

estimated assuming a 1.4 mph wind, and Stability Class D].

Consequence a measure of the expected effects of an incident

outcome case [e. g., an ammonia cloud from a 10 lb/sec leak under

Stability Class D weather condition, and 1.4 mph wind traveling in a

northerly direction] will injure, say, 50 people.

Source: CCPS, Guidelines for Chemical Process Quantitative Risk Analysis,

AIChE, New York (1989)

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Relationship between Incidents, Incident Outcomes, and Incident

Outcome Cases for a hydrogen cyanide (HCN) release.

INCIDENTS INCIDENT OUTCOMES INCIDENT OUTCOME CASES

5 mph Wind, Stability Class A

Toxic Vapor 10 mph Wind, Stability Class D

Atmospheric Dispersion 15 mph Wind, Stability Class Eoooetc.

Jet Fire

Tank Full

BLEVE of Tank 50% Full

HCN Tank oooetc.

After 15 min. Release

Unconfined Vapor After 30 min. Release

Cloud Explosion After 60 min. Release

o

ooetc.

100 lb/minRelease ofHCN froma Tank Vent

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1.3 Tasks of Hazard Assessment

a) Identification and description of hazards (undesired events) that could

lead to undesirable consequences.

b) Identification of the mechanisms that could lead to the hazardous event,

i.e. accident event sequence.

c) A qualitative estimate of the likelihood and/or consequence of eachaccident event sequence, i.e. estimate the extent of any harmful effects.

d) A quantitative estimate of risk, which can be compared with acceptable

risk to determine whether or not expenditure on particular safety

measure is justified.

e) Judgments about the significance of the identified hazards and theestimated risks, i.e. making a relative ranking of the risk of each hazard

and accident event sequence.

f) Making and implementing decisions or courses of action, including ways

of reducing the likelihood or consequences of undesired events.

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hazard, risk, safety

+ analysis, assessment, evaluation

= ?

Hazard Identification = (a) + (b)

Hazard Analysis = (a) + (b) + (c) + (d)

qualitative

Risk Analysis = (a) + (b) + (c) + (d)

quantitative

(Hazard Assessment orEvaluation)= (a) + (b) + (c) + (d) + (e) + (f)

qualitative

Risk Assessment = (a) + (b) + (c) + (d) + (e) + (f)

quantitative 19


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Figure 1: The Risk Assessment Procedure

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SystemDescription

HazardIdentification

AccidentProbabilitiesEstimation

RiskDetermination

RiskAcceptance ModifySystem

OperateSystem

AccidentConsequencesEstimation

No

Yes


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1.4 Risk Reduction

Possible Actions to Reduce Risk area change in the physical design and control system.

a change in the operating procedure.

a change in the process configuration or conditions.

a change in the process material.

a change in the testing, inspection/calibration and

maintenance procedure of the key safety items.

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Risk Reduction

The Risk Reduction Measures are classified to:

1. Substitution: those actions which eliminate the

hazard

2. Attenuation: those actions which reduce the

likelihood of its occurrence to an acceptable level.

3. Second Chance: those actions which eliminate or

reduce the hazard consequence.

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Risk Reduction

Example:Consider a reaction vessel where, in a HAZOP session, it wasdiscovered that if a certain impurity were introduced with one ofthe raw materials, there would be a sudden evolution of gas andan increase in pressure.

Solution: Eliminating the possibility of gas evolution by changing the raw

material responsible for the problem. (substitution)

Eliminating the possibility of gas evolution by altering one ofthe process conditions.(attenuation)

Fitting an appropriate pressure relief valve and vent system toprotect the plant.(second chance)

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1.5 Computation of Risk

The risk is simply computed by

R = P x S

R = risk,

P = probability (expressed as frequency, # of events/time),

S = severity (e.g., deaths, injuries, toxicity, etc.)

The term risk may be used qualitatively or quantitatively.

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Computation of Risk

In general form, the risk is computed by

fi = frequency of incident or rate at which the event occurs = (event / year)

xi = number of fatalities per event i = (deaths/event)

Ni = number of peoples exposed to event i= (number of exposed peoples per

event)

Pi = probability of fatalities among the exposed people = (deaths per exposed

people)

N = total number of peoples at risk

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n

i

iii

n

i

ii fPNN

fxN

R11

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Fatal Accident Frequency Rate (FAFR)

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)hours-exposed()employee(10

fatalitiesFAFR

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(108 employee-exposed hour) is based on the total working hours of

(1000 employees) x (2000 exposed-hours/year) x (50 years/employee)


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1 worker year = 50 work weeks/year x 40 hours/weeks =

2000 hours

Based on cases per 100 worker years = 100 workers x 2000

hours/worker = 200,000 hours-worker exposure to hazard

Two types of calculation (1) based on injuries and illness (2)

based on lost workdays

OSHA (1) = number of injuries & illness x 200,000 / total

hours work by all employees during period covered

OSHA (2) = number of lost workdays x 200,000 / total hours

work by all employees during period covered

OSHA Incidence Rate


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Fatality Rate per Person (FRP)

and Annual Fatal Risk (AFR)

FRP = Annual deaths (work related) / (Total number of

employees)

= (deaths/year)/(number of employees)

AFR = (Rate of fatal accidents/hour) (Average annual

hours of work, hours/year)

= accidents/year

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Given FAR = 2. If employee works 8 hour per shift, 300 days per

year, compute the fatality rate.

Fatality rate = 8 h/day x 300 days/year x 2 deaths/108 h

= 4.8 x 10-5 death per person per year

More rock climbers are killed travelling by car than are killed

during rock climbing. Is this statement supported by statistics?

Solution: From data in next table, for travelling by car, FAR = 57,

for rock climbing, FAR = 4000. Rock climbing produces more fatalities per exposed hours but

spend more time(exposed hour) travelling by car. Think about

this.

Example


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Industry OSHA IncidenceRate

FAR(deaths/108hrs)

Chemical 0.49 4.0

Vehicle 1.08 1.3

Steel 1.54 8

Coal Mining 2.22 40

Construction 3.88 67

Agricultural 4.53 100

Activity FAR (deaths/108hrs) Fatality Rate

Travelling by car 57 17 x 10-5

Rock climbing 4000 4 x 10-5

OSHA and FAR


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1.6 Acceptable and Maximum Risks

Most treatment of acceptable risk deal primarily

with the risk of death. This may appear somewhat

arbitrary. But there is justification for this approach:

1. Data on fatalities are most possibly recorded and arerelatively straightforward.

2. (Number of fatalities) (Number of other injuries)

3. Measures which reduce death from a particular hazard

tend to reduce injuries as well.

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Risk Acceptability


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Acceptable Risk to Public

Voluntary: 10-5/person/year

Involuntarily:

Natural Disaster: 10-5/person/year

Man-made: 10-7/person/year

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Risk Acceptability


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Societal Risk Acceptability


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Maximum Risk to Public

The Maximum Risk to Public is averaged over the

whole population (Average Risk):

10-7/person/year

For anyone in public(Individual Risk):

10-5 to 10-6/person/year

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