Definitions

1
Definitions Accident a specific unplanned event
or sequence of events that has a specific
undesirable consequence consequence.
Consequences the results of an accident event
sequence. It is originally considered to be the
fire, explosion, and release of toxic material
that results from the accident, but not the
health effects, economic loss, etc., which is the
ultimate result. Battelle, Guidelines for
Hazard Evaluation Procedures, AIChE, New York
(1985).
2
EXAMPLES

• Feyzin, France, 1966
• Fixborough, England, 1974
• Bhopal, India, 1984

3
TABLE 1-1. ELEMENTS OF ACCIDENTS
Hazards
Initiating Event/Upsets
Intermediate Events
Accident

(System or Operator
Responses to Upsets) Consequences


Propagating
Ameliorative Significant
Machinery and Equipment
Process Parameter Safety
System Inventories of
Malfunctions
Deviations Responses
a) Flammable Materials a) Pumps,
Valves a) Pressure
a) Relief Valves b)
Combustible Materials b) Instruments,
Sensors b) Temperature
b) Back-up Utilities Fires c)
Unstable Materials
c) Flow
Rate c) Back-up
Components d) Toxic Materials

d) Concentration d)
Back-up Systems Explosions e) Extremely
Hot or
e) Phase/State
Change Cold Materials



Impacts f) Inerting Gases (Methane, Carbon
Monoxide) Highly Reactive
Containment Failures Containment
Failures Mitigation System
Dispersion of


Responses
Toxic a) Reagents
a) Pipes
a) Pipes a) Vents
Materials b) Products
b) Vessels
b) Vessels
b) Dikes c) Intermediate
c) Storage Tanks
c) Storage Tanks
c) Flares Dispersion
of Products
d) Gaskets
d) Gaskets, Bellows, etc. d) Sprinklers
Highly d) By-products

e) Input/output or

Reactive

venting

Materials
4
Hazards
Initiating Event/Upsets
Intermediate Events
Accident

(System or
Operator Responses to Upsets)
Consequences

Propagating
Ameliorative Reaction Rates


Control Responses Especially
Sensitive to Human Errors
Material Releases
Operator Responses a) Impurities
a) Operations
a) Combustibles
a) Planned b) Process Parameters
b) Maintenance
b) Explosive Materials b) Ad Hoc
c)
Testing c)
Toxic Materials

d) Reactive Materials

Loss of Utilities
Ignition/Explosion Contingency
Operations
a) Electricity
Operator Errors
a) Alarms
b) Water

b) Emergency
Procedures
c) Air
a) Omission
c) Personnel Safety
d) Steam
b) Commission
d) Evacuations

c)
Diagnosis/Decision-Making e) Security

External Events External
Events External Events

a) Floods
a) Delayed Warning a) Early
Detection
b) Earthquakes
b) Unwarned b)
Early Warning
c) Electrical Storms
d) High
Winds
e) High Velocity Impacts
f) Vandalism

Method/Information Errors
Method/Information Failure Information Flow

a) As Designed a)
Amount a)
Routing
b) As Communicated
b) Usefulness b)
Methods

c) Timeliness
c) Timing
5
DEFINITIONS
Incident The loss of containment of material or
energy (e. g., a leak of 10 1b/sec of ammonia
from a connecting pipeline to the ammonia tank,
producing a toxic vapor cloud). Incident
Outcome The physical manifestation of the
incident for toxic materials, the incident
outcome is a toxic release, while for flammable
materials, the incident outcome could be a BLEVE
(Boiling Liquid Expanding Vapor Explosion), flash
fire, unconfined vapor could explosion, etc. (e.
g., for a 10 1b/sec leak of ammonia, the incident
outcome is a toxic release). Incident Outcome
Case 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 outcome e. g., a concentration of
3333 ppm (v) of ammonia 2000 ft downwind from a
10 1b/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 1b/sec leak under Stability Class
D weather condition, and 1.4 mph wind traveling
in a northerly direction will injure 50
people). CCPS, Guidelines for Chemical Process
Quantitative Risk Analysis, AIChE, New York (1989)
6
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
E

o


o


o


etc.
Jet
Fire


Tank Full
BLEVE of
Tank 50
Full
HCN Tank
o


o


o


etc.


After 15 min. Release

Unconfined Vapor
After 30 min. Release

Cloud Explosion
After 60 min. Release


o

o


o


etc. The relationship
between incidents, incident outcomes, and
incident outcome cases for a hydrogen cyanide
(HCN) release.
100 1b/min Release of HCN from a Tank Vent
7
Definitions Hazard a physical situation
with a potential for human injury, damage to
property, damage to environment or some
combination of these. (IChem E) a
characteristic of the system/plant/process that
represents a potential for an accident.
(AIChE) Risk the likelihood of a
specified undesirable event occurring within a
specified period or in specified circumstances.
(IChem E) a measure of potential economic
loss or human injury in terms of the probability
of the loss or injury occurring and the
magnitude of the loss or injury if it occurs.
(AIChE)
8
TYPICAL HAZARDS
9
TASKS OF HAZARD ASSESSMENT

1. Identification of undesired events.
2. Analysis of the mechanisms by which undesired
   events could occur.
3. Consideration of the extent of any harmful
   effects.
4. Consideration of the likelihood of the undesired
   events and the likelihood of specific detrimental
   outcomes. Likelihood may be expressed as
   probability or frequency.
5. Judgements about the significance of the
   identified hazards and estimated risks.
6. Making and implementing decisions or courses of
   action, including ways of reducing the likelihood
   or consequences of undesired events.

10
Risk Assessment Procedure
System Description
Hazard Identification
Accident Probabilities Estimation
Accident Consequences Estimation
Risk Determination
No
Risk Acceptance
Modify System
Yes
Operate System
11
hazard, risk, safety analysis,
assessmentevalution ?
Hazard identification (1) (2) Hazard
Analysis (1) (2) (3) (4)

???
qualitative Risk Analysis (1)
(2) (3) (4)
???

quantitative (Hazard Assessment) or (Hazard
Evaluation) (1) (2) (3)
(4) (5) (6)
???
qualitative Risk Assessment (1)
(2) (3) (4) (5) (6)

???
quantitative
12
Hazard Identification and Assessment
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 them.
13
Results of Hazard Identification and Assessment

• Identification and description of hazards which
  could lead to undesirable consequences.
• Identification and description of the mechanisms
  leading to the hazardous event, i. e. accident
  event sequence.
• A qualitative estimate of the likelihood and/or
  consequence of each accident event sequence.
• A quantitative estimate of risk, which can be
  compared with acceptable risk to determine
  whether or not expenditure on particular safety
  measure is justified.
• A relative ranking of the risk of each hazard and
  accident event sequence.
• Some suggested approaches to risk reduction.

14
Possible Actions to Reduce Risk

• A change in the physical design and control
  system.
• A change in the operating procedure.
• A change in process configuration or conditions.
• A change in the process material.
• A change in the testing, inspection/calibration
  and maintenance procedure of key safety items.

15
Classification of Risk Reduction Measures

1. Those actions which eliminate hazard
   (substitution)
2. Those actions which reduce the likelihood of its
   occurrence to an acceptable level. (attenuation)
3. Those actions which eliminate or reduce its
   consequence. (second chance)

16
Example Consider a reaction vessel where, in a
HAZOP session, it was discovered that if a
certain impurity were introduced with one of the
raw materials, there would be a sudden evolution
of gas and an increase in pressure.

• Solution
• Eliminating the possibility of gas evolution by
  changing the raw material responsible for the
  problem. (substitution)
• Lowering the possibility of gas evolution by
  altering one of the process condition.
  (attenuation)
• Fitting an appropriate pressure relief valve and
  vent system to protect the plant. (second chance)

17
MATRIX RELATING HAZARD EVALUATION PROCEDURES TO
HAZARD EVALUATION PROCESS STEPS (upper left)
18
MATRIX RELATING HAZARD EVALUATION PROCEDURES TO
HAZARD EVALUATION PROCESS STEPS (lower left)
19
MATRIX RELATING HAZARD EVALUATION PROCEDURES TO
HAZARD EVALUATION PROCESS STEPS (upper right)
20
MATRIX RELATING HAZARD EVALUATION PROCEDURES TO
HAZARD EVALUATION PROCESS STEPS (lower right)
21
What is acceptable risk?
22
Acceptable RiskMost treatment of acceptable
risk deal primarily with the riskof death. This
may appear somewhat arbitrary. But there is
justification for this approach

• Data on fatalities are most possibly recorded and
  are relatively straightforward.
• (number of fatalities) ? (number of other
  injuries)
• measures which reduce death from a particular
  hazard tend to reduce injuries as well.

23
Computation of Risk
where, fi the rate at which the event occurs
(event/year) xi number of fatalities per
event i (death/event) Ni number of peoples
exposed to event i (number of exposed
peoples/event) Pi the probability of
fatalities among the exposed people
(death/exposed people) N total number of
peoples at risk
24
Fatal Accident Frequency Rate (FAFR)
Based on the total working hours of 1000
employees (2000 hr/year and 50 year/person).
25
Table 9.2 Fatal Accident Rates in different
industries and jobs in the U.K.

Fatal Accident Rate (FAR)

(deaths/108
exposed hours) Clothing and footwear
industry
0.15 Vehicle industry
1.3
Chemical industry
3.5(a)
British industry
4
Steel industry
8
Agricultural work
10 Fishing

35 Coal mining

40(b) Railway shunting

45 Construction work
67
Air crew
250
Professional boxers
7000 Jockeys
(flat racing)
50000 (a). This value of
the FAR for the chemical industry predates
Flixborough. If the Flixborough
fatalities are averaged over 10 years the value
becomes 5. (b). This value is
now appreciably less. Sources
Sowby (1964), Pochin (1975), Kletz (1971,1976d)
26
Table 9.3 Fatal Accident Rates for the chemical
industry in different contries

Fatal Accident Rate (FAR)

(deaths/108
exposed hours) France

8.5 West Germany

5 United Kingdom (before
Flixborough) 4
(including
Flixborough) 5
United States
5
Sources Sowby (1964), Pochin (1975), Kletz
(1971,1976d)
27
Table 9.4 Fatal Accident Rates for some
non-industrial activities

Fatal Accident Rate (FAR)

(deaths/108 exposed
hours) Staying at home
3
Travelling by bus

3 by train

5 by car

57 by bicycle

96 by air

240 by moped
260
by motor scooter
310
by motor cycle
660 Canoeing

1000 Rock climbing

4000 Sources Sowby (1964), Pochin
(1975), Kletz (1971,1976d)
28
Maximum Risk to Employees
(Kletz, 1986)


(U.S.)
29
Fatality Rate per Person per Year
30
Table 9.5 Death rates for some voluntary and
involuntary risks (after Kletz, 1976d)

Fatality rate Reference

(deaths per person

per year) Voluntary risk Taking contraceptive
pill 2 ? 10-5
Gibson (1976c) Playing
football
4 ? 10-5 Pochin
(1975) Rock climbing
4 ? 10-5
Pochin (1975) Car driving
17 ? 10-5
Roach (1970) Smoking (20
cigarettes/day) 500 ? 10-5
Pochin
(1975) Involuntary risk Meteorite
6 ? 10-11
Wall (1976) Transport of
petrol and chemicals (U.K.) 0.2 ? 10-7
Aircraft crash (U.K.)
0.2 ? 10-7
Gibson (1976c) Explosion of
pressure vessel (U.S.A.) 0.5 ? 10-7
Wall (1976) Lightning (U.K.)
1 ?
10-7 Bulloch
(1974) Flooding of dikes (Netherlands)
1 ? 10-7
Turkenburg (1974) Release from nuclear power
station (at 1 km) (U.K.)
1 ? 10-7
Fire (U.K.)
150 ? 10-7
Melinek (BRE 1974 CP 88/74) Run over by
road vehicle 600 ?
10-7 Leukemia
800 ?
10-7 Gibson (1976c)
31
Acceptable Risk to Public Voluntary

10-5/person/year Involuntarily
Natural Disaster
10-5/person/year
Man-made
10-7/person/year Maximum Risk to
Public (Kletz) Averaged over the whole population
(average risk)
10-7/person/year For anyone in
public (individual risk)
10-5 to 10-6/person/year
32
OSHA Incidence Rate

• OSHA Incidence Rate (injury or sickness)
• OSHA Incidence Rate (loss of work days)
• Based on the total working hours of 100 employees
  in 1 year (2000 hr/year).

(total number of injuries/sicknesses )
(total working hours of all employees) 200,000
(total number of lost work days)
(total working hours of all employees) 200,000