Lecture 12
Safety Practices in Chemical and
Nuclear Industries
Hazard Evaluation
Dr. Raghuram Chetty
Department of Chemical Engineering
Indian Institute of Technology Madras
Chennai- 600 036.
Introduction
A hazard is defined to be a characteristic of a system,
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Petrochemic
al Plant
process and plant, that represent a potential for an
accident.
An accident is defined as an unplanned event or
sequence
of
events
that
has
undesirable
consequences.
The consequence could be a fire or explosion or
release of toxic material that could lead to health
effects, economic losses and public relation losses.
Effective hazard control requires analysis that is
systematic, comprehensive, and sufficiently easy to
carry out that it may be done by engineers associated
with design and operation of a plant.
These procedures should amongst other things be used
to identify accidents that could occur infrequently but
result in serious injury or loss when they do.
Introduction
Process / System
Hazard Evaluation
- Safety Review/Audit/Inspection
- Process/ System Checklists
Introduction
- Dow & Mond Indices
- Hazard Analysis (HAZAN)
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- HAZOP
Risk Assessment
- What can go wrong & how?
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- What are the chances?
Petrochemical - Consequences?
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Plant
Extremes
- Low probability
- Minimal consequences
Hazard identification
& evaluation
Scenario
identification
Accident
probability
Accident
consequences
Risk determination
risk &
hazard
acceptable
?
Y
Accept system
Modify design
Safety Layers of Protection
Focus on hazard
identification and
control
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Hazard still exist
Reduce the risk
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Petrochemical
Plant
Layers of protection
increase the complexity of
the process, and hence the
capital and operating cost.
Introduction
Centre for Chemical Process Safety (CCPS) of the
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American Institute of Chemical Engineers under
Guidelines for Hazard Evaluation Procedures
considers the following as most favor evaluation
procedures:
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Petrochemical
Plant
1.
Safety Review Methods
2.
Process/ System Checklists
3.
Relative Ranking Techniques (Dow & Mond
Indices)
Introduction
3. Preliminary Hazard Analysis (PHA)
4. What if Analysis
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5. Hazard and Operability studies (HAZOP)
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6. Failure Modes Effects Analysis (FMEA)
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7. Fault Tree Analysis (FTA)
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8. Event Tree Analysis (ETA)
Petrochemical
Plant
9. Cause-Consequence Analysis (CCA)
10. Human Reliability Analysis (HRA)
Qualitative methods
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The DOW and MOND methods provide a quick
and simple way of estimating risks in process
plants.
The procedure employed assigns penalties for
those processes or operations that can
contribute to an accident and assigns credits to
the safety features of the plant that can mitigate
the effects of an accident.
The penalties and credits are combined into an
index that indicates the relative ranking of the
plant risk.
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Petrochemical
Plant
Qualitative methods
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Preliminary Hazard Analysis (PHA) is a general,
qualitative study that yields a rough assessment of
the potential hazards and means of their
rectification within a system.
It is called preliminary because it is usually
refined through additional studies. PHA
contains a brief description of potential hazards
in system development, operation or disposal.
This method focuses on special attention on
sources of energy for the system and on
hazardous materials that might adversely affect
the system or environment.
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Application
Petrochemical
Plant
Qualitative methods
HAZOP (HAzard and OPerability studies) is a
procedural tool designed to highlight the deficiency
and shortcomings in the design and operation of
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industrial plants.
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Benefits: The HAZOP technique is a powerful
tool for hazard analysis. Its methodical approach
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ensures that deviations from design intent are
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detected and acted upon.
Petrochemical
Plant
Limitations: HAZOP utilises a team approach
and hence can not be conducted by a single
analyst. The team should be skilled and
multidisciplinary with good knowledge of the plant,
its intended design and operation.
Qualitative methods
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Failure Modes and Effects Analysis (FMEA) is
methodology for analyzing potential reliability
problems early in the development cycle where it
is easier to take actions to overcome these issues,
thereby enhancing reliability through design.
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Petrochemical
Plant
FMEA is a useful qualitative tool for failure
analysis and identification and can be used
extensively with other hazard identification
techniques such as HAZOP and fault tree
analysis.
Quantitative methods
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Petrochemical
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Fault Tree Analysis (FTA) is a reliability and logicbased methodology. It is used for identifying and
analysing the events that could lead to an accident
or an undesirable event.
Benefits and Limitations: FTA is a very useful
tool for studying the routes by which a hazard
can occur, although its implementation requires
skilled analysts. Furthermore, the quantification
of the fault tree depends on the accuracy of the
failure data and its availability.
Event Tree Analysis (ETA) is one of the logic tree
methods for hazard identification. Unlike fault tree
analysis, event tree analysis is a "forward thinking"
process.
Qualitative vs. Quantitative
Qualitative
Quantitative
The aim is a complete, detailed
description.
The aim is to classify features, count
them, and construct statistical models in
an attempt to explain what is observed.
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Researcher may only know roughly in
advance what he/she is looking for.
Researcher knows clearly in advance
what he/she is looking for.
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Recommended during earlier phases
of research projects.
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Application
Petrochemical
Plant
The design emerges as the study
unfolds.
Researcher is the data gathering
instrument.
Data is in the form of words, pictures
or objects.
Recommended during latter phases of
research projects.
All aspects of the study are carefully
designed before data is collected.
Researcher uses tools, such as
questionnaires or equipment to collect
numerical data.
Data is in the form of numbers and
statistics.
Hazard Evaluation
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Summary
The Hazard Evaluation (HE) techniques can be
used to identify different types of hazards within
the system components and to propose possible
solutions to eliminate the hazards. These
procedures are extremely useful in identifying
system modes and failures that can contribute to
the occurrence of accidents; they should be an
integral part of different phases of process
development
from
conceptual
design
to
installation, operation and maintenance.
The HE techniques vary in sophistication and
scope, and no single one will always be the best.
Hazard Evaluation
Qualitative methods (finds scenario)
These methods help a multi-disciplinary team
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Summary
(1) identify potential accident scenarios and (2)
evaluate the scenario in sufficient detail to
make a reasonable judgment of risk.
If
the team is not satisfied on the risk
assessment, a scenario identified in a
qualitative hazard review may be further
analyzed using one or more of the quantitative
techniques.
Hazard Evaluation
Quantitative methods (can help to decide, how
best to lower risk for selected scenario)
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Summary
These
do not identify possible accident
scenarios, but they instead aid in risk judgment
by providing more detailed, statistical
evaluations of the risk of a specific scenario.
Hazard Evaluation
Different techniques are used at different stages in
the development, design, construction, operation
and decommissioning of a process.
It goes without saying that the outcome of a hazard
evaluation depends on the experience, knowledge
and intelligence of the team conducting it.
The procedures, of themselves, only provide a
logical framework that may be used to uncover
hidden system failure modes and indicate how they
can be rectified.
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Plant
Hazard Evaluation
The team must have a clear understanding of a
system being considered, including its intended
function, interactions with personnel, other
components and the environment, sources of
energy, and materials used and produced.
Information
regarding
operating
conditions,
materials used, processed (toxic, flammable,
explosive) moving parts, electrical aspects should
be know to the team.
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Preparation,
modelling,
documentation.
evaluation
and
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R&D
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Conceptual Design
Procedure
Pilot Plant Operation
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Detailed Engineering
PetrochemicalConstruction/Start-up
Plant
Routine Operation
Expansion or Modification
Incident Investigation
Decommissioning
HRA
CCA
ETA
FTA
FMEA
HAZOP
What-If
PHA
Relative Ranking
Checklist
Safety Review
Typical Uses for HE Techniques
Add-on Safety
Knowledge of process
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Opportunities for installing
add-on safety features
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Petrochemical
Plant
Operation
Start up
Constrution
Detailed eng.
P ID
Flowsheet
Conceptual
Application
Research
Procedure
Opportunities for installing
inherently safer features
As a process goes through the phases of lifecycle, such as
research and development, design, construction, operation,
modification, and finally decommissioning, inherent and added
on safety have a varying emphasis.
The process development and conceptual design phases give
the best opportunities of implementing inherent safety.
Add-on Safety
Knowledge of process
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Opportunities for installing
add-on safety features
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Petrochemical
Plant
Operation
Start up
Constrution
Detailed eng.
P ID
Application
Flowsheet
Research
Procedure
Conceptual
Opportunities for installing
inherently safer features
The possibility of implementing inherent safety
decreases as the design proceeds. Thus the inherent
safety characteristics should be evaluated as early as
possible to gain benefit.
Material Property Data Required for
Hazard Identification
Acute /Chronic toxicity
Inhalation
Oral
Dermal
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Carcinogenicity
Mutagenicity
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Teratogenicity
Exposure limits
TLV
PEL
STEL
IDLH
ERPG
Biodegradability
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Aquatic toxicity
Persistence in the environment
Odour threshold
Physical properties
Freezing point
Coefficients of expansion
Boiling point
Solubility
Physical properties
Vapour pressure
Density/ Specific volume
Corrosivity / erosivity
Heat capacity
Specific heat
Reactivity
Stability
Process materials
Desired /side/decomposed reactions
Kinetics
Materials of construction
Raw material impurity/contaminants
Decomposition products
Incompatible chemicals
Shock
Temperature
Light
Polymerisation
Flammability/ Explosivity
LEL/ LEF
UEL /UFL
Dust explosion parameters
Minimum ignition energy
Flash point Boiling point
Solubility
SAFETY REVIEWS
A Safety Review typically involves an inspection
often performed by a team, that is meant to identify
and evaluate plant hazards.
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The Review is usually a cooperative effort between
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Plant
plant personnel and the inspection team, and the
results usually address major risk situation rather
than routine housekeeping and morale problems.
SAFETY REVIEWS
The Safety Review is usually initiated with a
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preparatory phase in which a detailed description
of the plant and operating procedures is
assembled, together with information regarding
materials processed and stored, as well as
available records regarding accidents and injuries
that have occurred.
This may be followed by discussions with plant
engineers and operators to clarify problems and fill
in missing information.
SAFETY REVIEWS
Procedure
for periodic testing safety-related
equipment, and ensuring proper maintenance,
should receive particular attention, as should
emergency response plan.
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The second phase is to identity deficiencies and
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Petrochemical
Plant
problem areas, and to develop recommendations
for remedial action.
The final phase consisting of documenting the
results and following up with reviews to ensure that
the problems have been dealt with.
Application to a Petrochemical Plant
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Petrochemical
Plant
A petrochemical plant has been operating for 30
years and from a financial viewpoint it is desirable
that operation continues for another 15 years.
A Safety Review is initiated by management to
determine whether plant safety considerations
would allow this.
The Safety Review team assembles all the
available information regarding the original plant,
modifications in the last 30 years and plant
inspections, interviews with personnel and review
of specific safety equipment design, maintenance
and inspection procedures.
Three major deficiencies are identified.
Application to a Petrochemical Plant
Three major deficiencies were identified by Safety
Review team:
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1. Though
the plant has been modified
substantially to increase capacity and allow
more flexibility in operation, the emergency relief
systems, and systems to treat the relieved fluids
have not been re-evaluated.
The Safety Review therefore recommends
that all emergency relief and treatment
systems be re-evaluated and modifications
made as required.
Application to a Petrochemical Plant
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2. The control systems and interlocks date back to
the origins of the plant and do not meet current
standards.
The
Review therefore recommends that
modern control and interlock systems be
designed and installed.
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3.
Equipment layout and spacing do not meet currently
accepted industry standards. While some of the
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problems relate to the propagation of plant fires can
be addressed by installing heat-activated sprinkler
systems, the most serious arise from the location of
the control room- it is too exposed to fire and
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explosion.
The
Review
modifications
therefore
to
recommends
feedstock
storage
some
systems,
additional sprinkler systems in certain locations,
and construction of a new control room at an
appropriate distance from hazardous areas.
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Having said this the Review finds the plant in good
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condition from the viewpoint of maintenance,
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corrosion, and general wear and tear. It is concluded
that the plant could be operated for another 15 years
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if the deficiency noted were corrected.
PROCESS / SYSTEM CHECKLISTS
Checklists are generally used to indicate
compliance with standard procedures and identify
common hazards.
They are easy to use and can be applied at
various
stages
of
a
project
including
commissioning and operation.
It is a systematic means of communicating what is
required and controlling the development of a
project from initial design to plant shutdown.
The checklist are usually prepared from prior
experience using standard procedures, manuals
and a knowledge of the system plant.
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Checklist
Process/System Checklists
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Design
Construction
Startups
Operation
Shutdown
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Checklist
- Design
- Construction
- Startups
- Operation
- Shutdown
RELATIVE RANKING TECHNIQUES
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Relative
ranking
technique
Methods that may be used to quickly estimate
risks in process plants can be useful in identifying
process areas, conditions, and materials, that
contribute most to the overall hazards of a facility.
Alternative can be rapidly explored and the impact
of remedial actions evaluated.
RELATIVE RANKING TECHNIQUES
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Relative
ranking
technique
Because process areas can be ranked in terms of
the hazards they present, methods which allow
this to be done without a detailed risk analysis are
called Relative Ranking Techniques.
1.
Dow Fire and Explosion Index
2.
Mond Index
3.
Substance Hazard Index
4.
Material Hazard Index
5.
Chemical Exposure Index
6.
Threshold Planning Index
Relative Ranking Techniques
Dow Fire and Explosion Index
Considers factors involving material properties,
process conditions, operating characteristics,
safety and fire protection systems and other
aspects, in arriving at an index, for each process
unit, that characterizes the fire and explosion risk,
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Relative
ranking
technique
Mond Index
An extension of the DOW F&EI to enable a wider
range of processes and properties, as well as
aspects of toxicity to be covered.
Substance Hazard Index
This index addresses the risks associated with
toxic vapour releases.
Relative Ranking Techniques
Material Hazard Index
An index based on material vapour pressure and a
level of concern related to toxicity, flammability,
explosivity etc., used to determine threshold
quantities of materials above which a risk
management program is required.
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Relative
ranking
technique
Chemical Exposure Index
An index, developed by Dow Chemicals, that
defines the risk of material releases, based on
toxicity, volatility, molecular weight, various
process parameters and distance to area of
concern.
Dow Fire and Explosion Index (FEI)
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FEI is a leading hazard index
recognized by the chemical industry.
The Dow FEI is a ranking system that gives a
relative index to the risk of individual process units
due to potential fires and explosions.
It serves as a guide for the selection of fire and
explosion protection methods.
Relative
ranking
technique
Fire and
Explosion
Index
methodology
Dow Fire and Explosion Index (FEI)
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It assists in determining the spacing between
adjacent unit process.
It is a guide for insurance agencies to set insurance
rates.
It ranks individual process units where special safety
attention can be focused.
Relative
ranking
technique
Fire and
Explosion
Index
What Does the FEI Consider?
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Relative
ranking
technique
Fire and
Explosion
Index
1.
Six general process hazards.
2.
Twelve special process hazards.
3.
Nine process control credit factors.
4.
Four material isolation credit factors.
5.
Nine fire protection credit factors.
1. General process hazards
Exothermic reactions.
Endothermic processes.
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Material handling and transfer.
Relative
ranking
technique
Enclosed or indoor process units.
Access.
Drainage and spill control.
Fire and
Explosion
Index
2. Special process hazards
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Relative
ranking
technique
Fire and
Explosion
Index
Toxic materials.
Sub-atmospheric pressure (<500 mmHg).
Operating in or near flammable range.
Dust explosion.
Pressure.
Low temperature.
Quantity of flammable/unstable material.
Liquids or gases in process.
Liquids or gases in storage.
Combustible solids in storage.
2. Special process hazards
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Relative
ranking
technique
Fire and
Explosion
Index
Corrosion and erosion.
Leakage joints and packing.
Use of fired equipment.
Hot oil heat exchanger system.
Rotating equipment.
3. Process Control Credit Factors
Emergency power.
Cooling.
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Explosion control.
Relative
ranking
technique
Emergency shutdown.
Computer control.
Inert gas.
Operating instruction procedures.
Reactive chemical review.
Process hazard analysis.
Fire and
Explosion
Index
4. Material Isolation Credit Factors
Remote control valves.
Dump or blow down control.
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Drainage.
Relative
ranking
technique
Interlocks.
Fire and
Explosion
Index
5. Fire Protection Credit Factors
Leak detection.
Structural steel .
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Fire water supply.
Relative
ranking
technique
Special systems.
Sprinkler systems.
Water curtains.
Foam.
Hand extinguishers.
Cable protection.
Fire and
Explosion
Index
What-If Analysis
What-If analysis identifies: hazards, possible
accidents,
qualitatively
evaluates
the
consequences and determines the adequacy of
safety levels.
Systematic, but loosely structured, assessment:
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Relative
ranking
technique
Fire and
Explosion
Index
team of experts brainstorming
What-If
Analysis
generate a comprehensive review
typically performed by one or more teams with
diverse backgrounds and experience
Applicable to any activity or system
What-If Analysis
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Relative
ranking
technique
Fire and
Explosion
Index
What-If
Analysis
generates qualitative descriptions of potential
problems
in the form of questions and responses
lists of recommendations for preventing
problems
What-If Analysis
Example of What-If analysis: Line/Vessel of hydrogen fluoride
supply system
What If
Consequences
Safety Level
Scenario
Comments
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Relative
ranking
technique
Fire and
Explosion
Index
the pressure
relief valve
fails ?
Possible rupture
of HF cylinder with
personnel
exposure to HF
and blast effect,
possible fatalities.
None
Add pressure
alarm on
operator
console
Review training
records to make
sure all staff
have been
trained in
current
procedures.
What-If
Analysis
the operator
does not valve
off the empty
cylinder before
removing it?
HF release with
personnel
exposure,
possible fatalities.
None
Summary
Hazard Evaluation Techniques
Quantitative
Qualitative
Safety Review
Process / System Checklists
Dow Fire and Explosion Index
What-If Analysis.
