Lecture 13
Safety Practices in Chemical and
Nuclear Industries
Hazard and Operability Studies (HAZOP)
Dr. Raghuram Chetty
Department of Chemical Engineering
Indian Institute of Technology Madras
Chennai- 600 036.
Hazard and Operability Studies

Originated as a hazard identification technique for process
plants


commonly applied in petrochemical, nuclear and food
processing industries
Described as a system of imaginative anticipation of
hazards
Hazard and Operability Studies

HAZOP

always a team activity

concentrates
components

uses well-defined guide words to steer analysis

considers both plausible
consequences of deviations.
on
deviations
in
causes
flows
and
between
possible
Hazard Evaluation Techniques
1960 - 2001
1960 - 2001
1965 - 2001
1970 - 2001
1972 - 1974
1974 - 2001
Safety
Review
Check Lists
Relative
Ranking
PHA
What if
HAZOP
Walk
Through
Inspection
Historical
Lists
ICI Mond
Index
Yes / No
Dow FEI
Preliminary
Hazard
Analysis
Brainstorming
Hazardous Mtls
Hazardous Opns
Hazards
Operability
Analysis
Line by Line
Deviation
Analysis
This presentation only considers the HAZOP technique.
HAZOP is a technique which provides opportunities for people to let their
imaginations go free and think of all possible ways in which hazards or
operating problems might arise, but to reduce the chance that something is
missed it is done in a systematic way.
HAZOP – technical approach

Before HAZOP study is started, detailed information on the
process must be available. This includes process flow diagrams
(PFD), piping and instrumentation diagrams (P&IDs), detailed
equipment specification, materials of construction, mass and
energy balances

Scope of study explicitly extends to cover unusual circumstances
such as startup, shutdown and plant maintenance

Scope of study does not include making detailed design changes,
although recommendations and follow-up questions should be
produced

HAZOP results are recorded in a tabular format

HAZOP procedures are adopted fully or partly by many
companies around the world.
HAZOP

Identifying potential hazards and operability problems
caused by deviations from the design intent of both new
and existing process plants

HAZOP studies are carried out by an experienced,
multidisciplinary team.

Review all physical aspects of a process (lines,
equipment, instrumentation) to discover potential hazards.

HAZOP will identify hazards: do not give insight into the
probability that they will occur or cause harm.
HAZOP Team Members


Leader

planning and preparation

act as chairman for meeting

ensure follow-up work is
completed
document the analysis
Designer(s), process / project
engineers


 may be site representative, operator,
maintenance crew
 supply information about the context
in which the system will be used,
e.g. site facilities
 ask questions, and help decide
which issues affect safety
Recorder


 User(s)
 Expert(s)
understand and explain the
plant design
answer questions about the
plant and process
 key function is to explore
suggest deviations / causes /
effects
 need good knowledge of process
chemistry, or experience of similar
plant
 technical specialist
HAZOP - Procedure
A HAZOP Guideword is combined with a Process
Parameter to Identify

Deviation from intended design / operation

Causes of those deviation

Consequences of those deviations

Safeguard to prevent causes and mitigate

Actions ( recommendations ) for design or operation
changes to avoid deviation
HAZOP – Procedure in Detail

Begin with a detailed flow sheet. Break the flow sheet into a
number of process units. e.g. reactor area might be one,
storage tank another. Select a unit for study

Choose a study node (vessel, line, operating instruction)

Describe the design intent of the study node. e.g. vessel V1 is designed to store benzene feedstock and provide to
the reactor on demand
HAZOP – Procedure in Detail

Pick a process parameter: flow, level, temperature, etc.

Apply a guide word to the process parameter to suggest
possible deviation

If the deviation is applicable, determine possible cause and
note any protective systems

Evaluation of consequences of the deviation (if any)

Recommend action (what? by whom? by when?)

Record all information.
Common HAZOP Analysis Terminology
Term
Definition
Sections of equipment with definite boundaries (e.g.,
a line between two vessels) within which process
Process
Sections (or parameters are investigated for deviations. The
locations on P&IDs at which the process parameters
Study
are investigated for deviations (e.g., reactor)
Nodes)
Intention
Definition of how the plant is expected to operate in
the absence of deviations. Takes a number of forms
and can be either descriptive or diagrammatic (e.g.,
process description, flow-sheets, line diagrams,
P&IDs)
Common HAZOP Analysis Terminology
Simple words that are used to qualify or quantify
Guide Words the design intention and to guide and simulate the
brainstorming process for identifying process
hazards
Process
Parameter
Physical or chemical property associated with the
process. Includes general items such as reaction,
mixing, concentration, pH, and specific items such
as temperature, pressure, phase, and flow
Common HAZOP Analysis Terminology
Deviations
Departures from the design intention that are discovered
by systematically applying the guidewords to process
parameters (flow, pressure, etc.) resulting in a list for the
team to review (no flow, high pressure, etc.) for each
process section. Teams often supplement their list of
deviations with ad hoc items
Causes
Reasons why deviations might occur. Once a deviation
has been shown to have a credible cause, it can be
treated as a meaningful deviation. These causes can be
hardware failures, unanticipated process states (e.g.,
change of composition), external disruptions (e.g., loss
of power), etc.
HAZOP - Procedure
Start
Finish
YES
NO
Select a component
All components analysed?
YES
NO
Select a flow
All flows analysed?
YES
Suggest a deviation
using a guide word
Investigate and
document causes
Investigate and
document effects
NO
All guide words considered?
Record as non-hazardous
deviation, with a
justification
NO
Record as hazard. Make
recommendations for
action if necessary
Does deviation have plausible
causes and hazardous effects?
YES
HAZOP Guidewords
No or Not
More
Less
As well as
Part of
Reverse
Other than
No part of the intention is achieved
e.g. No flow to the reactor
Quantitative increase in the intent
e.g. More flow to the reactor
Quantitative decrease in the intent
e.g. Less flow to the reactor
All intentions achieved, but with additional
effects e.g. Impurities in flow (air, water)
Only some of the intention is achieved
e.g. Part of the reactants to the reactor
Exact opposite of the intention
e.g. Reverse flow into the reactor
Complete substitution
e.g. Another material besides reactants
in the reactor
Common HAZOP Parameters
Flow
Frequency
Pressure
Viscosity
Temperature
Voltage
Level
Information
Time
Mixing
Composition
Addition
pH
Separation
Speed
Reaction
Guide word and process parameter combination
Process
parameter
No,
None
More
Less
As
well
as
Part
of
x
x
x
x
x
Temperature
x
x
Pressure
x
x
x
x
x
x
pH
x
x
Viscosity
x
x
Flow
Concentration
x
x represent valid combination
x
Reverse Other
than
x
x
x
HAZOP Example – a P&ID
Valve (normally closed during
operation of the plant)
Manually operated valve
Valve (normally open during
operation of the plant)
Non-return valve
Hydrocarbon
from storage
Drain
Pump
LC
Automation (level
controller)
LC
PG
PG
Settling tank
Drain
Transfer pumps
(one working, one spare)
To reactor
Drain
HAZOP Example – output
Guide
Word
NO
Deviation
No flow
Possible Causes
No hydrocarbon available
from storage
Consequences
Loss of feed to reactor.
Transfer pump fails (motor
fault, loss of power, impeller As above
corroded etc.)
MORE
More flow
More
pressure
More
temperature
Level control valve (LCV)
fails to open, or LCV
bypassed in error
Settling tank overfills
Action Required
1) Ensure good
communication with storage
area
2) Install low level alarm on
settling tank
Covered by 2)
3) Install high level alarm
4) Check size of overflow
5) Establish locking-off
procedure for LCV bypass
when not in use
Isolation valve or LCV
Line subjected to full
closed when pump running pump pressure
6) Install kickback on pumps
High intermediate storage
temperature
7) Install warning of high
temperature at intermediate
storage
Higher pressure in
transfer line and settling
tank
HAZOP Example – Reactor
An exothermic reaction controlled by cooling water
HAZOP study is performed on this unit to improve the safety of the
process. Using study nodes as the cooling coil (process parameter: flow
and temperature) and the stirrer (Process parameter: agitation)
Item
Study node
Process
parameters
Deviations
(guide
words)
Possible causes
Possible
consequences
Action required
1. 1A
1. Cooling
1. Flow
1. No
1. Control valve fails
1. Loss of cooling,
1. Select valve to fail open
2. Install filter with
coils
closed
2. Plugged cooling coils
3. Cooling water
service failure
4. Controller fails and
closes valve
2.
3.
4.
5.
possible
runaway
–do–do–do–do-
5. Air pressure fails,
closing valve
1. 1B
1. High
1. Control valve fails
open
2. Controller fails and
opens valve
1. 1C
1. Low
1. Partially
plugged
cooling line
2. Partial water
source failure
3. Control valve
fails to
respond
1. Reactor cools,
reactant conc.
builds, possible
runaway on
heating
2.
– do 1. Diminished
cooling, possible
runaway
2.
–do3.
–do-
maintenance procedure
Install cooling water
flow meter and low flow
alarm. Install high
temperature alarm to
alert operator
3. Check and monitor
reliability of water
service
4. Place controller on
critical instrumentation
list
5. See 1A.1
1. Instruct operators and
update procedures
2. See 1A-4
1. See 1A.2
2. See 1A.2
3. Place valve on critical
instrumentation list
1D
As well
as
1. Contamination of water
supply
1E
Part of
1.
1F
Reverse
1. Failure on water source
resulting in backflow
2. Backflow due to high
backpressure
1G
Other
than,
1.
Low
1K
Tempera
ture
1L
2A
2B
Stirre
r
Agitatio
n
1. Not possible here
1. None
1. Loss of cooling,
possible runaway
2. –do-
1. See1A.2
2. Install check calve
1
Low water supply
temperature
1. None; controller
handles
1. None
High
1. High water supply
temperature
1. Cooling system
capacity limited
temp. increases
1. Install high flow
alarm and/or cooling
water high temp. alarm
No
1. Stirrer motor malfunction
2. Power failure
1. No mixing,
possible
accumulation of
unreacted materials
2. Monomer feed
continues, possible
accumulation of
unreacted materials
1. Interlock with feed
line
2. Monomer feed valve
must fail closed on
power loss
More
1. Stirrer motor controller
fails, resulting in high motor
speed
1. None
Covered under 1C
Not considered possible
Hazard and operability study report
Project title:
Sheet
Project number:
Date:
P&ID number:
Chairman:
Line number:
Study team:
Guide
word
Deviation
Cause
Conseque
nces
of
Safeguards
Action
Num
ber
By
Details
Reply
accepted
HAZOP Action Sheet
Project:
Project no:
Action no:
P&ID no:
Date:
Tape ref:
Action no:
Date for reply:
Description:
Reply:
Signed:
Issued
Return completed form to:
Date:
Returned
Complete
HAZOP Advantages

Meets regulatory requirements

Plant operates better

Less down time

Product quality improved

Employees are happier
HAZOP Weakness

HAZOP is very time consuming and can be laborious
with a tendency for boredom for analysts.

It tends to be hardware-oriented and process-oriented,
although the technique should be amenable to human
error application.

It tends to generate many failure events with
insignificance consequences and generate many failure
events which have the same consequences.

It stifles brainstorming although this is not required at
the late stage of design when it is normally applied.

HAZOP does not identify all causes of deviations and
therefore omits many scenarios.
HAZOP Weakness

It takes little account of the probabilities of events or
consequences, although quantitative assessment are
sometime added. The group generally let their collective
experiences decide whether deviations are meaningful.

HAZOP is poor where multiple-combination events can
have severe effects.

It tends to assume defects or deterioration of materials of
construction will not arise.

When identifying consequences, HAZOP tends to
encourage listing these as resulting in action by
emergency control measures without considering that
such action might fail. It tends to ignore the contribution
which can be made by operator interventions.
HAZOP Purpose


It emphasizes upon the operating integrity of a system,
thereby leading methodically to most potential and
detectable deviations which could conceivably arise in
the course of normal operating routine

including "start-up " and "shut-down" procedures

as well as steady-state operations.
It is important to remember at all times that HAZOP is
an identifying technique and not intended as a means
of solving problems nor is the method intended to be
used solely as an undisciplined means of searching for
hazardous scenarios.