Hazard and operability study: HAZOP decision making process – PHA Technique
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| Hazard and operability study: HAZOP |
Hazard and operability study: HAZOP decision making process – PHA Technique
The Hazard and Operability Study
(HAZOP) is a systematic technique to examine the risk of failure of the
complex process in the intervention of operators, i.e. It is used to identify
possible process deviations, operational difficulties, determine the causes of
the deviation and recommend preventive actions or controls.
The basic concept behind the HAZOP
study is that the process works well when operated under design conditions.
When deviations from process design conditions occur, operability problems and
accidents can occur.
Modern HAZOP was developed by
Lawley in the UK after industrial accidents such as the Flixborough disaster on
June 1, 1974, UK, which resulted in the loss of 28 lives. This technique
adopted worldwide after the 1984 Bhopal toxic gas tragedy, India, which
resulted in the loss of more than 2,500 lives.
HAZOP can be performed to configure new facilities, during change management
(MOC), i.e. the modification of existing facilities or for a new project or for
an expansion project.
HAZOP is one of the methodologies to perform the Process Hazard Analysis (PHA) which is a
comprehensive systematic review of a process to identify hazards, analyze the significant hazards, evaluate
the adequacy of safeguards and provide appropriate solution to mitigate them when justified.
It is not a method of reviewing the quality
or operation of the design, be carried out before your PHA meeting so that everyone has already
accepted the functionality of the design.
HAZOP, PHA and PSM relation
In 1984, a Methyl isocyanate, a toxic gas was
released at a Union Carbide pesticide plant in India that killed more than
2,500 people and hospitalized more than 125,000. The Bhopal India incident was
the worst industrial accident in history, as a result, American Society of
Chemical Engineers helping to develop OSHA 29CFR 1910.119, this is known
as Process Safety Management (PSM). OSHA adopted Process Safety Management in 1992
for any process that could catastrophically release flammable or explosive
reactive toxic chemicals or stores more than 10,000 pounds of flammable
liquids.
These are the building blocks of Process
Safety Management for emergency shutdown, safety and alarm systems, process
controls, and logic and procedures.
An adequate PHA assessment will collectively
focus design and safety operational teams to review risks, incorporate input
and perspectives, and develop stronger safeguards and procedures to prevent
accidents. Most of the time organizations prefer techniques like ‘What if’ or
‘HAZOP’.
HAZOP is a more comprehensive review of a complex P&ID system i.e. easy
to organize and document. The ‘What if’ technique is preferred for simpler or
more focused systems.
HAZOP vs HAZID
Hazard and operability (HAZOP) is a technique used to identify both
potential hazards and potential operability problems due to deviation in the
design intent. It helps to review the design intent of the particular
installation or process system by applying guide words related to the
parameters of each process to discover the causes of deviation, adequacy and
effectiveness of the safeguards and then recommend controls. Use a risk matrix
to determine the probability and consequences of failure.
HAZOP is generally conducted for new facilities, modification to existing
facilities through MOC, and every 5 years for operational facilities.
Hazard Identification (HAZID) is a technique
used to identify hazards based on standard checklists prepared with reference
to best industry practices, standards, rules and regulations, experience and
best engineering practices. Provides input to the primary risk assessment for
credible events or scenarios.
HAZID can be performed at any stage of the
installation, i.e. at initial stage of the new installation, in modification of
existing project, expansion, operational project, regular, etc.
HAZOP purpose
HAZOP is a proactive technique to identify and assess hazards, review the
adequacy of safeguards, and assess risk in facility design rather than waiting
for an accident to occur.
Risk is the product of probability and
consequence. The term probability is something that expresses uncertainty about
future events, while the consequences are the result, extent, or severity of
the event.
HAZOP reviews the process plan to pick up potential design and engineering
problems. It helps to review the control philosophy defined in P&ID to
describe how each piece of equipment is controlled and operated. The programmer
uses the control philosophy to program the PLC, a control philosophy that
describes the project process and the interactive control of the system. It
also describes the safety system alarms and shutdown systems.
The first section of the control philosophy
describes the general process in words. The control philosophy also includes
shutdown and alarm setpoints, function and type of PLC. It Includes control
loop tables that describe each loop function, loop variable, and parameters. A
safety integrity level (SIL) describes the various layers of safety protection
to prevent accidents.
HAZOP's study addresses hazards (safety, health, environment, property / asset) and issues
that affect operability.
HAZOP Applicability
1. For any stage in the life of a process
that has reasonably high levels of danger and complexity of the process
2. For flow type processes. It is not used
very effectively for the mechanical or electrical type of risk analysis of
industrial processes or facilities.
3. In general, hazard analysis in the process
industries, especially in change management (MOC).
4. For the expansion of existing projects,
new or new facilities of process industries.
Hazards and Operability (HAZOP) Steps
The HAZOP method uses guide
words to ask questions about any possible deviations from the mode of
operation, evaluating the related causes and consequences.
Before starting HAZOP, the
company must have the HAZOP procedure and the risk matrix.
Preconditions or preparation for HAZOP
When conducting a HAZOP it is
essential that there must be standardized format to record all important
identified causes. The consequences of each cause will be developed globally
without regard to safeguards, safeguards will be claimed only if they are
documented and proven and a consistent risk classification process must be
used.
HAZOP team with leader
For a successful HAZOP, the
collective knowledge of a multidisciplinary team helps to identify risk through
a creative and collaborative process, so high-quality decisions require shared
perspectives.
Team members with engineering and operational
skills are very important in conducting HAZOP in decision making,
but they do not always agree, therefore a facilitator (team leader) should act
as a decision leader with the tools and the right approach.
The team leader guides the team to make
rational, high-quality decisions by applying a simple decision process that
should be sufficient for all team members to understand but must be
comprehensive enough to address risk.
List of nodes with design intent
Break down the process installation design
into a number of simpler sections in P&ID and PFD. These sections are
called 'nodes' which are then individually reviewed. Prepare a list of nodes
along with the design intent to analyze each node in the design.
You must select a section or node to review,
you can highlight each node in the P&ID in colour, for each node you must
continue with the start of the process and then clearly discuss and understand
the function or design. Spend enough time to thoroughly analyze design intent
and goals.
Please note that this is not an opportunity
to change the design, but it is to ensure the suitability, reliability and
effectiveness of the safeguards. Gather relevant up-to-date documents for
chemicals, materials, and equipment data.
Here are the steps to perform HAZOP;
HAZOP Step-01: Select a design intent parameter
This is the first step where the team
documents the node description and design intent when referring to the control
philosophy. The process parameters can be pressure, temperature, flow,
composition, etc.
For a better understanding, let's take the
example of the first node, there is a liquid-gas separator container that
receives the saturated liquid mixture from the bottom and then passes the gas
to the compressor and the liquid to the tank. The main parameters are pressure
and flow. But, for example, here we are selecting pressure as a parameter to
evaluate first.
HAZOP Step-02: Select an applicable
To identify the deviation, guidewords for
each parameter are applied to each node in the process. The guide words should
be selected according to the parameters and the study. Here are the ideas for
selecting guides.
• MORE or HIGH: indicates that the value
increased more than the design intention
• LESS or LOW: Indicates that the value
decreases than the design intention
• NO or NO: Indicates complete stop or
failure of design intent.
• REVERSE: Opposed to design, logical or
directional intention.
• AS WELL: Modification of the value towards
the upper side or increase
• PART OF: Modification of the value towards
the lower side or decrease
• INSTEAD OR OTHER THAN: Indicate substitute
• EARLY: Indicate the time before the design
intention
• LATE: Indicates the delay of the design
intention
• BEFORE: Indicates the sequence or before
the order
• AFTER: Indicates the sequence or later to
order
There are several guidewords, but here for
our examples, we select the guideword as "high". So, the parameter is
"pressure" and the guide word is "high". Let's look at the
deviation in the next step
HAZOP Step-03: Generate the deviation
Here we discuss the process deviations from
its design intent. We can add various core deviations to each node, including
changes in flow, temperature, pressure, level, leaks, concentrations, among
other problems. Each deviation will be reviewed by the team. The team must
analyze the consequence of each deviation without any design safeguards, then
analyze the protection included in the current design.
Brainstorm the possible causes within the
node that could cause the deviation, don't analyze or criticize these causes,
you just need to capture the score and record all of them, i.e. make a list.
HAZOP Step-04: Risk identification
The risk identification process must be
systematic, selecting specific deviations from the process; otherwise, if we
only try to identify the risk by asking what can cause a hazardous event, then
we cannot conclude since the process can have thousands of causes of hazards.
We can consider the process deviation as high
temperature, high pressure, low level, misdirected flow, no or low flow, etc.
which are relevant to the identified process. Then the team should focus on a
specific deviation simply by asking the question, what can cause high pressure
in the process.
For example, there is a separator vessel that
separates the liquid from the gas once it is received. This vessel is provided
with a pressure control circuit and a level control circuit and a maximum
allowable working pressure is slightly above the set point of the pressure
relief valve.
Now apply the first guide word, ie "high
pressure", deviation from normal operation. Then find out the cause of the
deviation, and if the pressure control valve fails to closed position.
HAZOP Step-05: Identify the effect or consequences
This step is to confirm that the cause is
valid and then analyze it, otherwise remove it. When considering the
consequences, imagine that we are operating without safeguards, the operator is
not paying attention, the control valve is manual, position alarms and safe
interlocks are not working, and no one is following the procedures. It is vital
to develop and document the consequences fully and chronologically. You should
look at the worst possible reasonable scenarios. You must record all
consequences.
Identify the worst credible consequences
without safeguards and assign a level of severity. What will be the
consequences if overpressure of the vessel leads to rupture due to involuntary
closure of the pressure control valve? What is the level of exposure of
operators or other personnel at the facility and what will be the impact? Can
it kill someone? Can it cause a public relations disaster, a major loss of
property? Will it cost huge financing to repair or replace equipment, etc. Some
of the significant consequences may be considerable and important for the
company to manage risk based on the decision of the HAZOP team. Suppose here
that the team agreed that rupture of vessel can lead to fatality.
HAZOP Step-06: Assess probability
In our example, here we must evaluate the
probability of failure of the pressure control valve without safety safeguards.
The causes of failure of the control valve may be due to the logic controller
or the sensor outlet air.
Then based on data within the company, past
experience, or different operational and engineering experience, we have to
determine the probability of failure. It can be 5% or 10% or 20% or 90%
probability in a year.
HAZOP Step-07: Risk evaluation without safeguard
Once we have decided the probability and the consequences,
we must assess the risk without any safeguards. In our example of the separator
vessel rupture scenario, the probability of failure of the control valve is
high, and the severity is also high due to a possible fatality.
Consider that
in our organization's standard risk matrix, when there is high probability and
high consequences, the risk is considered high. According to the risk matrix,
high risk is not acceptable at all. Then we must decide on some control
measures to reduce the risk and bring it to an acceptable level.
HAZOP Step-08: Assess the safeguards
Now here at this stage, our main focus is to
reduce risk and bring it to an acceptable level by ensuring some safeguards. We
can reduce risk by reducing probability and / or consequences. In an earlier
stage we evaluated the risk without considering the safeguard, but in this
stage we have to evaluate the risk with the available safeguards.
So just check
if there is an alarm with operator intervention, if there is any pressure
relief valve or PSV available in the vessel, if there is any trip interlock, if
there is any automatic bypass system, if there is any automatic shutdown system
high pressure alarm trigger etc.
You must add safeguards that are included in
the P&ID and control philosophy. It first assesses global safeguards and
challenges their effectiveness, then we visualize the sequence of the accident,
consider the time, effects, and possible impact of stress and urgency in
reaction time. Safeguards are listed in priority.
First, it lists the elements that eliminate
the causes, then we list the elements that mitigate the consequences, and
finally, it documents human intervention, such as the emergency response.
If you check the layer of protection, the
first layer is the processed design, which is the most reliable. As you work
from the centre, you add alarms, safety instrumented systems (SIS), emergency
relief systems, and lastly, emergency response.
Each of these is an additional layer of
protection that can collectively prevent accidents. Safeguards include
instrumented protection such as control loops and alarms, non-instrumented
operator surveillance and mechanical design features, and others such as
training and preventive maintenance.
Now, for our examples, suppose there is PSV
available to deal with failure of pressure control valve, it will reduce
pressure. This protection can prevent the rupture of the vessel, that is, it
will reduce the probability of rupture of the vessel.
But based on industry experience and data, it
has been observed that the PSV can also fail, so we cannot consider it as the
most reliable or full proof system as it will not completely eliminate the risk
to an acceptable level.
HAZOP Step-09: Decision making
This is the stage in which we have to
conclude that the risk is acceptable, or we have to think of some other more
reliable safeguard, that is. recommendation to further reduce risk by reducing
probability. You should follow your organization's risk matrix to decide the
acceptability of risk, as it is the only document that will help you make a
decision on risk acceptance criteria. Based on the risk decision, you should
recommend safeguards when the risk is not acceptable.
Team must rank i.e. classify cause and
consequence. Team has first assigned a numerical level of severity without
safeguards, then assign a numerical probability of occurrence with safeguards
and finally assign a numerical risk ranking or rating from the risk
matrix.
Team must classify the probability of the
consequence according to these descriptions is the frequent, occasional,
possible, improbable, or improbable probability afterwards. Severity may be
high, medium, or low probability.
The risk ranking is based on the rating of
probability and impact of the consequence, then you must determine the level of
action required for each level of risk, after having evaluated these events,
team will propose recommendations for the highest risk scenarios.
HAZOP Step-10: Record and report
The team must prioritize ideas that prevent
the cause from occurring. If the cause cannot be avoided, the team will suggest
ideas to mitigate the consequences, the team should not attempt to design
solutions at this point.
But without commitment to action, there is no
value in recommendations. Therefore, there should be an action plan with
properly assigned responsibilities with a target date. Record all
recommendations and assigned responsible parties. It is important to assign
responsible parties before the end of the HAZOP.
Merits of Hazard and operability study (HAZOP)
1. It is more suitable to study the complex
process or a complicated procedure.
2. It helps to study and analyze large
processes in detail.
3. It is the most appropriate process to
analyze both safety and operability problems.
4. It is the most logical and gives a
constructive result to decide the control measures or safeguards.
Demerits of Hazard and operability study (HAZOP)
1. It is a complicated and time-consuming
process
2. Study sessions can be intensive and
exhausting, as it takes time to provide a full analysis.
3. In general, do not observe chronic or
occupational hazards.
4. People may spend time discussing
operational issues rather than safety.
Hazard and operability (HAZOP) study Example
Here are the sample of HAZOP sheet as we
studied above for the gas-liquid separator system
|
Sr. No.
|
Parameter
|
Guide
word
|
Deviation
|
Causes
|
Consequences
|
Safeguards
|
Recommendations
|
|
1
|
Pressure
|
High
|
High
pressure in vessel
|
Control
valve failure to close
|
Rupture
of vessel
|
PSV on
vessel
|
Automatic
shutdown system
|
|
2
|
|
|
|
|
|
|
|
Examples No-2: Cooling water chlorination
system
|
Sr. No.
|
Parameter
|
Guide
word
|
Deviation
|
Causes
|
Consequences
|
Safeguards
|
Recommendations
|
|
1
|
Flow
|
No
|
No flow
in chlorination loop
|
Pump
failure.
IV failed
to open
Loss of
electric power to pump
|
No
chlorin in tower basin reduces the desired concentration
|
Pump
malfunction alarm
|
Nil
|
|
2
|
Flow
|
More
|
High
flow in chlorination loop
|
Non
identified
|
NA
|
NA
|
Note:
Pump normally runs at full speed
|
Conclusion
Hazard and Operability (HAZOP) is the most appropriate Process Hazard
Analysis (PHA) technique to study the complex process to identify potential
hazards in the designed process and probable operability problems by analyzing
the causes of the deviation of the process and associated risk and review the
safeguards to prevent accidents during its operation stage.
The HAZOP team should be
competent and aware of the PHA technique led by the facilitator to facilitate
the decision-making process. A record must be kept for the full HAZOP
study and roles and responsibilities assigned for each recommendation
before the study is closed.
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