HAZards and OPerability analysis (HAZOP) simulates abnormal behavior by considering deviations and disturbances due to causes likely to impact immediate and surrounding plant resulting in consequences. The study team then decides whether the design has adequate features that can prevent occurrence or limit the consequential effects. If no such safeguards exist, then the team considers what actions are needed to remedy the situation.
Guide Word HAZOP is one of the most rigorous PHA methodologies, and many experts believe this approach gives the most complete analysis. Knowledge Based HAZOP can also be a valuable technique, provided that the engineering, plant and facility design standards and procedures are extensive and that the study team is highly experienced.
This section discusses the following topics:
Guide Word HAZOP on page 20.
The Cause-by-Cause Methodology on page 22.
The Deviation-by-Deviation Methodology on page 22.
Methodology for Guide Word HAZOP on page 23.
Assigning Nodes on page 23.
Examining Causes of Deviations on page 23.
Identifying Consequences, Safeguards and Recommendations on page 23.
Team Requirements on page 24.
Data Requirements on page 24.
Time Requirements on page 24.
Advantages of Guide Word HAZOP on page 24.
Limitations of Guide Word HAZOP on page 25.
Knowledge Based HAZOP on page 25.
Time Requirements of Knowledge Based HAZOP on page 25.
Advantages of Knowledge Based HAZOP on page 26.
Limitations of Knowledge Based HAZOP on page 26.
Guide Word HAZOP Guide Word HAZOP is a qualitative PHA method that identifies potential hazards and operating problems. This technique applies guide words to process
parameters to create deviations from the design intention.
Guide Word + Parameter = Deviation Example:
High + Flow = High Flow
The deviations apply to specific items, known as nodes. Risk analysts use Guide Word HAZOP to identify process and/or operational hazards as well as
unacceptable risk situations.
The following table presents some examples of guide words and parameters.
In this table, guide words are defined and then matched with parameters to create sample deviations:
The main focuses of Guide Word HAZOP are P&IDs and PFDs. Equipment specification, shut down logic, etc., and other process documentation are used as supporting documents. The analysis requires a multi-disciplinary team with members experienced in HAZOP, plant design, operation and maintenance. Study
Note: Not all guide words can be applied to all parameters. For example, it is invalid to combine the guide word “Reverse” with the parameter
“Temperature” because “Reverse Temperature” does not exist.
Guide Words Parameters
Guide Word Meaning Example Deviation
No, None Negation of design intent No flow
More, High Quantitative increase
(above design intent) More flow
Less, Low Quantitative decrease
(below design intent) Less flow
Part of Qualitative decrease
(below design intent) Part of stream composition is missing
As well as Qualitative increase (above design intent)
Reverse Logical opposite of
design intent Reverse flow
Other than Alternative mode (what
else can happen) Startup, shutdown, power failure
sessions must be dedicated to risk, not side issues, and must include a systematic review of all equipment items.
This methodology is applicable to continuous processes, batch processes and written procedures, such as operating instructions. The technique can be used for new designs at later stages, existing processes, revamp cases and plant
modifications. Some of the newer applications of Guide Word HAZOP include computer-controlled systems critiques.
Becoming familiar with Guide Word HAZOP is highly recommended before you begin using other PHA techniques. Once you learn how to use the Guide Word HAZOP methodology, the other techniques are relatively simple to follow.
The Cause-by-Cause Methodology
The Cause-by-Cause (CBC) methodology links each cause to consequences, safeguards and recommendations. CBC is an accurate HAZOP methodology because it is fully auditable.
In the following example, there are three causes, two consequences, three safeguards and two recommendations:
The Deviation-by-Deviation Methodology
The Deviation-by-Deviation (DBD) methodology records causes, consequences, safeguards and recommendations but does not show linking. This example is for the same situation as in the previous example:
Related
Cause 2 Consequence #1 Safeguard #1 Recommendation #1 Cause 3 Consequence #2 No safeguard Recommendation #2
Consequences Safeguards Recommendations
Cause 1 Consequence #1 Safeguard #1 Recommendation #1 Cause 2 Consequence #2 Safeguard # 2
Cause 3 Safeguard #3 Recommendation#2
Tip: You should use the CBC methodology wherever possible, because regulatory agencies emphasize the need for auditable documentation. The DBD approach may not satisfy such requirements.
Note: Other Guide Word HAZOP methodologies include the Exception Only method and the Recommendation (or Action) Only method. These techniques are not recommended due to their lack of thoroughness.
Methodology for Guide Word HAZOP
Guide Word HAZOP methodology consists of seven key steps:
Assigning Nodes You should assign nodes on a functional basis to reflect a specific function. Most nodes are of the “Line” type, but other categories include Vessels, Compressors, Tanks, and Reactors. Typical examples of nodes include the following:
• Transference or heating of a material.
• Increasing the potential energy by mechanical means, such as a pump.
• Separation of phases.
You may find it beneficial to join several types of nodes to form a single
compound node, such as Line + Pump + Heat Exchanger. Doing so may help you to avoid repetition and to maintain continuity and focus.
Examining Causes of Deviations
For each deviation, list all possible causes for the deviation from the design intention. Examples of this procedure might include the following:
Identifying Consequences, Safeguards and
Recommendations
For each cause, identify the potential consequences. If you want, you can use the following criteria to help rank each consequence:
Step Action
1. Assemble up-to-date P&IDs, PFDs, material⁄energy balances, plot plans, equipment specifications and so on.
2. Divide P&IDs systematically into nodes by line, vessel, pump, heat exchanger number and so forth.
Note: Nodes can consist of multiple items, such as Pump + Line + Heat Exchanger.
3. Prepare HAZOP Outline that lists each node with respective deviations by using guide words applied to parameters.
4. Record administrative information, such as company name, location, unit, team members and so on.
5. Assemble HAZOP team and explain process flow sheet, P&ID and so forth, as required.
6. For each node, examine each deviation for:
• Possible causes.
• Potential consequences.
• Effective safeguards.
• Recommendation items, if existing safeguards are not adequate.
• Remarks, if required.
7. Assign responsibilities for recommendations.
Deviation Possible Cause
Low/No Flow A level control valve failing in the closed position
High Temperature Uncontrolled runaway exothermic reaction in a reactor
High Pressure Loss of cooling water to a condenser
• Fire.
For each cause, identify the existing safeguards to prevent the cause from occurring and/or mitigate the associated consequences.
If the existing safeguards are not adequate to reduce risk to an acceptable level, you need to make recommendations.
Team Requirements A study team conducting a Guide Word HAZOP should ideally include the following individuals:
• Facilitator who is familiar with the Guide Word HAZOP methodology.
• Person who is entirely familiar with the process.
• Operations Supervisor.
• Maintenance Supervisor.
• Other specialists in areas such as instrumentation, controls, electronics, mechanical operations, and so forth.
• Scribe (optional).
Data Requirements A typical Guide Word HAZOP requires the following sources of information:
• Up-to-date drawings, P&IDs, PFDs or equivalent.
• Supporting documentation, such as:
• Equipment specifications.
Time Requirements Depending on the size and complexity of a system, a Guide Word HAZOP usually takes the following amount of time:
Advantages of Guide Word HAZOP
Guide Word HAZOP has the following advantages:
• Thorough and effective.
• Structured.
Preparation Team Sessions
Simple or small
system About one day Several days to one week
Large or complex
system Several days At least one month
Limitations of Guide Word HAZOP
Guide Word HAZOP has the following disadvantages:
• Time consuming.
• Can only be used for short study sessions, otherwise team members experience fatigue.
Knowledge Based HAZOP Knowledge Based HAZOP is a variation of Guide Word HAZOP and is based on the assumption that previous lessons learned, documented and applied can serve as a basis for evaluating and upgrading a facility, system, unit or process. Guide words are replaced for the most part by the knowledge and expertise of the team members and by detailed library checklists. This methodology compares the design to well-established designs and similar design practices that have proven integrity and performance and that are well documented from previous
experience. As a result, this approach may be inadequate for new processes with unproven track records.
When using Knowledge Based HAZOP, an organization should have the following:
• A well-proven track record.
• Experience with similar designs.
• Extensive design standards and procedures.
The following are the same as for Guide Word HAZOP:
• Team requirements.
• You can use risk matrices.
Time Requirements of Knowledge Based HAZOP
Depending on the size and complexity of a system, a Knowledge Based HAZOP usually takes the following amount of time:
Tip: Use the Knowledge Based Library Checklists that come with PHA-Pro. You can easily modify these libraries to suit your needs. For more information, see Chapter 6: Working with Libraries, Copy From and AutoType on page 137.
Tip: Although the use of guide words and parameters to form deviations is not an integral part of Knowledge Based HAZOP, you can apply these items as a last step in the analytical procedure. In doing so, treat the unit in question as a single node.
Preparation Team Sessions
Simple or small
system About one-half to one day One to three days
Advantages of Knowledge Based HAZOP
Knowledge Based HAZOP has the following advantages:
• Very effective with an experienced team.
• Time effective.
Limitations of Knowledge Based HAZOP
Knowledge Based HAZOP has the following disadvantages:
• May not be good with non-proven processes.
• Team dependent.
• Assumes good basic standards exist.
Large or complex
system One or two days At least two weeks
Preparation Team Sessions