Hazard and operability study (HAZOP)

Background.

A HAZOP study identifies hazards and operability problems. The concept involves investigating how the plant might deviate from the design intent. If, in the process of identifying problems during a HAZOP study, a solution becomes apparent, it is recorded as part of the HAZOP result; however, care must be taken to avoid trying to find solutions which are not so apparent, because the prime objective for the HAZOP is problem identification. Although the HAZOP study was developed to supplement experience based practices when a new design or technology is involved, its use has expanded to almost all phases of a plant's life. HAZOP is based on the principle that several experts with different backgrounds can interact and identify more problems when working together than when working separately and combining their results. The "Guide-Word" HAZOP is the most well-known of the HAZOPs; however, several specialisations of this basic method have been developed.

A hazard and operability study (or HAZOP) is a systematic, critical examination by a team of the engineering and operating intentions of a process to assess the hazard potential of mal-operation or mal-function of individual items of equipment and the consequential effects on the facility as a whole. It is quite normal to carry out safety reviews. These may take different forms. Experts may be consulted in isolation, without reference to each other. They may instead be gathered in lengthy meetings to discuss the particular topic. Hazops are meetings with a distinct structure, the structure imposing a certain organization, to enhance effectiveness. They are a generalized study technique, equally applicable to microchip manufacture, pharmaceutical synthesis, effluent plant operation or any process. They should not be seen, however, as a solution to all ills, the ultimate review. The procedure is only anther tool in the safety locker and should be seen as complementary to other techniques. Indeed, it is best applied as one stage of a multistage procedure, applying different techniques as relevant to each stage. It does not replace, but rather supplements, existing Codes of Practice. Neither can it totally substitute for experience. But, both Codes of Practice and experience are evolved from existing situations. Innovative developments require a review which investigates the unknown. Hazops are a systematic, logical approach to determining problems

Concept.

The HAZOP concept is to review the plant in a series of meetings, during which a multidisciplinary team methodically "brainstorms" the plant design, following the structure provided by the guide words and the team leader's experience. The primary advantage of this brainstorming is that it stimulates creativity and generates ideas. This creativity results from the interaction of the team and their diverse backgrounds. Consequently, the process requires that all team members participate (quantity breeds quality in this case), and team members must refrain from criticizing each other to the point that members hesitate to suggest ideas. The team focuses on specific points of the design (called "study nodes"), one at a time. At each of these study nodes, deviations in the process parameters are examined using the guide words. The guide words are used to ensure that the design is explored in every conceivable way. Thus the team must identify a fairly large number of deviations, each of which must then be considered so that their potential causes and consequences can be identified. The best time to conduct a HAZOP is when the design is fairly firm. At this point, the design is well enough defined to allow meaningful answers to the questions raised in the HAZOP process. Also, at this point it is still possible to change the design without a major cost. However, HAZOPs can be done at any stage after the design is nearly firm. For example, many older plants are upgrading their control and instrumentation systems.

The success or failure of the HAZOP depends on several factors:

• The completeness and accuracy of drawings and other data used as a basis for the study
• The technical skills and insights of the team
• The ability of the team to use the approach as an aid to their imagination in visualizing deviations, causes, and consequences
• The ability of the team to concentrate on the more serious hazards which are identified.

The process is systematic and it is helpful to define the terms that are used:

1. STUDY NODES - The locations (on piping and instrumentation drawings and procedures) at which the process parameters are investigated for deviations.
2. INTENTION - The intention defines how the plant is expected to operate in the absence of deviations at the study nodes. This can take a number of forms and can either be descriptive or diagrammatic; e.g., flowsheets, line diagrams, P&IDS.
3. DEVIATIONS - These are departures from the intention which are discovered by systematically applying the guide words (e.g., "more pressure").
4. CAUSES - These are the 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, human errors, an unanticipated process state (e.g., change of composition), external disruptions (e.g., loss of power), etc.
5. CONSEQUENCES - These are the results of the deviations should they occur (e.g., release of toxic materials). Trivial consequences, relative to the study objective, are dropped.
6. GUIDE WORDS - These are simple words which are used to qualify or quantify the intention in order to guide and stimulate the brainstorming process and so discover deviations. The guide words shown in the following table are the ones most often used in a HAZOP; some organisations have made this list specific to their operations, to guide the team more quickly to the areas where they have previously found problems. Each guide word is applied to the process variables at the point in the plant (study node) which is being examined.

WHY is a HAZOP carried out?

The reasons for carrying out hazard and operability studies, are:

1. Primarily, to identify hazards, and
2. To a lesser extent, to resolve these hazards.

In saying this, hazards are very generally defined. They are understood to be events, which: -

1. Lead to injury of people, either inside or outside the plant.
2. Injure the environment.

iii. Insult the environment. Harmful effects may not occur, but disturbance itself is unacceptable.

1. Damage the plant, an obvious hazard.
2. Result in loss of production quantity, quality or schedule.

In practice, some resolution of hazards is normally accepted. However, a careful balance must be maintained to ensure that the primary purpose of hazard identification is not compromised.

WHEN is a HAZOP carried out?

The timing of a hazard and operability study is determined by the objectives of a study, and in turn determines the benefits that may be gained. The outline concept of a process may be examined to highlight any major omissions or significant features. As further detailing is carried out, e.g. when the process design is complete, the full study procedure may best be applied. Operating procedures may be examined to ensure that all eventualities have been considered. Modifications including so-called “minor modifications”, generally benefit from a rigorous study. Often an apparently simple, uncomplicated modification can give rise to a greater problem than it was intended to solve. Existing plant and new equipment are other examples of topics that may benefit from study.

Therefore, a project may be studied several times in its life-time: -

Despite these comments there is quite a distinct benefit from carrying out a proper HAZOP Study in terms of the correct timing and to obtain the maximum cost benefit. Therefore, a hazop cannot be carried out before the line diagrams (or process instrumentation diagrams as they are often called) are complete. It should be carried out as soon as possible thereafter. If an existing plant is being studied the first step is to bring the line diagrams up to date or check that they are up-to-date. Carrying out a hazop on an incorrect line diagram is the most useless occupation in the world. It is as effective as setting out on a journey with railway timetable ten years out of date.

A hazop takes 1.5-3 hours per main plant item (still, furnace, reactor, heater, etc.). If the plant is similar to an existing one, it will take 1.5 hours per item but if the process is new it may take 3 hours per item. Meetings are usually restricted to 3 hours, twice per day, 2 or 3 or even 4 days per week, to give the team time to attend to their other duties and because the imagination tires after 3 hours at a stretch. The hazop on a large project may take several months, even with 2 or 3 teams working in parallel on different sections of the plant. It is thus necessary to either:

1. a) Hold up detailed design and construction until the hazop is complete, or
2. b) Allow detailed design and construction to go ahead and risk having to modify the detailed design or even alter the plant when the results of the hazop are known. Ideally, the design should be planned to allow time for

(a) but if completion is urgent

(b) may have to accepted - but this is not a widely accepted option due to the cost implications.

A preliminary hazop may be carried out on the flowsheet before detailed design starts. This will take much less time than the hazop of the line diagrams and will identify ‘area’ of the process of a particular hazardous nature. It provides a more “structured” and “systematic” approach than a preliminary design review - but NOT the detailed analytical data of a true P&ID HAZOP.

Introduction to the HAZOP Approach: -

The Hazard and Operability (HAZOP) Analysis technique is based on the principle that several experts with different backgrounds can interact in a creative, systematic fashion and identify more problems when working together than when working separately and combining their results. Although the HAZOP Study technique was originally developed for evaluation of a new design or technology, it is applicable to almost all phases of a process’s lifetime. The essential feature of the HAZOP Study approach is to review process drawings and/or procedures in a series of meetings, during which a multidisciplinary team uses a defined protocol to methodically evaluate the significance of deviations from the normal design intention.

The primary advantage of the brainstorming associated with HAZOP Study is that it stimulates creativity and generates new ideas. This creativity results from the interaction of a team with diverse backgrounds. Consequently, the success of the study requires that all participants freely express their views and good supportive teamwork practices are adopted. Participants should refrain from criticizing each other to avoid smothering the creative process. This creative approach combined with the use of a systematic protocol for examining hazardous situations helps improve the thoroughness of the study. The HAZOP study focuses on specific points of the process or operation called “study nodes,” process sections, or operating steps. One at a time, the HAZOP team examines each section or step for potentially hazardous process deviations that are derived from a set of established guide words. One purpose of the guide words is to ensure that all relevant deviations of process parameters are evaluated. Sometimes, teams consider a fairly large number of deviations (i.e. up to 10 to 20) for each section or step and identify their potential causes and consequences. Normally, all of the deviations for a given section or step are analysed by the team before it proceeds further. HAZOP Analysis studies can be performed on new projects as well as on existing facilities. For new projects, it is best to conduct a HAZOP Analysis when the process design is fairly firm. Normally, the system P&IDs are available so the team can formulate meaningful answers to the questions raised in the HAZOP Analysis process. Also, it is still possible to change the design without incurring major costs. However, HAZOP Analysis studies can also be performed at earlier stages of a process lifetime as long as the team members have adequate process documentation and knowledge upon which to base their analysis. But a HAZOP analysis performed at this early stage should not be a substitute for a thorough design review.

Duties of the Study Leader: -

1. Before the HAZOP Study: - Ask for two copies of the P and ID to be HAZOPed at least a week ahead of the study. Check the P and ID to ensure that;
2. The right members of the team have been requested to attend the HAZOP Study. (See who is listed on the HAZOP Schedule).
3. The Process is a continuous process rather than a batch process. If it is a batch process there are additional documents to be obtained, and additional work for the Study Leader to do before the study begins.
4. P and ID is developed to the point where it is ready to be HAZOPed (New Design). For an existing plant make sure the P and ID is up to date and is an accurate representation of the plant was built, rather than an indication of what the designers intended to have built.
5. The drawing is readily understandable and that by looking at it the Study Leader gets a good overall grasp of the overall process.
6. If there are non-standard symbols used on the P and ID, a key to the symbols used is available for reference.
7. Pre-HAZOP Meeting:- It is wise for the Study Leader to meet with the Process Engineer in advance of the actual study to assure himself that he has a good grasp of what the process is all about, and to agree a mutually acceptable basis for dividing up the P and ID into short sections suitable for individual study. At the pre-HAZOP meeting it is good to make up one copy of the P and ID with the division into short sections, and to record any other comments or explanations the Study Leader might wish to record as an aid memoir. The second copy of the P and ID will be put on the table at the HAZOP Study as a clean drawing, and any markings put onto it will be done in the presence of the HAZOP team. This second copy is then called the “HAZOP Master”. While the Study Leader should be independent of those who are associated with the project or plant, he should take time before the HAZOP Study to brief himself with details of the project or plant, so that he does not waste the time of the HAZOP team asking unnecessary questions.

1. During the HAZOP Study: The Study Leader acts as Chairman and is responsible for starting the meeting on time and terminating the session once it has run for three hours. He begins by calling on the Process Engineer to give the overall process description and the design intent for each P and ID. This should be a final revision as far as the Study Leader is concerned, but it ensures that others present, who may not have done any preparation in advance, have the background information against which to carry out the study. The Study Leader then defines the first section to be studied in detail and he may put a dotted yellow line onto the HAZOP Master to ensure that everyone is looking at the same section of the plant. He then calls on the Process Engineer to give the design intent and detailed information about the contents of the line, the design and operating temperature and pressure, etc. The Study Leader begins the review of this section by reading out the first guide word “NO FLOW”. When possible reasons for NO FLOW are established by the team he writes them down on his pad, and then ticks them off as they are studied in detail and at the same time, transferred onto the formal record of the HAZOP Study. When an action is agreed the Study Leader writes onto the HAZOP Master the number of the action in red, next to the item to be changed. If the change is agreed by the meeting he also marks up the HAZOP Master in red with the change to be made. When all the guide words have been used and the study of a sub-section is complete, the Study leader should yellow-off all the section completed. The Study Leader then defines the second section to be studied in detail and calls on the Process Engineer to give a detailed process description before starting again with the first guide word, ‘NO FLOW”.
2. After the HAZOP Study: - At the Study the Report Form is filled in by the Technical Secretary. However, before this Report Form is typed up for distribution it is helpful for the HAZOP Study Leader to check that the actions are stated clearly - so that they can be understood by persons not at the meeting - and that the scope of each action is completely defined. It may be helpful for the HAZOP Report to be signed by the Project Engineer and approved by the Study Leader. While checking the report the Study Leader should also check the HAZOP Master P and ID to ensure that all the modifications agreed at the study are shown clearly, and that the locations of all the actions are shown by the number drawn-on in red. Sometimes the HAZOP Study Leader (Chairperson) may also act as Technical Secretary.
3. Responsibilities of HAZOP Chairperson: -

• Ensure that the proper hazard evaluation is selected and correctly applied
• Organize an analysis and negotiate for resources
• Communicate with personnel at all levels in the organization
• Motivate a group to achieve a common goal
• Work with a wide range of personalities (possibly including highly defensive and argumentative individuals, quick and direct individuals, and rambling and talkative individuals) Interpret engineering drawings and understand process operations
• Ask questions and probe for further explanations without making team members defensive Maintain objectivity, honesty, and ethical conduct, reporting all significant findings regardless of the potential discomfort to the leader, team, or management
• Encourage, direct, and focus group discussions
• Judge the relative importance of issues and help the team drop those not worth pursuing
• Summarize issues, negotiate a compromise, and forge a consensus
• Appreciate different points of view and empathize with team members
• Remain impartial and maintain the respect of the team
• Manage the pace of team discussions and tactfully maintain the meeting schedule
• Sense team fatigue, boredom, unsuitability, etc., and implement corrective action
• Keep the team working together
• Suspend discussion of issues that cannot be resolved by the team
• Fulfil team members’ psychological needs without letting anyone ego, including the leader’s, dominate the team

 Overall Procedural Steps in a HAZOP Study: -

Introduction to the HAZOP Study Procedure: -

The outline steps for the overall HAZOP Study methodology are shown in the above diagram. A potential HAZOP Study Leader or HAZOP Study Chairperson must be aware of all of these. However, many people - engineers, chemists, project managers, process operators, maintenance staff, services engineers, contractors, equipment suppliers, control systems staff etc. etc. - will be required to attend and participate in HAZOP Studies. Consequently, as part of introducing HAZOP it is worthwhile early on in this course for us to look at two aspects of the study method relating to Blocks 7 and 9 in the diagram. Two methods of importance in the “practical side” of performing a HAZOP Study are:

• Defining each Pipe section to be studied. This should have been agreed previous to the actual HAZOP Study Meeting between the HAZOP Study Chairman and the Lead Process Engineer
• Application of the Guide Words

HAZOP GUIDE WORDS AND MEANINGS: -

Guide Word Meaning

No----------------------------------------------------------------Negation of the Design Intent

Less----------------------------------------------------------------------Quantitative Decrease

More-----------------------------------------------------------------------Quantitative Increase

Part Of---------------------------------------------------------------------Qualitative Decrease

As Well As-----------------------------------------------------------------Qualitative Increase

Reverse--------------------------------------------------------Logical Opposite of the Intent

Other Than--------------------------------------------------------------Complete Substitution

 These guide words are applicable to both the more general parameters (e.g. react, transfer) and to the more specific parameters (e.g. pressure, temperature, flow). With the general parameters, meaningful deviations are usually generated for each guide word. Moreover, it is not unusual to have more than one deviation from the application of one guide word. For example, "more reaction" could mean either that a reaction takes place at a faster rate, or that a greater quantity of product results. With the specific parameters, some modification of the guide words may be necessary. In addition, it is not unusual to find that some potential deviations are eliminated by physical limitation. For example, if the design intention of a pressure or temperature is being considered, the guide words “more” or "less" may be the only possibilities. Finally, when dealing with a design intention involving a complex set of interrelated plant parameters (e.g., temperatures, reaction rates, composition, or pressure), it may be better to apply the whole sequence of guide words to each parameter individually than to apply each guide word across all of the parameters as a group. Also, when applying the guide words to a sentence it may be more useful to apply the sequence of guide words to each word or phrase separately, starting with the key part which describes the activity (usually the verbs or adverbs). These parts of the sentence usually are related to some impact on the process parameters.

Guidelines for Using Procedure

The concepts presented above are put into practice in the following steps:

1. Define the purpose, objectives, and scope of the study
2. Select the team

3.- Prepare for the study

1. Carry out the team review
2. Record the results.
3. Follow up to ensure results are implemented.

It is important to recognize that some of these steps can take place at the same time. For example, the team reviews the design, records the findings, and follows up on the findings continuously. (Refer also to the diagrammatic representation of the HAZOP procedure attached).

Detailed Listing of HAZOP guide words:

Guide Word (By Node)

• High Flow, High level
• Low Flow, Low level
• Zero Flow, Empty
• Reverse Flow
• High pressure (venting, relief rate)
• Low pressure (venting, relief rate)
• High temperature
• Low temperature
• Impurities (Gaseous, Liquid, Solid)
• Change in composition / Change in concentration / Two phase flow / Reactions
• Testing (Equipment / Product)
• Plant items (Operable / maintainable)
• Electrical (Area Classification / Isolation / Earthing)
• Instruments (Sufficient for control / Too many / Correct location)
• Guide Word (Overview)
• Toxicity
• Services Required
• Materials of Construction
• Commissioning
• Start-up
• Shut-down (Isolation, Purging)
• Breakdown (Power failure, Air, Steam, Water, Vacuum, Fuel, Vents, Computer, Other)
• Effluent (Gaseous, Liquid, Solid
• Noise (Sources, Problem, Control measures)
• Fire / Explosion
• Safety Equipment (Personal, Fire detection, Firefighting, Means of escape)
• Quality and Consistency
• Output - Reliability and bottlenecks
• Efficiency - Losses
• Simplicity

The GUIDEWORDS are applied to a range of process PARAMETERS. Usually only a limited number of combinations of guidewords and process parameters are used. The most common process parameters are shown in the Table and the four in the first column are the ones most frequently used - FLOW, PRESSURE, TEMPERATURE and LEVEL, others will be tested and used on a case by case basis if required

Process Section: -

The section to be studied is usually a section of pipeline between two main process items on a P&ID (piping and instrumentation diagram) - for continuous process operations. Usually the analysis is carried out on final P&ID’s, that is, prior to “Issue for Construction”. Frequently the section of line undergoing a HAZOP Study may go through several other items of equipment which must be considered but providing there is no chemical change it is acceptable and normal to HAZOP in this way. Sometimes an additional chemical may even be added into the line (via a branch line, e.g. T junction on Y junction) and these “in-line” additions are usually included as part of the HAZOP of this particular section - but NOT always the branch line. When the Pipe section has been followed through to the equipment item it is usual to assess the equipment item as part of the “same section” by applying a number of equipment guidewords. The same method, of course, applies to the equipment item at the beginning of the process. The whole HAZOP process usually starts with the engineering drawing(s) at the BEGINNING of the process, the feeds being the raw materials. Often as many as 3 or 4 P&ID’s may be tabled at one session to enable the HAZOP team to identify where streams are coming from on one or more P&ID’s and where they are going to on the next one or two P&ID’s.

HOW are HAZOPs done?

Earlier, these studies were defined as examinations of engineering and operating intentions. An intention is the expected behaviour of a process and its associated hardware, under normal and abnormal conditions. It may be defined either diagrammatically or descriptively; diagrammatically in terms of flowsheets, P&ID’s. etc., or descriptively with operating instructions or design specifications. A very important assumption is that no hazard can arise from an intention that behaves as expected, i.e. no one deliberately builds in a hazard. Therefore, a hazard can arise only if there is a deviation from the expected behaviour. Hypothetical deviations are prompted by applying guide words, which will be explained shortly, to each intention. Consequently, the design basis is not explicitly challenged and process alternatives may not be recognized.

For example, it is proposed that excess pressure may exist in a line. Firstly, it must be established if there is a realistic cause of this deviation. If there is, the consequences must be considered. They may be trivial or significant. If significant, they must be evaluated to see if they constitute a hazard. In the example of line over-pressure, the excess may be within the line rating. This consequence is trivial. If the rating is exceeded, however, rupture may result. This is obviously a hazardous occurrence. The study procedure may be broken into several distinct steps and is shown in the diagram. We must define the scope of the study, select a team to carry it out, and make the necessary preparations before the examination itself can be carried out. Arising from the examination will be a number of follow-up activities. Finally, a detailed record of the study is also necessary; but now we will consider the “Application of the Guidewords” to a particular “Section” or “Study Node”.

HAZOP Terminology: -

Example of examination sheet: -

The following are a few of the safety points that came out of this early hazop (though that term was not used then; the exercise was described as a method study or hazard investigation). Some of the points are now included in design specifications but were not included at the time.

• By-passes around control valves which are connected to safety trips should be deleted. Use of a by-pass renders the safety trip useless.
• Nitrogen should be used for vacuum breaking to prevent the ingress of air into a hot system.
• Break tanks should be fitted in town water supply to prevent contamination by reverse flow.
• The relief valve system should be checked for places in which liquid could collect.
• A slip-plate should be fitted in the feed line to [vessel X] to prevent liquid leaking in before conditions are correct.
• Vent valves should be fitted to all blowing points so that the pressure can be blown off before hoses are disconnected.
• A vent valve should be fitted to a high pressure filter so that the pressure can be blown off before the filter is opened for cleaning.
• Extended spindles should be fitted to the valves on acid tanks to reduce the risk that operators may be splashed by leaks.
• Special equipment should be designed for charging and discharging catalysts and other auxiliary materials, to remove the dangers that go with improvisation.

Note that all these points are written as recommendations. Today most hazop teams would not say ‘should’ but simply ‘Delete by-passes... etc.’. More operating points than safety ones came out of the study. This was expected. The remit of the team was ‘To devote themselves full-time to obtaining and studying information from all sources and to take any necessary decisions on broad plant design aimed at ensuring that the phenol plant would start up quickly and satisfactorily; that it will produce its design output and quality of products; that it will operate safely and its effluents will be satisfactorily treated’. Today many, perhaps most, hazops produce more operating points than safety ones.

Process HAZOP worksheet: -

Analysis of Main Findings: -

An indication of the criteria used to determine whether or not action was chosen to be taken regarding the outcome of a deviation is required. The results of the HAZOP, giving deviations, consequences and actions required, should be provided. Those events on which the decision of no action was made should also be listed, along with the events for which consequence or risk analysis was considered necessary. The decisions made after such further analyses should also be given. Any alternative actions generated and considered should be detailed.

Actions Arising from the HAZOP: -

This section should highlight those actions which are potentially hazardous to plant personnel, the public or the environment or have the potential to jeopardise the operability of the plant. Also included should be a clear statement of commitment to modify the design or operational procedures in accordance with the identified required actions and a timetable for implementation. Justification as to why no action was chosen for any actions identified should also be made. The current status of the recommended actions at the time of the report should also be given together with the names/designations of persons responsible for their implementation.