Chapter 13
HAZOP Study
CHAPTER 13
HAZOP STUDY
Chapter 13
HAZOP Study
Introduction Traditionally, safety in the design of chemical plant relied upon the application of codes of practice practice design design codes and and checklists checklists based based on the wide wide experience experience and knowledge knowledge of profession professional al experts and specialists in the industry. However, such approaches can only cope with problems that have arisen before with the increasingly complexity of modern plant, these traditional approaches are likely to miss other issues which need to be considered at the design stage of a project. Hazard and Operability studies (HAZOPs) were developed by ICI during the 1960s as a technique to overcome this problem and to systematically identify potential hazards and operability problem problem in new designs designs for chemical chemical and petroche petrochemical mical plant plant in both both batch batch and continuous continuous processes processes HAZOPs HAZOPs can can also be used used for the modificatio modification n and review review of existing existing processe processes. s.
What are HAZOPs? HAZOPs are structured critical examination of plant and processes undertaken by an experienced team of company staff in order to identify all possible deviations from an intended design, along with the consequent undesirable effects concerning safety, operability and the environment. The possible deviations are generative by rigorous questioning, prompted by a series of stand, guide words, applied to the intended design.
Table 13.1 Standard Guide words and their generic meanings Guide Words
Meaning
No(not, none)
None of the design intent intent is achieved
More(more of, higher)
Quantitative increase in parameter
Less(less of, lower)
Quantitative decrease in parameter
As well as(more than)
An additional activity occurs
Part of
Only some of the design intention is achieved
Reverse
Logical opposite of the design intention occurs
Where else
Applicable for flow, transfer, sources and destination
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Before/After
The step(or some part of it)is effected out of sequence
Early/Late
The timing is different from the intention
Faster/Slower
The step is done/not done with the right timing
The deviation from the intended intended design are generated by coupling the the guide words with a variable parameter parameter or characte characteristic ristic of the plant of process process such such as reactant reactant reaction reaction sequence, sequence, temperat temperature, ure, pressure, pressure, flow, flow, phase etc etc in other other words. words. GUIDE WORD+PARAMETER=DEVIATION WORD+PARAMETER=DEVIATION For For exam exampl ple, e, when when consi conside deri ring ng a react reaction ion vess vessel el in which which an exoth exother ermi micc reac reacti tion on is to be undertaken and one of the reactants is to be added stepwise, the guideword ‘more’ would be coupled with the parameter ‘reactant’ and the deviation generated would be ‘thermal runaway’. Systematic examinations are made of each part of a plant or process using these guidewords having examine one part of the design recorded recorded any potential hazard and operability operability problems associated associated with it, the study progresses to focus on the next part of the design and recorded any potential hazards and operability problems associated within, the study progressed to focus on the next part of the design or the next step in the operating procedure procedure material or for referral outside for further consideration can then be made to overcome the problems which has been identified. The approach described above will generate hypothetical deviations from the design intention, the success or failure of study depends on four aspects: •
The accuracy of the design drawings and other data used as the basis for the study
•
The technical skills and expertise of the team
•
•
The ability of the team to use the approach as an aid to their imagination in visualizing possible possible deviations deviations causes causes and conseque consequences nces and The ability of the team to maintain senses of proportion, particularly
When assessing the seriousness of the hazards which are identified care thought must, therefore, be given to preparative work, team composition, keeping of records and so on.
PREPARATION FOR CARRYING OUT HAZOP The amount of preparation required for HAZOP depends upon the size and complexity of the plant. Typically, Typically, the data required consists of various drawings drawings in the form of line diagrams flow sheets plant layouts isometrics and fabrication drawing operating instructions, instrument sequence control charts, logic diagrams and compute programs. Occasionally there are plant manuals and
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equipment manufacturer’s manuals the data must be accurate and sufficiently comprehensive, in particular particular for the existing plant, the line diagrams diagrams must be checked to ensure that they are up-todate and those modifications have not been made since the pl plant ant constructed.
COMPOSITION OF THE TEAM TO CARRY OUT A HAZOP HAZOPs are normally carried out by a multi disciplinary team including chemical engineers and chemists with members being chosen for their knowledge and experience in design, operation, maintenance or health and safety. A typical team would have between 4 to 7 members each with a detailed knowledge of the way in which plant is intended to work, knowledge and expertise to bear systematically so that problems are less likely to be messed. HAZOP is technique of bringing fresh minds to work on problem. HAZOP studies generate recommendations for design changes: the team should have the authority to agree changes there and then as progress is slow if every change has to be referred elsewhere for a decision. It is also essential that the team leader is an expert of the HAZOP techniques. The team leader’s role is to ensure the team follows the procedure he or she needs to be skilled in leading a team of people people who may not normally normally be responsible responsible to him or her and the sort of person person who pays meticulous attention to detail. The team should have a secretary to prepare notes after each meeting and to circulate them before the next meeting. It is recommended that team leader should be an independent person .i.e. this should not be somebody who is closely associated with the plan under study. The team leader must have sufficient knowledge to guide the study properly but should not be expected to make this technical contribution. It is beneficial if team members have had some training in the HAZOP technique.
RECORD KEEPING It is usual to record each step of a HAZOP for all the physically meaningful deviations or if a sunset is used to include those requiring an action plus those which considered significant but required no action because the existing protection was deemed adequate. A particularly useful type of record is the ‘Hazard file’. This would normally include •
A copy of the date (flow sheets, original and final process and instrument diagrams running instructions bar sheets models etc) used by the team they have been examined
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•
A copy of all working papers, questions , recommendations, redesigns etc, produced by the team and others as a result of study, and
•
Confirmation that all the agreed actions have been carried out
•
The file should be retained on the plant to provide a source of information for future use
RESULT OF HAZOP On completion of HAZOP study the likely outcomes are: •
Some improvements in operation/maintenance, control programmers and instructions which may have been implements together together minor(low cost) hardware alterations alterations these will have have been put in hand as parts of the study.
•
Some proposed changes may await the result of a more detailed quantitative assessment. assessment.
•
Major recommendations have yet to be implemented, most possibly awaiting capital sanction.
•
The team members will already have both a better understanding of the plan/process and a better appreciation of potential hazard and risks than if the study had not been carried out.
BENEFITS OF HAZOP The circumstances when HAZOP is likely to produce benefits •
During the design of installation of any new plant or process, or major modification to an existing one.
•
When there are novel hazard such as environmental hazard and quality or cost issues associated with the operations.
•
Following a major incident involving involving fire, explosion, toxic toxic release etc, and
•
To justify why a particular code of practice, guidance not or industry code is not to be followed .
CAUTION Even the most rigorous HAZOP cannot be relied upon to foresee every hazard and some accidents may well occur in the future. When an accident occurs in a plant which has undergone a HAZOP study,
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•
Had the set of conditions conditions (deviations) which which led to the incident considered considered by the HAZOP study team If not, could the team reasonably have been expected to have does so? And
•
If such such deviat deviation ionss and their their causes causes had been been conced conceded, ed, had the team team made made reason reasonabl ablee judgments of the likely frequency of the events and had concluded that they were unlikely to occur and thus posed ‘acceptable risks’?
•
In such circumstances it is clearly Important to document all the outcomes of study in order to answer these questions
CONCLUSION HAZOPs HAZOPs are an essential essential tool for hazard hazard identific identification ation and have been used successfully successfully to improve the safety and operability of both new and existing chemical plant. The technique a not confined to the chemical and pharmaceutical Industries and has also been used successfully in a number of other industries, including the off-shore oil and food industries.
PERFORMANCE OF HAZOP Now in order to perform HAZOP study following table is used industrially, first the item number of process is mentioned then the deviation of that item is written on the column and causes, consequences and safeguards are written we have chosen to perform HAZOP on absorber in which CO 2 is removed with the help of water.
HAZOP OF AN ABSORBER Team: Production of Specialty Chemicals from Acid Gas waste Stream Meeting date: 21-05-12
Revision No. 1
ITEM NO
Deviation
Causes
Consequen ces
Safeguard s
Actions
AB1
Low Pressure
Unsuitable packing.
Low flooding efficiency, flood can occur
Use pressure controller at the upstream of the absorber
Use blower upstream and also use suitable packing for the absorber
Good absorption,
Use pressure controller,
Check blower
High Liquid loading
High Pr Pressure
Low pr pressure drop
Working
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Low temperature
Chocking can occur in the packing
Efficiency of absorption reduces also pressure drop increases
Use controller for the measurement of temperature of inlet gases and MEA
Use control valves and temperature controllers upstream the absorber
High temperature
Quencher is not working properly
Low absorption, damage to the packing
Use controller for the measurement of temperature of inlet gases and MEA
Use control valves and temperature controllers upstream the absorber
High Concentration of CO2
Change in wood composition, increase in CO 2 conversion
More water is required to remove CO2, increase operating cost, less water is required , also operating cost decreases
Check CO2 concentration after cracker, use wood of constant composition
Use controller for controlling concentration of CO2
Low Concentration of CO2
Less conversion in the cracker and more C remains as it is
