The purpose of a HAZOP study is to identify and evaluate any hazards within a planned process or operation that were not identified or designed out in earlier stages. The hazards may be several types, including those to people and property, both on- and off-site. It is also important to consider the potential effects to the environment. Regardless of the type of hazard, many have directly related financial consequences. HAZOP studies are also normally used to identify significant operability or quality problems and this will be included as a defined objective of the study. It is advisable to cover aspects of maintenance operations, including isolation, preparation, and removal for maintenance since these often create hazards as well as pose an operability problem. Where there are manual operations or activities, it may be necessary to analyse the ergonomics of the whole operation or activity in detail.
The HAZOP study must proceed in a carefully planned, systematic manner to cover all of the selected aspects of the process or operation. It is normal to cover a continuous operation by dividing it into sections and working from an upstream starting point. A batch process or a procedure is divided into sequential steps and these are taken in a chronological order.
It is essential that the multi-disciplinary team begins with a full understanding of the section or stage to be analysed, either knowing the existing situation or having sufficient information to be able to form an adequate conceptual model. A full description should be developed, including all the key parameters, and the HAZOP report should include the design description. Next, a design intention for the step is formulated and recorded. This should include a statement of the intended operational range (envelope) so that the team can recognize any situations lying outside this range as deviations. The design intention may be interlinked with the step description and the design parameters of the equipment. It is good practice to develop a comprehensive design intention, clearly linked to the drawings being used, which can be referred to, while scouting for deviations. A design intention may refer to equipment items in the section, materials, conditions, sources, and destination, to changes or transfers, as well as, the means of control and timing of a step. It not only refers to plant equipment but covers what is intended to be done within the section being analysed.
GUIDEWORDS
The next step is to generate a meaningful deviation by coupling a guideword and a parameter A deviation can be generated by taking a parameter and combining it with each guideword in turn to see if a meaningful deviation results (the parameter-first approach). The alternative approach is to take a guideword and try each parameter in turn (the guideword-first approach). The standard set of guidewords for process plant is listed in below table.
| Parameter | Guidewords That Can Give a Meaningful Combination |
| Flow | None; more of; less of; reverse; elsewhere; as well as |
| Temperature | Higher; lower |
| Pressure | Higher; lower; reverse |
| Level | Higher; lower; none |
| Mixing | Less; more; none |
| Reaction | Higher (rate of); lower (rate of); none; reverse; as well as/other than; part of |
| Phase | Other; reverse; as well as |
| Composition | Part of; as well as; other than |
| Communication | None; part of; more of; less of; other; as well as |
EVALUATING CONSEQUENCES
The consequences of each cause must be carefully analysed to see whether they take the system outside the intended range of operation. It is essential to fully identify all of the consequences, both immediate and delayed, and both inside and outside the section under analysis. It often helps to analyse how the consequences develop over a period of time, noting when alarms and trips operate and when and how the operators are alerted. This allows a realistic judgment on the likelihood and influence of operator intervention.
Where an effect occurs outside the section or stage being analysed, the team leader must decide whether to include the consequences in the immediate analysis or to note the potential problem and defer the analysis to a later, more suitable point, in the overall HAZOP study. Whichever approach is adopted it is important that consequences outside the study section are fully covered, however distant they may be.
SAFEGUARDS (PROTECTION)
There are variations in practice as to when the existing safeguards and protection are noted and taken into account. One approach is first to analyse the outcome ignoring the existence of any safeguards such as an alarm, trip, or vent. Then, when the worst outcome has been identified, the safeguards are noted and the team moves to considering the need for action. This approach has the advantage that the team is alerted to possible serious consequences and misjudgements of the need for protection are less likely. Against this, it can be argued that it is unrealistic to ignore the in-built safeguards of a well-designed operation. Whichever approach is adopted, it is good practice to make note of the safeguards in the detailed records of the study.
PROJECT CASE STUDY
Haldia Refinery refineries of M/s Indian Oil Corporation Limited (IOCL) is a fuels & lubes refinery, having a nameplate capacity of 8,000 thousand MT per annum (TMTPA), processing approximately 70% high sulfur crudes and balance low-sulfur crudes. Altogether, Process units of IOCL HR are categorized in block wise manner into FOB-BLOCK, LOB-BLOCK, DHDS-BLOCK, OHCU-BLOCK, DYIP-BLOCK, OFFSITE, TPS & ETP. The refinery was commissioned in 1975 with a crude capacity of 2,500 TMTPA and thereafter has been revamped in stages several times to the present capacity of 8,000 TMTPA. During the course of time, many revamps and modifications have occurred. As per OISD-GDN-206, Hazop study needs to be revalidated every 5 (FIVE) Years.
In summary, a retrospective HAZOP is a fantastic tool to periodically review process plants in order to identify new risks brought about by major changes to the plant or through a significant number of minor plant changes, to apply learnings from recent incidents from industry and to establish current operability issues. iFluids Engineering has successfully conducted a retrospective HAZOP study of all process units of IOCL, analysing the adequacy of safety & system integrity as per the current infrastructure available. Details on the Process Units is tabulated below.
| SL. No. | NAME OF PROCESS UNIT |
| 1 | Crude Distillation Unit (CDU I) |
| 2 | Crude Distillation Unit (CDU II) |
| 3 | Naphtha Hydro Treating (NHDT) |
| 4 | Catalytic Reforming Unit (CRU) |
| 5 | Kero Hydro Desulphurization (KHDS) |
| 6 | Vacuum Distillation Unit-1 (VDU I) |
| 7 | Propane De-Ashphalting Unit (PDA) |
| 8 | Furfural Extraction Unit (FEU) |
| 9 | Lube Hydro Finishing Unit (LHFU) |
| 10 | Visbreaking Unit (VBU) |
| 11 | Catalytic Iso-Dewaxing Unit (CIDWU) |
| 12 | Vacuum Distillation Unit-11 (VDU II) |
| 13 | BELCO |
| 14 | Fluid Catalytic Cracking (FCCU) |
| 15 | MERICHEM |
| 16 | NHDT |
| 17 | ISOM |
| 18 | Prime G+ |
| 19 | DHDS |
| 20 | Sulfur Recovery Unit-II |
| 21 | Old Amine Regeneration Unit |
| 22 | Sour Water Stripping Unit (Old Chain) |
| 23 | Sour Water Stripping Unit (New Chain) |
| 24 | New Amine Regeneration Unit |
| 25 | Sour Water Stripping Unit |
| 26 | Sulfur Recovery Unit-IV |
| 27 | Sulfur Recovery Unit-III |
| 28 | Sulfur Recovery Unit-V |
| 29 | Amine Regeneration Unit |
| 30 | HGU II |
| 31 | BOC NGU+ BHPV NGU |
| 32 | Sour Water Stripping Unit |
| 33 | DCU |
| 34 | LPGT |
| 35 | CGO-HDT |
| 36 | HGU II |
| 37 | NGU |
| 38 | OHCU |
| 39 | FPU |
| 40 | ETP-1 |
| 41 | ETP-2 |
| 42 | OM&S |