HAZOP

Hazard identification is the first step in a systematic process which aims toimprove the management of risk

Several of these identification methods are incorporated into Process Hazards Assessment

A HAZOP is a group technique for identifying hazards and operability problems. It can be applied to operating process plants and to plants in various stages of design

Hazard and Operability Studies OverviewOriginated as a hazard identification technique for process plants developed by ICI in mid 1960salso commonly applied in petrochemical, nuclear and food processing industriesDescribed as a system of imaginative anticipation of hazardsDiffers from Functional Failure Analysis in 4 key ways:always a team activityconcentrates on deviations in flows between componentsuses well-defined guide words to steer analysisconsiders both plausible causes and possible consequences of deviations

HAZOP technical approachIn the process industries, HAZOP is normally applied once the piping and instrumentation (P&I) diagram has been producedScope of study explicitly extends to cover unusual circumstances such as startup, shutdown and plant maintenanceScope of study does not include making detailed design changes, although recommendations and follow-up questions should be producedAs with FHA, results are recorded in a tabular formatNo reason why HAZOP cannot be applied to any system in which flows can be identified including information flow in software

HAZOP procedure

HAZOP Team Members 1Leaderplanning and preparationact as chairman for meetingsign off documentationensure follow-up work is completedRecorderparticipant in study (?)document the analysisDesigner(s), process / project engineersunderstand and explain the plant designanswer questions about the plant and processUser(s)may be site representative, operator, maintenance crewsupply information about the context in which the system will be used, e.g. site facilitiesask questions, and help decide which issues affect safetyExpert(s)key function is to exploreask questionssuggest deviations / causes / effectsneed good knowledge of process chemistry, or experience of similar plantTeam may recruit extra members (technical specialists) if required

HAZOP Team Members 2Team may recruit additional members when required

Typically to answer specific questions, e.g.control engineermechanical engineeroccupational health / hygienistenvironmental specialists

Different teams often appropriate for different stages of the lifecycle, but continuity (e.g. same leader) usually recommended

HAZOP guide wordsPhysical properties Temperature Flow rate Pressure Reaction rate Viscosity

HAZOP guide words 2Guide word Other Than has many interpretationsthis word particularly is often defined specifically for particular studyexamples includeplant operation phasesstartup, shutdown, maintenance, sampling and inspectioninstrumentationcontrol philosophy, location of instruments, alarms and tripsfailure of plant servicescooling water, fuels, power suppliesother external factorsweatherspare / non-installed / portable equipmentsafetyfire and leak detection, emergency planning

HAZOP example a bit of P&I

HAZOP example output

Hazard Study Phasing 1

Hazard Study Phasing 21 - Concept StudyPHI, checklist, materials and reagents study, first outline HAZOP2 - Process Flow StudyHAZOP3 - Detailed Design (P&I) StudyHAZOP4 - Detailed Design (Physical) StudyHAZOP update, Sneak, Fault Trees, FMEA5 - Audit of installed plant against designChecklist, review of HAZOP actions, inspections6 - Audit / review of processFinal review of evidence, completeness of HAZOP actions

HAZOP Pros and Cons Advantagesvery methodical. Use of guide words can give confidence in completeness of analysis.has both inductive and deductive phasesteam conclusions may carry more weight than those reached by individual analysts

Disadvantagescan produce lots of output. Structure is only obtained by reference back to P&I diagramsteam approach is expensive must be shown to be cost-effective

ICI ExperienceMore operability problems than hazards are usually foundthough difference is smaller with existing plants

Number of post-commissioning modifications to plant is reduced

Time to achieve intended production rates is reduced

For major plant, cost is typically about 1% of design cost0.1 - 0.2% of total plant cost