RASM Chapter 5

Information about RASM Chapter 5

Published on July 11, 2014

Author: pthomp

Source: authorstream.com

Content

PowerPoint Presentation: Risk Assessment and Safety Management INHERENTLY SAFER DESIGN Dr Pauline Thompson PowerPoint Presentation: Introduction to ISD Good practice to assess safety at the DESIGN STAGE don’t think about control of hazards afterwards Design team conduct qual /quant risk assessment e.g. HAZOP/FTA etc. Decide if need add on device or fundamental changes Usually best to pre-empt prior to construction PowerPoint Presentation: Inherent Safety Concept Simple idea that a process will be safer if hazards are eliminated or reduced at source don’t rely on bolt on safety devices consistent with hierarchy of risk control measures = common sense! e.g. many examples in books published by T Kletz : Hazard = falling down stairs at home - could build handrails - inherent safety approach would have been to build a bungalow instead! PowerPoint Presentation: Methods of Inherent Safety Some of the ways inherent safety can be achieved: Intensification or minimisation Reduce quantities – it cannot leak if it is not there e.g. minimise storage of hazardous materials specific equipment that requires lower volumes using more efficient equipment can often also be economic savings too Substitution Replace a hazardous material with one less hazardous e.g. choose processing method with safer intermediates Attenuation or moderation Using materials in a less hazardous form e.g. at lower temperatures/pressures/smaller units PowerPoint Presentation: Conflicts in Inherent Safety In practice a number of factors need to be considered: May be possible to improve in one respect but not in another e.g. intensification may lead to operation at a higher temperature/pressure to improve reaction rates and allow smaller reactors BUT this is contrary to the principle of attenuation as materials will be in a more hazardous condition. BUT it may be easier to design a smaller reactor for reliable operation PowerPoint Presentation: Conflicts in Inherent Safety Other example of conflicts include: Storage vs. transportation risks Process vs. product risks Process vs. supplier risks Containment vs. render harmless Reduced pipework vs. segregation Capital vs. running costs PowerPoint Presentation: Reversals in Inherent Safety Sometimes priorities of safety change: e.g . Chlorinated fluorinated carbon compounds (CFCs) replaced more toxic and flammable refrigeration chemicals but when they were thought likely to be linked to ozone depletion the choices were reversed Aerosols containing CFCs are still less dangerous than flammable hydrocarbons PowerPoint Presentation: “Friendlier” processes/equipment Kletz separates inherent safety from “friendlier”: Equipment designed to be difficult to misuse Reduce probability of human error e.g.: not possible to install back to front equipment reserved for specialist purposes e.g. emergencies clearly labelled identified with serial numbers labelled logically status clear from inspection common sense approach PowerPoint Presentation: Case studies - Flixborough 1 st June 1974 killed 28 people on the plant and damaged 2000 buildings up to 2 miles away reacted oxygen+cyclohexane = cyclohexanone+cyclohexanol then used in nylon manufacture in liquid phase at 150°C/10 bars single pass conversion was only ~6% so 6 reactors used each reactor on a plinth with a gravity drop to the next one connected by short pipes with bellows for thermal expansion liquid stirred with a paddle on shaft from top of reactor PowerPoint Presentation: Case studies - Flixborough Lead up to accident: few months before cyclohexane vapour found to be escaping from a leaky stirrer gland on Reactor 5 vapour condensed by pouring over process cooling water contained nitrates – promoted stress corrosion cracking of the mild steel reactor (known problem to metallurgists but not the engineers) damaged reactor removed for repair temporary pipe installed between reactors 4 & 6 narrower 20” longer dog leg pipe installed to allow for height difference (original 28”) pipe supported on scaffold between original bellows PowerPoint Presentation: Case studies - Flixborough Arrangement of temporary pipe PowerPoint Presentation: Case studies - Flixborough Onset of accident: rise in pressure pipe twisted tore the metal bellows liquid cyclohexane at 150°C and 10bar jetted out and vaporised (normal boiling point 81°C) 30-50 tonnes escaped before ignition source encountered 50 secs after leak massive vapour cloud explosion Destroyed plant and office block 100m away where most people were killed Fortunately occurred at ~5pm on a Saturday or more would have died PowerPoint Presentation: Case studies - Flixborough Photo after the accident PowerPoint Presentation: Case studies - Flixborough Lessons learnt: Modifications dangerous if not planned well. Pipework design is a specialised skill. Engineers designed modification “did not know what they did not know”. Bellows should not have been used in the modified design, use was contrary to the manufacturer’s recommendations. Allow expansion but not strong enough for tearing forces. Scaffold allowed more movement than bellows could handle Reduction in pipe diameter from 28 to 20 inches reduced the area for flow to nearly half of the original value, increasing liquid velocity and pressure drop. Presence of a dog-leg in the pipe caused constant twisting force in operation as liquid underwent momentum changes. PowerPoint Presentation: Case studies - Flixborough Lessons learnt: A Safety Management System should cover modifications which ensures that individuals are competent, that proper risk assessment has been carried out and that the plant management are aware of work in progress. Modifications at Flixborough seem to have been performed without written authorisation from a competent authority (e.g. permit to work system) and without the proposals being examined for potential safety problems. The only plan of the installed temporary pipe appears to have been a chalk drawing on the floor! A full risk assessment, including a HAZOP study, should be carried out for such modifications. PowerPoint Presentation: Case studies - Flixborough Far reaching consequences in raising awareness and tightening regulation of chemical plant operations Inherent safety issues at Flixborough: Large inventory of flammable material - this was a consequence of an inefficient reactor design that gave a low single-pass conversion and necessitated a high recycle ratio, giving a large inventory of highly flammable material. Plants should be laid out to protect personnel in the event of an accident. If at all possible, office and control areas, where the majority of employees work, should be located away from the most hazardous areas of the plant. The office block that was destroyed was a standard building with no special protective features and consequently could not survive a large explosion. PowerPoint Presentation: Case studies - Bhopal 3 rd Dec 1984 - 2,000 people killed and 10,000 injured (though final toll may have been 10,000 killed and 200,000 injured) owned by Union Carbide, now part of Dow Chemical Company made Sevin brand carbaryl pesticide from methyl isocyanate (MIC) made on site and stored in intermediate storage tanks water + chloroform entered storage tank and reacted with MIC water source = controversial (= leaky valve/vandalism?) cooling system not working? scrubber turned off? flare stack turned off for repair? water curtain for vapour not high enough? increased temperature and pressure released a cloud of MIC drifted into shanty town where the majority of the victims lived and thousands of people were exposed to the toxic MIC gas PowerPoint Presentation: Case studies - Bhopal Whatever the cause the following points are still valid. Inherent safety at Bhopal: Inventories could have been reduced. MIC was an intermediate and allowed to accumulate in large quantities. Closer control to balance production / utilisation may have eliminated need for intermediate storage. An alternative process route could have been used to avoid such a toxic intermediate (substitution). Plant should have been located far away from a population centre. Gas would have dispersed and been diluted with air and the number of persons affected would have been lower . PowerPoint Presentation: Summary Inherent safety - safer if hazards can be eliminated or reduced at source rather than relying on extra safety devices. T hree ways are 1) intensification or minimisation, 2) substitution and 3) attenuation or moderation. Compromise and conflicts are common. Life-cycle assessment may be needed to ensure that the overall risk is reduced. Designed to be “friendlier” as well as more inherently safe. ISD could have helped Flixborough and Bhopal accidents. Systematic ISD required at beginning of the design process. Currently not widely used - toolkits and aids are available Restraints include prescriptive regulations, time limits, focus of companies on capital expenditure rather than operational expenditure. PowerPoint Presentation: Discussion Questions Now we recommend: Read through these sections in the notes Tackle the discussion the questions at the end Work with your fellow students on the discussion boards PowerPoint Presentation: Any Questions ? If so – post them in the discussion boards

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