Accident Precursor Analysis and Management Reducing Technological Risk Through Diligence (2004) / Chapter Skim
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Section IV Risk Management7 Inherently Safer Design
Section IV Risk Management7 Inherently Safer Design
Pages 101-118
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Section IV Risk Management
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From page 103... ...
An example of how m mcident-investigation team responds to m accident precursor, in this case m example from the chemical process industry, will illustrate the application of this desigm philosophy, When designmg or operating any engineered system, whether a chemical plant, a consumer product, a machine, or any other system, the desigmer must first identify specific hazards associated with the operation. Preferably, formal hazard-identification techniques are used to identify hazards as pa t of the design process.
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The purpose of using these tools is to identify hazards md the specific accident scenarios associated with them before m accident occurs. In some cases, however, hazards or potential accident scenarios are identified through the recogmtion of accident precursors during operation.
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—A string trimmer would elimmate the hazard of a sharp, rapidly rotatmg cutting blade for cutting grass. (Sheep or goats might be considered an inherenrdy safer and more environmentally friendly technology for keeping grass trimmed.
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INEIERENTLY SAFER DESIGN STRATEGIES Four main strategies minimize, moderate, substitute, and snmplify have been developed to help designers identify inherently safer systems. The notemrzmg strategy involves reducing the amount of hazardous material or energy in the system, ideally to the pomt that the uncontrolled release of he entire inventory of material or energy would not cause significant damage.
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The new mimufacturmg process was continuous, essentially a "phosgene machine" When the consumer process needed phosgene, the new contmuous process was sta ted up and brought quickly to steady state to produce acceptable quality phosgene; the phosgene was then fed directly to the consumer process with no mtermediate storage (Delseth, 1998; Osterwalder, 1996)
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A fmal example of the moderating strategy is of a company that developed a process for m mufacturmg methyl acetate that used reactive distillation to reduce the number of major pieces of equipment from eight columns, one extraction
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11111ERENTLYSAFER DESIGN ~ Controls FIGURE I Poorly designed kitchen stove. Source: Normiut, 1992.
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and to allow people to disarm air bags (an inherent response that elimmated the hazard but exposed occupants to mcreased risk from a different hazard in the event of a serious collision)
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IPIHERE~LYSAFER DESIGN :3 enomue ~ _ Sullune rein FIGURE 3b 111 FIGURE 3 Process for manufacturmg methyl acetate.
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This is the inherently safer design approach. For a chemical process, for example, many checklists are available for specific types of equipment to help the desigmer identify inherently safer design strategies (CCPS, 1998)
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The exact circumstances that led to a specific incident are unlikely to recur in precisely the same way, even if nothmg is changed But smiler incidents are likely to occur, and lessons from m event m one facility c m result m rmprovements in other facilities that appear to have lithe m common with the facility where the incident occurred. CASE STUDY This precursor event at a chemical processmg pEmt illustrates how the design strategies I've described c m be applied.
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From page 114... ...
But other possibilities were also investigated, and subsequent laboratory work revealed that the cause was a decomposition reaction of the material in the pipe. The reaction was promoted by the presence of water in the combined waste stream and was initiated by a failure of the steam-pressure controller, resulting in maximum steam pressure and temperature on the pipe heat tracing.
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However, if existing vessels are replaced m the future, or if a new plamt is built, it may be feasible to build stronger vessels (higher design pressure) to conriun potential decomposition reactions.
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Furthemmore, this incident is an example of a general concern about reactive chemistry hazards, md particularly, reactive chemical st uctures. The investigation results were shared throughout the company, along with additional infommation about structures where the possibility for decomposition reactions should be evaluated.
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From page 117... ...
Responding to accident precursors requires a combmation of management and engireering-de iigm activities. Accident precursors provide new information about hazards, potential accident scenarios, and the effectiveness of existmg safeguards.
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