Release of Toxic Vapor

Bums and Hazzan demonstrated the use of event tree and fault tree analysis in tlie study of a potential accident sequence leading to a toxic vapor release at an industrial chemical process plant. Tlie initiator of tlie accident sequence studied is event P, the failure of a plant programmable automatic controller. Tliis event, in conjunction willi the success or failure of a process water system (a glycol cooling system) mid an opcmtor-nianual shutdown of tlie distillation system produced minor, moderate, or major release of toxic material as indicated in Fig. 

Tlie symbols W, G, O represent tlie events listed  

W failure of process water stcni G failure of glycol cooling system 

O failure of operator-niaiiual shutdoum of the distillation system 

Tlie symbols W. G, and O, the complements of W, G, and O, represent tlie corresponding successes. 

O failure of operalor-nianual shutdown of the distillation system 

G15CMB/G15CAM - 15 C glycol cooling system (GC) circulation pumps B/A mechanical failure 

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The seven single failures causing moderate release of toxic vapor... 

The second case study (Section 21.3), focusing on tlie healtli impact of the release of toxic vapors during transportation of a hazardous chemical, illustrated tlie derivation of a risk curve showing tlie relation between number of people affected and average aimual frequency of exposure to a potential letlial concentration of toxic vapors. 

In Section 21.4 tlie effects of the release of toxic vapors were considered in connection witli an accident sequence initiated by the failure of a plant programmable automatic controller. In tliis study, event tree analysis and fault tree analysis led to identification of tlie glycol cooling system circulation pumps as components meriting high priority for inspection. 

The safe operation of chemical facilities and protection of the people and the environment surrounding them is a requirement for creating long-term social and economic benefits for the community and industry. Many of the major accidents in the chemical process industry have involved the release of toxic vapors or ignition of flammable vapors. The purpose of this book is to address current engineering methods in practice for minimizing the consequences of these types of releases, once the event has occurred. 

Tlie healtli impact of a release of toxic vapor varies with the severity of tlie release, tlie population density along tlie route of tlie tank car, and weather conditions affecting dispersion. Table 21.3.3 shows, for each degree of severity, hypothetical estimates of tlie number of people affected, tlie likelihood of exposure to a potentially letlial concentration, and tlie product of tliis likelihood by the average aimual frequency of each release severity. Tliis latter product represents average annual frequency of e.xposure to a potentially letlial concentration. Average aiuiual frequency of exposure is plotted against tlie number of people affected in tlie risk curve shown in Fig. 21.3.1. 

HAZARD RISK Potential release of toxic vapors and solutions pose a threat to public safety flammable NFPA Code H 2 F 3 R 0. 

A crane accident in a Marathon Refinery resulted in a dropped object on a hydrofluoric reactor vessel and a large release of toxic vapor. The impact was limited. 

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