Análisis Ambiental

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I U the ReleaM Ia There Immediate1 I Instantaneous? Ignition? Does a Pool Form? Does the Pool Ignite?

FIGURE 1.12. Spill event tree for a flammable liquid release.

Since these bounds can be established without exhaustively examining all of the incident outcome cases possible, the experienced analyst can manage the number of cases to be examined without compromising thedesire to develop a quantitative understanding of the range—a feel for spread—of the risk estimate. 1.4.3. Tracking The development of some risk estimates, such as individual risk contours or societal risk curves requires a significant number of calculations even for a simple analysis. This can be time consuming if a manual approach is employed for more than a few incident outcome cases. Chapter 4,Section 4.4, describes risk calculation methods and provides examples of various simplifiied approaches. The techniques are straightforward, however many repetitive steps are involved, and there is a large potential for error. A computer spreadsheet or commercial model is generally useful in manipulating, accounting, labeling, and tracking this information. The case studies of Chapter 8 illustratethese grouping, accounting, labeling, and tracking processes.

1.5. Applications of CPQRA
No organization or society has the resources to perform CPQRAs (of any depth) on all conceivable risks. In order to decide where and how to use the resources that are avail-

able, it is necessary to select specific subjects for study and to optimize the depth of study for each subject selected. Thisselection process or screening technique is discussed (Section 1.5.1) along with its use for existing facilities (Section 1.5.2) and new projects (Section 1.5.3). 1.5.1. Screening Techniques In creating a screening program, it is helpful to determine the organizational levels that are most amenable to screening, and those where CPQRAs can be applied most effectively. Figure 1.13 illustrates thestructure of a typical CPI organization. It shows a hierarchical scheme, with the organization divided into facilities (plants), the facilities divided into process units, the process units divided into process systems and the process systems divided into pieces of equipment. A general observation is that the number of possible CPQRAs increases exponentially—but that the scope of each one narrows—movingfrom the top to the bottom of the hierarchy. Use of CPQRA is typically restricted to the lower levels of the hierarchy, and in those levels it is selectively applied. Methods are needed to screen—prioritize and select—process units, systems, and equipment for selective application of CPQRA. These methods must ensure that all facilities are considered uniformly in the screening process.Establishment of a prioritized listing of candidate studies allows efforts to focus on the most onerous hazards first and, depending on available resources, progress to less serious hazards. Certain listings are "zoned35 according to high, medium, and low levels of concerns, and studies placed into the lowest class receive attention only after all studies in higher classes have been executed. If a decisionis made to zone a priority list, it is important to establish zone cutoff criteria prior to screening in order to avoid bias. Bask estimates can be developed at any level of the typical CPI organization, but usually focus on specific elements of the lower levels of the hierarchy—for instance, the COMPANY HEADQUARTERS

MANUFACTURING FACILITIES

PROCESS UNITS PROCESS SYSTEMS PROCESS EQUIPMENTFIGURE 1.13. Structure of a typical CPI company.

risk from the rupture of a storage tank. The following discussions of screening methods show that methods are available to study various levels of the typical CPI organization. 1.5.1.1. PROCESS HAZARD INDICES Dow Chemical has developed techniques for determining relative hazard indices for unit operations, storage tanks, warehouses, etc. One...