UNDERSTANDING THE ASSUMPTIONS AND LIMITATIONS

Quantitative risk analysis is subject to several theoretical limitations. Table 13 lists five of the most global limitations of QRA. Some of these may be relatively unimportant for a specific study, and others may be minimized through care in execution and by limiting one s expectations about the applicability of the results. However, you must respect these limitations when chartering a QRA study and when using the results for decision-making purposes. 

The hazard evaluation step is where the issue of completeness primarily arises. It is impossible for the QRA analyst to identify and model all of the things that can possibly go wrong, and it is impractical to evaluate the frequency and consequences of every identified event. But you can reasonably expect trained and experienced practitioners using systematic 

Completeness There can never be a guarantee that all accident situations, causes, and effects have been considered. 

Model Validity Probabilistic failure models cannot be verified. Physical phenomena are observed in experiments and used in model correlations, but models are, at best, approximations of specific accident conditions. 

AccuracyAJncertainty The lack of specific data on component failure characteristics, chemical and physical properties, and phenomena severely limit accuracy and can produce large uncertainties. 

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In deriving equations, I have strived for rigor, clarity, and a minimum of mathematical complexity. I have attempted to clearly state the conditions under which each theoretical relation is valid because only by understanding the assumptions and limitations of a derivation can one know when to use the relation and how to adapt it for special purposes. I have taken care to be consistent in the use of symbols for physical properties. The choice of symbols follows the current recommendations of the International Union of Pure and Applied Chemistry (lUPAC) with a few exceptions made to avoid ambiguity. 

In all but a few cases derivations of equations and the method of analysis have been given so that the engineer will understand the assumptions and limitations involved. Also, example calculations and designs have been included to illustrate the use of the relationships and recommended procedures. 

If this assumption is correct, and only key areas of legislative control are necessary, can deregulation be a viable alternative to limitless prescriptive control Understanding the constraints and limitations that the chemical industry operate under may help future legislators act with balanced consideration of both the needs of the environment and the future of the chemical industry. 

Nowadays, commercial finite element programs are quite powerful and user friendly codes which are available for all platforms of operating systems. However, reliable results can only be obtained when the user of such codes understands the problem, how to model it, the behavior of finite elements, the assumptions and limitations of the code, and when the analyst is able to check for errors at all stages. It is not necessary that the end-user understands in detail all procedures and routines, but incorrect results, which may cause serious damage such as production delay, redesign or even collapse, can be avoided only by those who understand the basic ideas and single steps of the FEM (cf. O Chap. 25). 

Any model is as good as the assumptions that it is built upon. Assumptions are needed to simplify the model. It is important to understand the assumptions in order to understand the model s limitations and to accurately interpret its results. Common assumptions used in fuel cell modeling are ... 

Another way to evaluate risks is to calculate the sensitivity of the total risk estimates to changes in assumptions, frequencies, or consequences. Risk analysts tend to be conservative in their assumptions and calculations, and the cumulative effect of this conservatism may be a substantial overestimation of risk. For example, always assuming that short-term exposure to chemical concentrations above some threshold limit value will cause serious injury may severely skew the calculated risks of health effects. If you do not understand the sensitivity of the risk results to this conservative assumption, you may misallocate your loss prevention resources or misinform your company or the public about the actual risk. 

When used judiciously, the advantages of QRA can outweigh the associated problems and costs. Companies that prudently commission QRAs and conscientiously act on the resulting recommendations are better off for two reasons (1) they have a better base of information to make decisions and (2) their judicious use of QRA technology represents another demonstration of responsible concern for the health and safety of workers and the public. However, companies should resist the indiscriminate use of QRA as a means to solve all problems since this strategy could waste safety improvement resources, diverting attention from other essential safety activities. Once executives are able to interpret and use QRA results, they will appreciate that the quality of their decisions rests largely on their ability to understand the salient analysis assumptions and the limitations of the results. 

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