Quantitative risk analysis evaluation

The acronym for chemical process quantitative risk analysis. It is the process of hazard identification followed by numerical evaluation of incident consequences and frequencies, and their combination into an overall measure of risk when applied to the chemical process industry. It is particularly applied to episodic events. It differs from, but is related to, a probabilistic risk analysis (PRA), a quantitative tool used in the nuclear industry... 

Quantitative risk analysis (QRA) is a powerful analysis approach used to help manage risk and improve safety in many industries. When properly performed with appropriate respect for its theoretical and practical limitations, QRA provides a rational basis for evaluating process safety and comparing improvement alternatives. However, QRA is not a panacea that can solve all problems, make decisions for a manager, or substitute for existing safety assurance and loss prevention activities. Even when QRA is preferred, qualitative results, which always form the foundation for QRA, should be used to verify and support any conclusions drawn from QRA. 

The American Chemistry Council, formerly the Chemical Manufacturers Association (CMA), and the American Institute of Chemical Engineers Center for Chemical Process Safety (AIChE/CCPS) have jointly published Evaluating Process Safety in the Chemical Industry User s Guide to Quantitative Risk Analysis. This is a revised and updated edition of Evaluating Process Safety in the Chemical Industry A Manager s Guide to Quantitative Risk Analysis, published in 1989 by CMA. 

Chemical Process Quantitative Risk Analysis(CPQRA) The numerical evaluation of both incident consequences and probabilities or frequencies and their combination into an overall measure of risk. 

This chapter provides general information for performing qualitative or quantitative risk assessments on buildings in process plants. For detailed guidance on risk assessment techniques, the user is referred to other CCPS books on this subject, including Reference 3, Guidelines for Hazard Evaluation Procedures, Second Edition, and Reference 4, Guidelines for Chemical Process Quantitative Risk Analysis. 

The terminology used varies considerably. Hazard identification and risk assessment are sometimes combined into a general category called hazard evaluation. Risk assessment is sometimes called hazard analysis. A risk assessment procedure that determines probabilities is frequently called probabilistic risk assessment (PRA), whereas a procedure that determines probability and consequences is called quantitative risk analysis (QRA). 

It is difficult to identify all of the possible events and their consequences in a complex chemical processing plant without the application of systematic procedures and proper management techniques. Several hazard evaluation procedures have been developed. Most of these procedures are described in other AIChE/CCPS publications such as Guidelines for Hazard Evaluation Procedures [2,3] and Guidelinesfor Quantitative Risk Analysis [4]. Other publications on hazard evaluation techniques include [246,247]. 

GENERAL References AICHE/CCI S Guidelines for Chemical Process Quantitative Risk Analysis, 2d ed., American Institute of Chemical Engineers, New York, 2000. AICHE/CCPS, Guidelines for Hazards Evaluation Procedures, 2d ed., American Institute of Chemical Engineers, New York, 1992. Crowl and Louver, Chemical Process Safety Fundamentals with Applications, 2d ed., Prentice-Hall, Englewood Cliffs, N.J., 2002. Mannan, Lees Loss Prevention in the Process Industries, 3d ed., Elsevier, Amsterdam. 

Logic Model Methods The following tools are most commonly used in quantitative risk analysis, but can also be useful qualitatively to understand the combinations of events which can cause an accident. The logic models can also be useful in understanding how protective systems impact various potential accident scenarios. These methods will be thoroughly discussed in the Risk Analysis subsection. Also, hazard identification and evaluation tools discussed in this section are valuable precursors to a quantitative risk analysis (QRA). Generally a QRA quantifies the risk of hazard scenarios which have been identified by using tools such as those discussed above. 

The best avenue is to use input data that would be considered the WCCE for the incident under evaluation. One should then question if the output data provided is realistic or corresponds to historical records of similar incidents for the industry and location. In other cases where additional analysis is needed, several release scenarios (small, medium and large) can be examined and probabilities can be assigned to each outcome. This would then essentially be an Event Tree exercise normally conducted during a quantitative risk analysis. Certain releases may also be considered so rare an event they may be outside the realm of accepted industry practical protective requirements. 

Several qualitative approaches can be used to identify hazardous reaction scenarios, including process hazard analysis, checklists, chemical interaction matrices, and an experience-based review. CCPS (1995a p. 176) describes nine hazard evaluation procedures that can be used to identify hazardous reaction scenarios-checklists, Dow fire and explosion indices, preliminary hazard analysis, what-if analysis, failure modes and effects analysis (FMEA), HAZOP study, fault tree analysis, human error analysis, and quantitative risk analysis. 

For those issues or transportation scenarios requiring more detail and insight than a qualitative approach offers, the next level of analysis is a semi-quantitative evaluation. A semi-quantitative risk analysis includes some degree of quantification of consequence, likelihood, and/or risk level. Once evaluated, through the combination of consequence and likelihood, a risk-based decision can be made from the analysis or additional refinement in consequence or likelihood may be necessary. This refinement of the risk analysis may rely on additional information included as part of a semi-quantitative analysis or, as warranted, on some minimal level of quantitative techitiques described in the next chapter. 

At this point in the example, the eompany-wide reviews eompleted in Chapters 2 and 3 are replaeed with a first-level qualitative risk analysis foeused on the transportation aetivities of a single XYZ Cheinieal faeihty. To eontinue the example, a single XYZ Chemical facility is selected. This facility will continue to be evaluated as the level of analysis detail increases from qualitative to semi-quantitative in this chapter to quantitative risk analysis (Chapter 5). As the level of detail increases, the analysis will be directed at specific questions that remain following each level of analysis. This facility will continue to be the focus of Chapter 6 where the security vulnerabihty of the hazardous materials in transit is evaluated. Chapter 7 where risk reduction options are evaluated, and Chapter 8 where the ongoing management of risk continues in the future. 

One of the major benefits of a semi-quantitative risk analysis is that the technique can be applied and results understood by a wide range of stakeholders in the transportation field. Unlike full quantitative approaches, these types of risk analyses do not require specialized risk management experts. Even with the ease of application, however, personnel involved in these activities need to be knowledgeable in the operations under evaluation and the use of semi-quantitative... 

While qualitative techniques represent a very basic analysis, with little modification, they can be expanded and evaluated in a semi-quantitative risk analysis. Additionally, events and scenarios elevated for further analysis can be further evolved by considering the range of potential consequences (small to large) and/or likelihood (low to high probability of occurrence). 

In addition to the risk reduction benefits, the costs of risk mitigation options need to be evaluated. Due to the uncertainties associated with semi-quantitative and quantitative risk analysis results, a relative risk comparison, as compared to absolute measures of risk and benefits, is recommended. To conduct this type of relative comparison, incremental risk analysis can be used to evaluate the cost effectiveness of risk mitigation options, or determine the optimal combination of risk mitigation options. Figure 7.4 illustrates example results of this type of analysis, and uses the options from the F-N curve in Figure 7.3 as the basis for comparison. 

As part of the semi-quantitative risk analysis report to corporate, the analysis team included an evaluation of the recommended risk mitigation options. For chlorine, the second recommendation was not practical because the end user required ton containers thus, the transport of smaller cylinders was not an option for the customer. Therefore, this recommendation was removed from further consideration. Since ton containers are shipped via truck, an alternate route was discussed with the carrier—a new route that eliminated passing near higher-population densities with little change in the distance or conditions of the route. 

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