Process Accident Consequence Analysis

The analysis of the potential consequences of an accident is a useful way of understanding the relative inherent safety of process alternatives. These consequences might consider, for example, the distance to a benchmark level of damage resulting from a fire, explosion, or toxic material release. Accident consequence analysis is of particular value in understanding the benefits of minimization, moderation, and limitation of effects. This discussion includes several examples of the use of potential accident consequence analysis as a way of measuring inherent safety, such as the BLEVE and toxic gas plume model results shown in Figures 4, 5, and 6. 

Risk is defined as the combination of the expected frequency and consequence of accidents that could occur as a result of an activity. Risk analysis is a formal process of increasing one s understanding of the risk associated with an activity. The process of risk analysis includes answering three questions ... 

This section reflects on the limitations of the PSA process and draws extensively from NUREG-1050. These subjects are discussed as plant modeling and evaluation, data, human errors, accident processes, containment, fission product transport, consequence analysis, external events, and a perspective on the meaning of risk. 

Cause-consequence risk evaluation combines event tree and fault tree analysis to relate specific accident consequences to causes. Tlie process of cause-consequence evaluation usually proceeds as follows ... 

Risk is defined as a measure of human injury, environmental damage, or economic loss in terms of both the incident likelihood (probability) and the magnitude of the loss or injury (consequence) (AICHE/CCPS, Guidelines for Chemical Process Quantitative Risk Analysis, 2d ed., American Institute of Chemical Engineers, New York, 2000, pp. 5-6). It is important that both likelihood and consequence be included in risk. For instance, seat belt use is based on a reduction in the consequences of an accident. However, many people argue against seat belts based on probabilities, which is an incorrect application of the risk concept. 

In designing the Process Safety Management standard (PSM), OSHA looked at overall process safety from a broad system s view and identified 14 key elements that industry should address to minimize catastrophic accidents. The 14 elements cover the things that can cause a process failure and accidents. Some of the major causes of process accidents are a lack of training, lack of information about process equipment, lack of equipment inspections, poor coordination with contractors, and lack of employee participation in process planning and implementation. The program is triggered by above-threshold quantities of any of 136 chemicals. The purpose of the standard is to minimize the consequences of a catastrophic release of a toxic, flammable, reactive, or explosive chemical. The importance of this standard is that it requires safety analysis and names certain analytic techniques to use or their equivalent. 

If a flammable gas is released, a fire or an explosion may occur. Fires and explosions can take place inside the containment (vessels, pipework etc.) of a process plant and after release (vid. Fig. 10.1). The former are the subject of the analysis of the engineered systems of the plant, the latter the concern of accident consequence calculations. In the former case the expected frequency of the undesired events fire and explosion must be determined, the reasons for their occurrence and possible coimtermeasures as well as potential consequences such as the flight of fragments (vid. Sect. 10.9) and the possibility of impacts on other parts of the plant. In the latter case the consequences are considered. Mainly these are treated below. 

Level-2 PSA tasks (consequence analysis) are underway. The current effort for the invessel physical process includes a preliminary analysis of an event tree of ULOF accident in a large LMFBR particularly reflecting recent experimental and analytical knowledge. For the exvessel physical process, an analysis of source terms has been continued postulating a reactor vessel melt through event. A debris-concrete interaction has been mainly investigated by means of a sensitivity analysis of the relation between the initial conditions such as debris mass and temperature and the quantity of radiological nuclides to be released. 

Owners and operators of facilities the produce, process, and store extremely hazardous substances must develop a risk management plan (RMP) including an executive summary, registration information, off-site consequence analysis, five-year accident history, prevention program, and emergency response program. 

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