Accidents steps involved

The frequency analysis step involves estimating the likelihood of occurrence of each of the undesired situations defined in the hazard identification step. Sometimes you can do this through direct comparison with experience or extrapolation from historical accident data. While this method may be of great assistance in determining accident frequencies, most accidents analyzed by QRA are so rare that the frequencies must be synthesized using frequency estimation methods and models. 

Using the techniques referenced in Section 5.2, a detailed list of potential accident scenarios can be prepared. This list can then be refined to give the minimum number of scenarios that need to be assessed to adequately reflect the spectrum of possible incidents and to satisfy the requirements of the study. The steps involved may include ... 

Safety analysis is a process of preventing accidents which involve operation of system under development or it is a process of (a postdevelopment) demonstration of the system safety properties. Four major steps of the process are distinguished to identify (understand and model) (compare [N.G. Leveson, (1995)], [N.G. Leveson, (2004)], [J. Zalewski and all, (2003)]) ... 

Large-scale experimental releases of fission product activity are clearly ruled out because of the implications on the safety of the public, described in Section V,F, as are also the smaller scale releases referred to earlier in Section V. Therefore, for verification of our conclusions we have to rely on limited experience from those few accidents that have occurred and that have released fission products (see Section I,D), but much more on our store of knowledge of all the factors involved, i.e., types of reactor accidents (Section II) through fission product release (Section III) and dispersion of a release in the atmosphere (Section IV), to analysis of the radiation and radiobiological hazards and risks to exposed members of the population. In view of these several steps involved in the estimation of hazard, it is reassuring that the many different authors who have written on the topie reach conclusions which are generally similar and differ only in limited areas. 

Safety is always an important part of job training and especially so in a chlor-alkali plant. Trainees should be encouraged to pay close attention to the safety aspects of design and operation. One method recommended by King [7] is the use of job safety analysis (JSA). This isaformalized procedure in which analysts list the sequence ofbasic steps involved in a given task, consider the potential accidents that can occur, and then develop strategies and techniques of prevention. The National Safety Council of the United States [8] has developed standardized forms for JSA. 

This step involves a more detailed consideration of the exposure/impact assessment combined with the assessment of the likelihood of accidents and resulting consequences. This helps to evaluate the acceptability of current practices and procedures. 

The steps involved in a JSA/JHA process have been outlined in the previous pages. It should be especially clear that the main point of doing a JSA/JHA is to prevent accidents by anticipating and eliminating hazards. JSA/JHA is a procedure for determining the sequence of basic job steps, identifying potential accidents or hazards, and developing recommended safe job procedures. Table 12.2 provides an example JSA/JHA. 

The first three steps involve the execution of a risk assessment. An example of how this may be carried out will be presented in Chapter 24. We will here focus on the principles behind the legislation in the selection of safety measures or barriers. There is a clear relationship between the different types of safety measures according to EN 292 and Haddon s strategies for accident prevention (see Table 7.3). 

The next part of the procedure involves risk assessment. This includes a deterrnination of the accident probabiUty and the consequence of the accident and is done for each of the scenarios identified in the previous step. The probabiUty is deterrnined using a number of statistical models generally used to represent failures. The consequence is deterrnined using mostiy fundamentally based models, called source models, to describe how material is ejected from process equipment. These source models are coupled with a suitable dispersion model and/or an explosion model to estimate the area affected and predict the damage. The consequence is thus determined. 

In Steps 2 through 5 of Figure 5 you will use subjective judgment to consider whether the situation involves major hazards, familiar processes, large consequence potential, or frequent accidents. The... 

The cost of performing the hazard identification step depends on the size of the problem and the specific techniques used. Techniques such as brainstorming, what-if analyses, or checklists tend to be less expensive than other more structured methods. Hazard and operability (HAZOP) analyses and failure modes and effects analyses (FMEAs) involve many people and tend to be more expensive. But, you can have greater confidence in the exhaustiveness of HAZOP and FMEA techniques—their rigorous approach helps ensure completeness. However, no technique can guarantee that all hazards or potential accidents have been identified. Figure 8 is an example of the hazards identified in a HAZOP study. Hazard identification can require from 10% to 25% of the total effort in a QRA study. 

However, in the case of a root cause analysis system, a much more comprehensive evaluation of the structure of the accident is required. This is necessary to unravel the often complex chain of events and contributing causes that led to the accident occurring. A number of techniques are available to describe complex accidents. Some of these, such as STEP (Sequential Timed Event Plotting) involve the use of charting methods to track the ways in which process and human events combine to give rise to accidents. CCPS (1992d) describes many of these techniques. A case study involving a hydrocarbon leak is used to illustrate the STEP technique in Chapter 7 of this book. The STEP method and related techniques will be described in Section 6.8.3. 

The first step in a process plant building risk assessment is to identify specific accident scenarios that endanger building occupants. As discussed in Chapter 2 and illustrated in Table 2.1, accident scenarios are sequences of events that lead to an outcome of concern. The specific outcomes of concern are those involving explosions or fires that could impact buildings in process plants. 

Every operation is covered by a SOP. These procedures delineate the step by step process to be followed in conducting a hazardous operation. They identify and specify the safety equipment and clothing to be employed and the emergency procedures to be followed if an accident occurs. They identify the responsible individual for the operation and specify the number of operating personnel that can be present. The SOP is prepared by the operating personnel, reviewed by the laboratory director, reviewed by and co-approved by the Preventive Medical Activity when health hazards are involved, and approved by the MIRADCOM safety office. 

This chapter examines die history of accidents from early incidents to recent catastrophes. In conjunction widi diis review, die material will study the evolution of safety precautions, particularly as diey apply to chemical plants. A crucial part of any design project is the inclusion of safety controls. Wliedier die plans involve a chemical plant, a nuclear reactor, or a dmiway, steps must be taken to minimize the likelihood, or eonsequences, of accidents. It is also important to realize how accident plaiming lias improved in order to monitor today s adi anced teclmologies. This cliapter reviews a variety of actual accidents in order to provide an understanding of diese phenomena, which will supplement the subsequent chapters tliat deal widi diese subjects in significant detail. 

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