Credible scenarios

It may not be credible that multiple simultaneous failures and maloperations occur. In considering what is credible, the following points may be useful  

It will then be necessary to check whether or not the credible maloperations can lead to exothermic runaway. Information on this is given in reference 1. Where it is shown that runaway can occur, and it is decided that emergency pressure relief may be used as part of the basis of safety, then it will be necessary to carry out further work to identify the worst case for relief system sizing.  

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All single events that can actually and reasonably occur are credible scenarios. 

In Chapter 3, it was noted that remarkably few biochemical pathways lead from Basic Integrated Metabolism into the pathways that serve to produce substances with Type 11 and Type 111 properties. Using the concepts discussed, it is possible to offer one evolutionary scenario to explain why a few, rather than many pathways flow from Basic Integrated Metabolism. As in the case of most evolutionary arguments, it is not difficult to produce a credible scenario but hard to find supporting evidence simply because the postulated events would have occurred early in the evolution of life and there are no known biochemical fossils . However, some computer modelling has at least provided some support for the approach adopted. 

For the purposes of the Workbook, the worst case scenario is the credible combination of equipment failures and maloperations that gives rise to the largest calculated relief size compared with other credible scenarios. See Chapter 3. The worst case scenario is the basis for the relief system design. 

Once a credible scenario has been established, it is not recommended to take into account the use of instrumentation as a means of reducing the relieving load. 

Singh, J., Batch Runaway Reaction Relief Re-evaluation of credible Scenario, Process Safety Prog., Vol. 16, No. 4, pp. 255-261, 1997. 

Figure 1. Flow chart of the procedure for the identification of credible scenarios in Natech events (Cozzani et al. 2007).

B), and (C), respectively for minor, intermediate and severe LOC. In the present approach, credit factors should not be intended as failure frequencies of the equipment items, but only as factors summarizing the past performance of similar equipment items. These factors were introduced to allow the identification of worst credible scenarios in the analysis. 

The unit inherent hazard index UHI), is introduced to consider the safety scores of the equipment, expressed by the credit factors cfi,k)- Thus, the UHI is mostly determined by the estimated impact area of the worst credible scenario ... 

Uncertainty Evaluation/States of Nature The most credible scenarios for these three units can be found in Khan et al (2002) and its consequences has been summarized in Table 2. The most credible scenario for the first separator is a BLEVE followed by fire, for the Flash Drum unit is a VCE followed by fire and for the Drier unit the most credible scenario is a BLEVE followed by fire. These most credible... 

A scenario that has a relatively high probability of occurring for the situation under examination. As part of emergency response planning, credible scenario exercises or pre-planning is undertaken. In order to avoid undue and unrealistic postulation of events that may occur, a collection of credible scenarios is identified and assembled. These can be based on past loss histories or on risk assessments that have been undertaken for the specific aspect being reviewed. See also Pre-incident Planning Scenario Tabletop Drill. 

Statements of guidance and direction necessary for selecting appropriate strategies and the tactical direction of resources. Incident objectives are based on realistic expectations of what can be accomplished when all allocated resources have been effectively deployed. Incident objectives must be achievable aud measurable, yet flexible enough to allow strategic and tactical alternatives. See also Credible Scenario. Incident Report, Injury and Illness, OSHA... 

A description of the potential hazards and recommended actions to be taken by public emergency response agencies or private industry for response to emergency incidents at a specific facility or location (Figure P.3). See also Credible Scenario. Pre-startnp Safety Review (PSSR)... 

A postulated or assumed set of conditions or events. Most commonly used in an analysis or assessment to represent possible future conditions and/or events to be modeled, such as possible incidents and their effects on the surrounding environment. A scenario may represent the conditions at a single point in time or a single event, or a time history of conditions and/or events (including processes). See also Credible Scenario. Schedule Rating... 

The heavy-bunker approach would make maintenance difficult and in many cases would require workers to don full protective equipment before entering the confined area. It is not suitable for loading/unloading systems and is largely confined to the erection of small buildings for confinement of reactors that have the potential for a sudden release of considerable energy or quantities of chlorine. The Eimo Chlor conclusion is tiiat no credible scenario justifies (this approach) for imits containing liquid chlorine. ... 

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