Fault trees structure

Other approaches deal with the problem of automatically removing existing loops in fault trees. In (Ciarambino, di Torino, Contini, Demichela, Piccinini 2002), syntax rules are used to identify and remove loops. In (Domis Trapp 2008) Boolean structures are analyzed and loops are removed from the safety analysis model. However, these type of approaches require prior recognition by the analyst of the initiation of a loop. To enable automations for fault tree structures that do not require interactions... 

The logical structure of a fault tree can be described in terms of boolean algebraic equations. Some specific prerequisites to the application of this methodology are as follows. 

A significant development of the study was the use of event trees to link the system fault trees to (lie accident initiators and the core damage states as described in Chapter 3. This was a response to the ditficulties encountered in performing the in-plant analysis by fault trees alone. Nathan Villalva and Winston Little proposed the application of decision trees, which was recognized by Saul Levine a.s providing the structure needed to link accident sequences to equipment failure. 

FTED, fault free editor module, edits and manages fault trees in text format using a full screen, user-friendly, menu driven editor. FTED automatically verifies the FT logic and determines if all components are defined in the RDB. It transfers the tree structure from graphics to text format. 

Fault tree or equivalent analysis is key to PSA. Small logical structures may be evaluated by hand using the iciples of Chapter 2 but at some point computer support eeded. Even for simple structures, uncertainty analysis VIonte Carlo methods requires a computer. However, t of the codes are proprietary or a fee is charged for their... 

This stage involves representing the structure of the tasks in which errors with severe consequences could occur, in a manner that allows the probabilities of these consequences to be generated. The usual forms of representation are event trees and fault trees. 

As can be noted in Figure 21.7.2, steam and ediane are mi.xed before entering die reactor tubes where pyrolysis reacdons take place. All feed and product lines must be equipped with appropriate control devices to ensure safe operation. The FTA flow chart breaks down a TOP event (see descripdon of fault tree in Unit II) into all possible basic causes. Aldiough, diis mediod is more structured than a PHA, it addresses only one individual event at a dine. To use an FTA for a complete liazard analysis, all possible TOP events must be identified and investigated this would be extremely time consuming and perhaps urmecessary in a preliminary design. 

Fault trees developed by different individuals are usually different in structure. The different trees generally predict different failure probabilities. This inexact nature of fault trees is a considerable problem. 

The disciplines of engineering and quality control have long recognized the principles of root cause analysis. Some process safety tools for root cause analysis have been borrowed from these disciplines. For example, fault tree analysis was developed as an engineering tool, but its logic tree structure has been adapted to meet process safety requirements. 

Another type of logic tree, the event tree, is an inductive technique. Event Tree Analysis (ETA) also provides a structured method to aid in understanding and determining the causes of an incident.(i) While the fault tree starts at the undesired event and works backward to identify root causes, the event tree looks forward to display the progression of various combinations of equipment failures and human errors that result in the incident graphically. 

Method A involves a deductive search for all credible ways an occurrence could arise using timeline construction and a simplified fault tree approach. It can be viewed as an integrated method for systematically searching for all underlying root causes. The structured framework helps the investigator to keep on track, reach sufficient depth, and not stop prematurely at the symptoms or apparent causes. 

As a result of the AIC s efforts, we now have a process for investigating accidents in which we construct an event tree for each incident. The tree is quite similar to a fault tree from the quantitative risk analysis discipline, except that in the investigations we often sacrifice some structural rigor to get the most results in a reasonable time. Basically, the process uses a team to reconstruct the chronology of the incident and to construct the event tree. We try to include those who are most familiar with what actually happened, including the injured person(s) if any. We use the same basic method to investigate process failures, spills, injuries, or any other system failures. Emphasizing the system aspects of the failure removes much of... 

Another problem for fault trees is the uniqueness of the result. Fault trees produced by two different teams of analysts most often show a different structure. However, this problem is reduced as the detail in the problem definition increases. 

K. Parsaye and K. Y. Lin, "An Expert System Structure for Automatic Fault Tree Generation for Emeigency Feedwater Systems for Nuclear Power Plants," in Proceedings of the Second IEEE Westex Conference, Anaheim, Calif., June, 1987. 

The graphical structure of the fault tree enables the primary causes and secondary events that produce the hazards to be combined. It is then possible to compare the relative contributions of the different events to the probability of the hazardous outcome by employing the probability of occurrence of causes and events on the fault tree. 

The fault tree cited in literature for this process is shown in Fig. 26 (Battelle, 1985). Notice the similarity between Figs. 26 and 25, particularly in the structure of the two trees, and recognize that as a result of quantitative analysis. Fig. 26 has an and-gate as its top-level gate. More importantly, recognize that without complete quantification of the root causes, the fault tree given in Figure 26 may be incomplete. 

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