Fault tree analysis construction

The frequencies of plant damage and public consequence are calculated using plant logic combined with component fragilities. Event and fault trees are constructed to identify tiic accident sequences and the damage that may result from an earthquake. In performing a plant system and accident-sequence analysis, the major differences between seismic and internal events analysis are given in Table 5.1-4... 

Fussell, J. B. 1975, Computer Aided Fault Tree Construction for Electrical System and Fault Tree Analysis, SIAM, Philadelphia, PA, p 37. 

Figure 21.7.2 demonstrates die preliminary steps for a fault tree analysis (FTA) in addition, die TOP event, bounds, configurations, and unallowed events are specified, and die level of resolution is shown. Once all die limits have been detennined, die fault tree is constructed (Figure 21.7.3). Note tliat every branch of die fault tree ends in a basic fault or cause leading to die TOP event.

Unallowed events are those that are considered to be incredible or that, for some other reason, are not to be considered in the analysis. For example, wiring failures might be excluded from the analysis of an instrument system. Existing conditions are, for the purposes of the FTA, events or conditions considered certain to occur. The unallowed and existing conditions do not appear in the fault tree, but their effects must be considered in developing other fault events as the fault tree is constructed. 

Fault tree analysis is based on a graphical, logical description of the failure mechanisms of a system. Before construction of a fault tree can begin, a specific definition of the top event is required for example the release of propylene from a refrigeration system. A detailed understanding of the operation of the system, its component parts, and the role of operators and possible human errors is required. Refer to Guidelines for Hazard Evaluation (CCPS, 1992) and Guidelines for Chemical Process Quantitative Risk Assessment (CCPS, 2000). 

HAZOP and What-If reviews are two of the most common petrochemical industry qualitative methods used to conduct process hazard analyses. Up to 80% of a company s process hazard analyses may consist of HAZOP and What-If reviews with the remainder 20% from Checklist, Fault Tree Analysis, Event Tree, Failure Mode and Effects Analysis, etc. An experienced review team can use the analysis to generate possible deviations from design, construction, modification, and operating intent that define potential consequences. These consequences can then be prevented or mitigated by the application of the appropriate safeguards. 

EVALUATING THE FAULT TREE. After a fault tree is constructed, it can be input to a fault tree analysis computer program, such as FTAP, IRRAS, or WAM. The output from the computer program is a list of MCSs which cause the top event to occur. For each of the MCSs, the analysts describe the consequences associated with that cut set. Table 4.25 shows a typical worksheet used to document the consequences associated with MCSs. 

Fault Tree. When direct data allowing to calculate the probability of a failure mode are not available or this failure form is complex, it is proposed the elaboration of a fault tree. It is a method of multidisciplinar analysis that begins with the selection of a failure mode or event that is tried to avoid. The event is developed into its immediate causes, and the sequence of events continues until basic causes are identified. The fault tree is constructed showing the logical event relationships that are necessary to result in the top event. The fault tree reaches terminal events whose probability must be calculated or estimated. These events can be basic events, which do not require to be explained by means of other previous events, or events which are not developed because it is not considered necessary or for lack of information. 

The above layout would be cumbersome to construct for each fault tree generated, so many fault tree analysis programs allow CCF groups to be created. Basic events are added to the group and a Beta factor between 0 and 1 specified. This is normally represented visually on the fault tree by displaying a p symbol next to the included events, as shown in Fig. 4A-4A 4 below. 

As stated earlier, two types of fault trees will be constructed to demonstrate the use of this system safety analytical technique. The first, which will be referred to as a positive fault tree analysis, will identify the events necessary to achieve a top desired event of no accidents. The second, or negative fault tree, will be constructed to show those events or conditions that will lead to a top undesired event of a fire in a manufacturing facility. 

Fault tree analysis as applied to system safety relies on preliminary hazard analyses (PHA) or other analysis techniques to identify major undesirable events. After constructing a tree, a system safety team applies qualitative or quantitative analyses to elements. To perform quantitative analysis on a tree, team members must apply a probability to each event cause. Today, computer systems make the... 

Another limitation on the use of fault tree analysis is cost. Compiling the knowledge for, constructing the fault tree, and assigning probabilities to tree elements can be laborious and costly. 

Some government organizations require or apply system safety methods for construction projects. A project may require selective use of methods. Organizations apply system safety in some construction projects. Included are the Nuclear Regulatory Commission, the Department of Defense and its service agencies, the Federal Aviation Administration and others. Some projects may simply require use of preliminary hazard analysis that leads to a site safety plan for a project. Complex facilities that integrate specialized equipment into the project may require failure mode and effects analysis or even fault tree analysis. 

Fault Tree Analysis - working backwards from a failure or potential failure, this logical procedure identifies all the possible causes, and hence, the origins of that failure. The fault tree consists of branches connected by AND and OR nodes - all the branches below an AND node need to coincide for the event above the node to occur, but only one of the branches below an OR node is required to condition the same. In these terms, a cause-effect map of failure is constructed. The advantage of such analysis is that it codifies a common understanding of the intrinsic logic of failure possibility. 

The next step in the risk assessment process is to identify accident scenarios and develop the initiating events for those scenarios. A hazard analysis was performed and various hazards were identified. Of the hazards identified, the most significant were related to the uncontrolled release of cryogenic fluid or gas. With that information, a fault tree was constructed for the system with the top event designated as uncontrolled cryogenic release. An FMEA was performed on those components that were determined to be critical to the fault tree. 

Fault Tree Analysis (FTA)— A deductive technique that focuses on one particular incident, often called a top event, and then constructs a logic diagram of all conceivable event sequences (both mechanical and human) that could lead to that incident. It is usually a logic model that mathematically and graphically portrays various combinations of equipment faults, failures, and human errors that could result in an incident of interest, expressed in an annual estimation. 

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