Fault Tree Analysis analyses

P. O. Chelsau, ReHabihty Computation Using Fault Tree Analysis, TR32-1542, NASA, Airport, Md., 1971. 

Methods for performing hazard analysis and risk assessment include safety review, checkhsts, Dow Fire and Explosion Index, what-if analysis, hazard and operabihty analysis (HAZOP), failure modes and effects analysis (FMEA), fault tree analysis, and event tree analysis. Other methods are also available, but those given are used most often. 

Fault Tree Analysis. Fault trees represent a deductive approach to determining the causes contributing to a designated failure. The approach begins with the definition of a top or undesired event, and branches backward through intermediate events until the top event is defined in terms of basic events. A basic event is an event for which further development would not be useful for the purpose at hand. For example, for a quantitative fault tree, if a frequency or probabiUty for a failure can be deterrnined without further development of the failure logic, then there is no point to further development, and the event is regarded as basic. 

It is important in fault tree analysis to consider only the nearest contributing event. There is always a tendency to jump immediately to the details, skipping all of the intermediate events. Some practice is required to gain experience in this technique. 

Failure Mode and Ejfect Analysis (FMEA) This is a systematic study of the causes of failures and their effects. All causes or modes of failure are considered for each element of a system, and then all possible outcomes or effects are recorded. This method is usually used in combination with fault tree analysis, a quantitative technique. FMEA is a comphcated procedure, usually carried out by experienced risk analysts. 

Fault Tree Analysis Faiilt tree analysis permits the hazardous incident (called the top event) frequency to be estimated from a logic model of the failure mechanisms of a system. The top event is traced downward to more basic failures using logic gates to determine its causes and hkelihood. The model is based on the combinations of fail-... 

Frequency Estimation There are two primary sources for estimates of incident frequencies. These are historical records and the apphcation of fault tree analysis and related techniques, and they are not necessarily applied independently. Specific historical data can sometimes be usehiUy applied as a check on frequency estimates of various subevents of a fault tree, for example. 

In some instances, plant-specific information relating to frequencies of subevents (e.g., a release from a relief device) can be compared against results derived from the quantitative fault tree analysis, starting with basic component failure rate data. 

Identification and quantitative estimation of common-cause failures are general problems in fault tree analysis. Boolean approaches are generally better smted to mathematically handle common-cause failures. 

Design teehniques (for example, FMEA or Fault Tree Analysis (FTA))... 

Today there are many tools available to aid in problem solving or f ure analysis. These include the Weibull Analysis, Failure Mode i Effect Analysis, and Fault Tree Analysis, to name a few. One of the m widely accepted is the Weibull analysis. This method can provide accurate engineering analysis based on extraordinary small samples [1]. 

It is interesting that NASA in their review of WASH-1400 Draft (included in W.ASH-1400 Final Appendix II), indicated that they had discontinued the use of fault tree analysis in fas or of the FMEA. 

This section describes the most commonly used method for complex systems analysis - fault tree analysis. The previous section introduced cutsets as physically cutting through an RED, here, cuiscis. ire presented mathematically. The symbols of fault trees are introduced and a heuristic... 

A simple example of fault tree analysis applied to an internal combustion engine (Figure 3.4.4-2) is the Figure 3.4.4-3 fault tree diagram of how the undesired event "Low Cylinder Compression" may occur. The Boolean equation of this fault tree is in the caption of Figure 3.4.4-3. Let the occurrence of these events be represented by a 7, non-occurrence by 0, and consider that there may he a long history of occurrences with this engine. Several sets of occunrence.s (trials) are... 

A failure modes and effects analysis delineates components, their interaction.s ith each other, and the effects of their failures on their system. A key element of fault tree analysis is the identification of related fault events that can contribute to the top event. For a quantitative evaluation, the failure modes must be clearly defined and related to a numerical database. Component failure modes should be realistically and consistently postulated within the context of system operational requirements and environmental factors. 

Comptete all gate inputs. Beginning a new gate before completing a previous gate leads to confusion and incompleteness. These rules are made concrete by an example of fault tree analysis. 

The branching probability at a node is determined by either fault tree analysis of the event system or by data from operating experience. 

System reliability can be analyzed in a number of other ways. Objections to fault tree analysis are ... 

An early version of MET methodology was applied in the Interim Reliability Evaluation Program (IREP) that analyzed the ( ill vert Cliffs and Arkansas Nuclear lessons learned in IREP and other applications. Although MET is an extension of the fault tree analysis (Section 3.4,4), it warrants a. separate discussion (see NUREG/ CR 3268). Objectives of MET are ... 

The codes listed in Tables 3.6-1 and 3.6-2 are useful for various purposes of s primarily fault tree analysis. The following discusses their features. 

Most codes for fault tree analysis in the public domain are available from the Argonne Code Center. EPRI codes are available under special arrangements SETS... 

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