Qualitative Fault Tree Evaluation

Qualitative FTA consists of determining the minimal cut sets and common cause failures. The qualitative analysis reduces the FT to a logically equivalent form, by using the Boolean algebra, in terms of the speciflc combination of basic events sufficient for the undesired top event to occur (Henley and Kumamoto (1992)). In this case each combination would be a critical set for the undesired event. The relevance of these sets must be carefully weighted and major emphasis placed on those of greatest significance. 

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Several qualitative approaches can be used to identify hazardous reaction scenarios, including process hazard analysis, checklists, chemical interaction matrices, and an experience-based review. CCPS (1995a p. 176) describes nine hazard evaluation procedures that can be used to identify hazardous reaction scenarios-checklists, Dow fire and explosion indices, preliminary hazard analysis, what-if analysis, failure modes and effects analysis (FMEA), HAZOP study, fault tree analysis, human error analysis, and quantitative risk analysis. 

This kind of plant design is also called single error forgiving or error tolerating design. For this purpose of identifying the error propagating pathways this qualitative use and evaluation of a fault tree analysis is especially suitable. 

The quantification is done on the basis of a qualitative analysis, which is reflected by the fault tree of Fig. 11.3, and its evaluation in terms of probabilities. The fault tree of Fig. 11.3 has the following minimal cut sets... 

However, when someone tries to model the component events it gets complicated to model the human error or to isolate and determine the probabihty for the raw events. Since a probabihty wiU be as accurate as the inputs, the fault tree would be only a qualitative (and not a quantitative) evaluation. 

Fault tree analysis is a technique by which the system safety engineer can rigorously evaluate specific hazardous events. It is a type of logic tree that is developed by deductive logic from a top undesired event to all subevents that must occur to cause it. It is primarily used as a qualitative technique for studying hazardous events in systems, subsystems, components, or operations involving command paths. It can also be used for quantitatively evaluating the probability of the top event and all subevent occurrences when sufficient and accurate data are available. Quantitative analyses shall be performed only when it is reasonably certain that the data for part/component failures and human errors for the operational environment exist. 

After all the causal events are listed on a fault tree, the FTA allows the analyst to evaluate each event separately or in combination with other events on the tree. This provides the user with a powerful tool capable of determining, through deduction, which event or set of events led to the top event. When more than one contributing event is identified, as is usually the case, their respective locations on the fault tree are collectively referred to as a cutset. Identification and qualification of one or multiple cutsets within a fault tree facilitates the evaluation process. Essentially, the cutset isolates specific events in the system and allows for a qualitative examination of the relationship between the cutset, as a whole, and its effect on the top event. 

The FTA is a technique that can be used to identify those events that can or must occur in order to realize a desired or undesired outcome. The technique uses a deductive approach to event analysis as it moves from the general to the specific. The FTA has great utility in its ability to distinguish between those events that must occur (represented by an AND gate) and those that simply can occur (represented by an or gate) in order for the top event to occur. The information charted on a fault tree provides a qualitative analysis by demonstrating how specific events will alfect an outcome. If probability data are known for these events, then the FTA can also provide quantitative information to further evaluate the likelihood of achieving the top event. 

FTA is a tool employed in the analysis of complex systems to estimate the likelihood of a hazardous event. It has been applied, for example, in safety evaluations of nuclear power plants, space missions, air, rail, highway, marine and pipeline transport, liquefied natur gas, chemical manufacturing, and other hazardous material facilities. With this method, all material, personnel, and environmental factors of a complex system can be systematically presented. A well-constructed fault tree enables us to discover failure combinations that would not normally be discovered and provides for both qualitative and quantitative evaluation. 

Evaluate the fault tree—Conducts quantitative and qualitative analysis of the fault tree through cut sets and Boolean algebra... 

As stated earlier, the fault tree is a model of the system fault state. There are qualitative and quantitative tools to evaluate the tree. Qualitative analysis of fault trees is conducted through the use of cut sets and simple Boolean algebraic manipnlation. Trees are quantified by applying probabilities or frequencies of occurrence of each event fault. The event faults are then combined through Boolean manipulation, and the top-event probabUity is determined. You may wish to review a math book and become familiar with Boolean algebra and probability theory. The U.S. Nuclear Regulatory Commission s Fault Tree Handbook (Roberts et al., 1981) and NASA s Fault Tree Handbook with Aerospace Applications (Stamatelatos et al, 2002) are excellent references as well. 

Bow-Tie Analysis (BTA) a type of qualitative process hazard analysis.The methodology is an adaptation of three conventional system safety techniques Fault Tree Analysis, Causal Factors Charting, and Event Tree Analysis. Existing safeguards (barriers) are identified and evaluated for adequacy... 

Once the Fault Tree for each identified deviation is elaborated, the qualitative evaluation of combinations of basic events that cause the occurrence of the top event can be executed. In other words, the cut set of tree that cause the top event are identified. To identify the minimum cut is necessary to apply the main rules of Boolean algebra. 

Numerical probabilities should not be indicated for software errors in fault trees. Any software analysis in an FTA should be expressed in terms of DALs to protect against software errors. The analysis should be evaluated for compliance on a purely qualitative basis. When the probability of an undesired event needs to be calculated, SAE ARP4761 (para 4.1.2) advises as follows ... 

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