Qualitative fault tree analysis

What are the input requirements for fault tree analysis (qualitative and quantitative) ... 

Identification and quantitative estimation of common-cause failures are general problems in fault tree analysis. Boolean approaches are generally better suited to mathematically handle common-cause failures. The basic assumption is that failures are completely independent events, but in reality dependencies will exist and these are categorized as common cause failures (CCFs). Both qualitative and quantitative techniques can be applied to identify and assess CCFs. An excellent overview of CCF is available (AIChE-CCPS, 2000). 

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. 

The estimated impact is then compared to hazard acceptance criteria to determine whether the consequences are tolerable without additional loss prevention and mitigation measures. If the identified consequences are not tolerable, the next step is to estimate the ffequency/probability of occurrence of the identified failure modes leading to loss of containment. For simple cases, frequency estimates are combined with consequences to yield a qualitative estimate of risk. For complex cases, fault tree analysis is used to estimate the frequency of the event leading to the hazard. These estimates are then combined with the consequences to yield a measure of risk. The calculated risk level is compared to a risk acceptance criterion to determine if mitigation is required for further risk reduction. 

Module 3 (Description) a dozen employees already have been trained in qualitative fault tree analysis by an external training institute. 

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. 

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. 

Fault tree analysis (FTA) is a deductive method, which usually serves for quantification. Just like any method of systems analysis it requires in the first place a qualitative investigation of the system under analysis. After system failure or more generally the undesired or unwanted event (e.g. toxic release) has been defined, logic relationships with the so-called primary or basic events are identified and represented by a fault tree (vid. Fig. 9.8). The primary event may represent the failure of a technical component, an operator error or an impact from outside the plant like flooding or the spreading of a fire from neighbouring installations. 

Stated that fault tree analysis searches for the conditions of plant failure and therefore may be regarded as the antithesis of the design process, which aims at identifying the conditions for functioning. Hence, it proves useful for identifying design weaknesses in both its qualitative and quantitative parts. Eventually, the synthesis of design and safety analysis leads to a better and safer plant. 

Qualitative models consider cause and effect relationships, express component malfunctions in a qualitative manner and link them with deviations in measurement data. (Qualitative fault diagnosis is based on fault tree analysis [15, 21], or... 

Fault tree analysis is used to examine, in detail, how a specific unwanted event could occur. It is a deductive, top-down effort normally reserved for examining critical types of failures or mishaps. Fault tree analysis can be qualitative or quantitative. 

There are two basic approaches to fault tree analysis. The qualitative approach is used to determine, using deductive logic, the ways in which the undesired top event could occur. The quantitative approach adds reliability or probability of failure data. 

Fault tree analysis is used primarily as a tool for conducting system or subsystem hazard analyses, even though qualitative or top-level (that is, limited number of tiers or detail) analyses may be used in performing preliminary hazard analyses. Generally, FTA is used to analyze failure of critical items (as determined by a failure mode and effects analysis or other hazard analysis) and other undesirable events capable of producing catastrophic (or otherwise unacceptable) losses. 

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. 

Part V will cover several techniques for working on prevention that apply multiple factor models. Multiple factor models may use quantitative or qualitative analysis. Statistical techniques, such as factor analysis, multiple regression analysis and other multivariate methods may be useful. Fault tree analysis, failure mode and effects analysis and other approaches help identify characteristics that together can lead to undesired events. 

Fault Tree Analysis This method uses qualitative and/or quantitative analysis of all undesirable outcomes of a process. It uses a diagrammatic approach to visualize what may go wrong. The method starts with low-level component failures and works upward to higher-level failures. In a quantitative application, probabilities become assigned to failures. The probabilities allow calculations for the likelihood of a higher level failure. 

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... 

NFPA has developed a Fire Safety Concepts Tree At the top of the tree are fire safety objectives, followed by actions to achieve the objectives. Elements of the tree connect using AND and OR gates, similar to fault tree analysis (Figure 36-10). A Fire Safety Concepts Tree can help analyze buildings and designs using qualitative and quantitative procedures. 

Fault Tree Analysis (FTA) is a well known and widely used safety tool, implementing a deductive, top down approach. It starts with a top level hazard, which has to be known in advance and "works the way down" through all causal factors of this hazard, combined with Boolean Logic (mainly AND and OR gates). It can consider hardware, software and human errors and identifies both single and multiple points of failure. Both a quantitative and qualitative analysis is possible. 

When the analysis is completed by setting up the fault tree, the prognosis procedure is qualitative. Up to this step the fault tree analysis provides... 

E.D. van Breukelen, R.J. Hatnarm, E.G. Overbosch, Qualitative Fault Tree Analysis Applied as a Design Tool in a Low Cost Satellite Design Method and Lessons Learned, May 2006. CCPS, Layer of Protection Analysis Simplified Process Risk Assessment, Wiley Publications. 

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