Fault tree analysis quantitative evaluation

Allowing time in the early stages of design for critical reviews and evaluation of alternatives would involve studies such as an early hazard and operability (HAZOP) study, using flowsheets, before final design begins,4 Fault tree analysis, quantitative risk assessment (QRA), checklists, audits, and other review and checking techniques can also be very helpful. These techniques are extensively discussed in the technical literature and will not be discussed in detail here. 

Process Hazards Analysis. Analysis of processes for unrecogni2ed or inadequately controUed ha2ards (see Hazard analysis and risk assessment) is required by OSHA (36). The principal methods of analysis, in an approximate ascending order of intensity, are what-if checklist failure modes and effects ha2ard and operabiHty (HAZOP) and fault-tree analysis. Other complementary methods include human error prediction and cost/benefit analysis. The HAZOP method is the most popular as of 1995 because it can be used to identify ha2ards, pinpoint their causes and consequences, and disclose the need for protective systems. Fault-tree analysis is the method to be used if a quantitative evaluation of operational safety is needed to justify the implementation of process improvements. 

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. 

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. 

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). 

HAZAN, on the other hand, is a process to assess the probability of occurrence of such accidents and to evaluate quantitatively the consequences of such happenings, together with value judgments, in order to decide the level of acceptable risk. HAZAN is also sometimes referred to as Probabilistic Risk Assessment (PRA) and its study uses the well-established techniques of Fault Tree Analysis and/or Event Tree Analysis ... 

A systems hazards analysis (SHA) is a systematic and comprehensive search for and evaluation of all significant failure modes of facility systems components that can be identified by an experienced team. The hazards assessment often includes failure modes and effects analysis, fault tree analysis, event tree analysis, and hazards and operability studies. Generally, the SHA does not include external factors (e.g., natural disasters) or an integrated assessment of systems interactions. However, the tools of SHA are valuable for examining the causes and the effects of chemical events. They provide the basis for the integrated analysis known as quantitative risk assessment. For an example SHA see the TOCDF Functional Analysis Workbook (U.S. Army, 1993-1995). 

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. 

Failure Modes and Effects Analysis and Fault Tree Analysis (FTA) are used less widely than Hazop but sometimes allow more comprehensive and quantitative analysis of certain situations. Table 16.4 lists these techniques and their applications at different stages in the hazard evaluation process. No one technique is efficient or even useful at... 

Although this guidance focuses on the LOPA technique, other techniques such as fault tree analysis or detailed quantitative risk assessment, used separately, may be a more appropriate alternative under some circumstances. Quantified methods can also be used in support of data used in a LOPA study. It is common practice with many dutyholders to use detailed quantified risk assessment where multiple outcomes need to be evaluated to characterise the risk sufficiently, where there may be serious off-site consequences, where the Societal Risk of the site is to be evaluated, or where high levels of risk reduction are required. 

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

Fault Tree Analysis. Fault tree analysis, the most complex of the commonly used methods, is employed to determine the possible causes of a preselected undesired event. Through the use of logic diagrams and failure rate data, the team can make a quantitative evaluation of the frequency of the undesired event. 

Representation Having defined what the operator should do (via task analysis) and what can go wrong, the next step is to represent this information in a form which allows the quantitative evaluation of the human-error impact on the system to take place. It is usual for the human error impact to be seen in the context of other potential contributions to system risk. Human errors and recoveries are usually embedded within logical frameworks such as fault tree analysis and event tree analysis. 

In a more quantitative sense, cause-consequence analysis may be viewed as a blend of fault tree end event tree analysis (discussed in tlie two preceding cliapters) for evaluating potential accidents. A major strengtli of cause-consequence analysis is its use as a communication tool. For example, a cause-consequence diagram displays the interrelationships between tlie accident outcomes (consequences) and Uieir basic causes. The method can be used to quantify the expected frequency of occurrence of the consequences if the appropriate chita are available. 

Probability analysis Way to evaluate the likelihood of an event occurring. By using failure rate data for equipment, piping, instruments, and fault tree techniques, the frequency (number of events per unit time) can be quantitatively estimated. 

The starting point in event tree analysis is the initiating event. The quantitative evaluation of the event tree requires condition probabilities. These may be based on reliability data, historical records, experience, or from fault trees. 

When the likelihood of an event is known and a probability value has been assigned, then analysis of these events on a fault tree will also yield quantitative results. As cutsets are identified, the probability of occurrence as a result of cutset interactions can be quantified and the associated risk can be more readily evaluated. 

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. 

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. 

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