Fault tree analysis , acceptable

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]. 

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 ... 

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. 

A methodical examination of a process, plant and procedure which identifies hazards, assesses risks and proposes measures which will reduce risks to an acceptable level. (May use inter alia Hazops. Fault Tree Analysis, Check-lists, Event Tree Analysis. FMECA, etc). 

Instrumentation and Control (I C) systems are very often subject of probabilistic examination either within separate structural reliability analysis or Probabilistic Safety Assessment of a whole technological complex (e.g. Nuclear Power Plant). Use of programmable components in the design of these systems represents a challenge and utilizes the methods, which have been developed for components with a different behaviour. The typical method used for above mentioned examination is Fault Tree Analysis (FTA) (Vesely et al., 1981). The way of software faults modelling within Fault Trees vary a lot between particular models and there is no generally accepted modelling technique. 

The typical manager has no clear idea of the risks he/she is actually assuming in conduct of the operations. An important feature of a MORT fault tree analysis is that it surfaces these risks, forcing a series of choices on management (a) to eliminate certain risks (or reduce their probability) or (b) to accept the remaining risks as known and permanent characteristics of the operation. 

FTA, fault tree analysis LOPA, layer of protection analysis NR, not recommended PFDavgt probability of failure on demand SIL, safety integrity level X, acceptable. ... 

Where multiple, diverse hazards exist, the practical approach is to treat each hazard independently, with the intent of achieving acceptable risk levels for all. In the noise and toluene example, the hazards are indeed independent. In complex situations, or when competing solutions to complex systems must be evaluated, the assistance of specialists with knowledge of more sophisticated risk assessment methodologies such as Hazard and Operability Analysis (HAZOP) or Fault Tree Analysis (FTA) may be required. However, for most applications, this author does not recommend that diverse risks be summed through what could be a questionable methodology. 

Faults are analyzed through a graphical representation of causality known as Fault Tree Analysis (FTA). Faults are used to analyze the effect of failures on the system, subsystem, or operating environment (i.e., to facilities, equipment, or personnel). Failures are associated with a quantitative analysis of the design of the system. Hazards are assessed qualitatively, aud must be analyzed and either eliminated or reduced to an acceptable level of risk through a mitigation process. The relationship between faults, failures, and hazards may best be understood as follows not aU faults are failures and not aU failures present a hazard to the system. 

Risk is the product of the probability of a release, thepjpbability of exposure, and the consequences of the exposure. Risk is usually described graphically, as shown in Figure 11-15. All companies decide their levels of acceptable risk and unacceptable risk. The actual risk of a process or plant is usually determined using quantitative risk analysis (QRA) or a layer of protection analysis (LOPA). Other methods are sometimes used however, ORA and LOPA are the methods that are most commonly used. In both methods the frequency of the release is determined using a combination of event trees, fault trees, or an appropriate adaptation. 

The system safety case of corrrse varies from sector to sector. The core of a nuclear system safety case is (i) a deterministic analysis of the hazards and farrlts which could arise and cause injury, disability or loss of life fiom the plarrt either on or off the site, and (ii) a demonstration of the sufficiency and adequacy of the provisions (engineering and procedural) for ensuring that the combined frequencies of such events will be acceptably low. Safety systems will feature amongst the risk reducing provisions comprised in this demonstration, which will thus include qualitative substantiations of compliance with appropriate safety engineering standards supplemented (where practicable) by probabihstic analyses of their reliabilities. Other techniques which may be used for stracturing the safety case include fault and event tree analysis, failure mode and effects analysis (FMEA) and hazard and operability studies (HAZOPS). 

The system models consist of system-level fault trees and detailed generic-component fault trees. The component fault trees are used to model component-specific failures and system dependencies. The PRA team constructed the fault trees in such a manner that they can be used for the same function but with different success criteria. Thus, one fault tree can be used for the same function in different accident sequences. This was accomplished by the extensive use of flags. This method of system analysis is acceptable. 

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