Fault tree analysis steps

Common Fault Tree Symbols and Fault Tree Analysis Steps... 

Tlie patli tliat an accident lakes tlirougli tlie above tliree steps can be determined by means of a fault tree analysis. ... 

Figure 21.7.2 demonstrates die preliminary steps for a fault tree analysis (FTA) in addition, die TOP event, bounds, configurations, and unallowed events are specified, and die level of resolution is shown. Once all die limits have been detennined, die fault tree is constructed (Figure 21.7.3). Note tliat every branch of die fault tree ends in a basic fault or cause leading to die TOP event.

Figure 21.7.2 Fault tree analysis (FTA) preliminary steps, ethylene plant.

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. 

The conceptual system selected in step 3 is designed. Reliability and maintainability of this design are assessed. Various methodologies, such as design review, failure mode and effect analysis, fault tree analysis, and probabilistic design approach, can be applied at this step. Reliability is a design parameter and must be incorporated in the system at the design step. 

The steps Usted below are needed for carrying out a fault tree analysis. The first step is largely a matter of organization. It is not dealt with here a detailed treatment is foimd in [21]. In what follows the remaining steps are discussed in detail and illustrated by a number of examples and case studies. 

Step la This CMA process is performed to verify that AND events (e.g. in the Fault Tree Analysis (FTA) or Dependence Diagrams (DD), or any quahtative probabihty declaration) are truly independent. 

The fault tree analysis describes a hazardous top event and the basic event which maybe leads to such a top event in a top-down method. The methods are dev-ided in static fault tree analysis and dynamic fault tree analysis. The static fault tree analysis describes the system top event in static way. In further steps it is not possible to describe functional system redundancy with this static Fault Tree Analysis (FTA). Especially if cold and hot spares are integrated or if triggers are used, the static fault tree analysis is unsatisfying these requirements. Therefore it is more suitable to use the extended Dynamic Fault Tree Analysis. The DIFTree (Dynamic Innovative Fault Tree) software package could be a helpful tool for the system development... 

Domenech et al., in press presents an approach to integrate CCP effectiveness assessment into predictive modeling based on the performance of the coupled control-monitoring system. Following this focus, this paper shows in an appHcation example of honey, how to aggregate control cells, using a model based on fault tree analysis. This structure allows integrating the main parameters of one step and its control in order to know... 

The first step in performing a fault tree analysis is to collect the appropriate project description documents, existing hazard analyses, and guidance documents and carefully review them to determine the limits, scope, and ground rules for the FTA.This review includes defining the system to be analyzed, the depth or indenture levels to be included in the effort, and, of course, the nature of the undesired event or failure to be studied. 

The following seven steps are generally used to perform fault tree analysis [1,39] ... 

Software System Hazard Analysis This type of analysis is conducted similar to a hardware system hazard analysis (SHA), analyzing software functional processing steps to determine whether they may have any particular hazardous effect on the system. The analysis utilizes a hazard-risk index to illustrate the severity of each potential failure. The main advantage to this method is in its ability to positively identify safety-critical hardware and software functions as well as consider the effect of the human element in system software operations. The results of the software SHA, which identifies single-point failures or errors within a system, can often be used to assist in the development of a software fault tree analysis or, to some degree, a system FMEA. However, as with the other various SWHA techniques briefly described above, this method is also time-consuming and costly to perform. 

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... 

Consequence spectmm, accidental event definition step-by-step procedure, probability frequency of outcomes for manual and automatic procedures for event tree analysis (ETA), fault tree analysis (FTA), and LOPA, QRA and HRA with focus on E/E/PE. Discussions on relevant standards like lEC 31010, 60812, 61025, 61508/61511, 61582, ISA 84. Life cycle (LC) analysis SIS standard. Also batch process ISA 88. 

The preceding is just an example to show how Little JIL can be applied in a manufacturing process. In tbe example, it is only one failure mode and type has been depicted. Tbere could be a large number of failure modes/artifact-related failure modes or can easily be turned into artifact-related failure modes. Also, if a step is done incorrectly, one would expect that failure to be evident in one or more of the out artifacts associated with that step. For detailed and effective analysis, an artifact flow graph (AFG) may be developed from by traversing the process tree with necessary algorithm. Derived FMEA information is obtained from AFG. When the system is large, one may take the help of fault tree analysis (FTA) to determine one failure... 

Our approach solely consists of 8 different process steps which are all not new in the area of safety engineering. Thus, established safety analysis techniques (such as Hazard and Operability Studies (HAZOP), Failure Mode and Effects Analysis (FMEA), Fault Tree Analysis (FTA), etc.) can be applied and no new safety analysis methodologies need to be developed. 

Fault Tree Analysis (FTA) is a well known and effective method for analyzing hardware systems 1-3. This paper describes a possible use of FTA in a software embedded system for temperature control. The analysis is first applied to the hardware and software systems but concentrates on the software section afterwards, two critical events were detected by FTA and steps were taken to overcome them. Although this work describes a specific situation it can be applied to many control systems. 

The systems risk analysis is performed in two steps. As a first step there is a FMEA and a conventional fault tree analysis. This is done for two reasons. The FMEA is done at a very early point in the project. At this early stage, the system architecture definition is not yet complete. High level errors and possible unsafe states are... 

So far we have seen how useful fault tree analysis can be for analyzing the risk of a known top event. The next step is to build in the possible risk reduction measures and predict the new risk frequency for the top event. It s easy to do this using the general approach shown in the next diagram. 

Various methods may be used individually or in a combination to carry out Step 1 of the FSA approach. Such typical methods include Preliminary Hazard Analysis (PHA), Fault Tree Analysis (FTA), Event Tree Analysis (ETA), Cause-Consequence Analysis (CCA), Failure Mode, Effects and Criticality Analysis (P MECA), HAZard and OPerability analysis (HA2X)P), Boolean Representation Method (BRM) and Simulation analysis (Henley and Kumamoto (1996), Smith (1992), Villemeur (1992), Wang (1994)). The use of these methods as safety analysis techniques has been reviewed in Chapter 3. 

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