Fault-Tree Analysis FTA

The fault tree symbols and dieir definitions are presented in Table 

Standiird Usage Basic initiating fault requiring no furtlier development 

Modified ADL Usage Represents initiating event and tlierefore lias a yearly rate of occurrence 

Standard Usage Event wliich is not developed any further as it is not required or data is unavailable 

Modified ADL Usage Represents contributing events having taken place 

Standard Usage Event normally expected to occur 

Fault Tree Analysis (FTA) is a formal deductive procedure for determining combinations of component failures and human errors that could result in the occurrence of specified undesired events at the system level (Ang and Tang (1984)). It is a diagrannnatic method used to evaluate the probability of an accident resulting from sequences and combinations of faults and failure events. This method can be used to analyse the vast majority of industrial system reliability problems. FTA is based on the idea that  

A failure in a system can trigger other consequent failures. 

A problem might be traced backwards to its root causes. 

The identified failures can be arranged in a tree structure in such a way that their relationships can be characterised and evaluated. 

The most widely used analytical technique, FTA, is a symbolic logic diagram graphically depicting the cause-and-effect relationships of a system (Ferry, 1988). 

The hierarchy of fault tree events can be classified as follows  

Head event (top event) The event at the top of the tree that is to be analyzed 

Secondary event The effect that is caused on another component, device, or outside condition 

Basic event An event that occurs at the element level and refers 

This is a widely used method in the industrial sector, particularly in nuclear power generation, to perform reliability and safety analysis of engineering 

A fault tree is a logical representation of the relationship or primary events that may cause the occurrence of a specified undesirable event, known as the top event, and it is depicted using a tree structure with normally OR and AND logic gates. The method is described in detail in Ref. [9], and an extensive list of publications on the method is available in Ref. [38]. 

The fault-tree method was developed in the early 1960s at the Bell Telephone Laboratories to conduct analysis of the Minuteman Launch Control System [3]. Some of the main objectives of performing FTA are as follows [3, 5]  

There are many prerequisites associated with FTA, and some of the main ones are as follows [3]  

FTA starts by idenhfying an undesirable event, known as a top event, associated with a system/item under consideration. Fault events that can cause the occurrence of the top event are generated and coimected by logic operators such as AND and OR. The AND gate provides a True output (i.e., fault) when all the inputs are true. Similarly, the OR gate provides a true output (i.e., fault) when one or more inputs are true. 

The construction of a fault tree proceeds by generating fault events in a successive manner until the fault events need not be developed any further. These fault events are called basic or primary events. A fault tree is a logic structure that relates the top event to the basic or primary fault events. During the construction of a fault tree, one question that is successively asked is, How could this fault event occur  

Basic fault tree symbols (a) AND gate, (b) OR gate, (c) basic fault event, (d) resultant event. 

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Design teehniques (for example, FMEA or Fault Tree Analysis (FTA))... 

How do you then design an effective system There are several techniques you can use. Failure Modes and Effect Analysis (FMEA), Fault Tree Analysis (FTA), and Theory of Constraints (TOC) are but three. The FMEA is a bottom-up approach, the FTA a top-down approach, and TOC a holistic approach. 

The use of event trees is sometimes limiting for liazard analysis because it may lack die capability of quantifying die potendal of die event occurring. Tlie analysis may also be incomplete if all inidal occurrences are not identified. Its use is beneficial in examining, rather dian evaluating, die possibilities and consequences of a failure. For this reason, a fault tree analysis (FTA) should supplement diis, to establish die probabilities of die event tree branches. Tliis topic was introduced in a subsection of Cliapter 16. 

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.

All team members should be familiar with PrHA objectives, the PrHA method to be used, and their roles in performing the PrHA. A 1- or 2-hour overview at the beginning of the first team review session is generally sufficient for this purpose. However, the more demanding PrHA methods, such as fault tree analysis (FTA), require more training and/or a greater depth of experience than less-rigorous methods, such as what-if and checklist analyses. 

Three hazard evaluation procedures using logic diagrams are (1) fault-tree analysis (FTA), (2) event-tree analysis (ETA), and (3) cause-consequence analysis (CCA). Appropriate references are [2,3,251,261]. 

Fault tree analysis (FTA) and event tree analysis (ETA) are the methods most commonly applied quantitatively. Since they only address the likelihood of undesired events, these methods are often combined with consequence severity calculations in a quantitative risk analysis, as described by CCPS (1999b). Layer of protection analysis (LOPA) uses a semiquantitative, order-of-magnitude approach. It is documented with worked examples in CCPS (2001b). 

Fault Tree Analysis (FTA) Scenario- based Deductive By undesired event Can analyze complex processes with multiple safeguards and operator interactions Only looks at events that precede the selected top event Highest... 

Many deductive investigation techniques use logic tree diagrams. A partial list of these methods includes fault tree analysis (FTA), causal tree... 

The overall concept of all of the following tools is that of risk analysis or risk assessment. Risk analysis helps to decide whether an aspect is GMP-critical or not. The risk analysis can be performed in a formal or more informal way. Following are two popular and import types of risk analysis. Another method, the fault tree analysis (FTA), has recently been used in the area of computer validation. This method is not described here, as it is a complex form of risk analysis. 

Deductive methods, such as the Fault Tree Analysis (FTA) that proceeds by starting from the top event and looking for failures that may cause it to happen. These methods are based on questions of the type How can it happen ... 

The rest of this section outlines the core sub-processes to support safety analyses (compare [F. Redmill, (2004)], [N. G. Leveson, (2004)], [Ch. Blechinger, (2004)], [J. Zalewski and all, (2003)]). The processes place techniques, such as Hazard And Operability Studies (HAZOP), Failure Modes and Effects Analysis (FMEA) and Fault Tree Analysis (FTA), into context. 

Identification can be as simple as asking what-iP questions at design reviews. It can also involve the use of a checklist outlining the normal process hazards associated with a specific piece of equipment. The major weakness of the latter approach is that items not on the checklist can easily be overlooked. The more formalized hazard-assessment techniques include, but are not limited to, hazard and operability study (HAZOP), fault-tree analysis (FTA), failure mode-and-effect analysis (FMEA), safety indexes, and safety audits. 

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