Fault tree analysis examples

Fault Tree Analysis. Fault trees represent a deductive approach to determining the causes contributing to a designated failure. The approach begins with the definition of a top or undesired event, and branches backward through intermediate events until the top event is defined in terms of basic events. A basic event is an event for which further development would not be useful for the purpose at hand. For example, for a quantitative fault tree, if a frequency or probabiUty for a failure can be deterrnined without further development of the failure logic, then there is no point to further development, and the event is regarded as basic. 

Frequency Estimation There are two primary sources for estimates of incident frequencies. These are historical records and the apphcation of fault tree analysis and related techniques, and they are not necessarily applied independently. Specific historical data can sometimes be usehiUy applied as a check on frequency estimates of various subevents of a fault tree, for example. 

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

A simple example of fault tree analysis applied to an internal combustion engine (Figure 3.4.4-2) is the Figure 3.4.4-3 fault tree diagram of how the undesired event "Low Cylinder Compression" may occur. The Boolean equation of this fault tree is in the caption of Figure 3.4.4-3. Let the occurrence of these events be represented by a 7, non-occurrence by 0, and consider that there may he a long history of occurrences with this engine. Several sets of occunrence.s (trials) are... 

Comptete all gate inputs. Beginning a new gate before completing a previous gate leads to confusion and incompleteness. These rules are made concrete by an example of fault tree analysis. 

Although risk analysis of new facilities is required by Ref. 39, the method of conducting the analysis is left quite open. The reference suggests fault hazard analysis, fault tree analysis, or sneak circuit analysis. Ref. 41 is an example of a thorough hazards evaluation and risk analysis for a new facility at Radford Army... 

Process hazard analysis (PHA) Any of a number of techniques for understanding and managing the risk of a chemical process or plant. Examples of PHA techniques include HAZOP, checklists, what-if methods, fault tree analysis, event tree analysis, and others. 

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). 

The PHA procedure can be conducted using various methodologies. For example, the checklist analysis discussed earlier is an effective methodology. In addition, Pareto analysis, relative ranking, pre-removal risk assessment (PRRA), change analysis, failure mode and effects analysis (FMEA), fault tree analysis, event tree analysis, event and CF charting, PrHA, what-if analysis, and HAZOP can be used in conducting the PHA. 

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). 

The disciplines of engineering and quality control have long recognized the principles of root cause analysis. Some process safety tools for root cause analysis have been borrowed from these disciplines. For example, fault tree analysis was developed as an engineering tool, but its logic tree structure has been adapted to meet process safety requirements. 

Figu re 1.8 Example fault tree analysis for the collision of a car with a deer. 

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). 

The term near miss —which may better be called near hit —describes an incident that did not result in an actual loss but that had the potential to do so. For example, if an object is dropped from a crane but no one is hurt then the incident is a near miss. In terms of fault tree analysis, a near miss is an event in which one or more of the inputs to an AND gate was negative. 

In conventional fault tree analysis, one of the biggest benefits of the technique is that it highlights common cause events, i.e., those events that occur in two or more places on the tree and thus bypass safeguards. A common example would be electrical power failure. Loss of power could cause equipment to fail and could also lead to failure of some of the backup systems. 

The primary events of the fault tree may be further decomposed. For example, the failure of the pump motor Ml might be caused by a failure of its stator or rotor windings, cables or such like. This would make sense if the motor itself were the object of the fault tree analysis. In practice the degree of decomposition (degree of detail) is determined by the boundaries (deUmitation) of the reliability data for describing component behaviour, which are needed for quantifying a fault tree. 

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