System fault tree analysis

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 fault tree is a graphic technique used to analyze complex systems. Fault tree analysis attempts to describe how and why an accident or other undesirable event lias occurred. It may also be used to describe how and why an accident or otlier undesirable event could take place. 

Topics Include methods lor calculating damage resulting from the physical effects of accidental releases, using risk assessment Information to specify safety control systems, fault tree analysis, hazards of trace substances, warehouse fires, human exposure to process systems, and solutions to human factor problems. 

Fault trees are excellent troubleshooting tools. Properly used, they can aid in determining what failed or what was less than adequate in a particular system. Fault tree analysis is a tool used to determine how and why the particular failure occurred. 

Because so much of aviation is controlled by people, human factor analysis tools are at the heart of the aviation industry. Different types of human factors analyses are used in air navigation, such as air traffic control, crew resource management in the cockpit, and even appropriate design and maintenance of aircraft systems. Fault tree analysis, fault hazard analysis, FMEA, and different probabilistic risk tools are also used in the detailed design of safety critical subsystems. 

Failure Mode and Ejfect Analysis (FMEA) This is a systematic study of the causes of failures and their effects. All causes or modes of failure are considered for each element of a system, and then all possible outcomes or effects are recorded. This method is usually used in combination with fault tree analysis, a quantitative technique. FMEA is a comphcated procedure, usually carried out by experienced risk analysts. 

Fault Tree Analysis Faiilt tree analysis permits the hazardous incident (called the top event) frequency to be estimated from a logic model of the failure mechanisms of a system. The top event is traced downward to more basic failures using logic gates to determine its causes and hkelihood. The model is based on the combinations of fail-... 

A significant development of the study was the use of event trees to link the system fault trees to (lie accident initiators and the core damage states as described in Chapter 3. This was a response to the ditficulties encountered in performing the in-plant analysis by fault trees alone. Nathan Villalva and Winston Little proposed the application of decision trees, which was recognized by Saul Levine a.s providing the structure needed to link accident sequences to equipment failure. 

This section describes the most commonly used method for complex systems analysis - fault tree analysis. The previous section introduced cutsets as physically cutting through an RED, here, cuiscis. ire presented mathematically. The symbols of fault trees are introduced and a heuristic... 

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. 

The branching probability at a node is determined by either fault tree analysis of the event system or by data from operating experience. 

System reliability can be analyzed in a number of other ways. Objections to fault tree analysis are ... 

The systems list, across the top of the event tree, specifies the systems that must be analyzed to obtain the branching probabilities of the event tree. For complex reliable systems, fault tree or equivalent analysis may be needed to obtain system probability from component probabilities. For less reliable systems, the branching probability may be obtained from plant records with cautions regarding system interactions. 

Fault tree analysis of mitigating systems to develop logic models of how system failure may occur ... 

Crosetti, P. A., 1971, Fault Tree Analysis for Reactor Systems, Inst. Power Ind. 14 p 54. 

Fussell, J. B. 1975, Computer Aided Fault Tree Construction for Electrical System and Fault Tree Analysis, SIAM, Philadelphia, PA, p 37. 

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 FMEA approach is a bottom-up approach, looking at component failures and establishing their effect on the system. An alternative approach is to use a top-down approach such as Fault Tree Analysis to postulate system failure modes and establish which processes, procedures, or activities are likely to cause such failures. 

INTEGRATION WITH HARDWARE ANALYSIS. The error probabilities obtained from the quantification procedure are incorporated in the overall system fault trees and event trees. 

The complexity of the system directly affects the time and cost requirements for tlie fault tree analysis. The larger tlie modeling processes the longer tlie time needed to detemiine a resolution of tlie analysis. Complex systems mean many potential accident events and larger problems. 

Tliis cliapter is concerned willi special probability distributions and tecliniques used in calculations of reliability and risk. Tlieorems and basic concepts of probability presented in Cliapter 19 are applied to llie determination of llie reliability of complex systems in terms of tlie reliabilities of their components. Tlie relationship between reliability and failure rate is explored in detail. Special probability distributions for failure time are discussed. Tlie chapter concludes with a consideration of fault tree analysis and event tree analysis, two special teclmiques lliat figure prominently in hazard analysis and llie evaluation of risk. 

Bums and Hazzan demonstrated tlie use of event tree and fault tree analysis in tlie study of a potential accident sequence leading to a toxic vapor release at an industrial chemical process plant. The initiator of tlie accident sequence studied is event P, the failure of a plant programmable automatic controller. Tliis event, in conjunction willi the success or failure of a process water system (a glycol cooling system) mid an operator-manual shutdown of tlie distillation system produced minor, moderate, or major release of toxic material as indicated in Fig. 21.4.1. The symbols W, G, O represent tlie events listed ... 

In Section 21.4 tlie effects of the release of toxic vapors were considered in connection witli an accident sequence initiated by the failure of a plant programmable automatic controller. In tliis study, event tree analysis and fault tree analysis led to identification of tlie glycol cooling system circulation pumps as components meriting high priority for inspection. 

Prior to installing a new shutdown system, however, a fault tree analysis was performed on the proposed modifications. From this study, it was concluded that the overall frequency of brittle fracture was lowered from 5x10"4 to 5 x 10-5 (occurrences/year). Using this new frequency in the calculation for aggregate risk would result in revised outcome frequencies and F-N data points, as shown below. 

Failure mode and effect analysis (FMEA) A hazard identification technique in which all known failure modes of components or features of a system are considered in turn and undesired outcomes are noted. It is often used in combination with hazard and operability (HAZOP) studies or fault tree 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). 

Fault Tree Analysis (ETA)—Estimation of the hazardous incident (top event) frequency from a logic model of the failure mechanisms of a system. 

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