Fault tree analysis evaluation

This excellent, 44-page Canadian booklet [18] lists 58 references covering, but not limited to Emergency Planning, Process Hazards Reviews, Fault Tree Analysis, Evaluation of Toxic Vapor Cloud Hazards. 

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 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. 

An early version of MET methodology was applied in the Interim Reliability Evaluation Program (IREP) that analyzed the ( ill vert Cliffs and Arkansas Nuclear lessons learned in IREP and other applications. Although MET is an extension of the fault tree analysis (Section 3.4,4), it warrants a. separate discussion (see NUREG/ CR 3268). Objectives of MET are ... 

Cause-consequence risk evaluation combines event tree and fault tree analysis to relate specific accident consequences to causes. Tlie process of cause-consequence evaluation usually proceeds as follows ... 

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. 

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. 

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... 

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]. 

Several qualitative approaches can be used to identify hazardous reaction scenarios, including process hazard analysis, checklists, chemical interaction matrices, and an experience-based review. CCPS (1995a p. 176) describes nine hazard evaluation procedures that can be used to identify hazardous reaction scenarios-checklists, Dow fire and explosion indices, preliminary hazard analysis, what-if analysis, failure modes and effects analysis (FMEA), HAZOP study, fault tree analysis, human error analysis, and quantitative risk 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). 

Hazard and risk analysis is a vast subject by itself and is extensively covered in the literature [22]. In order to plan to avoid accidental hazards, the hazard potential must be evaluated. Many new methods and techniques have been developed to assess and evaluate potential hazards, employing chemical technology and reliability engineering. These can be deduced from Fault Tree Analysis or Failure Mode Analysis [23], In these techniques, the plant and process hazard potentials are foreseen and rectified as far as possible. Some techniques such as Hazards and operability (HAZOP) studies and Hazard Analysis (HAZAN) have recently been developed to deal with the assessment of hazard potentials [24]. It must be borne in mind that HAZOP and HAZAN studies should be properly viewed not as ends in themselves but as valuable contributors to the overall task of risk management... 

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 ... 

Fault tree analysis, Guidelines for Hazard Evaluation Procedures," Section 2.8 p. 2-18, AlChe, 1985. 

Failure rates for both equipment and peoples responses are assigned and the frequency and severity of a TOP Event can be calculated. Should the risk be found to be unacceptable, additional process safety hardware or additional procedures can be recommended. Then, calculations can be made to determine the benefits of the additional hardware or procedures. The Fault Tree Analysis method of evaluation is very sophisticated and a detailed explanation is beyond the scope of this book. 

Allowing time in the early stages of design for critical reviews and evaluation of alternatives would involve studies such as an early hazard and operability (HAZOP) study, using flowsheets, before final design begins,4 Fault tree analysis, quantitative risk assessment (QRA), checklists, audits, and other review and checking techniques can also be very helpful. These techniques are extensively discussed in the technical literature and will not be discussed in detail here. 

The Process Hazards Analysis team takes a systematic approach to identify potential process hazards and to document them [51]. The Hazardous-Operation Analysis (Haz-Op) is a method by which the process procedures, process and instrument diagrams, and process flow diagrams are evaluated for operability and safety. Fault-Tree Analysis (FTA) is also a method, which investigates the assessment of what-if scenarios and failure conditions. The outcomes of this analysis are recommendations for the col-... 

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). 

This kind of plant design is also called single error forgiving or error tolerating design. For this purpose of identifying the error propagating pathways this qualitative use and evaluation of a fault tree analysis is especially suitable. 

EVALUATING THE FAULT TREE. After a fault tree is constructed, it can be input to a fault tree analysis computer program, such as FTAP, IRRAS, or WAM. The output from the computer program is a list of MCSs which cause the top event to occur. For each of the MCSs, the analysts describe the consequences associated with that cut set. Table 4.25 shows a typical worksheet used to document the consequences associated with MCSs. 

Other tools Information from any of the methods for gathering information can be summarized using a tree diagram. A tree diagram is similar to a cause-and-effect diagram when causes of an event are being evaluated. Standard symbols are used with tree diagrams for applications like fault tree analysis or failure mode and effects antilysis (FMEA). 

This approach is illustrated by the development of event trees and fault tree analysis. In fault tree analysis, the probability of an accident is estimated by considering the probabihty of human errors, component failures, and other events. This approach has been extensively applied in the field of risk analysis (Gertman and Blackman 1994). THERP (Swain and Guttman 1983) extends the conditioning approach to the evaluation of human reliability in complex systems. 

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