Fault hazard analysis example

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... 

Figure 11.1 shows an example of a fault hazard analysis worksheet that can be modified to a specific system or subsystem. An explanation of each column on the worksheet is also provided. 

Figure 2.24 Example fault hazard analysis worksheet.

One illustrative example is presented in tliis final cliapter. It lias been adopted from tlie outstanding work of Kavianaian et al. and is concerned with an ethylene production plant. Tlie solution involves a prcliminaiy hazards analysis (PHA) and tlie development of a fault tree for tlie process. 

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 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). 

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). 

While a preliminary functional decomposition of the system components is created to start the process, as more information is obtained from the hazard analysis and the system design continues, this decomposition may be altered to optimize fault tolerance and communication requirements. For example, at this point the need... 

For example, the FTA approach is logical and rational. The persons building the tree assume that the base events, and the manner in which they interact with one another, have been defined before the analysis starts. However, fault tree analysts will often identify new incident scenarios and find new types of hazard. In other words, this logical/rational approach to hazards analysis can also be creative and imaginative. ... 

Analytical trees are also very useful as feeder documents for several hazard analysis techniques, for example, failure mode and effects analysis (Chapter 14), fault tree analysis (Chapter 15), energy trace and barrier analysis (Chapter 13), and project evaluation tree analysis (Chapter 16), the primary hazard analysis tools for many projects. Virtually any analytical technique or any type of analysis can be simplified by starting with the analytical tree as a base document. 

The SSHA evaluates hazardous conditions, on the subsystem level, which may affect the safe operation of the entire system. In the performance of the SSHA, it is prudent to examine previous analyses that may have been performed such as the preliminary hazard analysis (PHA) and the failure mode and effect analysis (FMEA). Ideally, the SSHA is conducted during the design phase and/or the production phase, as shown in Chapter 3, Figure 3.4. However, as discussed in the example above, an SSHA can also be done during the operation phase, as required, to assist in the identification of hazardous conditions and the analysis of specific subsystems and/or components. In the event of an actual accident or incident investigation, the completed SSHA can be used to assist in the development of a fault tree analysis by providing data on possible contributing fault factors located at the subsystem or component level. 

Examples of system safety analyses include routine hazard spotting job safety analysis hazard and operability studies design safety analysis fault-tree analysis and simulation exercises using a computer. 

As discussed previously, a quantified approach is recommended. For this example, fault-tree analysis was performed to determine the risk reduction provided by the protection layers for each initiating cause for the revised design. A summary of the fault-tree analysis is presented below in Table F.3. The hazard rate due to all causes, TT-1, TIC-1, and CV-1, is 2.2E-04. 

System Structure Analysis. After the identification of subsystems to be examined and the definition of undcsired events within the context of preliminary hazard analysis, events which lead to incidents are investigated. These event sequences can be represented as logic structure in a block diagram, a flow diagram, a fault tree, or a decision table. In the presentation which follows (Table 4.9.). a decision table was used. It contains, column by column, the combinations of system states which lead to the undesired event. The presentation permits qualitative identification of weak points in the system. In general, for example, the probability of a system state will decline with the growing number of failed components. The logic structure presentation could form the basis for further quantitative analyses. 

For this paper we treat hazard assessment as a combination of two interrelated concepts hazard identification, in which the possible hazardous events at the system boundary are discovered, and hazard analysis, in which the likelihood, consequences and severity of the events are determined. The hazard identification process is based on a model of the way in which parts of a system may deviate fi om their intended behaviour. Examples of such analysis include Hazard and Operability Studies (HAZOP, Kletz 1992), Fault Propagation and Transformation Calculus (Wallace 2005), Function Failure Analysis (SAE 1996) and Failure Modes and Effects Analysis (Villemeur 1992). Some analysis approaches start with possible deviations and determine likely undesired outcomes (so-called inductive approaches) while others start with a particular unwanted event and try to determine possible causes (so-called deductive approaches). The overall goal may be safety analysis, to assess the safety of a proposed system (a design, a model or an actual product) or accident analysis, to determine the likely causes of an incident that has occurred. 

Fault tree for overpressure example (Fig. VII/1.2.2-1). BPCS, basic plant control system C Valve, control valve E/E/PE, electrical/electronics/programmable electronics IPL, independent protection layer PHA, plant hazard analysis SIS, safety instrumentation system. 

Literature on the many techniques for making risk assessments is abundant. For example, in ANSI/ASSE Z690.3. Risk Assessment Techniques—reviews are included of 31 techniques. Examples are such as Primary Hazard Analysis, Fault Tree Analysis, Hazard and Operably Studies, Bow Tie Analysis, Markov Analysis, and Bayesian Statistics. Uncomplicated systems that could be introduced to supervisors and front-line employees are not as prevalent. Such a system is contained in an extension of the previously cited European Community bulletin. It follows. 

Where multiple, diverse hazards exist, the practical approach is to treat each hazard independently, with the intent of achieving acceptable risk levels for all. In the noise and toluene example, the hazards are indeed independent. In complex situations, or when competing solutions to complex systems must be evaluated, the assistance of specialists with knowledge of more sophisticated risk assessment methodologies such as Hazard and Operability Analysis (HAZOP) or Fault Tree Analysis (FTA) may be required. However, for most applications, this author does not recommend that diverse risks be summed through what could be a questionable methodology. 

The remainder of this chapter will discuss HAZOP and what-if techniques in detail and illustrate specific examples of how they are applied. Chapter 7 will address fault tree analysis and Chapter 8 will discuss failure modes effects and criticality analysis. An excellent reference manual for these techniques is the Guidelines for Hazard Evaluation Procedures, published by the American Institute for Chemical Engineers CCPS (2008). 

Do not hesitate to combine some of these tools. For example, when doing a hazard analysis or HAZOP, add a hnman factors analysis (as a subset of the overall hazard analysis) if hnman operators play a significant role. Or, if the HAZOP has identified particularly dangerous deviations of the process resulting from a failure in the system, do an FMEA or fault tree analysis of just that critical subsystan. 

The next step is to identify initiating events for scenarios. These are the events of interest. They are identified and studied by any one, or a combination, of the system safety analysis techniques discussed in Chapters 5 through 9. For example, a hazard analysis can identify the events that present the hazards of most concern. A fault tree can further refine how the event could occur, and failure mode and effect analysis (FMEA) can give specific failure information about particular components that led to that event. 

Chapters 5 through 9 describe the different safety analysis tools available. Hazard Analysis, H AZOF, What-If, Fault Tree Analysis, Failure Modes, and Effects Analysis, Human Factors, Software Safety, and other safety tools are described with realistic worked examples. The chapters detail how to use them, give examples, describe common mistakes in using them, and also provide best practices and tips of how to apply them judiciously. 

As a simple example of selecting an appropriate SIL, assume that the maximum tolerable frequency for an involuntary risk scenario (e.g., customer killed by explosion) is 10 pa (A) (see Table 2.1). Assume that 10 (B) of the hazardous events in question lead to fatality. Thus the maximum tolerable failure rate for the hazardous event will be C = A/B = 10 pa. Assume that a fault tree analysis predicts that the unprotected process is only likely to achieve a failure rate of 2 x 10 pa (D) (i.e., 1/5 years). The FAILURE ON DEMAND of the safety system would need to be E = C/D =10 column of Table 1.1, SIL 2 is applicable. 

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