Event tree construction

The success of some systems depends on other systems. An event tree is constructed by placing support systems before the supported systems. This may require iterating the event tree construction to get appropriate ordering. If the system dependency is in the fault trees, it is not reflected in the event tree and only becomes apparent when the PSA is calculated. 

Event tree construction and analysis (graphic and text) ... 

A dependency analysis determines which systems depend on other systems. This may require iterating the event tree construction to put a support system before the systems affected. 

Event tree construction is well depicted in Fig. V/2.2-1, where various barriers have also been shown. The figure also depicts how barriers influence the outcome probabilities. 

Table V/2.2-1 Steps Involved in Event Tree Construction...

Fault Tree Construction. Eault tree constmction begins at the TOP event and proceeds, level by level, until all fault events have been developed to their basic contributing causes (BASIC events). The analyst begins with the TOP event and, for the next level, determines the immediate. 

In constructing the event tree, the analyst considers the functions that are required to prevent damage states, health consequences considering the relationships between safety functions. For example, if RCS inventory is not maintained, the heat-removal functions are depicted as failed state.s that may lead to core melt. 

Direct Introduction - This involves locating, in the event tree, support systems (e.g., electric power) before the systems they affect. The event tree is constructed so that systems requiring the support systems are bypassed as failed. This is the technique in WASH-1400. 

Edit Graphics - constructs and edits event trees using the graphical Event Tree Editor. 

FEP, fault tree, gvent tree and piping instrumentation diagram (P ID) editor, accesses ihe package of graphical tools for risk assessment. These tools include the event tree, fault tree and P ID graphical editors. The event tree and fault tree editors are in PSAPACK the P ID editor is in FEP. The event tree editor is used for graphical construction and modification of event trees. The fault tree editor does the same for fault trees the P ID editor does the same for construction and modification of P IDs used in a PSA. 

The construction of event trees for eaithquake-induced initiating events follows the metliods described in Section 4.3. From... 

If the PSA were constructed in the ZIP format, these common cause effects would be incorporated by changing the event tree branching probabilities as a function of water depth a.s the flood fails the various systems. 

It is not feasible to construct event trees for 30 or 40 initiators, thus some sort of grouping and consolidation is needed. LCXIAs are grouped according to the systems required to respond to... 

DPSE, like all subsequent Ontario Hydro PSAs, used the SETS code consequently it is constructed entirely of fault trees, although simple event trees were drawn to describe the accident sequences which were then redone as high-level fault trees for ten Fuel Damage Categories (ITX s) and five Plant Release Categories (PRCs) into which the system-level fault trees were merged to get the results in Table 11.2-1. 

The PSA (Miller, 1990, Wyss, 1990a, 1990b) consisted of three steps 1) issues important to safety were identified by "brainstorms" constructed as an accident progression event tree, 2) deterministic calculations were performed on the issues when information was not available from previous calculations or similar systems, and 3) information from step 2 was used to elicii e.vpert Judgement of the issues identified in step 1. 

The first step of the analysis is to identify the human actions and equipment failures that can lead to the failure of interest. An HRA event tree (Figure 5.14) is then constructed to depict the potential human errors (represented by capital English letters) and the potential equipment failures (represented by capital Greek letters). The series of events that will lead to the failure of interest is identified by an Fi at the end of the last branch of the event tree. All other outcomes are considered successes even though the propane release is not isolated in outcomes S2 and S3, so the process must be shut down. 

This study is a good reference for the construction of fault/event trees of systems that are affected by valve performance. The valve failure modes are identified, the associated mechanisms are described in detail, and preventive measures are offered. 

Tlie following simplified example, constructed by Hendershot, will facilitate tlie transition to tlie case studies. Suppose tliat a risk assessment is being conducted at a chemical plant to detenuine the consequences of two incidents (tlie initiating events of die event tree shown in Fig. 21.1.1) defined as... 

Construction of the event tree taking into account the interaction between the critical factors and the initiating event... 

The fault tree is a graphic representation of the relationships between basic events and the selected top event. Table 4.24 presents the standard symbols used in fault tree construction to show these relationships. 

As a result of the AIC s efforts, we now have a process for investigating accidents in which we construct an event tree for each incident. The tree is quite similar to a fault tree from the quantitative risk analysis discipline, except that in the investigations we often sacrifice some structural rigor to get the most results in a reasonable time. Basically, the process uses a team to reconstruct the chronology of the incident and to construct the event tree. We try to include those who are most familiar with what actually happened, including the injured person(s) if any. We use the same basic method to investigate process failures, spills, injuries, or any other system failures. Emphasizing the system aspects of the failure removes much of... 

Other considerations for fault tree construction are (/) assume that faults propagate through normally operating equipment. Never assume that a fault is stopped by the miraculous failure of another piece of equipment. (2) Gates are connected through labeled fault events. The output from one gate is never connected directly into another. 

Inductive methods, such as check lists, Failure Mode and Effect Analysis (FMEA), event trees, decision tables, Analysis of Potential Problems (APP). These methods proceed from an initial cause of the deviation and construct a scenario ending with the final event. They are based on questions of the type What if ... 

HAZOP and What-If reviews are two of the most common petrochemical industry qualitative methods used to conduct process hazard analyses. Up to 80% of a company s process hazard analyses may consist of HAZOP and What-If reviews with the remainder 20% from Checklist, Fault Tree Analysis, Event Tree, Failure Mode and Effects Analysis, etc. An experienced review team can use the analysis to generate possible deviations from design, construction, modification, and operating intent that define potential consequences. These consequences can then be prevented or mitigated by the application of the appropriate safeguards. 

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