Intermediate Events

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. 

Figure 4 shows a fault tree for a flat tire on an automobile. The top event, the flat tire, is broken down into two immediate contributing events, road debris and tire failure. The contributing event, road debris, is a basic event. This event, which caimot be broken down into other events unless additional information is provided, is enclosed in a circle to denote it as a basic event. The other event, tire failure, is enclosed in a rectangle to denote it as an intermediate event. 

The next step is to define the intermediate event, tire failure. There are two events which could contribute a worn tire resulting from much usage or a tire that is defective owing to a manufacturing problem. These are both basic events because additional information is needed for any further definition. 

It is important in fault tree analysis to consider only the nearest contributing event. There is always a tendency to jump immediately to the details, skipping all of the intermediate events. Some practice is required to gain experience in this technique. 

The principal problem in using fault trees is that for reasonably compHcated processes the analysis is most likely to produce a huge fault tree. Eault trees involving hundreds or even thousands of intermediate events are not uncommon. The effort involved in fault tree development can also be substantial, requiring several years. 

The resulting fault tree is shown in Figure 6, in which the top event is defined in terms of two intermediate events failure of the tank system or failure of the pumping system. Failure in either system would contribute to the overall system failure. The intermediate events are then further defined in terms of basic events. All of the basic events are related by AND gates because the overall system failure requires the failure of all of the individual components. Failures of the tanks and pumps are basic events because, without additional information, these events cannot be resolved any further. 

Fig. 6. A fault tree for the pumped storage example of Figure 5. For a real system the tank and pump failures would be more precisely defined, or set as intermediate events having further definition by subsequent basic events and more detailed failure modes.

The INTERMEDIATE event represeats a fault eveat that results from the iateractioas of other fault eveats that are developed through logic gates such as those defiaed above. 

A given event sequence can proceed to various incident outcomes, depending on the sequence of intermediate events, as shown by Table 2.1. For example, a release of flammable vapor could result in a vapor cloud-explosion, flash fire, jet fire, or harmless dispersion. Other incident outcomes that this book addresses are briefly described below. 

Identify intermediate events that either propagate or mitigate the developing accident scenario. 

For each initiating event, determine the various accident pathways defined by the credible combinations of intermediate events. 

Process Hazards Initiating Events Intermediate Events Incident Outcomes... 

FTA is a deductive method that uses Boolean logic symbols (i.e., AND gates, OR gates) to break down the causes of the top event into basic equipment failures and human errors. The analysts begin with the top event and identify the causes and the logical relationships between the causes and the top event. Each of the causes, called intermediate events, is examined in the same manner until the basic causes for every intermediate event have been identified. 

A fault tree is, itself, a Boolean equation relating basic events to the top event. The equation can be analyzed quantitatively or qualitatively by hand or by using computer code(s). If it is analyzed quantitatively, the probabilities or frequencies of the intermediate events and the top event are calculated. If it is analyzed qualitatively, a list of the failure combinations that can cause the top event is generated. These combinations are known as cut sets. A minimal cut set (MCS) is the smallest combination of basic events that, if they occur or exist simultaneously, cause the top event. These combinations are termed "minimal" because all of the basic events in a MCS must occur if the top event is to occur. Thus, a list of MCSs represents the known ways the top event can occur, stated in terms of equipment failures, human errors, and associated circumstances. 

For instance, a flat tire on an automobile is caused by two possible events. In one case the flat is due to driving over debris on the road, such as a nail. The other possible cause is tire failure. The flat tire is identified as the top event. The two contributing causes are either basic or intermediate events. The basic events are events that cannot be defined further, and intermediate events are events that can. For this example, driving over the road debris is a basic event because no further definition is possible. The tire failure is an intermediate event because it results from either a defective tire or a worn tire. 

The flat tire example is pictured using a fault tree logic diagram, shown in Figure 11-12. The circles denote basic events and the rectangles denote intermediate events. The fishlike symbol represents the OR logic function. It means that either of the input events will cause the output state to occur. As shown in Figure 11 -12, the flat tire is caused by either debris on the road or tire failure. Similarly, the tire failure is caused by either a defective tire or a worn tire. 

INTERMEDIATE Event An event that results from the... 

Now consider any one of the new intermediate events. What events must occur to contribute to this single event Write these down as either intermediate, basic, undeveloped, or external events on the tree. Then decide which logic function represents the interaction of these newest events. 

Continue developing the fault tree until all branches have been terminated by basic, undeveloped, or external events. All intermediate events must be expanded. 

The main disadvantage of using fault trees is that for any reasonably complicated process the fault tree will be enormous. Fault trees involving thousands of gates and intermediate events are not unusual. Fault trees of this size require a considerable amount of time, measured in years, to complete. 

Event sequence A specific, unplanned sequence of events composed of initiating events and intermediate events that may lead to an incident. 

Intermediate events — synthesis of RNA and DNA, imbibition of water, and stimulation of certain enzymes. 

Recently detailed analyses by Stein et al.[7] have determined that the observed polish rates for tungsten are inconsistent with the known oxidation rates of tungsten in specific oxidants. Closer analysis shows a more complex sequence of events. However, at this point, the intermediate events are not well understood and await further investigation. 

The reader should note that a fault tree includes the following (1) works backward from an undesirable event or ultimate consequence to the possible causes and failures, (2) relates the occurrence of an undesired event to one or more preceding events, (3) chain links basic events to intermediate events that are in turn connected to the top event, (4) is used in the calculation of the probability of the top event, (5) is based on the most likely or credible events that lead to a particular failure or accident, and (6) analysis includes human error as well as equipment failure. 

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