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Bow Tie Analysis


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Bow Tie Analysis

The material presented here is taken from the next edition of Process Risk and Reliability Management - currently in draft form.

The bow tie technique is being used increasingly within the process industries not only to analyze risk but also to communicate hazard and risk findings to a broad audience. The technique has been in use since the 1970s, and has been incorporated into the Hazards Effects Management Plan (HEMP) methodology used by the Shell Oil Company. It generates graphical diagrams that have a rather fanciful resemblance to actual bowties (they were also at one time called butterfly diagrams).

The bow tie technique does not offer a new or different way of analyzing risk. The reason for its increased use is that the diagrams that it creates greatly assist in the communication of the hazards analysis process - particularly to non-specialists. Also, because the technique identifies the barriers that normally prevent an accident from occurring, it can be used in incident investigation (Philley 2006).

Figure 1 shows the structure of a bow tie diagram. It consists of a fault tree (the left hand side) and an event tree (the right hand side).

At the center of the diagram is an undesirable Top Event. Using the standard example from Chapter 1 to do with high level in Tank, T-100, the Top Event would be “High Level in T‑101 leading to overflow of RM-12”. To the left of the Top Event are threats, i.e., conditions that could cause the Top Event to occur. In the case of high level in T-100, threats will include instrument failure and operating error.

Figure 1
Bow-Tie Diagram

Bow Tie Analysis

Whipple and Pitblado (2008) note that each barrier is equivalent to a Fault Tree AND Gate (see Chapter 8 for an explanation of these terms). Entering each of the AND Gates are a “Demand on Control” and a “Control Fails”, as illustrated in Figure 2.

Figure 2
Barrier AND Gate

Bow Tie Analysis

Using the T-100 example once more, the control maybe a high level switch that shuts off the incoming flow. The “demand” would be a requirement that the switch be activated when the liquid in T-100 reaches a certain level. If this control fails then the likelihood of a tank overflow occurring is increased because this barrier has been overcome.

Returning to Figure 1, once the Top Event has occurred, a series of recovery or control steps are in place to reduce of eliminate the consequences. For T-100, one of the consequence of “Tank Overflows” is that toxic RM-12 may enter the drainage system and cause a serious environmental problem. This is “Consequence 1” in Figure 1. Two control measures are available. The first is a hazardous drain system that diverts spilled material to a special treating area. If the drain system does not work say because it is plugged then a second control is the use of a vacuum truck to suck up the spilled RM-12 before it enters the general drainage system.

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