Page 532 - Corrosion Engineering Principles and Practice

498 C h a p t e r 1 2 C o r r o s i o n a s a R i s k 499

Event

Conditional Fault Basic Undeveloped

Transfer

In Out

Gate

AND OR Inhibit

FIGURE 12.11 Fault tree symbols for gates, transfers, and events.

• OR gate: The output is true only if one or more of the input
events occur (add probabilities on the input, therefore
increasing resulting probability).
• Inhibit gate (hexagon): One input is a lower fault event and the
other input is a conditional qualifier or accelerator (direct
effect as a decreasing (< 1) or increasing factor (> 1)).
FTA emphasizes the lower-level fault occurrences that directly or
indirectly contribute to a major fault or undesired event. The technique
is one of “reverse thinking” where the analyst begins with the final
undesirable event that is to be avoided and identifies the immediate
causes of that event [11]. By developing the lower-level failure
mechanisms necessary to produce higher level occurrences, a total
overview of the system is achieved. Once completed, the fault tree
allows an engineer to fully evaluate a system safety or reliability by
altering the various lower-level attributes of the tree. Through this
type of analysis, a number of variables may be visualized in a cost-
effective manner.
Tracing the chain of events leading to the final outcome can
indicate where extra monitoring, regular inspection, and protective
schemes (e.g., temperature and pressure sensors, and alarms) would
be most efficient at preventing an impending failure. Fault tree
analysis is a very useful tool for studying the routes by which an
accident can occur, and is particularly effective at identifying accident
scenarios due to secondary and tertiary causes. However, it requires
a great deal of skill and effort to implement. For this reason it is