The propagation of faults in process plants: 9, trip systems in fault tree synthesis

Studies on the Digraph-Based Approach for Fault-Tree Synthesis. 2. The Trip Systems

“…Generally speaking, there are only two basic diagraph structures that can be identified in various trip systems (Hunt et al, 1992), i.e., the NFFL and the NFBL. In this work, the term "feedforward loop" (FFL) is used to represent a digraph configuration in which two or more paths start from a common node and converge at another node, and "feedback loop" (FBL) is a path which starts and ends at the same node.…”

“…Also, the procedure for building fault trees associated with other undesirable events in the trip systems has not been developed explicitly in any of the previous works. Finally, notice that in order to assess the risk associated with functional failures, three separate fault trees have to be constructed first and a final tree can then be assembled from portions of the previous there (Hunt et al, 1992). Thus, it is easy to see that the proposed method represents an improvement of the conventional approaches and the resulting software should be more efficient.…”

“…This is due to the fact that, although the diagraph structures of trip systems are similar to those of the regular control loops, the corresponding control logic is not the same. On the other hand, it should be noted that Lees and his co-workers (Hunt et al, 1992) have already developed useful faulttree construction techniques for trip systems with a different approach in modeling, i.e., the mini-fault trees. However, in applying their method, separate models have to be adopted to analyze the functional and operational failures associated with the same system.…”