Studies on the Digraph-Based Approach for Fault-Tree Synthesis. 1. The Ratio-Control Systems

The development of a systematic diagraph-based fault-tree synthesis procedure for complex ratiocontrol systems is presented in this paper. First, the feasibility of the established techniques in handling the single-NFBL (NFBL = negative feedback loop) ratio-control systems is verified with an example. Next, the diagraph structure of systems with multiple NFBLs is described and analyzed in detail. On the basis of qualitative simulation of the fault propagation patterns, the corresponding generalized fault-tree structures are then established. It can be observed clearly from the results of steady-state analysis that none of the existing procedures are capable of producing the correct fault trees for the more complex ratio-control systems. Also, to demonstrate the effectiveness of our techniques, successful application of the proposed structure to a caprolactam reaction process is shown. Finally, the resulting fault tree is compared with one obtained from a single-NFBL system and the trade-off between the two in terms of system safety is assessed accordingly.