How to avoid the generation of loops in the construction of fault trees

This paper examines the question of the generation of loops in the construction of fault trees (FTs) starting from the printouts of a hazard operability studies (HazOp) analysis. Some examples of the formation of a loop are presented and the rules for its elimination are illustrated. The easiest way is to apply simple syntax rules in the graphic construction of a FT. This, however, presupposes prior recognition by the analyst of the initiation of a loop. The safest way, therefore, is to eliminate the problem at its root by deriving the FT straight from the modules of a well-structured HazOp procedure, such as recursive operability analysis.