Operability analysis (OA), developed as far as its ＇quantitative＇ aspects, was employed to examine two critical features of a pressure-regulating installation (PRI) of a branched gas mains: ＇sudden blow-off＇ and ＇continuous discharge of gas＇ from the vent valve. An estimate was obtained of the probability of their occurrence by an event tree directly extracted from an OA, not handled in the classic manner, but treated as a tool for a sequential logical analysis. This full integration of qualitative and quantitative methods ensures the essential features for a probabilistic safety analysis, namely: systematism, completeness and congruence; numerical reliability and verification.