Make your Petri nets understandable: Reliability block diagrams driven Petri nets

For more than 30 years, the Petri nets (PNs) have proven to be very powerful for safety/dependability modeling and calculations. The number of publications is increasing and a standard is even going to be issued soon. Nevertheless, the dissemination is slow and the PNs are not yet commonly used by reliability engineers. As a matter of fact, the current PNs are often intricate and difficult to understand even if, in reality, they model simple systems. This is discouraging both for the PN designers and the PN readers and this is certainly one of the causes impeding the dissemination of this approach. Hopefully, some simple additional graphical rules can be easily implemented to undertake the PN modeling of large industrial systems while keeping the readability and the understandability all along the building process. The aim of this publication is to deal with the graphical aspects of Petri nets and it proposes first some very simple tricks and guidelines to structure and improve the drawing of standard PNs. It explains how the introduction of predicates and assertions allows developing modules (i.e., generic sub-PNs) in order to build the PNs in a modular way. Then it shows how reliability block diagrams (respectively flow diagrams) may be used as guidelines to build large PNs by using the above modules. It describes the RBD driven PNs which are very effective to model safety systems (e.g. safety instrumented systems) and explains how to extend them to the flow diagram (FD) driven PNs which allow to undertake simplified production availability modeling and calculations.