Hierarchical PFDs

Process Flow Diagrams (PFDs) use a notation that is similar to extended Function Flow Block Diagrams (eFFBDs) except that PFDs show operational sequences that include:

• The system under consideration
• The users of this system
• The other systems with which this system interacts

An operational sequence shows how the system is intended to be used. These three aspects of an operational sequence are shown in the PFD as:

• System operations, to show operations performed by the system
• User operations, to show operations performed by users of the system
• Agent operations, to show operations performed by other systems with which the systems interacts

PFDs have been extended:

• To be hierarchical
• To include a new operation symbol
• To have associated hierarchical consistency checks
• To allow user and agent operations to be encapsulated in a new environment symbol

The operation symbol can be expanded into a lower level PFD, and is the means to create a PFD hierarchy. At the higher levels, it is expected that PFDs will mainly contain operations. As these are decomposed into lower-level PFDs, it is expected that more and more of the operational sequence will be allocated to the system (when they will be shown as system operations), or to the users of the system (where they will be shown as user operations), or to the systems with which the system interacts (when they will be shown as agent operations).

The environment symbol allows user or agent operations to be enclosed inside it. This allows the PFD to show explicitly which user or external system in the environment is to be responsible for performing each of the user and agent operations: