For downstream markets, process flow diagrams (PFDs) and piping and instrumentation diagrams (P&IDs) are two important tools used in designing systems.

These diagrams provide a common language and notation to describe the equipment and connections of a process in a system. Feasibility studies, safety assessments, and brownfield modifications for refineries and plants all depend on accurate PFDs and P&IDs for better efficiency, greater safety, and maximum return on investment.

These two types of diagrams serve different, yet equally important, purposes. PFDs show relatively simple connections and relationships among the various major components of a process as well as the calculated design values for those components. The PFD is essentially a relationship diagram with system ratings and operational specifications. Design detail is relatively low with information like pipe classes, code classes, and safety relief valves being omitted.

P&IDs are similar to PFDs in that they show the relative locations of process equipment and their associated connections in a process. However, P&IDs tend to show more process details, including all valves, controllers, and pipe specifications. The P&ID is essentially a more detailed functional relationship diagram minus the operational specifications. Detail is relatively high but not extensive in its notes and descriptions, excluding fine details like temperature, pressure, and relative elevations.

Neither a PFD nor a P&ID should be considered a three-dimensional, to-scale architectural or control diagram. They act as standardized diagrams for understanding the different processes of a refinery or plant design. For three-dimensional representations and modeling of plant subsystems and piping layouts, companies turn to software solutions like Intergraph’s SmartPlant3D or PROCAD’s PapriCAD 3D.