A process flow diagram (PFD) is a diagram commonly used in chemical and process engineering to indicate the general flow of plant processes and equipment. The PFD displays the relationship between major equipment of a plant facility and does not show minor details such as piping details and designations. Another commonly used term for a PFD is a flowsheet.
Another definition can be that it is a very detailed map of every step in a process, including hidden factory steps.
Typically, PFDs of a single unit process will include the following:
- Process piping
- Major equipment items
- Connections with other systems
- Major bypass and recirculation (recycle) streams
- Operational data (temperature, pressure, mass flow rate, density, etc.), often by stream references to a mass balance.
- Process stream names
Generally they do not include:
- Pipe classes or piping line numbers
- Instrumentation details
- Minor bypass lines
- Controllers like Level Control or Flow Control
- Isolation and shutoff valves
- Maintenance vents and drains
- Relief and safety valves
Process flow diagrams of multiple process units within a large industrial plant will usually contain less detail and may be called block flow diagrams or schematic flow diagrams.