A Process flow diagram (PFD) is a diagram 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 or minor instrumentation.
PFD's are sometime called flowsheets
Typically, process flow diagrams will include:
Primary Process piping
Major equipment items
Control valves and other major valves
Connections with other systems
Major bypass and recirculation streams
Operational data (temperature, pressure, flow, density, etc.), often by stream references to a mass balance.
They generally cover a single part of the process and several are required for a complete process.